<%BANNER%>

Ultrafast Spectroscopy of Novel Materials

University of Florida Institutional Repository
Permanent Link: http://ufdc.ufl.edu/UFE0021382/00001

Material Information

Title: Ultrafast Spectroscopy of Novel Materials
Physical Description: 1 online resource (150 p.)
Language: english
Creator: Hardison, Lindsay Michelle
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2007

Subjects

Subjects / Keywords: cpes, nanorods, spectroscopy, ultrafast
Chemistry -- Dissertations, Academic -- UF
Genre: Chemistry thesis, Ph.D.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: My research focused on steady state and time-resolved photophysical characterization of a series of semiconductor nanoparticles and water soluble conjugated polyelectrolytes. Several studies have shown that the electronic structure and relaxation dynamics in CdSe nanocrystals are not only size but are also shape and passivation dependent; however, there is no detailed comparison of the photophysical properties of ZnCdSe particles with different relative amounts of Zn. This dissertation presents data collected for colloidal CdSe, CdSe/ZnSe and ZnCdSe nanoparticles with rod-like architectures synthesized and investigated in our labs to determine how size, shape, passivation and composition affect the quantum confinement and dynamics. In addition, a series of different polymer repeat unit lengths of a linear conjugated polyelectrolyte (CPE) with a carboxylate ionic side chain have been synthesized and their photophysical properties have been explored. Spectral shifts and line broadening exhibited within the Raman spectroscopy, UV-Vis spectroscopy and photoluminescence aided in determining the extent of alloying and compositional disorder created during the alloying process. The photoluminescence quantum yield of ZnCdSe nanorods is higher than that from pristine CdSe nanorods indicating a higher binding energy of the exciton. This effect is speculated to be due to increased localization of the exciton as a result of fluctuations in the composition, ultimately resulting in increases in luminescence efficiencies. Moreover, time-resolved photoluminescence characterized lifetimes of nanoparticles with similar shape but different composition. Emission of an inhomogeneous population distribution (different sizes, shapes or composition) leads to the simultaneous probing of particles with different decaying rates. A stretched exponential function, I(t)= A*exp-(t/?)^beta, can be used to describe these systems, where beta < 1 corresponds to disperse populations. In the experiments presented here, the photoluminescence data yields small beta values, independent of the emitted photon energy. Photoluminescence decay lifetime, ?, of the samples increased with alloying time due to compositional disorder leading to exciton localization. The dynamics of each nanorod was studied by absorption changes using ultrafast pump-probe spectroscopy. An excitation wavelength dependence study has been conducted to gain insight into the intraband/interband relaxation in core/shell nanorods with small valence band offsets. Determination of the dynamics and mechanisms of these systems will be useful for the study of fundamental physics and light emitting applications such as LEDs, photovoltaic devices, lasing and fluorescence tagging. CPEs are soluble in polar solvents and their conformational properties can be tuned to enhance their emissive behavior for sensing and device applications. It was found that polymer concentration, solvent, aggregation inducer and chain length, all affect the quenching efficiency; therefore, this dissertation examines energy transfer mechanism responsible for this behavior using ultrafast upconversion. Upon excitation of the aggregates from energetically higher lying isolated chains, the fluorescence lifetimes result in multi-exponential behavior due to the competition between the radiative and non-radiatve decay.
General Note: In the series University of Florida Digital Collections.
General Note: Includes vita.
Bibliography: Includes bibliographical references.
Source of Description: Description based on online resource; title from PDF title page.
Source of Description: This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Statement of Responsibility: by Lindsay Michelle Hardison.
Thesis: Thesis (Ph.D.)--University of Florida, 2007.
Local: Adviser: Kleiman, Valeria D.

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2007
System ID: UFE0021382:00001

Permanent Link: http://ufdc.ufl.edu/UFE0021382/00001

Material Information

Title: Ultrafast Spectroscopy of Novel Materials
Physical Description: 1 online resource (150 p.)
Language: english
Creator: Hardison, Lindsay Michelle
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2007

Subjects

Subjects / Keywords: cpes, nanorods, spectroscopy, ultrafast
Chemistry -- Dissertations, Academic -- UF
Genre: Chemistry thesis, Ph.D.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: My research focused on steady state and time-resolved photophysical characterization of a series of semiconductor nanoparticles and water soluble conjugated polyelectrolytes. Several studies have shown that the electronic structure and relaxation dynamics in CdSe nanocrystals are not only size but are also shape and passivation dependent; however, there is no detailed comparison of the photophysical properties of ZnCdSe particles with different relative amounts of Zn. This dissertation presents data collected for colloidal CdSe, CdSe/ZnSe and ZnCdSe nanoparticles with rod-like architectures synthesized and investigated in our labs to determine how size, shape, passivation and composition affect the quantum confinement and dynamics. In addition, a series of different polymer repeat unit lengths of a linear conjugated polyelectrolyte (CPE) with a carboxylate ionic side chain have been synthesized and their photophysical properties have been explored. Spectral shifts and line broadening exhibited within the Raman spectroscopy, UV-Vis spectroscopy and photoluminescence aided in determining the extent of alloying and compositional disorder created during the alloying process. The photoluminescence quantum yield of ZnCdSe nanorods is higher than that from pristine CdSe nanorods indicating a higher binding energy of the exciton. This effect is speculated to be due to increased localization of the exciton as a result of fluctuations in the composition, ultimately resulting in increases in luminescence efficiencies. Moreover, time-resolved photoluminescence characterized lifetimes of nanoparticles with similar shape but different composition. Emission of an inhomogeneous population distribution (different sizes, shapes or composition) leads to the simultaneous probing of particles with different decaying rates. A stretched exponential function, I(t)= A*exp-(t/?)^beta, can be used to describe these systems, where beta < 1 corresponds to disperse populations. In the experiments presented here, the photoluminescence data yields small beta values, independent of the emitted photon energy. Photoluminescence decay lifetime, ?, of the samples increased with alloying time due to compositional disorder leading to exciton localization. The dynamics of each nanorod was studied by absorption changes using ultrafast pump-probe spectroscopy. An excitation wavelength dependence study has been conducted to gain insight into the intraband/interband relaxation in core/shell nanorods with small valence band offsets. Determination of the dynamics and mechanisms of these systems will be useful for the study of fundamental physics and light emitting applications such as LEDs, photovoltaic devices, lasing and fluorescence tagging. CPEs are soluble in polar solvents and their conformational properties can be tuned to enhance their emissive behavior for sensing and device applications. It was found that polymer concentration, solvent, aggregation inducer and chain length, all affect the quenching efficiency; therefore, this dissertation examines energy transfer mechanism responsible for this behavior using ultrafast upconversion. Upon excitation of the aggregates from energetically higher lying isolated chains, the fluorescence lifetimes result in multi-exponential behavior due to the competition between the radiative and non-radiatve decay.
General Note: In the series University of Florida Digital Collections.
General Note: Includes vita.
Bibliography: Includes bibliographical references.
Source of Description: Description based on online resource; title from PDF title page.
Source of Description: This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Statement of Responsibility: by Lindsay Michelle Hardison.
Thesis: Thesis (Ph.D.)--University of Florida, 2007.
Local: Adviser: Kleiman, Valeria D.

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2007
System ID: UFE0021382:00001


This item has the following downloads:


Full Text
xml version 1.0 encoding UTF-8
REPORT xmlns http:www.fcla.edudlsmddaitss xmlns:xsi http:www.w3.org2001XMLSchema-instance xsi:schemaLocation http:www.fcla.edudlsmddaitssdaitssReport.xsd
INGEST IEID E20101206_AAAACY INGEST_TIME 2010-12-06T15:21:10Z PACKAGE UFE0021382_00001
AGREEMENT_INFO ACCOUNT UF PROJECT UFDC
FILES
FILE SIZE 6983 DFID F20101206_AABMMU ORIGIN DEPOSITOR PATH hardison_l_Page_139thm.jpg GLOBAL false PRESERVATION BIT MESSAGE_DIGEST ALGORITHM MD5
eb0ca25ba9dd969861e73a66711b69ff
SHA-1
acb036bd4e3b9bc0f3b55f371a122da10bfa31fe
142750 F20101206_AABLKH hardison_l_Page_148.jp2
7fc99821185ff44fa4624b7ce75e0dc4
ec250359cd96657e87c698c142623227d5f1d8c2
1051972 F20101206_AABLJS hardison_l_Page_133.jp2
c27fd60c21d78a1606fa86167104bb7d
ca1924bf366724175fee4a1df98a354376242ff3
30640 F20101206_AABMMV hardison_l_Page_140.QC.jpg
2edc828080536c3b2982b340e4e9d235
49deade17518af4a8edf7910e37f969843b0388a
79521 F20101206_AABLKI hardison_l_Page_149.jp2
64e78b3688798532e5595b5dab8adaf2
4635f4cfa016c38f248758bc4a7db8edee3e030e
133570 F20101206_AABLJT hardison_l_Page_134.jp2
53598b24ba359f0f4ba72e04e1e0704c
8e8405c8a3ef4404a0f73896c59c1a1cb1102255
6779 F20101206_AABMMW hardison_l_Page_141thm.jpg
cc442205f708636f9425d9fd870d6dff
c7c594c5daf5769e312e7a45af6f733617ef166d
73450 F20101206_AABLKJ hardison_l_Page_150.jp2
df48345a104cdd401f3312f41f45124a
6202d0571b35e279f5ee8c86fa9a2142e63a5144
140091 F20101206_AABLJU hardison_l_Page_135.jp2
c1908b6682d2ed75e3e0f0c229e5bc52
17231c7cea3df11fc1468dd3c9c330db9ac161a8
6914 F20101206_AABMMX hardison_l_Page_143thm.jpg
88d3b7b9e266a1694767c81d8b2aea6b
e5e591877e604b87fb46ade626b0e200f3d83d1e
1053954 F20101206_AABLKK hardison_l_Page_001.tif
7b6ef44edaa59681f5e8c87f2e4ee196
eb8b7209bddf8ea7d6394127675d03841b0ced91
138221 F20101206_AABLJV hardison_l_Page_136.jp2
b3b084513ac471b286db916d85643060
bf9428cdf14dacd235a43365c2a2ee2797d647cc
29333 F20101206_AABMMY hardison_l_Page_144.QC.jpg
5deab0cafa44529e3455f6bace7b48b6
192edae79001322e703073fed579f12fc1f146a4
135764 F20101206_AABLJW hardison_l_Page_137.jp2
485a5aa6c31f99aaec9afee94dd4d25a
32fd9dd924dc1d379a446dd1b50326fcd48e71cc
31044 F20101206_AABMMZ hardison_l_Page_145.QC.jpg
6c46d330c463efc5267d549c5ffac196
0f8d6b85e4b0d9cc04615f837039b1ea95717d84
F20101206_AABLKL hardison_l_Page_002.tif
b5fcbb03fc123423edd7ab5f9041847f
b0093c91cba480cb74e59c7660088e0f8c77c892
137286 F20101206_AABLJX hardison_l_Page_138.jp2
4adc067f4d82c5d9626f8f206a7f44ff
2da1c5000151d110c2c9fa7ce82d28e1658680f2
25271604 F20101206_AABLLA hardison_l_Page_017.tif
3e92c021ad4e78be4b9d0f6e6a4817bd
1f41d17d34ee2923965ee6a26cedb702863ffd22
F20101206_AABLKM hardison_l_Page_003.tif
91b414acfe65ad8544f92cbf59652b87
f5581d407575e9db494ca2dfa84dc207aabd3ff4
1051946 F20101206_AABLJY hardison_l_Page_139.jp2
57a7cdccf017aa24c63748f9ac7ad244
96d40a2026762dfe2866fd34c53f39c756691acc
F20101206_AABLLB hardison_l_Page_018.tif
ece8bac84b9afc3fb907e4e954a97637
52c76adf51b089ea2d305172f7e3fda789a849ce
F20101206_AABLKN hardison_l_Page_004.tif
de40d6deb946bf6e868b3149b1b5809f
ead0e39062df9d58aea5dc07a51517c557bda0ca
143256 F20101206_AABLJZ hardison_l_Page_140.jp2
c3b40b65d40d15f300f29d4d5b8a6231
5172d425536fe60d2faf127212c59f1f5c6add05
F20101206_AABLLC hardison_l_Page_019.tif
cff84179351431c9b30d9053987a10a5
1d40ce2d5f22ca0a2952ca194af16e1de3c8a67f
F20101206_AABLKO hardison_l_Page_005.tif
666fa940549bb9ed07eef715a114d3e8
250be80a9c77ba0a617bda0f79959e8da4e593de
F20101206_AABLLD hardison_l_Page_020.tif
9f0fc1cf39ffddd06f5e637e35315f47
061529359770bf4801d9b6e55c8dacc48b3118cc
F20101206_AABLKP hardison_l_Page_006.tif
4088846505c0473f2d39148e954b2cfd
a4bd7124d20130a6e5f8838dd7c44c85ae77f118
F20101206_AABLLE hardison_l_Page_021.tif
c281eed193e262f1c43f82bae50adc4a
9f9d7e4f6e4f8a97654db75ed7ae418a65e40157
F20101206_AABLKQ hardison_l_Page_007.tif
6a751d28d3e923326768443092ea2308
a4de601cfe1b206d9bbaf3662b56ba193f4d8f5f
F20101206_AABLLF hardison_l_Page_022.tif
cb22b6ad3270d4f3c39535b00b26a934
466e4ab5aedca96bcb22e21ed8c5c168b86341ab
F20101206_AABLKR hardison_l_Page_008.tif
abb2ec603489b0457694b7e063bd6fef
290905a85f8ffb8186fc4f178ee4ddc02c75c4b3
F20101206_AABLLG hardison_l_Page_023.tif
cbdc8639a6005cc1a68fd1106871b9e2
b44b3a8da039797b53b6aa552ceae78399aacf25
F20101206_AABLKS hardison_l_Page_009.tif
fa9fd941f2e65dfe707de6b5223d9837
b05799d41e8bd355ded14cb73e107caa7ef32b97
F20101206_AABLLH hardison_l_Page_024.tif
40d8945d79edaf588efa7cc9a7974f06
a4f7a86627e30d6d1e2b5afcbb6fe17640b9807f
F20101206_AABLKT hardison_l_Page_010.tif
d8db3819d6534ba161134bc02c55f9d2
af144a8c4ab11588e67819b6a1eb4104d09678dc
F20101206_AABLLI hardison_l_Page_025.tif
a36f26f94058e7e3491cc09528d81a0e
4a9808a6d527f6c302372bf0245e9febc25a6982
F20101206_AABLKU hardison_l_Page_011.tif
201798a78a1f7f11f72bd82a1681b051
6cc36298aabfe64f7b124075db6330f9d5dabe11
F20101206_AABLLJ hardison_l_Page_026.tif
338ee1a642bda31a53d619c177c7fba5
c028b22a73326191bdf64e192af6d8d9052ab426
F20101206_AABLKV hardison_l_Page_012.tif
4fd3208aa5989e8fbdcad13b6e8b3480
a7c5bfd9ab7b9c977fccff06113e22ecf9e93875
F20101206_AABLLK hardison_l_Page_027.tif
cb063e785960eac72e2d0464055456c8
6f1c0c1b2d7d689cef60504e73b389a0756e3de0
F20101206_AABLKW hardison_l_Page_013.tif
2f8a05096545a6e4ead2cf5d3fe061c8
6f38b3f699b35219f1e976295d38c02625444436
F20101206_AABLLL hardison_l_Page_028.tif
781c9de1f3b1e68f84638d819daff858
27b77d32b71c02d68cbe6d94d612cb2d7c4afa74
F20101206_AABLKX hardison_l_Page_014.tif
116b8faa68a9914b6f2686edd58edc86
f655fa44a0f11fb854a6216b83df3e44b835508b
F20101206_AABLMA hardison_l_Page_043.tif
d548d652772a3e892fa6846ad91fb97c
8743f3674c3fef72b8350bfcdeb4668c87150736
F20101206_AABLKY hardison_l_Page_015.tif
ba0b6a7fe049e111dcafaff76a07de8c
0ccc2f18f1e834dd85bf145c42f422481bffe1f3
F20101206_AABLMB hardison_l_Page_044.tif
160c2d3c53f4abd42b3ca495ea31cbdb
bafc0afb1d8de5f1cf91dd985678a6b373f34a91
F20101206_AABLLM hardison_l_Page_029.tif
2520031cfe2e32eb68a8d7be4f688c63
b8dc5624fbd52533994ad45fd22c60a465641e7b
F20101206_AABLKZ hardison_l_Page_016.tif
3e9f9a3c32859fa8389e89516664077f
fa7a20cccaa977883a1738f2aa180dbd0ab8611d
F20101206_AABLMC hardison_l_Page_046.tif
961cf72c623ff1b31e8d7abc299e75a2
095c006c0d1ee9b2f6380f6f0a6d393724ad3df1
F20101206_AABLLN hardison_l_Page_030.tif
32fae330e715acbea37f26fbb421d940
7c4368428cc0b3177a5f3b19641d02939a0c3939
F20101206_AABLMD hardison_l_Page_047.tif
f84b345beaf3cb506212d846fccf7377
c0769fe84ee1792ad65a11d9ca89c04603efaf1c
F20101206_AABLLO hardison_l_Page_031.tif
86c315364479efa189453d9bb441e638
70a4da19debc09febdebc8824394554ab3d82e5f
F20101206_AABLME hardison_l_Page_048.tif
1bf60a122e5f61157c100af9989c3c9d
d45758e74865df3bc25015e04b5892dd929f65a9
F20101206_AABLLP hardison_l_Page_032.tif
8a971499b3d2c595c0fdb117746fb74d
94d800ca73d7cbdce4663a1656ca6a47543857bd
F20101206_AABLMF hardison_l_Page_049.tif
b7c14ea8b26c66fb55bba990f7d551fa
f780fa73be992ec5aa4a38c440d25cf6edc493c6
F20101206_AABLLQ hardison_l_Page_033.tif
ef013a4b34c4a68f663cd050d35b9543
6459b6e7cb602b4c57266d4fc60d49bf265577e1
F20101206_AABLMG hardison_l_Page_050.tif
404ad6a8e9d1430eae2631a87da0e7ed
3b182d8236b953c928357406882fe0a08172770a
F20101206_AABLLR hardison_l_Page_034.tif
5d897b90e168b31958ad50fef651a2e9
49b86e4e70876225f5741743a712f2d0c368933a
F20101206_AABLMH hardison_l_Page_051.tif
ef6b311a6b3a3d7965e2ef4cf63c69d6
374d563fbd6e926ad266a64a41075a884f893a29
F20101206_AABLLS hardison_l_Page_035.tif
a5fc946448dd64cee3c4ff64cf611cf7
2b648e28ace9fbf345bdd88c11c968569dd3f12c
F20101206_AABLMI hardison_l_Page_052.tif
2275692bf965a9248b88a9e840743343
450c4064cd1b97125ad22a5ba0ac7d8bac3a9b4f
F20101206_AABLLT hardison_l_Page_036.tif
fcc796b635e75cbbb6d8734c54cc65da
03feb56ccaba2f03befc6e7eef45567fce2928e9
F20101206_AABLMJ hardison_l_Page_053.tif
2a35b1fb89d867d3a0428d28ecb7140d
1eeba5bf611fccd4b788632b4bea15704eb07f83
F20101206_AABLLU hardison_l_Page_037.tif
dd20f222b89017f02fe87a04bb55c1e7
0b970f64f25e560012a3695a9faf6627ec65443a
F20101206_AABLMK hardison_l_Page_054.tif
9a6bbd63cf7f49077fe9c68fefe2e9c4
eb4cf6c787f58fc2492af602243acc17b406ca53
F20101206_AABLLV hardison_l_Page_038.tif
0e95ca842621e83d88bc0245afbf3a91
54784b4f36dba88117e8743a2cf8c2d163b5290b
F20101206_AABLML hardison_l_Page_055.tif
9e6129ee042602380583848e96091f44
fa5a07a9aff2aa209c8e0a33ba89e0e899f16305
F20101206_AABLLW hardison_l_Page_039.tif
87ebb955e2f815681d997e0f51994a36
829fdbb48d9a5aea81c2c35c1c5672c248e04a6b
F20101206_AABLNA hardison_l_Page_070.tif
53918f67a10ad7a17e26473a361bb104
06de1850d4ab89d23c0183fdcd6cb62ad62fa24d
F20101206_AABLMM hardison_l_Page_056.tif
d4c266ad14fad888bab60e4936ce360f
7560704c8623de0a1c84d760ac2b9ea469bfe89f
F20101206_AABLLX hardison_l_Page_040.tif
c8b0a7e0247577af411343e7813d73f0
4907da77fe3762dcf6ea6e9b37bc87f20e82d2ac
F20101206_AABLNB hardison_l_Page_071.tif
170b72f7c167f0fb884f35d7923298eb
4225b7edcdebd5627efd2515c13dc66c2e4c8d38
F20101206_AABLLY hardison_l_Page_041.tif
1e11645be9fc0f8f8aec303335d69781
cb617d334a8d5198e687e8192f86a09505da21b5
F20101206_AABLNC hardison_l_Page_072.tif
8c1d62bc49993211bb185aecac174370
eed5e596f7e7f1fd3e3cf3b52b5dfadd3068c91d
F20101206_AABLMN hardison_l_Page_057.tif
d14064698e9faba4f6c8d43685b04ab7
5b3ce069f071c09322e2ac2669d66ef98ea98d95
F20101206_AABLLZ hardison_l_Page_042.tif
9b749ad35c1a91468d8560acaaca5852
5318af8f71e9798701fdb0a3170664ffed66e65f
F20101206_AABLND hardison_l_Page_073.tif
08572ef0b5e2759a0dc5a3b7eccb9021
0bc5d6c615caa8da6633e6a8a69cafa294ce48a1
F20101206_AABLMO hardison_l_Page_058.tif
84082edb6257184d3579ff300e9a4bcc
c2cd23c848590528f6996b467569fcc50eebab00
F20101206_AABLMP hardison_l_Page_059.tif
2125105f812ad7f24c4fd1581f7c937e
4a32a4a18ffb0fee6c9de91add8775bf42372d71
F20101206_AABLNE hardison_l_Page_074.tif
62e2fd7c0c429aa8396a3cd455ad04c2
0812bfae68fae69c4823e3b81d034938f005cc4c
F20101206_AABLMQ hardison_l_Page_060.tif
533f6624265c35c6efea17a2dbfd8893
30f963a9fbe25953a1a94c2a3fde6bdc8e4b4d36
F20101206_AABLNF hardison_l_Page_075.tif
8666273449094a52ce33f782f2d2a2ab
1101295eeec9caa92dcbfc655961927fbc9d75f0
F20101206_AABLMR hardison_l_Page_061.tif
d2ffc6c0d06d902447ef225e50ac167e
287118e686acef255fa78dd10670831b79006dd6
F20101206_AABLNG hardison_l_Page_076.tif
53307eae82ddf5d1be965e95d0080848
ac753b5dff23964f452ec097f2254ad89776db3e
F20101206_AABLMS hardison_l_Page_062.tif
556da6d570a7c4bb3438db38475bdfb8
1909e99952a4be275b9a1d0919246a8c9deba240
F20101206_AABLNH hardison_l_Page_077.tif
edc5c2e11b8fc86dc7b91f9707d5bb1c
49ae9fdda7b83ebedae17954d15f80e24061b237
F20101206_AABLMT hardison_l_Page_063.tif
4c4e44249121519136e86738be4c51ef
b7f8f6c1ca9236c7e6dbb3f0a35c9cf455da366c
F20101206_AABLNI hardison_l_Page_078.tif
88da86eb4df1beab36148b30dbb7055f
d3b386ed320ad3facbc213b862b44e5cbfc868db
F20101206_AABLMU hardison_l_Page_064.tif
1b20f8a403e863137a4af66865dfb0fa
7933a876aff0d9d7810af51ff2108a56682d82fc
F20101206_AABLNJ hardison_l_Page_079.tif
ce68e41655727f480bf4db33b5c55968
9e9d8ba5e6d3283c328a24141c2d1a2cc90320de
F20101206_AABLMV hardison_l_Page_065.tif
7c4bff3f151047c5d588cc19463c631a
b3cb20486475c4a1dbb300f53f38289d23c50638
F20101206_AABLNK hardison_l_Page_080.tif
c95f96a5ccee6ebe88c422fe3c6b5321
455179641753940ec57ac190d2773654a359d36a
F20101206_AABLMW hardison_l_Page_066.tif
aed974345f4f622768c1d4c9ecb4756b
79d546936af1fe5d9d59faaab453905d9172efa4
F20101206_AABLNL hardison_l_Page_081.tif
e419ee2bd10979a3b16f9e546e3a851c
9f03667e30b1a2d3bc72f7c14dcb260bf1469270
F20101206_AABLMX hardison_l_Page_067.tif
57931fd2049414dce569d7b0f7e7e6d9
b6e85fb94bf9ac9d18fc67d929e8e06ee5932f87
F20101206_AABLOA hardison_l_Page_096.tif
c6660294c73d67209caeb431cfe00d84
2ac52eb91571fbb1a4148e65b768c0aec7df9111
F20101206_AABLNM hardison_l_Page_082.tif
0aadd13d0cb2c6fd667ac0fbf0a573d6
2a5d24d67a64cddd27f176519d80c9aea633ce2e
F20101206_AABLMY hardison_l_Page_068.tif
1e001041aafb5d55ba256aadfeef4d42
07b8118784ffc3edd1384043b3ef3381bb8566cd
F20101206_AABLOB hardison_l_Page_097.tif
cb6674ba7f148861fb2587c1de1cb414
2e8a9fc070e90285248f47629d1ae9d19dd186d9
F20101206_AABLNN hardison_l_Page_083.tif
e0a7cdfa1e40ec8336d473a33875ecbe
750c1e1057c5c9ba03773d06a129d157416135de
F20101206_AABLMZ hardison_l_Page_069.tif
c6d527bb7e2a9d75fc8c09105bbe393a
d5714054d3fd94967292eb8d05a76d6838fedf7e
F20101206_AABLOC hardison_l_Page_098.tif
fe452d6b8a8d13592253c7bf837de7e3
ebde243cca07e726182559f9116571c028a5a280
F20101206_AABLOD hardison_l_Page_099.tif
bcfa82d5155eff91bd48e4230ac758cc
3d286e73346587ab095de846fefbaaafdae0f7b7
F20101206_AABLNO hardison_l_Page_084.tif
9bf046f4f0368d7d82922880531f2452
e6bebe0a388add45e4bbf2bf72c195b86dbd7d40
F20101206_AABLOE hardison_l_Page_100.tif
6a7f3e8c6e42bee4fb0d9144e782125b
ec99f5237bc9aa4abd44c8d3aeb9935b3e92da42
F20101206_AABLNP hardison_l_Page_085.tif
db9ac9a3f796f76cad6b69715a9151e5
dd8a6d4ea8d9810502f6f88fb49951d9c2d4905c
F20101206_AABLOF hardison_l_Page_101.tif
67787e8309750d0f070d390bd1a4b264
1bbd35950c812b5f0a0844359e0c27135e004685
F20101206_AABLNQ hardison_l_Page_086.tif
5acb46c93c5978e69022e80806d9dc09
2905da23b27b6c1c724f56e888a2674dcc7d7a77
F20101206_AABLOG hardison_l_Page_102.tif
1ce393c3fea58b3b4c95cc11b0f7da02
addc091125d1f9217a805e89100230915b5f9965
F20101206_AABLNR hardison_l_Page_087.tif
05b0d461d97b410cf317afc6f14f0bc2
4364b3c107b995d165ded7048d194561ac728277
F20101206_AABLOH hardison_l_Page_103.tif
ceda375320a5151dffe074e56c3fdfc6
6c11937dc0ba70562d50e896f453db222afdfc96
F20101206_AABLNS hardison_l_Page_088.tif
42582270ee7b82e42862f833ec82edf5
73cdcb3464267b73db408f23f27883f9c22c585e
F20101206_AABLOI hardison_l_Page_104.tif
c3743db8a3aaea60318b4907b2262629
b5e41e0b3dc9730d8451364967664d5e44002101
F20101206_AABLNT hardison_l_Page_089.tif
3bb4412a316c77d48bdba1d5eb9f5ba3
b33ec7c6896f1e9f287846bdef8372e7bccbc1a1
F20101206_AABLOJ hardison_l_Page_105.tif
25eb9ee74a86dd2a7a087f0294df50ba
401ab13f3d7e4db601abda6e48ec1d147c143adb
F20101206_AABLNU hardison_l_Page_090.tif
5c2108e09c6557e9cbb27985557891e3
aa5faaca6a51ee01ffe83d70c1ce8623c39adbd0
F20101206_AABLOK hardison_l_Page_106.tif
9ce25b1aa661d87e97740d7ab90473ee
c51e4100c951da0d929a6598ebb2c1f8b18f05a1
F20101206_AABLNV hardison_l_Page_091.tif
3a53555a551f231073fab262a8afefec
da820704689e9409e4d28136caaca7ac60ec6e79
F20101206_AABLOL hardison_l_Page_107.tif
e5be05abb98263c5994eba8b6845e50b
763257e5ec3a421dee3078b5816665ab15f5a5f8
F20101206_AABLNW hardison_l_Page_092.tif
c5968d344bd8309be78b5ffbc55b7c7f
3653288da24e620bb925f8b49902bae7b5d25360
F20101206_AABLPA hardison_l_Page_122.tif
ae6cdd7009bb463013b71bb894db8d75
5bde361a213dadafa9a2810bddb3fcc93c174dd5
F20101206_AABLOM hardison_l_Page_108.tif
0407f69c9d195d35da68bc05b8dd8c8a
18887d1cda3e7c6a6767573b4cadd8cda813c7e7
F20101206_AABLNX hardison_l_Page_093.tif
12572fc677b70d169a686b4f5aa83e34
d5f1bc5d194166c547b1c425efc15d4112985f3a
F20101206_AABLPB hardison_l_Page_123.tif
c04cfcbb1bded5cc71a3ad886dc7257d
d39346cfa38fdf779161ca9448a0adc12dd3ba4c
F20101206_AABLON hardison_l_Page_109.tif
8bdcc3696a432fc67537d2d7efc77c6b
35c42c17d75d43ff23bb5242b0ef23166296c779
F20101206_AABLNY hardison_l_Page_094.tif
3f04c698f9b3cadf7627a2b79e25b947
5927e64eb1f2635ebcdb97ac9fe33c728c969d01
F20101206_AABLPC hardison_l_Page_124.tif
14ef07af3ecb43df0c1327c7bc143135
bfb3d63ef1c94e0e7f0c754f0d9aa8dcb60ae71a
F20101206_AABLOO hardison_l_Page_110.tif
afa4b0e22f437fb545c7e2b271b30cb2
6f9a1faa3cabf56e82c34e283e86eeb38d192c12
F20101206_AABLNZ hardison_l_Page_095.tif
f17fb8e08dc1060362fb68037b4d4e0b
75f9999f086da86261e969c8bffc519828648bbb
F20101206_AABLPD hardison_l_Page_125.tif
00234008a2333d7b2875a3771462f698
3edd9a8c29fc53a00206df214cccfc422995f4dc
F20101206_AABLPE hardison_l_Page_126.tif
4830236af9731e9468c5dba73acdc47a
bac3a0c59a5c520b2236626c747fba9700b3f90d
F20101206_AABLOP hardison_l_Page_111.tif
f562f8c54b7bb49581a08bf60b1b475e
4b594d998858967dd07481c3ab8f58267ff6a933
F20101206_AABLPF hardison_l_Page_127.tif
7664ca303455bf0934e15bf4d1508ae4
1537572cd89eda02d0433d7173ecbfa1de16b033
F20101206_AABLOQ hardison_l_Page_112.tif
d374b8038ebbc1c7cfdd0fe370ed3149
d8760494761fa88bbd280ae36f16619864fa5cf1
F20101206_AABLPG hardison_l_Page_128.tif
6d3b3b3470698485d4a4812299768367
e52e0f989c5f5822518d0c270a2a88d644feff08
F20101206_AABLOR hardison_l_Page_113.tif
09ef22bf8b2b3a0ce4eda4e65793f5a7
4a508e74081e5827df5fae0c28f5d837d80efb02
F20101206_AABLPH hardison_l_Page_129.tif
66b9a651db2bd7f9c2ba62ec1f85f3a4
b27a6a176770b1ec8b9f57ea4af00e2f9e3777f9
F20101206_AABLOS hardison_l_Page_114.tif
1e6596d0385f76cb8102461dbbfd066f
0b50ca492b95212bd994395d5bcbfc92e4c3ba86
F20101206_AABLPI hardison_l_Page_130.tif
468ccea453373fbc4588abd4eb50322f
1d69abb90ccffa548bbd2dbab635d7f20426743c
F20101206_AABLOT hardison_l_Page_115.tif
78888d8758988d82bd9b8c5b190f47f0
49c71d725df9305b54361f76b0982f89fb88316f
F20101206_AABLPJ hardison_l_Page_131.tif
4a67656da4422251ceb038398e40acb6
d5438d3eaef406a8d5818751921df2290bd4cfdb
F20101206_AABLOU hardison_l_Page_116.tif
8153e99e856dd84a1a813e71bf7c5052
82d77f04df07cb79ac69c31926d3dc4d5e9acf77
F20101206_AABLPK hardison_l_Page_132.tif
4ae5707701664cb5103c2822765f31ea
764ac7cb82346d3837c3aba1cf9462f9917cebad
F20101206_AABLOV hardison_l_Page_117.tif
021d6b91370640da9fc63fb471d788ce
04fcca2678ec0916c3874a3b3e1817dbb5260cb6
F20101206_AABLPL hardison_l_Page_133.tif
db1e8c417ddb53acb2b1fb651ae02484
ae320f4cecb038d634025382b9079bcfb8b27b0e
F20101206_AABLOW hardison_l_Page_118.tif
3db4dbb3646b6df92199bc1b4606672c
ca12997f024bda60b2c7645dec466ae8a0de3100
F20101206_AABLPM hardison_l_Page_134.tif
05eb7ef8bc03163cb89b888e298d6b85
42dccae7c30542d52a2a181779dc3bb94ab34336
F20101206_AABLOX hardison_l_Page_119.tif
e468a4ae459da47d75ea5743139a8f6e
ba28d8a7df9c03dc7d7b966c2c076983d989d34f
F20101206_AABLQA hardison_l_Page_149.tif
ff9536371853929c66df3722de0fcd88
8954d8c3cafc7bc999c0d06e7a88fa3b6731366b
F20101206_AABLPN hardison_l_Page_135.tif
eb41fc9d4571a35f308c3e16fdbdae60
b517d9b1bf431bda4e1fcc45a36618e043064631
F20101206_AABLOY hardison_l_Page_120.tif
2c02761ddcf488d35a6fb0f53e717a67
f198d6316d1b4b157efca8c766708fdd5a887303
F20101206_AABLQB hardison_l_Page_150.tif
77f1abed3150b0c16a72407e4d2c6118
0cd10b31f8cfed54c813961623ed88ace703a463
F20101206_AABLPO hardison_l_Page_136.tif
7c89dc05f9f9b582b9ac0dcce68cd23f
60b1d6bb28f6a0455e840df8485a6976d33991f7
F20101206_AABLOZ hardison_l_Page_121.tif
55d11e679bcd4844b2c05a7b9f867e97
4c2adca023341a6ea9dacaed892f8a96df24e45d
7319 F20101206_AABLQC hardison_l_Page_001.pro
6ff9c92b30557d7b4913dd3fcbe689e8
ff08e303e7dbaecad826d02f066f93d010a2b04e
F20101206_AABLPP hardison_l_Page_137.tif
e1f899146c7681acefa4b99463e579ea
6a7f7b61ee506e4dca012556250218be0c71d630
962 F20101206_AABLQD hardison_l_Page_002.pro
9fa6f35a148e1a9c4b36656a487854a4
f4c7418e0675759c2966c7d3d2238104cfd63f0f
598 F20101206_AABLQE hardison_l_Page_003.pro
9d26e23e317032d8c036bfe8a39fe14a
92cd5fe08a58079c3f15fb40abecb4e3fbb6f20c
F20101206_AABLPQ hardison_l_Page_138.tif
604e34b7c36b73336250031b9bb57615
2772ab05c4d578985247cfcfe30c256ea710b25d
51266 F20101206_AABLQF hardison_l_Page_004.pro
a0b80a4bdf3da4a8f25ab9efd81ee099
04fc5989195b650fc7fe7b904973f4b75e9278f6
F20101206_AABLPR hardison_l_Page_139.tif
e5dfdd8084b93f6b994d7f3885b9d18c
6fcf28dd23187ff670a1d7a9ab56c2206cb150e6
54385 F20101206_AABLQG hardison_l_Page_005.pro
75d0d6b58a9479f85bfcbc54377077a4
f6bc1a9eb82fa012752efbfed00d76cdfac17f2e
F20101206_AABLPS hardison_l_Page_140.tif
839949dfabd9175f1b73f9a244347976
21ebb7ada6707e7ee51a6adb13eeb40bc39ca0bc
20633 F20101206_AABLQH hardison_l_Page_006.pro
d1289933de8963bc027f0e373ab4e525
423ae31ef884bc7f44a75f1196b71db06b1699c5
F20101206_AABLPT hardison_l_Page_141.tif
4d235e2e2812dab296833fc2efaeddfc
a7a2440f3db7f42d8ddf50eeef26ae5e0da686de
74841 F20101206_AABLQI hardison_l_Page_007.pro
90f54df3d6e457a7407c5762b9cd881c
5fe6590c53a86af73eee49f84b534a9c8553849b
F20101206_AABLPU hardison_l_Page_142.tif
52e00ac8f10c2b0f5f82f5626ed928a7
398650d3f6f5df8fe7b71c82c6deabcd0dbb7922
60925 F20101206_AABLQJ hardison_l_Page_008.pro
95421a6851138ac6d242bcc55b511420
2f49bf04bb35954f895d223cab788c52ef8dad9e
F20101206_AABLPV hardison_l_Page_143.tif
32121054902873c0fed5f0feb61e2800
84ff3158ed1a59085cd1f2f341591faefe4d8df8
8558 F20101206_AABLQK hardison_l_Page_009.pro
a2599b24519b23e5d19479ab5042ee52
b45c22a603ba5ea083f8b201173aac43188d2c9c
F20101206_AABLPW hardison_l_Page_144.tif
dc159cb2e5170867c13104641b66ae32
e88b2b1a0c5b5276fa79c0ca89e40bca82fb7154
58430 F20101206_AABLQL hardison_l_Page_010.pro
8f12d9a989d97771f87f059620c13c30
a0b5506d57b0c796cf00737e6b643c2d57d3f7cd
F20101206_AABLPX hardison_l_Page_145.tif
1d9f285463ec9786dc3d9430b94ad44a
4b2c06e83e05ccc090ce7397e1cc604f66b1f3ad
43399 F20101206_AABLRA hardison_l_Page_025.pro
d6cc57854e1c95736d62c8e5a80b5100
d4c508a14df2d5632d1376b4307aa1499f003800
64512 F20101206_AABLQM hardison_l_Page_011.pro
98649ec1aa6f3a65f0656f3bffb2199c
7c47bb10950a5b24d7f78d443953efe38afac771
F20101206_AABLPY hardison_l_Page_146.tif
3511c16347ab540420654f136aa2eb2f
70d45c70f5ee58aa4dc9f508d26dd01c93589f5a
25501 F20101206_AABLRB hardison_l_Page_026.pro
1782ecb7338fd7d6ab5bd67242d1f994
b508f764995fd7cd5a780049d057d81929163037
46394 F20101206_AABLQN hardison_l_Page_012.pro
520e024a9d0031fa8c217e9115e1584a
838d3b7ad0f0b7a2dcf771de78e79c692a6394fb
F20101206_AABLPZ hardison_l_Page_148.tif
0d302b5f4949f9da287e2861801fee95
6bb63e58da196ed2a0b3aa16d56e30b70b8d4f13
49682 F20101206_AABLRC hardison_l_Page_027.pro
13029e46891259c5c0f443e49b1de3be
3faa2efe6cc79deb2582adc640d794926b34e47a
45678 F20101206_AABLQO hardison_l_Page_013.pro
0ecf64370e013b5a7e168bd8aa45fb9c
4228565a1b13827bceb476eac2ca5bbb903514af
34810 F20101206_AABLRD hardison_l_Page_028.pro
310fb17fe35514f946e62a67fea37b04
30df1cbf0645be8f13fd00cf2f7b3871d1e81085
51946 F20101206_AABLQP hardison_l_Page_014.pro
5e47a2b6edebc6a83a6962a1bb7580db
15cffb8eefec0ff5720712728747b0a53001b956
57876 F20101206_AABLRE hardison_l_Page_029.pro
650a3399d1f3e0406c3669c2f30c2cb9
f279ea87de230d7176ef5ed34860a7ca4a778351
49223 F20101206_AABLQQ hardison_l_Page_015.pro
61448ca14ad8a78daaca8fd485234bf4
d89e2985d84e9ad6669c2af1437af8a6de2374f2
37171 F20101206_AABLRF hardison_l_Page_030.pro
a4b2edf847aea908de911f980b32776c
5eb1bace1cdf8e06821cbcfc88345c048851ead6
48629 F20101206_AABLRG hardison_l_Page_031.pro
08701c4363b67a63f97e41c22beb6932
f867182dc7e7cccccd764bf1b86bbca159c9e101
52823 F20101206_AABLQR hardison_l_Page_016.pro
b40b14cf98219bf04b29f680d170e57a
5ad80de7d2064d27c46ec43b06c6a9cab95d0e1b
55131 F20101206_AABLRH hardison_l_Page_032.pro
ddd4e2a6e7db82f77f6a6423863ecdea
92995a71a04a02931d58898b762ec5ec39a41a9f
35706 F20101206_AABLQS hardison_l_Page_017.pro
110b1ebba48f67b1c236b008f78a341c
f6673e3ff33cf39870e251a338b647e3e23c3820
27567 F20101206_AABLRI hardison_l_Page_033.pro
f42b777ec5ab96999e4e758c6f2d7c1e
bafcf4e63c583c2c9927495412995c9895a7fb5c
52254 F20101206_AABLQT hardison_l_Page_018.pro
8f6991552cfff0ae3faef07c2cad0d73
911d7ee87439b3066319a9e5f4be2da7c7d5b189
54239 F20101206_AABLRJ hardison_l_Page_034.pro
2a9f9448d2cb8b91789fe78b7c4e6443
c0c67e1f703924229cc2b2978166f1fb77b6d7bf
50040 F20101206_AABLQU hardison_l_Page_019.pro
a2272544cf1af7d3e7106c6bfa587502
fa6a9d357505a8c5300170bc11e47a058ccb4418
45884 F20101206_AABLRK hardison_l_Page_035.pro
483e3cbc7b51a172842cee6be78964a9
626148cfe22340542ca3efcb439a51c4faeca0df
52429 F20101206_AABLQV hardison_l_Page_020.pro
9bb8e3dedb30632e624b36c02edfa28c
091a3f842ef4880fd3068d835754907ce7f512cd
56134 F20101206_AABLRL hardison_l_Page_036.pro
48d96441365ae200ee1f1be616b1e7c5
e6125918ce1dc6bf573568a80df5abda18da0d4d
25945 F20101206_AABLQW hardison_l_Page_021.pro
73445a6ec4942afd7a308e134acaa9b8
4bb359c67ca71d4213bc3ca4e8bb990ccbe81a62
36600 F20101206_AABLSA hardison_l_Page_052.pro
70f8f86785579ea7b9caeb5029cfc7cb
35b50a2e50d581898ebd27da8fb217a3c260649d
55524 F20101206_AABLRM hardison_l_Page_037.pro
9e76e3e114e73f72da725eae45cac10f
78af490ee59c25fd31dc01a6782329a3ce5e9157
42341 F20101206_AABLQX hardison_l_Page_022.pro
39033d4b1fcb15f3c6ee6d6b61c61625
b93ffe4230992af283889c32cafd5c3946dae629
33666 F20101206_AABLRN hardison_l_Page_038.pro
f13c4a1a808c192deba86e0aa6573e30
19ef47158c43317a1a4339ceb3f5e43152e234d4
47431 F20101206_AABLQY hardison_l_Page_023.pro
d7af09283035625d9d8ae7aa9a439862
2df4b4dca1aa22a16fb387ce82f0b548ebd93ac6
36187 F20101206_AABLSB hardison_l_Page_053.pro
cedfa6829636b19d4c2b16833ebef201
80bd411b6bc06a8032d2644fcf65d948a80813ec
49807 F20101206_AABLRO hardison_l_Page_039.pro
8a333961480fa800ba3b3d4306cc212d
1e21fb0e02475bd7c3b46c5724392a73b08989e6
38320 F20101206_AABLQZ hardison_l_Page_024.pro
527e37ad8a1d019347a4fccb4b6e7eb5
be19a830eca07e819cf7724099d717a89cc1a937
55180 F20101206_AABLSC hardison_l_Page_054.pro
4b56b789be3d497dd0b387e28e08acbd
e6dc5ea6704e7d37807f0aa8f971699e484cab90
57766 F20101206_AABLRP hardison_l_Page_040.pro
9fef0a159c44bc3b01bae95f4fcaef54
bcd61bb2b0a28b9cf154b757b2296c0e49301444
38477 F20101206_AABLSD hardison_l_Page_055.pro
8c127fabf906038b88f542086ddc9b4c
160d4ef554a237ece94f8c5c4644c9f61940a8e0
50910 F20101206_AABLRQ hardison_l_Page_041.pro
7487203db33703a921d20de5687ee549
8dd23e59fe8c2754db49c5196cc10d8328f4f89c
54786 F20101206_AABLSE hardison_l_Page_056.pro
6e269b1f46a36ace0221d106d4263e5a
aa1a6bcce66b44836f271111a1e1702a8e73a174
50830 F20101206_AABLRR hardison_l_Page_042.pro
9994cf2bd833bcecbd289f1be4f90e71
1657ecee937c2620b275d8114205f90722718f71
29504 F20101206_AABLSF hardison_l_Page_057.pro
15b4625e6f282208c5520c8e4abe33fb
265d71f93ead8d0997ddfb281b994fae46f1c924
49756 F20101206_AABLSG hardison_l_Page_058.pro
282d46dac262c91881bcf81343c5de85
352c73bb37627c7f40f767520bbc902a6c1ed0b4
51436 F20101206_AABLRS hardison_l_Page_043.pro
c7bed4448d571f97a6d75b021325d96b
dded448f80c04292c634b31df22dcaf39aa50075
12332 F20101206_AABLSH hardison_l_Page_059.pro
b572f239ebca2d78aab997b308e39c8a
2214c54059a771df07185ced0cc19dc71e35ff80
35926 F20101206_AABLRT hardison_l_Page_044.pro
2aa1bc34c4617509296c463452ef423d
2b9dd1d063c5254aa8260e6734fc3cc055117b29
30758 F20101206_AABLSI hardison_l_Page_060.pro
c643839722cec320332904a473320713
3ae344722c5a704a82bfb90c66eb24a1c80fbe8b
54895 F20101206_AABLRU hardison_l_Page_045.pro
74c7d97b3826f08113fbe7e8f4c2d799
4090a76e68583d35279c8eeff8d180adf5723a31
36942 F20101206_AABLSJ hardison_l_Page_061.pro
999176597cd8775d3fd5e63cea9c3b5d
a6d33ee738cee994679e8d965e5d55ecbdcd6d0d
54319 F20101206_AABLRV hardison_l_Page_046.pro
b8d0bf0c5e2fdf25157f6458cf231430
17a27fbe575c2315547e6ceaa001031c514cca3d
53050 F20101206_AABLSK hardison_l_Page_062.pro
59e424099432bc64fddbec215c172aae
ad22480388a4908d2477d93a7952417995247577
35420 F20101206_AABLRW hardison_l_Page_047.pro
4c9085b2c85bec9b0ddad455c35bf2a8
cd590843b4139a9c77226ce1e0daf1467ab21957
52977 F20101206_AABLSL hardison_l_Page_063.pro
f69660ab30d00e96812a2b2f57d44213
074e1138fab91da6c7f7fb18fd2cd20cfcbd2c42
37837 F20101206_AABLRX hardison_l_Page_048.pro
455f258fa94a230ce6fc1789194c9c9b
42ab536552c531fa73406cb8a5b7869eda896f2d
32893 F20101206_AABLTA hardison_l_Page_078.pro
53210255126c543c82f42e5eec3df10d
638d0edde5d5da16b67fde438187f9e799728e89
41615 F20101206_AABLSM hardison_l_Page_064.pro
2d8b2aa5afd28d7af68527ea01d8ca53
6d54852091c5c7de38cf8b6bed666024959b8b28
8291 F20101206_AABLRY hardison_l_Page_050.pro
a5c531741ea7a1182c384de653a65242
ffb3be812318f9b64a274c5b61f6a1b2d10d3014
55969 F20101206_AABLTB hardison_l_Page_079.pro
37b1c487740dcca1efc78715952d6c2c
f111fba5d4888316a18a6b0841886ffe9529bacb
56105 F20101206_AABLSN hardison_l_Page_065.pro
02d572f7d3abf4c80ed68367e14bc495
761f0e3c5194d78f2739a200f5337fdf87c6202d
27872 F20101206_AABLRZ hardison_l_Page_051.pro
aedeefe54366f8ceb4e3fb79db6f3d76
51142ca97bce036ea91a1b18273e5c6534eba080
55031 F20101206_AABLTC hardison_l_Page_080.pro
3a2831dce7204a792b2a67db3a6a3cef
94bbcc0b17d85767d6944e4929b1e13cd8145541
54857 F20101206_AABLSO hardison_l_Page_066.pro
ca60851e9a2e47cd73e19a25730d9c9c
400f22940207767fb56796afb2dde0a4fa7427f5
58145 F20101206_AABLTD hardison_l_Page_081.pro
caf503eb375dad1d528387ea74ba0878
c648a199a27277c254f0300ee0d4423cfdcd8502
53295 F20101206_AABLSP hardison_l_Page_067.pro
fb6e5aea3134ffa6e1172d988db8a6df
e6f23c1ba1df0fab8e72e7bb57ac4a0fdc2d2e66
51854 F20101206_AABLTE hardison_l_Page_082.pro
977d68407f33d1744a11888363937c60
ec6ed07a9baddb9efe5e0eacab9b4fde48abdd4e
47143 F20101206_AABLSQ hardison_l_Page_068.pro
53e7af059d817e8c6161559e7811a487
d167790d06d3092719f8cec6191c2f929ec63d99
17859 F20101206_AABLTF hardison_l_Page_083.pro
619bf8cc58000994e763cc43715a139f
bf054d48f93924d681bf11d3545497ca20bafc3c
32467 F20101206_AABLSR hardison_l_Page_069.pro
bb9da67242f54d9282e2d6402ced18f7
8c1e623cbe3cdf98067144bbeb16959325224d6c
32932 F20101206_AABLTG hardison_l_Page_084.pro
0ce15567d7999d98a330db3104f33889
dead3d77147ad624f1971db3c6a6c51c53cdc16b
55814 F20101206_AABLSS hardison_l_Page_070.pro
79eea37a2f25e80f780e904569ad2ea3
9d214baee91dafc247d3c7ccf12a358fc8f028c4
23179 F20101206_AABLTH hardison_l_Page_085.pro
bd8cef12c4306eee6409a60b80fe5fa1
bb18797dcc771fea258f11a4e7b021ab4a7db6b9
51954 F20101206_AABLTI hardison_l_Page_086.pro
0ba4f49d8c1a181b31f9ed20110d1915
a0ed234eea407db404b6753eabcad1d93a312cb9
29448 F20101206_AABLST hardison_l_Page_071.pro
57cae72fe5e7db54c8e44a1a53ae66b3
f8747d9f7861df689d3c035668ed70e084adc41d
57663 F20101206_AABLTJ hardison_l_Page_087.pro
24a11924d9548fb849db58ddba0a1283
06cfee3e5dbf8e774bc542770edc6ccdd09eb9dd
54802 F20101206_AABLSU hardison_l_Page_072.pro
7472a40995f4c7f87e8cf3350adbc29e
cfe6cbf6c6523597ee396dfe2c36b52562014008
54723 F20101206_AABLTK hardison_l_Page_088.pro
1ea388b61417c152cf936ee7bd0f1ddd
89262a50ae8cf6aa01ed319ea56b721643b5d735
8891 F20101206_AABLSV hardison_l_Page_073.pro
b4569eb08fd6539ab53ec19eaf416b4c
46bc06f83cff6f56b252829e7e098bcf869f544f
45464 F20101206_AABLTL hardison_l_Page_089.pro
d55bb144587d4b47260c0662446ce6d8
4b6fddfaa7590489049cebb7f932dc2a8fbb9ee1
32128 F20101206_AABLSW hardison_l_Page_074.pro
161cf12f1ae8b276e5418736b386df24
9d3a991e5c50f965e84ca0dc0de70d97379d7044
12633 F20101206_AABLUA hardison_l_Page_104.pro
a1a6979e45673de34e6c2fd82c2d7e68
a776d81e001e4122fa71efaa0cf7e3a54fa72e91
51159 F20101206_AABLTM hardison_l_Page_090.pro
ce57c78ba806be3e3ef3ec7f7bf51293
8e6765abc85d427705072498d5fecdc342eff195
55840 F20101206_AABLSX hardison_l_Page_075.pro
8bf358b9fcfc2583014b6f6b983910ac
bb647203d7e7a1d273b9a0022ce650c1652ad033
56973 F20101206_AABLUB hardison_l_Page_105.pro
36e7e69b596c08736e68247525e5497b
84cca3261220c387ef73c05e131d90abe87a41e3
34304 F20101206_AABLTN hardison_l_Page_091.pro
e736262cfdb365c4570adf1121f267eb
7b922516a612994889f66e21f1d2594c927e7801
38708 F20101206_AABLSY hardison_l_Page_076.pro
30ebadcd303eafa9efd5e107187a99bc
3df1ade84293116ece0ae89e36668d42a273c314
21663 F20101206_AABLUC hardison_l_Page_106.pro
75fbe76c6a9e1413fd91b88511d03d88
3d99e0dc66dd9b93aa727e952aff61e7cee4976c
54725 F20101206_AABLTO hardison_l_Page_092.pro
5d8ffd297eece1f9c151f441bbbefc15
539ff97d7b545e2f7f7d166a1673a4c67cd3ddf4
28256 F20101206_AABLSZ hardison_l_Page_077.pro
29a09901ae455ee701240fc8ac28d2b5
05e5afc815d37be8a69ae772faac41631fcdfe54
53133 F20101206_AABLUD hardison_l_Page_107.pro
0db86393c40966b6c3a27e2f95175e99
429141b87547b27d8de29657d5bc8335ef50976c
50196 F20101206_AABLTP hardison_l_Page_093.pro
a78acba8661173a5a19b0d9a6f014c01
0ae35bc81c59cdc6b11e20740aa41450cf2efdbe
38522 F20101206_AABLUE hardison_l_Page_108.pro
613e850e151ecfc02e0704f4eb05da1e
7e91304b5a184910ec4dc8fa08e4da195788ed64
30118 F20101206_AABLTQ hardison_l_Page_094.pro
fb9dc12a3a16f4e1a1dc4844332ecbc4
48f1546df3f08e43388da481618e73e08b0992d3
29986 F20101206_AABLUF hardison_l_Page_109.pro
a926ede00e2925e195369b5a6a4ef06f
705beb5fbdeba2d3ab9a35316f185ea430dcaa0d
48652 F20101206_AABLTR hardison_l_Page_095.pro
c5bfa699953f809c8d59f4e5f7564b9f
f29192db3c9d5a4b2503c394cdf3e997b5f5460f
36771 F20101206_AABLUG hardison_l_Page_110.pro
96623a49799cd7b89f5feade784ab56e
0ef880eed1478ca4e10a9c60e762063b5ab96323
45465 F20101206_AABLTS hardison_l_Page_096.pro
57917d6388f8857f207eed49d26a5cb3
8b197d899a1aa77bbce876560682de087b89cda3
1648 F20101206_AABMAA hardison_l_Page_110.txt
de7043bf704dcb5fea6ea84fe847ff04
94f38065dbffc8a2575237b9922c94d449c5ba54
31948 F20101206_AABLUH hardison_l_Page_111.pro
dd189a206e52590e126dab87eed3bf60
087057a71e1299a080b3c2af1751eb3e6df1780d
28585 F20101206_AABLTT hardison_l_Page_097.pro
927d84de2c2ca1b974881427e903f10b
836cff96edf46fc463efc2afb36841281b2fe990
1468 F20101206_AABMAB hardison_l_Page_111.txt
ff9298c8b487110f389ab357f6dda00f
1c047250bd527e38795fdc017c6600a34465e1e2
29011 F20101206_AABLUI hardison_l_Page_112.pro
18c1040d80bc7064bf4140664d249ca1
a41a285bda7c9bdb356146fb40fd34c1beecb229
1441 F20101206_AABMAC hardison_l_Page_112.txt
702a96e6a1c4d9809d7798e638dbfa9a
7d928ea32644d2a649687570c64d8bfcff12023c
35336 F20101206_AABLUJ hardison_l_Page_113.pro
ca1a5b8d9cd81a21cc923b09c53a275e
831a020bc86c77c11189e6200232bcd33ecc465f
54120 F20101206_AABLTU hardison_l_Page_098.pro
e163ada6807ed11b2731d4380c40556d
daf6422302b1e9b777f16fb3d788ac8f77a368ff
1669 F20101206_AABMAD hardison_l_Page_113.txt
52070e5da04c558dac4b76739253575c
503a95fbae037d1797c88e5fb3a641db345cbecd
49403 F20101206_AABLUK hardison_l_Page_114.pro
108e303b9ded192118fae0d20343586e
7a44578b92cc3704b5025b6b7203b5791ca319f2
30451 F20101206_AABLTV hardison_l_Page_099.pro
f9f5536577ca946afd5cab617047308f
a8645b68325c881d9ef3a2ca167d5cf553ad122c
2200 F20101206_AABMAE hardison_l_Page_114.txt
2cc6e766c23b93fcdc34f94be9f6d7a9
5af897f449cd25edbd4ff1c1a3ad2a5dee0e639e
56421 F20101206_AABLUL hardison_l_Page_115.pro
8198e53b800fed0abd4a6401caa79b46
8cecc17f4944c7aca6f2578ccbac1eed52ae6de4
27694 F20101206_AABLTW hardison_l_Page_100.pro
e9eb3901d714c9435497fc43205ddf16
eb5266795399fbb90dec0dd7b55597f0b65fbb9e
2207 F20101206_AABMAF hardison_l_Page_115.txt
497441903cf5bc652e05cdf1442c43ba
4297bffec23b5faaae377fb2562f5d346661d84b
24543 F20101206_AABLUM hardison_l_Page_116.pro
fa2d9ec24ed130f6c12529f69442e74a
294774ad62fc5fb87cab0aa3803ba5ce2f457d60
57134 F20101206_AABLTX hardison_l_Page_101.pro
4c268e7916bd6714b24a026c0c19e102
e9ce4bb92542e5bc57c6e09da638e3c34487cd0f
1150 F20101206_AABMAG hardison_l_Page_116.txt
345bc6b779d88463fe03ec4a53447482
968db8c022bb5c356b017eb63128b02aff5cf25a
54043 F20101206_AABLVA hardison_l_Page_130.pro
6f6e8cb6801b9259afce246b072c5f46
448d7bf053902abbfe2183c98bfe26b4f1b4b0ed
42390 F20101206_AABLUN hardison_l_Page_117.pro
0e5c2a34aec718401a1b14fc36e06e48
062fb13a215f8d21785c0576f54016cbda53bc5b
42005 F20101206_AABLTY hardison_l_Page_102.pro
4618a7591a2869f4c08d7f3345f9817b
1771ca327691491954ec036316258b8d496b350b
2011 F20101206_AABMAH hardison_l_Page_117.txt
28fa2ec6e9da470eca41f2ce4f36aded
31dd66fd9568316fa19b7f2fa3cc06240d04fb21
31210 F20101206_AABLVB hardison_l_Page_131.pro
3c9e188cce3d46a6b4de19338e4df8c1
38ab38f6277e8cdcf0dd91f98c3616ce22a174cf
34284 F20101206_AABLUO hardison_l_Page_118.pro
31a7f5751e03c720578efb328830b085
e9e2f04a15032af8815ec7134690d19e5d0f87e4
47002 F20101206_AABLTZ hardison_l_Page_103.pro
6c7dc975ea2c47c344d1566977d9f548
79ba01cf4210f01b8f211cea037b1e86be228154
1505 F20101206_AABMAI hardison_l_Page_118.txt
785b4cd898da03a40001b920495d2b4f
09b8db8ab423aeff13a0a384e3637045cdbabc5a
4950 F20101206_AABLVC hardison_l_Page_132.pro
72b6c587bead75a2ba8a532f33695d18
c0c5c3e45c880e9166de44523e4d0786e5aed0c1
31787 F20101206_AABLUP hardison_l_Page_119.pro
e02480c4714fa7439d0921ce75d1a102
a11ca535716f743f12de02b66053b738aedd59b9
1426 F20101206_AABMAJ hardison_l_Page_119.txt
d99b59d1bd35e62c10f4771bee50b52c
47def2fbc997a272bbadf21773842cb36c059fef
52304 F20101206_AABLVD hardison_l_Page_133.pro
a08c40c411204025a3206fffbd39f306
4603d1d7b96c6187381ff79e1af757fe239fd71c
35832 F20101206_AABLUQ hardison_l_Page_120.pro
4bcc9a999759ddc27c84f8a81ce28cbf
8f9871d4b0f4b338c0378f1bb55f7e289107cc01
1594 F20101206_AABMAK hardison_l_Page_120.txt
16e3e0ea16b750e9feaa222733123795
9c988874725e743db8619a207c5d8384f6cbda3d
62587 F20101206_AABLVE hardison_l_Page_134.pro
0b0c1f400579d4455484494e377dee50
61c6b236c6c874f63f5c3e385244502b5afe5930
55164 F20101206_AABLUR hardison_l_Page_121.pro
3850f6d30597abadcf87b9c7ce24fdf0
940805981dc6dff17780e8a06ae695f8081e738c
2689 F20101206_AABMBA hardison_l_Page_136.txt
f9cb07c59b706323ef4d2dfa021a9ec5
64c9590d559602fb870be723e33cc7b3e85e652f
2164 F20101206_AABMAL hardison_l_Page_121.txt
4d8ef8f8e77d3d5073797e826f01e9ea
b34b5761096e8d084b8a2f9c62b7e9e85fb0c316
65488 F20101206_AABLVF hardison_l_Page_135.pro
b78a2443e55fb3e94de5aa63b14a6269
6818d526d3f1b5babbe3005a45998d5f15b3072b
55952 F20101206_AABLUS hardison_l_Page_122.pro
b4e3d10af71ea429996f207615f3f4d3
76aab72edc5cee645bde9f84ef9cd6a4d29fdece
2227 F20101206_AABMAM hardison_l_Page_122.txt
03fb01fadb75a7a5c96b0f2f29c238c1
089f12431d4c43acf4ecfd9e5a55ad4ede75b895
64332 F20101206_AABLVG hardison_l_Page_136.pro
9646c0a1fd17b2a7b3bb539ee5b1adcf
e6c45d0868e1ac31ac79c0192b2155dc785953bf
6756 F20101206_AABLUT hardison_l_Page_123.pro
7887d1f88de26d1254e096ac82b80cbf
523809d5a8f6051aa0525f2095af13c362d5cecd
2659 F20101206_AABMBB hardison_l_Page_137.txt
9156e93b5198d31809dd1c0b53dac50b
74aeaabc739c5c8b14dd624d2da4a01695b0d1b6
404 F20101206_AABMAN hardison_l_Page_123.txt
0c50c3e567f83153089b76cbd6b32e5d
a63b28c130bf979721f31a19f7c6727a28f1fe0e
63673 F20101206_AABLVH hardison_l_Page_137.pro
fd4e865956dbabcdbc6b7b9f57659ec5
e29020e2cda6730442bad1e94d77da426b06aba7
43127 F20101206_AABLUU hardison_l_Page_124.pro
c4f1dd674ad17b243f305a59029727a1
f3359d1a6935b61e7bcf19c5b4410e59acf5f791
2749 F20101206_AABMBC hardison_l_Page_138.txt
b73670a54e8aea6137ff99789475aa3f
11475252f11c724bba8852aa8aaef757e5785838
1805 F20101206_AABMAO hardison_l_Page_124.txt
05f890705154e2cd0c5c807df55ad4c1
2745fdd6594298effd47d8170c290698480b92c3
65028 F20101206_AABLVI hardison_l_Page_138.pro
417d87c987228f89a1b7489ecd14afee
9549eebd9ed163927404b6d967b7695d929f75d4
2435 F20101206_AABMBD hardison_l_Page_139.txt
33a5e6c92123b578c8d17b389ddb958b
31c852d31db838676debb55061d54138060d3ae6
2174 F20101206_AABMAP hardison_l_Page_125.txt
6cf25d3fcb98eee858551f10e7bd97d6
5182c25d8529e679fa7f5c0067b06c4436cef9ab
F20101206_AABLVJ hardison_l_Page_139.pro
5ded4e432341bc7d634fbda4be2e1dd8
3efa254d97d3744b11eeabf8524185131577a8e4
55218 F20101206_AABLUV hardison_l_Page_125.pro
a591f0e3f803692b422cfabc6a88a93f
9c6d2efa69f259e103550ad4c99f5e0693f966c0
2879 F20101206_AABMBE hardison_l_Page_140.txt
d9849d4a166402ab6c7ec8a92f4bd245
4be6de7aa3e172554912ffc7e86575a1b8bd793b
2109 F20101206_AABMAQ hardison_l_Page_126.txt
302f08d202d97a8ce8dc2468e113a574
adbc2456f3f88ee6626b2bf02dc737f057169a3f
68649 F20101206_AABLVK hardison_l_Page_140.pro
3c0f1a62b7233dec60ef78e41cf4dd14
12d70fa82a40eaef5857fdf7cc4a770dda2be280
53664 F20101206_AABLUW hardison_l_Page_126.pro
15a9ed2c92ebe9024ac85451678a50d7
983375d4c2863e54ec3c39ce93ce3df9b1b6b038
2621 F20101206_AABMBF hardison_l_Page_141.txt
cbc1ce4291d9b21fec2777d6f6cac739
b85e212131bfe165af05c162fca21c3dfd37e040
2084 F20101206_AABMAR hardison_l_Page_127.txt
b0044256a8fa722f997056e8251e2a80
3543e5ac0c62d3bf9a677c2b20c27ad6200f446e
61781 F20101206_AABLVL hardison_l_Page_141.pro
00c56b9c2347ceeef01ca5b7dc3eae78
89f1138dd07b0a8f5dbd8a09f80d23ff62df19f2
53008 F20101206_AABLUX hardison_l_Page_127.pro
1498384bd3b8c3e0d0dee7eca0b50f38
c7676bdc9c00db0b263732607757a63f05c1b433
2584 F20101206_AABMBG hardison_l_Page_142.txt
6745805bb44c4028aaa0bc8cac0568c6
80bed1a0bff4fa20e5d9a17861fced68d5757207
824 F20101206_AABLWA hardison_l_Page_006.txt
d6c78a60de2ddf53d74a3ad06e32247f
66cee2cda47e82ddbebd9f93f97383786e9b9611
2212 F20101206_AABMAS hardison_l_Page_128.txt
86291c295ffe2b1f077917fac98cdae7
ead8d216879d1c8b082eaeb2dcedf2763c665b1f
61277 F20101206_AABLVM hardison_l_Page_142.pro
f248fd5260fbb82331f607e12c35576b
f2a270f4812185654d276cdad4d816b95bf738ee
56335 F20101206_AABLUY hardison_l_Page_128.pro
bc91062e9039d92f4325c7fa1721a23f
1cdcc89558db9a92cb20129d5891ad1f16753a7d
2695 F20101206_AABMBH hardison_l_Page_143.txt
0997b41e168dd4fb1483b458c114a3a1
a27ce466840ac27202a341705bcf9b97933913db
3323 F20101206_AABLWB hardison_l_Page_007.txt
abc85b22db53ed6d3cf1c057a2a293c2
b1a50c57e2e69986871e057c82116bffaaa94f3a
2181 F20101206_AABMAT hardison_l_Page_129.txt
1b061ff5b5ee94b472331d120bd4d0cb
c9adcb5a6d3a4fbd4515fff23dbfa94606aa1392
63905 F20101206_AABLVN hardison_l_Page_143.pro
7217d0b2b2e8728dc75d3cd80e395424
87c854964863d1415e02eaa6fd5c03ff07edb6fa
2668 F20101206_AABMBI hardison_l_Page_144.txt
6d764d49b149fb63f506a0678acdcbf8
304d4fef7134b68a7ad6d6e165570feec243045c
2600 F20101206_AABLWC hardison_l_Page_008.txt
646b119544db922f56d91842c121bd5e
4ffe6891d06fbc24c57d8438c10bad7dac13d543
2126 F20101206_AABMAU hardison_l_Page_130.txt
6471f64b7aa34727fce4e7f80dc75562
992a7f73e874fe6cf8074a9203debc1d3c12a8c6
63632 F20101206_AABLVO hardison_l_Page_144.pro
e4325eeb83be92ad4000f7cd9f818886
c29ae58e2514c4fdce78281421b1e6fdd7acd994
54957 F20101206_AABLUZ hardison_l_Page_129.pro
fce3c99e2354b54c5fd88e69ce83a58d
54d5d5b635b632c54f491ad671769918b0bc4304
2927 F20101206_AABMBJ hardison_l_Page_145.txt
6b0f489abb10daa1d492503d772c4521
872d67d1ca9be0294fe7b072cbf999ca5536a01a
348 F20101206_AABLWD hardison_l_Page_009.txt
da41a8034782cb3262f16b4babb17852
de0b67b9257240f58a9d5a5c219843f2811e4e9d
1427 F20101206_AABMAV hardison_l_Page_131.txt
2f9062187d0fd19d7db6852faf860c7c
a51649c2c66e94bff44d1e501f7d1d1c4587822d
69882 F20101206_AABLVP hardison_l_Page_145.pro
165ee4d50d0c9be32d2a10de3b5f6366
8750b922d2504af20ddea2ba1cd849c6d2ed4415
2651 F20101206_AABMBK hardison_l_Page_146.txt
e0e5bf98aa26a4b966ef7ba9cbb863ae
084338b5820e981c507003664e72953466c9705c
2438 F20101206_AABLWE hardison_l_Page_010.txt
d64796db8d4d3274625902af9e314067
86ae995fd1c673da90daa7f8219b1c45ec834216
249 F20101206_AABMAW hardison_l_Page_132.txt
25385c6b7231d5e22258aa41d7f538d7
c948cd1d9dc318dde223960aa4b8676621952654
62428 F20101206_AABLVQ hardison_l_Page_146.pro
9535df5106162049349941b77345824d
fbbfa6f6371b5256f56668d636738b214407665e
2381 F20101206_AABMBL hardison_l_Page_147.txt
bc456afc7a39e78704d11cc9d1375b31
a7be374eb8f63edcd02696063015fe4351ef7037
2714 F20101206_AABLWF hardison_l_Page_011.txt
26ee30c6aa41c2532592bc92010ec3e9
b2c831a3dc09edff40ca4c112e2fec00ea499a3a
2226 F20101206_AABMAX hardison_l_Page_133.txt
6808471c0b34ac9ca6b2760f7e592c9a
08951179d9c934a7ac40b4823f319ced8f4fb4d2
56659 F20101206_AABLVR hardison_l_Page_147.pro
5c65ed4936392e8b8b1bb180808186f1
daf7d23eb7e9d1a77142fe1647a22afd24f08f75
27856 F20101206_AABMCA hardison_l_Page_146.QC.jpg
6db1bdc3b6f3acd63c8aed46871bbc0d
91e3bd4498654241de921cb5e86af33085b67645
2870 F20101206_AABMBM hardison_l_Page_148.txt
a409c6e325e528f80770af4eabfee914
5df851256f3ef7e34c5f6af0a9ae0165198e2e83
1927 F20101206_AABLWG hardison_l_Page_012.txt
9d4fdbfca928f37eb81d4924111aa90d
f4929dea0fb303c2eb35e9c665b5acb29d6b7c90
2626 F20101206_AABMAY hardison_l_Page_134.txt
aa8b9cf9e7396e6d729d0aad75ab74cc
10fbe7f7f942c30e5293c7362e7d067cbb59901e
67853 F20101206_AABLVS hardison_l_Page_148.pro
d50cdb4611e47061b2e55f13ce6f38b8
3a7db92e0914aac3999f0f30e3330988095f4bd8
27952 F20101206_AABMCB hardison_l_Page_034.QC.jpg
1844d4fadb4db3b0a8b7c5325b387ea2
ed8de2afe30f48670f1d5722c47a0e01ed4b9fe5
1565 F20101206_AABMBN hardison_l_Page_149.txt
d62309944edfe9e3d71e4ab36480928b
1d556bbb4d4cf1f370681af6ba584d0d0efb47e0
1989 F20101206_AABLWH hardison_l_Page_013.txt
584f8c0239b05f5912edd83935ca0744
eb1115d28b7d8eec225303bada36c828d1745526
2762 F20101206_AABMAZ hardison_l_Page_135.txt
977cb4b8a0ff0a1badce60213ea4aa90
24e23cae7b739084b7a5d58ddc45efa3ef79e26f
36100 F20101206_AABLVT hardison_l_Page_149.pro
7f1de1ee7457827d84c11f18289efbd3
8e17ef21924baff97e666685f5317be62e951d82
1329 F20101206_AABMBO hardison_l_Page_150.txt
dac98baa5aa8d12fe92f20d1ce79df89
72fe6d7bed4a2f1089ee00ac2a76c69bcb7c0669
2050 F20101206_AABLWI hardison_l_Page_014.txt
a73762ce123d8e84ce4fd4dc38e7dd07
e528b37560a99710bcae766f6c0153586e855026
32644 F20101206_AABLVU hardison_l_Page_150.pro
2d347a5158cb384719cba497181ce017
b1ae8d7f979b43e10b23dc85379488673dc10096
22628 F20101206_AABMCC hardison_l_Page_124.QC.jpg
67ab00acc8665ca04b44324041948364
a693170fdc32faec8af50923d22c6ee5e696c263
1512 F20101206_AABMBP hardison_l_Page_001thm.jpg
ab1b75f56df00bcb7d0fb349cd4afa58
f7891aa21c9141064bfbf161edf45c793903577c
2057 F20101206_AABLWJ hardison_l_Page_015.txt
626044e37b1c01817d58317bca406d2f
367f6c0b493ff62c61849677b64dd10e071c71d2
413 F20101206_AABLVV hardison_l_Page_001.txt
e3c96d575d21f7081eb10425574e4db6
5cc6c0216014a1f17ed4127fc97f743d46273a29
3991 F20101206_AABMCD hardison_l_Page_149thm.jpg
5d28ba5dc9de87bdb9b644888622cbac
477d678369dbbb15386327b3dffdc8235cebba0e
12518931 F20101206_AABMBQ hardison_l.pdf
d4a82c9590ec2de44be212e246e76dc6
529dfe306aa3465f7095a9a512194640b910e08d
2124 F20101206_AABLWK hardison_l_Page_016.txt
e12cc438cacccdd8762af2eb7e198130
26ad863a92fdb3ed69d394987fd1014d697548c2
5416 F20101206_AABMCE hardison_l_Page_099thm.jpg
cd722ccffb7d9b0c1d904037c0ab69a3
2993525b43b5fb4c1207f49932950a46f0533088
26358 F20101206_AABMBR hardison_l_Page_061.QC.jpg
6d71d9af37bba09aa89821fc3b3d0cee
055a58ad44f820e69ebaa64d66c17dd2e6c8debf
1579 F20101206_AABLWL hardison_l_Page_017.txt
3b0e9c3ee1c0e866aeca08d1f6156ce2
dd49fdaddb305b19f630c11a98f3e39bf77a4da2
91 F20101206_AABLVW hardison_l_Page_002.txt
4319ebd865abe6553ab57acb245771a1
f966224adcb557a629898501afc52cb2b1fac4d6
22094 F20101206_AABMCF hardison_l_Page_119.QC.jpg
327185a8ad2f5f846607ee09d76f4130
2316c075095b8d310d82b49089b301f94e7e7a44
24564 F20101206_AABMBS hardison_l_Page_120.QC.jpg
9fff94f83827b2f89c78bdbe0a202c20
f0b47f8a482296a785aebeac7ed50cecf2376fc4
2059 F20101206_AABLWM hardison_l_Page_018.txt
b05dddf1526c2d2f6e3e937b035240f0
81d3a5f4e64460e9357dbe9a8e1f4648d74812bc
77 F20101206_AABLVX hardison_l_Page_003.txt
c0fa3df3a92f681eb090f61ce2d4b584
69f9b7770cbb58247e5aa83212e07ac0fc26d502
5264 F20101206_AABMCG hardison_l_Page_097thm.jpg
26eef596b549657bb29ecc9b720d1cd5
ce65fb66534601389345e7f69a6d14508b7aef2d
F20101206_AABLXA hardison_l_Page_032.txt
1034e57bde4d90c332ff7d3ede0f24eb
b03a30cf5631310c5704d04bd3ed1251e0cdc901
5785 F20101206_AABMBT hardison_l_Page_052thm.jpg
77ea81ad265600cd56357e0582344682
a3921d8b97abbf8209d0a9823c3a47ac2a171b68
1987 F20101206_AABLWN hardison_l_Page_019.txt
7b51bf8483aac50b702a09dd89d53730
b652d0d6d29f5d00870c96ac41141f2dae892f86
2055 F20101206_AABLVY hardison_l_Page_004.txt
f6e16911b403942f64b57adc9530591d
a8bd482f8cf4437b29fd7aae269b6de9d3a80fb3
6227 F20101206_AABMCH hardison_l_Page_069thm.jpg
c869078989d0cdb5fe1ceb06f1bf9223
777360d7cba1b483addd92b7f40f8f60563ee21d
1473 F20101206_AABLXB hardison_l_Page_033.txt
7ad17949c60c5901d56ad5eaa6677021
c1550f3755ab4c5d2f8e32cca1fb8283bfd0b615
19715 F20101206_AABMBU hardison_l_Page_097.QC.jpg
2574ad781f23872f0f51c91ee68eb1a0
9f018637d1939efdf40188b91f808d0dfae08962
2099 F20101206_AABLWO hardison_l_Page_020.txt
b040d68a4ecae64af60270b1e27ec307
0d0c2196b532d16198fb609ed66c9ad94ad194b4
2139 F20101206_AABLVZ hardison_l_Page_005.txt
724e58168b7161916b25a1553648e517
901165dc743d570c841330fa194b3d1d3bcb3611
4953 F20101206_AABMCI hardison_l_Page_012thm.jpg
65d7cfeb5384abfdd4c84a3af5bef052
29e8ff1dfdacbb2918f1ca94c026b06154ba9d42
2128 F20101206_AABLXC hardison_l_Page_034.txt
0753c9f1144f30faac2fa295a665fb0a
5c32cafc6909c5a20ce359c67c846fc2e874c100
F20101206_AABMBV hardison_l_Page_086.QC.jpg
c803967378f7e7d6040dbf4ef9a0569e
dd2355b3a603c0dca3ae614402969c6dd2cb9a2b
1247 F20101206_AABLWP hardison_l_Page_021.txt
48b0762f720c13bd632a38b8d093cd02
14f9d807ae422d00d024e9ca639dceb67ae4a7d5
17831 F20101206_AABMCJ hardison_l_Page_150.QC.jpg
6edbb00e87f52e1b4790200365af9904
ef956390ba3330002a9528de6447e48f1d302d7e
1991 F20101206_AABLXD hardison_l_Page_035.txt
6cca03076f9373beed56adc26e0baa0c
b50a0905c5e343fe59ca2ed7ce5e89b54637411b
6811 F20101206_AABMBW hardison_l_Page_114thm.jpg
183b8d76c2d9234d6299dcdc6d9b3e54
62839374dc5fd4d64434e637e8fd23ca01163fb8
1755 F20101206_AABLWQ hardison_l_Page_022.txt
b2ef28b25fc52e7f98df10833c5ec7d9
281383384403fe8a2a00e5cd6c253701ddb2cd13
20798 F20101206_AABMCK hardison_l_Page_012.QC.jpg
b8ab0839e37cc1e3a846d4098894912c
71e94e30ac0851dc769c13d3d5ce562ee97b3b71
2248 F20101206_AABLXE hardison_l_Page_036.txt
d13089c3266b119ff1bf860a333ddef3
dc8192f9ae352b39cacdd6e78449bd62cbfa3bdd
920 F20101206_AABMDA hardison_l_Page_003.QC.jpg
255f92e994b26316953cd7a461990d77
76b69e9c88c9737f05352df435a2664c6cd0bbea
1895 F20101206_AABLWR hardison_l_Page_023.txt
813b84c8cefdcdc7d4c293c074c4b345
01a5d9a39980e0be5e56f3eea7e439eba993d3f1
21814 F20101206_AABMCL hardison_l_Page_113.QC.jpg
50efc4174b75184cd0f590bf0ee09c81
7b8e75198faed35dadf9b2b2247a30e402a2edb6
2175 F20101206_AABLXF hardison_l_Page_037.txt
eb0b3afa641cff9de16dba9c74c69e4a
f8d1c34fb20598715d9fdd540949e29f15643285
5216 F20101206_AABMBX hardison_l_Page_009.QC.jpg
a83394c705e92a804d6526ee1070bd15
a0b26592d30ebe8a37d4bc491a3ce1a4a8dc4c31
F20101206_AABMDB hardison_l_Page_042thm.jpg
65bf9b631e9b27532400e68b82ca2abd
0f6f07c1f3fbbd56bd3fdd211449f105d531bfd8
1699 F20101206_AABLWS hardison_l_Page_024.txt
610c89f10699bbe87f4043245343aac5
6cdb7f6b9cf1e81d13535f60f8cc3495d384a68e
24604 F20101206_AABMCM hardison_l_Page_028.QC.jpg
61a036579ed3dff0ac609ef859fab96b
6737feb976d4b1ad0ff02bb28c0b2344f8d478e8
1607 F20101206_AABLXG hardison_l_Page_038.txt
13cae39f405a053743a28fc56a5369c6
5d9cae0729d9449ea92abf8614689e2626bf2b07
27728 F20101206_AABMBY hardison_l_Page_041.QC.jpg
65c05e7f3d2275343cd3e1e8c0c74e95
8592b7c8e75cb5ee487b964e5cef3d62c1ecf64f
5658 F20101206_AABMDC hardison_l_Page_060thm.jpg
38cdd657d56c8bbd90a5010f71a715a6
0c2b94464481389217a66b2f9a39fc8ee6735a94
1871 F20101206_AABLWT hardison_l_Page_025.txt
e67217e96211caac8da900142d8a9027
262266fa520a318b9038801bd52a0eb57892021f
60148 F20101206_AABLAA hardison_l_Page_026.jpg
a19260df9d5b31934f8c0179699aedf7
5903f97ead18a27945e2e3420231573fe3c3e5ff
6879 F20101206_AABMCN hardison_l_Page_128thm.jpg
2e22c5f15e9fce2fb46ea54293017633
cbf55031cca8c65ec36ddfeaaa1f5e4a9222f59e
2002 F20101206_AABLXH hardison_l_Page_039.txt
9c4427747e20512195b1fcf7e03858fd
0008fc3bd232e70b190bc0c7bfc5e7ba2391a58a
25832 F20101206_AABMBZ hardison_l_Page_058.QC.jpg
075beb9fd904fda66e0ca9e9f0afa04c
c83374b6b292339c4cd0a6f45d6d0169bc8a8662
1096 F20101206_AABLWU hardison_l_Page_026.txt
e217ff0b81e3b9f3df513adf6531fe9a
c31842e66bdc86f48a8a39428150e1ce83dd33e8
27092 F20101206_AABMCO hardison_l_Page_035.QC.jpg
64e45d8f00bd816f295720f412658611
85668530489addccbca0cb41aadb93bcdd6a3a86
2252 F20101206_AABLXI hardison_l_Page_040.txt
10b2a2bd9488262fdc81085e8f83e09b
466a9430753d49c5c11b6ba7edf53283f7f4407d
4995 F20101206_AABMDD hardison_l_Page_007thm.jpg
867b9e5055229c34ca27b97e74ddc181
b86522c85e5f23e4c5432efb59e086cd414f849c
2104 F20101206_AABLWV hardison_l_Page_027.txt
6fb93c9051d7418127adb6a7daca0bc1
92fb9f3ab9c8393bd037d7c624d11ff415c2f932
84087 F20101206_AABLAB hardison_l_Page_027.jpg
4a46f4520d942a0541e4b9a3ddece977
c47eab0ee52e32098957b643ea804bbb84a89aac
28116 F20101206_AABMCP hardison_l_Page_138.QC.jpg
f0201d140ccd7d35973b012a18fedde5
cef765ab04d069dc0366641350652b4241acbdf3
2032 F20101206_AABLXJ hardison_l_Page_041.txt
6dc9a80d37cbf2a9f69ae7cc5faf91fc
fb504c59217cf59d7058df845d5d186576bc1140
6713 F20101206_AABMDE hardison_l_Page_036thm.jpg
068a2de1f44b85a13753ad02df441b70
bab25d7e44c63dc276288f0974f7efe2f3955cd0
1471 F20101206_AABLWW hardison_l_Page_028.txt
b96080e4a459c86c15d2cc682751cf9c
3386e629f9cfda1994161089722ff7cd4fab8aa7
76222 F20101206_AABLAC hardison_l_Page_028.jpg
5c7ec068efc51d08369b9723a8ef1ea4
6f2af7d9f184b29718a816458b7ed93853c41409
23611 F20101206_AABMCQ hardison_l_Page_094.QC.jpg
cacc0466de2870bca952d52d92a8cb89
4042f94a38a923473659fe501a1d9f69ffc73b1a
2000 F20101206_AABLXK hardison_l_Page_042.txt
00551b752f3ab3582365254a76a46008
4c7bf283db00b79276221d7f22989f5903442bda
7032 F20101206_AABMDF hardison_l_Page_061thm.jpg
caf50574bbc3acabf1ac304cb6e86458
8c58afa60b614aa247d97a2aeedcc90ee726624c
93261 F20101206_AABLAD hardison_l_Page_029.jpg
f4e0e081324d5a6bfc128d0d4cd3f7b1
c8276333818bf954c697339fe0d0159224cd1706
6162 F20101206_AABMCR hardison_l_Page_055thm.jpg
596fba41c5ed60f2f97d472f0f3e7a8e
3356b7fe98a5537e40120f1a7db74024af9478e4
2024 F20101206_AABLXL hardison_l_Page_043.txt
23b11698e064bf7c9913620a3328c9ca
170867cde3ccaa535058bdbe30e53e38cf28991f
19247 F20101206_AABMDG hardison_l_Page_033.QC.jpg
d6ee415a73e0bf8a466584a43f389e2a
f7fb0c1f45737863584d1db500e3d06aaab2f1ab
2269 F20101206_AABLWX hardison_l_Page_029.txt
3ac360e3a65501d9a729cd20da2b4e54
6d4a37bc841b75e295511b834c63ac10cfb7d762
74240 F20101206_AABLAE hardison_l_Page_030.jpg
8bdda95600adb120165079a95e12508b
ec89464612f6dfc57b04fb8bc9d8bbd212c058ab
1959 F20101206_AABLYA hardison_l_Page_058.txt
85a655c15f3bbf2debef176e38bea989
0bb118d2053cf06d92d0158a22b04e39f854ed1e
22968 F20101206_AABMCS hardison_l_Page_021.QC.jpg
87ecbf6c8fa34463973e956f0b920829
2bd5d5883e508e15cc0562f99b54e0c710529675
1561 F20101206_AABLXM hardison_l_Page_044.txt
7f7d53c775b67bba8833949799170a9c
884667913d7a1c41fa67eaf52aa2189430078e2f
5768 F20101206_AABMDH hardison_l_Page_053thm.jpg
7c2a8ab4994ab572557d44c39ef0de06
e827b0f2872c6398069d4f274c7807f4abed1c2c
1756 F20101206_AABLWY hardison_l_Page_030.txt
0a34111ddc63bd4883d8142053bcbd93
26a59e7d28cbec43b6068ca1754d5e27150c8be1
79701 F20101206_AABLAF hardison_l_Page_031.jpg
6041ea49fb07b8dafdc431e4fe4e8eb7
2c91bca34d73682faa94af8bce21a940f864992c
689 F20101206_AABLYB hardison_l_Page_059.txt
e06436ddd8a98cfcd3bda1c4da569c93
7d6455f2c0724ee4a7d38c66066c8a202237ee9b
6582 F20101206_AABMCT hardison_l_Page_067thm.jpg
7f83a768ce563aa0f9ee0e07248fb87a
b5d73a70bbdbcf7f13bda8eb8fe3c0c15dcfc282
2160 F20101206_AABLXN hardison_l_Page_045.txt
a06ff6ec7c673e7140cf000bab959751
9aba161e62d750ccd834cc36b8e10ce7b653da2b
24235 F20101206_AABMDI hardison_l_Page_068.QC.jpg
30845f51f0973a45ff2a1a6209a532b8
2d01c6f45278db19294bd6a66132d0801ed98192
1963 F20101206_AABLWZ hardison_l_Page_031.txt
04473c3fba223b3a251a57124ca6928f
bfade40770837e041f1a90de1333bcb5faf15f17
89293 F20101206_AABLAG hardison_l_Page_032.jpg
f2cf52bac2dcfa9eb41274f2185c3860
4b1c55176ce7d59f0b188385c23c30ceb4df55aa
1672 F20101206_AABLYC hardison_l_Page_060.txt
679ead282b0cc8f232a16e08839e132e
cde5423c13f6841f257296eb73d606f4243c58dd
6994 F20101206_AABMCU hardison_l_Page_135thm.jpg
2974994780688cc800109c2158754757
d30cd21f852e668e21814a9beeccb4199cba73ee
F20101206_AABLXO hardison_l_Page_046.txt
b4566333769299aff35add869c67a0e5
9f7cd9b6dd4b2947143cecb223b971ef477a3001
19186 F20101206_AABMDJ hardison_l_Page_109.QC.jpg
74d307102ce8926c87075967ace14828
37b72616daafbeddfe776c92aa44c0b81901ac80
57093 F20101206_AABLAH hardison_l_Page_033.jpg
771b8b4d833427ab3e0a93eca345921f
e456bb448f7e9062afc607b4e97e5a6f3caccc49
1577 F20101206_AABLYD hardison_l_Page_061.txt
416692bed9b9699310d0011eb175628e
904ad7bb60ff1ae95a583783f00dcc6a7349c40c
27128 F20101206_AABMCV hardison_l_Page_062.QC.jpg
87ca34f4e20f96bde882b682753c8f67
4effc2ba36129bb90b9fe21674201d2e37ae2fb9
1708 F20101206_AABLXP hardison_l_Page_047.txt
4ffa72c83850678d7ab93a152a65e0fd
91a3710e5522b2d2c1b81a81cae9f08cb4b032f7
6687 F20101206_AABMDK hardison_l_Page_041thm.jpg
d79d9b3d236d4afa48ac3958fbf34311
990e49ac92981c4b565c07df7b08a6b7f3ed961a
6644 F20101206_AABMCW hardison_l_Page_004thm.jpg
c2abc5d8eb9ae6f7070c3a061096e892
c86551759f9ec7228da439a0f9725bbfc34ce93c
89890 F20101206_AABLAI hardison_l_Page_034.jpg
7f5a7cabaa4190705710a3288a99f7f5
f6e2c7b03afa7ac252c2c0c138e4ccfd04eb7592
2130 F20101206_AABLYE hardison_l_Page_062.txt
37b842136beffb553e64bd3035620913
7e50c15e03c7faf6dd8cfafe43c9f9b3a4db741b
1611 F20101206_AABLXQ hardison_l_Page_048.txt
5056d5c6ae412caa80ae53f712f485c2
faf44b03d0d8bb10e0c2856709b53184aa739658
5031 F20101206_AABMEA hardison_l_Page_033thm.jpg
8502e6bdae86457371148195b45966a4
563dca65b9d43d2cb4e5fb8fb8ad4f97e290aa3a
6271 F20101206_AABMDL hardison_l_Page_120thm.jpg
8d3f9fa856e57ea591074144b98bb425
95dea2d15a8305a3c07c4355b6e67d3f6c67871d
469 F20101206_AABMCX hardison_l_Page_002thm.jpg
c11d794d56c6560c293540e63e0fef9e
2cd1a245838f4ed01a2bc08821bdd89b64f25230
83616 F20101206_AABLAJ hardison_l_Page_035.jpg
ff0357fbb97cd51448cf3ffbee8bcaac
6eb2825bc9207a1be5568f0a75f4c5fd502545e2
2178 F20101206_AABLYF hardison_l_Page_063.txt
d28ce7199d278d301cd6f8da9a99ab3b
c622bdb89637c5d41378a290c28a68025351eb83
1969 F20101206_AABLXR hardison_l_Page_049.txt
f1f2ec3eb4ab7eea3fc6f40a928bbc64
75d8c500ed4f5b820589c6189ad600ef348966f2
27011 F20101206_AABMEB hardison_l_Page_032.QC.jpg
83ba4de4dd536973be50186f469a8776
cc6d080ae5876e3bd58476fcdd4c66a62f710e0a
406 F20101206_AABMDM hardison_l_Page_003thm.jpg
9683c122cb0360d244d457f24271b586
9f550da7cef7f22c4503ac23c5a03f7032a3308a
28586 F20101206_AABMCY hardison_l_Page_092.QC.jpg
f29737835acecb9d071d5f92bd14ed10
276dc5498f26b1b8fd97ba9ad1df1be99d1c4912
92485 F20101206_AABLAK hardison_l_Page_036.jpg
26aaaa601eef8f34adcf444de052feb6
d7fcae313e506a24a40194f51138725fd5a44a73
2046 F20101206_AABLYG hardison_l_Page_064.txt
0dcc7ed7d55a6b0eaf8527e4328cdd26
98d1587b6c19982c1fd3833d4329d3882a442fcd
577 F20101206_AABLXS hardison_l_Page_050.txt
71e6c19192fa2a62b2462d8e2b6858fc
275a45c3be46d50ffd0155cfd21e8485936eb5bb
28324 F20101206_AABMEC hardison_l_Page_125.QC.jpg
3cb25a38bafcd31bc26b07054ca19d9b
6648bda6bd61d7a4b26a51e5cc47abc1c1ed4c12
24291 F20101206_AABMDN hardison_l_Page_030.QC.jpg
84263c3e064dff427cf96fde4a82e6bf
016de2da27d582339497d3f7709cebd6bf06f0e4
23312 F20101206_AABMCZ hardison_l_Page_096.QC.jpg
3bac3202f7f4b54e490a1af8c952e3e8
5ad2c47c0d13edb12a7a5fa05ef9c1ece927eba5
90661 F20101206_AABLAL hardison_l_Page_037.jpg
858e4170ccd53698b5f91dc91007a40f
9a94e2e59ef27a5c722ef7ec82d26a027af8b34c
2196 F20101206_AABLYH hardison_l_Page_065.txt
075280d0661966270b6c9d9f8a75b47d
153359fe68a1009df000d66517ecfb3c2eafba2f
1204 F20101206_AABLXT hardison_l_Page_051.txt
5e74beb32542d9a342e6604d55544cbb
9057a69ce3b2f123760ac5a90316efae6c517c1b
69213 F20101206_AABLBA hardison_l_Page_052.jpg
a4b0362c3197d419219bb458ed819674
78bc0bda8eed5ade2f5ebcad174d9b7516c8c611
5854 F20101206_AABMED hardison_l_Page_103thm.jpg
5d91be31bded15cfe9e44a8ebe2446d1
b1f2a6a498ba8aaa35b494c9550fa0d371ed7f52
22510 F20101206_AABMDO hardison_l_Page_103.QC.jpg
55958ba9b1d2ce4c797ab292bde4e9b2
1e00a05381f3882eca2ef30dd841979fd355e3cd
72734 F20101206_AABLAM hardison_l_Page_038.jpg
ce76df4464015c14e662b0e44227a244
fedca361fe923d4d5b1802e0918acad03e115f73
2155 F20101206_AABLYI hardison_l_Page_066.txt
041c8987229d6531e519af980498ec3f
1613277b73ddc3e99c35666ad286c209b0a2c8c3
1725 F20101206_AABLXU hardison_l_Page_052.txt
1852837fe6abd636dfab36baf35bb8aa
5756eb898a3e20aecc886eff7850accdcff4fbea
68081 F20101206_AABLBB hardison_l_Page_053.jpg
6630cac7b532ec24c73c186e2367b8a7
bb4453280157491e76aa55be78212bd096217cfb
6429 F20101206_AABMDP hardison_l_Page_125thm.jpg
da3fbf6bcee9ef8b987f1a642da4f5b3
c2f2c26a34a930aa45a4487774f70984aaf04d27
80817 F20101206_AABLAN hardison_l_Page_039.jpg
7a39f7499d865c984cd3e3dad56f414a
08c1ded63879c9e2ecaba12c7960e76d3024dcbf
2115 F20101206_AABLYJ hardison_l_Page_067.txt
235bee4cd1792bc2ecbcca10cfc2461b
bc0cc794bae4c9c0eb3d5cca65fa6a4edd896145
F20101206_AABLXV hardison_l_Page_053.txt
22abae5a1d19ec789f26e250c5cc9eff
934dab148f5413bd73b372acf7bb4a84c7d890e5
6373 F20101206_AABMEE hardison_l_Page_018thm.jpg
5f515c87cd3fdcaa6cf96b75394e4eab
c326ffc7fbed08f1f857be8145ee7e09d5e7d59e
16618 F20101206_AABMDQ hardison_l_Page_008.QC.jpg
b47d27e6276092613a4ed29ae4390f1a
c67f56635aaca64cbe0d683aa1abbae0d7e86dbc
94242 F20101206_AABLAO hardison_l_Page_040.jpg
54ccb2c8c0d995378b8299e35e97a129
d1e61bc321c9ed1944afe8b90296c97c53dd2ebd
1921 F20101206_AABLYK hardison_l_Page_068.txt
8a7f41cec6e4ed276ba12a26631fbe0a
df2d3e43436cc3fcca79c5bf82cafbdde5125ce2
2161 F20101206_AABLXW hardison_l_Page_054.txt
c7d70462e489f5f14c418cee18a75336
6e508c827b30840adce9b98f872fae40a6a8bbe8
92022 F20101206_AABLBC hardison_l_Page_054.jpg
9c05bdc1d32d75e0d3bc3b871fb6ebfc
6accb6f9c4d206d93d2e63938cc5ba880b5b808d
27584 F20101206_AABMEF hardison_l_Page_056.QC.jpg
d9a1421368b92f6afd964be6931363a2
a82e3642e744ea79f2793eecc9753c3e3becd4bf
5844 F20101206_AABMDR hardison_l_Page_023thm.jpg
87d7d1be2818ebfae8990e52b8189a7d
97fff3ed7cd723a64fbfd0c844d5a13ef5f9d949
86204 F20101206_AABLAP hardison_l_Page_041.jpg
6dfd3be8d6887d45003a6b022dd33b5b
e08be1542bb4e6f07eb970bd3befd4dcd608e512
1482 F20101206_AABLYL hardison_l_Page_069.txt
0f44a3dd5aaa56ad6e2a12cb0a696e55
ded40bb98ad69832bc7497570c484b232de764f4
1707 F20101206_AABLXX hardison_l_Page_055.txt
7edebebbf21b9c078ad0613c27ccd21d
3a9fa150dbb01d1016cf87bc4f4001ae36d0fac2
76167 F20101206_AABLBD hardison_l_Page_055.jpg
28ca4389e01dc98993e078c330e8a9ff
0ec1a9ece282b5803961d7b5564c2767a9ddca24
29243 F20101206_AABMEG hardison_l_Page_143.QC.jpg
41745af185b78f446e973dc346a97a18
e6068a0b3f041d8f521e1c27d0173b05fda16c9a
7011 F20101206_AABMDS hardison_l_Page_148thm.jpg
c16d9cf8d1d4f7fe7966f87eb1dd3465
0d83e0bf1968cf226fb99ffff679508f86d95a82
87462 F20101206_AABLAQ hardison_l_Page_042.jpg
028da4c9fc6e2d09de03de2f5d4fdb8c
ca64ee11651ff6cc772da199ed82a4c634821a44
1548 F20101206_AABLZA hardison_l_Page_084.txt
9e301603d59490eb4356ea04720698c8
9a1c3bee60a242362c11dd290d17636f755eba5b
2185 F20101206_AABLYM hardison_l_Page_070.txt
9455dcef59eb3109421d641a092d9f4a
df300f95d99941af22a6cb35829028698c93e479
89474 F20101206_AABLBE hardison_l_Page_056.jpg
1a83a5c7f551771e4124c02184aa33df
4af5991a093c8ccc4e06f5ce1daa5695eefdecd7
27745 F20101206_AABMEH hardison_l_Page_072.QC.jpg
6439c6f503045de275809d42b351a6c8
2436ea6717c11a9fe556ee0f6e61ccdf52ec0307
7082 F20101206_AABMDT hardison_l_Page_136thm.jpg
84056fe2044c757a6201a2a26f53ecfa
1e6f0f6a288b085ee257db5b1a458cbbc00ab44e
86816 F20101206_AABLAR hardison_l_Page_043.jpg
8ead627b14ed9c930b8aa1b735700fdf
3d122a914fc2851bfdc5a531bb2905b343cc3ce6
971 F20101206_AABLZB hardison_l_Page_085.txt
8972c34a523568e1af59cb31d1d0ee63
ca6df2112ef3be92db0a631221c8b605915c6111
1340 F20101206_AABLYN hardison_l_Page_071.txt
832bda3d0aa5bada0d0b50a036029ea4
ea9071bbf9d8076e34107e0cf4516d8ba7357824
2150 F20101206_AABLXY hardison_l_Page_056.txt
d041022d6d12c2e2843d98360c22195a
7e0259485903636f19a7b86bb11987890303d69a
81127 F20101206_AABLBF hardison_l_Page_057.jpg
1ebee8ef901b3e2bc568b012b27d7f0b
3f83513b000052881b61bd10886b7b3e1c2b1651
28322 F20101206_AABMEI hardison_l_Page_037.QC.jpg
972a3086ef28cec0befb38c89fe84d7d
7b6aa0593d589a3a52d5917518bc0a1c627e2cfb
F20101206_AABMDU hardison_l_Page_070.QC.jpg
f2cf0631ee46d83ff6ad00e3c1d3ddf5
4be84c3012b523b8b06e734214ca06c5f0a02dbb
71708 F20101206_AABLAS hardison_l_Page_044.jpg
149e4bea615f78e7b2040416ef81a185
2d0b3b1aa9423f713abafa18e3eaf0579fc29284
2141 F20101206_AABLZC hardison_l_Page_086.txt
7b5a997d3ddefeebfb73436e15a21320
57feefd1ec5bf482ae2ea283ae1d0a366a78d1d6
2151 F20101206_AABLYO hardison_l_Page_072.txt
0aea0116b64ac4f212fe2b530dd7346c
7c016c993921d8bfae453e60549bb7c0e933295c
1332 F20101206_AABLXZ hardison_l_Page_057.txt
5ee322af257f88745f58b177aa47849b
04bbf307ec9b05ab67e8b8ee9007ebbf45849425
83297 F20101206_AABLBG hardison_l_Page_058.jpg
4d44febb9b52d88f6c5139e472568c48
f09ac37fc71f3f55e52cf3fc3cee6ce1fa4967c6
6171 F20101206_AABMEJ hardison_l_Page_090thm.jpg
f7678218230fab1d647a8c369c51b358
6c974c7a4537af04c399cde37812fe876b304e73
6725 F20101206_AABMDV hardison_l_Page_130thm.jpg
a2ffb69c3443b41d648466d2ae6cac69
b77d1b86996fd9905dfe511cf69bb2225a1eae3c
89551 F20101206_AABLAT hardison_l_Page_045.jpg
5928c47ac12b34dff7c463516d2ba893
233f62f38ead9ce8dbb09e56e7771c70826f998e
2249 F20101206_AABLZD hardison_l_Page_087.txt
4fbdd9598d0b9976aba520d87f614fee
f51c22edb44be5a4953985cf553522bfcd864016
427 F20101206_AABLYP hardison_l_Page_073.txt
e5679627078a0fdb4c3f3f7a2bd35c86
67809d35e695ee45446fb4d9a32205316c2ce565
52484 F20101206_AABLBH hardison_l_Page_059.jpg
433a0414afa390bb36889a171d2cf7fb
f78ac6610ade905783417af57512599e4de23548
6838 F20101206_AABMEK hardison_l_Page_115thm.jpg
3c9b0e927243263d12e23cbe05c0ed2f
4eeea900e2c92e227e75cd4a9158a600198b9f6a
27898 F20101206_AABMDW hardison_l_Page_137.QC.jpg
5e4094407fc9e8406c4fccffd1db6c06
e19be5a6142b819de3c6a3d00b447c26d2772c8c
89763 F20101206_AABLAU hardison_l_Page_046.jpg
9b1e51fae35b373ed966e051a6505081
a04d82d072d4d9ce9bee6ba37c0318d3638dc2d9
F20101206_AABLZE hardison_l_Page_088.txt
031b7c25e75357aa65b3dd4d69f981cc
cf79b0ee517984ae6558f17b2276722ca2b54178
1479 F20101206_AABLYQ hardison_l_Page_074.txt
79da8247afc1e7aded01f0600498e418
571dff75b6521dafbd9d95b840b8bb7735f917a1
64409 F20101206_AABLBI hardison_l_Page_060.jpg
e02fab72247fc1da60f656acf60bdfec
15aaed53853e345c005e7d22c581b5db85329424
5830 F20101206_AABMFA hardison_l_Page_119thm.jpg
fe36c82ba3bbc5bb9927acf2eadd1545
badaea64f284100aca994f634d501380814c06dc
5923 F20101206_AABMEL hardison_l_Page_038thm.jpg
f86fd92d8dba90dccf59f4e5982770ed
85a06eb3b14bda025715d3d1653c328a2ce4c179
6689 F20101206_AABMDX hardison_l_Page_065thm.jpg
38b7627e9ac8dde0e50bb59ec123e441
505dbba9dab8b27af5768664cb2f5239086b9f65
75499 F20101206_AABLAV hardison_l_Page_047.jpg
48cee62d3fd8eecfbf9492f33dd0bdbd
76af04be7a94b4574ba511efe8c6b8ff63d92b6c
1833 F20101206_AABLZF hardison_l_Page_089.txt
01326296dc13bf596cf83258ee75f21d
a5a8bcc39fba12f798c425f961047f6f78e82d43
2195 F20101206_AABLYR hardison_l_Page_075.txt
c620a19465dbb14ba0d22acac321e4b7
e29a3f9acb1eceb500cd2e76a703ddb67651564c
83859 F20101206_AABLBJ hardison_l_Page_061.jpg
e29b945e07f7b1da61d03a669c3cb2b5
d29120b1232a2bb069227b836cc15f042ec788ee
3137 F20101206_AABMFB hardison_l_Page_085thm.jpg
24895fda554b0d7ee4411e025f3ae07b
6e5fe2d3fc28038a1f9d02bb2a8547c9cbc9c0b1
6404 F20101206_AABMEM hardison_l_Page_086thm.jpg
42d4df6b32760e7a0e6342d075df0ac1
cd56abd52a7504881ac43f8d71a00fc2aafcdee9
6748 F20101206_AABMDY hardison_l_Page_126thm.jpg
abadf75771b25a77ac324adca69ab492
28287df2e5b1b2abe1c469803926b8c9ef6df801
80395 F20101206_AABLAW hardison_l_Page_048.jpg
1825ea22e5771ab01cf435303e039396
fd90dd5de343760f91e1944b7575590b1e46a806
2015 F20101206_AABLZG hardison_l_Page_090.txt
86855617a3b1bee234fa13df70fb6672
49f212e3b5624081a84738b6354c7b100aca6997
1982 F20101206_AABLYS hardison_l_Page_076.txt
7f846b888df92f776fb352e52acfa97b
94e9b8508ce2cf5359f091c9f3760728c1757eff
88514 F20101206_AABLBK hardison_l_Page_062.jpg
383618a3ceba1cc12bc818d3a715a296
a87353fb832e6ff043f8af132d1e4583d277bd9a
24825 F20101206_AABMFC hardison_l_Page_133.QC.jpg
456633fa589a31780113a59c4337d093
3557b0991ffd74c2f5e39722c772aa44df958dee
6305 F20101206_AABMEN hardison_l_Page_044thm.jpg
31402d131cf95e04764fb58b4f337f5d
d9e868a20671d37182499d7dfaf28c5607cdb31d
17375 F20101206_AABMDZ hardison_l_Page_073.QC.jpg
d122a26cf775fd553a25b99c18b59fd6
3acaf3ebd2570a9018de8d4c6a5c2d45c21863d1
84517 F20101206_AABLAX hardison_l_Page_049.jpg
7a32a7f9bb9828210201769f9f0e16cb
85653fed3ebeee6b570dca86f5df5be2253876db
1524 F20101206_AABLZH hardison_l_Page_091.txt
4855d6a48870d566165299b311e7d0f5
3d9fcbca59f26ea3dbda32963b38060ebfc41ba4
1335 F20101206_AABLYT hardison_l_Page_077.txt
ffa184eded2f50a9c4960dc4343f55d9
4a2ddafbaef02f7f07639bdc897014ba6a3b15b5
66375 F20101206_AABLCA hardison_l_Page_078.jpg
b876c89443d5a0f1be25a269cfa59798
b551aa88137b3e4881ec1438fd242bbbf39b362a
89956 F20101206_AABLBL hardison_l_Page_063.jpg
d8b520b7469b60e863323f5283ca6861
e187cecccb656b8876c0a0159d119651cb3397f0
6899 F20101206_AABMFD hardison_l_Page_142thm.jpg
c11a0c9c1499fe0473a39d35dd69452d
92aeaedc7174841cc93077feb27ce979480234b7
5893 F20101206_AABMEO hardison_l_Page_110thm.jpg
5d240b65498a301b7fb5f2764ece8d4d
d69fb5aaf264d42e555317b763267e598636cdd8
35752 F20101206_AABLAY hardison_l_Page_050.jpg
0873ee7785b176455b7c279ac3855660
001cb962d9b0d25135405c44ba57fd78465524b7
2157 F20101206_AABLZI hardison_l_Page_092.txt
2de545ccea4137f7e4f5cd31527814e0
9879b21a1dbd434557c8d4677f00a0bf63df3e51
1501 F20101206_AABLYU hardison_l_Page_078.txt
5881d8842467e8fb14c582544f24fdd2
114db2a52bd0dcec29f0651905ca899486274beb
91084 F20101206_AABLCB hardison_l_Page_079.jpg
39ce9968a3bd489511179ce6b6bddc1d
3ed9365c79a56d61a046e70d0d2ca3c9cbe36c9f
80464 F20101206_AABLBM hardison_l_Page_064.jpg
232e4638ef155c90b52b222bb75c0d13
a3bbe9d86d961e447a1eda2e4fee043fab4230a5
14599 F20101206_AABMFE hardison_l_Page_104.QC.jpg
d9466d8c10cd18f579768a96ad8a448b
b5cb4be2ba424f8d5d840f29a1ae67ad4f35c93b
6608 F20101206_AABMEP hardison_l_Page_034thm.jpg
2ab9f85523cad32f080b88ec7cde3629
525faa998a2f73e4dc444d97bac5ef9857522598
62950 F20101206_AABLAZ hardison_l_Page_051.jpg
d4b40b786ca86a3bc655d417d420e929
5a163f71fedcb59787cfe9abbbe626b32ecc334a
2017 F20101206_AABLZJ hardison_l_Page_093.txt
daa246dee7de79a602550d19978aaab7
d2811a0fec4e19c916ad0156b31c3079c74e01b9
2192 F20101206_AABLYV hardison_l_Page_079.txt
4f7abba4a03002225e16aeff15d61816
443cc911a6506fcbea84eda2b2028dae7f24c2c8
89799 F20101206_AABLCC hardison_l_Page_080.jpg
5f2fc0996befb933caf7d6f2cfeecdf1
640804e336b7fd7b822ae1d07e79fe8caca3f1a8
92032 F20101206_AABLBN hardison_l_Page_065.jpg
4b8f7b8d56ec642707c7239972279bad
3c7653a14683733d89b4843859ed28eb77bea006
24322 F20101206_AABMEQ hardison_l_Page_048.QC.jpg
f01a04dc2ef7f21f48324c8cf06a82d0
bf38c84be9c4e8f07053fb4a39ed66b918c21cf8
1368 F20101206_AABLZK hardison_l_Page_094.txt
277d8313dbb5c44ada0ce4af8e5ee859
36aba4842cbc0923f05293187d4d3fd027b93b58
F20101206_AABLYW hardison_l_Page_080.txt
56bd9a364b2b0375adca8dd7251f220f
a95d8bd9187b5ba4c43dae2b543e3a054152f161
89843 F20101206_AABLBO hardison_l_Page_066.jpg
2849b7f55e4cbc6083884c53cbe57451
5ddb75fe55aa11fef0d561b0024b17d4e50e9271
6389 F20101206_AABMFF hardison_l_Page_047thm.jpg
22202972e1a308642dfe6058569700a5
7217755125027c7fe7e4986f4a23e44b6595d7e9
27544 F20101206_AABMER hardison_l_Page_005.QC.jpg
aac641655c9b6a048bb92eb2b8e2473e
e2fabd8cfaf849dde268f5aaf28ae2f59b596cf6
1964 F20101206_AABLZL hardison_l_Page_095.txt
c66bcfe290c5e374581901c96f78b8b9
2c83be2982d48cee9c56e680aad39ee1c0cb1e1b
2273 F20101206_AABLYX hardison_l_Page_081.txt
1266aff863f1c830ca9f41e4e02c730a
f8414356086d68d84f07d6285fc5c8ce8efb8098
94681 F20101206_AABLCD hardison_l_Page_081.jpg
fc3d89d59aa3cf8565e55a85e80172a4
ec0d6db36071a99e1d40ddd27b29c4c9d62943d9
88458 F20101206_AABLBP hardison_l_Page_067.jpg
857d7487ad6965af52f66b9829b37b78
3c68d1682a490de97cc2e628f544bb2e002882a0
6628 F20101206_AABMFG hardison_l_Page_122thm.jpg
91f84e6287682ef00f8ad394b3b54c27
06311dc4d6eed9f06d6b9cd40a9abf60ebc49825
6323 F20101206_AABMES hardison_l_Page_020thm.jpg
33499493bf55e838983cb750d10d10d6
4a68e02d67326b6359fd8ea3f52179e8e5e393af
1859 F20101206_AABLZM hardison_l_Page_096.txt
ab2cbf36b42aec828eb12466b27b670d
e39088190d37047d24a7bc513b334e8bac4c72f1
2043 F20101206_AABLYY hardison_l_Page_082.txt
79fb17a24b618e120c912a99c24dcc6b
2ecb9250864a23eb17045f62eba9f065915a0f14
83976 F20101206_AABLCE hardison_l_Page_082.jpg
29f3aa7697bce1defcf355941582362c
93293db51afdbe787a1d141a1f53742f9a89411e
79270 F20101206_AABLBQ hardison_l_Page_068.jpg
5c92e2246210bc9b2b7c5501efc1bb82
6da35acb38c49c9d21bc745968f4d640314c52fc
5800 F20101206_AABMFH hardison_l_Page_084thm.jpg
f6863c6eb09791df49f9717b7759fee8
dcad346baaae1f6308836f4e092d1c3a9d7abf37
6398 F20101206_AABMET hardison_l_Page_133thm.jpg
2b5a9949af12007ea64214e54205f1bb
504f28336cf15c70e3a4a36213bd235c79aaf5e8
1460 F20101206_AABLZN hardison_l_Page_097.txt
81e214259adf6204d03b0edf63cb5585
2f2ea0f1f92c01e35a89e4bd118e938adbd33d0b
44747 F20101206_AABLCF hardison_l_Page_083.jpg
0647be2b2a4c91d2b3d48b511eef06f8
5fe64ec3c8585b34e9e8ad2f8ebc71bfceaa2249
69502 F20101206_AABLBR hardison_l_Page_069.jpg
67745c81ad60125275ca7ba538640c38
17ad03b887e731a67f85d3c5fc8242d923dd1d8a
1472 F20101206_AABMFI hardison_l_Page_009thm.jpg
2ae28c74ea12e309c3d8523db1871cbd
569c2de6df62bc136ac352f557bc26dc7f538c6a
5782 F20101206_AABMEU hardison_l_Page_026thm.jpg
10d4ee7270ac00997be30bec881b736a
201f1e3d502012560fbcdf720e99aec1729edf2a
F20101206_AABLZO hardison_l_Page_098.txt
8b50ce845b7fcc3c91a216e75a547cfe
f44a42f19878744486965f83596bb10d723cfae1
932 F20101206_AABLYZ hardison_l_Page_083.txt
c2fb4813c3733e08c5c5d8082e2b4963
eb498ad57151722de8cb66069ed4a9bf80332598
65100 F20101206_AABLCG hardison_l_Page_084.jpg
273bc82a2be03c6b48baf8b0348091c5
b5353285d835647146d5599107b042920d7ddf63
91059 F20101206_AABLBS hardison_l_Page_070.jpg
31d01a18c170e564758052a95333d39f
ad59d39975dc1c4fce86c1d1371b8e489e88726b
6138 F20101206_AABMFJ hardison_l_Page_015thm.jpg
0bc75a57afe20b6c6315e9a6ac5a1ef7
0124c3fec8c8e86ce5b61fb552c0d6242c2f9232
5835 F20101206_AABMEV hardison_l_Page_001.QC.jpg
9c1a8ff307b10a6a7684c8ed3bff7651
79d9a2485e77fa3dddd462362a217b3b93fa12dd
1259 F20101206_AABLZP hardison_l_Page_099.txt
13f552d487546443bdaa844a061780a8
ace987a07cab1fcecfbd27ccd9792f5cef23f6b3
40939 F20101206_AABLCH hardison_l_Page_085.jpg
c9b12eaf774730a71faad09def7dd217
7b773ab2c29f0e6b77b9074c724d4dcd7463c281
83331 F20101206_AABLBT hardison_l_Page_071.jpg
1e2b0b1746b433a66000965788c506c8
8d6a55859c35eb6fdc36d8668f62b6bd81decc27
20630 F20101206_AABMFK hardison_l_Page_007.QC.jpg
e601f0b68708d720d7815059d900b748
b2d392a41e783fd7dc1a05cb0261b087c8cccd83
6213 F20101206_AABMEW hardison_l_Page_011thm.jpg
0a96953a6fffb41343646ba8013fb597
b14cfaa181c7fa2b3247a3ff961946dcb5912266
1349 F20101206_AABLZQ hardison_l_Page_100.txt
98ef19bb05ec21b9ae1a6eaa4e26c785
74aab1b03e2e05275ee27505c768056e4a9257f9
87420 F20101206_AABLCI hardison_l_Page_086.jpg
0d4a8549f2fc47b0094b285c680a5886
74360f8ee93fbb8eedaf9ea2cb04bdeff570c4fc
90524 F20101206_AABLBU hardison_l_Page_072.jpg
439a5624f04964c6c649cedf70d11f52
7518d7dc0d01fffe0f432a87566a1ed03bf0b613
28114 F20101206_AABMGA hardison_l_Page_139.QC.jpg
03ea1e7e43f6ef2e3bcf2253144e8e92
01936e20b4252a9d6088cb2bf74d3a0aae288b31
28633 F20101206_AABMFL hardison_l_Page_075.QC.jpg
397b2c2792dc99f253e36de2be84923f
5a8cef813df649df27ae16cc15dacae04105d350
29057 F20101206_AABMEX hardison_l_Page_036.QC.jpg
4889c0334e419e74a1531ee096f4798c
0273369b3fd900ff75a02f0f1fd4f56c7accc050
2245 F20101206_AABLZR hardison_l_Page_101.txt
f5e479e2d2c98b84ec2f60cb534bf495
7a4c91aa7433342d5884033ee0f0248d8dce4ab1
93299 F20101206_AABLCJ hardison_l_Page_087.jpg
6b0049ced15a7f71c60b0a3c7e5dae5a
80d340edbf3eb4fb364242b66b3f2a44d20d3cc4
61458 F20101206_AABLBV hardison_l_Page_073.jpg
b57e763acea644dd77d14d5e63818741
69c349e468f3bc87f6697f65dc8f9757dc33fe91
24394 F20101206_AABMGB hardison_l_Page_110.QC.jpg
94b3ee340e8408f87aa4444f88db155a
3cc13e79e6130b1e18d924661e5da69541c4e97a
6801 F20101206_AABMFM hardison_l_Page_054thm.jpg
358298bff7ca0eba906c2c49fa1c7171
49a905e049d0d75d496ddf4150d2b5ce66ff21a1
28959 F20101206_AABMEY hardison_l_Page_128.QC.jpg
63a04b1dbfdc0f33559f2ed382b8d30b
c05da558926e3cdb4fd8776cfd947efcf97db034
1787 F20101206_AABLZS hardison_l_Page_102.txt
523249151cf1df3394a7d980aaa54ee2
c25b86e51e2a4cfdf25f5fbb32b812790bda3755
90640 F20101206_AABLCK hardison_l_Page_088.jpg
8ec24511b368ed5ef3b8ed32a5b4cd21
d9335cdb9af184357088ecee22f917aa4a8e40a9
77692 F20101206_AABLBW hardison_l_Page_074.jpg
90125f2aba252a478ca8ea1a8dff330f
fe9ed3c563afc5dadd356047c55ce99f3f523ac4
7119 F20101206_AABMGC hardison_l_Page_140thm.jpg
8c867cfab303d6d9d77c4d7f3cc3ee4c
f4e8710f12db251bc53090ec79404e77fd077c6f
26695 F20101206_AABMFN hardison_l_Page_019.QC.jpg
da5e9cd00fca7d27d82e3f292c8c1446
ad18f0ad92681d1095ecea586678d491b9cca504
6362 F20101206_AABMEZ hardison_l_Page_043thm.jpg
b8b68a83e20cd711a20622b3d465e758
d51fa52ed5be9cf2ce9cc842fc81ee18e1d2689f
1868 F20101206_AABLZT hardison_l_Page_103.txt
a3bf95c28fe7d1f43b442a4c9efa340f
bef664cb0d21fea00ebbd0d41d4861b73cdb5385
43390 F20101206_AABLDA hardison_l_Page_104.jpg
d154b856a4f109db144e2b3746d583e8
aac7da6d027059e7659e667e2c433090b292de6d
76074 F20101206_AABLCL hardison_l_Page_089.jpg
86746489cd6b04cf9aefbd3740c85e1f
d015e3e6fd220b1938779322e6fa87f941b193e2
92154 F20101206_AABLBX hardison_l_Page_075.jpg
cff13b49bc198d76b5b44d42d902baec
1765efa8318731c21581d66d457d8a0aaeb67ba4
26741 F20101206_AABMGD hardison_l_Page_049.QC.jpg
7d27ad1ad4d81fd5a4b80f91975be975
53ce3ffaaf47ec6ce17be203dfc4dd7ed4b5783f
26328 F20101206_AABMFO hardison_l_Page_014.QC.jpg
856185da5baa6ecf7620b7786a437e49
6aaeed7f2f2b3a05a6bb52d6b4c18e60cd7b81b7
576 F20101206_AABLZU hardison_l_Page_104.txt
18dd9a54c3f55f3c5df8253bae443a40
13fa53c6a42a1f8dd0d6ae31186df85e0a04f960
53305 F20101206_AABLDB hardison_l_Page_106.jpg
25c5f01098e88dc347b1450217335a80
980f7d98e9f239d6379b319a74afbbc250fdf719
84977 F20101206_AABLCM hardison_l_Page_090.jpg
d786631150b9071f233819e1f6c56027
b10862cdd34d33b7493ae2e691593056c881bc77
81939 F20101206_AABLBY hardison_l_Page_076.jpg
ebe7470ca23d9f342b109b0dfb0e5fc3
4c1c0e45edc6558ddad86305bd93d6032fdcb1ef
24617 F20101206_AABMGE hardison_l_Page_074.QC.jpg
315ed81a106cbb27077cd991c3627fc7
87d73470efdeff3e70d3d614a734f5d2f5d202ef
6667 F20101206_AABMFP hardison_l_Page_137thm.jpg
4506fa0bb764fc59db739340b943dcea
b0fd1298f07ea0e796a8ae2f6fdf26923eb55835
F20101206_AABLZV hardison_l_Page_105.txt
a59a414907b43d173ec70a1d59b0f01a
d1650c30b61fa30edd48671b521d46338666e716
88816 F20101206_AABLDC hardison_l_Page_107.jpg
8833f2582fd64513ec34e22b11535bd6
96288e6b88cf88c2e81a5e6de7c24cc0e016b13f
74149 F20101206_AABLCN hardison_l_Page_091.jpg
a408064073193c94813bf3ac20b3b7e6
7b0582f277060a5f185559e3863023075a6d0eb9
71104 F20101206_AABLBZ hardison_l_Page_077.jpg
73f4544687d6563fbaf76317aa63483f
94ca4300561c50e105142e358b563fabd84d9be9
F20101206_AABMGF hardison_l_Page_062thm.jpg
c6b81a75acbf93dba040b7b6c4c188a0
1321de0ee8041e5c6b34fc2c69c2d87499395a1b
5940 F20101206_AABMFQ hardison_l_Page_078thm.jpg
74dd9742a66409d6f655726cf6881c41
2af70352391eee6d3cb307a0a5a46e8e9975639e
1401 F20101206_AABLZW hardison_l_Page_106.txt
804b66992ebe2fc87145249e61feabdb
ed5f3506bf66fe4273e72a24941481faeb4f755d
78785 F20101206_AABLDD hardison_l_Page_108.jpg
11894f294acb9c06f7c1e868cacd0a4a
d00564229d5ccd47640f7e6ae7334d66421d286a
91942 F20101206_AABLCO hardison_l_Page_092.jpg
3832b8e66c0ea3e16eb462a96e14a320
a3e1cd961e9abaa8341048d4ca62b710243e8c4d
6685 F20101206_AABMFR hardison_l_Page_129thm.jpg
380a450c87aaec5a45c48759da9c8e32
34166289de64e7164bc39a8d2bf43fbdc5bd6055
85086 F20101206_AABLCP hardison_l_Page_093.jpg
144f0e794cd0dc16cd664c7217dce3f1
3f88c829e59d9fb29615672c8dddd28068ff277f
2095 F20101206_AABLZX hardison_l_Page_107.txt
f463701c10937ab5720297a15db24aac
c0891fa354580eeefbefef23129968f2ec8e71b9
6850 F20101206_AABMGG hardison_l_Page_080thm.jpg
a6b8d1ac2c2d1185bf0f50bd274b8836
054e2cdcaef4468a97adbbfc1b56bc9fac0d9374
5565 F20101206_AABMFS hardison_l_Page_102thm.jpg
af26d3538bb15180954d901223276520
f977f704db9315c33e4f1227581d3f71d3060fec
60495 F20101206_AABLDE hardison_l_Page_109.jpg
277794bb917c08575434ab6b58cae581
1c2ad4001d4fb11cbb3d621e9795fd7fcf7e1392
70695 F20101206_AABLCQ hardison_l_Page_094.jpg
7c53225f740025d7e03ba460260ac9be
7d0c88795203bd38b7cc603512e730174ed6d024
1710 F20101206_AABLZY hardison_l_Page_108.txt
85186b7d406d097ba48cefadd068a4a7
232e776c4f657421856aa9bfb531699743ca3fc0
19916 F20101206_AABMGH hardison_l_Page_112.QC.jpg
2041c22a2e552ee66041ed35d53c1d38
281a8ba3176e8f12928a0ff48c121ddaba56a354
27570 F20101206_AABMFT hardison_l_Page_114.QC.jpg
67959f50d4b1e05d9b2f9027d9e64fb1
0587edb9ae8300e6d2adbff3db50f2b7785d6c60
75646 F20101206_AABLDF hardison_l_Page_110.jpg
e698a759255d30319a65a126fa7dd371
b9343cfc84f7311ea461b31a42948548ff52539a
83260 F20101206_AABLCR hardison_l_Page_095.jpg
354f73b673dbbfbcc93085fb7f042e1d
bf6b6ab90ccfb1b446d89d276c66340d97c2525a
1421 F20101206_AABLZZ hardison_l_Page_109.txt
6c751e0f9c5a3037fbb1482828f67fbd
0f19c96d3f56578853b4502f8d02027f8cd5c5e5
6974 F20101206_AABMGI hardison_l_Page_081thm.jpg
d20e4d2ca0a89b5c58836d568a5f9354
8a36cee4dc16fdf91b994c067a4b7c4edf920dd8
28117 F20101206_AABMFU hardison_l_Page_141.QC.jpg
b521892f08971678cbb7863c155c389d
340918f3b5268ad0f611a540dd5348d0dc516584
68222 F20101206_AABLDG hardison_l_Page_111.jpg
5ebb72843369aee061ac6bfd7eb61a9a
eca8f716336b4cd102995620c9c1a7bbdc37cd89
73972 F20101206_AABLCS hardison_l_Page_096.jpg
a6eca3301f40a91b0ffdd85eade159a5
c53133e8f8060d323b8d8c9634bba1bdc70e616d
12887 F20101206_AABMGJ hardison_l_Page_085.QC.jpg
cdf4fc6ade947049cae4298e1dd18c18
653b174a4241d24d2ca875c70c8eada959fbcc8b
5335 F20101206_AABMFV hardison_l_Page_112thm.jpg
400e45fa990382342cbd4be83b469b3e
22da46fa0f51099c9b924f47d6f4a0f81a49e445
58160 F20101206_AABLDH hardison_l_Page_112.jpg
a5e845542a23d97dc262d12133e59311
0e7f4e81d9b8cca60c5dfc5211cf6c514b727ea6
62798 F20101206_AABLCT hardison_l_Page_097.jpg
2190f59cbd55ea8e614e635205d6a18b
86957201399964bd49c7480465fd55ea8058a6be
19461 F20101206_AABMGK hardison_l_Page_024.QC.jpg
bde5ed6e5df7a3e1387061ac39cca61f
0e6d841c115d81e3792664c136ab795835823f9d
27747 F20101206_AABMFW hardison_l_Page_067.QC.jpg
33e03d4da51089fb9da14e6f8c16e696
80765d553fe4ebfb995b88bed11329a093d0bee6
64712 F20101206_AABLDI hardison_l_Page_113.jpg
159598b79fb814fc376111a6f582eb71
210aec01513552d3846e75ab54c573dd9b9fb81f
88344 F20101206_AABLCU hardison_l_Page_098.jpg
cb4e22dd3fe6f687f1dcf7b6737c9844
bbf4d9b46097ab3a18def1674f4c2ac71b5161e8
25979 F20101206_AABMHA hardison_l_Page_015.QC.jpg
b5a69fc992bc5dbe6e3e8fa5acae8212
19dc3c857bea025577df4d2cf98ef30ad5b55093
23374 F20101206_AABMGL hardison_l_Page_077.QC.jpg
a531d2c0704cbafb78a2f6d8fe3e6622
47a4e65c8effc63013989fdf46d66708e44c04bd
30089 F20101206_AABMFX hardison_l_Page_105.QC.jpg
019d45f6c8059febc2df0b1d5fd09737
3206c7c8781f7096bf7d3210190771805f84fcc5
61494 F20101206_AABLCV hardison_l_Page_099.jpg
89a87505d695354d290bd7144e254bf9
a0155b059625f54a1424166186a8e07e9f388516
88289 F20101206_AABLDJ hardison_l_Page_114.jpg
c33ab077f23d337bbc8db01a90a4f982
bdc99bb64fe6b4a7ad573538a7ecadea464eaac7
6276 F20101206_AABMHB hardison_l_Page_016thm.jpg
e4e04fc8e74caf90135f3b2d8ee888d0
a6bc055b9966d14ce52a4b23362a32ef126305cb
29056 F20101206_AABMGM hardison_l_Page_121.QC.jpg
cae807c933000c6ff5517035abc712dc
2a90e5c3f2072d7a27166d57b97de38c2860c3bb
5491 F20101206_AABMFY hardison_l_Page_022thm.jpg
fb8210375733a7e60d8c2a3a9eeb271f
6c2fdaf2983baea404dc07dfc4329e17ecd62f33
49752 F20101206_AABLCW hardison_l_Page_100.jpg
d94c1c3b38306d4f190f3e43c7767032
4b177f5f189cbd1c5b2cf5abd4a8f8d597073c6e
91809 F20101206_AABLDK hardison_l_Page_115.jpg
6cc3e1633c218e0186277ecf96aa7602
4ef4bf0f119c5a1a9b2d56138ccfc88579388ede
23400 F20101206_AABMHC hardison_l_Page_025.QC.jpg
879253053950a9fabe31f5de62246b99
a9d0f479eadb4a0cacb0c74438fd45629085ac41
6676 F20101206_AABMGN hardison_l_Page_064thm.jpg
d93c80a64796d2af1644c18488300083
3da173a3d5b906e5d082f7be104aa03b46ee03b9
F20101206_AABMFZ hardison_l_Page_037thm.jpg
7cf0bac4894c6f9bf09a781be856e98e
1a10d925a9baf05a94c97b6d069ccfa8bb53db60
93021 F20101206_AABLCX hardison_l_Page_101.jpg
0f11198a2a4e43391fcfb2232596351b
f83c17169c591d2cf3ed7ece770376d521001165
65587 F20101206_AABLEA hardison_l_Page_131.jpg
3156bc6a2642bb7642509a74308b2670
405579d19ca0edd6f78d0ba48ed76b551ddf1d01
70283 F20101206_AABLDL hardison_l_Page_116.jpg
95fb2f0eab822040c7afd3977e097402
1d6cf160782b578ed2c809f6741ae142475dcad0
24102 F20101206_AABMHD hardison_l_Page_055.QC.jpg
9506be89f23e352c2f74fa46d55c5140
fc4e9c99d21fd071f33dd36e4d236af96ae92959
6763 F20101206_AABMGO hardison_l_Page_121thm.jpg
96de6af88eea2afadf3ffa76b6edb152
d6a1bb7939818c82b7a736705d3360df7f2e15cb
F20101206_AABLCY hardison_l_Page_102.jpg
c6e11d51a66b0b99c9de493c122a2ed7
72845b81de72952cf30513ac171d1e567060b3c6
20999 F20101206_AABLEB hardison_l_Page_132.jpg
bb820d20a5d4fe8373df1a2d6989f765
340d051c9b4dcf57792e7b04642517e1d7e789fb
80368 F20101206_AABLDM hardison_l_Page_117.jpg
353339baa3e6f0291cda3641581ba2a1
328c0e7dcaceba8d1f9d5fab8ed2cbded2bf1b3d
20420 F20101206_AABMHE hardison_l_Page_099.QC.jpg
22795587d2c0107f6b37f18d083add75
9990e10bb3483cd7d4d537286e9741e753fc2818
27026 F20101206_AABMGP hardison_l_Page_076.QC.jpg
1dc9173e2731c2b03df0c31a9b7963d5
acb2fb618dd098c8cb8985c2b360e583fc9bd560
78802 F20101206_AABLCZ hardison_l_Page_103.jpg
6ced17cda9b247dd06da858c0f76def5
30c999d79a6bb571eef40230f87840bb63f5b826
88172 F20101206_AABLEC hardison_l_Page_133.jpg
556db01b362f1a399b461619c9fd3596
40385902aeb2f2e3816bec7306fa628c28b720ee
71680 F20101206_AABLDN hardison_l_Page_118.jpg
ce7470cab3e02ee3dc566a9315ac17c8
af853265f3d672c6146da009fd6c980cc2d50977
26484 F20101206_AABMHF hardison_l_Page_117.QC.jpg
cb2eebc5d96e377ae6fc3876408a5e4c
71b0416a9d2cc38efb18b657c9f61788fd70601a
6787 F20101206_AABMGQ hardison_l_Page_146thm.jpg
6a14820747f985bcf68a0263aee7b2aa
3517f83e1a803852ca752773c339051060bb449c
101977 F20101206_AABLED hardison_l_Page_134.jpg
67ac2d031d9fd45d77a54054004e2e3d
9f5e88b2af8c69856c17457ed11d8a1136906d0e
69500 F20101206_AABLDO hardison_l_Page_119.jpg
ca21936a2d9d4472ed1132d5060ac9ea
29650c5ea8391f9722f76ffe2a3a8fc007879785
6414 F20101206_AABMHG hardison_l_Page_107thm.jpg
a44563429fab006ad3f64d843e101e08
80d14042ea0c9807127d1e13d5ec54026834cab2
5361 F20101206_AABMGR hardison_l_Page_051thm.jpg
176cbe21eae6c9e4ddfe23399dd70b61
6d4c42541ff6f01cb0b66b5f3542d9ecba16ae28
108730 F20101206_AABLEE hardison_l_Page_135.jpg
c1a01277c364e402b84f5c37eeab28d3
4551f7be58f3d420cc09b294c4ab2c055d561f40
75926 F20101206_AABLDP hardison_l_Page_120.jpg
1183b40621d8b4914c1df645a34abaaa
58803110afc429421d87b7e951dcf12dd896b6bf
26496 F20101206_AABMGS hardison_l_Page_020.QC.jpg
3ed26651eceffaaf8095742b1230ad13
8c6467f1150c47737fae65089621d3214ebf3a29
90969 F20101206_AABLDQ hardison_l_Page_121.jpg
3147d06581c9a7ad24293e3e3c6fa128
2302b346b12b1301b028efbef368221186aaa2a0
27723 F20101206_AABMHH hardison_l_Page_127.QC.jpg
bf0a5c1958c85b97fe3b298d680bacee
d75d4c0f43e2a4a2a419b9bf5d5d30b1a4ee16c4
26535 F20101206_AABMGT hardison_l_Page_095.QC.jpg
198134d83511c23670413771bfb47b17
f0c194933819c1c5b551870eaf2be3be2954bc8d
113991 F20101206_AABLEF hardison_l_Page_136.jpg
f8dc9d8e22f96016ff257c7a55315b36
f2d1de62979b923378ca6860e5b9fe68f3c230cf
92436 F20101206_AABLDR hardison_l_Page_122.jpg
8445083d4b1410fc68748b8bd5fe4185
007ab5cff886152a3014e57c7170cfb77953034a
28276 F20101206_AABMHI hardison_l_Page_054.QC.jpg
b9f44ecc74ae442bc47185373b0d6cf0
69e748289c9aee9006cce0a386e77a6d043068b0
22279 F20101206_AABMGU hardison_l_Page_017.QC.jpg
1c6a62c80e50e0f217a49f4d0afd9aef
2c422b78d5c1d6cb633efb12752823517905be67
100949 F20101206_AABLEG hardison_l_Page_137.jpg
6bf8a62de6ffe4e1d9465466270cbc9a
ae152a131cac5aad9949a56801f012e5d5634a22
F20101206_AABLDS hardison_l_Page_123.jpg
e3f719a0b8c852449164958e727975fd
878e23910364876fbc09ba98b2a4c83d1489528f
5774 F20101206_AABMHJ hardison_l_Page_013thm.jpg
7733b7863f20e557c56e45460e2d1ba1
c28a351005bcf96348212f46e647be3bf9081507
28864 F20101206_AABMGV hardison_l_Page_135.QC.jpg
04fb00470fb84fb048ed3a18d12cd874
b163f80156569e1ae6edf8a30bee8c1be0b6bcb0
103795 F20101206_AABLEH hardison_l_Page_138.jpg
afd9541f0ce3ce73f2b0e706987d1084
1e4103c022a7adb409150eb97229673c7b94eef3
18794 F20101206_AABKZB hardison_l_Page_001.jpg
3dbe1d41046e61b445f7d7073082421c
64c64cce8d5bbc3b6de75196f12b1c396f321716
74269 F20101206_AABLDT hardison_l_Page_124.jpg
95f81fd1170b9b3e49b810214326a563
057bf03347859674aacee50f8b09d938d6922821
18365 F20101206_AABMHK hardison_l_Page_106.QC.jpg
f9cb8660649cd20c4a69a7dcef89a87e
103c8435efe9c8e0d33e30bb634ae55cbee7cf9e
7279 F20101206_AABMGW hardison_l_Page_145thm.jpg
d157d9840995239e48f1c1830b1b36af
bc87a4fd8e72f6c352f40c4fa8e27fdeb40823d4
100226 F20101206_AABLEI hardison_l_Page_139.jpg
75a8501e2a931b3d5bc18bde9514d5c1
4b96d91c19a1dd58a446800ab27f2958e3b7f9ea
3878 F20101206_AABKZC hardison_l_Page_002.jpg
9c98c376b0094dd2efd70c452275a09a
4f127710c199ade36676ce58cabd38d3abd6b4af
90964 F20101206_AABLDU hardison_l_Page_125.jpg
c35d61f826ded72ca3324ca3ee9d0b1e
2c4089d3cd2cfb1b6e07104a672376ada0603088
5967 F20101206_AABMIA hardison_l_Page_017thm.jpg
01e408b248ff261f93d944d48f792679
10a014173af4f87ee815395cfd5900ec79f93d69
5351 F20101206_AABMHL hardison_l_Page_124thm.jpg
8d8fc6418557a8d091f9e180e1b8a220
3bb9f631d05f06e4a7f8cb41110772e88f6feefc
27713 F20101206_AABMGX hardison_l_Page_080.QC.jpg
261ca3a46290bea502572577870109b4
5aa0f1908c492380b29016caebf84be0a7807548
115476 F20101206_AABLEJ hardison_l_Page_140.jpg
a1a0cb4da83bd28fd50843ae1cacf191
065e88c47396a2e16629299e65de23590dfc756a
3033 F20101206_AABKZD hardison_l_Page_003.jpg
f97034a062fed1f0d5af258d6cdb0684
5b1b145c7d426ee81d8fade2b6fad8c388224dbb
87934 F20101206_AABLDV hardison_l_Page_126.jpg
0657a027517c1c6c33c4d291a3efea49
a597e536781ca63f80d2dfa3d117c70ee3a89be6
6711 F20101206_AABMIB hardison_l_Page_032thm.jpg
1030af3971c6872d309766f11b13a22a
748af5bae8c6dfa710478fd606260b6376f6f8fd
6456 F20101206_AABMHM hardison_l_Page_049thm.jpg
a923d8a64e3eeed5530232fa8b6340a7
cd7e6fb095b67346df6f4f04166bb7aeb00281c8
24180 F20101206_AABMGY hardison_l_Page_108.QC.jpg
f80d598e7e6fe9276bc3aa378500297e
7a1f1087a1aa97f8d51b484e974c590cd91ea7d4
F20101206_AABKYQ hardison_l_Page_147.tif
eb07b19b3e86a46f2d083fc085376e0c
b07ff21c2ac7fe55ffdaaa06f314a7c2f2a844f2
102079 F20101206_AABLEK hardison_l_Page_141.jpg
4267413c8b0b580ad608241a9a22ae1b
0d1bfb09fe479720fec6d56913a9438b279def12
86190 F20101206_AABKZE hardison_l_Page_004.jpg
2be377896e1754ac0cdf3b4ad394dbd6
02a7362abef861984be687c7c49dd381bf1cfcbe
89088 F20101206_AABLDW hardison_l_Page_127.jpg
8a3cb01dd8b95bd8c20a2c1d4fe7cf60
2db71d04de89ea9cfbdff76fa83f9ca05a67ef54
28628 F20101206_AABMIC hardison_l_Page_079.QC.jpg
0f4fde255ad2e42cfa082ef0f04ba2b6
33c48f3918980571c03c1ed2214aba76967cc17e
22133 F20101206_AABMHN hardison_l_Page_052.QC.jpg
cb5c94d8c3604c7548dd90a78ef7e375
ddc2326b8c831c29807395effa77a9eae8e586ce
27274 F20101206_AABMGZ hardison_l_Page_107.QC.jpg
0b5c551b05bec8c6604fce0aae5aaf49
2cd47682d165d19ad7be3de9bd08fe5c73e2f887
F20101206_AABKYR hardison_l_Page_045.tif
56191f5d67b90b3e013c94cb6bb499e1
c01bf5da718785eeb5179f50055f024a8440d1ab
1051982 F20101206_AABLFA hardison_l_Page_008.jp2
46fe88110c074b9b385e7c647618d287
2f47ea116016a3c35d2f50e73091aeb720d59c1e
106442 F20101206_AABLEL hardison_l_Page_142.jpg
9f4384abb58c7d2ff2a703b98104cf43
1bd68d4bc39b94ee12cd3c71df3566608410808c
F20101206_AABKZF hardison_l_Page_005.jpg
5f7a1fa8821cb1b2ae91a993447e3f35
b1f0e6ef2b7f6823937b47205e78baeec91f2488
92848 F20101206_AABLDX hardison_l_Page_128.jpg
b965dc7f615dfb6a53e227828e95ca0c
94a366a606473bda5d20fddce9e9fa3248af0c98
6846 F20101206_AABMID hardison_l_Page_087thm.jpg
0e9a6d977bfb385c2c5e9dad3e707511
157f8571aec1e0eafa91762c6fca82a1e14be621
6812 F20101206_AABMHO hardison_l_Page_138thm.jpg
f440b08e1dea376d90806ec12b509c1b
17d451fbdd026a9168c980d52bc89b2a21968de3
1051980 F20101206_AABKYS hardison_l_Page_011.jp2
c978cda5775db3261766cf2ceba7a552
16a19b7060def7814dfe3f9d8d4f14e82c9f35c2
334375 F20101206_AABLFB hardison_l_Page_009.jp2
c59171de0f781b32a920e4f9c3d14262
b3fb4b4af73a161eeba97b73c498ffe36ab209f1
108214 F20101206_AABLEM hardison_l_Page_143.jpg
d10da254f0cdf8acb82af6ffae6a7464
edf6d9c733e1e55113cc3d38bb2a3a5ed0959b22
37464 F20101206_AABKZG hardison_l_Page_006.jpg
0b26217b53475fa465743edf2ff43d36
87d7cc586aaf1d24ccd5eedffb5032b53d188a47
91976 F20101206_AABLDY hardison_l_Page_129.jpg
ee875bce4b7f24b05281030a8b54a179
b524019155d2ce03853e3dcbcc2823ace64db9ef
7268 F20101206_AABMIE hardison_l_Page_071thm.jpg
0b20a345a5250b9dcf2e749d94e3cb54
fbc6b960247ba2c374a58d3e717c8b12e083cc68
6875 F20101206_AABMHP hardison_l_Page_029thm.jpg
a58732776a7749291a636f42087284af
a66e8db9f67b30d22bdb01fe3c3c59bcbbdccadf
120812 F20101206_AABKYT hardison_l_Page_101.jp2
8ae0cf12f22d03adce5a1593caa333c6
2e38b22b6c1cd682f476612be1e116a5f40c2a30
1051984 F20101206_AABLFC hardison_l_Page_010.jp2
e65a4778555b1bb50a04f0b3267f6a18
e9b2d6bab202b58cf866d8fccd483f33fb9d8b89
108059 F20101206_AABLEN hardison_l_Page_144.jpg
66987b44822f847659fcf6cab5870ad7
9d0f020db445aee0235cc180256cbc4100b10b92
95142 F20101206_AABKZH hardison_l_Page_007.jpg
3f161839cfdd8cad3767b72659ec2590
51a830755657b986d378d73a7382791cd18cae6b
90213 F20101206_AABLDZ hardison_l_Page_130.jpg
85d1d0c33446181dc5626a171dd86499
bd6ccb3c6c51d3900bd707021e390bb96a949923
29467 F20101206_AABMIF hardison_l_Page_081.QC.jpg
6f2a1997b5bff172e246d4bdadac5964
1e5326ebe2b41c44d8f9bb2101b135c551039d68
26793 F20101206_AABMHQ hardison_l_Page_093.QC.jpg
cf5b43fbb3552050a52bc7b987772e92
7dcba4f0ec54593f6636476ede07130bdad3bd68
1001414 F20101206_AABKYU hardison_l_Page_118.jp2
a3b63e93a5ac3f7674167c9d057689c8
67f73f479eb2ef74e649bf1bf61aba371896b200
1051983 F20101206_AABLFD hardison_l_Page_012.jp2
8edacf9071066ba783c4c682a3307e4d
1e0d1384ed335111e034d9e939432f8920093af0
117694 F20101206_AABLEO hardison_l_Page_145.jpg
d616583a07581df0b849308b9880f8b1
bcd53161a79502c01049f8579c340fbb655df4eb
78565 F20101206_AABKZI hardison_l_Page_008.jpg
5a13dc3c15809db4c19ff6f09da3b981
d70c5535786a1320eb2ca76a3c3270e4640d660b
27353 F20101206_AABMIG hardison_l_Page_018.QC.jpg
0281851af903288b4b7039056c3ef3af
c2a8241168eebfaabdf873c10c709bc639b71bbe
F20101206_AABMHR hardison_l_Page_045thm.jpg
ebd9e2d78a10e4017be7dc35654bcf1c
766050e5fe674555d9aa09837a86a26e49bda1dc
95289 F20101206_AABKYV hardison_l_Page_105.jpg
1c7ae35d010577de9ad0a9e8a483626f
03e3e7aec664aa2b4e303fdff8b0c859ef3aa5c7
101634 F20101206_AABLFE hardison_l_Page_013.jp2
f2f575fd7285ab4e076710dc1d533a68
ae501dec94918a4dcb04941861f4e851f697ce81
103061 F20101206_AABLEP hardison_l_Page_146.jpg
146c1202af3d017246c7dc21b7071d65
02121756f2cf6a656d577438a1592f55d744e173
17456 F20101206_AABKZJ hardison_l_Page_009.jpg
e9cc8c34d574153835a09aef9ec34076
198a0c18c0a0bdfa404eec74e1ffcf59289a3ddf
28420 F20101206_AABMIH hardison_l_Page_045.QC.jpg
fa9d7a6731c8d59a246a17443c0becd1
5f2166001789477556a887de03e9b471c2362fda
25154 F20101206_AABMHS hardison_l_Page_011.QC.jpg
bb652c5d61b00556114e8a2bd15b749d
d44ddb61d7d5711b8d0d3af30912a301a165d523
49883 F20101206_AABKYW hardison_l_Page_049.pro
ac7d8c8a77a1482562bf6f31d0b03ed4
88430993bd151f95eaf17ba81a5088cc9ef09461
113065 F20101206_AABLFF hardison_l_Page_014.jp2
adfd86e1f28fe039be6bb5e64e96ce82
efe0c603e6d3ab58e29fdac65a27cc7b594bcf2a
93918 F20101206_AABLEQ hardison_l_Page_147.jpg
3523559899ebdb03688836d2f52523ca
2f00aacf063ff8e7ab901fc3f4b095588dd7261a
92342 F20101206_AABKZK hardison_l_Page_010.jpg
76ad081a5d28aef77c3e64d40bd76b7d
632c57de4e6605be6a6e2334c9b6afe55657a3b5
28716 F20101206_AABMHT hardison_l_Page_142.QC.jpg
dd653a95b2db32607f865e9834f34b1b
b2ec9a6b51d686fae6462122785eb74d5fa69741
22905 F20101206_AABKYX hardison_l_Page_001.jp2
ff7f637cae17b4b811cf5e0bd3231a8d
cf67a35f983c2b1c8a4973ec0cde911026c6bd8f
110294 F20101206_AABLER hardison_l_Page_148.jpg
fdff6a8656d45683e90e3e2f59e50ce1
bbc2048c7b5401fc4704f70b1aca8b090b103eae
96203 F20101206_AABKZL hardison_l_Page_011.jpg
b5b506c02cbdeb81f276ede0c9e2f44b
6639eccc2e39dcb6566cdffd842127dfe4ef0ead
5870 F20101206_AABMII hardison_l_Page_089thm.jpg
56a07f9b1951f8c84d04cd58b3eb2cca
47e3857a8a3789802fbb16b538162c7f7bb1e4b3
5761 F20101206_AABMHU hardison_l_Page_010thm.jpg
c54bf81840f4e1b8f33fcc7008c382cd
59f07faf0369ddbb106c1b43e63ab94a1018c6bb
174577 F20101206_AABKYY UFE0021382_00001.mets FULL
05d1e23ab76ed1b7311b333172677119
791f53f7873f365bfd4cf2659ffe13870a7f0bd8
108921 F20101206_AABLFG hardison_l_Page_015.jp2
bedd5f14edfdc88050b0ee4c651bd959
a5e9929807c08db9c75a6bb5648bdb7a715c2c99
62206 F20101206_AABLES hardison_l_Page_149.jpg
44be52304f11951fd78db76dec0a4f7a
c56a54acd87ca01fb923de6c5e82b44a27bc5055
74549 F20101206_AABKZM hardison_l_Page_012.jpg
e3390fa8dbb79c6394b34f63ab9ee7fa
bb6476a48924e5921e9d58a9850beec2b4dccb9b
6642 F20101206_AABMIJ hardison_l_Page_082thm.jpg
582888da41fbeed0817ecf8ed28c69d7
32cbaebf7910bef1e1beda7c88656b4f6313ccbc
6739 F20101206_AABMHV hardison_l_Page_079thm.jpg
da0dd74ef1e7d31e0fc44a7b2afaf2f5
9a65bd44f28be34de0b559e595d96e2e8303b868
114316 F20101206_AABLFH hardison_l_Page_016.jp2
65a3a0920a5d80273f5a36066f4dbfc9
0adddded3113780e6f0c83830edd5a841eceb9d1
57386 F20101206_AABLET hardison_l_Page_150.jpg
48129a25a86831e56b33e046a0cfd522
94d8d3f8a028edff8b69114768975ae19cf3443d
78690 F20101206_AABKZN hardison_l_Page_013.jpg
775f27fe73465f0f0ee465e4dfa2819a
f4191b9c8f35c25116a05ab6ca1750ebe714083d
21838 F20101206_AABMIK hardison_l_Page_102.QC.jpg
5b77df43ba24db66dcabf57a48c0617f
0619fd6cd3cd0b667c12b3460813875144842e09
5504 F20101206_AABMHW hardison_l_Page_025thm.jpg
66419c523af3a527d4ebbb6a979ec601
1d9cfc8ad9a89ecf86e74b50ac6cdc35abeefc7a
861981 F20101206_AABLFI hardison_l_Page_017.jp2
4d63c63373ebc9e560c036c49b4a1768
00e15ab2d53c89ea1098270eb0d90e9c306ea58f
5554 F20101206_AABLEU hardison_l_Page_002.jp2
7b65e0a8a73ff61f33608aea8a2b8610
1828414eeddf1a06bd6682f719ac7e3fd917b715
85621 F20101206_AABKZO hardison_l_Page_014.jpg
182207216100f4e19953a1192894cf56
5be721b62bb5235bc761fde79fa614d3e5286f68
27470 F20101206_AABMJA hardison_l_Page_016.QC.jpg
58bc5e09c9c14e8ab870f90b7fab7d58
fffcb9d588de16d147c8df282175ae29c5a0b3bb
29342 F20101206_AABMIL hardison_l_Page_040.QC.jpg
7a339536dbbad9b05b957b1ccc68678e
c4189928a08d1046802290fd23dd7ed49f0d9b87
15838 F20101206_AABMHX hardison_l_Page_100.QC.jpg
2de026eb63ee50094c943542e715bade
ed0916ddc81c8a5f3d49b3c5469ae6ed2ede8275
112149 F20101206_AABLFJ hardison_l_Page_018.jp2
cd3e5c9ea30a446bb5d0f0d8d7e11350
cbcdd6c6121e028530437fc5d671a4a7da12284d
4335 F20101206_AABLEV hardison_l_Page_003.jp2
3f408f1850de5f36976d113336eaf238
ecd21efc2900394f7a02bb04c941cb14a351c2f0
82640 F20101206_AABKZP hardison_l_Page_015.jpg
598e7236eb6e108a0f14d13ae2de8598
97812dc2e471f5b3bd0fbc91b09ef2de51538d1a
6808 F20101206_AABMJB hardison_l_Page_101thm.jpg
5b2f4e19fcabf1c8882d0bede71d352a
bf28db8ccf904fab8fa274ac1ad2461b922acb50
29770 F20101206_AABMIM hardison_l_Page_136.QC.jpg
737d770f4134275e481c69bbbba184ad
c37b394731e9999c83ceb65529976248ca5c225e
6683 F20101206_AABMHY hardison_l_Page_075thm.jpg
ac44c49623d321c38875fc0e39dd46cf
7a08d391da0af6b7c7b2704cd1af0fb4fd9be717
107681 F20101206_AABLFK hardison_l_Page_019.jp2
08b648891f4bedce2d0804c6351e7ebe
b31e6781f5a70baff6d1dae5ef4fb12a62028337
109526 F20101206_AABLEW hardison_l_Page_004.jp2
2ea6908c74e66d2c832cd39747614089
3284ac17c640b7b04b8815d6fb3da6a0bff96e40
87233 F20101206_AABKZQ hardison_l_Page_016.jpg
4cde64990090ef7e587d7a794c260d0e
8ac0c32457dbbf9b644c0efbc4a8d8fdb0b09d0f
5671 F20101206_AABMJC hardison_l_Page_096thm.jpg
937c4e5847cfd66c653811ad2f67dd51
fe4c83f699b224e918d07fe1dc4619a76099b545
6127 F20101206_AABMIN hardison_l_Page_058thm.jpg
11b2bd21ba95904c47e55cdb4f86757b
3828b0d23970d6a04b022fedea87134730ce68c0
15742 F20101206_AABMHZ hardison_l_Page_083.QC.jpg
fb8852f0a018eaca6dc636ea5692aa94
af005457e55f22a407c7834e191bcc8a219863e6
110827 F20101206_AABLFL hardison_l_Page_020.jp2
cf4210e5eab0200f4139e8f5ce547f96
4be5b61e7d1b4f4fb51f6121b40639172be3e5f1
113077 F20101206_AABLEX hardison_l_Page_005.jp2
f38feb57646a90e7e9f216c2d6bd75ec
51af8aa869d015b1b16719c928f9b16013cc768c
70952 F20101206_AABKZR hardison_l_Page_017.jpg
dc6f8917df481e395e655ea00fd60e14
32d9026666009500e36fee0d0939db92040f3ca9
1051892 F20101206_AABLGA hardison_l_Page_035.jp2
e840b6a4ebb8baf8f6de0ced6b0a0f78
e3a9301c91f5e8890233c2b459d003ead3ffe58b
6568 F20101206_AABMJD hardison_l_Page_028thm.jpg
4d42446c58b8d202fce6d31d42be1f99
029d079833951bb12a5106e6363340a61e3d27c2
20895 F20101206_AABMIO hardison_l_Page_026.QC.jpg
bd205f8f7130f6b1ed03194049e803f8
a870442254da983bf693d416d6f4c1a6560888d9
925972 F20101206_AABLFM hardison_l_Page_021.jp2
fe09484332396eda0710eac9368dec52
0241fe363a3e547b937a95d93b8a7c64c0824336
47093 F20101206_AABLEY hardison_l_Page_006.jp2
009170f9ed822b996ad44661fcfea941
9351b44c6fdf1adaac0fc3e1cd64f9a99d1d5779
87234 F20101206_AABKZS hardison_l_Page_018.jpg
67895fb84dfb88c6be125ea24a01156a
53965edcb0b57dfac3fceac312e244f0d24c704e
121452 F20101206_AABLGB hardison_l_Page_036.jp2
aa3fd37f03565a1b78454a69da9aedcd
13cce1805a8cec0b98fec82a20f9b8dc1c59fc03
6898 F20101206_AABMJE hardison_l_Page_076thm.jpg
2553a9c29740fff9ffe82a1d1d1a3317
ed990bb6f77718e04e701239ac123d879f2ae1b0
28975 F20101206_AABMIP hardison_l_Page_087.QC.jpg
cbc40fe3ac9fe7420e260f52aa110eac
38614dc4244d7c463a05fdfcc22355d814117fd3
92486 F20101206_AABLFN hardison_l_Page_022.jp2
19736622e952b6bfddac10d1c33662a8
b07e865b73cdc2722ccae5dd144dd0ab4f9fc589
1051979 F20101206_AABLEZ hardison_l_Page_007.jp2
b3689b98487ad7405fd4eb870cb3de65
a55fd3a8c326be44a27cc3842a1db926e3241ac3
81740 F20101206_AABKZT hardison_l_Page_019.jpg
18c60fd433386913b1012f40f457a104
60e79087a123439bdff4927bd12e8f67ef32ea1f
117808 F20101206_AABLGC hardison_l_Page_037.jp2
77b2806e179519c1987b600bad509283
33801a79d7effff91bcdae76957cde3757018113
24860 F20101206_AABMJF hardison_l_Page_039.QC.jpg
e0303bd3b8cb1727429cbf0c7a8c80d5
b462912304897a67206380ccf054e7ed36eb1a22
2502 F20101206_AABMIQ hardison_l_Page_132thm.jpg
9891fda1b60bd33bc92ca0892f005cb1
87f1d0d5cdb9726023a9f369f5f231dcb86530fb
102353 F20101206_AABLFO hardison_l_Page_023.jp2
bdce5ecab497b4ef70c7e571cf198c91
85be230bc0903fbe006df57880c2ac41ce87c978
86009 F20101206_AABKZU hardison_l_Page_020.jpg
fc4090bbb5a7fa3fea94a4586fc322e0
e2c6dcfec96b41505c6f2bb62afd02dd2c87244c
953031 F20101206_AABLGD hardison_l_Page_038.jp2
70eaf23d466e9c8b19b2206bb9adff66
b6f7b7b48081e2e5b8427deb7fb6d58212944f42
6834 F20101206_AABMJG hardison_l_Page_144thm.jpg
fe90fa2be7ea63a38a5f1d81141621b1
66d9f141fb55ab80f3a9204f68721f3a90b98729
21500 F20101206_AABMIR hardison_l_Page_084.QC.jpg
d9d5bcd491c06d4e41f6487825d44103
e79f8d76469aefbab4d81fd7760d6b58d6dd2d23
84356 F20101206_AABLFP hardison_l_Page_024.jp2
fca84a948e3d905e128a3201a0f70a01
894bb696c7a4119defaed27aeae34ca8b4182687
69061 F20101206_AABKZV hardison_l_Page_021.jpg
2e1391d90201a5624af70ab8d336f736
19185ddd2d663f76831d93addec9073c536dd114
107490 F20101206_AABLGE hardison_l_Page_039.jp2
e7f52a012b9e0d093e59fe829d8c83fa
4d44b2cac6b215562fa631f190c87623986d5236
5287 F20101206_AABMJH hardison_l_Page_059thm.jpg
de7160c533a7520c4a2f55c683d3903e
bdf90e3ee6cb8a1838a60e3037887cc5d8822dc0
23462 F20101206_AABMIS hardison_l_Page_013.QC.jpg
34f6768187a68b16aaec84684df743e6
5369c2361405965176370b6e225601ae4f29361d
92760 F20101206_AABLFQ hardison_l_Page_025.jp2
49b894e343f5215946884ad881db0ce5
14b26c61f2531ee759eb1f3dc46be8e2b8372a95
69359 F20101206_AABKZW hardison_l_Page_022.jpg
cf41f9c66d8b47c287287e3fc5e8966d
527d33f26a78d82ffa22dae1ab02916f209e6856
1051934 F20101206_AABLGF hardison_l_Page_040.jp2
f22275e90479eb8e19304cd3fe3cd913
c79085e9b373927bddb677e34a34419befecdd08
6944 F20101206_AABMJI hardison_l_Page_070thm.jpg
ebb99384a4871c41e305fe399d8de2c1
95c3a780a58f076b141ec51f8a0bb36b148d027b
6639 F20101206_AABMIT hardison_l_Page_092thm.jpg
47e51e8f4b49e55b18ac5732c8302e41
15a457ce88d12cd68bd9b2e2c53dec68a6376484
792713 F20101206_AABLFR hardison_l_Page_026.jp2
a98ea47ad14048931fcc36774532fe6f
112f146f30ae032a25c9e161e6d2521c77a2c369
78932 F20101206_AABKZX hardison_l_Page_023.jpg
5cfb84abb7069da1dc53edb01c7a5929
f0cf4ab22b197b4f20dda8c32ba4eca2deb06fd5
113600 F20101206_AABLGG hardison_l_Page_041.jp2
41d392a8b3783a34c5b9c42b8dd40df5
bed5974d0f3fa939a6278e72fee02db3d4f03afc
26165 F20101206_AABMIU hardison_l_Page_057.QC.jpg
623f1a59abbdbe34d7ef8eb6481b8bb3
39d9f353683bd69fc28ce3cf1f1a26c639b3b68a
109494 F20101206_AABLFS hardison_l_Page_027.jp2
66e2d009b7cbf43a20971855cd683d47
f2a3e2622dc6da71992a677757f4652a40b28150
64735 F20101206_AABKZY hardison_l_Page_024.jpg
27144ee565f9fb53d13f7b8a318ec0d7
1404ef22c10cbbc7217107ce2fe1cd28870abb87
6703 F20101206_AABMJJ hardison_l_Page_091thm.jpg
4c21fe8d56944f7b8641638b4be2ee27
072e0019bece71d066ade1d729295ae927fcbc68
26889 F20101206_AABMIV hardison_l_Page_090.QC.jpg
5eb05ab52bb25c7518dbd6ed5785fa2f
4b602b9511ab0318a184fbb800ecf875ca86c685
944750 F20101206_AABLFT hardison_l_Page_028.jp2
690b3481adc398d591f55b95569894b6
bb004ce58b534211632e61d6a90e5e7c93de4e93
71692 F20101206_AABKZZ hardison_l_Page_025.jpg
ce5bf513911cacba18282d685826d914
3b10df8e4eaefc225bd52ebea35178066ed731d3
1051944 F20101206_AABLGH hardison_l_Page_042.jp2
e1c43e6e33d8931eeb0be5fc9e85eb3b
ee448d80b27474890a468e0b7804921b241b89a3
226250 F20101206_AABMJK UFE0021382_00001.xml
edd966023d5886a59475a6ec39a6801b
6c2f651fdb6f35c4e1a78df699ee82bcfce5d440
5296 F20101206_AABMIW hardison_l_Page_131thm.jpg
cb8f79d24244a2070330a18c4f6d3180
31689830a648228d2a2e8c58ae89934ea6086cdf
121556 F20101206_AABLFU hardison_l_Page_029.jp2
49ff8d4a3904c0d2701eca73d1e19f3e
9acf7493af5bb0492d5240792caea6320e0eefea
111781 F20101206_AABLGI hardison_l_Page_043.jp2
9062bee44077d2b7056fb73bb7d745ad
343e9496d6e6bc4fcafb8bf01abc78db3b11d7b9
29067 F20101206_AABMKA hardison_l_Page_029.QC.jpg
54c06cdbc534fba21024096644c52979
1a324c4ee40ab5b3515cb5282e13334afde3190e
1108 F20101206_AABMJL hardison_l_Page_002.QC.jpg
69bb08cf2f7c27061009c8213c4ebc6e
d1d26b597eae7dc68a08e3b116f3683758f2d41a
4328 F20101206_AABMIX hardison_l_Page_050thm.jpg
984702fa277c20a25c4e78ba2e3d9eb5
753073f7b79ac84923a9cb007dbe70d86a930015
F20101206_AABLFV hardison_l_Page_030.jp2
246ec6bff6c556a0f25e680b0e768d5c
26a6ef9b6905f5b3e0c8660792a327279018c695
921633 F20101206_AABLGJ hardison_l_Page_044.jp2
840dc997037b33e5439986a34ae4f446
2bb8842bc18f0eb08fac1492d3c77d0137584483
6372 F20101206_AABMKB hardison_l_Page_030thm.jpg
c2c47a1b46c11f806228e82c02781ac2
c1e0cf7116dfd2b39dc3e6f5332e83b76c83a6a0
26744 F20101206_AABMJM hardison_l_Page_004.QC.jpg
505ba1e1b57d8dad73c4623f816eaa83
b03a301a8614190efe3fce07716d4116a1dd0494
21992 F20101206_AABMIY hardison_l_Page_078.QC.jpg
80f45acfa1ee962fff4af60c88030e7f
54326b2eabdf178406abed5125cfbefe25b7496b
104765 F20101206_AABLFW hardison_l_Page_031.jp2
342885b307bb51521f40a69ef8de852c
0c316da814072e282c70987177867cf6153ecd49
117703 F20101206_AABLGK hardison_l_Page_045.jp2
e144b449841f8b55959e2cb1b9c5fe02
3a72bbad0dc6a6723bd806374b314c317edefa1b
5904 F20101206_AABMKC hardison_l_Page_031thm.jpg
7cfb8d6037814a0db158044890813e45
917fec008f3252a34ac7f0e9d49b5778567e7f98
6744 F20101206_AABMJN hardison_l_Page_005thm.jpg
0ceba906f040ac12463288c2ae0320b2
02d7feaa3201351c748a7be90ec0cc9f08fab100
28971 F20101206_AABMIZ hardison_l_Page_122.QC.jpg
8e9bb4c3006e62f55833f860917d2d8d
e1614c5cc5ce5ee41730c1eed00d616e413e63e3
114293 F20101206_AABLFX hardison_l_Page_032.jp2
93835d8e9b335e18e59f64a5166adc36
c04d600279d73f81879a115d1faa270683e66a64
1051971 F20101206_AABLHA hardison_l_Page_061.jp2
06d5d9f2388a7c7c33731b1db24bc408
3db6a0b65f4b916784ef669884730332117ff07a
1051917 F20101206_AABLGL hardison_l_Page_046.jp2
418f9f4a28b4e83ebf74c3558c188421
d17ee70808876c12eafe0a13f0a47f49b922b894
25314 F20101206_AABMKD hardison_l_Page_031.QC.jpg
b0862183f23c6c1585d4bfb085447d40
c5522a02f4633ff4fc7ddbc1feaf25c1038af447
3077 F20101206_AABMJO hardison_l_Page_006thm.jpg
5be62a27cb408c980508319c17b07551
a286c838ead5db5d5f150d7fba77eaa337c74d67
716086 F20101206_AABLFY hardison_l_Page_033.jp2
66b880b00b559c5a87e302c9b956867b
aeec1a451734316c7197a40235f5a9ce04b2d75f
115889 F20101206_AABLHB hardison_l_Page_062.jp2
c58916da4d077ef3b73bb317c839b7b8
f775b1ccce047d42a1fe1bdb97d6fa1d346f509a
95500 F20101206_AABLGM hardison_l_Page_047.jp2
994fe7c0812c3eba739a8860e6f7e4fa
b130e0417763beece1d55b813b629baf1e25daa9
6718 F20101206_AABMKE hardison_l_Page_035thm.jpg
9ca9650d09519e328b2a1340a3e18c56
029df4b223e3ffd45c567e82002c43ad5702383a
11745 F20101206_AABMJP hardison_l_Page_006.QC.jpg
13db290f459c51a37e15b5c8b8f06653
f024aba5bb1fdf26066befdbd776ef3327e672bb
116639 F20101206_AABLFZ hardison_l_Page_034.jp2
c0641502ff8ca60e3777db2ba988bd13
12457d40b570e7866ced151bd3748b6b4e7ffb66
114969 F20101206_AABLHC hardison_l_Page_063.jp2
5a05133e001a17d79c9cf76e0959fe18
8a3599e885e6b6592d2e0b6aaeb3227b22796a3b
1051963 F20101206_AABLGN hardison_l_Page_048.jp2
32711a08e7adf6ee34688390f2607edb
8eb47be6b626b69528bd88981d07fa1b72877cb1
23001 F20101206_AABMKF hardison_l_Page_038.QC.jpg
e694de5ab9649d3a3b2366b1f12ff810
4ca3a3f838e681b7f154b98c3469094223c0d822
4209 F20101206_AABMJQ hardison_l_Page_008thm.jpg
04079d2db32965056c5f55916c83aa85
dbd3d72c3fc3dddbc020d3618cf37e702b2b234e
1051933 F20101206_AABLHD hardison_l_Page_064.jp2
2663e3a0317355e4eec916eb9cac536c
98315a8626acfa73342f8ef2b73515046d18e4fc
109168 F20101206_AABLGO hardison_l_Page_049.jp2
698de532639947971186b98535541a68
b511f4592253fd8aebc4369ab51b8814868e8ac3
6212 F20101206_AABMKG hardison_l_Page_039thm.jpg
886e9d486dab14317f6222d726f14aa5
0e2d92ec8eac61055302a772bf7a1c71e34cc086
25307 F20101206_AABMJR hardison_l_Page_010.QC.jpg
9c6efed38aac347a370d9528a1d34cef
9431ad4755c65b8ebe5140c8338eb83ee28e3163
120210 F20101206_AABLHE hardison_l_Page_065.jp2
129a4609c5b626f448a8ca2580dcb8dc
cdac8bc60e4b8f89a493a8391f9319f1eb4d1502
39125 F20101206_AABLGP hardison_l_Page_050.jp2
0f60c1d26d442dba4c69d861107dee89
545790c3135341c2f252d29782d2badb2d89faa8
6939 F20101206_AABMKH hardison_l_Page_040thm.jpg
ebb49c463c152c341ba2facc2bf1ab03
8e6df4e3d0c475660ae789e28bf9773e37a07c9f
F20101206_AABMJS hardison_l_Page_014thm.jpg
547c265f6c15452861f38d5f2354673d
5abac707d7c7457fdd05e5139df57d4a4cf9e141
118203 F20101206_AABLHF hardison_l_Page_066.jp2
df3918cd4bd1365c9d8acdac339701d0
afcd05effc15ecbb994a43101a22fa977b02db6e
779895 F20101206_AABLGQ hardison_l_Page_051.jp2
5c580640460c75184a75b35b94036d9f
75653df77218a93647bcbd78f2165b94c22a0fa1
27793 F20101206_AABMKI hardison_l_Page_042.QC.jpg
d33e0903ae56c0d98c9eab2acad90c3f
c8a28124ecd238018d355b383fadbb9440da6c94
6036 F20101206_AABMJT hardison_l_Page_019thm.jpg
c9c0f7e69cd4c455d27ac6d8f44c5fdb
bcdebccc20b5b35198b5d45bf2b04a7cd7b752c8
115039 F20101206_AABLHG hardison_l_Page_067.jp2
8d525bff3a08850b72953687bbd79a88
c7c3984936b62ec59ba9be7799d40ce2f37b70e8
918001 F20101206_AABLGR hardison_l_Page_052.jp2
6e5cd4550d35c6304a6f84cf0e59cf07
5447342bcc3a917f8c8511bdba48fe2adedd6eda
27302 F20101206_AABMKJ hardison_l_Page_043.QC.jpg
68e46225c63a7036b4701e80ad322fc6
4765bc6de08764bcd5e079d2cb863eecf38620c3
6219 F20101206_AABMJU hardison_l_Page_021thm.jpg
630a7b0c261391163d3a44cdd5871622
02fd4eb0d2e313acb08907483fcdd33a1783af5a
103337 F20101206_AABLHH hardison_l_Page_068.jp2
9fbe9109a907d1496ffeda7fefc9dcb1
bbc6c363ba81177f85d2dc2fbdee2a755197d79d
891629 F20101206_AABLGS hardison_l_Page_053.jp2
b24ab598b9171ea89a3cbabd97dc9d9c
e5ec0717c7db7ac963407a0543ab5bb3f5f04617
22838 F20101206_AABMJV hardison_l_Page_022.QC.jpg
9fa05ec01c08564b3322a648a00a818e
2c0d71b8b43ab2921047157a01242d56a90b511b
119036 F20101206_AABLGT hardison_l_Page_054.jp2
c35450939eb442f0e749d5e9a1c38a5f
9673a810afe1b599dd4687ed3d9eb6a02c1ab817
23607 F20101206_AABMKK hardison_l_Page_044.QC.jpg
ee71944c442ba04789c971df481c2af5
79540bf320fa56a1d57989fd92e595a073fab72e
24756 F20101206_AABMJW hardison_l_Page_023.QC.jpg
f7afc36eb8df6dd3f40cb3e73648d90d
0541a3f4651826ff3ca9c7ee062ede32990d8ff4
859062 F20101206_AABLHI hardison_l_Page_069.jp2
3961e81c2a863a47717676050a89728d
6145a9ca7de60f8c6f9ad95619ff206e207220e2
1040050 F20101206_AABLGU hardison_l_Page_055.jp2
1fb63bf7e3edc62beb0e5a491bc1cbbf
74b694b2bab413ac8453c772fec7a1a13f1a659d
6816 F20101206_AABMLA hardison_l_Page_066thm.jpg
86dce9a28c2311f2c0303196d1b694cc
e3b59f92ab0bf93b482b8e0797e014e959b7bd30
6651 F20101206_AABMKL hardison_l_Page_046thm.jpg
b66b02992cc082fe47a15eaa92c68eae
83c0f2a636d23f185740e853b5b4e624840ddd1c
5130 F20101206_AABMJX hardison_l_Page_024thm.jpg
8e6322f47f71cc79d18d0041dcc0b1f2
3d6e48129270734d7c052a83c1da10698985cebc
119850 F20101206_AABLHJ hardison_l_Page_070.jp2
8b26e43a2eb5a6216cc7eab71f59d99b
b7289c754f4698dd10dc0f61a7ef72d59f54f980
117423 F20101206_AABLGV hardison_l_Page_056.jp2
31aeb34fd223c4797d717672bc0cc39d
5b428b23207956cadc479cfd5593923fcd46e042
F20101206_AABMLB hardison_l_Page_066.QC.jpg
973d47b44cd65451b38b8bbba17be3cd
ad64f05060721fbd7da10a067e4039fd586ada1f
28657 F20101206_AABMKM hardison_l_Page_046.QC.jpg
6de35bf6df490f739fa99254bd7b7642
ae25e14206669b834c26abeec6f1bb5d7747f34a
6134 F20101206_AABMJY hardison_l_Page_027thm.jpg
027150a504c812895915c951dc952d03
cb36533d80e330c3132eeb92b8513d4e03bb4a75
F20101206_AABLHK hardison_l_Page_071.jp2
2ac82ac8b2020d8f6516b84059a1d523
4ceaea14ba529ca503aa39d55a1f620225d63d90
F20101206_AABLGW hardison_l_Page_057.jp2
b5117219c495153b87720a7c6b0f5954
bf957ae1d31331300e773ca5007dbf7d8ed145a8
6041 F20101206_AABMLC hardison_l_Page_068thm.jpg
21894e1ddb3b907b28c68f64a7178c4a
d581e942bbb155b17532d85abba53a739fd27c5a
24176 F20101206_AABMKN hardison_l_Page_047.QC.jpg
87c85c1928862ee083402e55a1160454
ac0b6c79c5f8321adae483a90e8f45d593b8a4d6
26586 F20101206_AABMJZ hardison_l_Page_027.QC.jpg
aa8b5bdec695c9c65d68d023a751130e
a5260a4266d0f18fa397ec861bc223759ce37a45
123202 F20101206_AABLIA hardison_l_Page_087.jp2
17cc2d061dcb64666d35637c142e723b
609a147b9821d65b2e46d3a42557cc4eb46d2c62
117713 F20101206_AABLHL hardison_l_Page_072.jp2
cf54a36c02c164d527b8a0d0872d2c9f
9bc9c126773b232f670bb8c8c45ee0920bedb5ec
107351 F20101206_AABLGX hardison_l_Page_058.jp2
a958e1988a9087af59016cd37f12881d
f6980c03bc8b1641fc23cd0ab015793d2ddb8bd2
22829 F20101206_AABMLD hardison_l_Page_069.QC.jpg
792a3d0f77de16ba3d8b5b87bd6e13e9
e218703e03680db133cd7698271a3d21cc3608ef
6428 F20101206_AABMKO hardison_l_Page_048thm.jpg
3b413d9986177677837751a311c3fd85
1b0609f1abf17d95a9befe16e9c5f90dbec92d6c
119636 F20101206_AABLIB hardison_l_Page_088.jp2
78b33c0d252709ad5260ed7c83b746cc
63c8c541fa3a9137bdb40f328ef43c7295fb0099
1051974 F20101206_AABLHM hardison_l_Page_073.jp2
9ec5e4032decb5c9fd4b1fe499c246a2
987d7100065eb2eb2052359647a405c57fd0c0b9
831131 F20101206_AABLGY hardison_l_Page_059.jp2
ce8d811acf8e6ccb8018897371ff4840
4520e1d2a203a9f570387baf7843d04b9cee4f10
28033 F20101206_AABMLE hardison_l_Page_071.QC.jpg
8746d90aa32729e92e258dd3c4def7b3
77c17d38a5f2310795223777a549ce163d81f468
12130 F20101206_AABMKP hardison_l_Page_050.QC.jpg
d7be4544d08899f876e74c2b9d3e45a9
fefb01c5345861c38ab7bc3a6bc2858265fa2b50
97972 F20101206_AABLIC hardison_l_Page_089.jp2
76c7215d90879dd097fb0829aee9ed49
6f0e4ea5d3a8c569f49d93f6cd0d5c1b23ad743e
1045766 F20101206_AABLHN hardison_l_Page_074.jp2
4c189031e86d52dad20103536abc13d7
7506d4e69db5b527b5b8e60230df16be1206f132
822224 F20101206_AABLGZ hardison_l_Page_060.jp2
829da081a7e88305e4418e1b0eacc76a
64a34adae623d5c1c6693b03645dd53336c8bcd6
6896 F20101206_AABMLF hardison_l_Page_072thm.jpg
9ab7d168e1d278a33d51118bff7f75a5
c948642f6a19f267658ee42af36f08a97e41c99d
20318 F20101206_AABMKQ hardison_l_Page_051.QC.jpg
f9f38b98d58f81fa2c1c8ee3903098fd
e57cdadfe29ba9b8888d81edf687f52bb6781a12
110991 F20101206_AABLID hardison_l_Page_090.jp2
724a4a017aee41df240c91cc13b7a83f
6e2147a08be0a209202ef8e603f4b263619e581a
118615 F20101206_AABLHO hardison_l_Page_075.jp2
16757427cbede00aa4b725eeb15709bb
207a40b716e00528154d34b9e9e8d41410e3bd51
5399 F20101206_AABMLG hardison_l_Page_073thm.jpg
585eda11ce7e00f96234c575e37d8a9f
ed81f03f375d82c72271207d0cd155ac8be7178a
F20101206_AABMKR hardison_l_Page_053.QC.jpg
da7b78f0dbfde3120e33a5b8594b4ef6
d96b20dcabfc9671003a52f8c32270a6455d2bc1
1051959 F20101206_AABLIE hardison_l_Page_091.jp2
1954a4f7dcba4a36c0b2400a09b35e68
af2792883ef73c4622ca676171af18836586309e
F20101206_AABLHP hardison_l_Page_076.jp2
11503d477dfed667abc59fe9f8102c0b
fe9d9b2a772bab5e4dc07f814ae255d544083f50
6320 F20101206_AABMLH hardison_l_Page_074thm.jpg
35e6fce1217a0bbeb27e15e4da52354c
a84af728d6ff777d542aa6b5619fdfbf2c9e7d8b
6749 F20101206_AABMKS hardison_l_Page_056thm.jpg
429092a5c2fc6e4764dd39b267409b09
cce24c5f4ad006c480c5f4a6066289b01876e84f
119040 F20101206_AABLIF hardison_l_Page_092.jp2
a1eaa7b68e14ea169f788a0b5dd53192
9d583ebed27e40864acb8a0209584340c1b67a22
980934 F20101206_AABLHQ hardison_l_Page_077.jp2
212af10e426097a1d3cd464909774b65
f86c91bfc0a5783bafb2748d5f3ddbd3027a78a0
6129 F20101206_AABMLI hardison_l_Page_077thm.jpg
f85e2804b060cc7263d05c05d618671d
5a6187ba57a22d3cae2c503cd186d97e33e72ed1
7514 F20101206_AABMKT hardison_l_Page_057thm.jpg
5ea63332c227f3d96369f9502ad596e9
1d60591de00bca47f8dcd51cd3cb02f959b4d211
110308 F20101206_AABLIG hardison_l_Page_093.jp2
25394de4903c0f972964d97fbb7e7ce1
db2ec37ca34998e8144efad8cbbff388eb50f701
869577 F20101206_AABLHR hardison_l_Page_078.jp2
f9ceac8b046dc1c7081fa6c4a149fcb5
f7fe0eba8682f40e1b64113d991a29a281a5eec5
27196 F20101206_AABMLJ hardison_l_Page_082.QC.jpg
177db0bfd1b8e21bc81af963d036f242
4f43e4ea1900ad62e4cb3de924964c3b8da395ab
18174 F20101206_AABMKU hardison_l_Page_059.QC.jpg
f4b62ed9e0d3dba200adffac752ded86
411c0945a3fdba1e411351630b246d8a4b4742d5
120194 F20101206_AABLHS hardison_l_Page_079.jp2
b8d22f588bf65fb553a7fbc219e07da6
32954f95d48f902f26ac6e04d4cbc2aedb2b8d99
989585 F20101206_AABLIH hardison_l_Page_094.jp2
5b1e103f16c5ca72a508dbc02666455e
f7434ea3124617c7bf0e5da2c51870a0dfff39f8
4593 F20101206_AABMLK hardison_l_Page_083thm.jpg
20d916cf54d2a0999a845a1a96ec5bc7
aaed07266090bf777ac8519c63840639ff48964d
21420 F20101206_AABMKV hardison_l_Page_060.QC.jpg
a019cef8302be777e7d630ddb739d609
f8a82ab8dd748dd974a5fcc161b48f8d2d1176e8
118186 F20101206_AABLHT hardison_l_Page_080.jp2
bd5d2637337c677942486008e7159189
fb3bcbf5f1ad95e15287161e2a8d3c255dece482
109136 F20101206_AABLII hardison_l_Page_095.jp2
4d814e927047cf21509dd0e79bb41382
e1ad64ed61dd16b1952d53c4cf797c882776275b
6415 F20101206_AABMKW hardison_l_Page_063thm.jpg
0c500990971e3ed43516eda9d038428b
34ac04f3c4d481cf4caa1b8d0ce28b791cbc6b7e
F20101206_AABLHU hardison_l_Page_081.jp2
5504117986c458a5a1b636b28c2e7de4
ca2a72c9ae1b625effdfbb9106b244f71a4cfc9f
5139 F20101206_AABMMA hardison_l_Page_109thm.jpg
66915abe429867eb59c5dca308e3ffa5
199508f44162595c28e6e4da71c4de264ef1af03
6678 F20101206_AABMLL hardison_l_Page_088thm.jpg
fef184dbc06d46229a3f84100a961d1b
8bdcefc0503e04ffbedd71aae558226c981422d5
28127 F20101206_AABMKX hardison_l_Page_063.QC.jpg
dd4c012268837c28378a361a92d40abe
b10d02d5e6df5b139160de22bcc0b8fb250b217f
111274 F20101206_AABLHV hardison_l_Page_082.jp2
98a26ad76d0b7cf025527869e46c1b9e
5c6e8931dab9d446b86b0aa9e02917ca50cc890b
97290 F20101206_AABLIJ hardison_l_Page_096.jp2
ebc5e5270f98bfe0076236a16e56fc74
72bb0045e791f7a18d0a487e5188252d8b6e3647
5889 F20101206_AABMMB hardison_l_Page_111thm.jpg
2d6fb9b3411f0ecb5cbd4d1b810e1a94
949226e7c904602591f0f31f9dc654332961119e
28198 F20101206_AABMLM hardison_l_Page_088.QC.jpg
bfcb6e3d9a8ea23f64e77ea712467dd8
2bb44b842747a047181194368a8bf70213cd2db0
26050 F20101206_AABMKY hardison_l_Page_064.QC.jpg
8f0c05e8113b056bd7c434d048e38729
8502d2fcba95eeb53ca0d0de99f3246954c68ac7
714924 F20101206_AABLHW hardison_l_Page_083.jp2
b8c6133f3f99113ff0da6274227681cc
74024a47eb1d4de8e2126c77273a710c22a90c10
826111 F20101206_AABLIK hardison_l_Page_097.jp2
007ac9d0f3d72ef04a8bca922d1f650a
2b46432e3e5681da5c238a03d0c516b35f785971
22585 F20101206_AABMMC hardison_l_Page_111.QC.jpg
40fa48980f486631d2a4e3fd5b793837
bc2c44e79f3c439e61c719567b44208be99c3a01
24193 F20101206_AABMLN hardison_l_Page_089.QC.jpg
1a60de6763a0b638cb869766c5c7df3c
73e9f3a0dbd5292947bafad6ee8684c9db4a18d3
28475 F20101206_AABMKZ hardison_l_Page_065.QC.jpg
67c90aab76b76c4305f6cc12ba53db5a
81483128994fa5caff4525c4a619332071a270fc
813616 F20101206_AABLHX hardison_l_Page_084.jp2
a13b916b1e4a3d3268d8e3284c40f0f0
f5dbbe83c8173d3930c9764758cab63138996203
F20101206_AABLJA hardison_l_Page_114.jp2
7e1214538f9b3b087f3796d27c7a8f5b
18d935825201bd2187c547da66d76a75b4ea4810
113947 F20101206_AABLIL hardison_l_Page_098.jp2
8b0b5131252f2c405b3c442c7a63cbae
35a36b7b2209801b4166cbc178198bf7b67e1041
5357 F20101206_AABMMD hardison_l_Page_113thm.jpg
7a70f98a6f6a0c07310bc07f98f3462c
7bb94240be2070868b15b734d2f591c86072da55
25596 F20101206_AABMLO hardison_l_Page_091.QC.jpg
9c92625039f94492a972d770967d217b
c825b1de1bb4e2d41bde84a16a86ddb8c16eb0be
53972 F20101206_AABLHY hardison_l_Page_085.jp2
2ed665211537e735768e206f6b8a6683
4ce0406f17d0727128de0aa9ae293a5dbade76b8
121031 F20101206_AABLJB hardison_l_Page_115.jp2
387ee129821597affb8de66923d6c1a9
cd640ce91a76c2fd46340cd2e837dc77db3d0c8e
765645 F20101206_AABLIM hardison_l_Page_099.jp2
f6f440d80c2686f63735788c2537fd54
68b1e94d9851ee86c0d8b502563d08334566dd26
28539 F20101206_AABMME hardison_l_Page_115.QC.jpg
fa7f5184a7945a51660aa20923fa44ac
844b8c026912b29acda7724232d65685ac3e427f
6366 F20101206_AABMLP hardison_l_Page_093thm.jpg
50cce9d8a7399572b960c02d62b09f34
fc00ad99a329b4ce472547af01f1800f90eeb505
113348 F20101206_AABLHZ hardison_l_Page_086.jp2
ced1278348f8c2f1e80dd36d1098462a
8e78b5ec2472fadba82355e779e2dd3188dde0f3
1041837 F20101206_AABLJC hardison_l_Page_116.jp2
f572f7de3e8260f52706812a4af15424
fcca99caf31b706c146a007d5e71bc82f8cbc969
63376 F20101206_AABLIN hardison_l_Page_100.jp2
6b616bcba01d69a6f7842f62f362e53b
c0f392a8ff4d7fa6d6a3e851f1eb3e9db890d553
F20101206_AABMMF hardison_l_Page_116thm.jpg
35d5a91267e77bd8dd71244a202cfbda
104283eeca7e6eba1508a35b7bf8d6b407d70575
6289 F20101206_AABMLQ hardison_l_Page_094thm.jpg
91582f36b1b7641e3776da9f9e905b1e
37146c549b5f6831c55ab5c077c796495e318e2a
1051985 F20101206_AABLJD hardison_l_Page_117.jp2
686dc3ffddf844bb5896f5fbe3053e0a
8fd69daa776f255dda9046aa925152f0356761dc
91416 F20101206_AABLIO hardison_l_Page_102.jp2
8911ce90787a99f6275c1d20b0256775
0035752abd73b831ba35d696de6cf18e72c93071
23362 F20101206_AABMMG hardison_l_Page_116.QC.jpg
f00853f3b918f6d0cf20551152e6c487
1f8b3ef6ee3e68f57cbf07beac2e895c89afd005
6229 F20101206_AABMLR hardison_l_Page_095thm.jpg
16048c38c600e73d033101da65abbc65
d59080d9680ccc7d87ab1ce01e4d2d273cdc5312
1007702 F20101206_AABLJE hardison_l_Page_119.jp2
e02bbb647c087ec155bef514ba2ab109
3c775e28c73f75812c99586fb1e602886f9adca8
101687 F20101206_AABLIP hardison_l_Page_103.jp2
3075bc091b4e2a36431aaa1a2a001426
846bf80a8a3717e21c1c7b247fa7f4aff893a2c8
6844 F20101206_AABMMH hardison_l_Page_117thm.jpg
d0c0ce2843c899958656fff92ac5b7f8
1e5f693e3aef4b590c5f4f2af6bfc2b1f55054ba
6526 F20101206_AABMLS hardison_l_Page_098thm.jpg
cbec4dfc425e9bea367f2c5c00eceb42
539c32fe37fd6f009e1d64ece4480e904a776ba4
1032281 F20101206_AABLJF hardison_l_Page_120.jp2
40660c99a21231f30f0378ce335911c7
332317c4d693b73610994216c37d566e4605867e
468722 F20101206_AABLIQ hardison_l_Page_104.jp2
e2e806e45ada87921ae1b5b6f44158e4
2a5f8599cc70fd1c7df7bf344af19f16b6f9388d
5900 F20101206_AABMMI hardison_l_Page_118thm.jpg
efb3f8c1e44615e61219bb829f4e62fb
93334c9a2e7100635c3d865a44d13e51aaecf856
27608 F20101206_AABMLT hardison_l_Page_098.QC.jpg
3eb19a5b8f69aa79b85c2cd9ee71d1f7
19ded9b795ca0b46df54eeebb9228c0277f19f33
119990 F20101206_AABLJG hardison_l_Page_121.jp2
c41872cba0ac9ab1f6eabb1da896fea5
98b651725ca15d742558d73a4db08520e1371873
122932 F20101206_AABLIR hardison_l_Page_105.jp2
d36e6fb2e6df897bc58ddecff958ff70
baf6f6f591f496eed9f733319b046d30944125de
23385 F20101206_AABMMJ hardison_l_Page_118.QC.jpg
978fcc9d181391b5e0c95e4d3b030092
543d32732c969f14711f81ea22395d5fa428fcf3
4129 F20101206_AABMLU hardison_l_Page_100thm.jpg
c8cf7498c10bad96df18aa8ba3daeca3
3e223b1c366895bcaceb2e00c96220e5ed4ae61d
120853 F20101206_AABLJH hardison_l_Page_122.jp2
13985ac08caf6affadf3c9fd0bbd8e5f
ab29a5125782cced8a2562a24e817bea4898b65e
804949 F20101206_AABLIS hardison_l_Page_106.jp2
286d50ec3981eb1f11d831f45530400a
d252d87e36a909b1dd99e02b081e1446185c46b7
4006 F20101206_AABMMK hardison_l_Page_123thm.jpg
9e547c3592b4bf3f944d9a0e7084723f
86c2ea727bb786aa80ac7219316a2e8010c0c80d
29063 F20101206_AABMLV hardison_l_Page_101.QC.jpg
99717cc028c49b5f70492ccb854891ae
b8d19068d27c1647935a2a5baaf073b93975c22d
443225 F20101206_AABLJI hardison_l_Page_123.jp2
cae6281bc6428363065891e7a8f1516f
f51af38f4248e35230e4676a537e036f449bc70d
115486 F20101206_AABLIT hardison_l_Page_107.jp2
1bd47c3bd9c5926fb07af3310066319d
a74ec0735642cf6020ee55afe38d093c861d13e8
12542 F20101206_AABMML hardison_l_Page_123.QC.jpg
7bb585474c3926a03515850f0d5a4a66
d5d33bb23ecbfe3f06b34b65a8c1e3012f4568bd
4463 F20101206_AABMLW hardison_l_Page_104thm.jpg
5f8c3f55d259051bf629fa0f19ef0be5
daa2d8d2451ac2c6c85996153059d5da7ec06066
95974 F20101206_AABLJJ hardison_l_Page_124.jp2
af56e0442e006cde22e959ad3f3b6187
b85a8dcdd966551ee7088812b91ba4ceec2200f7
1051949 F20101206_AABLIU hardison_l_Page_108.jp2
84139bacddc045213aa893c2d225f623
7007a92964c10ffe429fd775faf824862598b70e
6425 F20101206_AABMNA hardison_l_Page_147thm.jpg
c57c45afc012d37ee4932087eee5625c
7df14c96eac2db527e43eee0cf2a441830f808b7
6979 F20101206_AABMLX hardison_l_Page_105thm.jpg
9b73b35f0786ccb75a26dcca26059c3d
bd84be764db347ca499e31ba713032ad54c0cf8c
826534 F20101206_AABLIV hardison_l_Page_109.jp2
89f1069324f2d9da8c729fb602f09664
a36f6ea7faff06f19095222f6268e873a90313d3
26450 F20101206_AABMNB hardison_l_Page_147.QC.jpg
87ebd155f9c4cdc16b41ef20f6b29d7b
350888bad038e7ea3ae9cb2f02c5cbef6f76eed1
27682 F20101206_AABMMM hardison_l_Page_126.QC.jpg
5614c005083af20d7b166cb593e8434a
96c777c21a271c9635e9eedb35aa7a779fc179c1
5122 F20101206_AABMLY hardison_l_Page_106thm.jpg
36a78207e0d744f84bd971ff6e39e6c4
5e14456fcd529633ecf0f6cb9082b66d285235c0
118210 F20101206_AABLJK hardison_l_Page_125.jp2
00535999a085d01eb0e6dc203761b8af
93e54d58ae88c6673c318dba56a9d03e64e6778f
1019571 F20101206_AABLIW hardison_l_Page_110.jp2
6c33efa9e82cf875ad3aedc293801d76
9d1f4597b88e9a4a5b38a4776a03ac9b26bcfe72
29942 F20101206_AABMNC hardison_l_Page_148.QC.jpg
5ac66736c57b1565311870e1582fb69d
9b2c5a64294284afb2d7585737c31d857a36e069
6368 F20101206_AABMMN hardison_l_Page_127thm.jpg
e525e45187b8b1995d599420dc86c4bb
c02c531aa627815313f95d5c0ded190f9eb6ed13
5931 F20101206_AABMLZ hardison_l_Page_108thm.jpg
e76b2bac33e2d248e9d1fa208265bf18
0058b31f0d26d1500969850e0673987c966b451b
130019 F20101206_AABLKA hardison_l_Page_141.jp2
53ef43bc4d8f729344470ec1eee77781
ed39998079b7f7b65d6fd667a54c7e747233abba
116561 F20101206_AABLJL hardison_l_Page_126.jp2
6325d54522def96fb47ad085606beb41
20fb5d2f5ea0f005230880688c7877116c430eb6
958057 F20101206_AABLIX hardison_l_Page_111.jp2
caa105cc9e1677c04b046b0bed5eee65
3b48ab72f57a19ce95ab279a09d70c53dd162535
16633 F20101206_AABMND hardison_l_Page_149.QC.jpg
34838a59a722659711e9b5030a523a9c
735e8f80987263dc8a6e1882edf1e5a89a73c790
29094 F20101206_AABMMO hardison_l_Page_129.QC.jpg
5c8d3727f9bed7ca29b9bd50f0d1179c
2e0b12c89e494ad14d2fa79b5c32fc8acb3c7040
129774 F20101206_AABLKB hardison_l_Page_142.jp2
42642978ed996494dfc03568d88a897e
a3c83a36c32a6de561276e10f615f24687e5b0d5
116375 F20101206_AABLJM hardison_l_Page_127.jp2
fd374ae00f6a93a59a327755f5a7c999
f9dfde36d85647238c179dfef29e562a41dea6b2
826002 F20101206_AABLIY hardison_l_Page_112.jp2
ab740c1754f3cea1a5d2758d24038db6
7686bac1917ceae48adfb445586ad29e489cef90
4323 F20101206_AABMNE hardison_l_Page_150thm.jpg
ae56909e87fa4b5394a6f54756552f11
b32f8e52c69ceec47ac073d566caa57aae50049e
28049 F20101206_AABMMP hardison_l_Page_130.QC.jpg
25cb901f4521f6ce6c7885939c5c038d
6aeec83bf8f71a80aaf5ae26ed23f9f854ed5656
136154 F20101206_AABLKC hardison_l_Page_143.jp2
ddd5ba6581fe7e5391bf411ee19aa91e
d6bacc54d4c4bb73d3c5622436b7d9a101dcdd74
F20101206_AABLJN hardison_l_Page_128.jp2
6653ac3e798da218390fc58b3e7b1ee1
c89a68518086bd3505d828974b5c6b103535f27b
85134 F20101206_AABLIZ hardison_l_Page_113.jp2
1f9e79eac903a83852b7ea23fe919490
929deaccc9644f91e8237e1ceddaa284aaee7fbf
20541 F20101206_AABMMQ hardison_l_Page_131.QC.jpg
445e42ae0c2c5b3c31c01aa8e9b0e653
35b9a09484aa9cca233e0f1bb94f66881902e228
136413 F20101206_AABLKD hardison_l_Page_144.jp2
46c1a016f1ca601e4b1a49747cdcfd84
49009bab000ce85265f217a34ded314180226b63
119176 F20101206_AABLJO hardison_l_Page_129.jp2
6eb2ee82997d5d3ec58e393928e8daff
b4efb7f7e8b1dbad3f88f719fbef5284916c1cfb
7992 F20101206_AABMMR hardison_l_Page_132.QC.jpg
991219b0ea17a0fb886f8cf50e040bb2
5b4ec3929dca33ba9e0953f6fc29522a8500e5a3
147053 F20101206_AABLKE hardison_l_Page_145.jp2
776a7dd0eee0129a55d4496c19871d85
a3e7b1d7cefcb5704be1bceac0427c7b49ff51e8
117187 F20101206_AABLJP hardison_l_Page_130.jp2
585465ddbd8c53c1dcb63f1bba01c3dd
d94d089b2df2c45e6ef0eb936b4e71de5b0e69f2
6795 F20101206_AABMMS hardison_l_Page_134thm.jpg
4907df6973a04326fea5d2b49aa52a33
eb5e3a9e8ce77b78866bdcd99dc2c583ec0a9c4f
134204 F20101206_AABLKF hardison_l_Page_146.jp2
17ca07b3e3bfe20dac23530cca2ace60
b3244a7013c0193e826a80ad74725028fc9e5f48
859850 F20101206_AABLJQ hardison_l_Page_131.jp2
e66353d7954bacef0779f1995d483d22
e2a4de015feb565841b50cbd129fcc68f60faea6
27934 F20101206_AABMMT hardison_l_Page_134.QC.jpg
f323c0e7ac312e508cbbc356c32a24ab
dab38bfc5efb59d1f7386fa1b3cd5004aed0aa7a
124323 F20101206_AABLKG hardison_l_Page_147.jp2
131fd9b72c1bba8d2361808626f06bce
29336f4e8b3266dab16d6c1a53f6bafffc32e4de
225505 F20101206_AABLJR hardison_l_Page_132.jp2
be74677dfca36f4dc1a5c40fb167e3b1
2f07f2ba8a511f7a7abec2d72b536350dd948a83







UJLTRAFAST SPECTROSCOPY OF NOVEL MATERIALS


By

LINDSAY M. HARDISON


















A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
DOCTOR OF PHILOSOPHY

UNIVERSITY OF FLORIDA

2007

































O 2007 Lindsay M. Hardison

































To my family









ACKNOWLEDGMENTS

As I reflect on the number of years that have led up to this moment of earning a Ph.D. in

Physical Chemistry, I realize there are numerous people to recognize and say thank you to

because without their support and encouragement I would not have made it to this point.First and

foremost, I thank the Lord because without His mercy nothing is possible. I thank my advisor,

Professor Valeria D. Kleiman for her guidance, patience and consistent motivation throughout

my journey. I appreciate the effort and time she has put into helping me pursue this degree

including letting me explore new possibilities and career building activities such as working for a

summer at Corning, Inc as an intern. Valeria has continually believed in me and my work even

when I did not and it is this type of support and enthusiasm that has enabled me to finish this

proj ect.

I thank my supervisory committee members Professors Philip Brucat and Nico Omenetto

for their guidance and thought provoking discussions. My gratitude goes to Dr. Kirk Schanze,

Dr. Hui Jiang and Xiaoyoung Zhao for their CPE collaboration. I appreciate them providing the

polymers and their willingness to help satisfy the needs of the proj ect. I am also grateful for Dr.

Paul Holloway and Dr. Hyeokjin Lee asking for our assistance in their nanorod proj ect; it has

been an experience I truly enjoyed.

I express gratitude to the members, past and present, of the Kleiman Group. Thanks goes

to Dr. Juirgen Muiller for giving me a fundamental understanding of the transient absorption. I

thank Dr. Evrim Atas for her ongoing friendship, Turkish cooking and being my upconversion

mentor. There are no words that describe how much I appreciate Daniel Kuroda. I thank him for

not only his ability to answer all of my questions but also for his constant support and

encouragement and of course, his BBQing skills. I also want to thank "Cochuk", Aysun Altan,









for helping me out in so many ways, especially in the last few weeks, it has been greatly

appreciated.

I have dedicated this dissertation to my family. To me, family means more than just blood

relatives, it is who you believe supports you and will be there for you always. To start, I want to

thank Coach Dr. Nancy Bottge. She was such an influential person in my life and taught me that

I must "stick to the fight when hardest hit". Her "don't quit" attitude is one of the reasons for this

success. She had not only been a mentor and a coach but a friend that I could turn to and who

taught me so many invaluable lessons. I extend extreme gratitude to Chad Mair who has been

beside me each step of my life in the past five years. I have been blessed with having such a

good friend that I love so much that I consider him to be my brother. I can count on him for

anything and know that without him, this achievement in my life may not have been possible. He

has been the best work out partner, best friend and best colleague a girl could ask for. Jana

Vanderloop, my best friend of ten, going on forever, years has also been a rock for me to lean on.

I will always be sure to appreciate our "inner randomness" because without it, life is too serious.

I enj oy the fun we have on a daily basis, it keeps me sane. From day one, Richard Farley and I

have battled our way through the trials and tribulations of grad school. I thank him for his

companionship, sense of humor and open mindedness. I thank Roxy "Rory" Lowry and Todd

Prox for their friendship, laughs and their ability to give me different perspectives on all

situations I run into in life. I also appreciate the encouragement and advice that I received from

my friend Jim Reynolds. I thank Megan Meyer for her intense sarcasm because no matter what

mood I am in, it always puts a huge smile on my face.

My time here would not have been the same without the social activities provided by all

my friends in Gainesville. I am extremely grateful that I was a part of such a fun group that









includes Sophie, Merve, Roxy, Richard, Rob, Neil, Eric, Megan, Meg etc... Their laughter and

craziness will be greatly missed, especially during the fall at tailgating. Thanks to M.I.A and

Whoever Shows Up, the two best intramural softball teams in University of Florida history for

making me feel a little bit younger. I have enjoyed playing for five years and will miss the

teammates that have helped form our dynasty.

Finally, I thank my parents Craig Hardison and Susan Keller for allowing me to make my

own decisions so that I could become the independent woman I am today. They have always

believed that I could do anything I put my mind to. I thank my sister Brynn, for terrorizing me as

a child but growing up to become a wonderful young woman that I can call my friend.












TABLE OF CONTENTS


page

ACKNOWLEDGMENTS .............. ...............4.....


LIST OF TABLES ............ ....._._. ...............9.....


LIST OF FIGURES .............. ...............10....


AB S TRAC T ............._. .......... ..............._ 13...


CHAPTER


1 INTRODUCTION .............. ...............15....


Study Overview ......_._ ................ ...............15.......
Photophysics Concepts ................. ...............16.......... .....
Energy Transfer ................. ...............18.......... ......
Radiative Energy Transfer............... ...............19
Non-radiative Energy Transfer............... .... ................1
Random Walk Migration (Intrachain Energy Transfer) ................. ........................20
Emission Measurements ................. ...............22.................
Transient Absorption .............. ...............24....


2 QUANTUM NANOPARTICLES .............. ...............27....

Overview. ............ _.... .....__ _.... .............2
Bulk vs. Quantum Semiconductors .............. ...............27....
Size and Shape Dependence .............. ...............33....
Passivation ........._._... .... ...._.__......_. .............3

Composition Changes: Interdiffusion................ ... ..... ... ..... ........3
Experimental Methods: Nanorod Synthesis and Composition Characterization ...................41
Preparation of ZnCdSe Nanorods. ............ .....__ .......__ ........ 4
Steady State Instrumentation ................. ......__ ..... ............4
Time-Resolved Photoluminescence Instrumentation ......___ .......___ ................43
Results and Discussion .............. .. ...............46...

Synthesi s of ZnCd Se Nanorods ............._. ...._... ...............46...
Structure of ZnCd Se Nanorods ............_. ...._... ...............47...
Effect of Alloying on the Phonon Spectra ....._.._._ ........___ .....__ ..........4
Photoluminescence and Absorption Properties ................. ...............................52
Time-Resolved Photoluminescence (TRPL) ................. ........__ ......... 55.... ....
Sum m ary ................. ...............62........ ......


3 QUANTUM PARTICLE ELECTRONIC STRUCTURE ................. ......__ ..........._._.63

Introducti on .........._.... .. .. .. _. ..... ... ...............63

Experimental Methods: Transient Absorption............... ...............6












Re sults............... .... ........ ........ .. ............. .............6
CdSe versus CdSe/ZnSe Core/Shell .....__.....___ ..........__ ............6

Core/Shell Excitation Dependence ............_....._ ....._ ............7
Core/Shell versus Alloys .............. ...............75....
Discussion ............. ...... ._ ...............78...
Summary ............. ...... ._ ...............85...


4 CONJUGATED POLYELECTROLYTES (CPES) ................ ...............86................


Introducti on ................. ...............86.................

Quenching PPE-CO2- ............. ...... ...............93...
Experimental M ethods............... ... .. .......... ...............9
Synthesis of Variable Chain Lengths of PPE-CO2- ......____ ...... .___ ................95
Photophysical M ethods ................................................9
Photophysics of Variable Chain Length PPE-CO2- Polymers ................ ............ .........105
Steady State Characterization............... ...........10
Time-Resolved Fluorescence ............_ ..... ..__ ...............112...
Isotropic Up conversion ................. ...............112.....__ ......
Time-Resolved Anisotropy ..........._..._ ...............119.....__ ......
Potential Kinetic M odel ..........._...__........ ...............121....

Summary ..........._...__........ ...............122.....


5 CONCLUSIONS AND FUTURE WORK ..........._.._........_........__ ...........12


Nanoparticle Conclusions and Future Work............... ...............124.
Conclusions .............. ...............124....
Outlook/Future Work ...................... ...............125
PPE-CO2 COnclusions and Future Work ........._..__......_ .. ...............129.
Conclusions ........... .... .. .................... ...............12
Outlook/ Future Work (Hyperbranched PPE-CO2-) ................ .......... ...............130


LIST OF REFERENCES ................. ...............133................


BIOGRAPHICAL SKETCH ................. ...............150......... ......










LIST OF TABLES


Table page

2-1 Comparison of P and z value of CdSe/ZnSe and ZnCdSe nanorods .................. ...............61

4-1 Experimental conditions for wavelength dependence study .............. ....................9

4-2 Detection dependence decay times ................. ...............113........... ...











LIST OF FIGURES


Figure page

1-1. Jablonski diagram. .............. ...............17....

1-2. Generalized diagram for spectral overlap of donor emission and acceptor absorption
and the energy transfer between resonant transitions of donor and acceptor. Adapted
from B. Valeur.(2)............... ...............2

1-3. Signals in transient absorption measurements ................. ...............26..............

2-1. The band theory of solids .............. ...............28....

2-2. The nanocrystal band gap size dependence ................. ...............30..............

2-3. The absorption spectra of TOPO/TOP passivated CdSe nanocrystals with radii from 1.2
to 4. 1 nm .............. ...............33....

2-4. Nanorod with each axi s labeled ................. ...............35..............

2-5. Electronic potential step of valence and conduction bands ................. ................. ...._3 8

2-6. Time-resolved photoluminescence ................. ...............44........... ....

2-7. Powder X-ray diffraction patterns of CdSe nanorods, CdSe/ZnSe core/shell nanorods,
and ZnCdSe alloyed nanorods .............. ...............47....

2-8. High resolution-transmission electron microscopy image and histogram of size
distribution of ZnCdSe nanorods. ..........._......_ .....__ ...........4

2-9. Raman spectra of LO phonon mode of CdSe nanorods and CdSe/ZnSe core/shell
nanorods ........... __..... ._ ...............50....

2-10. Raman LO phonon spectra of ZnCdSe nanorods after annealing at 2700C.............._._......5 1

2-11. The UV-Vis absorption spectra CdSe, CdSe/ZnSe core-shell and ZnCdSe nanorods......52

2-12. The photoluminescence spectra of CdSe/ZnSe core/shell and CdSe nanorods...................53

2-13. The photoluminescence spectra of CdSe/ZnSe core/shell and ZnCdSe nanorods .............55

2-14. The broad band photoluminesce ................. ...............57........... ..

2-15. The CdSe/ZnSe core/shell photoluminesence .............. ...............59....

2-16. The time-resolved photoluminescence decay curves .............. ...............59....











2-17. The In[1n(lo/It)] versus In(time) of CdSe/ZnSe coreshell nanorods and ZnCdSe alloy
nanorods ............ ...... .. ...............60..

2-18. High resolution-transmission electron microscopy images of CdSe/ZnSe Core/Shell
and ZnCd Se Nanorods .............. ...............61....

3-1. Electronic structure in semiconductor nanoparticles. ....._.. .........._... ................ ...6

3-2. Transient absorption .............. ...............69....

3-3. The broad band transient absorption spectra for CdSe and CdSe/ZnSe core/shell rods at
various pump delay times .............. ...............71....

3-4. The kinetic traces corresponding to the l S, 1P and 2S bands .............. .....................7

3-5. The time-resolved excitation dependence collected for the core/shell sample. ....................74

3-6. The broad band transient absorption spectra for CdSe/ZnSe and ZnCdSe nanorods at
various time delays. ............. ...............76.....

3-7. The 1S and 1P composition dependence. ............. ...............77.....

3-8. The comparison of the 1S band for the CdSe/ZnSe core/shell and 3 hr ZnCdSe alloy ........78

3-9. Valence and conduction band offsets for various materials. (75) .............. .....................8

3-10. CdSe/ZnSe core/shell potential kinetic model ....__ ......_____ .......___ ..........8

3-11. ZnCdSe alloy potential kinetic model. ............. ...............84.....

4-1. The intrachain energy transfer of excitation to quencher molecule along polymer
b ackb one. ............. ...............9 1....

4-2. The PPE-CO2~ pOlymer repeat unit ............ ...... .._ ...............91

4-3. The Stern-Volmer plot of 10 CLM 185 PRU PPE-CO2-.......____ ........__ ........... ....94

4-4. Fluorescence up-conversion .............. ...............97....

4-5. Transition moments. ............. ...............99.....

4-6. Photoselection............... ............10

4-7. Berek polarization compensator. .............. ...............104....

4-8. Berek compensator used as a half-wave plate ....__ ......_____ .......___ ...........0

4-9. The chain length absorption shift for PPE-CO2- in methanol ....._____ ....... .....__.........106











4-10. The emission spectra 10CLM PPE-CO2- in methanol ................. ........__ ........._.._. 106

4-11. The emission of 10 CLM 35 PRU PPE-CO2- ............. .....................108

4-12. The excitation spectra 10 CLM 8 PRU PPE-CO2-. ............ ...............109.....

4-13. The excitation spectra 10 CLM 35 PRU PPE-CO2- in methanol .........._.._. ......_.._.......110

4-14. The excitation spectra 10 CLM 35 PRU PPE-CO2- in water ................. ........___.........111

4-15. The excitation spectra 10 CLM 35 PRU PPE-CO2- in methanol with Ca2+. ................... .....112

4-16. The time-resolved fluorescence decay of 8 PRU PPE-CO2- in methanol .........._............114

4-17. The time-resolved fluorescence decay of 30 CLM PPE-CO2- with different polymer
repeat units in methanol. ........._.. ........... ...............116..

4-18. The time-resolved fluorescence decay of 30 CLM PPE-CO2- (35 PRU) with and without
Ca2+ .......... ...............117......

4-19. The time-resolved fluorescence decay of 10 CLM PPE-CO2- (8 PRU) with and without
Ca2+ ............ ... ...............118..............

4-20. The time-resolved fluorescence decay of 30 CLM PPE-CO2- 35 PRU with different
quenchers ................. ...............119......... ......

4-21. The anisotropy of 8 PRU PPE-CO2-.........._._. ......... ...............12

4-22. Possible kinetic model for all PPE-CO2- PRU chains .............. ...............123....

5-1. The absorption spectra of the hyperbranched PPE-CO2- and the linear PPE-CO2-.............13 1

5-2. The photoluminescence spectra of hyperbranched PPE-CO2- ................ ......._... .......132









Abstract of Dissertation Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Doctor of Philosophy

UJLTRAFAST SPECTROSCOPY OF NOVEL MATERIALS

By

Lindsay M. Hardison

December 2007

Chair: Valeria D. Kleiman
Major: Chemistry

My research focused on steady state and time-resolved photophysical characterization of a

series of semiconductor nanoparticles and water soluble conjugated polyelectrolytes. Several

studies have shown that the electronic structure and relaxation dynamics in CdSe nanocrystals

are not only size but are also shape and passivation dependent; however, there is no detailed

comparison of the photophysical properties of ZnCdSe particles with different relative amounts

of Zn. This dissertation presents data collected for colloidal CdSe, CdSe/ZnSe and ZnCdSe

nanoparticles with rod-like architectures synthesized and investigated in our labs to determine

how size, shape, passivation and composition affect the quantum confinement and dynamics. In

addition, a series of different polymer repeat unit lengths of a linear conjugated polyelectrolyte

(CPE) with a carboxylate ionic side chain have been synthesized and their photophysical

properties have been explored.

Spectral shifts and line broadening exhibited within the Raman spectroscopy, UV-Vis

spectroscopy and photoluminescence aided in determining the extent of alloying and

compositional disorder created during the alloying process. The photoluminescence quantum

yield of ZnCdSe nanorods is higher than that from pristine CdSe nanorods indicating a higher

binding energy of the exciton. This effect is speculated to be due to increased localization of the









exciton as a result of fluctuations in the composition, ultimately resulting in increases in

luminescence efficiencies.

Moreover, time-resolved photoluminescence characterized lifetimes of nanoparticles with

similar shape but different composition. Emission of an inhomogeneous population distribution

(different sizes, shapes or composition) leads to the simultaneous probing of particles with

different decaying rates. A stretched exponential function, I(t)= A~exp[-(t/z) ], can be used to

describe these systems, where P <1 corresponds to disperse populations. In the experiments

presented here, the photoluminescence data yields small P values, independent of the emitted

photon energy. Photoluminescence decay lifetime, z, of the samples increased with alloying time

due to compositional disorder leading to exciton localization.

The dynamics of each nanorod was studied by absorption changes using ultrafast pump-

probe spectroscopy. An excitation wavelength dependence study has been conducted to gain

insight into the intraband/interband relaxation in core/shell nanorods with small valence band

offsets. Determination of the dynamics and mechanisms of these systems will be useful for the

study of fundamental physics and light emitting applications such as LED's, photovoltaic

devices, lasing and fluorescence tagging.

CPEs are soluble in polar solvents and their conformational properties can be tuned to

enhance their emissive behavior for sensing and device applications. It was found that polymer

concentration, solvent, aggregation inducer and chain length, all affect the quenching efficiency;

therefore, this dissertation examines energy transfer mechanism responsible for this behavior

using ultrafast upconversion. Upon excitation of the aggregates from energetically higher lying

isolated chains, the fluorescence lifetimes result in multi-exponential behavior due to the

competition between the radiative and non-radiatve decay.









CHAPTER 1
INTTRODUCTION

Study Overview

The overall goal of my study was to investigate light-matter interactions in a series of

novel materials. The proj ects presented are an interesting opportunity to explore the exciton

dynamics and energy transfer processes in inorganic semiconductor nanoparticles and organic

conjugated polymers. The first chapter will briefly discuss fluorescence principles and excitation

energy transfer (interchain) and random walk (intrachain) energy transfer. This discussion is

helpful to understand quenching of conjugated polyelectrolytes. Also, the signals that can be

detected using femtosecond transient absorption (pump-probe) spectroscopy are described in

detail. The information provided is advantageous for the reader to understand the data presented

concerning the excited state of the nanorods.

Chapter 2 describes the motivation for the synthesis and characterization of an array of

CdSe/ZnSe core/shell and ZnCdSe alloyed nanorods. Background into the size, shape,

passivation and composition dependence is presented. Moreover, synthetic steps, x-ray

diffraction, high-resolution transmission electron microscopy, Raman spectroscopy, steady state

absorption, photoluminescence and time-resolved photoluminescence data are included. The

ternary alloy composition is confirmed and the qualitative trends based on compositional

disorder are discussed.

Femtosecond transient absorption (Chapter 3) was used to explore the excited state

behavior of the same series of nanorods. A comparison between CdSe and CdSe/ZnSe core/shell

nanorods is made to show how passivation alters the exciton behavior. Also, excitation

wavelength dependence is presented for the CdSe/ZnSe shell to elucidate the influence an

interfacial state (determined in Chapter 2) has on the dynamics of the photo-generated exciton. In









addition, the effects alloying has on the excited state of the ZnCdSe nanorods are discussed. Two

models, one for the core/shell and one for the alloy, are proposed to describe the relaxation

processes observed in each of the experiments.

The dynamics of the energy transfer from isolated to aggregated species in a series of

different polymer repeat unit sizes of a conjugated polyelectrolyte, PPE-CO2 Synthesized by

Xiaoyoung Zhao in Dr. Schanze's lab are discussed (Chapter 4). Anisotropy measurements

confirm that this polymer is very rigid and the conjugation length is longer than expected. The

data is analyzed to extract the influence aggregation has on the isolated chain emission.

Finally, Chapter 5 summarizes each project and states general conclusions drawn from the

results collected for this dissertation. Suggestions are made for potential applications for which

semiconductor nanoparticles presented in this dissertation may be useful. Also, an additional

molecule, similar to the PPE-CO2~, iS presented as the next step in a series of polymers to

investigate for chemo-or bio sensors.

Photophysics Concepts

Spectroscopy methods, whether they be time-resolved or steady state, provide numerous

ways to measure emission of materials that are intended to be used in a wide variety of

applications including opto-and electronic, biomedical, and chemical research. This dissertation

focuses on the photophysical properties of nanocrystals and energy transfer mechanisms that

induce the amplified quenching capabilities of conjugated polyelectrolytes.

It is important to know the multiple photophysical processes that an excited chromophore

can undergo between the absorption and emission of light. These processes are dictated by the

probability that a transition from an initial state to a Einal state can occur. By using time-

dependent perturbation theory, Fermi's Golden Rule for transitions between two states

corresponds to a transition rate equal to: (1, 2)









k, rl= A A(Y, H 'F d (1-1)

where p corresponds to the density of final states that are available to interact with the initial

states via the perturbation, H'. This perturbation can alter the positions or motions of particles

and restructure the initial state so that is looks like the final state. Thus, Fermi's Golden Rule is

simply a transition rate probability between an initial and final state which depends on the

magnitude of a perturbation.(1)

The electronic transitions can be visualized along with the processes that can occur

between these states in a general Jablonski diagram seen in Figure 1-1.

S,


S,



S, A T1




So
Electronic ground state
Figure 1-1. Jablonski diagram. A = photon absorption; F = fluorescence (emission); P =
phosphorescence; S = singlet state; T = triplet state; IC = internal conversion; ISC =
intersystem crossing, IVR = internal vibrational relaxation. Adapted from B. Valeur.
(2)
Absorption of a photon by the ground state, So, promotes an electron to the vibrational

levels of an upper singlet excited state, S1, S2, Or higher, via a spin-conserved, allowed transition.

Subsequently, the excitation can be transferred to an isoenergetic vibrational manifold of a lower

singlet excited state with the same spin multiplicity, for example S2 to S1. This process is aided










by the overlap between the wavefunctions of the vibrational levels participating in the process.

This radiationless passage is termed internal conversion and occurs quite rapidly, usually within

a few picoseconds or less after light absorption, which is significantly faster than typical

fluorescence lifetimes. Based on Kasha' s rle,(2) the excitation will rapidly relax to the lowest

vibrational level of the first singlet excited state, S1, via internal vibrational relaxation (IVR);

therefore, the fluorescence emission, in most organic molecules, comes from the lowest excited

vibrational level. Radiationless decay (which releases heat) and intersystem crossing (ISC) to a

triplet excited state (resulting in phosphorescence) can also occur. ISC is a non-radiative

transition that involves two electronic states that are equally energetic but have different

multiplicities. However, the magnitude of coupling between the orbital magnetic moment and

spin magnetic moment (spin-orbit coupling) can be large enough so that this normally forbidden

transition may occur.(2)

Energy Transfer

One of the motivations behind the work presented in this dissertation is to identify the

mechanisms responsible for the amplified quenching observed in conjugated polyelectrolytes.

Aside from relaxation, the excited state of a chromophore, D*, can relax to its ground state after

transferring the photoexcited energy to an acceptor molecule, A, via a bimolecular process:

D* + A -D +A*

This process strongly depends on two conditions: (1) the emission of the excited donor should

overlap with the absorption of the acceptor and (2) the natural lifetime of the excited donor must

be slower than the energy transfer process. Once energy transfer has occurred, the photoexcited

chromophore, A*, has the ability to play a part in photochemical reactions or display sensitized

emission.(1, 2) There are several different types of energy transfer mechanisms used to describe









the diffusion of energy in molecules. In this dissertation only an overview of the radiative and

non-radiative (interchain) energy transfer and random walk diffusion (intrachain) are discussed.

Radiative Energy Transfer

A two step process that involves emission of a photon from the excited state of the donor

molecule followed by the same photon being absorbed by the ground state of the acceptor is

called radiative energy transfer.

Step 1: D* D + hu

Step 2: hu + A -A*

This type of energy transfer is the least complicated since it does not involve the interaction of

the donor and acceptor molecules. For this mechanism to be effective, the quantum yield of the

donor must be high in the spectral region of the absorption of the acceptor. To further enhance

radiative energy transfer, it is beneficial to have a high concentration and extinction coefficient

of the acceptor in addition to a large spectral overlap between the emission of the excited donor

and ground state absorption of the acceptor. The emission spectra of a donor molecule that

undergoes radiative transfer will experience a decrease in its fluorescence intensity in the

spectrally overlapped region and can lead to repeated absorption and emission if the donor and

acceptor molecules are identical (self-absorption/reabsorption). If there is adequate absorption

and emission overlap, the fluorescence lifetimes can increase.(1, 2) An example of this process is

shown in Figure 4-8, where we observe self-absorption in a conjugated polyelectrolyte solution

that is highly concentrated.

Non-radiative Energy Transfer

Non-radiative energy transfer occurs in a single step and just as radiative energy transfer,

depends on the spectral overlap between the donor' s emission and acceptor' s absorption spectra

but relies more on their coupled resonances (Figure 1-2). As seen in the Jablonski diagram in









Figure 1-1, several vibronic transitions of one state or in this case, a donor molecule, can be

isoenergetic to the corresponding transitions of the acceptor (D*- D and A A*). In general,

the non-radiative transfer rate is given by Eqn 1-1, (Fermi's Gold Rule) where the density (p) is

not only related to the coupling of the initial and final states capable of a transition (determined

by Frank-Condon factors) but also by the non-inhomogeneously broadened spectral overlap, J, of

the donor emission, ID(v),and acceptor absorption, E,(v), determined using Eq 1-2.(1, 2)


J =il ID 4 (v)dv (1-2)


This integral assumes that the relaxation within the excited state vibrational manifold is faster

than the energy transfer process and that energy transfer abides by the Franck-Condon principle

(vertical transition). As the number of resonant transitions between the donor and acceptor

increases, the likelihood for a non-radiative energy transfer process to occur increases since these

transitions are proportional to the overlap integral (Figure 1-2).(1, 2)

Random Walk Migration (Intrachain Energy Transfer)

In some cases, a quenching of the fluorescence occurs but can not be explained by a

bimolecular energy transfer mechanism. Upon excitation of a molecule, an excited state electron

and ground state hole pair are created, termed "exciton". If the molecule consists of multiple

segments that are equivalent in energy or are a cascade of energies (like a polymer with repeating

chromophores), this exciton can diffuse from one segment to another while remaining bound.

The exciton undergoes a mechanism that involves a "hopping" from one segment to another

within the same polymer. The "random walk" or intrachain energy transfer implies that the

electron and hole move together, and will always be located within the same chromophore thus

charge separation does not occur. Also, during the course of the energy diffusion, the energy is









not dissipated. This type of energy transfer between chromophores is difficult to measure

primarily because directly collecting the fluorescence of an acceptor after exciting a donor

chromophore of the same species with similar energies can be complicated.

DONOR ACCEPTOR








A A"











31 21 1 "11 2*1 3




D A

resonan~t tra~nsitions

Figure 1-2. Generalized diagram for spectral overlap of donor emission and acceptor absorption
and the energy transfer between resonant transitions of donor and acceptor. Adapted
from B. Valeur.(2)

Eugene Rabinowitch, a biophysicist, likened the random walk to a steel ball being shot into

a pinball machine where the ball bounces around within the machine but eventually either falls to

the bottom (fluorescence) or falls into a "play hole," (trap).(3) Scientifically speaking, the

exciton can either fluoresce by recombining the electron and hole, can be dissipated by internal









conversion in one of the chromophores or it can reach a "trap". A "trap" (or "energy sink") is

considered a lower energy state that is too deep for the exciton to overcome so from there it will

recombine radiatively or non-radiatively but it will not undergo any more diffusion.(4, 5) In the

case of conjugated polymers, this trap could be due to a kink in the polymer chain, a defect on a

chromophore, or a particularly low energy chromophore. If trapping happens the exciton cannot

reach its desired destination, an external quencher, an analyte or a material that can separate

charges, which would be detrimental for bio- and chemo sensors and photovoltaics.

Emission Measurements

Conjugated polyelectrolytes are intended for use as fluorescence based-sensors and rely on

changes of emission intensity and/or lifetime. Sensors of this nature are some of the most

common due to the ease of measurement and low detection limits.(6, 7) The fluorescence

quantum yield (OF) is an important parameter that is defined as the ratio of the numbers of

emitted photons, Ne;;zz, to the number of absorbed photons, Nabs. If all pOSsible pathways are

considered, the quantum yield is calculated as follows:(2)


0 Now, krad krad d(13
F ab k,,, krad kre + krse + kET f


where krad is the rate constant of the fluorescence emission, kre, kise, and kET are the internal

conversion, intersystem crossing and energy transfer rate constants, respectively. If the non-

radiative decay (k,;r) is the only competing process with the fluorescence emission, the quantum

yield is given by: (2)

kva
O ra (1-4)
Fkrad + knv










Thus, the greater the non-radiative decay rate constant, the smaller the quantum yield, and vice

versa. The time that it takes for the excited state of a molecule to decay to 1/e of its initial value

is the lifetime of the excited state, which is given by:


S=~ (1-5)
kvad + k,,,

The fluorescence emission intensity, quantum yield, and lifetime can be negatively

affected by numerous quenching processes including collisions with heavy atoms, electron

transfer, energy transfer, excimer formations, aggregate formation, and dynamic collisions.(2)

The quenching processes discussed above can be measured but the results of these experiments

can be hard to interpret. Instead, fluorescence anisotropy is employed to understand amplified

quenching observed in conjugated polyelectrolytes.

Direct measurement of the random walk "hopping" of excitons is a difficult task since this

process can compete with other energy transfer processes. Time-resolved anisotropy is type of

measurement that measures the decay of polarized fluorescence, which gives a better

understanding of the random intrachain energy migration in a material. The sensitivity to

depolarizing the transition dipole moments between an absorbing and emitting molecule is

directly related to the loss of anisotropy. Excitation with light polarized in a particular direction

will only excite molecules with the same orientation. For example, a vertical excitation will

preferentially excite molecules with vertical transition dipole moments. Anisotropy values will

not change if as the exciton migrates there is no change in the dipole moments between the

chains that absorb and emit. However, if these dipoles do change, as the exciton "hops", it loses

its original orientation and the fluorescence signal depolarizes.(4, 5)









Transient Absorption

Changes in population of different energy states can be examined using femtosecond time-

resolved pump-probe spectroscopy. A pump pulse excites the sample, causing a depopulation of

the ground state. A broad-band probe is transmitted through the excited volume of the sample,

monitoring the populations of various excited states. The time-dependency of the technique is

introduced by varying the delay time between the probe pulse and the excitation pulse.

Simple absorption measurements can be accomplished by measuring the log of the ratio of

the intensity of an incident beam that enters the solution, lo, and intensity of the beam exiting the

solution, I.


I,
A =log "
"I


log T


(1-6)


where A is the absorbance and T is the transmittance. This ratio leads to the number of photons

absorbed which is based on the sum of the absorption cross section, o (cm2), Of all the molecules

in the path of the incident beam.(2)


(1-7)


I, N ocl I, 1
In -- or log -" ocl
I 1000 I 2303

sinc N~oleule/cm) =N, (molecules/mol) c (mol/L)
1000 (cm3/L)

where, Na is Avogadro's number, c is the sample concentration (M) and I is the optical path

length (cm) within the sample. The molar absorption coefficient (extinction coefficient) is

defined as:(2)


E = Units = M^1 cm l
2303

So, for a dilute solution, the absorption of light can be described using Beer' s law:


(1-8)





(1-9)


--cl .
2303


A = ECl or A









In ultrafast experiments, shot-to-shot laser fluctuations can hinder the detection of small

transient signals. To overcome this limitation, a shot-to-shot normalization is utilized by having a

second beam (reference) transmitted through the sample without overlapping with the pump. For

transient absorption, the change in transmission (AT) is defined as the transmission of the probe

in the presence of the pump (It, pum) minus the transmission of the probe in the absence of the

pump (It~., n pm). The normalized change in transmission is given by:


t, pump/ /opm
AT Tpump Tnopump leernc referenc
T nopump Jt,nopump/I(-0




this can be converted to changes in absorption using Eq 1-11.

-AT
AA =l g 1 (1-11)


There are multiple types of signals that can be observed in data collected using this

technique. In Figure 1-3 the possible transitions associated with particular changes in the

absorption spectra are shown.

Bleach (1) The ground state of the sample will absorb photons creating a bleach of the

ground state. When the pump is on, the depleted ground state will absorb less photons, and the

measured AA will be negative. If measuring transmission, the probe will transmit more, creating

a positive change in AT/T. This bleach signal will appear at transitions observed in the steady

state absorption spectrum. A bleach signal appears instantaneously after the excitation.

Photoinduced absorption (2) If the photoexcited state absorbs a photon, the beam

probing that state will be attenuated, creating a positive change in absorption and a negative

change in transmission.









Stimulated emission (3) The pump will excite a population to some higher lying states

and the probe will stimulate them to emit. If measuring the change in transmission, this signal

will be positive since more light is apparently being transmitted through the sample. If measuring

AA, since the probe is stimulating emission it corresponds to a negative absorption. A stimulated

emission signal might not appear instantaneously, since the relaxation of the initially excited

state will occur before stimulated emission. Although stimulated emission can appear at

wavelengths other than the steady state absorption spectrum, it can be difficult at times to

distinguish between the bleach and stimulated emission signals due to overlap of the absorption

and emission positions.


8 A




meamammm
t=


- A~

p p
t


O


Steady A
State



Time aAA
Resolved


Figure 1-3. Signals in transient absorption measurements


A A<



SAo ee


LZ7 Excitation


t<0









CHAPTER 2
QUANTUM NANOPARTICLES

Overview

Continuous advancements in the synthetic methods for the production of colloidal

semiconductor nanoparticles of different size, shape, and composition have greatly improved

their process-ability and functionality. The effects that the physical features of these materials

have on their corresponding photophysics have been the focus of numerous scientific

investigations in hopes that the inherent characteristics of the new systems will make them useful

for applications extending from basic fundamental physics studies (8), photovoltaics (9),

optoelectronics (10-12) to photocatalysis.(13-15) Many researchers have employed ultrafast

time-resolved techniques to elucidate the dynamics within various colloidal semiconductor

quantum confined materials.(16, 17) These fundamental investigations are important for

understanding and influencing the direction and development of the area of nanoparticle science.

New and precise synthetic methods have provided the ability to control the size, shape, and

composition in order to manipulate the electronic or optical properties of nanoparticle materials.

In 2004, we started collaborating with Professor Paul Holloway's research group to focus

on the study of exciton dynamics in various semiconductor nanomaterials. A member of Dr.

Holloway's group, Dr. Hyeokjin Lee, had synthesized CdSe, CdSe/ZnSe core/shell, ZnCdSe rods

and ZnCdSe dots. Aside from CdSe, (13, 16-20) the literature lacked any information concerning

the dynamics within these materials; it was an excellent opportunity for our lab to conduct

cutting edge research in this area of materials science.

Bulk vs. Quantum Semiconductors

The primary bulk semiconductors used in solid-state electronic applications include

materials such as silicon, gallium arsenide and cadmium selenide.(21) The relationship between
















Atomic
Diatomic
Hybrid Orbitals





s Antibondin




Bonding


the created exciton and electronic properties is of significant interest for the design and

engineering of useful bulk and nanoscale semiconductor materials. Consider a summary of the

band theory of solids presented in Figure 2-1.


Mol. Bulk Quantum
)rbitalsl Solid Solids

,UMO CB" IC



iOMO VB g


Discrete States Density of States Discrete States

Figure 2-1. Band theory of solids

Silicon has four sp3 hybridized atomic orbitals. Neighboring atoms contribute orbitals

which combine to form highest occupied molecular orbitals (bonding orbitals, o) and lowest

unoccupied molecular orbitals (antibonding orbitals, o*). The total number of occupied and

unoccupied orbitals is equal to the number of atomic orbitals present within the crystal. As more

atoms are added, a density of orbital energies develops reducing the spacing between the states in

each band. This increase in density results in a continuum of energies separated by a gap. In a

bulk solid, the highest occupied orbitals form the valence band and the lowest unoccupied

orbitals form the conduction band. The minimum energy required to excite an electron from the

top of the valence band to bottom of the conduction band is the band gap energy of the

semiconductor (E,).(21)









The electronic and optical properties of a material are due to electron motion within

molecular orbitals. The energy absorbed by an unbound electron (not confined) within the

density of states in a bulk material is not quantized. Therefore, the energy released by this

electron is converted into kinetic energy. A semiconductor can be photoexcited with a photon,

exciting an electron from the valence band into the conduction band of the material, leaving a

hole of opposite electric charge behind, separated by distance consisting of several atoms within

the material. These distances are within the nanometer scale and are called the Bohr exciton

radii. This radius, combined with a high dielectric constant results in a small binding energy. The

electron can be bound to the hole due to Coulombic forces and if these interactions are strong

enough, a Coulomb correlated, bound 'quasi-particle' called an exciton (electron-hole pair) is

formed. If the size of the electron-hole pair is approximately the same as the Bohr radius, and it

is larger than the lattice spacing within the crystal, a Wannier-type exciton is formed. This

exciton can diffuse through the material until it is trapped, annihilated (under multi-excitation

conditions) or recombined. If the wavefunctions of the electron and hole extend over a large

number of atoms, the Coulombic attraction becomes negligible resulting in unbound charge

carriers which have slightly higher energies than the bound electron-hole pair. (13, 21, 22)

If the size of bulk semiconductor is significantly decreased to the point where is it similar

to the size of the Bohr exciton radius, then the motion of the exciton will become confined in

multiple dimensions (quantum confinement) since it will have less "room" to move. The energy

spacing between the various confined (bound) electron and hole states within the corresponding

bands becomes quantized and the separation between these energy states will increase as the size

of the particle (space) decreases due to stronger confinement.(13, 17, 21, 22) In addition, the

energy separation of the electron states is larger than the separation of the hole states since the








hole has a larger reduced mass compared to the electron and the density of states in the valence

band is larger than in the conduction band. Overall, due to quantum confinement, as the size of

the semiconductor nanoparticle decreases, the band gap increases resulting in changes in the

absorption and emission colors.(13, 22, 23) (Figure 2-2)




CB rrr
CB

CB



E E.

VB
VB
VB

Figure 2-2. Nanocrystal band gap size dependence
Just as in most quantum systems, there are multiple attempts to describe the electronic

states mathematically. One particular way, used in quantum confinement of quantum dots,

assumes that the quantum dots themselves are larger than the lattice constants of the crystal

structure, which implies that the effective mass of the charge carriers remains unchanged despite

the difference in size of the quantum dot compared to bulk. This is known as the effective mass

approximation (EMA) and is utilized by most researchers in this area.(22-25) Since, the effective

masses of the carriers are considered to be constant; any modifications to the optical properties

of the quantum dots observed will be due to quantum confinement. II-VI and III-V semi-

conductor quantum dots are considered to be in the strong confinement regime because their

dimensions are generally larger than the lattice constant but less than or equal to the Bohr radius









size (1 1.2 nm for CdSe). Strong confinement of the electronic wavefunctions results in an

increase in the Coulomb interactions between the electron and hole. If the materials are larger

than this radius but smaller than their bulk counterparts, they are considered to be within the

weak confinement regime.(24) Nanowires, are a perfect example of materials in this regime due

to the extension of the c-axis and smaller confinement potential in that direction.

Considering EMA and the augmentation of Coulomb interactions between electrons and

holes, excitons within nanometer sized semiconductors can be compared to motion of a particle

in a 3-D box; as the size of the box decreases, the kinetic energy and excitation energy increases.

Figure 2-2 depicts how the quantum dot band gap varies as the size of the dot (box) changes. The

enhanced exciton confinement within smaller dots increases the amount of energy necessary to

promote an electron to the conduction band, overcoming the band gap barrier.

The Schroidinger equation is used to consider the energy of the electronic states in quantum

dots :


H Y(r) =EY(r) (2-1)


where H is the hydrogenic Hamiltonian for a Wannier-type exciton:(21, 26)

A2V A2V e2
H = h-(2-2)
2 n? 2m n? he h~

where me and nmh represent the electron and hole mass respectively, the distances between the

electron and hold from the center of the quantum dot are re and rh, and E is the dielectric

constant of the semiconductor. Since the center of mass and reduced mass motions cannot be

separated into independent coordinates, analytical solutions for Eq. 2-1 and 2-2 are impossible.

Therefore, different methods such as perturbation theory (26, 27) or a variational calculation (28)

are utilized to describe energy in quantum systems resulting in the following equation:(28)










A r 1 1 18
Em =2i -+- --- .82 0.25E (2-3)
m 2R m ;,e Ryd

Due to the quantum dot size dependence resulting in blue shifts as the dot size decreases, this

equation evaluates the adjusted quantum dot band gap, Emm,, with respect to the quantum dot

radius, R and the bulk exciton binding energy, E rd.(26-28)

As a result of quantum confinement properties exhibited by semiconductor nanoparticles,

their electronic states are discrete and well-defined; therefore, their electronic states can be

described in an atomic-like fashion. Three quantum numbers including spin are derived from the

Schroidinger equation (Eqn 2-2) to evaluate the electronic states of the quantum dots. Using the

effective mass model,(25) the electron and hole state notation is nLe and nLF, TOSpectively where

n is the principal quantum number (1, 2, 3, etc), and L is the envelope wave function angular

momentum (S, P, D, etc) used to distinguish energy states of the electrons and holes.(23) The

hole total angular momentum, F [for a value of F, the state is (2F+1)-fold degenerate], where F

=2L+S and S represents spin.(1 7) In CdSe, the valence band is six-fold degenerate if the spin is

considered since this band originates from p-atomic orbitals from within the selenium atoms.(25)

The fine structure of the lowest exciton state within the valance band can be revealed (29) if the

nanocrystal is non-spherical(30) or the crystal field effects(23) and exchange interactions(29) are

considered.(17) It is well known that within CdSe quantum dots, the three lowest electron energy

states are 1Se, 1Pe, and 1De and the first three hole states are 1S3 2, 1P3 2, 2S3 2.(25) Thus, the

three lowest energy bands in ideal CdSe quantum dots are labeled as 1S [1S(e)-1S3 2(h)], 2S

[1S(e)-2S3 2(h)] and 1P [1P(e)-1P3 2(h)]. It is possible to disrupt the ideal selection rules for

spherical quantum dots, An=0, AL=0, a 2 and AF=0, 1, (31) via strong hole-state mixing (31) or

by breaking the symmetry, which alters the degeneracy and splitting of the excited states of the










semiconductor quantum dots, ultimately changing the behavior of the excitons.(1 7) The

absorption spectra of five colloidal CdSe nanocrystals with different radii (1.2, 1.7, 2.3, 2.8, and

4.1 nm) is shown in Figure 2-3 (17) and illustrates not only the quantum dot band gap

dependence but the features corresponding to the optical transitions that arise from the coupled

electron and hole electronic states previously discussed.(16-19)



R = I.2 nm (6 = 4 4%)


1.7 nm lr


10 2.3 nm (6%)












4 ~e~ -7I:h



O f ISe)- 2S.~Ilt IP(e) IP3,2h)
0 ~ e- : ISlc)25.,

2.0 2.2 2.4 2.6 2.8 3.0 3.2
Photon energy leV)

Figure 2-3. Absorption spectra of TOPO/TOP passivated CdSe nanocrystals with radii from 1.2
to 4. 1 nm.(1 7)

Size and Shape Dependence

Quantum confinement or the "quantum size effect" is a property that in recent years has

revolutionized the semiconductor industry. This effect leads to unique electronic and optical

properties making quantum dots differ from their bulk counterparts. II-VI semiconductor









quantum dots have been the focus of several photophysical studies (18-20) stimulating interest in

other types of quantum particles. Fabrication of nearly spherical particles with various

compositions in addition to synthesis of rod shaped (32) and even multifaceted tetrapods (33) has

been achieved. Manufacturing such materials can be achieved by two different methods: 1.

bottom-up and 2. top-down. The first method utilizes synthetic routes that adjust ratios of the

chemicals needed to make the nanoparticles with passivation or capping materials.(33-42) In the

latter, the bulk semiconductor is "cut down" to scale using laser ablation-condensation or

lithographic techniques although these methods are extremely expensive.(43) The materials

presented in this thesis have been prepared by a "bottom-up" approach in an attempt to

synthesize better materials while enhancing their process-ability.

Dependence on the sensitivity to size and shape is important when considering the tunable

optical properties of quantum nanoparticles. In particular, the size dependence and

photoluminescence tunability in the visible region of CdSe quantum dots has been studied

extensively.(37, 44-48) New methods for synthesis of rod-shaped CdSe nanoparticles have

opened the door for shape-dependent applications such as polarized LEDs (49, 50). In particular,

Alivisatos' group synthesized quantum confined colloidal nanoparticles with rod-like

architectures by using various surfactants that bind to different faces of the crystal.(51) For

example, colloidal CdSe rod lengths can be varied from 5 nm up to 100 nm while maintaining a

2 to 10 nm diameter, which preserves lateral confinement of carriers in the nanocrystal.

Alivisatos determined that the band gap depends mainly on the width (a or b axis) and slightly on

the length (c-axis) (Figure 2-4). (52, 53) However, a comparison of the dynamics within CdSe

dots and nanorods has proven to be useful in understanding the electronic structure differences

that occur when the c-axis is elongated.









El-Sayed et al. synthesized and compared CdSe rods with aspect ratios ~ 3 (length/width)

and dots of 4.2 nm diameter.(53) TEM images show that the particles are different although the

steady state absorption spectra do not indicate significant differences. Electron-hole dynamics

measured by femtosecond pump-probe, although still not completely understood, show quite

different behavior for rods versus dots. This is confirmed by the increase in the number of bands

in the deconvoluted absorption spectra of the quantum rods.(1 7, 18, 53) Moreover, they observed









Traps Front
Zoomed
View

Figure 2-4. Drawing of a nanorod with each of the axis labeled. The front zoomed view shows
that the surface curvature is not smooth, leading to surface traps.

a significant increase in the carrier relaxation time in the quantum rods compared to the quantum

dots.(53) Nanodots have a higher order of symmetry, which is lost in the rods. Extension of the

c-axis results in a splitting of the degenerate level in the symmetric quantum dot (30, 53-60) and

that energy level splitting could be one reason for El-Sayed' s results. Due to the large surface-to-

volume ratio at the surface in nanorods, electron and holes have a high probability of being

trapped by surface impurities. However, the quantum dot curvature can create a larger number of

localized surface trap states than the elongated nanorods,(53) which allow for the carriers to have

"free" motion in the c-direction, reducing the probability of the carrier to be trapped as quickly.

In some cases, the impurities present enable the materials to be used in oxidation-reduction

chemistry, more specifically photocatalysis, (15) photodegradation and detoxification of









chemical and environmental pollutants.(61) For optical applications such as photovoltaics or

LEDs, it is important that these surface traps do not contribute to the exciton trapping within the

material. Several groups have worked on developing passivation techniques that will enable

enhancement of their photophysical characteristics without altering their confinement behavior.

Passivation

Modiaication of semiconductor nanocrystal surfaces plays an important role in their

electronic and optical properties and has been the subj ect of extensive investigations. (34, 40, 62-

65) The dangling bonds present on the surface of the nanocrystals negatively influence the

optical properties but passivation has been proven to improve various confinement properties

such as high quantum efficiency and luminescence stability. Due to a high surface-to-volume

ratio, even pristine, bare CdSe quantum dots tend to result in low luminescent yields (0.6%) and

poor stability.(66) The ratio leads to augmentation of the electron and surface state

wavefunction overlap which creates localized midgap surface state traps resulting in non-

radiative decay and decreasing the overall photoluminescence quantum yield (Figure 2-4).(67)

However, in certain applications (68), in which the charge carrier-interface interaction is crucial,

the high surface-to-volume ratio is beneficial. (14, 15, 69)

If the surface of colloidal nanoparticles is coated with an appropriate passivating agent,

e.g., organic molecules, this competition may be sufficiently reduced to dramatically extend the

band-edge lifetime and enhance the luminescence efficiency. (37, 53, 64, 70) However, due to

several drawbacks including imperfect surface passivation and exchange reactions causing

photodegradation, organic coating is not sufficient for improving quantum yields.(16, 71)

Using the diffusion-controlled colloidal growth method developed by Bawendi and co-

workers, (37) CdSe quantum dots have been passivated with various shells, among these is

ZnS,(39) which narrows the fluorescence emission and improves their efficiency. Epitaxial










overgrowth of a higher band gap inorganic shell (Figure 2-5) creates a "step" to confine the

exciton to the core.(39-41, 65) This increased confinement has been employed to enhance the

quantum yield of the dots to over one order of magnitude and to increase its stability against

surface oxidation. (34, 38, 72) Various examples using ZnS, CdS and ZnSe as shell layers

include CdSe/ZnS (39, 40), CdSe/CdS (11), CdSe/ZnSe (73), CdS/ZnS (34) and InAs/ZnSe (74).

Passivation using multiple shells (34, 70, 75) or "onion like" structures (76) has also been

achieved.

It has been observed that the absorption and emission of a nanocrystal that is passivated

with a ZnS shell exhibits a shift to longer wavelengths by approximately 10 to 20 nm as

compared to the unpassivated core.(39, 40, 66, 77) Dabbousi et al. explained this phenomenon

by considering charge carriers in a spherical box. The observed shifts to lower energies result

from the tunneling of the lighter electron wave function into the shell while the hole remains in

the core. If this happens, the exciton is "delocalized" in the particle resulting in decreased

confinement and excited state energy. For the electron to be able to penetrate into the shell, it

must be able to overcome the valence band offset (barrier height), the energy difference between

the valance band of the core and the valence band of the shell, that is present between the core

and shell. If this offset is small, the shifts towards lower energies can be large.(39, 66) For

nanoparticles passivated with an inorganic material, a critical thickness is present that is

dependent on the size of the core and lattice mismatch between the core and shell. (34, 38, 72)

This thickness influences the ability of the electron to tunnel to the surface potentially resulting

in little to no shifts in band gaps or confinement potentials compared to unpassivated particles.

However, it is possible to surpass this thickness allowing the electron to tunnel into the shell

layer which can decrease the overall quantum yield and shift the absorption and emission










spectrum to lower energies.(72, 78) This is an important factor in the analysis of our CdSe/ZnSe

core/shell nanorods which will be discussed in Chapter 3.

organic
molecule









offset *


band ~~t~~as~ ~ hN
offset**



A) B)

Figure 2-5. Electronic potential step of valence and conduction bands, HOMO and LUMO levels
of A) inorganic core and B) inorganic core/shell nanocrystals, both with surface
attachment of organic molecules. Adapted from H. Lee.(66)

Atom dislocations induced by interfacial strain as a result of the lattice mismatch between

the core and shell can also have a negative effect on the luminescence quantum yield because

they can behave as sites that cause non-radiative recombination. The defects that arise from the

core/shell interface can be reduced resulting in higher quantum yields by either growing a

nanocrystal that is comprised of one core and two shells, such as a CdSe core passivated with a

CdS/ZnS shell/shell structure (34, 66, 79) or by "photoannealing"(34, 66) which reduce strain or

diffuse defects to the surface, respectively. For example, irradiating CdSe/ZnS core/shell

nanocrystals with UV light caused an increase in the photoluminescence quantum yield by

reducing the number of vacancies present at the interface.(34, 66)










Composition Changes: Interdiffusion

It has been an on-going goal to develop synthetic methods to produce highly luminescent

quantum confined materials with increased stability in the blue-green spectral region. Therefore,

some focus on synthesizing binary or core/shell materials has shifted to mixed ternary

heterostructures. This would allow for an extra degree of freedom (size and composition) to

achieve particular confinement characteristics, such as photoluminescence tunability in fewer

synthetic steps. If the cations in the shell were to exchange with the cations in the core

(interdiffusion), the optical properties can be changed significantly. More specifically, changes to

the energies of the valence and conduction bands, band gap and confinement potentials are to be

expected.

Temperature can influence the rates of chemical reactions and can be described using an

Arrhenius equation. In solid state, diffusion is thermally activated thus the diffusion coefficient,

D, can be determined using Eq. 2-4.(80)


D = D,e (2-4)

E4 represents the activation energy and Do is the diffusion coefficient when the temperature is

considered to be infinite. (80) Since temperature influences diffusion in solid state materials it is

important to determine the optimal conditions that will produce the desired ternary

heterostructure when alloying a core/shell material. This poses a problem for II-VI band gap

materials since the experimental values for D, and E4 for interdiffusion are limited.

Several groups have determined experimental values for diffusion lengths in bulk and

quantum well structures. For example, Martin (81) investigated the diffusion lengths for Cd

diffusion into ZnSe that were annealed for 1 hour at temperatures in the range of 300 to 550oC

and concluded that the optical properties, such as photoluminescence, should exhibit changes at









temperatures as low as 350oC for a diffusion distance required for one monolayer of CdSe (~0.3

nm).(66) ZnSe/CdSe and ZnS/CdS superlattice structures investigated by Parbrook et al.

exhibited diffusion at annealing temperatures greater than 400 and 450oC rather than below

400oC.(66, 82) The negligible changes observed in the range of 340oC~400oC are further

confirmed by investigations to determine diffusion lengths for Cd in ZnSe/CdSe quantum wells

by Rosenauer et al.(66, 83) and Strapburg et al.(66, 84) If the same experiments are conducted

in nanocrystals the diffusion behavior is quite different compared to quantum wells or bulk

materials; the alloyingg point" (the temperature at which the core/shell nanocrystals begin to

alloy (48)) in nanocrystals has been shown to occur at lower temperatures. For example, Zhong

et al.(48, 66) observed alterations to the band gap and blue shifts in the photoluminescence in the

range of 270~290oC in colloidal CdSe/ZnSe core/shell nanoparticles.(66) Additionally, at

temperatures greater than 290oC, the alloying process can take as little as five minutes to

complete.(48) There are several reasons that can account for the smaller alloyingg point"

temperature. Recall that as the size of the nanoparticle decreases, the surface-to-volume ratio

increases resulting in a large number of atoms being exposed to the surface or located in the

interfacial region between the core and shell; therefore, diffusion at lower temperatures

compared to bulk materials can be expected. Diffusion in colloidal nanoparticles can be also be

aided by the desire for the surface atoms to minimize their energy by reorganizing to reduce their

surface area.(22, 66) The interface between the core and shell can have imperfections or defects

that will also increase the diffusion rates. However, these imperfections can lead to a decrease in

the photoluminescence quantum yields. Finally, diffusion can be influenced by the crystal field

strength of the nanoparticle. The crystal field strength will scale with the size of the nanoparticle;

therefore, bulk crystals have larger crystal field strengths than quantum particles. Based on this









principle, the interaction between atoms "far" from one another in nanocrystals is weak which

will reduce the activation energy ultimately enhancing the diffusion in these nanomaterials.(66,

85, 86)

In our samples, we were able to achieve interdiffusion of Zn from a ZnSe shell into a CdSe

core by alloying at a temperature (270~290oC) determined to be effective by Zhong et al.(48)

Despite the fact that ZnCdSe quantum dots have been grown on ZnSe (87) and GaAs (88)

substrates to determine how radius affects the fluorescence lifetime, little has been published on

the compositional affects on the dynamics of colloidal ZnCdSe nanoparticles.

From TEM images we observe that diffusion of Zn into the CdSe core does not change the

shape of the rods significantly and the single phonon mode observed by Raman backscattering

indicate that the ZnCdSe materials are complete quantum rod alloys, not composites.

The differences that arise in both steady state absorption and photoluminescence in

addition to time-resolved photoluminescence measurements make investigating these ternary

systems using ultrafast techniques extremely appealing (Chapter 3). In the current chapter, the

synthesis, characterization, steady state photophysics and time-resolved photoluminescence

measurements are described to begin to explain the carrier relaxation within CdSe, CdSe/ZnSe

core/shell and ZnCdSe alloy quantum rods.

Experimental Methods: Nanorod Synthesis and Composition Characterization

The synthesis, XRD, TEM, Raman, and some optical characterization of each of the

materials studied in this dissertation were carried out by Dr. Hyeokjin Lee in the Department of

Materials Science at the University of Florida.

Preparation of ZnCdSe Nanorods

CdSe nanorods were synthesized using the method described by Peng. (46) In this method,

CdO, trioctylphosphine oxide (TOPO) and tetradecylphosphonic acid (TDPA) were heated in a









three-neck flask on a Schlenk line under a N2 atmosphere to 350oC while stirring. After the

solution became optically clear, it was cooled to room temperatures. The solid Cd-TDPA

complex was used after aging for 24 hr without further purification. This Cd-TDPA complex was

heated in a three-necked flask under a N2 atmosphere to 280oC while stirring, and selenium

dissolved in trioctylphosphine (TOP) was inj ected quickly. After inj section, the temperature of the

mixture was kept at 250oC for the 30 min growth of CdSe nanorods, and then cooled to 180oC.

(42)

For shell growth, ZnO was dissolved in oleic acid (Zn-oleate) at 350oC and cooled to room

temperature, and then TOP was added to prevent solidification. In addition, Se was dissolved in

TOP (Se-TOP). The Zn-oleate and Se-TOP solutions were mixed by stirring for ten minutes at

room temperature, and this mixture was loaded into a syringe and inj ected drop-by-drop into the

reaction flask over 1.5 hr. After inj section was complete, the solution was stirred at room

temperature for another ten minutes. For alloying, the reaction vessel was heated with stirring to

270oC for up to 3 hrs. After heating for 1, 2 or 3 hrs, a sample was immediately cooled and

diluted with toluene to stop alloying, then was precipitated with methanol/toluene co-

solvents. (42)

Steady State Instrumentation

High-resolution transmission electron microscope (HR-TEM) images were collected using

a JEOL 2010F microscope for imaging and direct determination of the average and distribution

of the nanorod dimensions. To prepare TEM samples, the nanocrystals were dispersed in toluene

and deposited onto formvar-coated copper grids. X-ray diffraction (XRD) patterns were obtained

using a Philips APD 3720 X-ray diffractometer and used for determination of both the crystal

structure and size. Raman spectra were measured at 300K in the backscattering geometry, using









the 532 nm line from a Verdi 8 doubled Nd-YAG solid state laser in a Ramanor U-1000 Jobin-

Yvon Raman spectrometer.(42)

Absorption spectra were collected with a Shimadzu UV-2401PC spectrophotometer.

Photoluminescence was measured at room temperature using nanorods suspended in toluene

using a Fluorolog Tau 3 spectrofluorometer (Jobin Yvon Spex instruments, S.A. Inc.). The

photoluminescence quantum yield was determined using Rhodamine 6G organic dye standard.

(42)

Time-Resolved Photoluminescence Instrumentation

Relaxation processes of colloidal nanocrystals were explored using time-resolved

photoluminescence. A commercial Ti-Sapphire (Ti-Sa) laser system consisting of a Ti-Sa

oscillator (Tsunami, Spectra-Physics) and subsequent amplifier (Spitfire, Spectra-Physics) with a

repetition rate of 1 k
parametric amplifier (OPA) to generate excitation pulses. For this experiment, since 400 nm is at

the limit of both the signal and idler, we must use the second harmonic of the amplifier (800 nm)

to achieve stable and high energy pulses. The residual 800 nm is directed to a horizontal BBO

crystal (output of the Spitfire is polarized in the horizontal direction). A general schematic is

provided in Figure 2-6. The second harmonic (400 nm) is then fed through a prism compressor,

resulting in pulse lengths less than 100 fs (FWHM). The excitation beam is focused to a diameter

of ~150 Clm at the sample position and its energy was set to~- 56 nJ yielding a fluence of 3 17

CLJ/cm2. The optical density of each solution was 0.075/mm at 400 nm. Sample solutions of

colloidal nanorods dissolved in toluene were placed in a quartz cuvette with a 2 mm path length

and continuously stirred to guarantee excitation of a new sample volume with every laser shot.

Broad band luminescence (grating range: 438 to 718 nm) from the sample was collected using a








2 in. lens and then focusing this into the entrance slit of a monochromator. Time-resolved

photoluminescence spectra were recorded with an intensified charge-coupled device (ICCD)
(Andor iStar coupled to a Shamrock 303i spectrograph) with a 4 ns gate. The 4 ns collection
window electronically scans to map the temporal evolution of the photoluminescence. The

exposure time at each time step is 0.5 seconds.

from OPA, delay stage


Pump

Mlonochromator
Lens


o



Figure 2-6. Time-resolved photoluminescence
The standard mechanical shutters commercially available are unable to gate at ultrafast

speeds. Instead, the image intensifier found in the ICCD acts not only as an amplifier but as an
electronic gate, opening and closing on a nanosecond timescale. There are three maj or

components of an image intensifier: photocathode, microchannel plate (MCP) and phosphor
screen. The limitations of each of these determine how well the intensifier can perform. The
incident image is first captured by the photocathode which subsequently emits a photoelectron
and is then pulled to the MCP by an electric field. A high voltage is applied to the MCP causing

the photoelectron to ricochet along the channel walls creating an avalanche of secondary
electrons which exit the MCP as an electron cloud. Typical intensifications can be as high as









10,000. An additional voltage forces the electron cloud to hit the phosphor screen located at the

front of the fiber optic exit window. The voltages between the photocathode and MCP can be

controlled in a manner so that the image intensifier can be quickly turned off and on, effectively

creating an electronic gate.(89, 90)

As photons hit the surface of the CCD sensor, electrons are generated which are stored in

individual pixels. The maximum number of electrons that one pixel can accumulate during

integration is considered to be the full well capacity. A 16 bit analog to digital output converter

which is capable of digitizing 65,536 levels (216) Of light is used to read out the pixels.(89, 91-

93) The dynamic range is the number of steps or levels of light intensity that can be represented

per bit.(93) Without using an image intensifier (gain), the CCD dynamic range (maximum and

minimum signal intensities that can be measured simultaneously) is defined as the full well

capacity per pixel divided by the read noise.(89, 90, 92, 94) For this system, the full well

capacity per pixel is 300,000 electrons and the read noise is 4 electrons resulting in a potential

dynamic range of 75,000 to 1. This value exceeds the upper constraint of the digitizer so the

dynamic range is instead limited to 65,000 to 1. Dynamic ranges can vary since the read noise

depends on the read out rate. If the read out is fast, the read noise can be high and the dynamic

range can be low or vice versa. A slower read out rate will reduce the read noise (high read noise

will affect the quality of the image). Depending on the application, cameras that have a high well

capacity and low read noise (high dynamic range) in addition to a large analog to digital

conversion capability are optimal.(90, 92) The response of this camera is considered to be linear

(1) within its full dynamic range.(89)

The dynamic range of the CCD serves as the base dynamic range of the ICCD camera

system. As gain is added, the dynamic range is reduced.(90) For example, if the gain is set to 50










(counts/electron) a multiplication factor is employed which reduces the dynamic range from

65,000 counts to 13 10 counts (65,536 divided by 50). Gain can be advantageous if used in

appropriate amounts. For instance, the read noise produced by the CCD section of the camera is

no longer an issue. However, the dark current that is created thermally by the photocathode prior

to the amplification stage is still present and can also be amplified when gain is applied. Thus,

even though high gains will lead to enhanced signals, the noise is also increased. It is also

possible to have too little gain, which can sacrifice the well depth with no significant signal

amplification. A balance between the system gain and dynamic range is necessary to achieve the

best signal to noise ratio. Once gain is applied, the response of the CCD remains linear within its

dynamic range; however, the signal to noise equation is changed by the gain noise factor of

1.4.(89-92, 94)

Results and Discussion

Synthesis of ZnCdSe Nanorods

Combinations of surfactants such as TDPA and TOPO are generally used to prepare

nanocrystals since they have strong binding energies that ultimately raise the surface energies of

a crystal face compared to another. (42, 95, 96) Previously, Zn-TDPA and Cd-TDPA in a TOPO

solution were utilized in order to synthesize ZnCdSe nanorods. However, this method was not

successful, which most likely resulted due to the different reactivity with Se-TOP leading to a

lack of crystallite shape control.(42, 73, 97) It is also suggested that the temperature be higher

and reaction time be longer in order to promote a more thorough complexation of ZnO with

TDPA. When synthesizing the ZnCdSe ternary heterostructures, it is important to first prepare

CdSe nanorods. Once the CdSe rod has been grown, a Zn-Oleate and Se-TOP mixture was used

to grow the shell overtop the core. Zn-oleate and Se-TOP mixture was slowly added to prevent

homogeneous nucleation. It is extremely important that the temperature be controlled properly.









When the temperature was > 210oC, the emission blue shifted because the shell began alloying

causing a blue shift in the emission. If the temperature was kept too low, for example <170oC,

only a small shell grew because the Zn-oleate complex reacted too slowly with TOP-Se to grow

a shell, resulting in very weak emission. Finally, diffusion of Zn from the shell to the core is

instigated by raising the temperature slowly to 270oC to form ZnCdSe alloys.(42)

Structure of ZnCdSe Nanorods

X-Ray diffraction patterns for hexagonal CdSe, CdSe/ZnSe, and ZnCdSe nanorods are

shown in Figure 2-7.

(0 02) (1 10) (1 03) (1 12)
(1 00)
(101)





B)I




A)

20 30 40 50 60
2Theta

Figure 2-7. Powder X-ray diffraction patterns of A) CdSe nanorods, B) CdSe/ZnSe core/shell
nanorods, and C) ZnCdSe alloyed nanorods. Adapted from H. Lee.(66).

The crystal structure for this series of rods can be extracted from this experiment. In each of the

materials measured, the (002) diffraction peak is not as broad as the (001) diffraction peak. The

(002) peak is assigned to the plane that is perpendicular to the extended c-axis in rod-shaped

materials. The lattice spacings for CdSe, CdSe/ZnSe, and ZnCdSe were 7.01 A+, 6.94 A+, and 6.77

A+, respectively. These values are extremely interesting since in CdSe/ZnS nanoparticles the ZnS









shell exhibits an 11% smaller lattice parameter causing the core to be compressed, whereas the

lattice parameter in the c-axis for the core/shell material was ~ 1% smaller compared to the

CdSe. After interdiffusion of Zn into the core, the lattice parameter and the lattice mismatch

strain are reduced due to a lattice contraction. Also, after addition of Zn into the core, the

diffraction peaks shifted to a larger 26 indicating a smaller interplanar spacing.(38, 42)

A HR-TEM image of the ZnCdSe nanorods is shown in Figure 2-8. The diameter and

lengths of the nanorods measured from such images has been included in the histogram.
















20 nm scale bar 5 nm scale bar

Figure 2-8. HR-TEM image and histogram of size distribution of ZnCdSe nanorods. Lattice
fringe from a nanorod is shown in the lower right corner. Adapted from H. Lee.(66)

From this graph, the average diameter is ~6 nm and the average length is ~13 nm resulting in an

aspect ratio equal to ~ 2. 1 nm for the alloys.(42, 66) When using XRD, the diffraction patterns

can exhibit broadening effects due to particle size. Using the Debye-Scherrer formula, the

average crystallite size in A can be determined:


D, (2 -5)
/7 cos B

Where k is a correction factor to account for particle shapes, and P is the observed width at half

the maximum peak intensity and 6 is the Bragg angle. It must be noted that the observed width









includes additional sources of broadening, arising from the experimental setup and

instrumentation.(98, 99) Using Eq. 2-5, the particle sizes of ZnCdSe nanorods calculated were a

diameter of 5.5 nm and a length of 1 1.8 nm resulting in an aspect ratio of ~2. 1 nm, which agree

well with HR-TEM data shown in Figure 2-8.(42, 66)

Effect of Alloying on the Phonon Spectra

The compositional changes to the structure of a material that arise when adding a shell or

alloying by diffusion through the dependence of the phonon frequencies have been studied using

Raman spectroscopy.(42, 100, 101) The Raman peaks detected from CdSe nanorods are shown

in Figure 2-9 A. The peak at ~206 cml is from the CdSe LO phonon (42, 101, 102) which is 4

cm-l shifted compared to the bulk CdSe (210 cm- ) which is due to the quantum confinement of

the optical phonons in the nanorods.(42, 100-102) A broad "shoulder" (~180cm- ) appears to the

left of the main mode which arises from the non-spherical geometry of the CdSe nanorods. (42,

103, 104)

The Raman peak for CdSe/ZnSe core/shell nanorods is shown in Figure 2-9 B. The

original CdSe LO phonon mode is still detected with the addition of the ZnSe shell mode at ~247

cm- ). A new interfaciall ZnCdSe" is also detected and corresponds to a frequency ~23 5 cml

The small, unresolved Raman peaks on either side of the CdSe phonon mode can be assigned to

isolated atom-impurity modes when Zn and Cd atoms interchange with one another (Zn in CdSe

~190 cm-l and Cd in ZnSe ~218 cm )~. (42, 105)

The effects of alloying time (1, 2 or 3 hrs at 270oC) on the Raman spectra are shown in

Figure 2-10. After alloying, one mode is present at ~223 cm l, ~228 cm-l and ~226 cml (1, 2 and

3 hrs, respectively), which is similar to the interfacial layer observed in the core/shell material

and the one phonon-mode behavior for bulk ZnCdSe. (42, 106, 107)















A)











150 200 250
Raman shift(cm l)






B) mOCdSe


CO C

C U. O ZnSe
i ZnCdSe
E *






150 175 200 225 250 2.'5
Raman shift(cm-1)

Figure 2-9. Raman spectra of LO phonon mode of A) CdSe nanorods and B) CdSe/ZnSe
core/shell nanorods. Adapted from H.Lee.(66)





Odse LO pkaonn i


ZnSe LO phonon


C)


I Illill



180 200 220 2-0 260 260
Raman shift (cn?)

Figure 2-10. Raman LO phonon spectra of ZnCdSe nanorods after annealing at 2700C for A) 1,
B) 2, or C) 3 hrs. Adapted from H. Lee.(66)

Detection of only one mode that is only slightly shifted compared to the bulk in these

materials indicates that the interface between the CdSe core and ZnSe shell is no longer present,

implying that the material is in fact an alloy and not a composite. The narrow particle

distribution and uniform composition observed in the XRD is confirmed from the relatively

sharp single-mode peak. The broader peak observed after only one hour of alloying compared to

the 2 and 3 hour is primarily due to compositional disorder. As the alloying continues from one

to two hours, the Zn continues to diffuse resulting in a ~5 cm-l shift of the Raman peak. After 3

hours of alloying this peak shifts back ~2 cm-l due to compositional disorder (42, 108, 109) and

stress relaxation by thermal annealing(110).(42)










Photoluminescence and Absorption Properties

Significant differences can be seen in the absorption spectra for CdSe core, CdSe/ZnSe

core/shell and ZnCdSe alloyed nanorods. The absorption spectra of nanorods are shown in

Figure 2-1 1. It presents two absorption peaks on top of a broad absorption. These absorption

peaks correspond to confined states although they are not as sharp or as well resolved as peaks

reported for CdSe quantum dots.(17) Due to the loss of symmetry in nanorods, confinement

along the c-axis is not as strong as it can be in dots which results in a large distribution of energy

levels in the conduction and valence bands.(53, 111) In addition, the compositional disorder

indicated from the Raman data will lead to broader features in the absorption and emission

spectra. Therefore, it is difficult to determine the exact energy spacing between the first and

second absorption peaks. (42)
0.25


0.20 :


0.15-


S0.10-


0.05-
A)
0.oo C B

300 400 500 600 700
Wavelength
Figure 2-1 1. UV-Vis absorption spectra of A) CdSe nanorods, B) CdSe/ZnSe core-shell
nanorods, and C) ZnCdSe nanorods alloyed at 2700C for 3hrs. Adapted from H.
Lee.(66)

For CdSe core and CdSe/ZnSe core/shell nanorods, the absorption edge is at ~650 nm and

~645 nm, respectively. A second peak is observed at ~520 nm. These features correlate to optical










transitions involving the electron and hole quantized states.(22) Direct assignment of these

peaks is difficult and needs to be evaluated using the effective mass approximation. The

photoluminescence spectra from CdSe core and CdSe/ZnSe core/shell nanorods, with peaks at

642 nm and 63 8 nm respectively, are shown in Figure 2-12. With the addition of the ZnSe shell,

CdSe photoluminescence quantum yields increased from 0.6% to 15% due to passivation of non-

radiative surface states.(42) Increasing the shell thickness up to a critical thickness of an

inorganic shell with a higher bandgap has been shown to increase the photoluminescence

quantum yield in rods.(34, 38, 72) Defects at the core/shell interface due to lattice strain

relaxation from shells thicker than the critical value will actually decrease the quantum yield.(39,

42, 65, 72)
6x106


5x106
A)
4x106


v,3x106


2x106


1x106
B)

450 500 550 600 650 700 750 800
Wavele ngth(nm)

Figure 2-12. Photoluminescence spectra of A) CdSe/ZnSe core/shell nanorods and B) CdSe
nanorods. Adapted from H. Lee.(66)

It is intriguing that a 4 nm blue shift from core to core-shell emission occurs because

Mokari and Banin (78) have reported a~-10 nm red shift for CdSe/ZnS core/shell quantum rods.

They attribute this shift to tunneling of the electron wave function into the ZnS shell delocalizing









the electron, lowering the confinement energy and ultimately decreasing the energy of the

exciton levels.(112) Based on the Raman data presented, the formation of interfacial ZnCdSe

results in a decrease in size of the CdSe core (56) resulting in increased localization and a blue

shifted emission. This is further supported from alloy formation resulting in a blue shift of the

photoluminescence peak.(42)

The energies of the corresponding absorption features from alloyed ZnCdSe (3hrs at

2700C) is considerably blue shifted to ~555 nm and ~465nm (Figure 2-11 C). These features

originate from the states similar to those in the core and core/shell nanorods but with a larger

band gap due to the formation of ZnCdSe. Figure 2-13 presents the photoluminescence spectra of

the alloyed ZnCdSe samples. Upon annealing at 2700C the photoluminescence spectra shift to

higher energies. After one hour of annealing, the peak appears at 610 nm. Further annealing (2

and 3 hours) produces a much larger blue shift, 510 and 565 nm, respectively. This behavior is

consistent with the variation in composition indicated by the broad Raman peak (Figure 2-10). In

addition to the energy shift, the alloys present changes in bandwidth and intensity. As alloying

time increases, the width of the photoluminescence band is reduced. The change in intensity does

not follow a trend, with the 3 hour alloyed sample presenting a sharp increase in photo-

luminescence intensity. Quantum yield measurements are ~8, 5 and 10% for 1, 2 and 3 hrs,

respectively. These values are higher than CdSe rods (0.6%) but lower than the core/shell sample

(15%). Composition disorder in ternary alloy nanorods will lead to localization of excitons

compared to binary samples.(42, 113) Such localization effects are known to improve the

photoluminescence efficiency by increasing the overlap integral of the electron and hole

wavefunctions. On the contrary, the quantum yield values are lower compared to the core/shell

due to the lack of surface passivation on the ZnCdSe nanorods. This is consistent with the










quantum yield decreasing after 1 and further after 2 hours of annealing, since diffusion will be

reducing the gradient in Zn (i.e. reducing the high concentration at the surface and increasing the

low concentration in the middle of the nanorods). However, annealing for 3 hours increased the

quantum yield over that from samples annealed for 1 or 2 hours. This increased quantum yield

and full-width-half maximum (FWHM) reduction can be attributed to annealing of crystalline

defects and reduction of stress, consistent with the Raman data. Defects found in the crystal are

known to act as "traps", reducing emission efficiency(114, 115). (42)
6x1 06


5x106A)1
4x1 06


3x1 06 B) \


2 x1061



1x106



400 450 500 550 600 650 700 750 800
Wavelength(nm)
Figure 2-13. Photoluminescence spectra from A) CdSe/ZnSe core/shell nanorods and ZnCdSe
nanorods alloyed at 270oC for B) 1, C) 2, and D) 3 hrs. Adapted from H. Lee.(66)

Time-Resolved Photoluminescence (TRPL)

The linewidths of optical transitions can be inhomogeneously broadened due to effects that

act differently on different radiating or absorbing particles.(2) Emission from an inhomogeneous

population (different sizes, shapes or composition) leads to the simultaneous probing of particles

with different decaying rates. Monitoring time-resolved photoluminescence at different

wavelengths not only identifies the states emitting but also extracts their decay rates. The









formation of ZnCdSe alloys is not perfectly uniform and contributes to broad, inhomogeneous

photoluminescence spectra and a distribution of decay rates.

Figure 2-14 shows the broad band luminescence spectra of the core/shell and alloyed

nanorod samples as a function of time. This data was collected on a nanosecond time scale using

a 4 nanosecond instrument response and 0.4 ns time step. The fast decays (< 4 ns) are not

detectable due to this limitation. From this data we were able to extract time traces at the

maximum wavelength to determine the corresponding decay rates of each sample. Figure 2-15 A

shows the broad band luminescence of the core/shell sample at two different time steps, 4.4 ns

(black line) and 20 ns (red line). As the signal decays at the maximum wavelengths, the

photoluminescence values do not shift significantly but the broadening is slightly reduced.

Broadening also occurs since measurements of the nanorods samples were carried out at room

temperature. The time traces corresponding to three different wavelengths, 645, 670 and 630 nm

(black, red and green lines, respectively) for the core/shell sample are shown Figure 2-15 B. At

early decay times, the emission at different wavelengths is not the same but ends up being

identical after ~ 100 ns. Moving from the core/shell (A) to the alloy 3 hr (D) the band gap shifts

to higher energies and the broad band signal narrows.

Figure 2-16 shows the log plot of the time-resolved photoluminescence decay curves for

the nanorods samples. These decays are the normalized kinetic traces at the wavelengths

corresponding to the maximums of the photoluminescence. Several differences can be

highlighted. The signal at decay times less than four nanoseconds have to be deconvoluted with

the instrument response function. Since we are interested in extracting a characteristic lifetime

we do not consider this for analysis. It is seen from this plot that the core/shell decay (black line)

is much faster than the alloys. Also, the core/shell decay curve is almost a straight line,

















































II I I I I


I I


indicating a higher degree of homogeneity and a smaller distribution of decay rates compared to

the alloys. The alloys (green, red, and blue lines) do not show the same behavior as the

core/shell; instead, they exhibit two or more decay components (similar dynamics prior to 50 ns

but deviate after that). The data collection for alloy 3 hr (blue line) is shorter due to the lack of

detectable signal after 60 ns.


640

620

E 600

580


15 20


A)


,' 640


620


0 5 10 15 20 25 0 5 10
time (ns) time (ns)


C)


D)


0 5 10 15 20 25 0 5 10 15
time (ns) time (ns)

Figure 2-14. Broad band Photoluminesce of A): CdSe/ZnSe core/shell nanorods B) 1 hr ZnCdSe
C) 2hr ZnCdSe D) 3hr ZnCdSe

In dispersed systems such as polymers (116) or colloidal nanoparticles, it is easy to

believe that the relaxation behaves non-exponentially and that the large distributions of local

environments lead to variations of relaxation times. Multi-exponential functions are useful and

are more commonly utilized for fitting decay curves; however, a model is generally proposed


E









based on the number of exponentials and an extensive number of parameters required to fit the

decay curve resulting in exact decay lifetimes. This can become very complicated and can lead to

incorrect assignments of the photophysical processes occurring within the material. Jones et al.

found that they were able to fit their photoluminescence data collected from CdSe/ZnS core shell

quantum dot' s decays with a multi-exponential function, implying that there is an existence of

several discrete relaxation pathways, with individual lifetimes. However, they were not able to

claim the exact number and identity of such pathways.(117) For simultaneous measurements of a

large ensemble of relaxation times it is more advantageous to use a stretched exponential (non-

exponential) function to evaluate the distribution of relaxation times in such dispersive

systems.(43, 116) This type of equation encompasses both independent, single step processes in

addition to sequential multi-step processes.(116)


I(t) = Io exp -~l (2-6)


Where z is the characteristic lifetime and (0 < P < 1) is the dispersion exponent. Despite only

extracting an average lifetime from a non-exponential; the function provides a phenomenological

description that is considered purely empirical, fitting data with a minimum number of

parameters. These parameters can vary depending on the phenomenon of interest and external

variables such as temperature.(43) For the limiting case of P- 1, we get the single exponential

decay with the characteristic lifetime, z. For ideal, single quantum dots, we can expect P=1. It

should be mentioned that P<1 results from superposition of many exponential decays and as P

approaches zero, the distribution of decay times increases. This decay law can then be used to

compare different samples qualitatively in terms of non-uniformity or topological disorder.(42)























h(n m)











0 20 40 60 80 100 120 140


1.0-

A) 0 8
0.6-

2 0.4-

0.2-

0.0-
600


620 640 660 680


Time (ns)

Figure 2-15. CdSe/ZnSe Core/Shell Photoluminesence: A) Broad band spectra at 8.8 ns (
25 ns (-) and B) Kinetic traces for 645 (-), 670 (-) and 630 ( ) nm.


-) and


0.01


0 20 40 60 80 100 120 140 160
Time (ns)


Figure 2-16. TRPL decay curve of CdSe/ZnSe nanorod (-),
3hr alloy (-)


1 hr alloy (-


-), 2 hr alloy (


), and











Figure 2-17 shows a plot of the data for the nanorod samples in the form of an In[1n(lo/It)]

versus In(time) plot and fitting using a linear function. This type of plot is useful to determine the

extent of exponential behavior present in the material.(116) Starting with Eq. 2-6:





I t>
In( In a1 = /71n (2-8)

I(t~r )= rt(2






A) 1 r I 1B)



-2- T=300K --T=300K
p= 0.75 p= 0.58
-3- T- 173ns -3 I 7= 277ns

*~~cs cd zns clsel

Ln(Time(ns)) Ln(Time(ns))

0-o


c> -1-15-


T=300K
.* ~T=300K
.* p 0.4 ..p= 0.58
-3- r- 501nS -3.0- 7= 276ns


2 3 4 500 05 10 15 20 25 30 35 40 45
Ln(Time(ns)) Ln(Time(ns))


Figure 2-17. Equation In[1n(lo/It)] versus In(time) of A) CdSe/ZnSe coreshell nanorods, B)
ZnCdSe alloy nanorods 1hr, C) ZnCdSe alloy nanorods 2hr, and D) ZnCdSe alloy
nanorods 3hr. Adapted from H. Lee.(66)

When P, the slope, equals 1, the line should be straight indicating an exponential decay with a


well-defined rate.(116) Obtained values are summarized in Table 2-1. The fitted P of CdSe/ZnSe

coreshell nanorod is ~0.75, which reflects higher degree of ordered crystals. Difference of P










values between 1 and ~0.75 might be mainly due to size distribution. CdSe/ZnSe quantum wells

have demonstrated similar values.(42, 66, 118)

Table 2-1 Comparison of P and z value of CdSe/ZnSe and ZnCdSe nanorods. (42)
Nanorods hem(nm) P z (ns)
Core/Shell 645 0.75 173
ZnCdSe Alloy 1hr 625 0.58 277
ZnCdSe Alloy 2hr 570 0.48 501
ZnCdSe Alloy 3hr 566 0.58 276

A comparison of the TEM images of the core/shell and the 3 hr alloy are shown in Figure

2-18. The aspect ratio of the core/shell nanorods is 2.5 while for the alloy it is 2.1i, indicating that

the size distribution is not significantly changed during alloying process (Zn diffusion does not

change the shape or size significantly). Thus, the photophysical changes observed are due to the

annealing process altering the composition. Comparing CdSe/ZnSe coreshell nanorods to alloy

ZnCdSe nanorods, the P values are significantly decreased from 0.75 to 0.48~0.58 due to

increased disorder as a result of spatial fluctuations of Zn concentrations after the annealing

process. After three hours of annealing, the P value did increase, which is consistent with the

increased compositional homogeneity observed in the photoluminescence and Raman data.(42)


Alloying
~12800C







-20nm

CdSe/ZnSe Core/Shell Nanorods ZnCdSe Nanorods
(Aspect Ratio ~ 2.5) (Aspect Ratio ~ 2.1)
Figure 2-18. TEM images of CdSe/ZnSe Core/Shell and ZnCdSe Nanorods









Theoretical arguments predict that the radiative lifetime of bound excitons increases with

binding energy (114) and this is observed in our samples where the z's increase with alloying

time from 173 ns to 276 to ~501ns (Table 2-1). These time constants imply that in order to fully

characterize the long decay present in these samples, experiments on a much longer time scale

should be conducted. Exciton binding energy is increased by exciton confinement which is

obtained in this experiment by localization of the carrier wave function by changing the

composition of the quantum rods.(119) Therefore, it is expected that the luminescence decay

time (z) in these alloy nanorods will increase due to increased localization of excitons, i.e.

binding energies induced by compositional fluctuations.(42)

Summary

Green-yellow emitting ZnCdSe ternary alloy nanorods with relatively high quantum yield

are presented. The nanorods size and shape were characterized by XRD, TEM while the limited

alloying in core/shell nanorods and composition disorder was detected by Raman spectroscopy.

It has been shown that the quantum yield of ZnCdSe nanorods is a function of alloying time and

is significantly higher compared to CdSe nanorods, but is still lower than the core/shell nanorods.

The luminescent efficiency of these materials was discussed in terms of compositional disorder,

defects induced by the alloying process, and surface passivation by larger band gap surface

layers resulting from higher Zn concentrations near the surface. Time resolved emission

provided information regarding the role of diffused Zn. A stretched exponential function was

used to describe these systems, where P<1 corresponds to a distribution of decay rates.

Comparing CdSe/ZnSe core/shell nanorods to ZnCdSe alloy nanorods we found a significant

decrease in the p value. Photoluminescence decay lifetime, z, of the samples increased with

alloying time due to compositional disorder leading to exciton localization.(42)









CHAPTER 3
QUANTUM PARTICLE ELECTRONIC STRUCTURE

Introduction

Investigations into the behaviors of the electrons and holes in quantum nanoparticles have

been of great interest for several years.(10, 18, 31, 120-123) As seen in the previous chapter,

steady state spectroscopy does not give a clear understanding of the exciton behavior when

comparing rods to dots. On the other hand, there are methods that can measure the temporal

dynamics and the kinetics of photophysical processes. These methods are called time-resolved

spectroscopy techniques,(124) and they are a powerful tool that can bridge fundamental

parameters such as size, shape, composition and passivation to increasing quantum yields and

stability, reducing photodegradation, lowering the cost of fabrication (deposition and lithography

methods are expensive), making the synthesis safer, and improving their process-ability.

Whether in a conjugated molecule or semiconductor nanoparticle, upon excitation, an

electron and hole are created. Figure 3-1 shows a cartoon of molecular orbitals (MO) as linear

combinations of atomic orbitals. In conjugated molecules, the MOs near the "band gap" are

linear combinations of the same type of atomic orbitals whereas in nanocrystals the MOs can be

linear combinations of atomic orbitals from different atoms. When an electron is excited from

the HOMO (valence band) to the LUMO (conduction band), a hole is left behind in the HOMO.

In a CdSe nanocrystal, the HOMO has contributions from atomic orbitals from the Se2- whereas

the LUMO is a linear combination of atomic orbitals from Cd2+. Therefore, the created hole will

be located within the anion MOs while the electron will occupy the cation MOs. This in addition

to a high dielectric constant present in semiconductor nanocrystals means that the electron and

hole are correlated although at the same time the individual carriers can behave, i.e. be excited,

trapped or relax nonradiatively, to some extent, independently.(13)










MOs = Ic Electron
Intraband

~5~3 Transition

Cd2' Enegy 1 Interband

EnergyTransition

h'

Sel- Intra aend

Se2- Tasto
MOs = Ici


Figure 3-1. Electronic structure in semiconductor nanoparticles. Adapted from M. El-Sayed.(13)

After the initial excitation, the electron can only be further excited to higher states of the

cation MOs while the hole can only be further excited to other anion MOs (intraband transitions).

The recombination of the electron and hole from the conduction band to the valence band

involves an interband transition which can be directly detected in the visible spectrum. The

transient absorption signals detected in the visible region only reveal the behavior of this bound

exciton. Intraband relaxation of either electron or hole dynamics for strongly confined

nanocrystals are detected independently in different spectral regions. Since the energy spacing

between the levels within the cation MOs is much larger than the energy spacing between the

anion MOs the intraband excitation or relaxation of the hole intraband transfers is detected at

lower energies (infrared region) than the electron intraband transfers.(13)

The relaxation processes from higher to lower excited states within nanocrystals are

extremely intriguing and counterintuitive. Unlike in bulk materials where the cooling of the

photo-generated carriers is rapid and occurs via lattice phonons through its conduction band

continuum, carrier cooling in quantum particles must occur in discrete steps based on the nature









of the energy states of the materials.(125) Due to the quantization of nanocrystals, the spacing

between the energy levels for the electrons is quite large, reaching values much greater than

longitudinal optical (LO) phonons found in bulk semiconductors (~ 25 meV). Cooling via

phonons is possible; however, it requires a simultaneous emission of a substantial amount of

phonons, which has low probability. Therefore, it was assumed for many years that the

relaxation of these excitons should be inhibited, due to this "phonon bottleneck", resulting in

nanosecond cooling times.(122, 126) The electron relaxation from the 1P-to-1S in CdSe

nanocrystals occurs in the subpicosecond timescale (faster than even in bulk) thus bypassing this

bottleneck.(10, 17, 122) Klimov was able to extract population dynamics of the 1S and 1P states

determining that a 1P to 1S relaxation rate of ~ 300 fs and a 1S buildup time depends on the

confinement enhancement and decreases as the nanocrystal radius decreases.(18) This fast

relaxation process is Auger in nature in that the Coulomb interaction between the electron and

hole, which is increased due to quantum confinement, allows the electron to relax but transfer

excess energy to the hole, scattering it deeper into the valence band.(10, 31, 122) This strong

coupling between the electron and hole in quantum systems has allowed for predictions of

efficient electron-hole energy transfer to occur within 500 fs (12 7) to 2 ps (128) though it is

difficult to measure these values directly. Phonon assisted relaxation of the hole is more probable

due to smaller energy spacing within the valence band.(122)

Using infrared transient absorption(129) and terahertz spectroscopy(122) several groups

have attempted to correctly identify and confirm the Auger relaxation mechanism where the

electron transfers excess energy to the hole. The distinct photoabsorption features present in

transient spectra is very useful in identifying different photo-excited species. However, broad

band spectra are sometimes difficult to interpret and assignment of various species becomes










tedious and complicated. Moreover, luminescence up-conversion does provide for better time

resolution then other time-resolved techniques which allow for detection of ultrafast dynamic

processes. Time-resolved luminescence is not useful when trying to extract information about

charge carrier mobility and density or even carrier cooling since the dynamics of each carrier

must be measured separately. Although pump-probe in the infrared region has shown evidence

for the Auger cooling mechanism,(18) Terahertz spectroscopy is the first method that allowed for

direct measurement and quantifieation of hot cooling.(122) Hendry et al. determined that the

hole relaxation rate strongly depends on the amount of excess energy the electron provides. They

were able to confirm the Auger cooling mechanism and claim that this decay occurs on a 1 ps

timescale. (122)

This dissertation utilizes the information gathered by Hendry concerning the Auger cooling

mechanism, focusing on the exciton dynamics, i.e. interband relaxation for bare, core/shell and

ternary nanorods. From the experiments completed by Hendry, and our data, it is concluded that

the excess energy that the electron transfers to the hole, in addition to the small valence band

offset between the core and shell (0.07 eV), could be sufficient to cause the hole to tunnel into

the ZnSe shell and extend the electron and hole recombination times.

The ultrafast carrier dynamics in bare CdSe and core/shell CdSe/CdS/ZnS quantum rods

using femtosecond pump-probe spectroscopy has been conducted previously (72) to see how the

interface between the core and shell affects the competition between photoinduced absorption

and stimulated emission for lasing applications. Faster relaxation in the core samples was

observed due to surface traps. Upon passivation, the number of surface trap states decreases, at

the same time the shell introduces interfacial states due to the lattice strain mismatch between

core and shell.(72) To date, no studies have compared the ultrafast carrier dynamics in bare CdSe









rods and CdSe/ZnSe core/shell rods. In this work, we explore how addition of shell with only

one inorganic material with a small valence band offset affects the photophysical properties and

then compare this data to the exciton behavior in ternary alloy heterostructures.

Experimental Methods: Transient Absorption

Relaxation processes of colloidal nanocrystals were explored using femtosecond transient

absorption (TA). A commercial Ti-Sapphire (Ti-Sa) laser system consisting of a Ti-Sa oscillator

(Tsunami, Spectra-Physics) and subsequent amplifier (Spitfire, Spectra-Physics) with a repetition

rate of 1 k
excitation pulses. A portion of the amplifier output is split off to pump a 1 mm rotating CaF2

window to generate white light continuum probe with an effective bandwidth ranging from 310

to 750 nm. Prior to white-light generation, the probe polarization is tilted by 45 degrees with

respect to the pump pulse using a thin-film polarizer. A detailed description is available

elsewhere. (130) A general schematic is provided in Figure 3-2. The OPA idler/signal output is

used to produce excitation pulses (pump) through harmonic generation (450, 575, 610, 630 and

650 nm). This beam is then fed through a prism compressor, resulting in pulse lengths less than

100 fs (FWHM). The excitation beam is focused to a diameter of ~150 Clm at the sample position

and its energy was set to ~ 39 to 45 nJ yielding a fluence of 221 to 255 CIJ/cm2. Low fluences are

necessary to avoid multiple excitations (biexcitons). From previous works, it is known that a

signature of multiparticle interactions are decay rates that occur faster than 50 ps. Klimov also

observed that the decay rates increased as the number of excitons per nanoparticle

increased.(131) Experimentally, I verified the fact that multiple excitations per nanocrystal were

not initially created via a power dependence study. Our data show (Figure 3-4: no fast decay









observed in the kinetic traces prior to 100 ps) that we were able to assume that for the power

range utilized, multi-excitons are not initially generated.

Prior to interaction at the sample, a fraction of the probe pulse is split off. This reference

beam follows the same optical path as the probe but it probes only a sample volume that is not

excited by the pump pulse. The pump pulses were modulated by an optical chopper at a

frequency of 500 Hz and passed through a computer-controlled optical delay line to delay the

probe arrival time relative to the excitation. The pump and probe beams were spatially

overlapped at the sample. Probe and reference signals are collected in the presence and absence

of the excitation pulse and the ratio:

I I
probeprobe(3-1)



is recorded for each wavelength at every time step. A Glan-Thompson polarizer splits the

transmitted signal, into its polarization components, parallel (A ) and perpendicular ( A ) with

respect to the pump. The intensity at magic angle is calculated from the parallel and

perpendicular components measured simultaneously:

A. + 2 A
A ,(3-2)
magic angle 3

eliminating the influence of rotational times on the signal. Parallel and perpendicular transmitted

probe and reference signals were focused into a spectrograph attached to a charge-coupled

device (CCD) (Andor iStar coupled to Shamrock 303i spectrograph) for detection. Sample

solutions for TA measurements were placed in a quartz cuvette with a 2 mm path length and

continuously stirred to guarantee excitation of a new sample volume with every laser shot.

Changes in optical density (OD) were in the range of 1 to 50 mOD, and scans were repeated

multiple times to achieve acceptable signal-to-noise ratio. Each time step was averaged 250









times per scan. When twenty scans were completed, the total number of laser shots per point was

equal to 5000. Kinetic traces at particular wavelengths can be extracted from the full spectrum

collected using the CCD camera.

from OPA4, delay stage




Pumpr Mnochromator

Chopper ,
From Reference
I ""

White Probe
Lig ht
Generation P,
SREF


Figure 3-2. Transient absorption schematic

Results

Insight can be gained by investigating the spectral evolution caused by population density

changes in different energy levels using broad band femtosecond time-resolved absorption.(17,

53) A continuum probe results in a collection of the entire transient spectra at each delay time in

a single experiment. Therefore, photo-excited species can be detected and in principle, identified

based on their characteristic transient absorption features. In even a simple system, assignment

and interpretation of such photo-excited species can be difficult due to convoluted absorption

features within the transient spectrum.

CdSe versus CdSe/ZnSe Core/Shell

CdSe nanorods were synthesized using the method described by Peng.(46) Raman

spectroscopy is a useful tool for evaluating the structure and compositional homogeneity of









nanocrystals.(16, 1 7) In the previous chapter, the effect of adding an inorganic shell and

subsequent alloying have on phonon spectra was presented. It is clear that new modes appear

after the addition of the ZnSe shell. Despite no significant changes between the bare and

passivated samples in the steady state absorption, the Raman data shows that the structure of the

system has changed; therefore it is necessary to investigate the system using time-resolved

methods. More specifically, the linear absorption spectra indicate that confinement is maintained

after addition of the ZnSe shell. Meanwhile, the quantum yield is increased from 0.6% to 15% in

bare and core/shell materials respectively.(42) Clearly, surface traps are reduced, prompting a

time-resolved investigation into the changes in the carrier relaxation due to the change in

electronic structure.

Figure 3-3 depicts the transient signal collected at 0 fs, 400 fs, 800 fs, 2.47 ps, 200 ps, and

575 ps for bare CdSe and passivated CdSe/ZnSe core/shell samples. Both samples are excited at

450 nm well above the band gap and the 1S and 1P absorption bands seen in Figure 2-12.

Multiple transitions dominated by state-filling are observed, leading to transient bands at the

energies of the allowed optical transitions. Exact determination of the electron-hole transitions

which give rise to different resonances need to be determined by comparison with the states

theoretically calculated by an effective mass theory. (72) In this work, we assign the transitions

based on works done by Klimov,(1 7) Efros,(25) and Guyot-Sionnest(129). Using their notation,

B1 and B2 are assigned to the photobleach of the 1S [1 S(e)-1S3/2(h)] and 2S [1S(e)- 2S1/2(h)]

states respectively while B3 corresponds to the bleach absorption of the 1P [1P(e)-1P3/2(h)] state.

Meanwhile, the Al band is assigned to the photoinduced absorption that grows in after high

energy excitations cool from the 1P to the l S. Within 400 fs (red line), the carriers are

distributed throughout the cascade of energy states. As the delay time increases from 400 fs to









2.47 ps (blue line), carriers quickly relax from the higher energy states to the band gap state

resulting in a corresponding 1P to 1S relaxation.


Al






-0.01-
A) CdSe Rods

0.00- All





-0.02-

B) CdSa/ZnSe CS Rods

500 550 600 650 700



Figure 3-3. Broad band transient absorption spectra for A) CdSe Rods and B) CdSe/ZnSe
Core/Shell Rods at various pump delay times: 0 (-), .400 (-), .800 ( ), 2.47
(-), 200 ( ), 575 (-) ps.

Extracting the kinetic information from the broad band spectrum enables the comparison of

carrier relaxation trends at each optical transition (Figure 3-4). In both systems (CdSe and

CdSe/ZnSe), the decay lifetimes corresponding to the 1S and 2S bands are identical (>200 ps)

(black and red lines). The 1P decays rapidly and results in photoinduced absorption at longer

delay times, the origins of which will be explained in further detail in the discussion. The rise of

the PIA signal in bare CdSe rods is slower (>10 ps) than the passivated rods (< 2 ps). A "dip" is

observed in the passivated sample for both the 1S and 2S bands, which results from overlap of









multiple processes detected simultaneously. This effect is not observed in the core sample. For

the core/shell sample, it is proposed that the hole migrates to the valence band of the ZnSe shell

(valence band offset = 0.07 eV) due to the energy transfer during Auger relaxation of the

electron resulting in longer bleach decaying times. The insets of Figure 3-4 A and B compare the

higher energy, 1P state negative absorption decay and photoinduced absorption to the 1S and 2S

states bleach decays. It is interesting to note that the rise of the negative signal from the 2S and

1P states (black and green respectively) are identical but the 1P bleach decays rapidly (~ 1 ps)

into a positive signal, matching the rise of the bleach of the lower 1S energy state (red line). This

confirms a 1P to 1S relaxation process in both CdSe and CdSe/ZnSe.

Core/Shell Excitation Dependence

An excitation wavelength dependence study (450, 575, 610, 630, 650 nm) was conducted

to elucidate the influence on the kinetic processes of the ZnCdSe interfacial state previously

detected by Raman spectroscopy. Figure 3-5 shows the steady state absorption spectrum, with

the horizontal arrows signaling the excitation wavelength for each row of time-resolved spectra

presented on the right two columns. The transient spectrum showed on top indicates the

detection wavelength for each column. For example, the kinetic data on the top left plot presents

the transient signal in the area of the 1P state after excitation at 650 nm whereas the middle right

plot presents the transient signal of the 1S state after excitation at 575 nm. After excitation at the

same energies, the 1S band bleach rises with a time constant corresponding to the decay of the

1P bleach. In addition, the 1P detection shows photoinduced absorption present at a time delay

greater than 1 ps due to the ZnCdSe interfacial state. For excitations greater than 600 nm (lower

energies), the P band does not contribute to the dynamics; instead, the excitation simultaneously

populates both the S band and trap states.











-1 12 345


I n l l e I n
0 100 200 300 400 500
time (ps)


-~------


r


CdSe Rods


B)


-1 0 2 a


CdSe/ZnSe


I I


I )


It~


200


300


Figure 3-4. Kinetic traces corresponding to the 1S (
rods and B) CdSe/ZnSe rods.


-), 1P ( ) and 2S (-) bands for A) CdSe


time (ps)










Considering the kinetic traces in the first column (P detection), as the excitation energy is

decreased the bleach decay faster and the onset of the photoinduced absorption occurs at earlier

delay time. Eventually, (excitation at 650 nm) the bleach is no longer present, indicating that the

P transition is not being accessed. As seen in the kinetic traces in the second column (S

detection), as excitation energy is increased, the rise time of the negative absorptions are slower.

When the sample is excited at 650 nm, the rise is instantaneous, while at 450 it is greater than 1

ps. The energy level associated with the ZnCdSe interfacial state is then considered to expand

through the density of states between the CdSe and ZnSe band gaps.
Detection Wavelength
500 550 600 650 700



700



650, c. o I


5 600






4 50~ C o .






400
0 10 1:02
time (ps)

Figure 3-5. Time-resolved excitation dependence collected for the core/shell sample. Left graph:
linear absorption spectra and indicates the excitation wavelengths (450, 575 and 650
nm). Top graph: transient spectra at 2.47 ps. Kinetic traces in column 1 correspond
to the 1P band while column 2 correlate with the 1S band behavior









Core/Shell versus Alloys

Prior to our work, limited information appeared in the literature involving the synthesis of

colloidal ZnCdSe nanorods for use in optoelectronic devices. Green-yellow emitting ZnCdSe

nanorods were prepared by diffusion of Zn into the CdSe core. For alloying, the reaction vessel

containing CdSe/ZnSe nanorods was heated and stirred for up to 3 hrs. An aliquot was removed

after heating for 1, 2 and 3 hrs, immediately cooled and diluted with toluene to terminate the

alloying process and then precipitated with methanol/toluene co-solvents. The Raman data

presented in Chapter 2 indicate the enhancement of the ZnCdSe phonon mode but disappearance

of the CdSe and ZnSe modes, confirming the alloyed nanorod composition.(42)

Figure 3-6 compares the bleach spectra of the core/shell and ZnCdSe alloyed samples

using the delay times of 0.400, 2.47, 150 and 575 ps. Note that this data is presented differently

than in Figure 3-3. Each plot includes the four samples at one particular delay time. For example,

Plot 1 corresponds to the transients for each sample at a probe delay time of 400 fs. This data

indicate the occurrence of a transformation of the band gap, band structure and surface-trap

states as function of alloying.

At early time delays (0.400 ps Plot 1), the 1S band corresponding to the 2 hr alloy (green

line) is much more intense than all other samples and is significantly blue shifted compared the

core/shell nanorods (black line). After 2.47 ps (Plot 2) the 1S band in all samples is maximized

and the wavelength shift mentioned previously is much more evident. The 1 hr alloy remains

very broad even after 575 ps (Plot 4). The overall signal obtained from the alloys is not as

intense as the core/shell and the 1 and 2 hr alloys are not as intense as the 3 hr alloy.

Photoinduced absorption present in the core/shell materials does not appear in the alloys. This

will be discussed in further detail later. At 150 ps (Plot 3), the alloy bleach has noticeably

decayed; meanwhile the core/shell has not decayed significantly. Even after 575 ps, the











core/shell decay is very small (compare black line in Plots 2, 3 and 4) and photoinduced

absorption is still present. For the ZnCdSe samples there is no distinct band gap associated with

CdSe or ZnSe. The only band present corresponds to the ternary composition; thus, it does not

exhibit excited state absorption. As alloying time increases, the amount of Zn diffusing into the

CdSe increases resulting in the band gap shifts observed in both steady state and transient

measurements.





C/S 1S
1S
-0.02 -0.02 -1 S
2hr 1S
-0.03-1 -0.03-
1S
-0.04- PLOT 1 40 -0.4-PLOT 2 1 .7p

500 550 600 650 700 500 550 600 650 700


0.00 0.00- L^I

-0.01 -0.01

-0.02 -0.02

-0.03 -0.03

-0.04 -PLOT 3 150 pS~ -004 PLOT 4 575 ps

500 550 600 650 700 500 550 600 650 700
h (nm) h (nm)

Figure 3-6. Broad band transient absorption spectra for core/shell(--), Alloy 1 hr (-), 2 hr ( )
3 hr (-) at various time delays.

A comparison between the 1S and 1P bands of each of the samples is shown in Figure 3.7.

All samples were excited at 450 nm but the detection wavelengths are not the same due to the

band gap shifts induced by Zn diffusion. The transient spectrum at the top is shown as a guide

indicating the detection wavelength for the 3 hr alloy. The first column of kinetic traces

corresponds to the detection of the P band while the second column depicts the dynamics of the









S band. The amount of Zn diffusion into the CdSe increases from top to bottom as alloying time

increases. When Zn is mixed in with the core, the material has more ZnSe character. Looking at

the P band, when Zn diffusion occurs, the photoinduced absorption is eliminated. Also, the rise

of the P band bleach gets faster as more Zn is diffused into the CdSe core. The lack of

photoinduced absorption in each of the alloys also confirms that the interfacial state present in

the core/shell material is no longer present. This is in agreement with the Raman data in Chapter

2. As seen in column 2, the rise of the bleach also gets faster as alloying time is increased. The S

bleach decay of the alloys (Figure 3-8 red line) is significantly faster than the core/shell material

(black line) due to surface traps, since the inorganic passivation layer is no longer present.
Detection Wavelength
475 500 525 550 575 600


P S


P


I


S


Core/Shell



1 HR
Alloy aa.


2 HR
Alloy 0o


3 HR

0 0

TiT Te (ps)
Figure 3-7. The 1S and 1P composition dependence.














-0.25



E -o.so



-0.75



-1.00

0 100 200 300 400 500

time (ps)
Figure 3-8. Comparison of the 1S band for the CdSe/ZnSe core/shell (-) and 3 hr ZnCdSe alloy


Discussion

In nanocrystals, optical transitions resulting in ground state absorption changes are due to

state-filling effects while extremely fast transitions (<1 ps) result from Coulomb interactions, i.e.,

Stark Effect.(1 7 72, 121) Red shifts are observed in CdSe quantum rods at longer delay times;

they are identified as a convolution of the S-type states near the band gap stemming from the

inherent size distribution present in colloidal nanoparticle samples.(72) The relaxation dynamics

within these systems are strongly influenced by ensemble dynamics collectively creating

inhomogeneities and also multiple photoinduced processes leading to multi-exponential or non-

exponential behaviors.(10)

In analogy to bulk materials, cooling of hot electrons could occur via emitting LO

phonons, and this mechanism would result in slow decay rates since the spacing of the intraband

states is large in quantum systems resulting in a phonon bottleneck.(31, 111, 122) However,

several studies (17, 111, 132) including our results, demonstrate a different behavior. High










energy relaxation in quantum systems occurs via Auger cooling in 2 ps (lP to 1S relaxation).(18,

53, 122, 128, 129) In each of the samples measured, relaxation was successfully observed as a

corresponding decay of the 1P band and subsequent rise of the 1S band. Other possible cooling

mechanisms include multi-phonon relaxation and polaron effects.(128, 133, 134) Under our

experimental conditions, we create less than one electron-hole pair per nanorod, and thus those

mechanisms are not plausible. In quantum dots, strong electron-hole Coulomb attractions favor

energy transfer from the electron to the hole. (111, 122) Due to increased size dependence on

carrier cooling, hot electrons transfer kinetic energy to holes which can quickly and efficiently

undergo intraband cooling due to their relatively larger effective mass and smaller electronic

energy level spacing.(18, 122) Enhancement of the Auger electron-hole interaction would then

bypass any phonon bottleneck.(122, 127-129, 131) This can not be confirmed without directly

measuring the electron-to-hole energy transfer which has been successfully observed by Hendry

et al. using THz spectroscopy.(18, 122)

Defect states can be resonant with high energy levels as observed by Rosenthal et al.(67,

122) The presence of these states results in an efficient trapping mechanism and faster bleach

recoveries in core samples. These trap states are reduced by passivation with an inorganic

material as in core/shell rods.(72) The shell reduces the number of surface states present in the

bare rods preventing the exciton to sample these higher excited states resulting in higher

photoluminescence quantum yields.(78) The core and core/shell systems both exhibit

photoinduced absorption indicating the presence of an alternate "state" contributing to the

excited state signal. The high surface- to- volume ratio within the bare rods creates additional

electronic "trap" states leading to small photoinduced absorption features at energies higher than

the band gap but similar to the high energy 1P state. The onset of this photoinduced absorption










is generally slow (>10 ps) and is related to the rate at which the carrier is trapped either at the

surface or other defect sites. The trapped electron can then relax to the ground state via an

alternative radiative decay pathway or decay non-radiatively.(67, 72) The actual rates and overall

dynamics are indistinguishable because of the inhomogeneity and ensemble averages of the

samples; therefore, direct assignment of rates and pathways is extremely difficult. On the other

hand, photoinduced absorption within the core/shell rods arises from interface defects resulting

in states that can act as traps and/or non-radiative recombination sites caused by lattice strain

relaxation introduced between the core and shell.(38, 65, 72) It appears that after the initial

Auger cooling from the 1P to 1S state, some carrier populations sample the ZnCdSe interfacial

layer resulting in photoinduced absorption. The signal is strong and lasts more than 500 ps

(Figure 3-4).

Compositional disorder (42, 108, 109) in ternary alloy nanorod structures leads to

localization of excitons, (113) hence an increase in quantum yield for ZnCdSe versus CdSe

nanoparticles. Localization effects increase the overlap integral of the electron and hole

wavefunctions improving the luminescence efficiency of the material and decreasing the bleach

lifetime for the ternary alloy samples compared to the binary CdSe materials. The lower ZnCdSe

quantum yield versus the CdSe/ZnSe core/shell nanorods results from the lack of surface

passivation and crystal defects within the ZnCdSe nanorods due to Zn diffusion into the CdSe

core. Increasing the Zn character in the core causes a blue shift in the spectrum as alloying time

increases. Raman data presented in Figure 2-11 show that CdSe and ZnSe phonon modes are no

longer present; the only mode observed is the ZnCdSe state. The lack of or small amount of

photoinduced absorption confirms that the alloy nanorod composition is uniform and few

interfacial traps are present. Since the inorganic shell disappears, surface traps reoccur and









decrease the overall intensity (not as low as bare CdSe rods) and bleach lifetimes (recovery

occurs faster than in the CdSe bare rods). In addition to surface state traps, crystal defects are

known to act as non-radiative recombination centers, reducing the emission efficiency and

enhancing the bleach recovery.(66, 114, 115) The band gap shifts, band narrowing and increase

in the overall bleach amplitude can be attributed to stress relaxation by thermal annealing.(66,

110) This is consistent with the weaker, broadened 1S band bleach signal observed after 1 hr and

further after 2 hrs of annealing, since diffusion alters the distribution of Zn throughout the

nanorod, i.e. decreasing the amount of Zn present at the surface and increasing the amount to-

wards the middle. However, annealing for 3 hrs enhanced and narrowed the 1S band bleach

compared to the samples annealed for 1 or 2 hrs. This increased change is attributed to annealing

of crystalline defects and reduction of stress, consistent with the Raman data reported.(42)

Tunneling of the electron wavefunction into the ZnS shell has been reported in CdSe/ZnS

core/shell structures by Mokari and Banin(38) resulting in a ~10 nm red shift. This tunneling led

to a delocalization of the electron, lowering its confinement energy and consequently the energy

of the exciton levels.(39, 42) Raman data presented previously indicate formation of interfacial

ZnCdSe in as-grown CdSe/ZnSe core/shell nanorods. This reaction would be expected to

decrease the size of the CdSe core (42, 135) resulting in increased localization and a blue shift in

emission. In our experiments, addition of the ZnSe shell does not alter the band gap significantly

(only a 4 nm blue shift observed in photoluminescence). We have successfully engineered the

material to create electron and hole wave functions that experience a confinement potential that

localizes (Type-I localization) the electron wave function within the CdSe core despite addition

of a shell.(136) It has been shown that the rise of the 2S and 1P bands are identical indicating

that the hole is delocalized within the density of states located in the valence band of the CdSe









core due to the small intraband spacing. Additionally, the 2S and 1S decays are identical. In fact,

the "dip" observed for each of these bands in the core/shell sample after several ps is likely due

to the hole tunneling into the interfacial ZnCdSe and/or ZnSe valence band due to the small

valence band offset (the energy difference between the valence band of the core and the valence

band of the shell). The small valence band offset between CdSe and ZnSe does not guarantee

that the electron hole energy transfer does not cause the hole to transfer to the shell. If this were

the case, it could account for the extended recovery rate observed in the core/shell versus core

sample. However, we are unable to measure the mobility of the hole directly, so this is only a

prediction.

Figure 3-9 shows the valence and conduction band offsets of various bulk materials. Most

of the work presented in the literature deals with nanoparticles with ZnS (39, 13 7-140) or CdS

(65) as the inorganic shell. CdS has been used to passivate CdSe(65) since it has a small lattice

mismatch value (0.04 eV) compared to ZnSe (0.07 eV) or ZnS (0.11 eV). In order to prevent

penetration of the carriers into the shell or potential well barrier, the photo-generated exciton

created in the CdSe core should be confined by a material with valence and conduction potential

wells that are comparable. Several studies have attempted to calculate to the first order

approximation, a penetration length (L) that is proportional to:(75)

Lac (myo)-1/2 (3-5)

where m represents the effective mass of the charge carrier (electron or hole) and Vo is the band

offset. These values can be seen in Figure 3-9. (141-145) Light electrons are more easily

confined in heterostructures that have the same anion; therefore, the CdSe/ZnSe heterostructure

is better balanced than the CdSe/CdS resulting in larger oscillator strengths from enhanced

electron-hole wavefunction overlap.(75)











me 0.2 0.'13 0.16 0.28


'1.0


o.o


CBO (eV)
VBO (eV)
o~o dS OdSe ZnSe ZnS;


-1.0 -0.7 0.5-1 0.6-1.1' 0.5 my


Figure 3-9. Valence and conduction band offsets for various materials. (75)

From the observations above we propose two models associated with the exciton dynamics

within the CdSe/ZnSe core/shell and ZnCdSe alloy nanorods. In Figure 3-10, high energy

excitation results in very fast 1P to 1S relaxation times (~ 1ps). State filling within the

conduction band of the CdSe occurs.


CdSe


1P,


1S,


hy= 2.75 eV




0 .4 V I S1/1S 3 2 I
0.4~ e1P3 2


Figure 3-10. CdSe/ZnSe core/shell potential kinetic model.


ZnSe
PIA









md
ition


n n7 eV' VB Offset









From the evidence presented in this thesis, it is conceivable that during the Auger cooling of the

hot electron from the 1P to 1S state, excess energy could cause the hole to become delocalized

within the ZnCdSe interface or even in the ZnSe shell. The bleach recovery from the conduction

band to ground state valence band is the longest in the core/shell materials due to passivation of

surface traps and potentially due to the position of the hole wavefunction.

Once the materials are alloyed, the ZnCdSe in Figure 3-11 becomes the only inorganic

material present. Again, 1P to 1S relaxation is observed with high energy excitation followed by

subsequent interband relaxation from the conduction to valence band. However, based on the

work completed by Rosenthal et al.,(67) midgap surface states involving selenium dangling

bonds are present due to the lack of inorganic passivation. An electron relaxing from the surface

Se atoms to the valence band can immediately fill the vacancy left by the photogenerated

electron contributing to the deep trap emission observed at 700 nm.

1Pe

<1ps

1Se V \ Trap State





Interband
hy= 2.75 el Relaxation /Surface Trap





0.4 eV 1s/
1P3/2
S2S1/2

Figure 3-11. ZnCdSe alloy potential kinetic model.










Summary

Transient absorption spectroscopy was utilized to extract the exciton dynamics within

binary CdSe/ZnSe core/shell and ternary ZnCdSe nanorods. A comparison between the exciton

behavior in unpassivated CdSe core and CdSe/ZnSe core/shell materials, an excitation

wavelength dependence for the core/shell nanorods and the influence alloying has on the exciton

behavior are all presented. For all samples, at high energy excitation, a 1P decay and subsequent

1S rise is observed corresponding to a 1P to 1S relaxation process. Also, the introduction of a

midgap state in the core/shell material leads to photoinduced absorption after the 1P bleach

recovers. Upon low energy excitation, this midgap state is directly populated. Surface trap states

reappear in the alloyed heterostructures (no passivation) leading to faster bleach recoveries then

the core/shell materials.









CHAPTER 4
CONJUGATED POLYELECTROLYTES (CPES)

Introduction

xn-Conjugated polymers are an interesting class of materials with unique physical

characteristics that make them excellent candidates for various purposes including lasers (146),

LEDs (147), photovolatics (148), and transistors (149). To be useful for any application, a

fundamental understanding of their photophysical properties is necessary in order to continue to

improve their efficiency and efficacy. In recent years, xn conjugated polyelectrolytes (CPEs)

have been synthesized incorporating ionic solubilizing side groups enabling the polymer to be

dissolved in water and other polar solvents while preserving the photophysical properties

associated with the polymer backbone.(6, 7) In an effort to reduce exposure of the non-ionic

components to the environment, when CPEs are dissolved in a polar solvent such as water they

self-assemble into aggregates due to the interaction between the charged functional groups and

hydrophobic backbone. (150-154) This intra- or intermolecular xn-x stacking of the polymer chain

creates new, red shifted absorption and emission peaks, (155) decreases the overall fluorescence

quantum yield and competes with radiative emission processes from the isolated chains.(156)

Aggregates can also form in concentrated polymer solutions with nonpolar solvents.(14, 20, 26-

28) Addition of a metal cation such as Ca2+ acts as a cross-linking agent and it has been shown to

induce aggregation in methanol, improving the amplified quenching properties of CPEs.(155,

157-159)

Aggregation is easily confused with other types of interchain interactions of xn-electrons in

spatially close chromophores. It is extremely important to correctly define, understand and

identify the different types of interchain species that can be present within the conjugated

polymer solution or film. Within the literature, there is a discussion on the proper identification









of interchain species.(159) In interchain interactions, xn-electron density is delocalized among

numerous conjugated segments in different polymer chains. Depending on the physical

conformation of the chains, it is possible that the two interacting species be located on the same

chain. For example, if the polymer chains are extremely long, the conjugation segments from the

same chain can interact spatially as a result of xn-x stacking due to backfolding. Shared xn-

electrons between two polymer chromophores in their excited state that are next to each other

create a species named termed "excimer".(159-162) When neutral excitons are shared by two or

more adjacent chromophores in the ground and excited state, the intrachain species that is

formed is known as an "aggregate". The aggregate formations that interact electronically will

cause a significant change in the absorption spectra corresponding to an elongation of the xn-

electron delocalization resulting in lower energy peaks compared to isolated chains.(159) In

addition to aggregates, a "polaron pair" can be created after excitation resulting in an radical

cation (hole polaron) in one chromophore and a radical anion (electron polaron) on another.

(159, 163, 164) A significant redshift in the emission spectra is a photophysical indicator that an

excited interchain species is present within the conjugated sample due to delocalization of xn-

electrons creating a lower electronic state compared to the isolated chains. Since this

phenomenon occurs for each of the interchain interactions, it is hard to distinguish between the

various types. Detection and identification is further complicated for room temperature

fluorescence measurements due to the large numbers of non-radiative "trap" sites in conjugated

polymers resulting in very low emission quantum yields. (159, 160, 163) Aggregates can be

differentiated from the other species because they are the only ones that show a weak redshift in

the ground state detectable in the absorption spectrum. (159, 165, 166) This shift can be subtle,

especially if the aggregate absorption is symmetry of the transition is forbidden; therefore, the









controversy between discriminating between aggregates and excited state interchain species is

still ongoing.(159) Based on the spectral signatures present in our photophysical character-

ization, the species present in PPE-CO2~ are COnsidered to be aggregates.

Correct identification of the types of interchains species is extremely important when

considering charge transport and light emission applications of conjugated polymers. In order to

fully understand a system and be able to make synthetic improvements it is necessary to

characterize each of the species accordingly. Also, CPEs are opening the door to various

biological applications but aggregation must be considered because it is an extremely important

factor that influences the polymer quenching capabilities and ultimately their performance as

chemo- or biosensors.(130, 155, 157, 160, 161)

Zhao et al. has reported the synthesis and characterization of a series of variable band gap

poly(arylene ethynylene) (PAE) water soluble conjugated polyelectrolytes dissolved in methanol,

water and methanol/water mixtures.(167) By only varying the anionic side group, they achieved

band gap tunability within the visible region. Photophysical data collected in their study correlate

the CPE side chain structure to the extent of polymer aggregation when dissolved in each

solvent. The work done for this thesis focuses on the role aggregation plays in the intra- and

intermolecular energy transport within varying polymer repeat units (PRU) of PPE-CO2-. From

previous research in this area it has been determined that the quenching efficiency increases as

the amount of controlled aggregation increases.(130, 150, 155)

Several investigations, (158-160) including those done by Chen et al. (157) have alluded to

the idea that quenching of a conjugated polymer emission is the fundamental property necessary

to understand and characterize these materials to be useful for chemo- and bio sensors. More

specifically, anionic polymeric electrolytes can be efficiently quenched by cationic systems in









solution. The quenching efficiency is described by the conventional "Stern-Volmer"

relationship:(2)

rI
o __-1+ k ro[Q]= 1+ Ks [Q] (4-1)
OI

where 40 (lo) and O(1 are the steady state fluorescence quantum yields (fluorescence

intensities) in the absence and presence of the quencher molecule respectively, Ksy is the Stern-

Volmer constant, and [Q] is the quencher concentration. A "Stern-Volmer" plot is the

fluorescence intensity ratio (l0/1) versus Q. This plot is expected to be linear with the slope equal

to the Stern-Volmer constant. From this information, the quenching rate constant, k,, can be

calculated if the excited state lifetime, to of the neat sample is known.

From a time-resolved measurements point of view, the quenching mechanism is "static" if

there is no change in z when a quencher is added to the solution. The following relationship is

used if the lifetime does change:(2)


a= 1+k ro[Q]. (4-2)


This relationship enables one to determine if the fluorescence decay under dynamic quenching

conditions in the presence of the quencher molecule and provides the value of k,. In CPEs, the

quenching mechanism is both static and dynamic. In fact, the dynamic component does not

necessarily arise from the diffusion of the quencher in the solution but from the diffusion of the

excitation within the polymer chain. (130, 168)

It is well known that fluorescence within the visible spectrum from low concentrations of

CPEs can be superlinearly quenched when placed in the presence of an oppositely charged

electron- or energy quencher molecule (superquenching (146); amplified quenching (167,

169)).(151, 152, 154, 157, 158, 162, 163, 165, 166, 170) Amplified quenching may lead to the









development of more sensitive sensors but a complete explanation responsible for such high

quenching efficiencies within CPEs has yet to be determined. This intriguing effect is not only

due to ion-pairing between the polymer and quenchers (157, 163, 164, 1 70) as in typical Stern-

Volmer kinetics, but also inter- and intrachain energy transport mechanisms. More specifically,

the random walk diffusion of the excitation energy along the polymer backbone, (154, 157, 1 71,

1 72) energy transfer between the polymer and quencher (1 71) and energy transfer between the

isolated polymer species and aggregated chains all contribute to such unique behavior.(143, 146,

162, 165-167)

Energy transfer is strongly dependent on the spectral overlap between the donor emission

and acceptor absorption. The energy transfer is also very rapid. (168-1 71) If an aggregate inducer

or quencher is added to the polymer solution, the conformation can change resulting in a spatial

redistribution of several chromophores. This enables the excitation located on the polymer

backbone to easily migrate to the quencher located at a particular site lower in energy. Therefore,

one quencher molecule can have the ability to reduce the emission from a large number of

chromophores. (157, 1 73) The intrachain random walk model, which leads to excitation

migration towards the quencher molecule, is strongly dependent on the conjugation, polymer

chain lengths and transition dipole orientations (Figure 4-1). Using time-resolved anisotropy

fluorescence measurements, it is seen that after excitation, the energy or exciton "hops"/migrates

from shorter (high energy segments) to longer (low energy ones) and depolarizes along the way,

reducing the anisotropy value. The exciton will continue to funnel through the cascade of

chromophores until it is either "trapped" or it reaches the lowest energy level where it can

fluoresce or non-radiatively decay.









Several studies have investigated the influence aggregates have on the kinetics within

water soluble conjugated polymer systems. (139, 152, 154, 1 73-1 79) For example, Fakis et al.

has shown that energy transfer from isolated poly(fluorenevinylene-co-phenylenevinylene

(PFV-co-PV) (156) to aggregated chains is very rapid and efficient. They determined the isolated

chain fluorescence, the aggregate emission and energy transfer contributions to the overall decay.

In addition, the correlation between the concentration and energy transfer efficiency was

thoroughly examined. A reduction in the concentration causes the energy transfer efficiency and

energy transfer rates to decrease linearly.(156) In this thesis, we investigate the influence chain

length, solvent and metal cations have on the ultrafast emission of a carboxylated

poly(phenylene) vinylene (PPE-CO2 ) Shown in Figure 4-2 to determine the excitation transport

processes. The energy transfer mechanism between isolated and aggregated chains within the

PPE-CO2~ pOlymer is of particular interest.





SPolymer Q~tCuencher


Figure 4-1. Intrachain energy transfer of excitation to quencher molecule along polymer
backbone.


CO2Na+

O






PPE-CO2

Figure 4-2. PPE-CO2~ pOlymer repeat unit. PPE-CO2~ in methanol (left) and water (right)









Previously, a series of steady state, time-resolved, anisotropy measurements and numerical

models were conducted with a similar CPE, PPE-SO3-, to determine the rate and efficiency

exciton migration has on fluorescence quenching.(130) Using PPE-SO3~ aS a model polymer

which exhibited both long range and random walk kinetics, we have designed experiments in

which the results should indicate the type of energy transfer present within PPE-CO2-. Time-

resolved photoluminescence and time-resolved anisotropy measurements were employed to

monitor the potential exciton hopping that was previously observed in PPE-SO3-, determine the

rise time of the aggregate state emission and characterize the overall polymer decay.

We studied very short and long polymer repeat unit (PRU) PPE-CO2- chains with the

expectation that short chains would be less likely to aggregate. We find that even short chains (8

PRU) form solutions with both isolated and aggregated chains. Even though the only difference

between PPE-SO3- and PPE-CO2~ is their ionic group, their photophysics are quite different.

PPE-CO2~ Steady state photophysics correlate more with ladder-type (poly-paraphenylene)

(LPPP) polymers. (116, 166, 174-177)

In most cases, conjugated polymer chains are not "frozen" in one conformation, instead

they have a proclivity to twist and coil. A series of chromophores can be linked resulting in

different degrees of xn-electron delocalization depending on the planarity of the conjugated

segments. Even if there are slight twists or bends along the polymer backbone, it is possible for

the conjugation to not completely break, resulting in larger delocalization lengths.(130, 159) Just

as in semiconductor nanoparticles, a particle-in-a-box model (1-D for polymers) is used to

explain the delocalization of excitons along the polymer backbone. Conjugation lengths that are

long tend to have lower xn x~ transition energies and vice versa.(159) Longer conjugation

lengths can be due to polymer rigidity which can create small shifts between the absorption and









emission maximums (Stoke' s shift). Both LPPP (116, 166, 1 74-1 77) and PPE-CO2- exhibit very

small Stoke's shifts alluding to their rigidity resulting in long conjugation lengths and therefore

altering the dynamics and exciton hopping compared to PPE-SO3 -

Quenching PPE-CO2~

As mentioned previously, addition of a cation such as calcium has been shown to induce

various amounts of aggregation in CPE solutions. (1 78, 1 79) A perfect example of this effect is

observed when poly (2-methoxy-5-propyloxy sulfonate phenylene vinylene) (MPS-PPV)

fluorescence is quenched due to induced aggregation created by the divalent cation, Ca2+.(157)

Dr. Hui Jiang, as a part of Dr. Kirk Schanze's lab in the Department of Chemistry at the

University of Florida, has investigated the effects that additions of a divalent cation and an

electron acceptor quencher, methyl violagen (MV2+) have on the quenching of PPE-CO2 .(155)

Preliminary steady state absorption and quenching experiments of PPE-CO2- with MV2+ wr

conducted by Dr. Jiang and presented here to provide a better understanding of the CPE

presented in this thesis.

Figure 4-3 depicts the Stern-Volmer plots for PPE-CO2~ flUOrescence quenched by MV2+ in

various solutions with increasing Ca2+ COncentrations. As seen in previous works, (168, 180) the

quenching efficiency varies depending whether the polymer is dissolved in methanol or water. In

a water solution (closed squares), the quenching is extremely efficient, requiring less than 1 CLM

MV2+ to quench the fluorescence by 90%. More importantly, the superlineararity begins at very

low quencher concentrations. On the other hand, if the polymer is dissolved in methanol (open

squares), a well-defined "induction region" in which the slope (Ksv) is almost linear is observed,

and the quenching efficiency does not become superlinear until much higher concentrations

(~ > 3 CLM).(155)

















20 -0/
Quenching







O 1 2 3 4 5

[MV2+]/pIM

Figure 4-3. Stern-Volmer plot of 10 CpM 185 PRU PPE-CO2-. Quenching due to MV2+ in water
(m) and in methanol with 0 pMc (0), 2.5 CpM (0), 5.0 CpM (0), 7.5 pMc (A), or 10.0 CpM
(V) CaCl2. (155)

Addition of various amounts of calcium to methanol solutions is also shown in Figure 4-

3. As the concentration of calcium is increased, the "induction region" is reduced and the

superlinear form occurs at smaller quencher amounts.(155) A 1:1 stoichiometric ratio of 185-

PRU PPE-CO2- to calcium results in identical quenching behavior as if the polymer was

dissolved in water. In addition, the absorption was collected for the PPE-CO2- dissolved in

methanol in the presence of various MV2+ COncentrations. As the amount of quencher increases,

the amount of aggregation increases as indicated by a redshift in the absorption spectra.(155)

This behavior follows suit with other CPE-quencher systems. (154, 157, 180) The mechanism for

the observed amplified quenching can not be determined from steady state alone, hence the need

for time-resolved measurements.










Experimental Methods

Synthesis of Variable Chain Lengths of PPE-CO2

Xiaoyong Zhao, a member of the Schanze group, is responsible for the synthesis and some

of the steady state characterization of the various chain lengths of PPE-CO2~ inVCStigated within

this dissertation.

To polymerize a stoi chi ometri c mixture of 2, 5 -bi s-(dodecyl oxy-carb onylmethoxy)- 1,4-

diiodobenzene and 1 ,4-di-ethynylbenzene a "precursor route" in which a Sonagashira coupling

reaction is used to produce a poly(phenylene ethynylene) with a dodecyl ester protecting the

carboxyl group. Gel permeation chromatography of the ester precursor polymers showed that

the molecular weight (Mn) for the four polymer chain lengths investigated in this dissertation are

S5000, 24000, 74000 and 127000 g-moll corresponding to average degrees of polymerizations

(Xn) of 8, 35, 108, 185, respectively. The protected ester polymer precursor was then hydrolyzed

with (n-Bu)4NOH to provide for the water-soluble conjugated polyelectrolyte PPE-CO2-. The

final polymer product was purified using dialysis against DI water for 4 days. All of the

polymers have polydispersity indices of ~ 2.(181)

Photophysical Methods

UV-Visible absorption spectra were recorded using a Lambda 25 spectrophotometer form

Perkin Elmer. Steady-state excitation and emission spectra were obtained with a Fluorolog-3

spectrofluorometer from Jobin Yvon. A 1-cm square quartz cuvette was used for all spectral

measurements. Concentrations varied from 10 to 30 CLM and were dissolved in spectroscopic

grade methanol.

Time-resolved anisotropy and fluorescence dynamics measurements were performed using

a femtosecond upconversion apparatus. An optical parametric amplifier (OPA) pumped by a

commercial Ti:Sa laser system consisting of a Ti:Sa oscillator (Spectra-Physics, Tsunami) and a









subsequent Ti:Sa amplifier (Spectra-Physics, Spitfire) with a repetition rate of 1 k
produce excitation pulses. More specifically, the output of the Ti:Sa amplifier feeds an OPA, and

the fourth harmonic of the signal is tuned to 375 nm. The excitation beams is fed through a prism

compressor, yielding an instrument response function of 225 fs. The instrument response

function (IRF) is determined by the cross-correlation of the excitation and gate pulses.

The upconversion setup used for these experiments is described in detail elsewhere.(182,

183) Briefly, a fraction of the 800 nm Ti:Sa amplifier that is leftover from the OPA is used as a

time delayed gate pulse (30 CLJ/pulse). After excitation, the sample fluorescence is collected using

a pair of off-axis parabolic mirrors and focused and spatially overlapped with the gate pulse in a

nonlinear crystal (0.5 mm BBO), resulting in the sum frequency of the two electromagnetic

fields (Figure 4-4 A).

The up-conversion signal has a photon frequency given by:


Usuns =gate + luo .U + (4-3)

This is also written as:

111

sunt gate fluo(4)

Detection wavelength is chosen by tuning the nonlinear crystal to a particular angle. Table

4-1 includes a list of each detection wavelength and crystal angles used for the experiments

discussed in this chapter. The resultant signal is then focused into a monochromator, detected

with a photomultiplier and the signal is gated with an integrating boxcar. When the gate pulse is

temporally and spatially overlapped with the fluorescence signal, the nonlinear crystal behaves

as an optical gate. Therefore, scanning the gate pulse with respect to the excitation pulses enables































Figure 4-4. Fluorescence Up-Conversion Technique A) Illustration of the upconversion principle
B) Up-converted fluorescence signal generated in a nonlinear crystal only while the
delayed gate pulse is present. (182)

Table 4-1.Experimental conditions for wavelength dependence study
Micrometer PMT
Emission h (nm) Monochromator h (nm) Micrometer Position (filters)
Detected Position w spacer

430 279.5 4.23 7.50 Blind
450 287.5 2.94 6.19 Blind
475 296.5 1.70 4.95 Blind
500 307.0 0.45 3.79 Blind/Vis
515 313.5 2.45 Vis (1 UG-11)
533 320.0 1.86 Vis (1 UG-11)
550 325.0 1.60 Vis (2 UG-11)
590 340.0 1.46 Vis (2 UG-11)


this optical gate to integrate different windows of time. The fluorescence signal is temporally


mapped at these varying time delays (Figure 4-4 B). (182)


A)


Crflu
Luminescence \ ..7
/ ...**** sum
,'/'*.. Up-converted signal
Gate Pulse / *.
Non-linear **
Gate crystal


The upconverted fluorescence signal intensity is determined by the convolution of the

fluorescence and gate pulse intensities:


I,,,, (r) = Io Ifa(t)Igate (r- t)dt


(4-5)









where z represents the time delay between the arrival of the gate pulse with respect to the sample

fluorescence. This optical gating technique is very advantageous because the time resolution is

dependent only on the width of the gate and pump pulses, not the detection system.(182) The

optical path length was 2 mm and the concentration of samples did not exceed 30 pLM yielding an

optical density ~ 0.45/mm. A circulating cell was used to ensure that a fresh volume of sample

was excited with every laser shot and a maximum of 100 nJ of energy per shot were used.

Anisotropy is the measurement of the extent of polarization that a material maintains after

being excited with polarized light. When the emission anisotropy is nonzero, the emission of the

material is polarized. The transition dipole moment (ya) of a molecule dictates which orientation

or direction molecules will absorb light. Light that is polarized consists of an electric field (E)

that oscillates in a particular direction. Excitation of a material with linearly polarized light

results in an excitation probability function that is proportional to the square of the scalar product

of the molecules dipole moment and the electric field vector (ya. E or cos2 8A) (Figure 4-6). The

phenomenon of polarized emission is dependent on the absorption and emission transition dipole

moments which can be oriented at different angles to one another. When the angle between the

two vectors is 900, the excitation probability is zero and maximized if they are parallel. After

creating an exciton in a high energy electronic state of an anisotropic material, it relaxes to the

first singlet state (Kasha' s Rule (2)) via internal conversion. Regardless of the orientation of the

transition moment of the high energy initial state, the emissive transition moment at the first

singlet state will remain the same (Figure 4-5). If the absorption and emission moments are

identical, the anisotropy will not be lost; however, if they differ, Figure 4-5, the anisotropy value

will change.(2)





Molecules can be excited selectively simply by arranging the electric field vector of the

incident light so that its orientation is relatively similar to their dipole moments. This method is

referred to as photoselection. For example, if a laser pulse with a polarization set to vertical is

used to pump a sample, only the molecules with vertical dipole moments will be excited.

Multiple processes can cause depolarization within molecules.(2)

These include: Adapted from B.Valeur.(2)

* Absorption and emission transition moments are not parallel
* twisting vibrations
* Brownian motion
* Energy transfer to other molecules with different transition moment orientations
* Molecular rotations


So-, S 2

Transition moments


f LS So-S,


'o
Absorption Fluorescence


Figure 4-5. Transition moments. Adapted from B. Valeur.(2)

Anisotropy measurements become very useful tools for determining information

concerning molecular size, shape and flexibility in addition to the viscosity of the solvent. The

fundamental anisotropy (ro) is the theoretical anisotropy of a material that does not undergo any

motion or loss of polarization.(2)










2 3cos2a_
ro (4-6)
5 2



O No Absorption




Maximum Absorption






OA Absorption COS28A



Figure 4-6. Photoselection. Adapted from (2)

For a spherical obj ect, if the absorption and emission transition dipole moments are

parallel (a = 00), ro should equal to 2/5 (0.4); however, if they are perpendicular (a = 900) the

lower limit is -1/5 (-0.2). These values correspond to the limiting values. If all emission

polarization is lost (due to any of the processes listed above) a value of zero anisotropy is

expected. The temporal behavior of the anisotropy can provide useful information regarding the

polarization loss mechanism.(2)

Fluorescence anisotropy decay measurements were conducted by rotating excitation

pulses with respect to a fixed polarization detection scheme. A Berek compensator is used to

excite the molecule with a beam polarized parallel and perpendicular with respect to the detected

fluorescence intensities. The anisotropy value (r) was then calculated using:

I, -I
r = "(4-7)
I, + 27,









The Model 5540 Berek polarization compensator from New Focus" was used in these

experiments to convert and control the pump polarization. A compensator such as this utilizes

the principal that different wavelengths of light propagate at different speeds through a medium

and that this velocity depends on the index of refraction. This compensator can cause a %4-wave

or '/-wave retardance for wavelengths in the ultraviolet (200 nm) to the infrared (1600 nm). The

compensator has a 12 mm aperture and was directly mounted to a post. The Berek compensator

is made up of a single birefringent uniaxial plate with an adjustable tilt angle to impose velocity

changes on incident light resulting in retardation. The velocity changes are both tilt angle and

wavelength dependent. The extraordinary axis, ne, is oriented perpendicular to the plate while the

ordinary axis, no, is parallel (Figure 4-7). If no tilt is imposed, the incident light remains normal

to the plate. As the light propagates through the medium, its velocity remains unaffected by the

polarization and is only dependent on the ordinary index of refraction. If the plate is tilted to a

particular angle, 8R, the velocity of the propagating light is changed. The axis oriented in the

plane of incidence is no longer ordinary, instead it has an "extraordinary" component, ne',

causing retardation. Polarized light that is perpendicular to the plane of incidence has a velocity

unaffected by the tilt. As a result, there is a retardance that is created between the ordinary and

extraordinary waves propagating in the polarization planes. The main advantage of using a Berek

compensator for polarization measurements is that it allows for simple and independent

adjustments for not only retardation but also plane of incidence orientation adjustments (which

are both wavelength dependent) as one unit. The retardation knob is used to set the tilt angle

while the orientation knob acts as a wave plate.(184, 185)

Due to group velocity dispersion, the retardance of the electric field is wavelength

dependent. Therefore, one must set the correct position of the Berek compensator polarization









axis. First, the tilt angle, OR, iS calculated then used to calculate the Retardation Indicator position

(I). From the Berek compensator manual we can derive the relationship between R and the tilt

angle. A summary is provided in this dissertation.

Consider a uniaxial crystal with an optical axis parallel to the plate surface. A normal

incident beam experiences a retardation (R) that is dependent on the path length (d), wavelength

(h) and the ordinary and extraordinary indices of refraction:(186)


R = (n n,) (4-8)


However, if the plate is tilted, the retardation equation becomes:(186)


R = (ne cos8, no cos 0) (4-9)


The tilt-induced extraordinary index of refraction from Figure 4-7 is determined by:(184, 185)

1 cosZ B sin" OR
-+ (4-10)
ne n; n;

The relationship between the optical axis of the medium, tilt angle, angle of incidence and

indices of refraction are used to derive the following equation for the retardance:(184)


2000 1 1- n sin O
R g s O 1 (4-11)


To use the Berek compensator as a half wave plate (1/2), R is fixed to 0.5. The tilt angle and

Retardation Indicator equations are purely empirical and are based on the crystal dimensions

only known by New Focus@ (187) and the dispersion relations for the indices of refraction

determined by Dodge (188) included in the Berek compensator manual. The indicator versus

wavelength graph corresponding to quarter and half wave retardance is included in the manual.

(184) The tilt angle is estimated using the following empirical equation:(184)









8 a sin (0.284J/1) (4- 12)

where h is the wavelength in micrometers. Once 8, is determined, the Retardation indicator

setting on the compensator can be calculated from the following empirical relationship:(184)


I= 50.2271sin -4~ (4-13)

After the retardation indicator is set, the "Orientation" (0) position must be rotated to the proper

position (Figure 4-8). For excitation at 375 nm, the tilt angle in radians was calculated to be

0. 1238 which led to a Retardation Indicator setting (I) of 6.57. To rotate polarizations by 900, the

retardance is turned to a V/2-WaVe Setting and the orientation positioned at 450 since the 1/2-wave

plate causes rotation of the plane of polarization by twice the orientation angle.(184) A polarizer

was used to verify the polarization of the beam exiting the compensator.

The Berek compensator was set to magic angle conditions to measure isotropic

fluorescence decay curves. Magic angle is a set condition that enables detection of the total

fluorescence intensity, not just emission proportional to I, orl The emission monochromator

depends on polarization; the observed signal is not proportional to the total intensity (which is

equal tol, + 2I In order to achieve the correct ratio, the excitation is oriented 54.70 from the

vertical since the cos2 (54.7) is 0.333 and sin2 (54.7) is 0.667 forming the correct sum for the

total intensity. This is especially important for fluorescence decay measurements because the

vertical and horizontal signals are usually very distinct due to molecular rotations, energy

transfer or some other polarization dependent process and if their intensities are not properly

weighted then incorrect population decay times are recovered.(189) The intensity at magic angle

is calculated as: (189)










I, + 2 I
I
magic angle


(4-14)


Tilted


8,R





LIGHT


LIGHT


Figure 4-7. Berek polarization compensator. Tilting the crystal causes retardance and
birefringence. Adapted from New Focus.(185)


Input: Wave Plate
Linearly Setting: 3/2
Polarized


Output:
Linearly
Polarized -
900 rotated


Figure 4-8. Berek compensator used as a half-wave plate. I
orientation. Adapted from New Focus.(184)


retardance indicator, O


No Tilt


ne)


no



o e









Photophysics of Variable Chain Length PPE-CO2~ POlymers

Steady State Characterization

The steady state photophysics pertaining to variable chain lengths of water soluble PPE-

CO2~ pOlyelectrolytes has been previously reported.(181) Inhomogeneous broadening (Figure 4-

9) is exhibited in the absorption spectra due to a distribution of excitation energies resulting from

slight variations and superpositions of absorptions of various segments with different

conjugation lengths. In addition, as the length of the polymer chain increases, the isolated xn -

x*" peak shifts towards the red possibly due to an extension of the conjugation length.(190) The

absorption maximums for the 8 PRU and 185 PRU are 404 and 432 nm respectively. Moreover,

the shoulder at 432 nm, which is assigned to aggregated species, becomes the absorption

maximum for the 108 and 185 PRU polymers. Structured vibronic features in the emission

spectrum are shown in Figure 4-10. The high energy emission corresponds to isolated chain

emission while the broader, low energy emission arises from the aggregate states (appearing as a

shoulder). The emission does not display the similar red shift seen in the absorption, instead the

fluorescence peak shifts are extremely small and decrease as the chain length is extended. The

SO S1 (0-0) transition corresponding to the 35 PRU is Stoke's shifted with respect to the 404

nm absorption by 20 nm. Meanwhile, the 185 PRU displays a Stoke's shift of only 4 nm between

the blue end of the emission (436 nm) and the red edge of the absorption (432 nm). The 185 PRU

sample undergoes self-absorption at 436 nm. A similar behavior has been observed in poly-

(para)-phenylene-ladder-type (LPPP) (116) in which the bridging present within the polymer

prevents the phenyl rings to twist, maintaining conjugation. The authors claim that the small

Stoke' s shift reflects the rigid geometry of the conjugated main chain resulting in reabsorption of

the So S1 (0-0) transition.(181) PPE-CO2~ pOlymers are geometrically rigid resulting in

comparable Stoke's shifts to the LPPP polymer. As the chain length increases from 35 to 185












PRU the weight of the aggregate emission at 520 nm increases due to an increase of the amount


of aggregate present in solution.



1.0-



0.8-



4 0.6-



0.4-



0.2



0.0 l l is l l
300 325 350 375 400 425 450 475 500

h (n m)


Figure 4-9. Chain length absorption shift for PPE-CO2- in methanol. Polymer repeat units 8 (-),
35 ( ), 108 (-), 185 (-)

1.2 -


1.0-


0.8 -


0.6-
Z
0.4 -


0.2-


0.0-

400 450 500 550 600 650 700

h (n m)
Figure 4-10. Emission spectra 10pM PPE-CO2- (methanol) excited at 380 nm for 35 PRU ()
and 185 PRU (-).


Despite the direct relationship between the increase in molar extinction values to the


increase in repeat units, the quantum yield for fluorescence decreases from ~ 0.6 (8 PRU) to ~









0.1 (108 PRU). Zhao et al. suggest that conformational, vibrational and rotational degrees of

freedom creating non-radiative decay channels lead to decrease fluorescence.(181) On the

contrary, it is clear that the large absorption red shifts due primarily to the rigidity of the

polymers lead to large emission and absorption spectral overlap. Moreover, the conformational

restrictions induced by this rigidity give rise to conjugation lengths longer than expected

ultimately reducing the degrees of freedom within the polymer and thus potentially makes the

stated reason an invalid argument to explain small quantum yields for long PPE-CO2- chains.

Figure 4-11 shows the emission spectrum of the PPE-CO2- (3 5 PRU). The black line

corresponds to the polymer dissolved in methanol and it shows the sharp bands characteristic of

isolated chain emission. The red line corresponds to a methanol solution in which 6 pLM of Ca2+

has been added. Ca2+ is an effective cross linker with the 2 carboxyl groups inducing aggregation

of the PPE-CO2-.(155) Emission from the aggregate can be clearly observed on the shoulder at

520 nm, as it grows relative to the unaggregated emission at 436 nm. Finally, when the CPE is

dissolved in water, the aggregated emission is mostly observed (green line). Overall, the red

shift, quenching and band broadening are due to aggregate formation of the polymer chains since

Ca2+ is a closed-shell ion and does not act as an electron or energy acceptor. (191-194) The small

Stoke' s shift previously mentioned results in excellent overlap of the isolated chain emission

with the aggregate absorption enhancing energy transfer from the higher energy isolated species

to the lower energy aggregates.

The excitation spectrum of a given chromophore is determined by monitoring the

fluorophore emission as it is excited at different wavelengths. Abramowitz et al. from the

Olympus" Microscopy Resource provides an excellent description of the excitation spectra

collection process.










"An emission wavelength is chosen and only emission light at that wavelength is
allowed to reach the detector. Excitation is induced at various excitation
wavelengths and the intensity of the emitted fluorescence is measured as a function
of wavelength. The result is a graph or curve which depicts the relative fluorescence
intensity produced by excitation over the spectrum of excitation wavelengths."
(195)

Excitation experiments at different detection wavelengths can be employed to identify the

species contributing to the emission which can be hidden due to inhomogeneous broadening

caused by the polydispersity present within polymeric samples. The absorption spectra lead one
1.2 .~~~.


1.0


0.8-


S0.6





0.0 I I III



400 450 500 550 600 650 700
h(nm)
Figure 4-11. Emission of 10 CLM 35 PRU PPE-CO2- in methanol (-), methanol with ~ 6 CLM
Ca2+ (-) and in water ()

to believe that no aggregates are present in the shorter polymer samples since neither a shoulder

or broadening are observed; however, excitation spectra indicate that this is not the case. Figure

4-12 presents the excitation spectra of the 8 PRU CPE in methanol detected a four different

wavelengths (430, 475, 510 and 590 nm). Detection at 430 (blue line) and 475 nm (green line)

show broad, featureless excitation spectra peaked at 396 nm. Upon shifting the detection

wavelength to 510 nm the excitation spectra becomes even broader and a small red-edge shift











begins to appear (440 nm). By shifting detection to 590 nm (red line), this red edge shift appears

more pronounced and it is due to the aggregate species.

From this data, we conclude that even in dilute solutions of CPE with small PRU lengths,

some aggregate is present and contributes to the overall energy transfer mechanism due to

spectral overlap. A small amount of the short, rigid chains are likely to "stack" on top of each

other rather than cluster up creating this red edge shift. It is also suggested in Zhao's work that

the reduction in quantum yield could be a result of the presence of aggregates which compete

with radiative decay channels.(181) Photophysical data presented here suggest that the presence

of aggregates is indeed a more suitable explanation because the presence of isolated and

aggregated chains is detected in small, dilute PRU samples in MeOH.
1.2 gg. g.


1.0


0.8 --



0.6 --


0.4 --


0.2 --


0.0 iiiil l
325 350 375 400 425 450 475 500

1 (nm)


Figure 4-12. Excitation spectra 10 pLM 8 PRU PPE-CO2-. Detection at 430 (-), 475 ( ), 510
(-), and 590 (-) nm










For longer PPE-CO2- PRU chains dissolved in methanol, the distinction between isolated

and aggregate emission bands based on the excitation spectra becomes clearer. For example,

Figure 4-13 shows the excitation spectra of 35 PRU PPE-CO2- in methanol. Detection at 430 nm

(blue line) results in a distinct peak at 390 nm. As the detection wavelength increases, the peak

broadens and shifts to the red. Detection at 590 nm (red line) clearly shows a new peak at 430

nm and this peak is attributed to the direct excitation of aggregated species. Figure 4-14 presents

the excitation spectra of the 3 5 PRU CPE dissolved in water detected a four different wave-

lengths (430, 475, 510 and 590 nm). Detection at 430 nm (blue line) show broad, featureless

excitation spectra peaked at 390 nm due to isolated chains. As the detection wavelength

increases, the peak broadens and shifts to the red. Upon shifting the detection wavelength to 510

nm (black line) the excitation spectra becomes even broader and a new peak appears (436 nm).

Detection at 510 (black line) and 590 nm (red line) exhibit more well-defined peaks at 436 nm,

comparable to the 185 PRU absorption spectrum in neat methanol.

1.0




0.8-





0.4-


0.2-


0.0 llIII
325 350 375 400 425 450 475 500
h (n m)

Figure 4-13. Excitation spectra 10 pLM 35 PRU PPE-CO2~ in methanol. Detection at 430 (-),
475 ( ), 510 (-), and 590 (-) nm










A similar trend is observed in Figure 4-15. After addition of 60% calcium to the methanol

solution, noticeable changes within the excitation spectra (at bluer wavelengths) indicate that the

calcium induces more aggregation within the polymer solution. Contribution from aggregates is

clearly evident when detecting at 475 (green line), 510 nm (black line), and 590 nm (red line),

although the signal collected at 590 nm does not increase significantly compared to the neat

methanol sample. When 35 PRU PPE-CO2~ is dissolved in water, it is confirmed that CPEs do

exist in water as aggregates, however; isolated chains are still present although their contribution

to the overall steady state fluorescence is reduced due to fewer free chains in solution,

reabsorption and energy transfer. The excitation spectra results presented here provide evidence

to support our assumption that multiple species contribute to the emission and overall

fluorescence decay in even dilute PPE-CO2~ Samples.



1.0




0.8--



co 0.6--


0.4--


0.2--


0.0 gggg..
325 350 375 400 425 450 475 500

h (nm)


Figure 4-14. Excitation spectra 10 pLM 35 PRU PPE-CO2- in water. Detection at 430 (-), 475
( ), 510 (-), and 590 (-) nm










Time-Resolved Fluorescence


Isotropic Upconversion

The fluorescence dynamics of PPE-CO2- with repeat unit lengths equal to 8, 35, 108 in

MeOH were excited at 375 nm and detected at magic angle at several different wavelengths.

Data was fit with a sum of exponentials using the following equation:


ft (-
I(t) = C A exp (-5


where A, represents the weight of each rate constant and r, is the associated time constant. Data

from these fits are summarized in Table 4-2.

1.2 g. .


1.0


0.8 --



S 0.6 --


0.4 --


0.2 --


0.0 ls llll
325 350 375 400 425 450 475 500

h (nm)

Figure 4-15. Excitation spectra 10 pLM 35 PRU PPE-CO2- in methanol with ~ 6 pLM Ca2+
Detection at 430 (-), 475 ( ), 510 (-), and 590 (-) nm

Figure 4-16 shows the time-resolved fluorescence decay of 8 PRU PPE-CO2- in methanol

at three different detection wavelengths (430, 436 and 450 nm) excited at 375 nm. Detection at

430 nm (blue line) shows a multi-exponential behavior which disappears as the detection









wavelength increases to 450 nm (red line). The isolated chain time constant of 531 ps (Table 4-2)

is extracted from the mono-exponential decay of the 8 PRU at intermediate wavelengths 450 (red

line). As the detection wavelength is increased from 450 to 550 nm (not shown) the behavior of

the exponential decay does not change. At all detection wavelengths the rise times are

comparable to our instrument response function. The first panel of Figure 4-16 shows the same

detection wavelengths on a shorter time scale. An extremely fast decay (< 1.5 ps) is observed

Table 4-2. Detection dependence decay times

PRU Det h (nm) zl1 (o ps) Amplitude zz2AG tpS) Amplitude
8 PRU 430 2713 36% 490+10 64%
436 1415 29 624119 71
450 531+7 100

35 PRU 430 1111 65% 17816 35%
450 3713 39 363110 59
500 3315 39 402117 61
550 3512 53 454116 47

108 PRU
Det h (nm)
430 1412 62% 20119 38%
450 4314 39 395111 63
500 42 (fixed) 39 333125 62
550 3916 55 551149 45

35 w 50% Ca
Det h (nm)
430 1812 61% 450117 40%
450 4315 29 468111 71
550 2614 53 611145 47

when detected at 430 nm (blue line) and its contribution to the overall signal diminishes as the

wavelength increases from 430 to 436 (magenta line) to 450 nm (red line). Emission at

wavelengths below 450 nm spectrally overlaps with the aggregate species absorption resulting in











efficient energy transfer from isolated chains to aggregates. Interestingly, detection at 590 nm


does not yield a significant change in the rise time (not shown) or decay compared to 450 nm


which had been expected if detecting emission from the aggregates. Dynamics observed at


wavelengths between 430 and 450 nm exhibit an additional intermediate decay time of 30 to 40


ps (middle panel). The amplitude of these time constants decreases as the wavelength increases.

From the excitation spectra it is clear that there is a small, yet significant amount of aggregate


within the sample below 450 nm facilitating energy transfer; hence, the intermediate time


constant is assigned to the energy transfer from the ensemble isolated to the aggregated chains.

1.6 a ,, ~ ~ ..,

1.4 -- -- -

1.2 -- -- -

1.0 -- -

E 0.8-- -

0.6 -- -

0.4 -- -

0.2 -- -- -

0.0 a - -
0 2 4 0 50 100 150 20t) 100 200 300 400 500 600

time (ps)


Figure 4-16. Time-resolved fluorescence decay of 8 PRU PPE-CO2- in methanol at three
different detection wavelengths. 430 (-), 436 (-), and 450 (-) nm, Fits (-)

The results for solutions of 35 and 108 PRU PPE-CO2- (30 CLM) are shown in Figure 4-17


for three distinct wavelengths (430, 450 and 550 nm) upon excitation at 375 nm. Detecting at


430 nm, the contribution from the very fast component is larger for the 35 PRU sample (black


line) compared to the 108 PRU sample (red line). As the detection wavelength is shifted to the


lower energies, contribution from this fast component to the overall signal is reduced.


Interestingly, the changes are more pronounced on the 35 PRU than on the 108 PRU sample.









Detecting at 550 nm, there is no contribution from the fast component to the 35 PRU signal, but

it is still present in the 108 PRU signal.

The photoluminescence rise follows the response function of the experimental setup. At

very long wavelengths one would expect to see a build up of population (rise time) due to energy

transfer from the isolated states however, the data collected do not show this slow rise time.(156,

196) This indicates that the singlet exciton is transferred from shorter (high energy) chains to

traps within the chains, not to aggregates. The right column in Figure 4-17 shows the

intermediate and long decays, observed at different detection wavelengths for the 35 and 108

PRU samples. Detection at 430 nm (top graph) presents an intermediate decay constant of 11+1

ps and a long decay of 178 & 6 ps. For longer detection wavelengths, the intermediate time

constant is ~ 30 to 40 ps while the long time decay lies between 350 and 450 ps. Results are

summarized in Table 4-2. The qualitative trend observed for the intermediate component is the

same for the 35 and 108 PRU but not 8 PRU. In the 450 to 590 nm detection region, as the

wavelength increases, the amplitude of the intermediate decay increases but the lifetimes do not

change significantly. These time-resolved emission signals result from an ensemble of isolated

CPE chains with different conjugation lengths. Meanwhile, the slow decay time increases as the

wavelength increases. A 350 to 450 ps time constant corresponds to the isolated chain natural

fluorescence lifetime (extracted from the 8 PRU detected at 450 nm); therefore, this component

is assigned to isolated chains not participating in the energy transfer process. Energy transfer is

not only dependent on spectral overlap of the donor fluorescence (isolated chain) and acceptor

absorption (aggregate chain), but also on the molecular distance between the two species.(2)

After initial energy transfer (<1.5 ps) from short conjugated segments to traps, the energy is then

transferred to the aggregate species (intermediate decay time). The change in amplitudes of the































12
10
08
06


00 -~

-02


intermediate decay time depends on the percentage of isolated chains transferring energy to the


aggregate species.

24L 14


~12 12
20 /~Imr a
10 'li',hn~??~~1.ru
16 08v-


0 1 2 3 4 5 0 100 70, 3Co 4aC sa eC




Figure 4-17. 30 CLM PPE-CO2- 35 PRU (-) and 108 PRU (-) with fits ( ) at various detection
wavelengths A) 430 nm, B) 450 nm, C) 550 nm


Figure 4-18 shows photoluminescence of a solution of 35 PRU PPE-CO2- in methanol

detected at 430 nm and the influence of addition of Ca2+. The first panel shows the fast decays,


the second panel shows the intermediate decays and the third panel shows the long decays. Ca2+


greatly influences the dynamics, dominating the ultrafast decay signal, altering the intermediate


decay amplitude and increasing the long decay as the detection wavelength increases. Rise times











are IRF-limited at all wavelengths. The augmented amount of aggregation increases the


amplitude of the energy transfer components. Jiang determined that Ca2+ induces the formation


of "loose" aggregates in methanol and more traps along the polymer backbone.(155) Our results


show that the addition of Ca2+ (red line) amplifies the energy transfer from the short isolated


chains to traps. Detecting at 430 nm, the long component increases from ~180 to ~ 450 ps while


the intermediate decays times remain relatively the same (Table 4-2).

1 : : 1.50 1.50 *
4.5-

4.0 -1 1.25 -1 1.25-

3.5 --
1.001 -1 1.00-
3.0-

E 2.5 --0.75 -U -1 0.75

Z2.0
0.50C C, -1 0.50-
1.5-

10-0.25 Vs.i -1 0.25-

0.5-
0.00 -1 0.00-
0.0 ~ ~ ~ 1 1 1 1 1
O 1 2 3 4 5 0 50 100 150 200 0 100 200 300 400 500 600

time (ps)

Figure 4-18. 30 ILM PPE-CO2- 35 PRU without (-) and with (-) Ca2+ at 430 nm.

Figure 4-19 presents the effect of the addition of Ca2+ to the dilute 8 PRU sample detected

at 450 nm upon excitation at 375 nm. The black line corresponds to the isolated chain emission


and the red line corresponds to the 8 PRU with the addition of 15 pM Ca2+. At 450 nm, the


addition of Ca2+ introduces a 1.5 ps decay time and an intermediate component (~ 30 to 50 ps)


not seen in the neat 8 PRU sample. The radiative decay rates measured with and without calcium


differ due to the presence of aggregate structures in conjugated polymers. As stated previously,


the addition of the dication calcium to the PPE-CO2-/methanol mixture causes aggregation,










bringing the molecules within the Coulombic coupling transfer radius facilitating extremely

efficient energy transfer.(197)

Cation-induced aggregation plays a critical role in amplified quenching (154, 155, 157,

168, 1 72, 180) due to ultrafast energy transfer. In addition, the Stern-Volmer behavior of CPEs

in the presence of polyvalent quencher ions such as MV2+ and a cation in solution have proven to

be superlinear. In fact, due to the loose type of aggregates formed when calcium is added to

PPE-CO2- solutions, small quenchers, like MV2+ have higher quenching efficiencies than in

other aggregate-inducing solutions, i.e., water. (155)


1.0


0.8 -I


0.6 --


S0.4 -'~v



0.2-


0.0 IIlIl
0 100 200 300 400 500 600
time (ps)
Figure 4-19. 10 CLM PPE-CO2- 8 PRU detected at 450 nm without (-) and with (-) Ca2+

We examined the effects on the energy transfer after addition of a quencher molecule to a

solution of PPE-CO2- with and without calcium. Figure 4-20 compares the fluorescence

dynamics a solution of 30CLM 35 PRU PPE-CO2- in methanol excited at 375 nm and detected at

450 nm when 15 CLM calcium, MV2+, and a mixture of the two are added. The concentration used

for MV2+ Samples was equivalent to the amount needed to quench the steady state fluorescence










by 80% (I/Io). The ultrafast decay that occurs in less than 1.5 ps is present in all samples

regardless of the presence of calcium (not shown). No significant changes in the excited state

lifetime at long timescales were detected despite a reduction in the photo-luminescence quantum

yield implying that the most important step in the decay mechanism facilitating amplified

quenching occurs in the first 2 ps in PPE-CO2~ pOlymers.


1.00-



0.75--



o 0.50--



0.25-



0.00--
) 50 100 150 200 250 300 350 400 450
time (ps)
Figure 4-20. 30 CLM PPE-CO2- 35 PRU with (-) Ca2+ at 450 nm 30 CLM PPE-CO2- 35 PRU with
MV2+ (80% quenched) at 450 nm and (-) 30 CLM PPE-CO2- 35 PRU with 15 CLM
Ca2+ and MV2+ (80% quenched)

Time-Resolved Anisotropy

To investigate the fluorescence depolarization, random walk migration, or intermolecular

energy transfer we measured the anisotropy dynamics of the 8 PRU polymers in methanol.

Figure 4-21 depicts the time-resolved fluorescence anisotropy of the 8 PRU detected at 430

(black line) and 450 (red line) nm upon excitation at 375 nm. An ultrafast loss of anisotropy (not

shown) followed by a constant anisotropy during the lifetime emission of the polymer is











observed. After the initial change in the first 5 ps from r ~ 0.4 to 0.2, both curves then remain

parallel to one another.

0.1 4gggg


0.3-


0.2--


0.1-


0.0


-0.1 --


-0.2 gggg.
0 100 200 300 400 500 600

time (ps)

Figure 4-21. Anisotropy of 8 PRU PPE-CO2-. Detection wavelengths 430 (-) and 450 (-) nm

Detection at 430 nm occurs in a region in which shorter conjugation lengths are present

allowing for a higher number of hops before finding traps along the polymer backbone resulting

in smaller anisotropy value (r~ 0.10) compared to the 450 nm (r~ 0.18). The 450 nm scan

corresponds to slightly longer conjugation and fewer hops. As shown in other works (130, 197) a

long polarization decay corresponds to aggregates emitting almost randomly polarized light

reducing the total polarization. However, in this polymer there is little to no depolarization due to

reorientation at 450 nm. Random walk migration is considered to occur at intermediate decay

times in PPE-SO3-, (130) but is not observed in PPE-CO2-. If random walk of excitations along

the polymer backbone were to occur, as the wavelength increased the hopping rate would

decrease due to a lack of lower lying states that are available for the excitation to jump. The

following factors clearly eliminate the possibility of random exciton migration in this PPE-CO2:










rigidity of the polymer, longer conjugation length, spectral overlap and proximity leading to

energy transfer from isolated to aggregated chains, no detection wavelength dependence for the

decays measured at various wavelengths and no depolarization at intermediate decay times even

in the presence of aggregates.

Potential Kinetic Model

The scheme presented in Figure 4-22 depicts a proposed model concerning the dynamics

mechanism in PPE-CO2-. This figure presents the model for the different polymer repeat units

collectively. The 8 PRU consists of mostly isolated chains. After excitation at 375 nm, energy is

transferred from the shorter conjugation lengths to traps located in the isolated chains. Due to the

spectral overlap between the isolated chains emission and aggregate absorption, this energy

transfer is detected primarily at wavelengths shorter than 450 nm. These detected emission

signals are convoluted with decays from an ensemble of different conjugation lengths, kinks and

other non-radiative recombination pathways within the isolated species. From the excitation

spectra of the 8 PRU (Figure 4-12) we can see that the red edge shift at 440 nm does not overlap

significantly with wavelengths greater than 450 nm. Energy transfer from the isolated chains to

the aggregates is not efficient resulting in a mono-exponential fluorescence lifetime from an

ensemble of isolated chains.

Longer polymer chains (35, 108, 185) contain a larger mix of isolated and aggregated

chains. After excitation at 375 nm, energy is transferred from the shorter conjugation lengths to

traps located in the isolated chains similar to the 8 PRU. Subsequently, energy transfer from this

ensemble to the aggregate species is observed within a wide spectral range. The number of traps

is increased upon addition of calcium resulting in a sharp increase in the ultrafast time

component but the energy transfer to the aggregate does not change significantly. Self-absorption

is observed in the 108 and 185 polymer repeat unit samples (more aggregated); however, this is a









radiative energy transfer process in which the photon emitted by the donor is then absorbed by

the acceptor. Therefore, it does not compete with other decay mechanisms and the fluorescence

decay time of the donor remains unchanged (refer to Chapter 1). Following the energy transfer,

the detected emission is dominated by the ensemble of decays from isolated chains and traps

located along the polymer backbone in addition to competing with non-radiative decay channels.

In summary, upon excitation of the aggregates from energetically higher lying isolated chains,

the fluorescence lifetimes result in multi-exponential behavior due to the competition between

the radiative and non-radiative decay. The integrated fluorescence collected for these

experiments does show emission from aggregates but it cannot be observed in the ultrafast time-

resolved experiments because of very long decay time constants and small contribution to the

overall signal.

Summary

The ultrafast time-resolved fluorescence of a series of PPE-CO2~ pOlymer repeat units is

presented. Using steady state UV-Vis, photoluminescence and excitation resources we

distinguished the species present in each solution. It was shown that even dilute, short PRU

chains do exhibit a small amount of aggregation. The addition of calcium or using water as the

solvent induces aggregation resulting in broad absorption/excitation spectra and the growth of a

red shoulder in the emission. To investigate the influence aggregation has on the fluorescence of

the polymers, we conducted a detection wavelength study using fluorescence upconversion. The

isolated chain emission was extracted from the 8 PRU at 450 nm (isolated chain emission and

aggregate absorption is minimal). In the presence of aggregates, an intermediate time constant on

the order of 30 to 40 ps is observed and is assigned to the energy transfer from the isolated to

aggregate species. At bluer wavelengths, a fast decay (< 1.5 ps) is observed and is attributed to

the transfer of excitation from shorter, high energy chains to longer, low energy chains and traps.









Time-resolved anisotropy confirmed that this polymer, no matter the PRU size, is extremely

rigid and has long conjugation lengths.

t,2~ 30 to 40 ps

,z< 1.5 ps






375 nmn z \,350 to 450









Figure 4-22. Possible kinetic model for all PPE-CO2~ PRU chains









CHAPTER 5
CONCLUSIONS AND FUTURE WORK

Nanoparticle Conclusions and Future Work

Conclusions

A complete steady state and time-resolved study of size, shape passivation and

composition dependence on colloidal semiconductor nanoparticles has been conducted in our

labs. Using pump-probe spectroscopy we were able to detect traps and interfacial states.

Confirmation of Auger-like cooling resulting in 1P to 1S relaxation (~ 1 ps) has been shown in

addition to interband relaxation (> 200 ps) have been measured for each nanoparticle system.

Comparisons between materials with different compositions were made finding higher

confinement potential in ZnCdSe alloys than in CdSe resulting in a lower probability of the

exciton to sample the surface and be trapped. Finally, utilizing each of the spectroscopic tools

available we were able to combine steady state and time-resolved data to construct qualitative

models describing the nanorod systems.

From this work we can conclude that passivation and alloying result in quantum yields

higher than for bare CdSe. The excitons are more confined in the alloy particles than in CdSe

rods. It is well known that the band gap is size (17) and shape dependent.(53) Despite breaking

the symmetry within the nanoparticle, confinement properties were maintained in our samples.

Passivation with an inorganic shell results in increased quantum yields and bleach signals

because the surface traps are eliminated. Finally, modifying the composition using one additional

synthetic alloying step has greatly improved the process-ability without drastically sacrificing

confinement characteristics.









Outlook/Future Work

Lasing/ Optical gain A population inversion in lasing media, in which the population of

electrons in the excited state must be greater than the population left in the ground state, is

necessary to achieve optical gain. It is clear that the size of these nanocrystals leads to an

enhancement of the carrier-carrier interactions leading to impressive optical properties in

systems with single and multiexciton states. It has been shown that new energy relaxation

pathways are created in nanocrystals compared to the bulk. For example, due to the high density

of states present in bulk materials, there is a lack of a phonon bottleneck which is also bypassed

in nanocrystals because of ultrafast non-radiative Auger recombination. Therefore, to attain

inversion within nanocrystals, a simultaneous excitation of the two electrons in the ground state

to an excited state must occur resulting in emission of multi-excitons via an Auger process which

dictates the decay of optical gain. (122, 198, 199)

Recently, Klimov has investigated the mechanisms for photogeneration and recombination

of multi-excitons in nanocrystals necessary for lasing and solar energy conversion. However,

since this recombination occurs in less than 1 ps, the optical gain lifetimes are in the picosecond

regime which is a drawback when designing materials for lasing. To improve optical gain it is

important to develop new materials that inherently diminish this phenomenon. It has been

suggested that increasing the nanocrystal volume fraction (packing density) in the optical gain

medium or using quantum rods instead of spheres will help reduce the influence that Auger

recombination has on these systems.(199)

Klimov has been using quantum rods to try to achieve population inversion for optical

gain. In order to do this, the Auger effects must be significantly reduced. He showed that

especially for CdSe based nanocrystal, the rods have slower Auger rates compared to quantum

dots with the same volume that emit in the red and orange spectral region.(199) It is hoped at










particular emission wavelengths, the Auger decay can be stifled in rod-shaped nanocrystals due

to not only the dependence that the confinement potential has on the length and size but also the

linear scaling of the decay time with rod volume.(52, 199, 200) For higher energy (shorter

wavelengths) quantum dots, the increased surface-to-volume ratio inhibits the Auger decay

suppression. Elongation of the nanocrystal in the c-direction has successfully increased the

optical gain lifetime since the effect that Auger has on the recombination behavior in rods is

decreased.(199, 201) CdSe/ZnSe core/shell materials have been thought to be used for this

application but it is more suitable to use inverted ZnSe/CdSe heterostructures in order to control

the electron-hole wavefunction overlap to increase the confinement energies and reduce Auger

recombination.(199, 202, 203)

Formulation of new alloy nanorods has opened the door for new investigations for their

potential applications. Extension of the c-axis in these ternary materials enables for higher

confinement potential in the blue-green region. It may be worth trying to determine how to alloy

the materials and then coat them to increase their photoluminescence quantum yield to make

them comparable to the core/shell or shell/core materials. These new alloy/shell materials can be

tunable based on the diffusion and have extended lifetimes necessary for charge-separated or

optical gain applications.

Light emitting diodes (LEDs) (11, 204-206). Although several improvements using

organic molecules for organic light emitting diodes (OLEDs) make them comparable to current

technologies, there are ongoing drawbacks and problems that must be overcome before these

devices can be commercially utilized. These include: a) difficulties tuning the colors since the

fluorescence is broad and b) synthesis of multiple molecules is required to obtain a broad range

of colors. Nanocrystals are being considered as attractive candidates to be used for LEDs since









their emission is not only tunable but considerably narrower than that from organic materials.

Moreover, nanocrystals have a higher probability of resisting photodegradation.(66)

Hybrid OLEDs have been developed in the past fifteen years, incorporating a polymer such

as PEDOT (207) or PPV (204) to transport charge to various nanocrystals (CdSe (204),

CdSe/CdS (11), CdSe/ZnS (208)) that act as the emission layer resulting in more stable and

efficient devices. Development of better materials and manipulating their interactions are the

main goals when working towards designing products that result in high electroluminescence

efficiencies. To achieve commercial quality devices the functionality must be improved by

enhancing the charge transfer between the polymers to the nanocrystal emission layer and

increasing the surface quality so the "traps" which cause non-radiative recombination are

reduced.(66) Experimentation with different combinations of polymer/nanoparticle blends is a

standard methodology to find devices that get rid of such adverse consequences. Within the

literature, most nanoparticles are spheres and their band gaps are tuned by only changing their

diameter.(20, 39, 48, 75, 78, 136, 138, 209-212) As the diameters are decreased, the band gap

energy does increase and emission in the blue-green region is achieved, however, the surface-to-

volume ratio is significantly increased which can lead to more surface "traps". Therefore, some

investigations into hybrid LEDs should incorporate not only size distributions to obtain tunability

but to investigate how the shape, passivation thickness and composition will affect the overall

efficiencies of the devices. A wide range of colors in the blue-green region can be achieved

simply by altering the alloying times in ZnCdSe nanorods. If a simple technique was developed

to passivate these alloy rods to reduce surface traps, they would allow for blue-green emission

wavelengths via a simple synthetic route (one batch).









Photovoltaics (213, 214) Although the cost of making quantum dot based photovolatics is

small, the efficiencies, due to recombination loses, are still too low for them to be used on a large

scale. Hybrid photovoltaic devices are integrated within the polymers to transport charge for

such applications as solar cells. Achieving charge separation and positive transport of the hole

and electron to the indium tin oxide (ITO) and aluminum electrode, respectively is the main goal

in photovoltaics.(66, 214) Instead of focusing on a binary system, a device utilizing ternary

compositions with varying degrees of Zn diffused into the core, may serve as more suitable

materials for photovoltaics. The variable Zn diffusion will create a gradient from the core to the

surface enabling the exciton to hop from one rod to another ultimately reaching the aluminum

electrode. Also, work might be directed to achieve charge separation within the nanocrystals.

The fact that the hole created after excitation within our CdSe/ZnSe quantum rods potentially

tunnels into the shell could help sustain charge separation and inhibit premature charge carrier

recombination. Investigations into the kinetics and mechanisms for creating and maintaining

charge separation in these materials are recommended since these processes are not completely

understood; however, it has been shown that the interface between nanocrystals and porous TiO2

supports highly efficient charge separation.(132, 199, 202)

Carrier multiplication The carrier-carrier interactions in nanoparticles lead to improved

exciton (carrier) multiplication (CM) which results from direct photogeneration of multiple

electron-hole pairs by single photons. This process is relatively new and the actual mechanism

behind carrier multiplication (if it truly exists) from a single excitation are currently being

debated. Klimov showed that seven excitons are produced in PbSe nanocrystals (QE = 700%

where 100% means 1 photon creates 1 e-h pair). This unique finding will be good for

photovoltaic cells and improve solar fuel technologies in the IR region. However, CdSe dots









have not been able to exhibit as efficient CM efficiencies in the visible region.(199)

Investigations into the effect that shape has on this phenomenon is recommended.

PPE-CO2 COnclusions and Future Work

Conclusions

The synthesis, characterization and time-resolved measurements conducted on this series

of PPE-CO2~ pOlymers with different chain lengths and extent of aggregation have led to several

interesting observations and conclusions. A complete steady state and time-resolved study of

length, solvent and aggregated inducer has been conducted in our labs. In particular, isotropic

and anisotropic fluorescence up-conversion was utilized to understand the quenching mechanism

within PPE-CO2-. We conducted a wavelength detection study based on exciting on the blue side

(isolated chain) of the absorption. Upon excitation of the aggregates from energetically higher

lying isolated chains, the fluorescence lifetimes result in multi-exponential behavior due to the

competition between multiple decay pathways. (116, 153, 156, 165, 215)

In all samples, the emission is inhomogeneously broadened. Detection of time-resolved

signals at all wavelengths is associated with the isolated chains emission, despite the

superposition of both species present within the samples. In addition, unlike PPE-SO3-, PPE-

CO2~ is Very rigid resulting in longer conjugation lengths than 4.5 PRU. This was determined not

by evaluating the behavior of the polymer chains as a function of chain length using steady-state

absorption but by evaluating the extremely slow time-resolved anisotropy decay. From the

excitation spectra collected for each polymer chain, it is seen that even a dilute PPE-CO2- with

only 8 polymer repeat units exhibits a slight amount of aggregation when detected at very red

wavelengths. Moreover, we conclude that the energy transfer from isolated to aggregated chains

is extremely fast, occurring in 30 to 40 ps for all samples. This component was significantly

enhanced when a poor solvent (water) or an aggregate inducer was used. An even faster










component (1.5 ps) was also observed and it was assigned to the transfer from shorter (high

energy) isolated chains to longer chains and traps within the isolated chain backbone.

Outlook/ Future Work (Hyperbranched PPE-CO2?

Within our collaboration with the Schanze and Reynolds groups, we have an opportunity

to investigate excitation and relaxation mechanisms for materials in solutions and in films using

both fluorescence up-conversion and broad band transient absorption techniques. Xiaoyong

Zhao, a student in the Schanze group, has synthesized a new hyperbranched PPE-CO2- in which

there are three carboxylate side chains attached on each side of the polymer backbone. Based on

excitation spectra data presented in Chapter 4, even dilute solutions of the linear 8 polymer

repeat unit displays some sort of aggregation within the sample. This new, hyperbranched

polymer is said to have no aggregation present even if dissolved in water. Figures 5-1 show the

absorption spectra of this new polymer when dissolved in different solvents and compared to the

linear 8 PRU (data collected by Xiaoyong Zhao). From this figure, it is necessary to look into the

excitation spectra of the hyperbranched polymer to compare to the dilute 8 PRU. The small

shoulder present in Figure 5-1 on the red side of the absorption is a small indication that there

may still be some aggregates present within this hyperbranched sample. However, this shoulder

could simply be an artifact of the size and repeat unit distributions present within polymeric

samples. The only way to be sure is to collect excitation spectra at various wavelengths and look

for red shifts that are characteristic of aggregate species. The polymer dissolved in water does

not alter the absorption spectra significantly. However, photoluminescence is quenched by nearly

half of its original intensity when dissolved in methanol (Figure 5-2). This is another indication

that the polymer could have some aggregate present despite the lack of structure in the

absorption spectrum.










1.0-

0.8-




0.4-

0.2-

0.0-


325 350 375 400 425 450 475 500

h (nm)

Figure 5-1. Absorption spectra of the hyperbranched PPE-CO2- in MeOH (-) and water (-)
and linear PPE-CO2- 8 PRU in MeOH ( )

If a complete understanding is necessary for these linear and hyperbranched polymers, it is

necessary to carry out similar methods of experimentation conducted in this dissertation. First, it

is imperative to make sure that the excitation spectra either does or does not indicate the presence

of aggregates. Second, a time-resolved detection wavelength dependence would be useful to

elucidate the dynamics of the system under various conditions. This data can then be compared

to the linear polymers. Once a comparison is made, further experiments can be designed to see

which polymer would be better for quenching and how calcium will affect the quenching

efficiency. Understanding these conjugated polyelectrolytes will help with designing new

materials for multiple applications including solar cells, LEDs and even chemo- and biosensors.

The hyperbranched CPE avenue is a new and exciting field and I recommend more time-resolved

experimentation be accomplished in this area.












7x105


6x105


5x105


4x105


3x105


2x10 -


1x10 -




400 450 500


550 600


650 700


h (nm)


Figure 5-2. Photoluminescence spectra of hyperbranched PPE-CO2- in MeOH (-) and water









LIST OF REFERENCES


(1). N. J. Turro, M~odern M~olecular Photochemistry (Univ Science Books, New York,
1991).

(2). B. Valeur, M~olecular Fluorescence Principles and Applications (Wiley-VCH,
Weinheim, 2002).

(3). G. S. H. Singhal, Janos; Rabinowitch, Eugene., Excitation-energy migration between
chlorophyll and b-carotene, Journal of Chemical Physics (1968) 49, 5206.

(4). Govindj ee, Excitation Energy Transfer and Energy M~igration : Some Basics and
Background, http:.//www.1ife.uiuc. edu/govindj ee/biochem494/foerster.htm, Online
Class Notes

(5). E. G. Rabinowitch, Phounpubesisl~\i (John Wiley & Sons, Inc., New York, 1969).

(6). G. D. Joly, L. Geiger, S. E. Kooi, T. M. Swager, Highly effective water-soluble
fluorescence quenchers of conjugated polymer thin films in aqueous environments,
Macromolecules (2006) 39, 7175.

(7). J. H. Wosnick, C. M. Mello, T. M. Swager, Synthesis and application of
poly(phenylene ethynylene)s for bioconjugation: A conjugated polymer-based
fluorogenic probe for proteases, Journal of the American Chemical Society (2005)
127, 3400.

(8). T. Kippeny, L. A. Swafford, S. J. Rosenthal, Semiconductor nanocrystals: A powerful
visual aid for introducing the particle in a box, Journal of Chemical Education (2002)
79, 1094.

(9). A. Hagfeldt, M. Gratzel, Light-induced redox reactions in nanocrystalline systems,
Chemical Reviews (1995) 95, 49.

(10). V. I. Klimov, D. W. McBranch, C. A. Leatherdale, M. G. Bawendi, Electron and hole
relaxation pathways in semiconductor quantum dots, Physical Review B (1999) 60,
13740.

(11). M. C. Schlamp, X. G. Peng, A. P. Alivisatos, Improved efficiencies in light emitting
diodes made with CdSe(CdS) core/shell type nanocrystals and a semiconducting
polymer, Journal ofApplied Physicsl (997) 82, 583 7.

(12). Y. Wang, N. Herron, Nanometer-sized semiconductor clusters Materials synthesis,
quantum size effects, and photophysical properties, Journal of Physical Chemistry
(1991) 95, 525.










(13). M. A. El-Sayed, Small is different: Shape-, size-, and composition-dependent
properties of some colloidal semiconductor nanocrystals, Accounts of Chemical
Research (2004) 37, 326.

(14). M. A. Fox, M. T. Dulay, Heterogeneous photocatalysis, Chemical Reviews (1993) 93,
341.

(15). L. N. Lewis, Chemical catalysis by colloids and clusters, Chemical Reviews (1993)
93, 2693.

(16). J. Aldana, Y.A. Wang, and X.G. Peng, Photochemical instability of CdSe
nanocrystals coated by hydrophilic thiols, Journal of the American Chemical Society
(2001) 123, 8844.

(17). V. I. Klimov, Optical nonlinearities and ultrafast carrier dynamics in semiconductor
nanocrystals, Journal of Physical Chemistry B (2000) 104, 61 12.

(18). V. I. Klimov, D. W. McBranch, Femtosecond 1P-to-1S electron relaxation in strongly
confined semiconductor nanocrystals, Physical Review Letters (1998) 80, 4028.

(19). N. Le Thomas, E. Herz, O. Schops, U. Woggon, M. V. Artemyev, Exciton fine
structure in single CdSe nanorods, Physical Review Letters (2005) 94, 016803.

(20). C. A. Leatherdale, W. K. Woo, F. V. Mikulec, M. G. Bawendi, On the absorption
cross section of CdSe nanocrystal quantum dots, Journal of Physical Chemistry B
(2002) 106, 7619.

(21). M. L. Steigerwald, L. E. Brus, Semiconductor crystallites A class of large
molecules, Accounts of Chemical Research (1990) 23, 183.

(22). A. P. Alivisatos, Perspectives on the physical chemistry of semiconductor
nanocrystals, Journal of Physical Chemistry (1996) 100, 13226.

(23). A. L. Efros, A. L. Efros, Interband absorption of light in a semiconductor sphere,
Soviet Physics Semiconductors-Ussr (1982) 16, 772.

(24). A. L. Efros, M. Rosen, The electronic structure of semiconductor nanocrystals,
Annual Review of2aterials Science (2000) 30, 475.

(25). A. I. Ekimov, F. Hache, M. C. Schanne-Klein, D. Ricard, C. Flytzanis, I. A.
Kudryaytsev, T. V. Yazeva, A. V. Rodina, and Al. L. Efros, Absorption and intensity-
dependent photoluminescence measurements on CdSe quantum dots- Assingment of
the 1 st electronic-transitions, Journal of the Optical Society of America B-Optical
Physics (1993) 10, 100.

(26). L. E. Brus, Electron electron and electron-hole interactions in small semiconductor
crystallites- The size dependence of the lowest excited electronic state Journal of
Chemical Physics (1984) 80, 4403.










(27). N. Chestnoy, R. Hull, L. E. Brus, Higher excited electronic states in clusters of ZnSe,
CdSe, and ZnS spin-orbit, vibronic, and relaxation phenomena Journal of Chemical
Physics (1986) 85, 2237.

(28). Y. Kayanuma, Quantum-size effects of interacting electrons and holes in
semiconductor microcrystals with spherical shape, Physical Review B (1988) 38,
9797.

(29). M. Nirmal, L. Brus, Luminescence photophysics in semiconductor nanocrystals,
Accounts of Chemical Research ( 1999) 32, 407.

(30). A. L. Efros, F. G. Pikus, V. G. Burnett, Density of states of a 2-dimensional electron-
gas in a long-range random potential, Physical Review B (1993) 47, 2233.

(31). A. J. Nozik, Spectroscopy and hot electron relaxation dynamics in semiconductor
quantum wells and quantum dots, Annual Review of Physical Chentistry (2001) 52,
193.

(32). X. G. Peng, L. Manna, W.D. Yang, J. Wickam, A.Kadavanich, A.P. Alivisatos, Shape
control of CdSe nanocrystals, Nature (2000) 404, 59.

(33). L. Manna, E. C. Scher, A. P. Alivisatos, Synthesis of soluble and processable rod-,
arrow-, teardrop-, and tetrapod-shaped CdSe nanocrystals, Journal of the American
Chemical Society (2000) 122, 12700.

(34). L. Manna, E. C. Scher, L. S. Li, A. P. Alivisatos, Epitaxial growth and photochemical
annealing of graded CdS/ZnS shells on colloidal CdSe nanorods, Journal of the
American Chemical Society (2002) 124, 7136.

(35). C. D. Donega, S. G. Hickey, S. F. Wuister, D. Vanmaekelbergh, A. Meijerink,
Single-step synthesis to control the photoluminescence quantum yield and size
dispersion of CdSe nanocrystals, Journal of Physical Chentistry B (2003) 107, 489.

(36). L. A. Swafford, Homogeneously Alloyed CdSxSel-x Nanocrystals: Synthesis,
Characterization, and Composition/Size-Dependent Band Gap, Journal of the
American Chemical Society (2006).

(37). C. B. Murray, D. J. Norris, M. G. Bawendi, Synthesis and Characterization of Nearly
Monodisperse Cde (E = S, Se, Te) Semiconductor Nanocrystallites, Journal of the
American Chemical Society (1993) 115, 8706.

(38). T. Mokari, U. Banin, Synthesis and properties of CdSe/ZnS core/shell nanorods,
Chentistry of2aterials (2003) 15, 3955.

(39). B. O. Dabbousi, J. RodriguezViejo, F.V. Mikulec, J.R. Heine, H. Mattoussi, R. Ober,
K.F. Jensen, M.G. Bawendi, (CdSe)ZnS core-shell quantum dots: Synthesis and
characterization of a size series of highly luminescent nanocrystallites, Journal of
Physical Chentistry B (1997) 101, 9463.










(40). M. A. Hines, P. Guyot-Sionnest, Synthesis and characterization of strongly
luminescing ZnS-Capped CdSe nanocrystals, Journal of Physical Chentistry (1996)
100, 468.

(41). Y. W. Cao, U. Banin, Synthesis and characterization of InAs/InP and InAs/CdSe
core/shell nanocrystals, Angewand'te Chenzie-1nternational Edition (1999) 38, 3692.

(42). H. Lee, L. M. Hardison, V.D. Kleiman, P.H. Holloway, H. Yang, Synthesis and
characterization of colloidal ternary ZnCdSe semiconductor nanorods, Journal of
Chemical Physics (2006) 125, 029901.

(43). M. Dovrat, Y. Goshen, J. Jedrzejewski, I. Balberg, A. Sa'ar, Radiative versus
nonradiative decay processes in silicon nanocrystals probed by time-resolved
photoluminescence spectroscopy, Physical Review B (2004) 69 155311.

(44). X. G. Peng, Mechanisms for the Shape-Control and Shape-Evolution of Colloidal
Semiconductor Nanocrystals Chens. Euro J (2002) 8, 334.

(45). Z. A. Peng, X. G. Peng, Formation of high-quality CdTe, CdSe, and CdS nanocrystals
using CdO as precursor, Journal of the American Chemical Society (200 1) 123, 1 83.

(46). Z. A. Peng, X. G. Peng, Nearly monodisperse and shape-controlled CdSe
nanocrystals via alternative routes: Nucleation and growth, Journal of the American
Chemical Society (2002) 124, 3343.

(47). L. H. Qu, Z. A. Peng, X. G. Peng, Alternative routes toward high quality CdSe
nanocrystals, Nano Letters (2001) 1, 333.

(48). X. H. Zhong, M. Y. Han, Z. L. Dong, T. J. White, W. Knoll, Composition-tunable
ZnxCdl-xSe nanocrystals with high luminescence and stability, Journal of the
American Chemical Society (2003) 125, 8589.

(49). X. Chen, A. Nazzal, D. Goorskey, M. Xiao, Z.A. Peng, X.G. Peng., Polarization
spectroscopy of single CdSe quantum rods, Physical Review B (2001) 64, 245304.

(50). J. T. Hu, L.S. Li, W.D. Yang, L. Manna, L.W. Wang, A.P.Alivisatos, Linearly
polarized emission from colloidal semiconductor quantum rods, Science (2001) 292,
2060.

(51). L. Manna, E. C. Scher, A. P. Alivisatos, Shape control of colloidal semiconductor
nanocrystals, Journal of Chester Science (2002) 13, 521.

(52). L. S. Li, J. T. Hu, W. D. Yang, A. P. Alivisatos, Band gap variation of size- and
shape-controlled colloidal CdSe quantum rods, Nano Letters (2001) 1, 349.

(53). M. B. Mohamed, C. Burda, M. A. El-Sayed, Shape dependent ultrafast relaxation
dynamics of CdSe nanocrystals: Nanorods vs nanodots, Nano Letters (2001) 1, 589.










(54). L. Brus, Electronic wave-functions in semiconductor clusters Experiment and
theory, Journal of Physical Chemistry (1986) 90, 2555.

(55). G. Cantele, D. Ninno, G. Iadonisi, Confined states in ellipsoidal quantum dots,
Journal ofPhysics-Condensed Ma'~tter (2000) 12, 9019.

(56). G. Cantele, D. Ninno, G. Iadonisi, Calculation of the infrared optical transitions in
semiconductor ellipsoidal quantum dots, Nano Letters (2001) 1, 121.

(57). Y. Kayanuma, Wannier excitons in low-dimensional microstructures Shape
dependence of the quantum size effect, Physical Review B (1991) 44, 13085.

(58). S. Legoff, B. Stebe, Influence of longitudinal and lateral confinements on excitons in
cylindrical quantum dots of semiconductors, Physical Review B (1993) 47, 13 83.

(59). D. Ninno, G. Iadonisi, F. Buonocore, Carrier localization and photoluminescence in
porous silicon, Solid State Communications (1999) 112, 521.

(60). A. D. Yoffe, Low-dimensional systems Quantum-size effects and electronic-
properties of semiconductor microcrystallites (zero-dimensional systems) and some
quasi-2-dimensional systems, Ad'vances in Physics (1993) 42, 173.

(61). C. E. Tyner, Application of solar thermal technology to the destruction of hazardous
wastes, Solar Energy Materals (1990) 21, 1 13.

(62). K. Hashizume, M. Vacha, T. Tani, Preparation and optical properties of capped-CdSe
nanocrystals, Journal of Luminescence (2000) 87-9, 402.


(63). L. Manna, L. W. Wang, R. Cingolani, A. P. Alivisatos, First-principles modeling of
unpassivated and surfactant-passivated bulk facets of wurtzite CdSe. A model system
for studying the anisotropic growth of CdSe nanocrystals, Journal of Physical
Chemistry B (2005) 109, 6183.

(64). D. J. Norris, A. Sacra, C. B. Murray, M. G. Bawendi, Measurement of the size-
dependent hole spectrum in CdSe quantum dots, Physical Review Letters (1994) 72,
2612.

(65). X. G. Peng, M. C. Schlamp, A. V. Kadavanich, A. P. Alivisatos, Epitaxial growth of
highly luminescent CdSe/CdS core/shell nanocrystals with photostability and
electronic accessibility, Journal of the American Chemical Society (1997) 119, 70 19.

(66). H. Lee, University of Florida (2005).

(67). D. F. Underwood, T. Kippeny, S. J. Rosenthal, Ultrafast carrier dynamics in CdSe
nanocrystals determined by femtosecond fluorescence upconversion spectroscopy,
Journal of Physical Chemistry B (2001) 105, 43 6.










(68). T. W. Roberti, N. J. Cherepy, J. Z. Zhang, Nature of the power-dependent ultrafast
relaxation process of photoexcited charge carriers in II-VI semiconductor quantum
dots: Effects of particle size, surface, and electronic structure, Journal of Chemical
Physics (1998) 108, 2143.

(69). B. Oregan, M. Gratzel, A low-cost, high-efficiency solar-cell based on dye-sensitized
colloidal TiO2 f11ms, Nature (1991) 353, 737.

(70). D. V. Talapin, A. L. Rogach, A. Kornowski, M. Haase, H. Weller, Highly
luminescent monodisperse CdSe and CdSe/ZnS nanocrystals synthesized in a
hexadecylamine-trioctylphosphine oxide-trioctylphospine mixture, Nano Letters
(2001) 1, 207.

(71). J. E. B. Katari, V.L. Colvin, and A.P. Alivisatos, X-Ray Photoelectron- Spectroscopy
of CdSe nanocrystals with applications to studies of the nanocrystal surface, Physical
Chentistry (1994) 98, 4109.

(72). A. Creti, M. Anni, M.Z. Rossi, G. Lanzani, G. Leo, F. Della Sala, L. Manna, M.
Lomascolo., Ultrafast carrier dynamics in core and core/shell CdSe quantum rods:
Role of the surface and interface defects, Physical Review B (2005) 72, 125346.

(73). P. Reiss, J. Bleuse, A. Pron, Highly luminescent CdSe/ZnSe core/shell nanocrystals
of low size dispersion, Nano Letters (2002) 2, 781.

(74). Y. W. Cao, U. Banin, Growth and properties of semiconductor core/shell nanocrystals
with InAs cores, Journal of the American Chemical Society (2000) 122, 9692.

(75). J. Bleuse, S. Carayon, P. Reiss, Optical properties of core/multishell CdSe/Zn(S, Se)
nanocrystals, Physica E-Low-Dintensional Systems & Nanostructures (2004) 21, 33 1.

(76). D. C. Pan, Q. Wang, J. B. Pang, S.C. Jiang, X.L. Ji, L.J. An, Semiconductor "nano-
onions" with multifold alternating CdS/CdSe or CdSe/CdS structure, Chentistry of
Materials (2006) 18, 4253.

(77). J. J. Li, J.T. Hu, W.D. Yang, A.P. Alivisatos, Large-scale synthesis of nearly
monodisperse CdSe/CdS core/shell nanocrystals using air-stable reagents via
successive ion layer adsorption and reaction, Journal of the American Chemical
Society (2003) 125, 12567.

(78). L. P. Balet, S. A. Ivanov, A. Piryatinski, M. Achermann, V. I. Klimov, Inverted
core/shell nanocrystals continuously tunable between type-I and type-II localization
regimes, Nano Letters (2004) 4, 1485.

(79). D. V. Talapin, A.L. Rogach, A. Kornowski, H. Weller, CdSe/CdS/ZnS and
CdSe/ZnSe/ZnS core-shell-shell nanocrystals, Journal of Physical Chentistry B
(2004) 108, 18826.










(80). W. E. Garner, Chentistry of the solid state (Butterworths Scientific Publications,
London, 1955).

(81). W. E. Martin, Photoluminescence Determinations of Cd Diffusion in ZnSe, Journal
ofApplied Physics (1973) 44, 5639.

(82). P. J. Parbrook, B. Henderson, K. P. Odonnell, P. J. Wright, B. Cockayne,
Interdiffusion in wide-bandgap Zn(Cd)S(Se) strained layer superlattices,
Semiconductor Science and Technology (1991) 6, 818.

(83). A. Rosenauer, T. Reisinger, E. Steinkirchner, J. Zweck, W. Gebhardt, High-resolution
transmission electron-microscopy determination of Cd diffusion in CdSe/ZnSe single-
quantum-well structures, Journal ofCrystal Gi 1,n thr (1995) 152, 42.

(84). M. Strassburg, M. Kuttler, U. W. Pohl, D. Bimberg, Diffusion of Cd, Mg and S in
ZnSe-based quantum well structures, Thin Solid Films (1998) 336, 208.

(85). W. Chen, J. O. Malm, V. Zwiller, R. Wallenberg, J. O. Bovin, Size dependence of
Eu2+ fluorescence in ZnS : Eu2+ nanoparticles, Journal ofAppliedPhysics (2001) 89,
2671.

(86). W. Chen, R. Sammynaiken, Y.N. Huang, Crystal field, phonon coupling and
emission shift of Mn2+ in ZnS : Mn nanoparticles, Journal of Applied Physics (2001)
89, 1120.

(87). C. X. Shan, X.W. Fan, J.Y. Zhang, Z.Z. Zhang, B.S. Li, Y.M Lu, Y.C. Liu, D.Z.
Shen, X.G. Kong, X.H. Wang, Growth and evolution of ZnCdSe quantum dots,
Journal of Vacuum Science & Technology B (2002) 20, 1 102.

(88). X. Y. Wang, J. Y. Zhang, A. Nazzal, M. Darragh, M. Xiao, Electronic structure
transformation from a quantum-dot to a quantum-wire system: Photoluminescence
decay and polarization of colloidal CdSe quantum rods, Applied Physics Letters
(2002) 81, 4829.

(89). U. Manual, A User's Guide to the Andor iStar (Andor Technology Limited, 2001).

(90). Andor, Andor Knowledge Lib rary, http://www.andor. com/l ib rary/di gital cam era s/

(91). J. Alford, Personal communication concerning the iStar CCD system,(2007)

(92). S. Cannistra, How to choose a CCD camera,
http://www. starrywonders. com/ccdcameraconsi derati ons. html

(93). M. C. Gino, Noise, Noise, Noise, http://www.astrophys-
as sist. com/educate/noi se/noi se.htm

(94). Apogee, CCD University, http ://www.ccd.com/ccdu.html










(95). E. C. Scher, L. Manna, A. P. Alivisatos, Shape control and applications of
nanocrystals, Philosophical Transactions of the Royal Society of London Series a-
Mathematical Physical and Engmneering Sciences (2003) 361, 241.

(96). A. Puzder, A.J. Williamson, N. Zaitseva, G. Galli, L. Manna and A.P. Alivisatos, The
effect of organic ligand binding on the growth of CdSe nanoparticles probed by Ab
initio calculations, Nano Letters (2004) 4, 2361.

(97). M. A. Hines, P. Guyot-Sionnest, Bright UV-blue luminescent colloidal ZnSe
nanocrystals, Journal of Physical Chemistry B (1998) 102, 3655.

(98). B. D. Cullity, and Stock, S. R., Elements ofX-ray Diffaction (Prentice Hall, New
York, ed. Third, 2001).

(99). M. M. a. F. Rashad, O. A., Synthesis and characterization of nano-sized nickel
ferrites from fly ash for catalytic oxidation of CO, Materials Chemistry and Physics
(2005) 94, 365.

(100). R. W. Meulenberg, T. Jennings, G. F. Strouse, Compressive and tensile stress in
colloidal CdSe semiconductor quantum dots, Physical Review B (2004) 70 235311.

(101). A. V. Baranov, Y.P. Rakovich, J.F. Donegan, T.S. Perova, R.A. Moore, D.V. Talapin,
A.L. Rogach, Y. Masumoto, I. Nabiev, Effect of ZnS shell thickness on the phonon
spectra in CdSe quantum dots, Physical Review B (2003) 68, 165306.

(102). Y. N. Hwang, S. Shin, H.L. Park, SH Park, U. Kim, H.S. Jeong, E. Shin, D. Kim,
Effect of lattice contraction on the Raman shifts of CdSe quantum dots in glass
matrices, Physical Review B (1996) 54, 15120.

(103). F. Comas, C. Trallero-Giner, N. Studart, G. E. Marques, Interface optical phonons in
spheroidal dots: Raman selection rules, Physical Review B (2002) 65, 073303.

(104). C. Trallero-Giner, A. Debernardi, M. Cardona, E. Menendez-Proupin, A. I. Ekimov,
Optical vibrons in CdSe dots and dispersion relation of the bulk material, Physical
Review B (1998) 57, 4664.

(105). R. G. Alonso et al., Raman-spectroscopy of 2 novel semiconductors and related
superlattices Cubic Cdl.- Mln Se and Cdl-xZnxSe, PhysicalReview B (1989) 40,
3720.

(106). V. V. Travnikov, V. K. Kaibyshev, Resonance exciton-phonon spectra in open
ZnCdSe/ZnSe nanowires: Raman scattering and hot luminescence, extended and
localized exciton states, Physics of the Solid State (2003) 45, 1379.

(107). R. Bhushan, V. Prasad, W. Meredith, G. Horsburgh, G.D. Brownlie, K.A. Prior, B.C.
Cavenett, W. Rothwell, A.J. Dann, Microprobe Raman study of the variation of LO
phonon frequency with the Cd concentration in the ternary compound Znl-xCdxSe,
Journal of Crystal Gi 1,n thr (1 996) 159, 1 03 .










(108). C. Ramkumar, K. P. Jain, S. C. Abbi, Resonant Raman scattering probe of alloying
effect in GaAsl-xPx ternary alloy semiconductors, Physical Review B (1996) 54, 7921.

(109). C. Ramkumar, K. P. Jain, S. C. Abbi, Raman-scattering probe of anharmonic effects
due to temperature and compositional disorder in III-V binary and ternary alloy
semiconductors, Physical Review B (1996) 53, 13672.

(110). T. Kummell et al., Size dependence of strain relaxation and lateral quantization in
deep etched CdxZn1-xSe/ZnSe quantum wires, Physical Review B (1998) 57, 15439.

(111). P. R. Yu, J. M. Nedeljkovic, P. A. Ahrenkiel, R. J. Ellingson, A. J. Nozik, Size
dependent femtosecond electron cooling dynamics in CdSe quantum rods, Nano
Letters (2004) 4, 1089.


(112). M. Achermann, J. A. Hollingsworth, V. I. Klimov, Multiexcitons confined within a
subexcitonic volume: Spectroscopic and dynamical signatures of neutral and charged
biexcitons in ultrasmall semiconductor nanocrystals, Physical Review B (2003) 68,
245302.

(113). Y. Kawakami, K. Omae, A. Kaneta, K. Okamoto, Y. Narukawa, T. Mukai, S. Fujita,
In inhomogeneity and emission characteristics oflInGaN, Journal ofPhysics-
Condensed Matter (2001) 13, 6993.

(114). H. S. Kim, R. A. Mair, J. Li, J. Y. Lin, H. X. Jiang, Time-resolved
photoluminescence studies of AlxGal-xN alloys, Applied Physics Letters (2000) 76,
1252.

(115). X. H. Zhong, Y. Y. Feng, W. Knoll, M. Y. Han, Alloyed ZnxCdl-xS nanocrystals with
highly narrow luminescence spectral width, Journal of the American Chemical
Society (2003) 125, 13559.

(116). R. F. Mahrt, T. Pauck, U. Lemmer, U. Siegner, M. Hopmeier, R. Hennig, H. Bassler,
E.O. Gobel, P.H. Bolivar, G. Wegmann, H. Kuz, U. Scherf, K.Mullen, Dynamics of
optical excitations in a ladder-type pi-conjugated polymer containing aggregate states,
Physical Review B (1996) 54, 1759.

(117). M. Jones, J. Nedeljkovic, R. J. Ellingson, A. J. Nozik, G. Rumbles,
Photoenhancement of luminescence in colloidal CdSe quantum dot solutions, Journal
of Physical Chemistry B (2003) 107, 1 1346.

(118). X. Chen, B. Henderson, K. P. Odonnell, Luminescence decay in disordered low-
dimensional semiconductors, Applied Physics Letters (1992) 60, 2672.

(119). R. Cingolani et al., Exciton spectroscopy in Znl-xCdxSe/ZnSe quantum-wells,
Physical Review B (1995) 51, 5176.










(120). S. A. Empedocles, M. G. Bawendi, Quantum-confined stark effect in single CdSe
nanocrystallite quantum dots, Science (1997) 278, 2114.

(121). V. Esch, B. Fluegel, G. Khitrova, H.M. Gibbs, J.J. Xu, K.Kang, S.W. Koch, L.C. Liu,
S.H. Risbud, N. Peyghambarian, State filling, coulomb, and trapping effects in the
optical nonlinearity of CdTe quantum dots in glass, Physical Review B (1990) 42,
7450.

(122). E. Hendry, M. Koeberg, F. Wang, H. Zhang, C.D. Donega, D. Vanmaekelbergh, M.
Bonn, Direct observation of electron-to-hole energy transfer in CdSe quantum dots,
Physical Review Letters (2006) 96, 125201.

(123). A. Shabaev, A. L. Efros, 1D exciton spectroscopy of semiconductor nanorods, Nano
Letters (2004) 4, 1821.

(124). C. Rulliere, Femtosecond Laser Pulses: Principles and Experiments, 2nd Edition
(2004).

(125). C. Burda, X. B. Chen, R. Narayanan, M. A. El-Sayed, Chemistry and properties of
nanocrystals of different shapes, Chemical Reviews (2005) 105, 1025.

(126). K. Brunner, U. Bockelmann, G. Abstreiter, M. Walther, G. Bohm, G. Trankle, G.
Weimann, Photoluminescence from a single GaAs/AlGaAs quantum dot Physical
Review Letters (1992) 69, 3216.

(127). L. W. Wang, M. Califano, A. Zunger, A. Franceschetti, Pseudopotential theory of
Auger processes in CdSe quantum dots, Physical Review Letters (2003) 91, 56404.

(128). A. L. Efros, V. A. Kharchenko, M. Rosen, Breaking the phonon bottleneck in
nanometer quantum dots Role of Auger-like processes, Solid State Communications
(1995) 93, 281.

(129). P. Guyot-Sionnest, M. Shim, C. Matranga, M. Hines, Intraband relaxation in CdSe
quantum dots, Physical Review B (1999) 60, R2181.

(130). J. G. Muller, E. Atas, C. Tan, K. S. Schanze, V. D. Kleiman, The role of exciton
hopping and direct energy transfer in the efficient quenching of conjugated
polyelectrolytes, Journal of the American Chemical Society (2006) 128, 4007.

(131). V. I. Klimov, A. A. Mikhailovsky, D. W. McBranch, C. A. Leatherdale, M. G.
Bawendi, Quantization of multiparticle Auger rates in semiconductor quantum dots,
Science (2000) 287, 1011.

(132). J. L. Blackburn, D. C. Selmarten, A. J. Nozik, Electron transfer dynamics in quantum
dot/titanium dioxide composites formed by in situ chemical bath deposition, Journal
of Physical Chemistry B (2003) 107, 14154.










(133). S. Sauvage, P. Boucaud, Rpsm Lobo, F. Bras, G. Fishman, R. Prazeres, F. Glotin,
J.M. Ortega, J.M Gerard, Long polaron lifetime in InAs/GaAs self-assembled
quantum dots, Physical Review Letters (2002) 88, 177402.

(134). P. C. Sercel, Multiphonon-assi sted tunneling through deep levels A rapid energy-
relaxation mechanism in nonideal quantum-dot heterostructures, Physical Review B
(1995) 51, 14532.

(135). M. Ohishi, K. Tanaka, T. Fujimoto, M. Yoneta, H. Saito, Alloying of CdSe/ZnSe
quantum dot grown by an alternate molecular beam supply, Journal of Crystal
Gia th1 l (2002) 237, 1320.

(136). X. H. Zhong, R. G. Xie, Y. Zhang, T. Basche, W. Knoll, High-quality violet- to red-
emitting ZnSe/CdSe core/shell nanocrystals, Chentistry of2aterials (2005) 17, 4038.

(137). B. P. Zhang, D. D. Manh, K. Wakatsuki, Y. Segawa, Nanostructures formed on
CdSe/ZnSe surfaces, Journal of Crystal G i,~Iron (200 1) 227, 645.

(138). V. V. Nikesh, S. Mahamuni, Highly photoluminescent ZnSe/ZnS quantum dots,
Semiconductor Science and Technology (2001) 16, 687.

(139). J. S. Steckel, J.P. Zimmer, S. Coe-Sullivan, N.E. Stott, V. Bulovic, M.G. Bawendi,
Blue luminescence from (CdS)ZnS core-shell nanocrystals, Angewandte Chemie-
International Edition (2004) 43, 2154.

(140). Z. H. Yu, L. Guo, H. Du, T. Krauss, J. Silcox, Shell distribution on colloidal
CdSe/ZnS quantum dots, Nano Letters (2005) 5, 565.

(141). C. Guenaud, E. Deleporte, A. Filoramo, P. Lelong, C. Delalande, C. Morhain, E.
Tournie, J.P. Faurie, Study of the band alignment in (Zn, Cd)Se/ZnSe quantum wells
by means of photoluminescence excitation spectroscopy, Journal ofAppliedPhysics
(2000) 87, 1863.

(142). V. I. Kozlovsky, Sadofyev, Yu G., Litvinov, V. G., Crystal and Solid State Physics,
Physics ofll-Y7 Compounds, Landolt-Borstein Series (Springer, Berlin, 1982).

(143). V. I. Kozlovsky, Y. G. Sadofyev, V. G. Litvinov, Band alignment in ZnCdTe/ZnTe
and ZnCdSe/ZnSe SQW structures grown on GaAs(100) by MBE, Nanotechnology
(2000) 11, 241.

(144). S. H. Wei, S. B. Zhang, A. Zunger, First-principles calculation of band offsets, optical
bowings, and defects in CdS, CdSe, CdTe, and their alloys, Journal ofApplied
Physics (2000) 87, 1304.

(145). S. H. Wei, A. Zunger, Calculated natural band offsets of all II-VI and Ill-V
semiconductors: Chemical trends and the role of cation d orbitals, AppliedPhysics
Letters (1998) 72, 2011.










(146). M. D. McGehee, A. J. Heeger, Semiconducting (conjugated) polymers as materials
for solid-state lasers, Advanced2\a~terials (2000) 12, 1655.

(147). R. H. Friend, R.W. Gymer, A.B. Holmes, J.H. Burroughes, R.N. Marks, C. Taliani,
D.D.C. Bradley, D.A. Dos Santos, J.L. Bredas, M. Logdlund, W.R. Salaneck,
Electroluminescence in conjugated polymers, Nature (1999) 397, 121.

(148). H. Spanggaard, F. C. Krebs, A brief history of the development of organic and
polymeric photovoltaics, Solar Energy Materials and Solar Cells (2004) 83, 125.

(149). H. Sirringhaus, N. Tessler, R. H. Friend, Integrated optoelectronic devices based on
conjugated polymers, Science (1998) 280, 1741.

(150). M. Fakis, Anestopoulos, D., Giannetas, V., et al, Influence of aggregates and solvent
aromaticity on the emission of conjugated polymers, Journal ofPhysical Chentistry B
(2006) 49, 24897.

(151). J. H. Hsu, W.S. Fann, H.F. Meng, E.S. Chen, E.C. Chang, S.A. Chen, K.W. To,
Decay dynamics of interchain excited states in luminescent conjugated polymer CN-
PPV, ChenticalPhysics (2001) 269, 367.

(152). F. J. Hua, E. Ruckenstein, Fluorescence study of aggregation in water of PEO-grafted
polydiphenylamine, Langmuir (2004) 20, 3954.

(153). L. O. Palsson et al., Photophysics of a fluorene co-polymer in solution and films,
Chemical Physics (2002) 279, 229.

(154). P. Wang, C. J. Collison, L. J. Rothberg, Origins of aggregation quenching in
luminescent phenylenevinylene polymers, Journal of Photochentistry and
Photobiology a-Chentistry (2001) 144, 63.

(155). H. Jiang, X. Y. Zhao, K. S. Schanze, Amplified fluorescence quenching of a
conjugated polyelectrolyte mediated by Ca2+, Langmuir (2006) 22, 5541.

(156). M. Fakis, Anestopoulos, D., Giannetas, V., et al, Femtosecond time resolved
fluorescence dynamics of a cationic water soluble poly(fluorenevinylene-co-
phenylenevinylene), Journal of Physical Chentistry B (2006) 110, 12916.

(157). L. H. Chen, D.W. McBranch, H.L. Wang, R. Helgeson, F. Wudl, D.G. Whitten,
Highly sensitive biological and chemical sensors based on reversible fluorescence
quenching in a conjugated polymer, Proceedings of the National Academy of Sciences
of the thrited States ofAnzerica (1999) 96, 12287.

(158). N. GronbechJensen, R. J. Mashl, R. F. Bruinsma, W. M. Gelbart, Counterion-induced
attraction between rigid polyelectrolytes, Physical Review Letters (1997) 78, 2477.










(159). B. J. Schwartz, Conjugated polymers as molecular materials : How chain
conformation and film morphology influence energy transfer and interchain
interactions, Annual Review of Physical Chentistry (2003) 54, 141.

(160). R. Jakubiak, C. J. Collison, W. C. Wan, L. J. Rothberg, B. R. Hsieh, Aggregation
quenching of luminescence in electroluminescent conjugated polymers, Journal of
Physical Chentistry A (1999) 103, 2394.

(161). S. A. Jenekhe, J. A. Osaheni, Excimers and exciplexes of conjugated polymers,
Science (1994) 265, 765.

(162). I. D. W. Samuel, G. Rumbles, C. J. Collison, Efficient interchain photoluminescence
in a high-electron-affinity conjugated polymer, Physical Review B (1995) 52, 11573.

(163). C. J. Collison, L. J. Rothberg, V. Treemaneekarn, Y. Li, Conformational effects on
the photophysics of conjugated polymers: A two species model for MEH-PPV
spectroscopy and dynamics, Macrontolecules (2001) 34, 2346.

(164). G. H. Gelinck, J. M. Warman, E. G. J. Staring, Polaron pair formation, migration, and
decay on photoexcited poly(phenylenevinylene) chains, Journal of Physical
Chentistry (1996) 100, 5485.

(165). J. W. Blatchford, S.W. Jessen, L.B. Lin, T.L, Gustafson, D.K. Fu, H.L. Wang, T.M.
Swager, A.G. MacDiarmid, A.J. Epstein, Photoluminescence in pyridine-based
polymers: Role of aggregates, Physical Review B (1996) 54, 9180.

(166). U. Lemmer, S. Heun, R.F. Mahrt, U. Scherf M. Hopmeier, U. Siegner, E.O. Gobel, K.
Mullen, H. Bassler, Aggregate Fluorescence in Conjugated Polymers, Chemical
Physics Letters (1995) 240, 373.

(167). X. Y. Zhao, M.R. Pinto, L.M. Hardison, J. Mwuara, J. Muller, H. Jiang, D.Witker,
V.D. Kleiman, J.R. Reynolds, K.S. Schanze,Variable band gap poly(arylene
ethynylene) conjugated polyelectrolytes, Macronsolecules (2006) 39, 6355.

(168). C. Y. Tan, E. Atas, J.G. Muller, M.R. Pinto, V.D. Kleiman, K.S. Schanze Amplified
quenching of a conjugated polyelectrolyte by cyanine dyes, Journal of the American
Chemical Society (2004) 126, 13685.

(169). B. S. Harrison, M. B. Ramey, J. R. Reynolds, K. S. Schanze, Amplified fluorescence
quenching in a poly(p-phenylene)-based cationic polyelectrolyte, Journal of the
American Chemical Society (2000) 122, 8561.

(170). G. H. Gelinck, E.G.J. Staring, D.H. Hwang, G.C.W. Spencer, A.B. Holmes, J.M.
Warman, The effect of broken conjugation and aggregation on photo-induced charge
separation on polyphenylenevinylene chains, Synthetic Metals (1997) 84, 595.

(171). C. H. Fan, S.Wang, J.W. Hong, G.C. Bazan, K.W. Plaxco, A.J. Heeger, Beyond
superquenching: Hyper-efficient energy transfer from conjugated polymers to gold










nanoparticles, Proceedings of the National Academy of Sciences of the thrited States
ofAnzerica (2003) 100, 6297.

(172). B. S. Gaylord, A. J. Heeger, G. C. Bazan, DNA detection using water-soluble
conjugated polymers and peptide nucleic acid probes, Proceedings of the National
Academy of Sciences of the thrited States of Anerica (2002) 99, 10954.

(173). M. Stork, B. S. Gaylord, A. J. Heeger, G. C. Bazan, Energy transfer in mixtures of
water-soluble oligomers: Effect of charge, aggregation, and surfactant complexation,
Advanced Ma'~terials (2002) 14, 361.

(174). A. Haugeneder, U. Lemmer, U. Scherf, Exciton dissociation dynamics in a
conjugated polymer containing aggregate states, Chentical Physics Letters (2002)
351, 354.

(175). G. Petekidis, G. Fytas, U. Scherf, K. Mullen, G. Fleischer, Dynamics of poly(p-
phenylene) ladder polymers in solution, Journal of Polynzer Science Part B-Polynzer
Physics (1999) 37, 2211.

(176). J. G. Muller, U. Lemmer, G. Raschke, M. Anni, U. Scherf, J.M. Lupton, J. Feldman,
Linewidth-limited energy transfer in single conjugated polymer molecules, Physical
Review Letters (2003) 91, 267403.

(177). B. Schweitzer, G. Wegmann, D. Hertel, R.F. Mahrt, H. Bassler, F. Uckert, U. Scherf,
K. Mullen, Spontaneous and stimulated emission from a ladder-type conjugated
polymer, Physical Review B (1999) 59, 4112.

(178). H. P. Gregor, L. B. Luttinger, E. M. Loebl, Metal-polyelectrolyte complexes .4.
complexes of polyacrylic acid with magnesium, calcium, manganese, cobalt and zinc,
Journal of Physical Chentistry (1955) 59, 990.

(179). I. B. Kim, U. H. F. Bunz, Modulating the sensory response of a conjugated polymer
by proteins: An agglutination assay for mercury ions in water, Journal of the
American Chemical Society (2006) 128, 28 18.

(180). C. Y. Tan, M. R. Pinto, K. S. Schanze, Photophysics, aggregation and amplified
quenching of a water-soluble poly( phenylene ethynylene), Chemical
Conanunications (2002), 5, 446.

(181). X. Y. Zhao, Jiang, H., Schanze, K.S., Water-Soluble Poly(phenylene ethynylene)s of
Variable Chain Length: Synthesis, Photophysics and Amplified Quenching, hr press
(2007).

(182). E. Atas, University of Florida (2006).

(183). E. Atas, Z. H. Peng, V. D. Kleiman, Energy transfer in unsymmetrical phenylene
ethynylene dendrimers, Journal of Physical Chentistry B (2005) 109, 13553.










(184). New Focus, M~odel 5540 User's Manual: Thze Berek Polarization Conspensator.

(185). New Focus, Polarization and Polarization Control.

(186). A. Yariv, and Yeh, P., Optical Waves in Crystals: Propagation and Control ofLaser
Radiation (Wiley-Interscience 2002).

(187). R. Raj asekaran, Personal conanunication for Berek Conspensator Crystal
Specifications, (2007)

(188). M. Dodge, Refractive properties of magnesium fluoride, Applied Optics (1984) 23,
1980.

(189). J. R. Lakowicz, Principles ofFluorescence Spectroscopy (Kluwer Academic/Plenum
Publishers, New York, ed. 2, 1999).

(190). R. E. Martin, F. Diederich, Linear monodisperse pi-conjugated oligomers: Model
compounds for polymers and more, Angewandte Chenzie-hiternational Edition (1999)
38, 1350.

(191). U. H. F. Bunz, J.M. Imhof, R.K. Bly, C.G. Bangcuyo, L. Rozanski, D.A.V. Bout,
Photophysics of poly [p-(2,5-didodecylphenylene)ethynylene] in thin films,
Macrontolecules (2005) 38, 5892.

(192). C. E. Halkyard, M. E. Rampey, L. Kloppenburg, S. L. Studer-Martinez, U. H. F.
Bunz, Evidence of aggregate formation for 2,5-dialkylpoly(p-phenyleneethynylenes)
in solution and thin films, Macrontolecules (1998) 31, 8655.

(193). J. Kim, D. T. McQuade, S. K. McHugh, T. M. Swager, lon-specific aggregation in
conjugated polymers: Highly sensitive and selective fluorescent ion chemosensors,
Angewandte Chenzie-hiternational Edition (2000) 39, 3868.

(194). J. Kim, T. M. Swager, Control of conformational and interpolymer effects in
conjugated polymers, Nature (2001) 411, 1030.

(195). M. Abramowitz, Johnson I. D., and Davidson, M. W., Fluorescence filter spectral
transmission~rtrtrtrt~t~t~ profiles,
http://www. olympusmi cro. com/primer/j ava/fluorescence/fluorocub es/index.html,

(196). V. F. Kamalov, I. A. Struganova, K. Yoshihara, Temperature dependent radiative
lifetime of J-aggregates, Journal of Physical Chentistry (1996) 100, 8640.

(197). D. Anestopoulos, Fakis, M etal, Excitation energy transfer in a cationic water-soluble
conjugated co-polymer studied by time resolved anisotropy and fluorescence
dynamics, Chemical Physics Letters (2006) 421, 205.










(198). A. A. Mikhailovsky, A. V. Malko, J. A. Hollingsworth, M. G. Bawendi, V. I. Klimov,
Multiparticle interactions and stimulated emission in chemically synthesized quantum
dots, Applied Physics Letters (2002) 80, 2380.

(199). V. I. Klimov, Mechanisms for photogeneration and recombination of multiexcitons in
semiconductor nanocrystals: Implications for lasing and solar energy conversion,
Journal of Physical Chentistry B (2006) 110, 16827.

(200). D. Katz, T. Wizansky, O. Millo, E. Rothenber, T. Mokari, U. Banin, Size-dependent
tunneling and optical spectroscopy of CdSe quantum rods, Physical Review Letters
(2002) 89, 086801.

(201). H. Htoon, J. A. Hollingworth, A. V. Malko, R. Dickerson, V. I. Klimov, Light
amplification in semiconductor nanocrystals: Quantum rods versus quantum dots,
Applied Physics Letters (2003) 82, 4776.

(202). S. A. Ivanov, J. Nanda, A. Piryatinski, M. Achermann, L.P. Balet, I.V. Bezel, P.O.
Anikeeva, S. Tretiak, V.I. Klimov, Light amplification using inverted core/shell
nanocrystals: Towards lasing in the single-exciton regime, Journal of Physical
Chentistry B (2004) 108, 10625.

(203). J. Nanda, S.A. Ivanov, H. Htoon, I. Bezel, A Piratinski, S. Tretiak, V.I. Klimov,
Absorption cross sections and Auger recombination lifetimes in inverted core-shell
nanocrystals: Implications for lasing performance, Journal ofAppliedPhysics (2006)
99, 013707.

(204). V. L. Colvin, M. C. Schlamp, A. P. Alivisatos, Light-emitting-diodes made from
cadmium selenide nanocrystals and a semiconducting polymer Nature (1994) 370,
354.

(205). B. O. Dabbousi, M. G. Bawendi, O. Onitsuka, M. F. Rubner, Electroluminescence
from CdSe quantum-dot polymer composites, ApplieadPhysics Letters (1995) 66,
1316.

(206). H. Mattoussi, L.H. Radzilowski, B.O. Dabbousi, E.L. Thomas, M.G. Bawendi, M.F.
Rubner, Electroluminescence from heterostructures of poly(phenylene vinylene) and
inorganic CdSe nanocrystals, Journal ofAppliedPhysics (1998) 83, 7965.

(207). H. S. Yang, P. H. Holloway, Electroluminescence from hybrid conjugated polymer -
CdS : Mn/ZnS core/shell nanocrystals devices, Journal of Physical Chentistry B
(2003) 107, 9705.

(208). S. Coe, W. K. Woo, M. Bawendi, V. Bulovic, Electroluminescence from single
monolayers of nanocrystals in molecular organic devices, Nature (2002) 420, 800.

(209). C. Bonati, M.B. Mohamed, D. Tonti, G. Zgreblic, S. Haacke, F. van Mourik, M.
Chergui, Spectral and dynamical characterization of multiexcitons in colloidal CdSe
semiconductor quantum dots, Physical Review B (2005) 71, 205317.










(210). C. Burda, M. A. El-Sayed, High-density femtosecond transient absorption
spectroscopy of semiconductor nanoparticles. A tool to investigate surface quality,
Pure and Applied Chentistry (2000) 72, 165.

(211). C. Burda, S. Link, M. Mohamed, M. El-Sayed, The relaxation pathways of CdSe
nanoparticles monitored with femtosecond time-resolution from the visible to the IR:
Assignment of the transient features by carrier quenching, Journal of Physical
Chentistry B (2001) 105, 12286.

(212). B. P. Zhang, Y.Q. Li, T. Yasuda, Y. Segawa, K.Edamatsu, T. Itoh, Time-resolved
photoluminescence of ZnCdSe single quantum dots, Journal ofCrystal G; 1,n thr
(2000) 214, 765.

(213). N. C. Greenham, X. G. Peng, A. P. Alivisatos, Charge separation and transport in
conj ugated-polymer/semi conductor-nanocry stal composites studi ed by
photoluminescence quenching and photoconductivity, Physical Review B (1996) 54,
17628.

(214). N. C. Greenham, X. G. Peng, A. P. Alivisatos, Charge separation and transport in
conjugated polymer cadmium selenide nanocrystal composites studied by
photoluminescence quenching and photoconductivity, Synthetic Metals (1997) 84,
545.

(215). L. M. Herz, C. Silva, R. T. Phillips, S. Setayesh, K. Mullen, Exciton migration to
chain aggregates in conjugated polymers: influence of side-chain substitution,
Chemical Physics Letters (2001) 347, 3 18.









BIOGRAPHICAL SKETCH

Lindsay Michelle Hardison was born on July 3, 1979, in Torrej on, Spain, where her father,

Craig Hardison was stationed as a member of the United States Air Force. She lived there with

her father and mother, Susan, until she was 16 months old. She then moved to Washington until

she started elementary school. Lindsay's father was relocated to Hampton, Virginia, where she

continued her early academic studies. After attending Hampton Christian High School for 3

years, Lindsay moved to Melbourne, Florida, in 1997 to begin her undergraduate studies at the

Florida Institute of Technology. In 2001, she graduated magna cum laude with a Bachelor of

Science degree in research chemistry. She then took a 1-year break from school and worked as

an analytical chemist at Midwest Research Institute in Palm Bay. Lindsay's drive to continue

her education and desire to learn brought her to the University of Florida in 2002. She began her

doctoral work under the supervision of Professor Valeria D. Kleiman in the area of ultrafast laser

spectroscopy of semiconductor nanoparticles and conjugated polymers. Her professional career

as a Ph.D. will begin in Hillsboro, Oregon, as a technologies development engineer at Intel@.





PAGE 1

1 ULTRAFAST SPECTROSCOPY OF NOVEL MATERIALS By LINDSAY M. HARDISON A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLOR IDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA 2007

PAGE 2

2 2007 Lindsay M. Hardison

PAGE 3

3 To my family

PAGE 4

4 ACKNOWLEDGMENTS As I reflect on the number of years that have led up to this moment of earning a Ph.D. in Physical Chemistry, I realize there are numer ous people to recognize and say thank you to because without their support and encouragement I w ould not have made it to this point.First and foremost, I thank the Lord because without His me rcy nothing is possible. I thank my advisor, Professor Valeria D. Kleiman for her guidance, patience and consiste nt motivation throughout my journey. I appreciate the effort and time sh e has put into helping me pursue this degree including letting me explore new possibilities and career building act ivities such as working for a summer at Corning, Inc as an intern. Valeria has continually believed in me and my work even when I did not and it is this type of support and enthusiasm that has enabled me to finish this project. I thank my supervisory committee members Prof essors Philip Brucat and Nico Omenetto for their guidance and thought provoking discussions. My gratitude goes to Dr. Kirk Schanze, Dr. Hui Jiang and Xiaoyoung Zhao for their CPE co llaboration. I apprecia te them providing the polymers and their willingness to help satisfy the need s of the project. I am also grateful for Dr. Paul Holloway and Dr. Hyeokjin Lee asking for our assistance in thei r nanorod project; it has been an experience I truly enjoyed. I express gratitude to the members, past a nd present, of the Kleiman Group. Thanks goes to Dr. Jrgen Mller for giving me a fundament al understanding of the transient absorption. I thank Dr. Evrim Atas for her ongoing friendshi p, Turkish cooking and being my upconversion mentor. There are no words that describe how mu ch I appreciate Daniel Kuroda. I thank him for not only his ability to answer all of my questions but also for his constant support and encouragement and of course, his BBQing skills. I also want to thank Cochuk, Aysun Altan,

PAGE 5

5 for helping me out in so many ways, especially in the last few week s, it has been greatly appreciated. I have dedicated this disserta tion to my family. To me, fami ly means more than just blood relatives, it is who you believe supports you and will be there for you always. To start, I want to thank Coach Dr. Nancy Bottge. She was such an in fluential person in my life and taught me that I must stick to the fight when hardest hit. Her dont quit attitude is one of the reasons for this success. She had not only been a mentor and a co ach but a friend that I could turn to and who taught me so many invaluable lessons. I extend ex treme gratitude to Chad Mair who has been beside me each step of my life in the past five years. I have been blessed with having such a good friend that I love so much that I consider him to be my brother. I can count on him for anything and know that without him, this achievement in my life may not have been possible. He has been the best work out partner, best friend and best colleague a girl could ask for. Jana Vanderloop, my best friend of ten, going on forever, years has also been a rock for me to lean on. I will always be sure to appreciate our inner ra ndomness because without it, life is too serious. I enjoy the fun we have on a daily basis, it k eeps me sane. From day one, Richard Farley and I have battled our way through the trials and tribulations of gr ad school. I thank him for his companionship, sense of humor and open mindedness. I thank Roxy Rory Lowry and Todd Prox for their friendship, laughs and their ability to give me different perspectives on all situations I run into in life. I also appreciate the encouragement and advice that I received from my friend Jim Reynolds. I thank Megan Meyer fo r her intense sarcasm because no matter what mood I am in, it always puts a huge smile on my face. My time here would not have been the same without the social ac tivities provided by all my friends in Gainesville. I am extremely gratef ul that I was a part of such a fun group that

PAGE 6

6 includes Sophie, Merve, Roxy, Richard, Rob, Neil, Eric, Megan, Meg etcTheir laughter and craziness will be greatly missed, especially during the fall at tailgating. Thanks to M.I.A and Whoever Shows Up, the two best in tramural softball teams in Univ ersity of Florida history for making me feel a little bit younge r. I have enjoyed playing fo r five years and will miss the teammates that have helped form our dynasty. Finally, I thank my parents Craig Hardison and Susan Keller for allowi ng me to make my own decisions so that I could become the inde pendent woman I am today. They have always believed that I could do anything I put my mind to. I thank my sist er Brynn, for terrorizing me as a child but growing up to become a wonderf ul young woman that I can call my friend.

PAGE 7

7 TABLE OF CONTENTS page ACKNOWLEDGMENTS...............................................................................................................4 LIST OF TABLES................................................................................................................. ..........9 LIST OF FIGURES................................................................................................................ .......10 ABSTRACT....................................................................................................................... ............13 CHAPTER 1 INTRODUCTION................................................................................................................... ...15 Study Overview................................................................................................................. .....15 Photophysics Concepts.......................................................................................................... .16 Energy Transfer...............................................................................................................18 Radiative Energy Transfer...............................................................................................19 Non-radiative Energy Transfer........................................................................................19 Random Walk Migration (Intr achain Energy Transfer)..................................................20 Emission Measurements..................................................................................................22 Transient Absorption........................................................................................................... ...24 2 QUANTUM NANOPARTICLES..............................................................................................27 Overview....................................................................................................................... ..........27 Bulk vs. Quantum Semiconductors........................................................................................27 Size and Shape Dependence...................................................................................................33 Passivation.................................................................................................................... ..........36 Composition Changes: Interdiffusion.....................................................................................39 Experimental Methods: Nanorod Synthesi s and Composition Characterization...................41 Preparation of ZnCdSe Nanorods....................................................................................41 Steady State Instrumentation...........................................................................................42 Time-Resolved Photoluminescence Instrumentation......................................................43 Results and Discussion......................................................................................................... ..46 Synthesis of ZnCdSe Nanorods.......................................................................................46 Structure of ZnCdSe Nanorods.......................................................................................47 Effect of Alloying on the Phonon Spectra.......................................................................49 Photoluminescence and Absorption Properties...............................................................52 Time-Resolved Photoluminescence (TRPL)...................................................................55 Summary........................................................................................................................ .........62 3 QUANTUM PARTICLE ELECTRONIC STRUCTURE..........................................................63 Introduction................................................................................................................... ..........63 Experimental Methods: Transient Absorption........................................................................67

PAGE 8

8 Results........................................................................................................................ .............69 CdSe versus CdSe/ZnSe Core/Shell................................................................................69 Core/Shell Excitation Dependence..................................................................................72 Core/Shell versus Alloys.................................................................................................75 Discussion..................................................................................................................... ..........78 Summary........................................................................................................................ .........85 4 CONJUGATED POLYELECTROLYTES (CPES)...................................................................86 Introduction................................................................................................................... ..........86 Quenching PPE-CO2 -..............................................................................................................93 Experimental Methods........................................................................................................... .95 Synthesis of Variable Chain Lengths of PPE-CO2 -.........................................................95 Photophysical Methods...................................................................................................95 Photophysics of Variable Chain Length PPE-CO2 Polymers..............................................105 Steady State Characterization........................................................................................105 Time-Resolved Fluorescence................................................................................................112 Isotropic Upconversion.................................................................................................112 Time-Resolved Anisotropy...........................................................................................119 Potential Kinetic Model.................................................................................................121 Summary........................................................................................................................ .......122 5 CONCLUSIONS AND FUTURE WORK...............................................................................124 Nanoparticle Conclusions and Future Work.........................................................................124 Conclusions...................................................................................................................124 Outlook/Future Work....................................................................................................125 PPE-CO2 Conclusions and Future Work..............................................................................129 Conclusions...................................................................................................................129 Outlook/ Future Work (Hyperbranched PPE-CO2 -)......................................................130 LIST OF REFERENCES.............................................................................................................133 BIOGRAPHICAL SKETCH.......................................................................................................150

PAGE 9

9 LIST OF TABLES Table page 2-1 Comparison of and value of CdSe/ZnSe and ZnCdSe nanorods......................................61 4-1 Experimental conditions for wavelength dependence study..................................................97 4-2 Detection dependence decay times.......................................................................................113

PAGE 10

10 LIST OF FIGURES Figure page 1-1. Jablonski diagram........................................................................................................ ..........17 1-2. Generalized diagram for spectral overl ap of donor emission and acceptor absorption and the energy transfer between resonant transitions of donor and acceptor. Adapted from B. Valeur.( 2 )..............................................................................................................21 1-3. Signals in transien t absorption m easurements.......................................................................26 2-1. The band theory of solids................................................................................................ ......28 2-2. The nanocrystal band gap size dependence...........................................................................30 2-3. The absorption spectra of TOPO/TOP pa ssivated CdSe nanocrystals with radii from 1.2 to 4.1 nm...................................................................................................................... ......33 2-4. Nanorod with each axis labeled........................................................................................... ..35 2-5. Electronic potential step of valence and conduction bands...................................................38 2-6. Time-resolved photoluminescence........................................................................................44 2-7. Powder X-ray diffraction patterns of CdSe nanorods, CdSe/ZnSe core/shell nanorods, and ZnCdSe alloyed nanorods...........................................................................................47 2-8. High resolution-transmission electron microscopy image and histogram of size distribution of ZnCdSe nanorods.......................................................................................48 2-9. Raman spectra of LO phonon mode of CdSe nanorods and CdSe/ZnSe core/shell nanorods....................................................................................................................... ......50 2-10. Raman LO phonon spectra of ZnCdSe nanorods after annealing at 270C........................51 2-11. The UV-Vis absorption spectra CdSe, CdSe/ZnSe core-shell and ZnCdSe nanorods........52 2-12. The photoluminescence spectra of CdSe/ZnSe core/shell and CdSe nanorods...................53 2-13. The photoluminescence spectra of CdSe/ZnSe core/shell and ZnCdSe nanorods..............55 2-14. The broad band photoluminesce..........................................................................................57 2-15. The CdSe/ZnSe core/shell photoluminesence.....................................................................59 2-16. The time-resolved photoluminescence decay curves..........................................................59

PAGE 11

11 2-17. The ln[ln(Io/It)] versus ln(time) of Cd Se/ZnSe coreshell nanorods and ZnCdSe alloy nanorods....................................................................................................................... ......60 2-18. High resolution-transmission electron mi croscopy images of CdSe/ZnSe Core/Shell and ZnCdSe Nanorods.......................................................................................................61 3-1. Electronic structure in semiconductor nanoparticles.............................................................64 3-2. Transient absorption..................................................................................................... .........69 3-3. The broad band transient absorption spectra for CdSe and CdSe/ZnSe core/shell rods at various pump delay times..................................................................................................71 3-4. The kinetic traces correspond ing to the 1S, 1P and 2S bands...............................................73 3-5. The time-resolved excitation dependence collected for the core/shell sample.....................74 3-6. The broad band transient absorption spec tra for CdSe/ZnSe and ZnCdSe nanorods at various time delays............................................................................................................76 3-7. The 1S and 1P composition dependence...............................................................................77 3-8. The comparison of the 1S band for the Cd Se/ZnSe core/shell and 3 hr ZnCdSe alloy ........78 3-9. Valence and conduction band offsets for various materials. ( 75 ).........................................83 3-10. CdSe/ZnSe core/shell potential kinetic model.....................................................................83 3-11. ZnCdSe alloy potential kinetic model.................................................................................84 4-1. The intrachain energy transfer of ex citation to quencher molecule along polymer backbone....................................................................................................................... .....91 4-2. The PPE-CO2 polymer repeat unit........................................................................................91 4-3. The Stern-Volmer plot of 10 M 185 PRU PPE-CO2 -..........................................................94 4-4. Fluorescence up-conversion............................................................................................... ...97 4-5. Transition moments....................................................................................................... ........99 4-6. Photoselection........................................................................................................... ...........100 4-7. Berek polarization compensator..........................................................................................104 4-8. Berek compensator used as a half-wave plate.....................................................................104 4-9. The chain length absorption shift for PPE-CO2 in methanol..............................................106

PAGE 12

12 4-10. The emission spectra 10 M PPE-CO2 in methanol.........................................................106 4-11. The emission of 10 M 35 PRU PPE-CO2 ......................................................................108 4-12. The excitation spectra 10 M 8 PRU PPE-CO2 -...............................................................109 4-13. The excitation spectra 10 M 35 PRU PPE-CO2 in methanol.........................................110 4-14. The excitation spectra 10 M 35 PRU PPE-CO2 in water...............................................111 4-15. The excitation spectra 10 M 35 PRU PPE-CO2 in methanol with Ca2+.........................112 4-16. The time-resolved fluorescence decay of 8 PRU PPE-CO2 in methanol.........................114 4-17. The time-resolved fluorescence decay of 30 M PPE-CO2 with different polymer repeat units in methanol...................................................................................................116 4-18. The time-resolved fluorescence decay of 30 M PPE-CO2 (35 PRU) with and without Ca2+ .............................................................................................................................. ...117 4-19. The time-resolved fluorescence decay of 10 M PPE-CO2 (8 PRU) with and without Ca2+............................................................................................................................... ...118 4-20. The time-resolved fluorescence decay of 30 M PPE-CO2 35 PRU with different quenchers...................................................................................................................... ...119 4-21. The anisotropy of 8 PRU PPE-CO2 -..................................................................................120 4-22. Possible kinetic model for all PPE-CO2 PRU chains.......................................................123 5-1. The absorption spectra of the hyperbranched PPE-CO2 and the linear PPE-CO2 -.............131 5-2. The photoluminescence spectra of hyperbranched PPE-CO2 ............................................132

PAGE 13

13 Abstract of Dissertation Pres ented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy ULTRAFAST SPECTROSCOPY OF NOVEL MATERIALS By Lindsay M. Hardison December 2007 Chair: Valeria D. Kleiman Major: Chemistry My research focused on steady state and time -resolved photophysical characterization of a series of semiconductor nanoparticles and water soluble conjugated polyelectrolytes. Several studies have shown that the electronic structur e and relaxation dynamics in CdSe nanocrystals are not only size but are also shape and passi vation dependent; however, there is no detailed comparison of the photophysical properties of Zn CdSe particles with different relative amounts of Zn. This dissertation presents data collected for colloidal CdSe, CdSe/ZnSe and ZnCdSe nanoparticles with rod-like architectures synthesized and inves tigated in our labs to determine how size, shape, passivation and composition aff ect the quantum confinement and dynamics. In addition, a series of different polymer repeat uni t lengths of a linear conj ugated polyelectrolyte (CPE) with a carboxylate ionic side chain ha ve been synthesized and their photophysical properties have been explored. Spectral shifts and line broadening exhi bited within the Raman spectroscopy, UV-Vis spectroscopy and photoluminescen ce aided in determining th e extent of alloying and compositional disorder created during the alloying process. The photoluminescence quantum yield of ZnCdSe nanorods is higher than that from pristine CdSe nanor ods indicating a higher binding energy of the exciton. This effect is specul ated to be due to increased localization of the

PAGE 14

14 exciton as a result of fluctuat ions in the composition, ultimate ly resulting in increases in luminescence efficiencies. Moreover, time-resolved photoluminescence char acterized lifetimes of nanoparticles with similar shape but different composition. Emissi on of an inhomogeneous population distribution (different sizes, shapes or composition) leads to the simultaneous probing of particles with different decaying rates. A stretched exponential function, I(t)= A*exp[-(t/ )], can be used to describe these systems, where <1 corresponds to disper se populations. In the experiments presented here, the photolumi nescence data yields small values, independent of the emitted photon energy. Photoluminescence decay lifetime, of the samples increased with alloying time due to compositional disorder l eading to exciton localization. The dynamics of each nanorod was studied by absorption changes using ultrafast pumpprobe spectroscopy. An excitation wavelength dependence study has been conducted to gain insight into the intraband/interband relaxation in core/shell nanorods with small valence band offsets. Determination of the dynamics and mechan isms of these systems will be useful for the study of fundamental physics and light emitting applications such as LEDs, photovoltaic devices, lasing and fluorescence tagging. CPEs are soluble in polar solvents and thei r conformational properties can be tuned to enhance their emissive behavior for sensing a nd device applications. It was found that polymer concentration, solvent, aggregati on inducer and chain length, all affect the quenching efficiency; therefore, this dissertation examines energy tr ansfer mechanism responsible for this behavior using ultrafast upconversion. Upon excitation of the aggregates from energetically higher lying isolated chains, the fluorescence lifetimes resu lt in multi-exponential behavior due to the competition between the radiativ e and non-radiatve decay.

PAGE 15

15 CHAPTER 1 INTRODUCTION Study Overview The overall goal of my study was to investigat e light-matter interactions in a series of novel materials. The projects presented are an in teresting opportunity to explore the exciton dynamics and energy transfer processes in inor ganic semiconductor nanoparticles and organic conjugated polymers. The first ch apter will briefly discuss fluores cence principles and excitation energy transfer (interchain) and random walk (i ntrachain) energy transfer. This discussion is helpful to understand quenching of conjugated po lyelectrolytes. Also, th e signals that can be detected using femtosecond transient absorpti on (pump-probe) spectros copy are described in detail. The information provided is advantageous for the reader to understand the data presented concerning the excited state of the nanorods. Chapter 2 describes the motivation for the sy nthesis and characterizat ion of an array of CdSe/ZnSe core/shell and ZnCdSe alloyed nanorods. Background into the size, shape, passivation and composition dependence is pres ented. Moreover, synthetic steps, x-ray diffraction, high-resolution transmission electr on microscopy, Raman spectroscopy, steady state absorption, photoluminescence and time-resolved photoluminescence data are included. The ternary alloy composition is confirmed and th e qualitative trends based on compositional disorder are discussed. Femtosecond transient absorption (Chapter 3) was used to explore the excited state behavior of the same series of nanorods. A co mparison between CdSe and CdSe/ZnSe core/shell nanorods is made to show how passivation alte rs the exciton behavior. Also, excitation wavelength dependence is presented for the CdSe /ZnSe shell to elucidate the influence an interfacial state (determined in Chapter 2) has on the dynamics of the photo-generated exciton. In

PAGE 16

16 addition, the effects alloying has on the excited st ate of the ZnCdSe nanorods are discussed. Two models, one for the core/shell a nd one for the alloy, are propose d to describe the relaxation processes observed in ea ch of the experiments. The dynamics of the energy transfer from isolat ed to aggregated species in a series of different polymer repeat unit sizes of a conjugated polyelectrolyte, PPE-CO2 -, synthesized by Xiaoyoung Zhao in Dr. Schanzes lab are discus sed (Chapter 4). Anis otropy measurements confirm that this polymer is very rigid and th e conjugation length is long er than expected. The data is analyzed to extract the influence a ggregation has on the isol ated chain emission. Finally, Chapter 5 summarizes each project and states general conclusions drawn from the results collected for this dissert ation. Suggestions are made for pot ential applications for which semiconductor nanoparticles presented in this dissertation may be useful. Also, an additional molecule, similar to the PPE-CO2 -, is presented as the next step in a series of polymers to investigate for chemo-or bio sensors. Photophysics Concepts Spectroscopy methods, whether they be time-re solved or steady state, provide numerous ways to measure emission of materials that ar e intended to be used in a wide variety of applications including opto-and el ectronic, biomedical, and chemi cal research. This dissertation focuses on the photophysical propert ies of nanocrystals and energy transfer mechanisms that induce the amplified quenching capabilitie s of conjugated polyelectrolytes. It is important to know the multiple photophysic al processes that an excited chromophore can undergo between the absorption and emission of light. These processes are dictated by the probability that a transition from an initial stat e to a final state can occur. By using timedependent perturbation theory, Fermis Golden Rule for transitions between two states corresponds to a transi tion rate equal to: ( 1, 2 )

PAGE 17

17 S0 S1S2Sn A F IC T1T2 ISC IC PElectronic ground state IVR S0 S1S2Sn A F IC T1T2 ISC IC PElectronic ground state IVR 2 2'22TifkVH (1-1) where corresponds to the density of final states that are available to interact with the initial states via the perturbation, H This perturbation can alter the positions or motions of particles and restructure the initial state so that is looks li ke the final state. Thus, Fermis Golden Rule is simply a transition rate probability between an initial and final state which depends on the magnitude of a perturbation.( 1 ) The electronic transitions can be visualized along with the processes that can occur between these states in a general Jabl onski diagram seen in Figure 1-1. Figure 1-1. Jablonski diagra m. A = photon absorption; F = fluorescence (emission); P = phosphorescence; S = singlet stat e; T = triplet state; IC = internal conversion; ISC = intersystem crossing, IVR = in ternal vibrational relaxation. Adapted from B. Valeur. ( 2 ) Absorption of a photon by the ground state, S0, promotes an electr on to the vibrational levels of an upper singlet excited state, S1, S2, or higher, via a spin-cons erved, allowed transition. Subsequently, the excitation can be transferred to an isoenergetic vibrational manifold of a lower singlet excited state with the same spin multiplicity, for example S2 to S1. This process is aided

PAGE 18

18 by the overlap between the wavefunctions of the vi brational levels particip ating in the process. This radiationless passage is termed internal c onversion and occurs quite rapidly, usually within a few picoseconds or less after light absorption, which is significantl y faster than typical fluorescence lifetimes. Based on Kashas rule,( 2 ) the excitation will rapidly relax to the lowest vibrational level of the firs t singlet excited state, S1, via internal vibrational relaxation (IVR); therefore, the fluorescence emission, in most orga nic molecules, comes from the lowest excited vibrational level. Radiationless decay (which releases heat) and intersystem crossing (ISC) to a triplet excited st ate (resulting in phosphor escence) can also occur. ISC is a non-radiative transition that involves two electronic states that are equally energe tic but have different multiplicities. However, the magnitude of coupl ing between the orbital magnetic moment and spin magnetic moment (spin-orbit coupling) can be larg e enough so that this normally forbidden transition may occur.( 2 ) Energy Transfer One of the motivations behind the work presen ted in this dissertation is to identify the mechanisms responsible for the amplified quenc hing observed in conjugated polyelectrolytes. Aside from relaxation, the excited state of a ch romophore, D*, can relax to its ground state after transferring the photoexcited energy to an acceptor molecule, A, via a bimolecular process: D* + A D +A* This process strongly depends on two conditions: (1) the emission of the excited donor should overlap with the absorption of th e acceptor and (2) the natural lifetime of the excited donor must be slower than the energy transfer process. Once energy transfer has occurred, the photoexcited chromophore, A*, has the ability to play a part in photochemical reactions or display sensitized emission.( 1, 2 ) There are several different types of ener gy transfer mechanisms used to describe

PAGE 19

19 the diffusion of energy in molecules. In this dissertation only an overvie w of the radiative and non-radiative (interchain) energy transfer and rand om walk diffusion (intrachain) are discussed. Radiative Energy Transfer A two step process that involves emission of a photon from the excited state of the donor molecule followed by the same photon being ab sorbed by the ground state of the acceptor is called radiative energy transfer. Step 1: D* D + h Step 2: h + A A* This type of energy transfer is the least compli cated since it does not i nvolve the interaction of the donor and acceptor molecules. For this mechanis m to be effective, the quantum yield of the donor must be high in the spectra l region of the absorption of th e acceptor. To further enhance radiative energy transfer, it is beneficial to have a high conc entration and extinction coefficient of the acceptor in addition to a large spectral ove rlap between the emission of the excited donor and ground state absorption of th e acceptor. The emission spectra of a donor molecule that undergoes radiative transfer will experience a decrease in its fluorescence intensity in the spectrally overlapped region and can lead to re peated absorption and emission if the donor and acceptor molecules are identical (self-absorption/r eabsorption). If there is adequate absorption and emission overlap, the fluorescence lifetimes can increase.( 1, 2 ) An example of this process is shown in Figure 4-8, where we observe self-abs orption in a conjugated polyelectrolyte solution that is highly concentrated. Non-radiative Energy Transfer Non-radiative energy transfer occu rs in a single step and just as radiative energy transfer, depends on the spectral overlap between the donors emission and acceptors absorption spectra but relies more on their coupled re sonances (Figure 1-2). As seen in the Jablonski diagram in

PAGE 20

20 Figure 1-1, several vibronic transitions of one state or in this case, a donor molecule, can be isoenergetic to the corresponding transitions of the acceptor (D* D and A A*). In general, the non-radiative transfer rate is given by Eqn 1-1, (Fermis Gold Rule) where the density ( ) is not only related to the coupling of the initial and final states capable of a transition (determined by Frank-Condon factors) but also by the non-in homogeneously broadened spectral overlap, J of the donor emission,()DI ,and acceptor absorption,()A determined using Eq 1-2.( 1, 2 ) 0()()DAJId (1-2) This integral assumes that the relaxation within the excited state vibrational manifold is faster than the energy transfer proce ss and that energy transfer abid es by the Franck-Condon principle (vertical transition). As the number of resonant transitions between the donor and acceptor increases, the likelihood for a non-ra diative energy transfer process to occur increases since these transitions are proportional to the overlap integral (Figure 1-2).( 1, 2 ) Random Walk Migration (Intrachain Energy Transfer) In some cases, a quenching of the fluorescen ce occurs but can not be explained by a bimolecular energy transfer mechanism. Upon exc itation of a molecule, an excited state electron and ground state hole pair are creat ed, termed exciton. If the molecule consists of multiple segments that are equivalent in energy or are a cascade of energies (like a polymer with repeating chromophores), this exciton can diffuse from one segment to another while remaining bound. The exciton undergoes a mechanism that involv es a hopping from one segment to another within the same polymer. The random walk or intrachain energy transfer implies that the electron and hole move together, and will always be located within the same chromophore thus charge separation does not occur. Also, during th e course of the energy diffusion, the energy is

PAGE 21

21 not dissipated. This type of energy transfer between chromophores is difficult to measure primarily because directly collecting the fluor escence of an acceptor after exciting a donor chromophore of the same species with similar energies can be complicated. Figure 1-2. Generalized diagram for spectral overlap of donor emission and acceptor absorption and the energy transfer between resonant transitions of donor and acceptor. Adapted from B. Valeur.( 2 ) Eugene Rabinowitch, a biophysicist, likened the random walk to a steel ball being shot into a pinball machine where the ball bounces around with in the machine but eventu ally either falls to the bottom (fluorescence) or falls into a play hole, (trap).( 3 ) Scientifically speaking, the exciton can either fluoresce by recombining the el ectron and hole, can be dissipated by internal

PAGE 22

22 conversion in one of the chromophores or it can reach a trap. A trap (or "energy sink") is considered a lower energy state that is too deep for the exciton to overcome so from there it will recombine radiatively or non-radiatively but it will not undergo any more diffusion.( 4, 5 ) In the case of conjugated polymers, this trap could be due to a kink in the polymer chain, a defect on a chromophore, or a particularly low energy chro mophore. If trapping happens the exciton cannot reach its desired destination, an external quenche r, an analyte or a material that can separate charges, which would be detrimental for bioand chemo sensors and photovoltaics. Emission Measurements Conjugated polyelectrolytes are intended for use as fluorescence based-sensors and rely on changes of emission intensity and/or lifetime. Se nsors of this nature are some of the most common due to the ease of measur ement and low detection limits.( 6, 7 ) The fluorescence quantum yield ( F) is an important parameter that is de fined as the ratio of the numbers of emitted photons, Nemit, to the number of absorbed photons, Nabs. If all possible pathways are considered, the quantum yield is calculated as follows:( 2 ) 1 1emitradradrad F absallradICISCET f lNkk Nkkkkk (1-3) where krad is the rate constant of the fluorescence emission, kIC, kISC, and kET are the internal conversion, intersystem crossing and energy transfer rate consta nts, respectively. If the nonradiative decay ( knr) is the only competing process with the fluorescence emission, the quantum yield is given by: ( 2 ) rad F radnrk kk (1-4)

PAGE 23

23 Thus, the greater the non-radiative decay rate co nstant, the smaller the quantum yield, and vice versa. The time that it takes for th e excited state of a molecule to decay to 1/e of its initial value is the lifetime of the excite d state, which is given by: 1radnrkk (1-5) The fluorescence emission intensity, quantum yield, and lifetime can be negatively affected by numerous quenching processes incl uding collisions with heavy atoms, electron transfer, energy transfer, excimer formations, aggregate formation, and dynamic collisions.( 2 ) The quenching processes discussed above can be m easured but the results of these experiments can be hard to interpret. Instead, fluorescen ce anisotropy is employed to understand amplified quenching observed in conj ugated polyelectrolytes. Direct measurement of the random walk hopping of excitons is a difficult task since this process can compete with other energy transfer pr ocesses. Time-resolved anisotropy is type of measurement that measures the decay of polarized fluorescence, which gives a better understanding of the random in trachain energy migration in a material. The sensitivity to depolarizing the transition dipole moments between an absorbing and emitting molecule is directly related to the loss of anisotropy. Excita tion with light polarized in a particular direction will only excite molecules with the same orient ation. For example, a vertical excitation will preferentially excite molecules with vertical transition dipole moments. Anisotropy values will not change if as the exciton migrates there is no change in the dipole moments between the chains that absorb and emit. However, if these dipoles do change, as the exciton hops, it loses its original orientation and the fluorescence signal depolarizes.( 4, 5 )

PAGE 24

24 Transient Absorption Changes in population of differe nt energy states can be examined using femtosecond timeresolved pump-probe spectroscopy. A pump pulse excites the sample, cau sing a depopulation of the ground state. A broad-band probe is transmitted through the excited volume of the sample, monitoring the populations of vari ous excited states. The time-dep endency of the technique is introduced by varying the dela y time between the probe pulse and the excitation pulse. Simple absorption measurements can be accomplished by measuring the log of the ratio of the intensity of an incident beam that enters the solution, I0, and intensity of the beam exiting the solution, I 0loglog I AT I (1-6) where A is the absorbance and T is the transmittance. This ratio leads to the number of photons absorbed which is based on the sum of the absorption cross section, (cm2), of all the molecules in the path of the incident beam.( 2 ) 00 3 31 ln or log 10002303 (molecules/mol) (mol/L) since N(molecules/cm) 1000 (cm/L)a a aINclI Ncl II Nc (1-7) where, Na is Avogadros number, c is the sample concentration (M) and l is the optical path length (cm) within the sample. The molar absorption coefficient (extinction coefficient) is defined as:( 2 ) 2303aN Units = M-1 cm-1. (1-8) So, for a dilute solution, the absorption of light can be described using Beers law: or 2303aN A clAcl (1-9)

PAGE 25

25 In ultrafast experiments, shot-t o-shot laser fluctuations can hinder the detection of small transient signals. To overcome this limitation, a shot-to-shot normalization is utilized by having a second beam (reference) transmitted through the sa mple without overlapping with the pump. For transient absorption, the change in transmission ( T ) is defined as the transmission of the probe in the presence of the pump ( It, pump) minus the transmission of the probe in the absence of the pump ( It, no pump). The normalized change in transmission is given by: ,, ,,tpumptnopump referencereference pumpnopump tnopump nopump referenceII II TT T I TT I (1-10) this can be converted to cha nges in absorption using Eq 1-11. log1 T A T (1-11) There are multiple types of signals that can be observed in data collected using this technique. In Figure 1-3 the po ssible transitions associated wi th particular changes in the absorption spectra are shown. Bleach (1) The ground state of the sample will absorb photons creating a bleach of the ground state. When the pump is on, the deplet ed ground state will absorb less photons, and the measured A will be negative. If measuring transmissi on, the probe will transmit more, creating a positive change in T/T. This bleach signal will appear at transitions observed in the steady state absorption spectrum. A bleach signal appe ars instantaneously after the excitation. Photoinduced absorption (2) If the photoexcited state absorbs a photon, the beam probing that state will be attenuated, creating a positive change in absorption and a negative change in transmission.

PAGE 26

26 S01 t<0 t=0 A<0 Excitation t = 0 t > 0 A 2 probeS0 A > 0 probe A3 1 2 probe A<0 probe ASteady State Time ResolvedAbs Fluo S01 t<0 t=0 A<0 Excitation t = 0 t > 0 A 2 probeS0 A > 0 probe A3 1 2 probe A<0 probe ASteady State Time ResolvedAbs FluoStimulated emission (3) The pump will excite a population to some higher lying states and the probe will stimulate them to emit. If measuring the change in transmission, this signal will be positive since more light is apparently being transmitted through the sample. If measuring A, since the probe is stimula ting emission it corresponds to a ne gative absorption. A stimulated emission signal might not appear instantaneously since the relaxation of the initially excited state will occur before stimulated emission. Although stimulated emission can appear at wavelengths other than the steady state absorptio n spectrum, it can be difficult at times to distinguish between the bleach and stimulated em ission signals due to overlap of the absorption and emission positions. Figure 1-3. Signals in transient absorption measurements

PAGE 27

27 CHAPTER 2 QUANTUM NANOPARTICLES Overview Continuous advancements in the synthetic methods for the production of colloidal semiconductor nanoparticles of different size, shape, and composition have greatly improved their process-ability and functionality. The effect s that the physical features of these materials have on their corresponding photophysics have been the focus of nu merous scientific investigations in hopes that the inherent character istics of the new systems will make them useful for applications extending from basic fundamental physics studies ( 8 ), photovoltaics ( 9 ), optoelectronics ( 10-12 ) to photocatalysis.( 13-15 ) Many researchers have employed ultrafast time-resolved techniques to elucidate the dyna mics within various colloidal semiconductor quantum confined materials.( 16, 17 ) These fundamental investig ations are important for understanding and influencing the di rection and development of the area of nanoparticle science. New and precise synthetic methods have provided the ability to control the size, shape, and composition in order to manipulate the electronic or optical propert ies of nanoparticle materials. In 2004, we started collaborating with Profe ssor Paul Holloways research group to focus on the study of exciton dynamics in various semiconductor nanomaterial s. A member of Dr. Holloways group, Dr. Hyeokjin Lee, had synthesi zed CdSe, CdSe/ZnSe core/shell, ZnCdSe rods and ZnCdSe dots. Aside from CdSe, ( 13, 16-20 ) the literature lacked any information concerning the dynamics within these materials; it was an excellent opportunity fo r our lab to conduct cutting edge research in this area of materials science. Bulk vs. Quantum Semiconductors The primary bulk semiconductors used in solid -state electronic appl ications include materials such as silicon, gallium arsenide and cadmium selenide.( 21 ) The relationship between

PAGE 28

28 sp3+* Atomic Hybrid Orbitals Diatomic Orbitals Bonding Antibonding Mol. Orbitals HOMO LUMO Bulk Solid CB VB EgDensity of States Discrete States Discrete States Quantum Solids EgCB VB sp3+* Atomic Hybrid Orbitals Diatomic Orbitals Bonding Antibonding Mol. Orbitals HOMO LUMO Bulk Solid CB VB EgDensity of States Discrete States Discrete States Quantum Solids EgCB VB the created exciton and electroni c properties is of significant interest for the design and engineering of useful bulk and nanoscale semiconductor materials. Consider a summary of the band theory of solids presented in Figure 2-1. Figure 2-1. Band theory of solids Silicon has four sp3 hybridized atomic orbitals. Nei ghboring atoms contribute orbitals which combine to form highest occupied molecular orbitals (bonding orbitals, ) and lowest unoccupied molecular orbita ls (antibonding orbitals, *). The total number of occupied and unoccupied orbitals is equal to th e number of atomic orbitals present within the crystal. As more atoms are added, a density of orbital energies de velops reducing the spacing between the states in each band. This increase in density results in a continuum of ener gies separated by a gap. In a bulk solid, the highest occupied orbitals form the valence band and the lowest unoccupied orbitals form the conduction band. The minimum en ergy required to excite an electron from the top of the valence band to bottom of the conduction band is the band gap energy of the semiconductor (Eg).( 21 )

PAGE 29

29 The electronic and optical properties of a material are due to electron motion within molecular orbitals. The energy absorbed by an unbound electron (not co nfined) within the density of states in a bulk material is not quant ized. Therefore, the energy released by this electron is converted into kinetic energy. A semiconducto r can be photoexcited with a photon, exciting an electron from the valence band into the conduction band of the material, leaving a hole of opposite electric charge be hind, separated by distance cons isting of severa l atoms within the material. These distances are within the na nometer scale and are called the Bohr exciton radii. This radius, combined with a high dielectri c constant results in a small binding energy. The electron can be bound to the hole du e to Coulombic forces and if these interactions are strong enough, a Coulomb correlated, bound quasi-particl e called an exciton (electron-hole pair) is formed. If the size of the electronhole pair is approximately the same as the Bohr radius, and it is larger than the lattice spacing within the cr ystal, a Wannier-type ex citon is formed. This exciton can diffuse through the material until it is trapped, annihilated (under multi-excitation conditions) or recombined. If the wavefunctions of the electron and hole extend over a large number of atoms, the Coulombic attraction becomes negligible re sulting in unbound charge carriers which have slightly higher en ergies than the bound electron-hole pair. ( 13, 21, 22 ) If the size of bulk semiconductor is significan tly decreased to the poi nt where is it similar to the size of the Bohr exciton radius, then th e motion of the exciton wi ll become confined in multiple dimensions (quantum confinement) since it will have less room to move. The energy spacing between the various confined (bound) elect ron and hole states within the corresponding bands becomes quantized and the separation between these energy states will increase as the size of the particle (space) decreases due to stronger confinement.( 13, 17, 21, 22 ) In addition, the energy separation of the electron states is larger than the separation of the hole states since the

PAGE 30

30 EgEgEg VB VB VB CB CB CB EgEgEg VB VB VB CB CB CBhole has a larger reduced mass compared to the el ectron and the density of states in the valence band is larger than in the conduction band. Overa ll, due to quantum confinement, as the size of the semiconductor nanoparticle d ecreases, the band gap increases resulting in changes in the absorption and emission colors.( 13, 22, 23 ) (Figure 2-2) Figure 2-2. Nanocrystal band gap size dependence Just as in most quantum systems, there are multiple attempts to describe the electronic states mathematically. One particular way, us ed in quantum confinement of quantum dots, assumes that the quantum dots them selves are larger than the lattice constants of the crystal structure, which implies that the effective mass of the charge carriers remains unchanged despite the difference in size of the quantum dot compared to bulk. This is known as the effective mass approximation (EMA) and is utilized by most researchers in this area.( 22-25 ) Since, the effective masses of the carriers are considered to be cons tant; any modifications to the optical properties of the quantum dots observed will be due to quantum confinement. II-VI and III-V semiconductor quantum dots are considered to be in the strong confinement regime because their dimensions are generally larger than the lattice c onstant but less than or e qual to the Bohr radius

PAGE 31

31 size (11.2 nm for CdSe). Strong confinement of the electronic wavefunc tions results in an increase in the Coulomb interactions between th e electron and hole. If the materials are larger than this radius but smaller than their bulk count erparts, they are consid ered to be within the weak confinement regime.( 24 ) Nanowires, are a perfect example of materials in this regime due to the extension of the c-axis and smaller confinement potential in that direction. Considering EMA and the augmentation of C oulomb interactions between electrons and holes, excitons within nanometer sized semiconductors can be compared to motion of a particle in a 3-D box; as the size of th e box decreases, the kinetic energy and excitation energy increases. Figure 2-2 depicts how the quantum dot band gap varies as the size of the dot (box) changes. The enhanced exciton confinement w ithin smaller dots increases the amount of energy necessary to promote an electron to the conduction ba nd, overcoming the band gap barrier. The Schrdinger equation is used to consider the energy of the electronic states in quantum dots: ) ( ) ( r E r H (2-1) where H is the hydrogenic Hamiltonian for a Wannier-type exciton:( 21, 26 ) 222 2 **22eh ehehe H mmrr (2-2) where me and mh represent the electron and hole mass resp ectively, the distances between the electron and hold from the center of the quantum dot are er and hr and is the dielectric constant of the semiconductor. Since the center of mass and reduced mass motions cannot be separated into independent coordinates, analyti cal solutions for Eq. 2-1 and 2-2 are impossible. Therefore, different methods su ch as perturbation theory ( 26, 27 ) or a variational calculation ( 28 ) are utilized to describe energy in quantum systems resulting in the following equation:( 28 )

PAGE 32

32 222 min 2111.8 0.25 2Ryd ehe EE RmmR (2-3) Due to the quantum dot size dependence resulting in blue shifts as the dot size decreases, this equation evaluates the adju sted quantum dot band gap,minE, with respect to the quantum dot radius, R and the bulk exciton binding energy, *RydE.(26-28) As a result of quantum confinement propert ies exhibited by semiconductor nanoparticles, their electronic states are discrete and well-defi ned; therefore, their electronic states can be described in an atomic-like fashion. Three quant um numbers including spin are derived from the Schrdinger equation (Eqn 2-2) to evaluate the electronic states of the quantum dots. Using the effective mass model,(25) the electron and hole state notation is nLe and nLF, respectively where n is the principal quantum number (1, 2, 3, et c), and L is the envelope wave function angular momentum (S, P, D, etc) used to distingui sh energy states of the electrons and holes.(23) The hole total angular momentum, F [for a value of F, the state is (2 F+1)-fold degenerate], where F =2L+S and S represents spin.(17) In CdSe, the valence band is sixfold degenerate if the spin is considered since this band originates from p-atomic orbitals from within the selenium atoms.(25) The fine structure of the lowest exciton stat e within the valance band can be revealed (29) if the nanocrystal is non-spherical(30) or the crystal field effects(23) and exchange interactions(29) are considered.(17) It is well known that within CdSe quant um dots, the three lowest electron energy states are 1Se, 1Pe, and 1De and the first three hole states are 1S3/2, 1P3/2, 2S3/2.(25) Thus, the three lowest energy bands in ideal CdSe quantum dots are labeled as 1S [1S(e)-1S3/2(h)], 2S [1S(e)-2S3/2(h)] and 1P [1P(e)-1P3/2(h)]. It is possible to disrupt the ideal selection rules for spherical quantum dots, n=0, L=0, 2 and F=0, (31) via strong hole-state mixing (31) or by breaking the symmetry, which alters the degener acy and splitting of the excited states of the

PAGE 33

33 semiconductor quantum dots, ultimately changing the behavior of the excitons.(17) The absorption spectra of five colloidal CdSe nanocrystal s with different radii (1.2, 1.7, 2.3, 2.8, and 4.1 nm) is shown in Figure 2-3 (17) and illustrates not only the quantum dot band gap dependence but the features corres ponding to the optical transitions that arise from the coupled electron and hole electronic states previously discussed.(16-19) Figure 2-3. Absorption spectra of TOPO/TOP passi vated CdSe nanocrystals with radii from 1.2 to 4.1 nm.(17) Size and Shape Dependence Quantum confinement or the quantum size eff ect is a property that in recent years has revolutionized the semiconductor industry. This e ffect leads to unique electronic and optical properties making quantum dots differ from their bulk counterparts. II-VI semiconductor

PAGE 34

34 quantum dots have been the focus of several photophysical studies (18-20) stimulating interest in other types of quantum particle s. Fabrication of nearly spherical particles with various compositions in addition to synthesis of rod shaped (32) and even multifaceted tetrapods (33) has been achieved. Manufacturing such materials can be achieved by two different methods: 1. bottom-up and 2. top-down. The first method utilizes synthetic routes that adjust ratios of the chemicals needed to make the nanoparticl es with passivation or capping materials.(33-42) In the latter, the bulk semiconductor is cut down to scale using laser ablation-condensation or lithographic techniques a lthough these methods are extremely expensive.(43) The materials presented in this thesis have been prepared by a bottom-up approach in an attempt to synthesize better materials while en hancing their process-ability. Dependence on the sensitivity to size and shap e is important when considering the tunable optical properties of quant um nanoparticles. In partic ular, the size dependence and photoluminescence tunability in the visible regi on of CdSe quantum dots has been studied extensively.(37, 44-48) New methods for synthesis of r od-shaped CdSe nanoparticles have opened the door for shape-dependent a pplications such as polarized LEDs (49, 50). In particular, Alivisatos group synthesized quantum confin ed colloidal nanopart icles with rod-like architectures by using various surfactants that bind to different faces of the crystal.(51) For example, colloidal CdSe rod lengths can be vari ed from 5 nm up to 100 nm while maintaining a 2 to 10 nm diameter, which preserves lateral confinement of carriers in the nanocrystal. Alivisatos determined that the band gap depends ma inly on the width (a or b axis) and slightly on the length (c-axis) (Figure 2-4). (52, 53) However, a comparison of the dynamics within CdSe dots and nanorods has proven to be useful in und erstanding the electronic structure differences that occur when the c-axis is elongated.

PAGE 35

35 a b c Front Zoomed View Traps + a b c a b c Front Zoomed View Traps + Front Zoomed View Traps +El-Sayed et al. synthesized and compared CdSe rods with aspect ratios ~ 3 (length/width) and dots of 4.2 nm diameter.(53) TEM images show that the pa rticles are different although the steady state absorption spectra do not indicate sign ificant differences. Electron-hole dynamics measured by femtosecond pump-probe, although s till not completely un derstood, show quite different behavior for rods versus dots. This is confirmed by the increase in the number of bands in the deconvoluted absorption spectra of the quantum rods.(17, 18, 53) Moreover, they observed Figure 2-4. Drawing of a nanorod with each of th e axis labeled. The front zoomed view shows that the surface curvature is not smooth, leading to surface traps. a significant increase in the car rier relaxation time in the quantum rods compared to the quantum dots.(53) Nanodots have a higher order of symmetry, whic h is lost in the r ods. Extension of the c-axis results in a splitting of the degene rate level in the symmetric quantum dot (30, 53-60) and that energy level splitting could be one reason for El-Sayeds results. Due to the large surface-tovolume ratio at the surface in nanorods, electr on and holes have a high probability of being trapped by surface impurities. However, the quantu m dot curvature can create a larger number of localized surface trap states than the elongated nanorods,(53) which allow for the carriers to have free motion in the c-direction, reducing the probab ility of the carrier to be trapped as quickly. In some cases, the impurities present enable th e materials to be used in oxidation-reduction chemistry, more specifically photocatalysis, (15) photodegradation and detoxification of

PAGE 36

36 chemical and environmental pollutants.(61) For optical applications such as photovoltaics or LEDs, it is important that these surface traps do not contribute to the exci ton trapping within the material. Several groups have worked on develo ping passivation techniques that will enable enhancement of their photophysical characteristics without alteri ng their confinement behavior. Passivation Modification of semiconductor nanocrystal su rfaces plays an important role in their electronic and optical propertie s and has been the subject of extensive investigations. (34, 40, 6265) The dangling bonds present on the surface of the nanocrystals negatively influence the optical properties but passivation has been pr oven to improve various confinement properties such as high quantum efficiency and lumines cence stability. Due to a high surface-to-volume ratio, even pristine, bare CdSe quantum dots tend to result in low luminescent yields (0.6%) and poor stability.(66) The ratio leads to augmentati on of the electron and surface state wavefunction overlap which creates localized midgap surface state tr aps resulting in nonradiative decay and decreasing the overall phot oluminescence quantum yield (Figure 2-4).(67) However, in certain applications (68), in which the charge carrier-interface interaction is crucial, the high surface-to-volume ratio is beneficial. (14, 15, 69) If the surface of colloidal na noparticles is coated with an appropriate passivating agent, e.g., organic molecules, this competition may be sufficiently reduced to dramatically extend the band-edge lifetime and enhance th e luminescence efficiency. (37, 53, 64, 70) However, due to several drawbacks including imperfect surface passivation and exchange reactions causing photodegradation, organic coating is not sufficient for improving quantum yields.(16, 71) Using the diffusion-controlled colloidal gr owth method developed by Bawendi and coworkers, (37) CdSe quantum dots have been passivat ed with various shells, among these is ZnS,(39) which narrows the fluorescence emission and improves their efficiency. Epitaxial

PAGE 37

37 overgrowth of a higher band gap inorganic shell (F igure 2-5) creates a step to confine the exciton to the core.(39-41, 65) This increased confinement has been employed to enhance the quantum yield of the dots to over one order of magnitude and to increase its stability against surface oxidation. (34, 38, 72) Various examples using ZnS, CdS and ZnSe as shell layers include CdSe/ZnS (39, 40), CdSe/CdS (11), CdSe/ZnSe (73), CdS/ZnS (34) and InAs/ZnSe (74). Passivation using multiple shells (34, 70, 75) or onion like structures (76) has also been achieved. It has been observed that the absorption and emission of a nanocrystal that is passivated with a ZnS shell exhibits a shift to longer wavelengths by approximately 10 to 20 nm as compared to the unpassivated core.(39, 40, 66, 77) Dabbousi et al. explained this phenomenon by considering charge carriers in a spherical box. The observed shifts to lower energies result from the tunneling of the lighter electron wave f unction into the shell wh ile the hole remains in the core. If this happens, the exciton is delo calized in the particle resulting in decreased confinement and excited state ener gy. For the electron to be able to penetrate into the shell, it must be able to overcome the valence band offset (barrier height), the energy difference between the valance band of the core and the valence band of the shell, that is present between the core and shell. If this offset is small, the shifts towards lower energies can be large.(39, 66) For nanoparticles passivated with an inorganic material, a critical thickness is present that is dependent on the size of the core and latti ce mismatch between the core and shell. (34, 38, 72) This thickness influences the abili ty of the electron to tunnel to the su rface potentially resulting in little to no shifts in band gaps or confin ement potentials compared to unpassivated particles. However, it is possible to surpass this thickne ss allowing the electron to tunnel into the shell layer which can decrease the overall quantum yi eld and shift the absorption and emission

PAGE 38

38 spectrum to lower energies.(72, 78) This is an important factor in the analysis of our CdSe/ZnSe core/shell nanorods which will be discussed in Chapter 3. A) B) Figure 2-5. Electronic potentia l step of valence and conducti on bands, HOMO and LUMO levels of A) inorganic core and B) inorganic core/shell nanocrystals, both with surface attachment of organic molecules. Adapted from H. Lee.(66) Atom dislocations induced by inte rfacial strain as a result of the lattice mismatch between the core and shell can also have a negative e ffect on the luminescence quantum yield because they can behave as sites that cause non-radiativ e recombination. The defects that arise from the core/shell interface can be reduced resulting in higher quantum yields by either growing a nanocrystal that is comprised of one core and two shells, such as a CdSe core passivated with a CdS/ZnS shell/shell structure (34, 66, 79) or by photoannealing(34, 66) which reduce strain or diffuse defects to the surface, respectively. For example, irradiating CdSe/ZnS core/shell nanocrystals with UV light caused an increa se in the photoluminescence quantum yield by reducing the number of vacanci es present at the interface.(34, 66) organic molecule Eg(core) Eg(shell) band offset band offset Organic molecule organic molecule Eg(core) Eg(shell) band offset band offset organic molecule Eg(core) Eg(shell) band offset band offset Organic molecule

PAGE 39

39 Composition Changes: Interdiffusion It has been an on-going goal to develop synt hetic methods to produce highly luminescent quantum confined materials with increased stability in the blue-green spec tral region. Therefore, some focus on synthesizing binary or core/s hell materials has shifted to mixed ternary heterostructures. This would allow for an extra degree of freedom (size and composition) to achieve particular confinement characteristics, such as photoluminescence tunability in fewer synthetic steps. If the cations in the shell we re to exchange with the cations in the core (interdiffusion), the optical properties can be ch anged significantly. More specifically, changes to the energies of the valence and conduction bands, band gap and c onfinement potentials are to be expected. Temperature can influence the rates of chemi cal reactions and can be described using an Arrhenius equation. In solid state, diffusion is th ermally activated thus the diffusion coefficient, D, can be determined using Eq. 2-4.(80) 0AE RTDDe (2-4) EA represents the activation energy and D0 is the diffusion coefficient when the temperature is considered to be infinite. (80) Since temperature influences diffusi on in solid state materials it is important to determine the optimal conditi ons that will produce the desired ternary heterostructure when alloying a core/shell material. This poses a problem for II-VI band gap materials since the experimental values for 0D and EA for interdiffusion are limited. Several groups have determined experiment al values for diffusion lengths in bulk and quantum well structures. For example, Martin (81) investigated the diffusion lengths for Cd diffusion into ZnSe that were annealed for 1 hour at temperatures in the range of 300 to 550oC and concluded that the optical properties, such as photoluminescence, shou ld exhibit changes at

PAGE 40

40 temperatures as low as 350oC for a diffusion distance required for one monolayer of CdSe (~0.3 nm).(66) ZnSe/CdSe and ZnS/CdS superlattice struct ures investigated by Parbrook et al. exhibited diffusion at annealing te mperatures greater than 400 and 450oC rather than below 400oC.(66, 82) The negligible changes obs erved in the range of 340oC~400oC are further confirmed by investigations to determine diffus ion lengths for Cd in ZnSe/CdSe quantum wells by Rosenauer et al.(66, 83) and Stra burg et al.(66, 84) If the same experiments are conducted in nanocrystals the diffusion behavior is quite different compared to quantum wells or bulk materials; the alloying point (t he temperature at which the co re/shell nanocrystals begin to alloy (48)) in nanocrystals has been shown to occur at lower temperatures. For example, Zhong et al.(48, 66) observed alterations to the band gap and blue shifts in the photoluminescence in the range of 270~290oC in colloidal CdSe/ZnSe core/shell nanoparticles.(66) Additionally, at temperatures greater than 290oC, the alloying process can take as little as five minutes to complete.(48) There are several reasons that can account for the smaller alloying point temperature. Recall that as the size of the na noparticle decreases, the surface-to-volume ratio increases resulting in a large number of atoms be ing exposed to the surface or located in the interfacial region betwee n the core and shell; therefore, diffusion at lower temperatures compared to bulk materials can be expected. Diffu sion in colloidal nanopart icles can be also be aided by the desire for the surface atoms to mini mize their energy by reorganizing to reduce their surface area.(22, 66) The interface between the core and sh ell can have imperfections or defects that will also increase the diffusion rates. However, these imperfections can lead to a decrease in the photoluminescence quantum yields Finally, diffusion can be in fluenced by the crystal field strength of the nanoparticle. The crystal field stre ngth will scale with the size of the nanoparticle; therefore, bulk crystals have larger crystal fiel d strengths than quantum particles. Based on this

PAGE 41

41 principle, the interaction between atoms far fr om one another in nanocrystals is weak which will reduce the activation energy ultimately e nhancing the diffusion in these nanomaterials.(66, 85, 86) In our samples, we were able to achieve interd iffusion of Zn from a ZnSe shell into a CdSe core by alloying at a temperature (270~290oC) determined to be effective by Zhong et al.(48) Despite the fact that ZnCdSe quant um dots have been grown on ZnSe (87) and GaAs (88) substrates to determine how radius affects the fluorescence lifetime, little has been published on the compositional affects on the dynamics of colloidal ZnCdSe nanoparticles. From TEM images we observe that diffusion of Zn into the CdSe core does not change the shape of the rods si gnificantly and th e single phonon mode observed by Raman backscattering indicate that the ZnCdSe materials are comp lete quantum rod alloys, not composites. The differences that arise in both stea dy state absorption and photoluminescence in addition to time-resolved photoluminescence meas urements make investigating these ternary systems using ultrafast techniques extremely appea ling (Chapter 3). In th e current chapter, the synthesis, characteri zation, steady state photophysics a nd time-resolved photoluminescence measurements are described to begin to explai n the carrier relaxation within CdSe, CdSe/ZnSe core/shell and ZnCdSe alloy quantum rods. Experimental Methods: Nanorod Synthesi s and Composition Characterization The synthesis, XRD, TEM, Raman, and some optical characterization of each of the materials studied in this dissertation were carried out by Dr. Hyeokjin Lee in the Department of Materials Science at the University of Florida. Preparation of ZnCdSe Nanorods CdSe nanorods were synthesized us ing the method described by Peng. (46) In this method, CdO, trioctylphosphine oxide (TOP O) and tetradecylphosphonic acid (TDPA) were heated in a

PAGE 42

42 three-neck flask on a Schlenk line under a N2 atmosphere to 350oC while stirring. After the solution became optically clear, it was cooled to room temperatures. The solid Cd-TDPA complex was used after aging for 24 hr without further purification. This Cd-TDPA complex was heated in a three-necked flask under a N2 atmosphere to 280oC while stirring, and selenium dissolved in trioctylphosphine (T OP) was injected quickly. After injection, the temperature of the mixture was kept at 250oC for the 30 min growth of CdSe nanorods, and then cooled to 180oC. (42) For shell growth, ZnO was dissolved in oleic acid (Zn-oleate) at 350oC and cooled to room temperature, and then TOP was added to prevent so lidification. In addition, Se was dissolved in TOP (Se-TOP). The Zn-oleate and Se-TOP soluti ons were mixed by stirring for ten minutes at room temperature, and this mixture was loaded into a syringe and injected drop-by-drop into the reaction flask over 1.5 hr. After injection was complete, the solution was stirred at room temperature for another ten minutes For alloying, the reaction vessel was heated with stirring to 270oC for up to 3 hrs. After heating for 1, 2 or 3 hrs, a sample was immediately cooled and diluted with toluene to stop alloying, then was precipitated with methanol/toluene cosolvents.(42) Steady State Instrumentation High-resolution transmission el ectron microscope (HR-TEM) images were collected using a JEOL 2010F microscope for imaging and direct determination of the av erage and distribution of the nanorod dimensions. To prepare TEM samples, the nanocrystals were dispersed in toluene and deposited onto formvar-coated copper grids. X-ray diffraction (XRD) patterns were obtained using a Philips APD 3720 X-ray di ffractometer and used for determination of both the crystal structure and size. Raman spectra were measured at 300K in the backsc attering geometry, using

PAGE 43

43 the 532 nm line from a Verdi 8 doubled Nd-YAG solid state laser in a Ramanor U-1000 JobinYvon Raman spectrometer.(42) Absorption spectra were collected with a Shimadzu UV-2401PC spectrophotometer. Photoluminescence was measured at room temp erature using nanorods suspended in toluene using a Fluorolog Tau 3 spectrofluorometer (Job in Yvon Spex instruments, S.A. Inc.). The photoluminescence quantum yield was determined using Rhodamine 6G organic dye standard. (42) Time-Resolved Photolumin escence Instrumentation Relaxation processes of colloidal nanocrys tals were explored using time-resolved photoluminescence. A commercial Ti-Sapphire (T i-Sa) laser system consisting of a Ti-Sa oscillator (Tsunami, Spectra-Physics) and subseque nt amplifier (Spitfire, Spectra-Physics) with a repetition rate of 1 kHz. The setup in our lab di rects the output of the amplifier into an optical parametric amplifier (OPA) to generate excitation pulses. For this experiment, since 400 nm is at the limit of both the signal and idler, we must use the second harmonic of the amplifier (800 nm) to achieve stable and high energy pulses. The residual 800 nm is directed to a horizontal BBO crystal (output of the Spitfire is polarized in the horizontal direction). A general schematic is provided in Figure 2-6. The sec ond harmonic (400 nm) is then fed through a prism compressor, resulting in pulse lengths less th an 100 fs (FWHM). The excitation beam is focused to a diameter of ~150 m at the sample position an d its energy was set to ~ 56 nJ yielding a fluence of 317 J/cm2. The optical density of each solution wa s 0.075/mm at 400 nm. Sample solutions of colloidal nanorods dissolved in toluene were place d in a quartz cuvette with a 2 mm path length and continuously stirred to guarantee excitation of a new sample volume with every laser shot. Broad band luminescence (grating range: 438 to 718 nm) from the sample was collected using a

PAGE 44

44 Pump from OPA, delay stage Lens Pump C C D from OPA, delay stage Lens Monochromator Pump from OPA, delay stage Lens Pump C C D from OPA, delay stage Lens Monochromator 2 in. lens and then focusing this into the en trance slit of a monochromator. Time-resolved photoluminescence spectra were r ecorded with an in tensified charge-c oupled device (ICCD) (Andor iStar coupled to a Shamrock 303i spectrog raph) with a 4 ns gate. The 4 ns collection window electronically scans to map the tempor al evolution of the photoluminescence. The exposure time at each time step is 0.5 seconds. Figure 2-6. Time-resolved photoluminescence The standard mechanical shutters commercially available are unable to gate at ultrafast speeds. Instead, the image intensifier found in th e ICCD acts not only as an amplifier but as an electronic gate, opening and closing on a nanosecond timescal e. There are three major components of an image intens ifier: photocathode, microcha nnel plate (MCP) and phosphor screen. The limitations of each of these determ ine how well the intensifier can perform. The incident image is first captured by the photocat hode which subsequently emits a photoelectron and is then pulled to the MCP by an electric fi eld. A high voltage is applied to the MCP causing the photoelectron to ri cochet along the channel walls creat ing an avalanche of secondary electrons which exit the MCP as an electron clou d. Typical intensifications can be as high as

PAGE 45

45 10,000. An additional voltage forces the electron cloud to hit the phosphor screen located at the front of the fiber optic exit window. The volta ges between the photocathode and MCP can be controlled in a manner so that the image intensifie r can be quickly turned off and on, effectively creating an electronic gate.(89, 90) As photons hit the surface of the CCD sensor, electrons are generated which are stored in individual pixels. The maximum number of el ectrons that one pixel can accumulate during integration is considered to be the full well ca pacity. A 16 bit analog to digital output converter which is capable of digitizing 65,536 levels (216) of light is used to read out the pixels.(89, 9193) The dynamic range is the number of steps or leve ls of light intensity that can be represented per bit.(93) Without using an image intensifier (g ain), the CCD dynamic range (maximum and minimum signal intensities that can be measured simultaneously) is defined as the full well capacity per pixel divided by the read noise.(89, 90, 92, 94) For this system, the full well capacity per pixel is 300,000 electrons and the read noise is 4 electrons resulting in a potential dynamic range of 75,000 to 1. This value exceeds the upper constraint of the digitizer so the dynamic range is instead limited to 65,000 to 1. Dynamic ranges can vary since the read noise depends on the read out rate. If the read out is fast, the read noise can be high and the dynamic range can be low or vice versa. A slower read ou t rate will reduce the read noise (high read noise will affect the quality of the image). Depending on the application, cameras that have a high well capacity and low read noise (high dynamic range) in addition to a large analog to digital conversion capability are optimal.(90, 92) The response of this camera is considered to be linear (1) within its full dynamic range.(89) The dynamic range of the CCD serves as th e base dynamic range of the ICCD camera system. As gain is added, the dynamic range is reduced.(90) For example, if the gain is set to 50

PAGE 46

46 (counts/electron) a multiplication factor is em ployed which reduces the dynamic range from 65,000 counts to 1310 counts (65,536 divided by 50) Gain can be advantageous if used in appropriate amounts. For instance, the read noise produced by the CCD section of the camera is no longer an issue. However, the dark current th at is created thermally by the photocathode prior to the amplification stage is still present and can also be amplified when gain is applied. Thus, even though high gains will lead to enhanced signa ls, the noise is also increased. It is also possible to have too little gain, which can sa crifice the well depth with no significant signal amplification. A balance between the system gain and dynamic range is ne cessary to achieve the best signal to noise ratio. Once ga in is applied, the response of the CCD remains linear within its dynamic range; however, the signal to noise equa tion is changed by the gain noise factor of 1.4.(89-92, 94) Results and Discussion Synthesis of ZnCdSe Nanorods Combinations of surfactants such as TDPA and TOPO are generally used to prepare nanocrystals since they have strong binding energies that ultimately raise the surface energies of a crystal face compared to another. (42, 95, 96) Previously, Zn-TDPA and Cd-TDPA in a TOPO solution were utilized in orde r to synthesize ZnCdSe nanorods. However, this method was not successful, which most likely resulted due to the different reactivity with Se-TOP leading to a lack of crystallite shape control.(42, 73, 97) It is also suggested that the temperature be higher and reaction time be longer in order to promot e a more thorough complexation of ZnO with TDPA. When synthesizing the ZnCdSe ternary hete rostructures, it is important to first prepare CdSe nanorods. Once the CdSe rod has been grow n, a Zn-Oleate and Se-TOP mixture was used to grow the shell overtop the core. Zn-oleate an d Se-TOP mixture was slowly added to prevent homogeneous nucleation. It is ex tremely important that the temp erature be controlled properly.

PAGE 47

47 2030405060 2030405060 2030405060 Counts (112) (110) (101) (102) (103) (002) (100) (c) (b) (a) 2Theta C) B) A)2030405060 2030405060 2030405060 Counts (112) (110) (101) (102) (103) (002) (100) (c) (b) (a) 2Theta C) B) A)When the temperature was 210oC, the emission blue shifted because the shell began alloying causing a blue shift in the emission. If the te mperature was kept too low, for example <170oC, only a small shell grew because the Zn-oleate comp lex reacted too slowly with TOP-Se to grow a shell, resulting in very weak emission. Finally diffusion of Zn from the shell to the core is instigated by raising the temperature slowly to 270oC to form ZnCdSe alloys.(42) Structure of ZnCdSe Nanorods X-Ray diffraction patterns for hexagonal Cd Se, CdSe/ZnSe, and ZnCdSe nanorods are shown in Figure 2-7. Figure 2-7. Powder X-ray diffraction patterns of A) CdSe nanorods, B) CdSe/ZnSe core/shell nanorods, and C) ZnCdSe alloyed nanorods. Adapted from H. Lee.(66). The crystal structure for this series of rods can be extracted from this experiment. In each of the materials measured, the (002) diffr action peak is not as broad as the (001) diffraction peak. The (002) peak is assigned to the pl ane that is perpendicular to th e extended c-axis in rod-shaped materials. The lattice spacings for CdSe, CdSe/ZnSe, and ZnCdSe were 7.01 6.94 and 6.77 respectively. These values are extremely inte resting since in CdSe/ZnS nanoparticles the ZnS

PAGE 48

48 01234567891011121314151617181920 0 10 20 30 40 50 60 70 80 90 100 Length Diameter Count01234567891011121314151617181920 0 10 20 30 40 50 60 70 80 90 100 nm 01234567891011121314151617181920 0 10 20 30 40 50 60 70 80 90 100 Length Diameter Count01234567891011121314151617181920 0 10 20 30 40 50 60 70 80 90 100 nmshell exhibits an 11% smaller lattice parameter causing the core to be compressed, whereas the lattice parameter in the c-axis for the core/she ll material was ~ 1% smaller compared to the CdSe. After interdiffusion of Zn into the core, the lattice parameter and the lattice mismatch strain are reduced due to a lat tice contraction. Also, after addition of Zn into the core, the diffraction peaks shifted to a larger 2 indicating a smaller interplanar spacing.(38, 42) A HR-TEM image of the ZnCdSe nanorods is shown in Figure 2-8. The diameter and lengths of the nanorods measured from such images has been included in the histogram. 20 nm scale bar 5 nm scale bar Figure 2-8. HR-TEM image and histogram of si ze distribution of ZnCdSe nanorods. Lattice fringe from a nanorod is shown in the lo wer right corner. Adapted from H. Lee.(66) From this graph, the average diameter is ~6 nm and the average length is ~13 nm resulting in an aspect ratio equal to ~ 2.1 nm for the alloys.(42, 66) When using XRD, the diffraction patterns can exhibit broadening effects due to particle size. Using the Debye-Scherrer formula, the average crystallite size in can be determined: coshklk D (2-5) Where k is a correction factor to account for particle shapes, and is the observed width at half the maximum peak intensity and is the Bragg angle. It must be noted that the observed width

PAGE 49

49 includes additional sources of broadening, arising from the experimental setup and instrumentation.(98, 99) Using Eq. 2-5, the particle sizes of ZnCdSe nanorods calculated were a diameter of 5.5 nm and a length of 11.8 nm resul ting in an aspect ratio of ~2.1 nm, which agree well with HR-TEM data shown in Figure 2-8.(42, 66) Effect of Alloying on the Phonon Spectra The compositional changes to the structure of a material that arise when adding a shell or alloying by diffusion through the dependence of th e phonon frequencies have been studied using Raman spectroscopy.(42, 100, 101) The Raman peaks detected from CdSe nanorods are shown in Figure 2-9 A. The peak at ~206 cm-1 is from the CdSe LO phonon (42, 101, 102) which is 4 cm-1 shifted compared to the bulk CdSe (210 cm-1) which is due to the quantum confinement of the optical phonons in the nanorods.(42, 100-102) A broad shoulder (~180cm-1) appears to the left of the main mode which arises from the non-spherical geometry of the CdSe nanorods. (42, 103, 104) The Raman peak for CdSe/ZnSe core/shell nanorods is shown in Figure 2-9 B. The original CdSe LO phonon mode is st ill detected with th e addition of the ZnSe shell mode at ~247 cm-1). A new interfacial ZnCdSe is also de tected and corresponds to a frequency ~235 cm-1. The small, unresolved Raman peaks on either side of the CdSe phonon mode can be assigned to isolated atom-impurity modes when Zn and Cd at oms interchange with one another (Zn in CdSe ~190 cm-1 and Cd in ZnSe ~218 cm-1). (42, 105) The effects of alloying time (1, 2 or 3 hrs at 270oC) on the Raman spectra are shown in Figure 2-10. After alloying, one mode is present at 223 cm-1, 228 cm-1 and 226 cm-1 (1, 2 and 3 hrs, respectively), which is similar to the interf acial layer observed in the core/shell material and the one phonon-mode behavior for bulk ZnCdSe. (42, 106, 107)

PAGE 50

50 A) B) Figure 2-9. Raman spectra of LO phonon mode of A) CdSe nanorods and B) CdSe/ZnSe core/shell nanorods. Adapted from H.Lee.(66) 150175200225250275 iZnCdSeZnSeCdSei(b) Raman IntensityRaman shift(cm-1)150200250 (a) IntensityRaman shift(cm-1)150175200225250275 iZnCdSeZnSeCdSei(b) Raman IntensityRaman shift(cm-1)150200250 (a) IntensityRaman shift(cm-1) A) B)150175200225250275 iZnCdSeZnSeCdSei(b) Raman IntensityRaman shift(cm-1)150200250 (a) IntensityRaman shift(cm-1)150175200225250275 iZnCdSeZnSeCdSei(b) Raman IntensityRaman shift(cm-1)150200250 (a) IntensityRaman shift(cm-1) 150175200225250275 iZnCdSeZnSeCdSei(b) Raman IntensityRaman shift(cm-1)150200250 (a) IntensityRaman shift(cm-1)150175200225250275 iZnCdSeZnSeCdSei(b) Raman IntensityRaman shift(cm-1)150200250 (a) IntensityRaman shift(cm-1) 150175200225250275 iZnCdSeZnSeCdSei(b) Raman IntensityRaman shift(cm-1)150200250 (a) IntensityRaman shift(cm-1)150175200225250275 iZnCdSeZnSeCdSei(b) Raman IntensityRaman shift(cm-1)150200250 (a) IntensityRaman shift(cm-1) A) B)

PAGE 51

51 A) B) C) A) A) A) B) C) Figure 2-10. Raman LO phonon spectra of ZnCdSe nanorods after annealing at 270C for A) 1, B) 2, or C) 3 hrs. Adapted from H. Lee.(66) Detection of only one mode that is only slig htly shifted compared to the bulk in these materials indicates that the inte rface between the CdSe core and Zn Se shell is no longer present, implying that the material is in fact an a lloy and not a composite. The narrow particle distribution and uniform compos ition observed in the XRD is confirmed from the relatively sharp single-mode peak. The broader peak observe d after only one hour of alloying compared to the 2 and 3 hour is primarily due to compositiona l disorder. As the alloying continues from one to two hours, the Zn continues to diffuse resulting in a 5 cm-1 shift of the Raman peak. After 3 hours of alloying this peak shifts back 2 cm-1 due to compositional disorder (42, 108, 109) and stress relaxation by thermal annealing(110).(42)

PAGE 52

52 300400500600700 0.00 0.05 0.10 0.15 0.20 0.25 300400500600700 0.00 0.05 0.10 0.15 0.20 0.25 300400500600700 0.00 0.05 0.10 0.15 0.20 0.25 Wavelength(c) (b) (a) Absorbance A) B) C) 300400500600700 0.00 0.05 0.10 0.15 0.20 0.25 300400500600700 0.00 0.05 0.10 0.15 0.20 0.25 300400500600700 0.00 0.05 0.10 0.15 0.20 0.25 Wavelength(c) (b) (a) Absorbance A) B) C)300400500600700 0.00 0.05 0.10 0.15 0.20 0.25 300400500600700 0.00 0.05 0.10 0.15 0.20 0.25 300400500600700 0.00 0.05 0.10 0.15 0.20 0.25 Wavelength(c) (b) (a) Absorbance 300400500600700 0.00 0.05 0.10 0.15 0.20 0.25 300400500600700 0.00 0.05 0.10 0.15 0.20 0.25 300400500600700 0.00 0.05 0.10 0.15 0.20 0.25 Wavelength(c) (b) (a) Absorbance 300400500600700 0.00 0.05 0.10 0.15 0.20 0.25 300400500600700 0.00 0.05 0.10 0.15 0.20 0.25 300400500600700 0.00 0.05 0.10 0.15 0.20 0.25 Wavelength(c) (b) (a) Absorbance 300400500600700 0.00 0.05 0.10 0.15 0.20 0.25 300400500600700 0.00 0.05 0.10 0.15 0.20 0.25 300400500600700 0.00 0.05 0.10 0.15 0.20 0.25 Wavelength(c) (b) (a) Absorbance A) B) C) Photoluminescence and Absorption Properties Significant differences can be seen in the absorption spectra for CdSe core, CdSe/ZnSe core/shell and ZnCdSe alloyed nanorods. The absorption spectra of nanorods are shown in Figure 2-11. It presents two absorption peaks on top of a broad absorp tion. These absorption peaks correspond to confined states although they are not as sharp or as well resolved as peaks reported for CdSe quantum dots.(17) Due to the loss of symmetry in nanorods, confinement along the c-axis is not as strong as it can be in dots which results in a la rge distributio n of energy levels in the conduction and valence bands.(53, 111) In addition, the compositional disorder indicated from the Raman data will lead to broader features in the absorption and emission spectra. Therefore, it is difficult to determin e the exact energy spacing between the first and second absorption peaks. (42) Figure 2-11. UV-Vis absorption spectra of A) CdSe nanorods, B) CdSe/ZnSe core-shell nanorods, and C) ZnCdSe nanorods alloyed at 270C for 3hrs. Adapted from H. Lee.(66) For CdSe core and CdSe/ZnSe core/shell nanorods, the absorp tion edge is at 650 nm and 645 nm, respectively. A second peak is observed at ~520 nm. These features correlate to optical

PAGE 53

53 450500550600650700750800 0 1x1062x1063x1064x1065x1066x106 450500550600650700750800 0 1x1062x1063x1064x1065x1066x106 IntensityWavelength(nm)(b) (a) A) B)450500550600650700750800 0 1x1062x1063x1064x1065x1066x106 450500550600650700750800 0 1x1062x1063x1064x1065x1066x106 IntensityWavelength(nm)(b) (a) 450500550600650700750800 0 1x1062x1063x1064x1065x1066x106 450500550600650700750800 0 1x1062x1063x1064x1065x1066x106 IntensityWavelength(nm)(b) (a) 450500550600650700750800 0 1x1062x1063x1064x1065x1066x106 450500550600650700750800 0 1x1062x1063x1064x1065x1066x106 IntensityWavelength(nm)(b) (a) 450500550600650700750800 0 1x1062x1063x1064x1065x1066x106 450500550600650700750800 0 1x1062x1063x1064x1065x1066x106 IntensityWavelength(nm)(b) (a) A) B)transitions involving the electr on and hole quantized states.(22) Direct assignment of these peaks is difficult and needs to be evaluate d using the effective mass approximation. The photoluminescence spectra from CdSe core and Cd Se/ZnSe core/shell nanorods, with peaks at 642 nm and 638 nm respectively, are shown in Figure 2-12. With the addition of the ZnSe shell, CdSe photoluminescence quantum yields increased from 0.6% to 15% due to passivation of nonradiative surface states.(42) Increasing the shell thickness up to a critical thickness of an inorganic shell with a higher bandgap has b een shown to increase the photoluminescence quantum yield in rods.(34, 38, 72) Defects at the core/shell in terface due to lattice strain relaxation from shells thicker than the critical value will actually decrease the quantum yield.(39, 42, 65, 72) Figure 2-12. Photoluminescence spectra of A) CdSe/ZnSe core/shell nanorods and B) CdSe nanorods. Adapted from H. Lee.(66) It is intriguing that a 4 nm blue shift from core to core-shell emission occurs because Mokari and Banin (78) have reported a ~10 nm red shift for CdSe/ZnS core/shell quantum rods. They attribute this shift to t unneling of the electron wave functi on into the ZnS shell delocalizing

PAGE 54

54 the electron, lowering the confinement energy and ultimately decreasing the energy of the exciton levels.(112) Based on the Raman data presented, the formation of interfacial ZnCdSe results in a decrease in size of the CdSe core (56) resulting in increased localization and a blue shifted emission. This is further supported from alloy formation re sulting in a blue shift of the photoluminescence peak.(42) The energies of the corresponding absorption features from alloyed ZnCdSe (3hrs at 270C) is considerably blue sh ifted to ~555 nm and ~465nm (Fi gure 2-11 C). These features originate from the states similar to those in th e core and core/shell nano rods but with a larger band gap due to the formation of ZnCdSe. Figur e 2-13 presents the photol uminescence spectra of the alloyed ZnCdSe samples. Upon annealing at 270C the photoluminescence spectra shift to higher energies. After one hour of annealing, the peak appears at 610 nm. Further annealing (2 and 3 hours) produces a much larger blue shift, 510 and 565 nm, respectively. This behavior is consistent with the variation in composition indi cated by the broad Raman peak (Figure 2-10). In addition to the energy shift, the alloys present changes in bandwidth and intensity. As alloying time increases, the width of the photoluminescence band is reduced. The change in intensity does not follow a trend, with the 3 hour alloyed sample presenting a sharp increase in photoluminescence intensity. Quantum yield measurements are ~8, 5 and 10% for 1, 2 and 3 hrs, respectively. These values are higher than CdSe rods (0.6%) but lower than the core/shell sample (15%). Composition disorder in ternary alloy na norods will lead to localization of excitons compared to binary samples.(42, 113) Such localization effect s are known to improve the photoluminescence efficiency by increasing the ov erlap integral of the electron and hole wavefunctions. On the contrary, the quantum yield values are lower compared to the core/shell due to the lack of surface passi vation on the ZnCdSe nanorods. This is consistent with the

PAGE 55

55 400450500550600650700750800 400450500550600650700750800 0 1x1062x1063x1064x1065x1066x106 400450500550600650700750800 400450500550600650700750800 400450500550600650700750800 Wavelength(nm) Intensity(d) (c) (b) (a) A) B) C) D) 400450500550600650700750800 400450500550600650700750800 0 1x1062x1063x1064x1065x1066x106 400450500550600650700750800 400450500550600650700750800 400450500550600650700750800 Wavelength(nm) Intensity(d) (c) (b) (a) A) B) C) D)400450500550600650700750800 400450500550600650700750800 0 1x1062x1063x1064x1065x1066x106 400450500550600650700750800 400450500550600650700750800 400450500550600650700750800 Wavelength(nm) Intensity(d) (c) (b) (a) A) B) C) D) quantum yield decreasing after 1 and further afte r 2 hours of annealing, since diffusion will be reducing the gradient in Zn (i.e. reducing the hi gh concentration at the surface and increasing the low concentration in the middle of the nanorods). However, annealing for 3 hours increased the quantum yield over that from samples annealed fo r 1 or 2 hours. This increased quantum yield and full-width-half maxi mum (FWHM) reduction can be attribut ed to annealing of crystalline defects and reduction of stress, c onsistent with the Raman data. Defects found in the crystal are known to act as traps, re ducing emission efficiency(114, 115). (42) Figure 2-13. Photoluminescence spectra from A) CdSe/ZnSe core/shell nanorods and ZnCdSe nanorods alloyed at 270oC for B) 1, C) 2, and D) 3 hrs. Adapted from H. Lee.(66) Time-Resolved Photoluminescence (TRPL) The linewidths of optical transitions can be inhomogeneously broadened due to effects that act differently on different radi ating or absorbing particles.(2) Emission from an inhomogeneous population (different sizes, shapes or composition) leads to the simultane ous probing of particles with different decaying rates. Monitoring ti me-resolved photoluminescence at different wavelengths not only identifies the states emittin g but also extracts their decay rates. The

PAGE 56

56 formation of ZnCdSe alloys is not perfectly un iform and contributes to broad, inhomogeneous photoluminescence spectra and a distribution of decay rates. Figure 2-14 shows the broad band luminescence spectra of the core/shell and alloyed nanorod samples as a function of time. This data was collected on a nanosecond time scale using a 4 nanosecond instrument response and 0.4 ns time step. The fast decays (< 4 ns) are not detectable due to this limitation. From this data we were able to extract time traces at the maximum wavelength to determine the corresponding decay rates of each sample. Figure 2-15 A shows the broad band luminescence of the core/shell sample at two different time steps, 4.4 ns (black line) and 20 ns (red line). As the si gnal decays at the maximum wavelengths, the photoluminescence values do not shift significantly but the broadening is slightly reduced. Broadening also occurs since measurements of the nanorods samples were carried out at room temperature. The time traces corresponding to three different wavelengths, 645, 670 and 630 nm (black, red and green lines, respectively) for th e core/shell sample are shown Figure 2-15 B. At early decay times, the emission at different wa velengths is not the same but ends up being identical after ~ 100 ns. Moving from the core/shell (A) to the alloy 3 hr (D) the band gap shifts to higher energies and the br oad band signal narrows. Figure 2-16 shows the log plot of the time-resolved photoluminescence decay curves for the nanorods samples. These decays are the nor malized kinetic traces at the wavelengths corresponding to the maximums of the photol uminescence. Several differences can be highlighted. The signal at decay times less than four nanoseconds have to be deconvoluted with the instrument response function. Since we are in terested in extracting a characteristic lifetime we do not consider this for analysis It is seen from this plot that the core/shell decay (black line) is much faster than the alloys. Also, the core /shell decay curve is almost a straight line,

PAGE 57

57 0510152025 620 640 660 680 D) C) B)nmnmtime (ns) time (ns)nm nmtime (ns)A)05101520 560 580 600 620 640 time (ns)0510152025 540 560 580 600 051015 540 560 580 600 A) C) D) B) 0510152025 620 640 660 680 D) C) B)nmnmtime (ns) time (ns)nm nmtime (ns)A)05101520 560 580 600 620 640 time (ns)0510152025 540 560 580 600 051015 540 560 580 600 0510152025 620 640 660 680 D) C) B)nmnmtime (ns) time (ns)nm nmtime (ns)A)05101520 560 580 600 620 640 time (ns)0510152025 540 560 580 600 051015 540 560 580 600 A) C) D) B)indicating a higher degree of hom ogeneity and a smaller distributi on of decay rates compared to the alloys. The alloys (green, red, and blue lines) do not show the same behavior as the core/shell; instead, they exhibi t two or more decay components (similar dynamics prior to 50 ns but deviate after that). The data collection for alloy 3 hr (blue line) is shorter due to the lack of detectable signal after 60 ns. Figure 2-14. Broad band Photoluminesce of A): Cd Se/ZnSe core/shell nanorods B) 1 hr ZnCdSe C) 2hr ZnCdSe D) 3hr ZnCdSe In dispersed systems such as polymers (116) or colloidal nanoparticles, it is easy to believe that the relaxation beha ves non-exponentially and that th e large distributions of local environments lead to variations of relaxation times. Multi-expone ntial functions are useful and are more commonly utilized for fitting decay curves; however, a model is generally proposed

PAGE 58

58 based on the number of exponentials and an extens ive number of parameters required to fit the decay curve resulting in exact de cay lifetimes. This can become ve ry complicated and can lead to incorrect assignments of the photophysical proce sses occurring within the material. Jones et al. found that they were able to f it their photoluminescence data colle cted from CdSe/ZnS core shell quantum dots decays with a multi-exponential functi on, implying that there is an existence of several discrete relaxation pathways, with individual lifetimes. However, they were not able to claim the exact number and identity of such pathways.(117) For simultaneous measurements of a large ensemble of relaxation times it is more advantageous to use a stretched exponential (nonexponential) function to evaluate the distribution of relaxation times in such dispersive systems.(43, 116) This type of equation encompasses bot h independent, single step processes in addition to sequential multi-step processes.(116) t I t Ioexp ) ( (2-6) Where is the characteristic lifetime and (0 1) is the dispersion exponent. Despite only extracting an average lifetime from a non-expone ntial; the function provides a phenomenological description that is consider ed purely empirical, fitting data with a minimum number of parameters. These parameters can vary dependi ng on the phenomenon of interest and external variables such as temperature.(43) For the limiting case of 1, we get the single exponential decay with the characteristic lifetime, For ideal, single quantum dots, we can expect =1. It should be mentioned that <1 results from superposition of many exponential decays and as approaches zero, the distribution of decay times in creases. This decay law can then be used to compare different samples qual itatively in terms of non-uniform ity or topological disorder.(42)

PAGE 59

59 Figure 2-15. CdSe/ZnSe Core/Shell Photoluminesen ce: A) Broad band spectra at 8.8 ns () and 25 ns ( ) and B) Kinetic tra ces for 645 (), 670 ( ) and 630 ( ) nm. Figure 2-16. TRPL decay curve of CdSe/ZnSe nanorod ( ), 1 hr alloy ( ), 2 hr alloy ( ), and 3hr alloy ( ) 0204060801001201400 1 2 3 4 Log PLTime (ns)600620640660680700 0.0 0.2 0.4 0.6 0.8 1.0 A) Norm PL(nm)B) 020406080100120140160 0.01 0.1 1 Norm Log PLTime (ns)

PAGE 60

60 012345 -4 -3 -2 -1 0 Ln(LnIo/It)Ln(Time(ns)) ZnCdSe alloy 2hr0.00.51.01.52.02.53.03.54.04.5 -4.0 -3.5 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 Ln(LnIo/It)Ln(Time(ns)) ZnCdSe alloy 3hr012345 -4 -3 -2 -1 0 Ln(LnIo/It)Ln(Time(ns)) CdSe/ZnSe Coreshell012345 -4 -3 -2 -1 0 Ln(LnIo/It)Ln(Time(ns)) ZnCdSe alloy 1hrT=300K = 0.75 = 173ns T=300K = 0.58 = 277ns T=300K = 0.48 = 501ns T=300K = 0.58 = 276ns(a) (b) (c) (d)012345 -4 -3 -2 -1 0 Ln(LnIo/It)Ln(Time(ns)) ZnCdSe alloy 2hr0.00.51.01.52.02.53.03.54.04.5 -4.0 -3.5 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 Ln(LnIo/It)Ln(Time(ns)) ZnCdSe alloy 3hr012345 -4 -3 -2 -1 0 Ln(LnIo/It)Ln(Time(ns)) CdSe/ZnSe Coreshell012345 -4 -3 -2 -1 0 Ln(LnIo/It)Ln(Time(ns)) ZnCdSe alloy 1hr012345 -4 -3 -2 -1 0 Ln(LnIo/It)Ln(Time(ns)) ZnCdSe alloy 2hr0.00.51.01.52.02.53.03.54.04.5 -4.0 -3.5 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 Ln(LnIo/It)Ln(Time(ns)) ZnCdSe alloy 3hr012345 -4 -3 -2 -1 0 Ln(LnIo/It)Ln(Time(ns)) CdSe/ZnSe Coreshell012345 -4 -3 -2 -1 0 Ln(LnIo/It)Ln(Time(ns)) ZnCdSe alloy 1hrT=300K = 0.75 = 173ns T=300K = 0.58 = 277ns T=300K = 0.48 = 501ns T=300K = 0.58 = 276ns(a) (b) (c) (d) A) B) C)D)012345 -4 -3 -2 -1 0 Ln(LnIo/It)Ln(Time(ns)) ZnCdSe alloy 2hr0.00.51.01.52.02.53.03.54.04.5 -4.0 -3.5 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 Ln(LnIo/It)Ln(Time(ns)) ZnCdSe alloy 3hr012345 -4 -3 -2 -1 0 Ln(LnIo/It)Ln(Time(ns)) CdSe/ZnSe Coreshell012345 -4 -3 -2 -1 0 Ln(LnIo/It)Ln(Time(ns)) ZnCdSe alloy 1hrT=300K = 0.75 = 173ns T=300K = 0.58 = 277ns T=300K = 0.48 = 501ns T=300K = 0.58 = 276ns(a) (b) (c) (d)012345 -4 -3 -2 -1 0 Ln(LnIo/It)Ln(Time(ns)) ZnCdSe alloy 2hr0.00.51.01.52.02.53.03.54.04.5 -4.0 -3.5 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 Ln(LnIo/It)Ln(Time(ns)) ZnCdSe alloy 3hr012345 -4 -3 -2 -1 0 Ln(LnIo/It)Ln(Time(ns)) CdSe/ZnSe Coreshell012345 -4 -3 -2 -1 0 Ln(LnIo/It)Ln(Time(ns)) ZnCdSe alloy 1hr012345 -4 -3 -2 -1 0 Ln(LnIo/It)Ln(Time(ns)) ZnCdSe alloy 2hr0.00.51.01.52.02.53.03.54.04.5 -4.0 -3.5 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 Ln(LnIo/It)Ln(Time(ns)) ZnCdSe alloy 3hr012345 -4 -3 -2 -1 0 Ln(LnIo/It)Ln(Time(ns)) CdSe/ZnSe Coreshell012345 -4 -3 -2 -1 0 Ln(LnIo/It)Ln(Time(ns)) ZnCdSe alloy 1hrT=300K = 0.75 = 173ns T=300K = 0.58 = 277ns T=300K = 0.48 = 501ns T=300K = 0.58 = 276ns(a) (b) (c) (d) 012345 -4 -3 -2 -1 0 Ln(LnIo/It)Ln(Time(ns)) ZnCdSe alloy 2hr0.00.51.01.52.02.53.03.54.04.5 -4.0 -3.5 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 Ln(LnIo/It)Ln(Time(ns)) ZnCdSe alloy 3hr012345 -4 -3 -2 -1 0 Ln(LnIo/It)Ln(Time(ns)) CdSe/ZnSe Coreshell012345 -4 -3 -2 -1 0 Ln(LnIo/It)Ln(Time(ns)) ZnCdSe alloy 1hrT=300K = 0.75 = 173ns T=300K = 0.58 = 277ns T=300K = 0.48 = 501ns T=300K = 0.58 = 276ns(a) (b) (c) (d)012345 -4 -3 -2 -1 0 Ln(LnIo/It)Ln(Time(ns)) ZnCdSe alloy 2hr0.00.51.01.52.02.53.03.54.04.5 -4.0 -3.5 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 Ln(LnIo/It)Ln(Time(ns)) ZnCdSe alloy 3hr012345 -4 -3 -2 -1 0 Ln(LnIo/It)Ln(Time(ns)) CdSe/ZnSe Coreshell012345 -4 -3 -2 -1 0 Ln(LnIo/It)Ln(Time(ns)) ZnCdSe alloy 1hr012345 -4 -3 -2 -1 0 Ln(LnIo/It)Ln(Time(ns)) ZnCdSe alloy 2hr0.00.51.01.52.02.53.03.54.04.5 -4.0 -3.5 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 Ln(LnIo/It)Ln(Time(ns)) ZnCdSe alloy 3hr012345 -4 -3 -2 -1 0 Ln(LnIo/It)Ln(Time(ns)) CdSe/ZnSe Coreshell012345 -4 -3 -2 -1 0 Ln(LnIo/It)Ln(Time(ns)) ZnCdSe alloy 1hrT=300K = 0.75 = 173ns T=300K = 0.58 = 277ns T=300K = 0.48 = 501ns T=300K = 0.58 = 276ns(a) (b) (c) (d) A) B) C)D) A) C) D) B)012345 -4 -3 -2 -1 0 Ln(LnIo/It)Ln(Time(ns)) ZnCdSe alloy 2hr0.00.51.01.52.02.53.03.54.04.5 -4.0 -3.5 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 Ln(LnIo/It)Ln(Time(ns)) ZnCdSe alloy 3hr012345 -4 -3 -2 -1 0 Ln(LnIo/It)Ln(Time(ns)) CdSe/ZnSe Coreshell012345 -4 -3 -2 -1 0 Ln(LnIo/It)Ln(Time(ns)) ZnCdSe alloy 1hrT=300K = 0.75 = 173ns T=300K = 0.58 = 277ns T=300K = 0.48 = 501ns T=300K = 0.58 = 276ns(a) (b) (c) (d)012345 -4 -3 -2 -1 0 Ln(LnIo/It)Ln(Time(ns)) ZnCdSe alloy 2hr0.00.51.01.52.02.53.03.54.04.5 -4.0 -3.5 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 Ln(LnIo/It)Ln(Time(ns)) ZnCdSe alloy 3hr012345 -4 -3 -2 -1 0 Ln(LnIo/It)Ln(Time(ns)) CdSe/ZnSe Coreshell012345 -4 -3 -2 -1 0 Ln(LnIo/It)Ln(Time(ns)) ZnCdSe alloy 1hr012345 -4 -3 -2 -1 0 Ln(LnIo/It)Ln(Time(ns)) ZnCdSe alloy 2hr0.00.51.01.52.02.53.03.54.04.5 -4.0 -3.5 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 Ln(LnIo/It)Ln(Time(ns)) ZnCdSe alloy 3hr012345 -4 -3 -2 -1 0 Ln(LnIo/It)Ln(Time(ns)) CdSe/ZnSe Coreshell012345 -4 -3 -2 -1 0 Ln(LnIo/It)Ln(Time(ns)) ZnCdSe alloy 1hrT=300K = 0.75 = 173ns T=300K = 0.58 = 277ns T=300K = 0.48 = 501ns T=300K = 0.58 = 276ns(a) (b) (c) (d) A) B) C)D)012345 -4 -3 -2 -1 0 Ln(LnIo/It)Ln(Time(ns)) ZnCdSe alloy 2hr0.00.51.01.52.02.53.03.54.04.5 -4.0 -3.5 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 Ln(LnIo/It)Ln(Time(ns)) ZnCdSe alloy 3hr012345 -4 -3 -2 -1 0 Ln(LnIo/It)Ln(Time(ns)) CdSe/ZnSe Coreshell012345 -4 -3 -2 -1 0 Ln(LnIo/It)Ln(Time(ns)) ZnCdSe alloy 1hrT=300K = 0.75 = 173ns T=300K = 0.58 = 277ns T=300K = 0.48 = 501ns T=300K = 0.58 = 276ns(a) (b) (c) (d)012345 -4 -3 -2 -1 0 Ln(LnIo/It)Ln(Time(ns)) ZnCdSe alloy 2hr0.00.51.01.52.02.53.03.54.04.5 -4.0 -3.5 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 Ln(LnIo/It)Ln(Time(ns)) ZnCdSe alloy 3hr012345 -4 -3 -2 -1 0 Ln(LnIo/It)Ln(Time(ns)) CdSe/ZnSe Coreshell012345 -4 -3 -2 -1 0 Ln(LnIo/It)Ln(Time(ns)) ZnCdSe alloy 1hr012345 -4 -3 -2 -1 0 Ln(LnIo/It)Ln(Time(ns)) ZnCdSe alloy 2hr0.00.51.01.52.02.53.03.54.04.5 -4.0 -3.5 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 Ln(LnIo/It)Ln(Time(ns)) ZnCdSe alloy 3hr012345 -4 -3 -2 -1 0 Ln(LnIo/It)Ln(Time(ns)) CdSe/ZnSe Coreshell012345 -4 -3 -2 -1 0 Ln(LnIo/It)Ln(Time(ns)) ZnCdSe alloy 1hrT=300K = 0.75 = 173ns T=300K = 0.58 = 277ns T=300K = 0.48 = 501ns T=300K = 0.58 = 276ns(a) (b) (c) (d) 012345 -4 -3 -2 -1 0 Ln(LnIo/It)Ln(Time(ns)) ZnCdSe alloy 2hr0.00.51.01.52.02.53.03.54.04.5 -4.0 -3.5 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 Ln(LnIo/It)Ln(Time(ns)) ZnCdSe alloy 3hr012345 -4 -3 -2 -1 0 Ln(LnIo/It)Ln(Time(ns)) CdSe/ZnSe Coreshell012345 -4 -3 -2 -1 0 Ln(LnIo/It)Ln(Time(ns)) ZnCdSe alloy 1hrT=300K = 0.75 = 173ns T=300K = 0.58 = 277ns T=300K = 0.48 = 501ns T=300K = 0.58 = 276ns(a) (b) (c) (d)012345 -4 -3 -2 -1 0 Ln(LnIo/It)Ln(Time(ns)) ZnCdSe alloy 2hr0.00.51.01.52.02.53.03.54.04.5 -4.0 -3.5 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 Ln(LnIo/It)Ln(Time(ns)) ZnCdSe alloy 3hr012345 -4 -3 -2 -1 0 Ln(LnIo/It)Ln(Time(ns)) CdSe/ZnSe Coreshell012345 -4 -3 -2 -1 0 Ln(LnIo/It)Ln(Time(ns)) ZnCdSe alloy 1hr012345 -4 -3 -2 -1 0 Ln(LnIo/It)Ln(Time(ns)) ZnCdSe alloy 2hr0.00.51.01.52.02.53.03.54.04.5 -4.0 -3.5 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 Ln(LnIo/It)Ln(Time(ns)) ZnCdSe alloy 3hr012345 -4 -3 -2 -1 0 Ln(LnIo/It)Ln(Time(ns)) CdSe/ZnSe Coreshell012345 -4 -3 -2 -1 0 Ln(LnIo/It)Ln(Time(ns)) ZnCdSe alloy 1hrT=300K = 0.75 = 173ns T=300K = 0.58 = 277ns T=300K = 0.48 = 501ns T=300K = 0.58 = 276ns(a) (b) (c) (d) A) B) C)D) 012345 -4 -3 -2 -1 0 Ln(LnIo/It)Ln(Time(ns)) ZnCdSe alloy 2hr0.00.51.01.52.02.53.03.54.04.5 -4.0 -3.5 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 Ln(LnIo/It)Ln(Time(ns)) ZnCdSe alloy 3hr012345 -4 -3 -2 -1 0 Ln(LnIo/It)Ln(Time(ns)) CdSe/ZnSe Coreshell012345 -4 -3 -2 -1 0 Ln(LnIo/It)Ln(Time(ns)) ZnCdSe alloy 1hrT=300K = 0.75 = 173ns T=300K = 0.58 = 277ns T=300K = 0.48 = 501ns T=300K = 0.58 = 276ns(a) (b) (c) (d)012345 -4 -3 -2 -1 0 Ln(LnIo/It)Ln(Time(ns)) ZnCdSe alloy 2hr0.00.51.01.52.02.53.03.54.04.5 -4.0 -3.5 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 Ln(LnIo/It)Ln(Time(ns)) ZnCdSe alloy 3hr012345 -4 -3 -2 -1 0 Ln(LnIo/It)Ln(Time(ns)) CdSe/ZnSe Coreshell012345 -4 -3 -2 -1 0 Ln(LnIo/It)Ln(Time(ns)) ZnCdSe alloy 1hr012345 -4 -3 -2 -1 0 Ln(LnIo/It)Ln(Time(ns)) ZnCdSe alloy 2hr0.00.51.01.52.02.53.03.54.04.5 -4.0 -3.5 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 Ln(LnIo/It)Ln(Time(ns)) ZnCdSe alloy 3hr012345 -4 -3 -2 -1 0 Ln(LnIo/It)Ln(Time(ns)) CdSe/ZnSe Coreshell012345 -4 -3 -2 -1 0 Ln(LnIo/It)Ln(Time(ns)) ZnCdSe alloy 1hrT=300K = 0.75 = 173ns T=300K = 0.58 = 277ns T=300K = 0.48 = 501ns T=300K = 0.58 = 276ns(a) (b) (c) (d) A) B) C)D)012345 -4 -3 -2 -1 0 Ln(LnIo/It)Ln(Time(ns)) ZnCdSe alloy 2hr0.00.51.01.52.02.53.03.54.04.5 -4.0 -3.5 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 Ln(LnIo/It)Ln(Time(ns)) ZnCdSe alloy 3hr012345 -4 -3 -2 -1 0 Ln(LnIo/It)Ln(Time(ns)) CdSe/ZnSe Coreshell012345 -4 -3 -2 -1 0 Ln(LnIo/It)Ln(Time(ns)) ZnCdSe alloy 1hrT=300K = 0.75 = 173ns T=300K = 0.58 = 277ns T=300K = 0.48 = 501ns T=300K = 0.58 = 276ns(a) (b) (c) (d)012345 -4 -3 -2 -1 0 Ln(LnIo/It)Ln(Time(ns)) ZnCdSe alloy 2hr0.00.51.01.52.02.53.03.54.04.5 -4.0 -3.5 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 Ln(LnIo/It)Ln(Time(ns)) ZnCdSe alloy 3hr012345 -4 -3 -2 -1 0 Ln(LnIo/It)Ln(Time(ns)) CdSe/ZnSe Coreshell012345 -4 -3 -2 -1 0 Ln(LnIo/It)Ln(Time(ns)) ZnCdSe alloy 1hr012345 -4 -3 -2 -1 0 Ln(LnIo/It)Ln(Time(ns)) ZnCdSe alloy 2hr0.00.51.01.52.02.53.03.54.04.5 -4.0 -3.5 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 Ln(LnIo/It)Ln(Time(ns)) ZnCdSe alloy 3hr012345 -4 -3 -2 -1 0 Ln(LnIo/It)Ln(Time(ns)) CdSe/ZnSe Coreshell012345 -4 -3 -2 -1 0 Ln(LnIo/It)Ln(Time(ns)) ZnCdSe alloy 1hrT=300K = 0.75 = 173ns T=300K = 0.58 = 277ns T=300K = 0.48 = 501ns T=300K = 0.58 = 276ns(a) (b) (c) (d) 012345 -4 -3 -2 -1 0 Ln(LnIo/It)Ln(Time(ns)) ZnCdSe alloy 2hr0.00.51.01.52.02.53.03.54.04.5 -4.0 -3.5 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 Ln(LnIo/It)Ln(Time(ns)) ZnCdSe alloy 3hr012345 -4 -3 -2 -1 0 Ln(LnIo/It)Ln(Time(ns)) CdSe/ZnSe Coreshell012345 -4 -3 -2 -1 0 Ln(LnIo/It)Ln(Time(ns)) ZnCdSe alloy 1hrT=300K = 0.75 = 173ns T=300K = 0.58 = 277ns T=300K = 0.48 = 501ns T=300K = 0.58 = 276ns(a) (b) (c) (d)012345 -4 -3 -2 -1 0 Ln(LnIo/It)Ln(Time(ns)) ZnCdSe alloy 2hr0.00.51.01.52.02.53.03.54.04.5 -4.0 -3.5 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 Ln(LnIo/It)Ln(Time(ns)) ZnCdSe alloy 3hr012345 -4 -3 -2 -1 0 Ln(LnIo/It)Ln(Time(ns)) CdSe/ZnSe Coreshell012345 -4 -3 -2 -1 0 Ln(LnIo/It)Ln(Time(ns)) ZnCdSe alloy 1hr012345 -4 -3 -2 -1 0 Ln(LnIo/It)Ln(Time(ns)) ZnCdSe alloy 2hr0.00.51.01.52.02.53.03.54.04.5 -4.0 -3.5 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 Ln(LnIo/It)Ln(Time(ns)) ZnCdSe alloy 3hr012345 -4 -3 -2 -1 0 Ln(LnIo/It)Ln(Time(ns)) CdSe/ZnSe Coreshell012345 -4 -3 -2 -1 0 Ln(LnIo/It)Ln(Time(ns)) ZnCdSe alloy 1hrT=300K = 0.75 = 173ns T=300K = 0.58 = 277ns T=300K = 0.48 = 501ns T=300K = 0.58 = 276ns(a) (b) (c) (d) A) B) C)D) A) C) D) B)Figure 2-17 shows a plot of the data for the nano rod samples in the form of an ln[ln(Io/It)] versus ln(time) plot and fitting using a linear function. This type of plot is useful to determine the extent of exponential behavior present in the material.(116) Starting with Eq. 2-6: () lnoItt I (2-7) 0lnlnln ()I t It (2-8) Figure 2-17. Equation ln[ln(Io/It) ] versus ln(time) of A) CdSe /ZnSe coreshell nanorods, B) ZnCdSe alloy nanorods 1hr, C) ZnCdSe a lloy nanorods 2hr, and D) ZnCdSe alloy nanorods 3hr. Adapted from H. Lee.(66) When the slope, equals 1, the line should be stra ight indicating an expo nential decay with a well-defined rate.(116) Obtained values are summarized in Table 2-1. The fitted of CdSe/ZnSe coreshell nanorod is ~0.75, which reflects high er degree of ordered crystals. Difference of

PAGE 61

61 CdSe/ZnSeCore/Shell Nanorods (Aspect Ratio ~ 2.5) Alloying ~280C 3 Hours ZnCdSeNanorods (Aspect Ratio ~ 2.1) 20nm CdSe/ZnSeCore/Shell Nanorods (Aspect Ratio ~ 2.5) Alloying ~280C 3 Hours ZnCdSeNanorods (Aspect Ratio ~ 2.1) 20nmvalues between 1 and ~0.75 might be mainly due to size distribution. CdSe/ZnSe quantum wells have demonstrated similar values.(42, 66, 118) Table 2-1 Comparison of and value of CdSe/ZnSe and ZnCdSe nanorods. (42) A comparison of the TEM images of the core/s hell and the 3 hr all oy are shown in Figure 2-18. The aspect ratio of the core/shell nanorods is 2.5 while for the alloy it is 2.1, indicating that the size distribution is not significantly change d during alloying process (Zn diffusion does not change the shape or size signifi cantly). Thus, the photophys ical changes observed are due to the annealing process altering the composition. Comp aring CdSe/ZnSe coreshell nanorods to alloy ZnCdSe nanorods, the values are significantly decrease d from 0.75 to 0.48~0.58 due to increased disorder as a result of spatial fluctuations of Zn concentrations after the annealing process. After three hours of annealing, the value did increase, which is consistent with the increased compositional homogeneity observed in the photoluminescence and Raman data.(42) Figure 2-18. TEM images of CdSe/ZnSe Core/Shell and ZnCdSe Nanorods Nanorods e m (nm) (ns) Core/Shell ZnCdSe Alloy 1hr ZnCdSe Alloy 2hr ZnCdSe Alloy 3hr 645 625 570 566 0.75 0.58 0.48 0.58 173 277 501 276

PAGE 62

62 Theoretical arguments predict that the radia tive lifetime of bound excitons increases with binding energy (114) and this is observed in our samples where the s increase with alloying time from 173 ns to 276 to ~501ns (Table 2-1). Th ese time constants imply that in order to fully characterize the long decay presen t in these samples, experiments on a much longer time scale should be conducted. Exciton binding energy is increased by exciton confinement which is obtained in this experiment by localization of the carrier wave function by changing the composition of the quantum rods.(119) Therefore, it is expected that the luminescence decay time ( ) in these alloy nanorods will increase due to increased localization of excitons, i.e. binding energies induced by co mpositional fluctuations.(42) Summary Green-yellow emitting ZnCdSe ternary alloy na norods with relatively high quantum yield are presented. The nanorods size and shape were characterized by XRD, TEM while the limited alloying in core/shell nanorods and composition disorder was detected by Raman spectroscopy. It has been shown that the quant um yield of ZnCdSe nanorods is a function of alloying time and is significantly higher compared to CdSe nanorods, but is still lower than the core/shell nanorods. The luminescent efficiency of these materials wa s discussed in terms of compositional disorder, defects induced by the alloying process, and surface passivation by larger band gap surface layers resulting from higher Zn concentrations near the su rface. Time resolved emission provided information regarding the role of di ffused Zn. A stretched exponential function was used to describe these systems, where <1 corresponds to a distribution of decay rates. Comparing CdSe/ZnSe core/shell nanorods to ZnCdSe alloy nanorods we found a significant decrease in the value. Photolumines cence decay lifetime, of the samples increased with alloying time due to compositional disord er leading to exciton localization.(42)

PAGE 63

63 CHAPTER 3 QUANTUM PARTICLE ELECTRONIC STRUCTURE Introduction Investigations into the behavi ors of the electrons and holes in quantum nanoparticles have been of great interest for several years.(10, 18, 31, 120-123) As seen in the previous chapter, steady state spectroscopy does not give a clear understanding of the exciton behavior when comparing rods to dots. On the other hand, th ere are methods that can measure the temporal dynamics and the kinetics of photophysical proce sses. These methods are called time-resolved spectroscopy techniques,(124) and they are a powerful tool that can bridge fundamental parameters such as size, shape, composition a nd passivation to increasi ng quantum yields and stability, reducing photodegradation, lowering the cost of fabrication (d eposition and lithography methods are expensive), making the synthesis safer, and improving their process-ability. Whether in a conjugated molecule or se miconductor nanoparticle upon excitation, an electron and hole are created. Fi gure 3-1 shows a cartoon of mol ecular orbitals (MO) as linear combinations of atomic orbitals. In conjugate d molecules, the MOs near the band gap are linear combinations of the same type of atomic orbitals whereas in nanocrystals the MOs can be linear combinations of atomic orbitals from diffe rent atoms. When an electron is excited from the HOMO (valence band) to the LUMO (conduction band), a hole is left behind in the HOMO. In a CdSe nanocrystal, the HOMO has contribu tions from atomic orbitals from the Se2whereas the LUMO is a linear combination of atomic orbitals from Cd2+. Therefore, the created hole will be located within the anion MO s while the electron will occupy the cation MOs. This in addition to a high dielectric constant pr esent in semiconductor nanocrysta ls means that the electron and hole are correlated although at the same time the individual carrier s can behave, i.e. be excited, trapped or relax nonradiatively, to some extent, independently.(13)

PAGE 64

64 Cd1+ Se1-h+ eInterband Transition Electron Intraband Transition Hole Intraband Transition Cd2+ Se2MOs = ci MOs = ci Energy Cd1+ Se1-h+ eInterband Transition Electron Intraband Transition Hole Intraband Transition Cd2+ Se2MOs = ci MOs = ci Energy Figure 3-1. Electronic structur e in semiconductor nanoparticles. Adapted from M. El-Sayed.(13) After the initial excitation, the electron can only be further excited to higher states of the cation MOs while the hole can only be further ex cited to other anion MOs (intraband transitions). The recombination of the electron and hole from the conduction band to the valence band involves an interband transition which can be di rectly detected in the visible spectrum. The transient absorption signals detect ed in the visible region only re veal the behavior of this bound exciton. Intraband relaxation of either electron or hole dyna mics for strongly confined nanocrystals are detected independently in diffe rent spectral regions. Since the energy spacing between the levels within the cation MOs is mu ch larger than the energy spacing between the anion MOs the intraband excitation or relaxation of the hole intrab and transfers is detected at lower energies (infrared region) than the electron intraband transfers.(13) The relaxation processes from higher to lowe r excited states within nanocrystals are extremely intriguing and counterin tuitive. Unlike in bulk materi als where the cooling of the photo-generated carriers is rapid and occurs via lattice phonons through its conduction band continuum, carrier cooling in quant um particles must occur in disc rete steps based on the nature

PAGE 65

65 of the energy states of the materials.(125) Due to the quantization of nanocrystals, the spacing between the energy levels for the electrons is qui te large, reaching values much greater than longitudinal optical (LO) phonons found in bul k semiconductors (~ 25 meV). Cooling via phonons is possible; however, it requires a simu ltaneous emission of a substantial amount of phonons, which has low probability. Therefore, it was assumed for many years that the relaxation of these excitons should be inhibite d, due to this phonon bo ttleneck, resulting in nanosecond cooling times.(122, 126) The electron relaxation from the 1P-to-1S in CdSe nanocrystals occurs in the subpico second timescale (faster than even in bulk) thus bypassing this bottleneck.(10, 17, 122) Klimov was able to extr act population dynamics of the 1S and 1P states determining that a 1P to 1S relaxation rate of ~ 300 fs a nd a 1S buildup time depends on the confinement enhancement and decreases as the nanocrystal radius decreases.(18) This fast relaxation process is Auger in nature in that the Coulomb interaction between the electron and hole, which is increased due to quantum confinement, allows the electron to relax but transfer excess energy to the hole, scattering it deeper into the valence band.(10, 31, 122) This strong coupling between the electron and hole in quant um systems has allowed for predictions of efficient electron-hole energy tran sfer to occur within 500 fs (127) to 2 ps (128) though it is difficult to measure these values directly. Phonon assisted relaxation of th e hole is more probable due to smaller energy spacing within the valence band.(122) Using infrared transient absorption(129) and terahertz spectroscopy(122) several groups have attempted to correctly identify and conf irm the Auger relaxation mechanism where the electron transfers excess energy to the hole. The distinct photoabsorption features present in transient spectra is very useful in identifying different photo-excited species. However, broad band spectra are sometimes difficult to interp ret and assignment of various species becomes

PAGE 66

66 tedious and complicated. Moreover, luminescen ce up-conversion does provide for better time resolution then other time-resolv ed techniques which allow for de tection of ultrafast dynamic processes. Time-resolved luminescence is not us eful when trying to extract information about charge carrier mobility and density or even ca rrier cooling since the dynamics of each carrier must be measured separately. Although pump-pr obe in the infrared region has shown evidence for the Auger cooling mechanism,(18) Terahertz spectroscopy is the first method that allowed for direct measurement and quan tification of hot cooling.(122) Hendry et al. determined that the hole relaxation rate strongly depends on the amou nt of excess energy the electron provides. They were able to confirm the Auger cooling mechanism and claim that this decay occurs on a 1 ps timescale. (122) This dissertation utilizes the information ga thered by Hendry concerning the Auger cooling mechanism, focusing on the exciton dynamics, i.e. interband relaxation fo r bare, core/shell and ternary nanorods. From the experiments complete d by Hendry, and our data, it is concluded that the excess energy that the electr on transfers to the hole, in addition to the small valence band offset between the core and shell (0.07 eV), coul d be sufficient to cause the hole to tunnel into the ZnSe shell and extend the elect ron and hole recombination times. The ultrafast carrier dynamics in bare CdSe and core/shell CdSe/CdS/ZnS quantum rods using femtosecond pump-probe spectrosc opy has been conducted previously (72) to see how the interface between the core and shell affects the competition between photoinduced absorption and stimulated emission for lasing applications Faster relaxation in the core samples was observed due to surface traps. U pon passivation, the number of surf ace trap states decreases, at the same time the shell introduces interfacial stat es due to the lattice strain mismatch between core and shell.(72) To date, no studies have compared the ultrafast carrier dynamics in bare CdSe

PAGE 67

67 rods and CdSe/ZnSe core/shell rods. In this wo rk, we explore how addition of shell with only one inorganic material with a small valence band offset affects the photophysical properties and then compare this data to the exciton beha vior in ternary all oy heterostructures. Experimental Methods: Transient Absorption Relaxation processes of colloidal nanocrystals were explored using femtosecond transient absorption (TA). A commercial Ti -Sapphire (Ti-Sa) laser system consisting of a Ti-Sa oscillator (Tsunami, Spectra-Physics) and subsequent amplifie r (Spitfire, Spectra-Physics) with a repetition rate of 1 kHz was used to pump an optical pa rametric amplifier (OPA) to generate tunable excitation pulses. A portion of the amplifier output is split off to pump a 1 mm rotating CaF2 window to generate white light continuum probe with an effective bandwidth ranging from 310 to 750 nm. Prior to white-light ge neration, the probe polarization is tilted by 45 degrees with respect to the pump pulse using a thin-film polarizer. A detailed description is available elsewhere. (130) A general schematic is provided in Fi gure 3-2. The OPA idler/signal output is used to produce excitation pulses (pump) th rough harmonic generation (450, 575, 610, 630 and 650 nm). This beam is then fed through a prism compressor, resulting in pulse lengths less than 100 fs (FWHM). The excitation beam is focused to a diameter of ~150 m at the sample position and its energy was set to ~ 39 to 45 nJ yielding a fluence of 221 to 255 J/cm2. Low fluences are necessary to avoid multiple exc itations (biexcitons). From previous works, it is known that a signature of multiparticle interactions are decay rates that occur faster than 50 ps. Klimov also observed that the decay rates increased as the number of excitons per nanoparticle increased.(131) Experimentally, I verified the fact that multiple excitations per nanocrystal were not initially created vi a a power dependence study. Our data show (Figure 3-4: no fast decay

PAGE 68

68 observed in the kinetic traces prio r to 100 ps) that we were able to assume that for the power range utilized, multi-excitons are not initially generated. Prior to interaction at the sample, a fraction of the probe pulse is split off. This reference beam follows the same optical path as the probe but it probes only a sample volume that is not excited by the pump pulse. The pump pulses were modulated by an optical chopper at a frequency of 500 Hz and passed through a comput er-controlled optical de lay line to delay the probe arrival time relative to the excitation. The pump and probe beams were spatially overlapped at the sample. Probe and reference signals are collected in the presence and absence of the excitation pulse and the ratio: probeprobe referencereference pumponpumpoffII II (3-1) is recorded for each wavelength at every time step. A Glan-Thompson polarizer splits the transmitted signal, into its polar ization components, parallel (A| |) and perpendicular (A) with respect to the pump. The intensity at magic angle is calculated from the parallel and perpendicular components measured simultaneously: magic angle2 3 AA A (3-2) eliminating the influence of rotational times on the signal. Parallel and perpendicular transmitted probe and reference signals were focused into a spectrograph attached to a charge-coupled device (CCD) (Andor iStar coupled to Shamrock 303i spectrograph) for detection. Sample solutions for TA measurements were placed in a quartz cuvette with a 2 mm path length and continuously stirred to guarant ee excitation of a new sample vo lume with every laser shot. Changes in optical density (OD) were in the range of 1 to 50 mOD, and scans were repeated multiple times to achieve acceptable signal-to-n oise ratio. Each time step was averaged 250

PAGE 69

69 times per scan. When twenty scans were completed, the total number of laser shots per point was equal to 5000. Kinetic traces at particular wa velengths can be extracted from the full spectrum collected using the CCD camera. CCD ARRAY White Light Generation PS PP REFChopper Pump Reference Probe from OPA, delay stage Chopper Pump Reference Probe from OPA, delay stage C C D From Amplifier Monochromator CCD ARRAY White Light Generation PS PP REFChopper Pump Reference Probe from OPA, delay stage Chopper Pump Reference Probe from OPA, delay stage C C D C C D From Amplifier Monochromator Figure 3-2. Transient absorption schematic Results Insight can be gained by inve stigating the spectral evoluti on caused by population density changes in different energy levels using br oad band femtosecond time-resolved absorption.(17, 53) A continuum probe results in a collection of the entire transient spectra at each delay time in a single experiment. Therefore, photo-excited species can be detected and in principle, identified based on their characteristic transient absorption features. In even a simple system, assignment and interpretation of such photo-excited species can be difficult due to convoluted absorption features within the transient spectrum. CdSe versus CdSe/ZnSe Core/Shell CdSe nanorods were synthesized us ing the method described by Peng.(46) Raman spectroscopy is a useful tool for evaluating the structure and compositional homogeneity of

PAGE 70

70 nanocrystals.(16, 17) In the previous chapter, the eff ect of adding an inorganic shell and subsequent alloying have on phonon sp ectra was presented. It is clear that new modes appear after the addition of the ZnSe shell. Despite no significant changes between the bare and passivated samples in the steady state absorption, the Raman data shows that the structure of the system has changed; therefore it is necessary to investigate the system using time-resolved methods. More specifically, the linear absorption spectra indicate that confinement is maintained after addition of the ZnSe shell. Meanwhile, the qua ntum yield is increased from 0.6% to 15% in bare and core/shell materials respectively.(42) Clearly, surface traps are reduced, prompting a time-resolved investigation into the changes in the carrier relaxation due to the change in electronic structure. Figure 3-3 depicts the transient signal collect ed at 0 fs, 400 fs, 800 fs, 2.47 ps, 200 ps, and 575 ps for bare CdSe and passivated CdSe/ZnSe core /shell samples. Both samples are excited at 450 nm well above the band gap and the 1S and 1P absorption bands seen in Figure 2-12. Multiple transitions dominated by state-filling ar e observed, leading to transient bands at the energies of the allowed optical transitions. Exact determination of the electron-hole transitions which give rise to different resonances need to be determined by comparison with the states theoretically calculated by an effective mass theory. (72) In this work, we assign the transitions based on works done by Klimov,(17) Efros,(25) and Guyot-Sionnest(129). Using their notation, B1 and B2 are assigned to the photobleach of the 1S [1S(e)-1S3/2(h)] and 2S [1S(e)2S1/2(h)] states respectively while B3 corresponds to the bleach absorption of the 1P [1P(e)-1P3/2(h)] state. Meanwhile, the A1 band is assigned to the photo induced absorption that grows in after high energy excitations cool from the 1P to the 1S Within 400 fs (red li ne), the carriers are distributed throughout the cascade of energy states. As the delay time increases from 400 fs to

PAGE 71

71 500550600650700 -0.03 -0.02 -0.01 0.00 B3 B2 B) CdSe/ZnSe CS Rods Anm)B3-0.01 0.00 A1 B1 B2 B1 A) CdSe Rods A12.47 ps (blue line), carriers quickly relax from the higher energy states to the band gap state resulting in a corresponding 1P to 1S relaxation. Figure 3-3. Broad band transient absorp tion spectra for A) CdSe Rods and B) CdSe/ZnSe Core/Shell Rods at various pump delay times: 0 (), .400 ( ), .800 ( ), 2.47 ( ), 200 ( ), 575 ( ) ps. Extracting the kinetic information from the broad band spectrum enables the comparison of carrier relaxation trends at each optical tran sition (Figure 3-4). In both systems (CdSe and CdSe/ZnSe), the decay lifetimes corresponding to the 1S and 2S bands are identical (>200 ps) (black and red lines). The 1P decays rapidly a nd results in photoinduced absorption at longer delay times, the origins of which will be explained in further detail in the discussion. The rise of the PIA signal in bare CdSe rods is slower (>10 ps) than the passivated rods (< 2 ps). A dip is observed in the passivated sample for both the 1S and 2S bands, which results from overlap of

PAGE 72

72 multiple processes detected simultaneously. This effect is not observed in the core sample. For the core/shell sample, it is proposed that the hole migrates to the valence band of the ZnSe shell (valence band offset = 0.07 eV) due to the en ergy transfer during A uger relaxation of the electron resulting in longer ble ach decaying times. The insets of Figure 3-4 A and B compare the higher energy, 1P state negative absorption decay and photoinduced absorption to the 1S and 2S states bleach decays. It is interesting to note that the rise of the negative signal from the 2S and 1P states (black and green resp ectively) are identical but the 1P bleach decays rapidly (~ 1 ps) into a positive signal, matching the rise of the bl each of the lower 1S energy state (red line). This confirms a 1P to 1S relaxation process in both CdSe and CdSe/ZnSe. Core/Shell Excitation Dependence An excitation wavelength dependence study (450, 575, 610, 630, 650 nm) was conducted to elucidate the influence on the kinetic processe s of the ZnCdSe interfacial state previously detected by Raman spectroscopy. Figure 3-5 show s the steady state absorption spectrum, with the horizontal arrows signaling the excitation wa velength for each row of time-resolved spectra presented on the right two columns. The tran sient spectrum showed on top indicates the detection wavelength for each column. For example, the kinetic data on the top left plot presents the transient signal in the area of the 1P state after excitation at 650 nm whereas the middle right plot presents the transien t signal of the 1S state after excitation at 575 nm. After excitation at the same energies, the 1S band bleach rises with a ti me constant corresponding to the decay of the 1P bleach. In addition, the 1P detection shows photoinduced absorption present at a time delay greater than 1 ps due to the ZnCdSe interfacial state. For excitations greater than 600 nm (lower energies), the P band does not contribute to the dynamics; instead, the excitation simultaneously populates both the S band and trap states.

PAGE 73

73 Figure 3-4. Kinetic traces corresponding to the 1S ( ), 1P ( ) and 2S () bands for A) CdSe rods and B) CdSe/ZnSe rods. 0100200300400500 -1 0 1 -1012345-1 0 1 norm Atime (ps)CdSe/ZnSe Dip time (ps) B)0100200300400500 -1 0 1 -1012345-1 0 norm Atime (ps)CdSe RodsA) time (ps)

PAGE 74

74 0 0 2 0 0 0123 0 0123 0 A400 450 500 550 600 650 700 01 Excitation Wavelength 500550600650700 1A Detection Wavelength APS time (ps)Considering the kinetic traces in the first column (P detection) as the excitation energy is decreased the bleach decay faster and the onset of the photoinduced absorption occurs at earlier delay time. Eventually, (excitation at 650 nm) the bleach is no longer presen t, indicating that the P transition is not being accessed. As seen in the kinetic traces in the second column (S detection), as excitation energy is increased, the rise time of the negative absorptions are slower. When the sample is excited at 650 nm, the rise is instantaneous, while at 450 it is greater than 1 ps. The energy level associated with the ZnCdSe interfacial state is then considered to expand through the density of st ates between the CdSe and ZnSe band gaps. Figure 3-5. Time-resolved excitation dependence coll ected for the core/shell sample. Left graph: linear absorption spectra and indicates th e excitation wavelengths (450, 575 and 650 nm). Top graph: transient spectra at 2.47 ps. Kine tic traces in column 1 correspond to the 1P band while column 2 corre late with the 1S band behavior

PAGE 75

75 Core/Shell versus Alloys Prior to our work, limited information appeared in the literature involving the synthesis of colloidal ZnCdSe nanorods for use in optoelectronic devices. Green-yellow emitting ZnCdSe nanorods were prepared by diffusion of Zn into the CdSe core. For alloying, the reaction vessel containing CdSe/ZnSe nanorods was heated and stirred for up to 3 hrs. An aliquot was removed after heating for 1, 2 and 3 hrs, immediately c ooled and diluted with toluene to terminate the alloying process and then precipitated with me thanol/toluene co-solvents. The Raman data presented in Chapter 2 indicat e the enhancement of the ZnCdSe phonon mode but disappearance of the CdSe and ZnSe modes, confirming the alloyed nanorod composition.(42) Figure 3-6 compares the bleach spectra of the core/shell and ZnCdSe alloyed samples using the delay times of 0.400, 2.47, 150 and 575 ps. Note that this data is presented differently than in Figure 3-3. Each plot includes the four samples at one pa rticular delay time. For example, Plot 1 corresponds to the transients for each samp le at a probe delay time of 400 fs. This data indicate the occurrence of a transformation of the band gap, band structure and surface-trap states as function of alloying. At early time delays (0.400 ps Plot 1), the 1S band correspo nding to the 2 hr alloy (green line) is much more intense than all other samples and is significantly blue shifted compared the core/shell nanorods (black line). After 2.47 ps (P lot 2) the 1S band in all samples is maximized and the wavelength shift mentione d previously is much more evident. The 1 hr alloy remains very broad even after 575 ps (Plot 4). The overa ll signal obtained from the alloys is not as intense as the core/shell and th e 1 and 2 hr alloys are not as intense as the 3 hr alloy. Photoinduced absorption present in the core/shell materials does not appear in the alloys. This will be discussed in further detail later. At 150 ps (Plot 3), the alloy bleach has noticeably decayed; meanwhile the core/shell has not d ecayed significantly. Even after 575 ps, the

PAGE 76

76 core/shell decay is very small (compare bl ack line in Plots 2, 3 and 4) and photoinduced absorption is still present. For the ZnCdSe sample s there is no distinct band gap associated with CdSe or ZnSe. The only band present corresponds to the ternary composition; thus, it does not exhibit excited state absorption. As alloying time increases, the amount of Zn diffusing into the CdSe increases resulting in the band gap shifts observed in both steady state and transient measurements. Figure 3-6. Broad band transi ent absorption spectra for co re/shell(), Alloy 1 hr ( ), 2 hr ( ), 3 hr ( ) at various time delays. A comparison between the 1S and 1P bands of each of the samples is shown in Figure 3.7. All samples were excited at 450 nm but the dete ction wavelengths are not the same due to the band gap shifts induced by Zn diffusion. The tran sient spectrum at the top is shown as a guide indicating the detection wavelengt h for the 3 hr alloy. The firs t column of kinetic traces corresponds to the detection of the P band while the second column depicts the dynamics of the 500550600650700 -0.04 -0.03 -0.02 -0.01 0.00 PLOT 4 PLOT 3 PLOT 2 400 fs PLOT 1 2hr 1S C/S 1S 500550600650700 -0.04 -0.03 -0.02 -0.01 0.00 1S 1S 1S 2.47 ps 1S500550600650700 -0.04 -0.03 -0.02 -0.01 0.00 A (nm)150 ps500550600650700 -0.04 -0.03 -0.02 -0.01 0.00 (nm)575 ps

PAGE 77

77 475500525550575600 P P S A S Detection Wavelength Time (ps)0 0 0 0 0 0 01230 01230 3 HR 2 HR Alloy 1 HR Alloy Core/ShellS band. The amount of Zn diffusion into the CdSe increases from top to bottom as alloying time increases. When Zn is mixed in with the core, the material has more ZnSe character. Looking at the P band, when Zn diffusion occurs, the photoi nduced absorption is eliminated. Also, the rise of the P band bleach gets faster as more Zn is diffused into the CdSe core. The lack of photoinduced absorption in each of the alloys also confirms that the interf acial state present in the core/shell material is no longe r present. This is in agreement with the Raman data in Chapter 2. As seen in column 2, the rise of the bleach also gets faster as alloying time is increased. The S bleach decay of the alloys (Figure 3-8 red line) is significantly faster than the core/shell material (black line) due to surface trap s, since the inorganic passivati on layer is no longer present. Figure 3-7. The 1S and 1P composition dependence.

PAGE 78

78 Figure 3-8. Comparison of the 1S band for the Cd Se/ZnSe core/shell () a nd 3 hr ZnCdSe alloy ( ) Discussion In nanocrystals, optical transitions resulting in ground state absorpti on changes are due to state-filling effects whil e extremely fast transitions (<1 ps) re sult from Coulomb interactions, i.e., Stark Effect.(17, 72, 121) Red shifts are observed in CdSe quantum rods at longer delay times; they are identified as a convolution of the S-t ype states near the band gap stemming from the inherent size distribution present in colloidal nanoparticle samples.(72) The relaxation dynamics within these systems are str ongly influenced by ensemble dynamics collectively creating inhomogeneities and also multiple photoinduced processes leading to multi-exponential or nonexponential behaviors.(10) In analogy to bulk materials, cooling of hot electrons could occur via emitting LO phonons, and this mechanism would result in slow decay rates since the spacing of the intraband states is large in quantum syst ems resulting in a phonon bottleneck.(31, 111, 122) However, several studies (17, 111, 132) including our results, demonstrate a different behavior. High 0100200300400500-1.00 -0.75 -0.50 -0.25 0.00 Norm Atime (ps)

PAGE 79

79 energy relaxation in quantum systems occurs via Auger cooling in 2 ps (1P to 1S relaxation).(18, 53, 122, 128, 129) In each of the samples measured, relaxation was successfully observed as a corresponding decay of the 1P band and subsequent rise of the 1S band. Other possible cooling mechanisms include multi-phonon re laxation and polaron effects.(128, 133, 134) Under our experimental conditions, we create less than one electron-hole pair per nanorod, and thus those mechanisms are not plausible. In quantum dots, strong electron-hole Coul omb attractions favor energy transfer from the electron to the hole. (111, 122) Due to increased size dependence on carrier cooling, hot electrons tr ansfer kinetic energy to holes wh ich can quickly and efficiently undergo intraband cooling due to their relatively larger effective mass and smaller electronic energy level spacing.(18, 122) Enhancement of the Auger electr on-hole interaction would then bypass any phonon bottleneck.(122, 127-129, 131) This can not be conf irmed without directly measuring the electron-to-hole energy transfer which has been successfully observed by Hendry et al. using THz spectroscopy.(18, 122) Defect states can be resonant with high en ergy levels as observed by Rosenthal et al.(67, 122) The presence of these states results in an efficient trappi ng mechanism and faster bleach recoveries in core samples. These trap stat es are reduced by passivation with an inorganic material as in core/shell rods.(72) The shell reduces the number of surface states present in the bare rods preventing the exciton to sample th ese higher excited states resulting in higher photoluminescence quantum yields.(78) The core and core/shell systems both exhibit photoinduced absorption indicati ng the presence of an alternat e state contri buting to the excited state signal. The high surfacetovolume ratio within the bare rods creates additional electronic trap states leading to small photoinduced absorption f eatures at energies higher than the band gap but similar to the high energy 1P st ate. The onset of this photoinduced absorption

PAGE 80

80 is generally slow (>10 ps) and is related to the rate at which the carrier is trapped either at the surface or other defect sites. The trapped electr on can then relax to the ground state via an alternative radiative decay path way or decay non-radiatively.(67, 72) The actual rate s and overall dynamics are indistinguishable be cause of the inhomogeneity a nd ensemble averages of the samples; therefore, direct assignment of rates an d pathways is extremely difficult. On the other hand, photoinduced absorption within the core/shell rods arises from interface defects resulting in states that can act as traps and/or non-radi ative recombination sites caused by lattice strain relaxation introduced between the core and shell.(38, 65, 72) It appears that after the initial Auger cooling from the 1P to 1S state, some carrier populations sample the ZnCdSe interfacial layer resulting in photoinduced ab sorption. The signal is strong and lasts more than 500 ps (Figure 3-4). Compositional disorder (42, 108, 109) in ternary alloy nanor od structures leads to localization of excitons, (113) hence an increase in quantum yield for ZnCdSe versus CdSe nanoparticles. Localization eff ects increase the overlap integr al of the electron and hole wavefunctions improving the luminescence efficien cy of the material and decreasing the bleach lifetime for the ternary alloy samples compared to the binary CdSe materials. The lower ZnCdSe quantum yield versus the CdSe/ZnSe core/shell nanorods results from the lack of surface passivation and crystal defects within the ZnCdSe nanorods due to Zn diffusion into the CdSe core. Increasing the Zn character in the core caus es a blue shift in the spectrum as alloying time increases. Raman data presented in Figure 211 show that CdSe and ZnSe phonon modes are no longer present; the only mode obser ved is the ZnCdSe state. The lack of or small amount of photoinduced absorption confirms that the alloy nanorod composition is uniform and few interfacial traps are present. Since the inorganic shell disappears, surface traps reoccur and

PAGE 81

81 decrease the overall intensity (not as low as bare CdSe rods) and bleach lifetimes (recovery occurs faster than in the CdSe bare rods). In addition to surface state traps, crystal defects are known to act as non-radiative recombination cen ters, reducing the emission efficiency and enhancing the bleach recovery.(66, 114, 115) The band gap shifts, band narrowing and increase in the overall bleach amplitude can be attributed to stress relaxation by thermal annealing.(66, 110) This is consistent with the weaker, broadene d 1S band bleach signal observed after 1 hr and further after 2 hrs of anneali ng, since diffusion alters the di stribution of Zn throughout the nanorod, i.e. decreasing the amount of Zn presen t at the surface and in creasing the amount towards the middle. However, annealing for 3 hrs enhanced and narrowed the 1S band bleach compared to the samples annealed for 1 or 2 hrs. This increased change is attributed to annealing of crystalline defects and reduc tion of stress, consistent w ith the Raman data reported.(42) Tunneling of the electron wavefunction into th e ZnS shell has been reported in CdSe/ZnS core/shell structures by Mokari and Banin(38) resulting in a ~10 nm red shift. This tunneling led to a delocalization of the electron, lowering it s confinement energy and consequently the energy of the exciton levels.(39, 42) Raman data presented previously indicate formation of interfacial ZnCdSe in as-grown CdSe/ZnSe core/shell nano rods. This reaction would be expected to decrease the size of the CdSe core (42, 135) resulting in increased localization and a blue shift in emission. In our experiments, addition of the Zn Se shell does not alter the band gap significantly (only a 4 nm blue shift observed in photolum inescence). We have successfully engineered the material to create electron and hole wave functions that experien ce a confinement potential that localizes (Type-I localization) th e electron wave function within the CdSe core despite addition of a shell.(136) It has been shown that the rise of th e 2S and 1P bands ar e identical indicating that the hole is delocalized within the density of states located in the valence band of the CdSe

PAGE 82

82 core due to the small intraband spacing. Additiona lly, the 2S and 1S decays are identical. In fact, the dip observed for each of these bands in the core/shell sample after several ps is likely due to the hole tunneling into the interfacial ZnCdSe and/or ZnSe valence band due to the small valence band offset (the energy difference between the valence ba nd of the core and the valence band of the shell). The small valence band offs et between CdSe and ZnSe does not guarantee that the electron hole energy transfer does not cause the hole to transfer to the shell. If this were the case, it could account for the extended recovery rate observed in the core/shell versus core sample. However, we are unable to measure the m obility of the hole directly, so this is only a prediction. Figure 3-9 shows the valence and conduction band offsets of various bu lk materials. Most of the work presented in the literatu re deals with nanoparticles with ZnS (39, 137-140) or CdS (65) as the inorganic shell. CdS ha s been used to passivate CdSe(65) since it has a small lattice mismatch value (0.04 eV) compared to ZnSe (0. 07 eV) or ZnS (0.11 eV). In order to prevent penetration of the carriers into the shell or potential well barr ier, the photo-generated exciton created in the CdSe core should be confined by a material with valence and conduction potential wells that are comparable. Several studies have attempted to calculate to the first order approximation, a penetration length (L) that is proportional to:(75) 1/2()OLmV (3-5) where m represents the effective mass of the charge carrier (electron or hole) and VO is the band offset. These values can be seen in Figure 3-9. ( 141-145 ) Light electrons are more easily confined in heterostructures that have the same anion; therefore, the CdSe/ZnSe heterostructure is better balanced than the CdSe/CdS resulti ng in larger oscillator st rengths from enhanced electron-hole wavefunction overlap.( 75 )

PAGE 83

83 ZnSe CdSe 1Se 1Pe 1S3/2 1P3/2 2S1/2 0.07 eV V.B. Offset ~ 1ps Core/shell interface PIA Interband Relaxation h = 2.75 eV 0.4 eV Figure 3-9. Valence and conduction band offsets for various materials. ( 75 ) From the observations above we propose two mo dels associated with the exciton dynamics within the CdSe/ZnSe core/shell and ZnCdSe alloy nanorods. In Figure 3-10, high energy excitation results in very fast 1P to 1S rela xation times (~ 1ps). State filling within the conduction band of the CdSe occurs. Figure 3-10. CdSe/ZnSe core/shell potential kinetic model.

PAGE 84

84 0.4 eV 1Se 1Pe 1S3/2 1P3/2 2S1/2 < 1ps Interband Relaxation h = 2.75 eV Trap State Surface Trap From the evidence presented in this thesis, it is conceivable that during the Auger cooling of the hot electron from the 1P to 1S state, excess en ergy could cause the hole to become delocalized within the ZnCdSe interface or even in the ZnSe shell. The bleach recovery from the conduction band to ground state valence band is the longest in the core/shell materials due to passivation of surface traps and potentially due to the position of the hole wavefunction. Once the materials are alloyed, the ZnCdSe in Figure 3-11 becomes the only inorganic material present. Again, 1P to 1S relaxation is observed with high en ergy excitation followed by subsequent interband relaxati on from the conduction to valence band. However, based on the work completed by Rosenthal et al.,( 67 ) midgap surface states involving selenium dangling bonds are present due to the lack of inorganic passivation. An electron relaxing from the surface Se atoms to the valence band can immediatel y fill the vacancy left by the photogenerated electron contributing to the deep tr ap emission observed at 700 nm. Figure 3-11. ZnCdSe alloy pot ential kinetic model.

PAGE 85

85 Summary Transient absorption spectrosc opy was utilized to extract the exciton dynamics within binary CdSe/ZnSe core/shell a nd ternary ZnCdSe nanorods. A comparison between the exciton behavior in unpassivated CdSe core and CdSe/ZnSe core/shell materials, an excitation wavelength dependence for the core/shell nanorods and the influence allo ying has on the exciton behavior are all presented. For all samples, at high energy excitation, a 1P decay and subsequent 1S rise is observed corresponding to a 1P to 1S relaxation pro cess. Also, the introduction of a midgap state in the core/shell material leads to photoinduced absorption after the 1P bleach recovers. Upon low energy excitation, this midgap st ate is directly populated. Surface trap states reappear in the alloyed heterostructures (no passiv ation) leading to faster bleach recoveries then the core/shell materials.

PAGE 86

86 CHAPTER 4 CONJUGATED POLYELECTR OLYTES (CPES) Introduction -Conjugated polymers are an interesting cl ass of materials with unique physical characteristics that make them excellent can didates for various purpos es including lasers ( 146 ), LEDs ( 147 ), photovolatics ( 148 ), and transistors ( 149 ). To be useful for any application, a fundamental understanding of their photophysical prope rties is necessary in order to continue to improve their efficiency and efficacy. In recent years, conjugated polyel ectrolytes (CPEs) have been synthesized incorporating ionic sol ubilizing side groups enabling the polymer to be dissolved in water and other polar solvents while preserving the photophysical properties associated with the polymer backbone.( 6, 7 ) In an effort to reduce exposure of the non-ionic components to the environment, when CPEs are di ssolved in a polar solven t such as water they self-assemble into aggregates due to the inte raction between the charged functional groups and hydrophobic backbone. ( 150-154 ) This intraor intermolecular stacking of the polymer chain creates new, red shifted abso rption and emission peaks, ( 155 ) decreases the overall fluorescence quantum yield and competes with radiative em ission processes from the isolated chains.( 156 ) Aggregates can also form in concentrated polymer solutions w ith nonpolar solvents. (14, 20, 2628) Addition of a metal cation such as Ca2+ acts as a cross-linking agent and it has been shown to induce aggregation in methanol, improving th e amplified quenching properties of CPEs.( 155, 157-159 ) Aggregation is easily confused with othe r types of interchain interactions of -electrons in spatially close chromophores. It is extremely important to correctly define, understand and identify the different types of interchain spec ies that can be present within the conjugated polymer solution or film. Within the literature, there is a discussion on the proper identification

PAGE 87

87 of interchain species.( 159 ) In interchain interactions, -electron density is delocalized among numerous conjugated segments in different polymer chains. Depending on the physical conformation of the chains, it is possible that the two interacting species be located on the same chain. For example, if the polymer chains are ex tremely long, the conjugation segments from the same chain can interact spatially as a result of stacking due to backfolding. Shared electrons between two polymer chro mophores in their exci ted state that are next to each other create a species named termed excimer.( 159-162 ) When neutral excitons are shared by two or more adjacent chromophores in the ground and ex cited state, the intrach ain species that is formed is known as an aggregate. The aggregate formations that interact electronically will cause a significant change in the absorption spectra corresponding to an elongation of the electron delocalization resulti ng in lower energy peaks compared to isolated chains.( 159 ) In addition to aggregates, a polaron pair can be created after excitation resulting in an radical cation (hole polaron) in one ch romophore and a radical anion (e lectron polaron) on another. ( 159, 163, 164 ) A significant redshift in the emission sp ectra is a photophysical indicator that an excited interchain species is present within the conjugated sample due to delocalization of electrons creating a lower electr onic state compared to the isolated chains. Since this phenomenon occurs for each of the interchain inte ractions, it is hard to distinguish between the various types. Detection and identification is further complicated for room temperature fluorescence measurements due to the large numbers of non-radiative trap sites in conjugated polymers resulting in very lo w emission quantum yields. ( 159, 160, 163 ) Aggregates can be differentiated from the other spec ies because they are the only ones that show a weak redshift in the ground state detectable in the absorption spectrum. ( 159, 165, 166 ) This shift can be subtle, especially if the aggregat e absorption is symmetry of the tran sition is forbidden; therefore, the

PAGE 88

88 controversy between discriminating between aggreg ates and excited state interchain species is still ongoing.( 159 ) Based on the spectral signatures pr esent in our photophysical characterization, the species present in PPE-CO2 are considered to be aggregates. Correct identification of the types of interc hains species is extremely important when considering charge transport and light emission applications of c onjugated polymers. In order to fully understand a system and be able to make synthetic improvements it is necessary to characterize each of the species accordingly. Also, CPEs are opening the door to various biological applications but aggreg ation must be considered because it is an extremely important factor that influences the polymer quenching capabilities and ultimately their performance as chemoor biosensors.( 130, 155, 157, 160, 161 ) Zhao et al. has reported the synthesis and char acterization of a series of variable band gap poly(arylene ethynylene) (PAE) water soluble conjuga ted polyelectrolytes di ssolved in methanol, water and methanol/water mixtures.( 167 ) By only varying the anionic side group, they achieved band gap tunability within the vi sible region. Photophysical data co llected in their study correlate the CPE side chain structure to the extent of polymer aggregation when dissolved in each solvent. The work done for this thesis focuses on the role aggregation plays in the intraand intermolecular energy transport within vary ing polymer repeat units (PRU) of PPE-CO2 -. From previous research in this area it has been determ ined that the quenching efficiency increases as the amount of controlled aggregation increases.( 130, 150, 155 ) Several investigations, ( 158-160 ) including those done by Chen et al. ( 157 ) have alluded to the idea that quenching of a conjugated polymer emission is the fundamental property necessary to understand and characterize these materials to be useful for chemoand bio sensors. More specifically, anionic polymeric electrolytes can be efficiently quenched by cationic systems in

PAGE 89

89 solution. The quenching efficiency is desc ribed by the conventi onal Stern-Volmer relationship:( 2 ) 00 01[]1[]qSVI kQKQ I (4-1) where 0 ( I0) and ( I ) are the steady state fluorescence quantum yields (fluorescence intensities) in the absence and presence of the quencher molecule respectively, KSV is the SternVolmer constant, and [Q] is the quencher conc entration. A Stern-Volmer plot is the fluorescence intensity ratio ( I0/ I ) versus Q. This plot is expected to be linear with the slope equal to the Stern-Volmer constant. From this information, the quenching rate constant, kq, can be calculated if the exc ited state lifetime, 0 of the neat sample is known. From a time-resolved measurements point of view, the quenching mechanism is static if there is no change in when a quencher is added to the so lution. The following relationship is used if the lifetime does change:( 2 ) 0 01[]qkQ (4-2) This relationship enables one to determine if the fluorescence decay under dynamic quenching conditions in the presence of the quencher molecule and provides the value of kq. In CPEs, the quenching mechanism is both static and dynami c. In fact, the dynamic component does not necessarily arise from the diffusion of the quenche r in the solution but from the diffusion of the excitation within the polymer chain. ( 130, 168 ) It is well known that fluorescence within the visible spectrum from low concentrations of CPEs can be superlinearly quenched when pl aced in the presence of an oppositely charged electronor energy quencher molecule (superquenching ( 146 ); amplified quenching ( 167, 169 )).( 151, 152, 154, 157, 158, 162, 163, 165, 166, 170 ) Amplified quenching may lead to the

PAGE 90

90 development of more sensitive sensors but a complete explanation responsible for such high quenching efficiencies within CPEs has yet to be determined. This intriguing effect is not only due to ion-pairing between the polymer and quenchers ( 157, 163, 164, 170 ) as in typical SternVolmer kinetics, but also interand intrachain energy transport mechanisms. More specifically, the random walk diffusion of the excitation energy along the polymer backbone, ( 154, 157, 171, 172 ) energy transfer between the polymer and quencher ( 171 ) and energy transfer between the isolated polymer species and aggregated chai ns all contribute to such unique behavior.( 143, 146, 162, 165-167 ) Energy transfer is strongly dependent on the spectral overlap between the donor emission and acceptor absorption. The energy transfer is also very rapid. ( 168-171 ) If an aggregate inducer or quencher is added to the polymer solution, th e conformation can change resulting in a spatial redistribution of several chromophores. This enables the excitation located on the polymer backbone to easily migrate to the quencher located at a particular site lowe r in energy. Therefore, one quencher molecule can have the ability to reduce the emission from a large number of chromophores. ( 157, 173 ) The intrachain random walk model, which leads to excitation migration towards the quencher molecule, is strongly dependent on the conjugation, polymer chain lengths and transition dipol e orientations (Figure 4-1). Using time-resolved anisotropy fluorescence measurements, it is seen that after ex citation, the energy or exciton hops/migrates from shorter (high energy segments) to longer (l ow energy ones) and depolarizes along the way, reducing the anisotropy value. The exciton wi ll continue to funnel through the cascade of chromophores until it is either trapped or it reaches the lowest energy level where it can fluoresce or non-radiatively decay.

PAGE 91

91 O O CO2 -Na+ CO2 -Na+ nPPE-CO2 Several studies have investigated the influe nce aggregates have on the kinetics within water soluble conjugated polymer systems. ( 139, 152, 154, 173-179 ) For example, Fakis et al. has shown that energy transfer fr om isolated poly(fluorenevinyleneco -phenylenevinylene) (PFV-co-PV) ( 156 ) to aggregated chains is very rapid and efficient. They determined the isolated chain fluorescence, the aggregate emission and ener gy transfer contributions to the overall decay. In addition, the correlation between the con centration and energy transfer efficiency was thoroughly examined. A reduction in the concentrati on causes the energy transfer efficiency and energy transfer rates to decrease linearly.( 156 ) In this thesis, we investigate the influence chain length, solvent and metal cations have on the ultrafast emissi on of a carboxylated poly(phenylene) vinylene (PPE-CO2 -) shown in Figure 4-2 to dete rmine the excitation transport processes. The energy transfer mechanism between isolated and aggregated chains within the PPE-CO2 polymer is of particular interest. Figure 4-1. Intrachain energy tr ansfer of excitation to quencher molecule along polymer backbone. Figure 4-2. PPE-CO2 polymer repeat unit. PPE-CO2 in methanol (left) and water (right) Polymer Quencher

PAGE 92

92 Previously, a series of steady state, time-re solved, anisotropy measurements and numerical models were conducted with a similar CPE, PPE-SO3 -, to determine the rate and efficiency exciton migration has on fluorescence quenching.( 130 ) Using PPE-SO3 as a model polymer which exhibited both long range and random walk kinetics, we have designed experiments in which the results should indicate the type of energy transfer present within PPE-CO2 -. Timeresolved photoluminescence and time-resolved anisotropy measurements were employed to monitor the potential exc iton hopping that was previ ously observed in PPE-SO3 -, determine the rise time of the aggregate state emission a nd characterize the overall polymer decay. We studied very short and long polymer repeat unit (PRU) PPE-CO2 chains with the expectation that short chains woul d be less likely to aggregate. We find that even short chains (8 PRU) form solutions with both isolated and a ggregated chains. Even though the only difference between PPE-SO3 and PPE-CO2 is their ionic group, their p hotophysics are quite different. PPE-CO2 steady state photophysics correlate more with ladder-type (p oly-paraphenylene) (LPPP) polymers. ( 116, 166, 174-177 ) In most cases, conjugated polymer chains ar e not frozen in one conformation, instead they have a proclivity to twist and coil. A seri es of chromophores can be linked resulting in different degrees of -electron delocalization depending on the planarity of the conjugated segments. Even if there are slight twists or bends along the polymer backbone, it is possible for the conjugation to not completely break, resulting in larger delocalization lengths.( 130, 159 ) Just as in semiconductor nanoparticle s, a particle-in-a-box model (1-D for polymers) is used to explain the delocalizatio n of excitons along the polymer b ackbone. Conjugation lengths that are long tend to have lower transition energies and vice versa.( 159 ) Longer conjugation lengths can be due to polymer rigidity which can create small shifts between the absorption and

PAGE 93

93 emission maximums (Stokes shift). Both LPPP ( 116, 166, 174-177 ) and PPE-CO2 exhibit very small Stokes shifts alluding to their rigidity result ing in long conjugation lengths and therefore altering the dynamics and excit on hopping compared to PPE-SO3 -. Quenching PPE-CO2 As mentioned previously, addi tion of a cation such as calci um has been shown to induce various amounts of aggregation in CPE solutions. ( 178, 179 ) A perfect example of this effect is observed when poly (2-methoxy-5-propyloxy sulfonate phenylene vinylene) (MPS-PPV) fluorescence is quenched due to induced aggr egation created by the divalent cation, Ca2+.( 157 ) Dr. Hui Jiang, as a part of Dr. Kirk Schanze s lab in the Department of Chemistry at the University of Florida, has investigated the eff ects that additions of a divalent cation and an electron acceptor quencher, methyl violagen (MV2+) have on the quenching of PPE-CO2 -.( 155 ) Preliminary steady state absorption a nd quenching experiments of PPE-CO2 with MV2+ were conducted by Dr. Jiang and presented here to provide a better unders tanding of the CPE presented in this thesis. Figure 4-3 depicts the Stern-Volmer plots for PPE-CO2 fluorescence quenched by MV2+ in various solutions with increasing Ca2+ concentrations. As seen in previous works, ( 168, 180 ) the quenching efficiency varies depend ing whether the polymer is dissolv ed in methanol or water. In a water solution (closed squares) the quenching is extremely effi cient, requiring less than 1 M MV2+ to quench the fluorescence by 90%. More importa ntly, the superlineararity begins at very low quencher concentrations. On the other hand, if the polymer is dissolved in methanol (open squares), a well-define d induction region in which the slope ( KSV) is almost linear is observed, and the quenching efficiency does not become superl inear until much higher concentrations (~ > 3 M).( 155 )

PAGE 94

94 [MV2+]/ M Increasing [Ca2+]90% Quenching 40 30 20 10I0/I 0 123 45 [MV2+]/ M Increasing [Ca2+]90% Quenching 40 30 20 10I0/I [MV2+]/ M Increasing [Ca2+]90% Quenching 40 30 20 10I0/I [MV2+]/ M Increasing [Ca2+]90% Quenching [MV2+]/ M Increasing [Ca2+]90% Quenching [MV2+]/ M Increasing [Ca2+]90% Quenching [MV2+]/ M Increasing [Ca2+] [MV2+]/ M [MV2+]/ M Increasing [Ca2+]90% Quenching 40 30 20 10I0/I 0 123 45 0 123 45 Figure 4-3. Stern-Vo lmer plot of 10 M 185 PRU PPE-CO2 -. Quenching due to MV2+ in water ( ) and in methanol with 0 M ( ), 2.5 M ( ), 5.0 M ( ), 7.5 M ( ), or 10.0 M () CaCl2. ( 155 ) Addition of various amounts of calcium to meth anol solutions is also shown in Figure 43. As the concentration of calcium is increa sed, the induction regi on is reduced and the superlinear form occurs at smaller quencher amounts.( 155 ) A 1:1 stoichiometr ic ratio of 185PRU PPE-CO2 to calcium results in identical quen ching behavior as if the polymer was dissolved in water. In addition, the absorption was collected for the PPE-CO2 dissolved in methanol in the presence of various MV2+ concentrations. As the amount of quencher increases, the amount of aggregation increases as indicat ed by a redshift in the absorption spectra.( 155 ) This behavior follows suit with other CPE-quencher systems. ( 154, 157, 180 ) The mechanism for the observed amplified quenching can not be determ ined from steady state alone, hence the need for time-resolved measurements.

PAGE 95

95 Experimental Methods Synthesis of Variable Chain Lengths of PPE-CO2 Xiaoyong Zhao, a member of the Schanze group, is responsible for the synthesis and some of the steady state characterization of the various chain lengths of PPE-CO2 investigated within this dissertation. To polymerize a stoichiometric mixture of 2, 5-bis-(dodecyloxy-carbonylmethoxy)-1,4diiodobenzene and 1,4-di-ethynylbenzene a precu rsor route in which a Sonagashira coupling reaction is used to produce a poly(phenylene et hynylene) with a dodecyl ester protecting the carboxyl group. Gel permeation chromatography of the ester precursor polymers showed that the molecular weight (Mn) for the four polymer chain lengths i nvestigated in this dissertation are 5000, 24000, 74000 and 127000 gmol-1 corresponding to average degrees of polymerizations ( Xn) of 8, 35, 108, 185, respectively. The protecte d ester polymer precursor was then hydrolyzed with ( n -Bu)4NOH to provide for the water-soluble conjugated polyelectrolyte PPE-CO2 -. The final polymer product was purified using dialysis against DI water for 4 days. All of the polymers have polydispers ity indices of ~ 2.( 181 ) Photophysical Methods UV-Visible absorption spectra were recorded using a Lamb da 25 spectrophotometer form Perkin Elmer. Steady-state excitation and emi ssion spectra were obtai ned with a Fluorolog-3 spectrofluorometer from Jobin Yvon. A 1-cm sq uare quartz cuvette was used for all spectral measurements. Concentrations varied from 10 to 30 M and were dissolved in spectroscopic grade methanol. Time-resolved anisotropy and fluorescence dyna mics measurements were performed using a femtosecond upconversion apparatus. An optical parametric amplifier (OPA) pumped by a commercial Ti:Sa laser system consisting of a Ti :Sa oscillator (Spectra-P hysics, Tsunami) and a

PAGE 96

96 subsequent Ti:Sa amplifier (Spectra-Physics, Spitfi re) with a repetition rate of 1 kHz is used to produce excitation pulses. More sp ecifically, the output of the Ti:S a amplifier feeds an OPA, and the fourth harmonic of the signal is tuned to 375 nm. The excitati on beams is fed through a prism compressor, yielding an instrument response function of 225 fs. The instrument response function (IRF) is determined by the cross-corre lation of the excitati on and gate pulses. The upconversion setup used for these experi ments is described in detail elsewhere.( 182, 183 ) Briefly, a fraction of the 800 nm Ti:Sa amplifie r that is leftover from the OPA is used as a time delayed gate pulse (30 J/pulse). After excitation, the sample fluorescence is collected using a pair of off-axis parabolic mirrors and focused a nd spatially overlapped with the gate pulse in a nonlinear crystal (0.5 mm BBO), resulting in th e sum frequency of the two electromagnetic fields (Figure 4-4 A). The up-conversion signal has a photon frequency given by: s umgatefluo (4-3) This is also written as: 111 s umgatefluo (4-4) Detection wavelength is chosen by tuning the non linear crystal to a part icular angle. Table 4-1 includes a list of each detection wavelength and crystal angles used for the experiments discussed in this chapte r. The resultant signal is then focu sed into a monochromator, detected with a photomultiplier and the signal is gated with an integrating boxcar. When the gate pulse is temporally and spatially overlapped with the fl uorescence signal, the nonlinear crystal behaves as an optical gate. Therefore, scanning the gate pulse with respect to the excitation pulses enables

PAGE 97

97 this optical gate to integrate different windows of time. The fl uorescence signal is temporally mapped at these varying time delays (Figure 4-4 B). ( 182 ) Figure 4-4. Fluorescence Up-Convers ion Technique A) Illustration of the upconversion principle B) Up-converted fluorescence signal generate d in a nonlinear crystal only while the delayed gate pulse is present. ( 182 ) Table 4-1.Experimental conditions for wavelength dependence study Micrometer PMT Emission (nm)Monochromator (nm) Micrometer Position (filters) Position w spacer 430 450 475 500 515 533 550 590 279.5 287.5 296.5 307.0 313.5 320.0 325.0 340.0 4.23 2.94 1.70 0.45 7.50 6.19 4.95 3.79 2.45 1.86 1.60 1.46 Blind Blind Blind Blind/Vis Vis(1 UG-11) Vis(1 UG-11) Vis(2 UG-11) Vis(2 UG-11) Micrometer PMT (nm)Monochromator(nm) Micrometer Position (filters) Position w spacer 430 450 475 500 515 533 550 590 279.5 287.5 296.5 307.0 313.5 320.0 325.0 340.0 4.23 2.94 1.70 0.45 7.50 6.19 4.95 3.79 2.45 1.86 1.60 1.46 Blind Blind Blind Blind/Vis Vis(1 UG-11) Vis(1 UG-11) Vis(2 UG-11) Vis(2 UG-11) Detected Micrometer PMT Emission (nm)Monochromator (nm) Micrometer Position (filters) Position w spacer 430 450 475 500 515 533 550 590 279.5 287.5 296.5 307.0 313.5 320.0 325.0 340.0 4.23 2.94 1.70 0.45 7.50 6.19 4.95 3.79 2.45 1.86 1.60 1.46 Blind Blind Blind Blind/Vis Vis(1 UG-11) Vis(1 UG-11) Vis(2 UG-11) Vis(2 UG-11) Micrometer PMT (nm)Monochromator(nm) Micrometer Position (filters) Position w spacer 430 450 475 500 515 533 550 590 279.5 287.5 296.5 307.0 313.5 320.0 325.0 340.0 4.23 2.94 1.70 0.45 7.50 6.19 4.95 3.79 2.45 1.86 1.60 1.46 Blind Blind Blind Blind/Vis Vis(1 UG-11) Vis(1 UG-11) Vis(2 UG-11) Vis(2 UG-11) Detected The upconverted fluorescence signal intensity is determined by the convolution of the fluorescence and gate pulse intensities: ()()()sumfluogate I ItItdt (4-5) gate flu sum Luminescence Gate Pulse Up-converted signal Non-linea r Crystal Excitation pulse Luminescence Gate pulse Up-converted signal A) B)

PAGE 98

98 where represents the time delay between the arrival of the gate pulse with respect to the sample fluorescence. This optical gating t echnique is very advantageous because the time resolution is dependent only on the width of the gate and pump pulses, not the detection system.( 182 ) The optical path length was 2 mm and the c oncentration of samples did not exceed 30 M yielding an optical density ~ 0.45/mm. A circul ating cell was used to ensure that a fresh volume of sample was excited with every laser shot and a maxi mum of 100 nJ of energy per shot were used. Anisotropy is the measurement of the extent of polarization that a ma terial maintains after being excited with polarized light. When the em ission anisotropy is nonzero, the emission of the material is polarized. The transition dipole moment (a) of a molecule dictates which orientation or direction molecules will absorb light. Light that is polarized consists of an electric field (E) that oscillates in a particular direction. Excita tion of a material with linearly polarized light results in an excitation probability function that is propor tional to the square of the scalar product of the molecules dipole moment and the electric field vector (a E or cos2 A) (Figure 4-6). The phenomenon of polarized emission is dependent on the absorption and emission transition dipole moments which can be oriented at different angles to one another. When the angle between the two vectors is 90, the excitation probability is zero and maximized if they are parallel. After creating an exciton in a high ener gy electronic state of an anisotr opic material, it relaxes to the first singlet state (Kashas Rule ( 2 )) via internal conversion. Regard less of the orientation of the transition moment of the high ener gy initial state, the emissive transition moment at the first singlet state will remain the same (Figure 45). If the absorption and emission moments are identical, the anisotropy will not be lost; however, if they differ, Figure 4-5, the anisotropy value will change.( 2 )

PAGE 99

99 S 2 S 1 S 0 Absorption Fluorescence S 0 S2S 0 S1 Transition moments S 2 S 1 S 0 Absorption Fluorescence S 2 S 1 S 0 S 2 S 1 S 0 Absorption Fluorescence S 0 S2S 0 S1 Molecules can be excited selectively simply by arranging the electric field vector of the incident light so that its orient ation is relatively similar to thei r dipole moments. This method is referred to as photoselection. For ex ample, if a laser pulse with a polarization set to vertical is used to pump a sample, only the molecules with vertical dipole moments will be excited. Multiple processes can cause depolarization within molecules.( 2 ) These include: Adapted from B.Valeur. (2) Absorption and emission transiti on moments are not parallel twisting vibrations Brownian motion Energy transfer to other molecules with di fferent transition mome nt orientations Molecular rotations Figure 4-5. Transition moments. Adapted from B. Valeur.( 2 ) Anisotropy measurements become very us eful tools for determining information concerning molecular size, shape an d flexibility in addition to the viscosity of the solvent. The fundamental anisotropy ( r0) is the theoretical anisotropy of a material that does not undergo any motion or loss of polarization.( 2 )

PAGE 100

100 No Absorption Maximum Absorption Absorption cos2A ANo Absorption Maximum Absorption Absorption cos2A A 2 023cos1 52 r (4-6) Figure 4-6. Photoselection. Adapted from ( 2 ) For a spherical object, if the absorption and emission transition dipole moments are parallel ( = 0), r0 should equal to 2/5 (0.4); howev er, if they are perpendicular ( = 90) the lower limit is -1/5 (-0.2). These values corre spond to the limiting valu es. If all emission polarization is lost (due to a ny of the processes listed above) a value of zero anisotropy is expected. The temporal behavior of the anisotro py can provide useful information regarding the polarization loss mechanism.( 2 ) Fluorescence anisotropy decay measurements were conducted by rotating excitation pulses with respect to a fixed polarization detection scheme. A Berek compensator is used to excite the molecule with a beam polarized paralle l and perpendicular with respect to the detected fluorescence intensities. The anisotropy value ( r ) was then calculated using: 2 I I r I I (4-7)

PAGE 101

101 The Model 5540 Berek polarization compensator from New Focus was used in these experiments to convert and control the pump polariz ation. A compensator su ch as this utilizes the principal that different wavelengths of light propagate at different speeds through a medium and that this velocity depends on the index of refraction. This compensator can cause a -wave or -wave retardance for wavelengths in the ultr aviolet (200 nm) to the in frared (1600 nm). The compensator has a 12 mm aperture and was direc tly mounted to a post. The Berek compensator is made up of a single birefringent uniaxial plate with an adjustab le tilt angle to impose velocity changes on incident light resulti ng in retardation. The velocity changes are both tilt angle and wavelength dependent. The extraordinary axis, ne, is oriented perpendicular to the plate while the ordinary axis, no, is parallel (Figure 4-7). If no tilt is imposed, the incident light remains normal to the plate. As the light pr opagates through the medium, its velocity remains unaffected by the polarization and is only de pendent on the ordinary index of refr action. If the plate is tilted to a particular angle, R, the velocity of the propagating light is changed. The axis oriented in the plane of incidence is no longer ordinary, in stead it has an extraordinary component, ne, causing retardation. Polarized light that is perpen dicular to the plane of incidence has a velocity unaffected by the tilt. As a result, there is a reta rdance that is created between the ordinary and extraordinary waves propagating in the polarizati on planes. The main advantage of using a Berek compensator for polarization measurements is that it allows for simple and independent adjustments for not only retardati on but also plane of incidence orientation adjustments (which are both wavelength dependent) as one unit. The retardation knob is used to set the tilt angle while the orientation knob acts as a wave plate.( 184, 185 ) Due to group velocity dispersion, the reta rdance of the electric field is wavelength dependent. Therefore, one must set the correct position of the Berek compensator polarization

PAGE 102

102 axis. First, the tilt angle,R is calculated then used to calcu late the Retardation Indicator position (I). From the Berek compensator manual we can derive the relationship between R and the tilt angle. A summary is provide d in this dissertation. Consider a uniaxial crystal with an optical axis parallel to the plate surface. A normal incident beam experiences a retardation (R ) that is dependent on the path length ( d ), wavelength ( ) and the ordinary and extraord inary indices of refraction:( 186 ) 0()ed R nn (4-8) However, if the plate is tilted, the retardation equation becomes:( 186 ) (coscos)eeood Rnn (4-9) The tilt-induced extraordinary index of refraction from Fi gure 4-7 is determined by:( 184, 185 ) 22 '22cossin 1RR eoennn (4-10) The relationship between the optical axis of the medium, tilt angle, angle of incidence and indices of refraction are used to derive the following equation for the retardance:( 184 ) 22 22 221sin 2000 sin1 1sineR oR oRn Rn n (4-11) To use the Berek compensator as a half wave plate ( /2), R is fixed to 0.5. The tilt angle and Retardation Indicator equations are purely empirical and are based on the crystal dimensions only known by New Focus ( 187 ) and the dispersion relations for the indices of refraction determined by Dodge ( 188 ) included in the Berek compensator manual. The indicator versus wavelength graph corresponding to quarter and half wave retardance is included in the manual. ( 184 ) The tilt angle is estimated usin g the following empirical equation:( 184 )

PAGE 103

103 1sin(0.284)R R (4-12) where is the wavelength in micrometers. OnceR is determined, the Retardation indicator setting on the compensator can be calculated from the following empirical relationship:( 184 ) 50.2271sin 4RI (4-13) After the retardation indicator is set, the Orientation (O) positi on must be rotated to the proper position (Figure 4-8). For excitation at 375 nm, th e tilt angle in radians was calculated to be 0.1238 which led to a Retardation Indicator setting (I) of 6.57. To rotate polarizations by 90, the retardance is turned to a -wave setting a nd the orientation positioned at 45 since the /2-wave plate causes rotation of the plane of polarization by twi ce the orientation angle.(184) A polarizer was used to verify the polarization of the beam exiting the compensator. The Berek compensator was set to magic angle conditions to measure isotropic fluorescence decay curves. Magic angle is a set condition that enables de tection of the total fluorescence intensity, not just emission proportional to I or I The emission monochromator depends on polarization; the obser ved signal is not proportional to the total intensity (which is equal to2 I I. In order to achieve the correct ratio, th e excitation is oriented 54.7 from the vertical since the cos2 (54.7) is 0.333 and sin2 (54.7) is 0.667 forming the correct sum for the total intensity. This is especially important for fluorescence decay measurements because the vertical and horizontal signals are usually very distinct due to molecular rotations, energy transfer or some other polarizat ion dependent process and if th eir intensities are not properly weighted then incorrect populati on decay times are recovered.(189) The intensity at magic angle is calculated as: (189)

PAGE 104

104 nonone ne none LIGHT LIGHTR= 0 RNo Tilt Tilted nonone ne none LIGHT LIGHTR= 0 RNo Tilt Tilted I O y xInput: Linearly Polarized Wave Plate Setting: /2 Output: Linearly Polarized 90rotated y x I O y xInput: Linearly Polarized Wave Plate Setting: /2 Output: Linearly Polarized 90rotated y x magic angle2 3 I I I (4-14) Figure 4-7. Berek polarization compensator. Tilting the crystal causes retardance and birefringence. Adapte d from New Focus.(185) Figure 4-8. Berek compensator used as a ha lf-wave plate. I = re tardance indicator, O = orientation. Adapte d from New Focus.(184)

PAGE 105

105 Photophysics of Variable Chain Length PPE-CO2 Polymers Steady State Characterization The steady state photophysics pertaining to va riable chain lengths of water soluble PPECO2 polyelectrolytes has been previously reported.(181) Inhomogeneous br oadening (Figure 49) is exhibited in the absorption spectra due to a distribution of excitation energies resulting from slight variations and superpos itions of absorptions of vari ous segments with different conjugation lengths. In addition, as the length of the polymer ch ain increases, the isolated peak shifts towards the red possibly due to an extension of the conjugation length.(190) The absorption maximums for the 8 PRU and 185 PR U are 404 and 432 nm respectively. Moreover, the shoulder at 432 nm, which is assigned to aggregated species, b ecomes the absorption maximum for the 108 and 185 PRU polymers. Stru ctured vibronic featur es in the emission spectrum are shown in Figure 4-10. The high ener gy emission corresponds to isolated chain emission while the broader, low en ergy emission arises from the a ggregate states (appearing as a shoulder). The emission does not disp lay the similar red sh ift seen in the absorption, instead the fluorescence peak shifts are extremely small and d ecrease as the chain length is extended. The S0 S1 (0-0) transition correspondin g to the 35 PRU is Stokes shifted with respect to the 404 nm absorption by 20 nm. Meanwhile, the 185 PRU disp lays a Stokes shift of only 4 nm between the blue end of the emission (436 nm) and the re d edge of the absorption (432 nm). The 185 PRU sample undergoes self-absorption at 436 nm. A similar behavior has been observed in poly(para)-phenyleneladder-type (LPPP) (116) in which the bridging present within the polymer prevents the phenyl rings to twist, maintaini ng conjugation. The authors claim that the small Stokes shift reflects the rigid ge ometry of the conjugated main chain resulting in reabsorption of the S0 S1 (0-0) transition.(181) PPE-CO2 polymers are geometrically rigid resulting in comparable Stokes shifts to the LPPP polymer As the chain length in creases from 35 to 185

PAGE 106

106 300325350375400425450475500 0.0 0.2 0.4 0.6 0.8 1.0 norm Abs (nm) PRU the weight of the aggregate emission at 520 nm increases due to an increase of the amount of aggregate present in solution. Figure 4-9. Chain length absorption shift for PPE-CO2 in methanol. Polymer repeat units 8 ( ), 35 ( ), 108 (), 185 ( ) Figure 4-10. Emission spectra 10 M PPE-CO2 (methanol) excited at 380 nm for 35 PRU ( ) and 185 PRU ( ). Despite the direct relationship between the increase in molar extinction values to the increase in repeat units, the qua ntum yield for fluorescence decreases from ~ 0.6 (8 PRU) to ~ 400450500550600650700 0.0 0.2 0.4 0.6 0.8 1.0 1.2 Norm PL (nm)

PAGE 107

107 0.1 (108 PRU). Zhao et al. sugge st that conformational, vibra tional and rotational degrees of freedom creating non-radiative decay channe ls lead to decrease fluorescence.(181) On the contrary, it is clear that the large absorption red shifts due primarily to the rigidity of the polymers lead to large emission and absorption spectral overlap. Moreover, the conformational restrictions induced by this rigi dity give rise to conjugation lengths longer than expected ultimately reducing the degrees of freedom within the polymer and thus potentially makes the stated reason an invalid argument to expl ain small quantum yields for long PPE-CO2 chains. Figure 4-11 shows the emission spectrum of the PPE-CO2 (35 PRU). The black line corresponds to the polymer dissolved in methanol and it shows the sharp bands characteristic of isolated chain emission. The red line corre sponds to a methanol solution in which 6 M of Ca2+ has been added. Ca2+ is an effective cross linker with th e 2 carboxyl groups inducing aggregation of the PPE-CO2 -.(155) Emission from the aggregate can be clearly observed on the shoulder at 520 nm, as it grows relative to the unaggregated emission at 436 nm. Finally, when the CPE is dissolved in water, the aggregated emission is mostly observed (green line). Overall, the red shift, quenching and band broadening are due to a ggregate formation of th e polymer chains since Ca2+ is a closed-shell ion and does not act as an electron or energy acceptor. (191-194) The small Stokes shift previously mentione d results in excellent overlap of the isolated chain emission with the aggregate absorption enhancing energy tr ansfer from the higher energy isolated species to the lower energy aggregates. The excitation spectrum of a given chromophore is determined by monitoring the fluorophore emission as it is exci ted at different wavelengths. Abramowitz et al. from the Olympus Microscopy Resource provides an excellent description of the excitation spectra collection process.

PAGE 108

108 An emission wavelength is chosen and onl y emission light at that wavelength is allowed to reach the detector. Excita tion is induced at various excitation wavelengths and the intensity of the emitted fluorescence is measured as a function of wavelength. The result is a graph or curve which depicts the relative fluorescence intensity produced by excitation over the spectrum of excitation wavelengths. (195) Excitation experiments at differe nt detection wavelengths can be employed to identify the species contributing to the emission which can be hidden due to inhomogeneous broadening caused by the polydispersity presen t within polymeric samples. The absorption spectra lead one Figure 4-11. Emission of 10 M 35 PRU PPE-CO2 in methanol (), methanol with ~ 6 M Ca2+ ( ) and in water ( ) to believe that no aggregates ar e present in the shor ter polymer samples since neither a shoulder or broadening are observed; howev er, excitation spectra indicate th at this is not the case. Figure 4-12 presents the excitation spectra of the 8 PR U CPE in methanol det ected a four different wavelengths (430, 475, 510 and 590 nm). Detecti on at 430 (blue line) and 475 nm (green line) show broad, featureless exci tation spectra peaked at 396 nm Upon shifting the detection wavelength to 510 nm the excitati on spectra becomes even broade r and a small red-edge shift 400450500550600650700 0.0 0.2 0.4 0.6 0.8 1.0 1.2 Norm PL (nm)

PAGE 109

109 325350375400425450475500 0.0 0.2 0.4 0.6 0.8 1.0 1.2 a.u.(nm)begins to appear (440 nm). By shifting detection to 590 nm (red line), this red edge shift appears more pronounced and it is due to the aggregate species. From this data, we conclude that even in dilute solutions of CPE with small PRU lengths, some aggregate is present and contributes to the overall energy transfer mechanism due to spectral overlap. A small amount of the short, ri gid chains are likely to stack on top of each other rather than cluster up creating this red edge shift. It is also suggested in Zhaos work that the reduction in quantum yield could be a resu lt of the presence of aggregates which compete with radiative decay channels.(181) Photophysical data presented he re suggest that the presence of aggregates is indeed a more suitable expl anation because the pres ence of isolated and aggregated chains is detected in small, dilute PRU samples in MeOH. Figure 4-12. Excitation spectra 10 M 8 PRU PPE-CO2 -. Detection at 430 ( ), 475 ( ), 510 (), and 590 ( ) nm

PAGE 110

110 325350375400425450475500 0.0 0.2 0.4 0.6 0.8 1.0 1.2 a.u. (nm)For longer PPE-CO2 PRU chains dissolved in methanol the distinction between isolated and aggregate emission bands based on the exc itation spectra becomes clearer. For example, Figure 4-13 shows the excitati on spectra of 35 PRU PPE-CO2 in methanol. Detection at 430 nm (blue line) results in a distinct peak at 390 nm. As the detection wavelength increases, the peak broadens and shifts to the red. Detection at 590 nm (red line) clearly shows a new peak at 430 nm and this peak is attributed to the direct excitation of aggregat ed species. Figure 4-14 presents the excitation spectra of the 35 PRU CPE dissolved in water detected a four different wavelengths (430, 475, 510 and 590 nm). Detection at 430 nm (blue line) show broad, featureless excitation spectra peaked at 390 nm due to is olated chains. As the detection wavelength increases, the peak broadens and shifts to the red. Upon shifting the det ection wavelength to 510 nm (black line) the excitation spectra becomes even broader a nd a new peak appears (436 nm). Detection at 510 (black line) a nd 590 nm (red line) exhibit more well-defined peaks at 436 nm, comparable to the 185 PRU absorption spectrum in neat methanol. Figure 4-13. Excitation spectra 10 M 35 PRU PPE-CO2 in methanol. De tection at 430 ( ), 475 ( ), 510 (), and 590 ( ) nm

PAGE 111

111 325350375400425450475500 0.0 0.2 0.4 0.6 0.8 1.0 1.2 a.u (nm)A similar trend is observed in Figure 4-15. Af ter addition of 60% calcium to the methanol solution, noticeable changes within the excitation spectra (at blue r wavelengths) indicate that the calcium induces more aggregation within the poly mer solution. Contribution from aggregates is clearly evident when det ecting at 475 (green line), 510 nm (b lack line), and 590 nm (red line), although the signal collected at 590 nm does not increase significantly compared to the neat methanol sample. When 35 PRU PPE-CO2 is dissolved in water, it is confirmed that CPEs do exist in water as aggregates, however; isolated chains are still present although their contribution to the overall steady state fluorescence is reduced due to fewer free chains in solution, reabsorption and energy transfer. The excitation sp ectra results presented here provide evidence to support our assumption that multiple specie s contribute to the emission and overall fluorescence decay in even dilute PPE-CO2 samples. Figure 4-14. Excitation spectra 10 M 35 PRU PPE-CO2 in water. Detection at 430 ( ), 475 ( ), 510 (), and 590 ( ) nm

PAGE 112

112 325350375400425450475500 0.0 0.2 0.4 0.6 0.8 1.0 1.2 a.u. (nm)Time-Resolved Fluorescence Isotropic Upconversion The fluorescence dynamics of PPE-CO2 with repeat unit lengths equal to 8, 35, 108 in MeOH were excited at 375 nm and detected at ma gic angle at several different wavelengths. Data was fit with a sum of exponent ials using the following equation: ()expi i it ItA (4-15) whereiA represents the weight of each rate constant and i is the associated time constant. Data from these fits are summarized in Table 4-2. Figure 4-15. Excitation spectra 10 M 35 PRU PPE-CO2 in methanol with ~ 6 M Ca2+. Detection at 430 ( ), 475 ( ), 510 (), and 590 ( ) nm Figure 4-16 shows the time-resolved fluorescence decay of 8 PRU PPE-CO2 in methanol at three different detection wa velengths (430, 436 and 450 nm) ex cited at 375 nm. Detection at 430 nm (blue line) shows a multi-exponential behavior which disappears as the detection

PAGE 113

113 wavelength increases to 450 nm (red line). The isolat ed chain time constant of 531 ps (Table 4-2) is extracted from the mono-expone ntial decay of the 8 PRU at intermediate wavelengths 450 (red line). As the detection wavelength is increased from 450 to 550 nm (not shown) the behavior of the exponential decay does not change. At a ll detection wavelengths the rise times are comparable to our instrument response function. The first panel of Fi gure 4-16 shows the same detection wavelengths on a shorter time scale. An extremely fast decay (< 1.5 ps) is observed Table 4-2. Detection dependence decay times PRU Det (nm) 1 (ps) Amplitude 2 (ps) Amplitude 8 PRU 430 27 36% 490 64% 436 14 29 624 71 450 531 100 35 PRU 430 11 65% 178 35% 450 37 39 363 59 500 33 39 402 61 550 35 53 454 47 108 PRU Det (nm) 430 14 62% 201 38% 450 43 39 395 63 500 42 (fixed) 39 333 62 550 39 55 551 45 35 w 50% Ca Det (nm) 430 18 61% 450 40% 450 43 29 468 71 550 26 53 611 47 when detected at 430 nm (blue line) and its cont ribution to the overall signal diminishes as the wavelength increases from 430 to 436 (magenta line) to 450 nm (red line). Emission at wavelengths below 450 nm spectrally overlaps with the aggregate species absorption resulting in

PAGE 114

114 024 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 Norm PLtime (ps)050100150200 0100200300400500600 efficient energy transfer from is olated chains to aggregates. In terestingly, detection at 590 nm does not yield a significant change in the rise time (not shown) or decay compared to 450 nm which had been expected if detecting emissi on from the aggregates. Dynamics observed at wavelengths between 430 and 450 nm exhibit an a dditional intermediate decay time of 30 to 40 ps (middle panel). The amplitude of these time c onstants decreases as th e wavelength increases. From the excitation spectr a it is clear that there is a small, yet significant amount of aggregate within the sample below 450 nm facilitating ener gy transfer; hence, the intermediate time constant is assigned to the energy transfer from the ensemble isol ated to the aggregated chains. Figure 4-16. Time-resolved fluor escence decay of 8 PRU PPE-CO2 in methanol at three different detection wavelengths. 430 ( ), 436 ( ), and 450 ( ) nm, Fits () The results for solutions of 35 and 108 PRU PPE-CO2 (30 M) are shown in Figure 4-17 for three distinct wavelengths (430, 450 and 550 nm) upon excitation at 375 nm. Detecting at 430 nm, the contribution from the very fast compone nt is larger for the 35 PRU sample (black line) compared to the 108 PRU sample (red line). As the detection wavelength is shifted to the lower energies, contribution from this fast component to the overal l signal is reduced. Interestingly, the changes are more pronounced on the 35 PRU than on the 108 PRU sample.

PAGE 115

115 Detecting at 550 nm, there is no contribution from the fast component to the 35 PRU signal, but it is still present in the 108 PRU signal. The photoluminescence rise follows the response function of the expe rimental setup. At very long wavelengths one would expect to see a build up of population (rise time) due to energy transfer from the isolated states however, the da ta collected do not show this slow rise time.(156, 196) This indicates that the singlet exciton is tr ansferred from shorter (h igh energy) chains to traps within the chains, not to aggregates. The right column in Figure 4-17 shows the intermediate and long decays, observed at diffe rent detection wavele ngths for the 35 and 108 PRU samples. Detection at 430 nm (top graph) pres ents an intermediate decay constant of 11 ps and a long decay of 178 6 ps. For longer detection wavelengths, the intermediate time constant is ~ 30 to 40 ps while the long time decay lies between 350 and 450 ps. Results are summarized in Table 4-2. The qualitative trend ob served for the intermediate component is the same for the 35 and 108 PRU but not 8 PRU. In the 450 to 590 nm detection region, as the wavelength increases, the amplitude of the interm ediate decay increases but the lifetimes do not change significantly. These time-resolved emission signals result from an ensemble of isolated CPE chains with different conjugation lengths. Meanwhile, the slow decay time increases as the wavelength increases. A 350 to 450 ps time consta nt corresponds to the is olated chain natural fluorescence lifetime (extracted from the 8 PRU dete cted at 450 nm); therefore, this component is assigned to isolated chains not participating in the energy transf er process. Energy transfer is not only dependent on spectral overlap of the donor fluorescence (isolated chain) and acceptor absorption (aggregate chain), but also on the molecular distance betw een the two species.(2) After initial energy transfer (<1.5 ps) from short conjugated segments to traps, the energy is then transferred to the aggregate species (intermediat e decay time). The change in amplitudes of the

PAGE 116

116 intermediate decay time depends on the percentage of isolated chains transferring energy to the aggregate species. Figure 4-17. 30 M PPE-CO2 35 PRU () and 108 PRU ( ) with fits ( ) at various detection wavelengths A) 430 nm, B) 450 nm, C) 550 nm Figure 4-18 shows photoluminescence of a solution of 35 PRU PPE-CO2 in methanol detected at 430 nm and the influence of addition of Ca2+. The first panel shows the fast decays, the second panel shows the intermediate decays and the third panel s hows the long decays. Ca2+ greatly influences the dynamics, dominating the u ltrafast decay signal, altering the intermediate decay amplitude and increasing the long decay as the detection wavelength increases. Rise times -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 C) A) B)0.0 0.4 0.8 1.2 1.6 2.0 2.4 Norm PL0.0 0.4 0.8 1.2 1.6 2.0 Norm PL-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 0123450.0 0.4 0.8 1.2 Norm PLtime (s)01002003004005006000.0 0.2 0.4 0.6 0.8 1.0 time (ps)

PAGE 117

117 012345 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 Norm PL050100150200 0.00 0.25 0.50 0.75 1.00 1.25 1.50 time (ps)0100200300400500600 0.00 0.25 0.50 0.75 1.00 1.25 1.50 are IRF-limited at all wavelengths. The augm ented amount of aggreg ation increases the amplitude of the energy transfer components. Jiang determined that Ca2+ induces the formation of loose aggregates in methanol a nd more traps along the polymer backbone.(155) Our results show that the addition of Ca2+ (red line) amplifies the energy tr ansfer from the short isolated chains to traps. Detecting at 430 nm, the long component increases from ~180 to ~ 450 ps while the intermediate decays times remain relatively the same (Table 4-2). Figure 4-18. 30 M PPE-CO2 35 PRU without () and with ( ) Ca2+ at 430 nm. Figure 4-19 presents the eff ect of the addition of Ca2+ to the dilute 8 PRU sample detected at 450 nm upon excitation at 375 nm. The black lin e corresponds to the isolated chain emission and the red line corresponds to the 8 PRU with the addition of 15 M Ca2+. At 450 nm, the addition of Ca2+ introduces a 1.5 ps decay time and an intermediate component (~ 30 to 50 ps) not seen in the neat 8 PRU sample. The radiativ e decay rates measured w ith and without calcium differ due to the presence of aggregate structures in conjugated polymers. As stated previously, the addition of the dication calcium to the PPE-CO2 -/methanol mixture causes aggregation,

PAGE 118

118 bringing the molecules within the Coulombic c oupling transfer radius facilitating extremely efficient energy transfer.(197) Cation-induced aggregation plays a critical role in amplified quenching (154, 155, 157, 168, 172, 180) due to ultrafast energy transfer. In addi tion, the Stern-Volmer behavior of CPEs in the presence of polyvalent quencher ions such as MV2+ and a cation in solution have proven to be superlinear. In fact, due to the loose type of aggregates formed when calcium is added to PPE-CO2solutions, small quenchers, like MV2+ have higher quenching e fficiencies than in other aggregate-inducing solutions, i.e., water. (155) Figure 4-19. 10 M PPE-CO2 8 PRU detected at 450 nm without () and with ( ) Ca2+ We examined the effects on the energy transfer after addition of a quencher molecule to a solution of PPE-CO2 with and without calcium. Figure 4-20 compares the fluorescence dynamics a solution of 30 M 35 PRU PPE-CO2 in methanol excited at 375 nm and detected at 450 nm when 15 M calcium, MV2+, and a mixture of the two are added. The concentration used for MV2+ samples was equivalent to the amount need ed to quench the steady state fluorescence 0100200300400500600 0.0 0.2 0.4 0.6 0.8 1.0 Norm PLtime (ps)

PAGE 119

119 050100150200250300350400450 0.00 0.25 0.50 0.75 1.00 Norm PLtime (ps) by 80% (I/I0). The ultrafast decay that occurs in less than 1.5 ps is present in all samples regardless of the presence of calcium (not show n). No significant changes in the excited state lifetime at long timescales were detected desp ite a reduction in the photo-luminescence quantum yield implying that the most important step in the decay mechanism facilitating amplified quenching occurs in the first 2 ps in PPE-CO2 polymers. Figure 4-20. 30 M PPE-CO2 35 PRU with () Ca2+ at 450 nm 30 M PPE-CO2 35 PRU with MV2+ (80% quenched) at 450 nm and ( ) 30 M PPE-CO2 35 PRU with 15 M Ca2+ and MV2+ (80% quenched) Time-Resolved Anisotropy To investigate the fluorescence depolarization, random walk migration, or intermolecular energy transfer we measured the anisotropy dynamics of the 8 PRU polymers in methanol. Figure 4-21 depicts the time-resolved fluorescence anisotropy of the 8 PRU detected at 430 (black line) and 450 (red line) nm upon excitation at 375 nm. An ultrafast loss of anisotropy (not shown) followed by a constant anisotropy duri ng the lifetime emission of the polymer is

PAGE 120

120 0100200300400500600 -0.2 -0.1 0.0 0.1 0.2 0.3 0.4 rtime (ps)observed. After the initial change in the first 5 ps from r ~ 0.4 to 0.2, both curves then remain parallel to one another. Figure 4-21. Anisotr opy of 8 PRU PPE-CO2 -. Detection wavelengths 430 () and 450 ( ) nm Detection at 430 nm occurs in a region in which shorter conjugati on lengths are present allowing for a higher number of hops before finding traps along the polymer backbone resulting in smaller anisotropy value (r~ 0.10) compar ed to the 450 nm (r~ 0.18). The 450 nm scan corresponds to slightly longer conjugation and fewer hops As shown in other works (130, 197) a long polarization decay corresponds to aggregat es emitting almost randomly polarized light reducing the total polarization. However, in this po lymer there is little to no depolarization due to reorientation at 450 nm. Random wa lk migration is considered to occur at intermediate decay times in PPE-SO3 -, (130) but is not observed in PPE-CO2 -. If random walk of excitations along the polymer backbone were to occur, as th e wavelength increased the hopping rate would decrease due to a lack of lower lying states th at are available for the excitation to jump. The following factors clearly eliminate the possibility of random exciton migr ation in this PPE-CO2 -:

PAGE 121

121 rigidity of the polymer, longer conjugation length, spectral over lap and proximity leading to energy transfer from isolated to aggregated ch ains, no detection wavelength dependence for the decays measured at various wavelengths and no depolarization at intermediate decay times even in the presence of aggregates. Potential Kinetic Model The scheme presented in Fi gure 4-22 depicts a proposed model concerning the dynamics mechanism in PPE-CO2 -. This figure presents the model for the different polymer repeat units collectively. The 8 PRU consists of mostly isolated chains. Afte r excitation at 375 nm, energy is transferred from the shorter conjugation lengths to traps located in the isol ated chains. Due to the spectral overlap between the isol ated chains emission and aggregate absorption, this energy transfer is detected primarily at wavelengths shorter than 450 nm. These detected emission signals are convoluted with decays from an ensemble of differen t conjugation lengths, kinks and other non-radiative recomb ination pathways within the isolat ed species. From the excitation spectra of the 8 PRU (Figure 4-12) we can see th at the red edge shift at 440 nm does not overlap significantly with wavelengths grea ter than 450 nm. Energy transfer from the isolated chains to the aggregates is not efficien t resulting in a mono-exponential fluorescence lifetime from an ensemble of isolated chains. Longer polymer chains (35, 108, 185) contain a larger mix of isolated and aggregated chains. After excitation at 375 nm, energy is tran sferred from the shorte r conjugation lengths to traps located in the isolated chains similar to th e 8 PRU. Subsequently, energy transfer from this ensemble to the aggregate species is observed wi thin a wide spectral range. The number of traps is increased upon addition of cal cium resulting in a sharp incr ease in the ultrafast time component but the energy transfer to the aggregate does not change significantly. Self-absorption is observed in the 108 and 185 polymer repeat unit samples (more aggregated); however, this is a

PAGE 122

122 radiative energy transfer process in which the photon emitted by the donor is then absorbed by the acceptor. Therefore, it does not compete w ith other decay mechanisms and the fluorescence decay time of the donor remains unchanged (refer to Chapter 1). Following the energy transfer, the detected emission is dominated by the ensemb le of decays from isolated chains and traps located along the polymer backbone in addition to competing with non-radiative decay channels. In summary, upon excitation of the aggregates fro m energetically higher lying isolated chains, the fluorescence lifetimes result in multi-exponen tial behavior due to the competition between the radiative and non-radiative decay. The in tegrated fluorescence collected for these experiments does show emission fr om aggregates but it cannot be observed in the ultrafast timeresolved experiments because of very long decay time constants and small contribution to the overall signal. Summary The ultrafast time-resolved fluor escence of a series of PPE-CO2 polymer repeat units is presented. Using steady stat e UV-Vis, photoluminescence a nd excitation resources we distinguished the species present in each soluti on. It was shown that even dilute, short PRU chains do exhibit a small amount of aggregation. The addition of calcium or using water as the solvent induces aggregation result ing in broad absorption/excitati on spectra and the growth of a red shoulder in the emission. To investigate th e influence aggregation has on the fluorescence of the polymers, we conducted a detection wavele ngth study using fluorescence upconversion. The isolated chain emission was extracted from th e 8 PRU at 450 nm (isolated chain emission and aggregate absorption is minimal). In the presence of aggregates, an intermediate time constant on the order of 30 to 40 ps is observed and is assigne d to the energy transfer from the isolated to aggregate species. At bluer wavelengths, a fast de cay (< 1.5 ps) is observed and is attributed to the transfer of excitation from shorter, high ener gy chains to longer, low energy chains and traps.

PAGE 123

123 Time-resolved anisotropy confirmed that this polymer, no matter the PRU size, is extremely rigid and has long c onjugation lengths. 375 nm Isolated Aggregate NR Isolated/traps1 < 1.5 ps 2 ~ 30 to 40 ps 3 ~ 350 to 450 ps 375 nm Isolated Aggregate NR Isolated/traps 375 nm Isolated Aggregate NR Isolated/traps1 < 1.5 ps 2 ~ 30 to 40 ps 3 ~ 350 to 450 ps Figure 4-22. Possible kinetic model for all PPE-CO2 PRU chains

PAGE 124

124 CHAPTER 5 CONCLUSIONS AND FUTURE WORK Nanoparticle Conclusions and Future Work Conclusions A complete steady state and time-resolv ed study of size, sh ape passivation and composition dependence on colloidal semiconducto r nanoparticles has been conducted in our labs. Using pump-probe spectroscopy we were ab le to detect traps and interfacial states. Confirmation of Auger-like cooling resulting in 1P to 1S relaxation (~ 1 ps) has been shown in addition to interband relaxation (> 200 ps) have been measured for each nanoparticle system. Comparisons between materials with differe nt compositions were made finding higher confinement potential in ZnCdSe alloys than in CdSe resulting in a lower probability of the exciton to sample the surface and be trapped. Finally, utilizing each of the spectroscopic tools available we were able to combine steady state and time-resolved data to construct qualitative models describing the nanorod systems. From this work we can conclude that passi vation and alloying resu lt in quantum yields higher than for bare CdSe. The excitons are more confined in the alloy particles than in CdSe rods. It is well known that the band gap is size (17) and shape dependent.(53) Despite breaking the symmetry within the nanopart icle, confinement properties were maintained in our samples. Passivation with an inorganic sh ell results in increased quant um yields and bleach signals because the surface traps are eliminated. Finally, modifying the composition using one additional synthetic alloying step has grea tly improved the process-ability without drastically sacrificing confinement characteristics.

PAGE 125

125 Outlook/Future Work Lasing/ Optical gain A population inversion in lasing me dia, in which the population of electrons in the excited state mu st be greater than the populati on left in the ground state, is necessary to achieve optical gain It is clear that the size of these nanocrystals leads to an enhancement of the carrier-carri er interactions leading to impressive optical properties in systems with single and multiexciton states. It has been shown that new energy relaxation pathways are created in nanocrystals compared to the bulk. For example, due to the high density of states present in bulk mate rials, there is a la ck of a phonon bottleneck which is also bypassed in nanocrystals because of ultraf ast non-radiative Auger recombin ation. Therefore, to attain inversion within nanocryst als, a simultaneous exc itation of the two electr ons in the ground state to an excited state must occur resulting in emi ssion of multi-excitons via an Auger process which dictates the decay of optical gain. (122, 198, 199) Recently, Klimov has investigated the mechan isms for photogeneration and recombination of multi-excitons in nanocrystals necessary fo r lasing and solar energy conversion. However, since this recombination occurs in less than 1 ps, the optical gain lifetimes are in the picosecond regime which is a drawback when designing materi als for lasing. To improve optical gain it is important to develop new materi als that inherently diminish this phenomenon. It has been suggested that increasing the nanoc rystal volume fracti on (packing density) in the optical gain medium or using quantum rods instead of spheres will help reduce the influence that Auger recombination has on these systems.(199) Klimov has been using quantum rods to try to achieve population i nversion for optical gain. In order to do this, the Auger effects mu st be significantly reduced. He showed that especially for CdSe based nanocry stal, the rods have slower A uger rates compared to quantum dots with the same volume that emit in the red and orange spectral region.(199) It is hoped at

PAGE 126

126 particular emission wavelengths, the Auger decay can be stifled in rodshaped nanocrystals due to not only the dependence that th e confinement potential has on th e length and size but also the linear scaling of the deca y time with rod volume.(52, 199, 200) For higher energy (shorter wavelengths) quantum dots, the increased surfac e-to-volume ratio inhibits the Auger decay suppression. Elongation of the na nocrystal in the c-direction has successfully increased the optical gain lifetime since the effect that Auge r has on the recombination behavior in rods is decreased.(199, 201) CdSe/ZnSe core/shell materials have been thought to be used for this application but it is more suitable to use inverted ZnSe/CdSe heterostructures in order to control the electron-hole wavefunction overlap to increas e the confinement energies and reduce Auger recombination.(199, 202, 203) Formulation of new alloy nanorods has opened the door for new investigations for their potential applications. Extensi on of the c-axis in these tern ary materials enables for higher confinement potential in the blue -green region. It may be worth trying to determine how to alloy the materials and then coat them to increas e their photoluminescence quantum yield to make them comparable to the core/shell or shell/core materials. These new alloy/shell materials can be tunable based on the diffusion and have extended lifetimes necessary for charge-separated or optical gain applications. Light emitting diodes (LEDs) (11, 204-206). Although several improvements using organic molecules for organic light emitting diodes (OLEDs) make them comparable to current technologies, there are ongoing drawbacks and pr oblems that must be overcome before these devices can be commercially utilized. These incl ude: a) difficulties tuning the colors since the fluorescence is broad and b) synthesis of multiple molecules is required to obtain a broad range of colors. Nanocrystals are bei ng considered as attractive candida tes to be used for LEDs since

PAGE 127

127 their emission is not only tunabl e but considerably narrower than that from organic materials. Moreover, nanocrystals have a higher proba bility of resisti ng photodegradation.(66) Hybrid OLEDs have been developed in the past fifteen years, incorporating a polymer such as PEDOT (207) or PPV (204) to transport charge to various nanocrystals (CdSe (204), CdSe/CdS (11), CdSe/ZnS (208)) that act as the emission layer resulting in more stable and efficient devices. Development of better materials and manipulating their interactions are the main goals when working towards designing produc ts that result in hi gh electroluminescence efficiencies. To achieve commer cial quality devices the functionality must be improved by enhancing the charge transfer between the pol ymers to the nanocrystal emission layer and increasing the surface quality so the traps which cause non-radiative recombination are reduced.(66) Experimentation with different combinati ons of polymer/nanoparticle blends is a standard methodology to find devices that get ri d of such adverse consequences. Within the literature, most nanoparticles are spheres and th eir band gaps are tuned by only changing their diameter.(20, 39, 48, 75, 78, 136, 138, 209-212) As the diameters are decreased, the band gap energy does increase and emission in the blue-gre en region is achieved, however, the surface-tovolume ratio is significantly increased which can lead to more surface traps. Therefore, some investigations into hybrid LEDs s hould incorporate not only size dist ributions to obtain tunability but to investigate how the shape, passivation thickness and composition will affect the overall efficiencies of the devices. A wide range of colors in the bl ue-green region can be achieved simply by altering the alloying times in ZnCdSe nanorods. If a simple technique was developed to passivate these alloy rods to reduce surface traps, they would allow for blue-green emission wavelengths via a simple s ynthetic route (one batch).

PAGE 128

128 Photovoltaics (213, 214) Although the cost of making quant um dot based photovolatics is small, the efficiencies, due to recombination loses, are still too low for them to be used on a large scale. Hybrid photovoltaic devices are integrated within the polymers to transport charge for such applications as so lar cells. Achieving charge separati on and positive transport of the hole and electron to the indium tin oxide (ITO) and aluminum electrode respectively is the main goal in photovoltaics.(66, 214) Instead of focusing on a binary system, a device utilizing ternary compositions with varying degrees of Zn diffus ed into the core, may serve as more suitable materials for photovoltaics. The variable Zn diffusi on will create a gradient from the core to the surface enabling the exciton to hop from one rod to another ultimately reaching the aluminum electrode. Also, work might be directed to achie ve charge separation w ithin the nanocrystals. The fact that the hole created after excitation within our CdSe /ZnSe quantum rods potentially tunnels into the shell could help sustain charge separation and i nhibit premature charge carrier recombination. Investigations into the kinetics and mechanis ms for creating and maintaining charge separation in these materials are recomm ended since these processes are not completely understood; however, it has been shown that the interface between nanocrystals and porous TiO2 supports highly efficien t charge separation.(132, 199, 202) Carrier multiplication The carrier-carrier interactions in nanoparticles lead to improved exciton (carrier) multiplication (CM) which resu lts from direct photogeneration of multiple electron-hole pairs by single photon s. This process is relatively new and the actual mechanism behind carrier multiplication (if it truly exists) from a single excitation are currently being debated. Klimov showed that seven excitons are produced in PbSe nanocrystals (QE = 700% where 100% means 1 photon creates 1 e-h pa ir). This unique finding will be good for photovoltaic cells and improve solar fuel technol ogies in the IR region. However, CdSe dots

PAGE 129

129 have not been able to exhibit as efficien t CM efficiencies in the visible region.(199) Investigations into the e ffect that shape has on this phenomenon is recommended. PPE-CO2 Conclusions and Future Work Conclusions The synthesis, characterization and time-reso lved measurements c onducted on this series of PPE-CO2 polymers with different chai n lengths and extent of aggr egation have led to several interesting observations and conclusions. A co mplete steady state and time-resolved study of length, solvent and aggregated inducer has been conducted in our labs. In particular, isotropic and anisotropic fluorescence upconversion was utilized to u nderstand the quenching mechanism within PPE-CO2 -. We conducted a wavelength detection st udy based on exciting on the blue side (isolated chain) of the absorpti on. Upon excitation of the aggreg ates from energetically higher lying isolated chains, the fluorescence lifetimes result in multi-exponential behavior due to the competition between multiple decay pathways. (116, 153, 156, 165, 215) In all samples, the emission is inhomogeneous ly broadened. Detection of time-resolved signals at all wavelengths is associated with the isolated chains emission, despite the superposition of both species present within the samples. In addition, unlike PPE-SO3 -, PPECO2 is very rigid resulting in l onger conjugation lengths than 4.5 PRU. This was determined not by evaluating the behavior of the polymer chains as a function of chain length using steady-state absorption but by evaluating the extremely slow time-resolved anisotropy decay. From the excitation spectra collected for each polymer chain, it is seen that even a dilute PPE-CO2 with only 8 polymer repeat units exhibits a slight amo unt of aggregation when detected at very red wavelengths. Moreover, we conclude that the energy transfer from isolated to aggregated chains is extremely fast, occurring in 30 to 40 ps fo r all samples. This component was significantly enhanced when a poor solvent (water) or an aggregate inducer was us ed. An even faster

PAGE 130

130 component (1.5 ps) was also observed and it was assigned to the transfer from shorter (high energy) isolated chains to l onger chains and traps within th e isolated chain backbone. Outlook/ Future Work (Hyperbranched PPE-CO2 -) Within our collaboration with the Schanze a nd Reynolds groups, we have an opportunity to investigate excitation and rela xation mechanisms for materials in solutions and in films using both fluorescence up-conversion and broad band transient absorption techniques. Xiaoyong Zhao, a student in the Scha nze group, has synthesized a new hyperbranched PPE-CO2 in which there are three carboxylate side chains attached on each side of the polymer backbone. Based on excitation spectra data presented in Chapter 4, even dilute solutions of the linear 8 polymer repeat unit displays some sort of aggregati on within the sample. This new, hyperbranched polymer is said to have no aggreg ation present even if dissolved in water. Figures 5-1 show the absorption spectra of this new polymer when dissol ved in different solvents and compared to the linear 8 PRU (data collected by Xiaoyong Zhao). From this figure, it is nece ssary to look into the excitation spectra of the hyperbra nched polymer to compare to the dilute 8 PRU. The small shoulder present in Figure 5-1 on the red side of the absorption is a small indication that there may still be some aggregates pres ent within this hyperbranched sample. However, this shoulder could simply be an artifact of the size and re peat unit distributions pr esent within polymeric samples. The only way to be sure is to collect excitation spectra at vari ous wavelengths and look for red shifts that are characteristic of aggreg ate species. The polymer dissolved in water does not alter the absorption spectra significantly. However, photoluminescence is quenched by nearly half of its original intensity when dissolved in methanol (Figure 5-2). Th is is another indication that the polymer could have some aggregate pr esent despite the lack of structure in the absorption spectrum.

PAGE 131

131 Figure 5-1. Absorption spectra of the hyperbranched PPE-CO2 in MeOH () and water ( ) and linear PPE-CO2 8 PRU in MeOH ( ). If a complete understanding is necessary for these linear and hyperbranched polymers, it is necessary to carry out si milar methods of experimentation conducte d in this dissertation. First, it is imperative to make sure that the excitation spec tra either does or does not indicate the presence of aggregates. Second, a time-resolved detec tion wavelength dependence would be useful to elucidate the dynamics of the system under variou s conditions. This data can then be compared to the linear polymers. Once a comparison is made further experiments can be designed to see which polymer would be better for quenching and how calcium will affect the quenching efficiency. Understanding these conjugated poly electrolytes will help with designing new materials for multiple applications including sola r cells, LEDs and even chemoand biosensors. The hyperbranched CPE avenue is a new and exc iting field and I recommend more time-resolved experimentation be accomplished in this area. 325350375400425450475500 0.0 0.2 0.4 0.6 0.8 1.0 Norm Abs (nm)

PAGE 132

132 Figure 5-2. Photoluminescence sp ectra of hyperbranched PPE-CO2 in MeOH () and water ( ) 400450500550600650700 0 1x1052x1053x1054x1055x1056x1057x105 PL(nm) Methanol Water

PAGE 133

133 LIST OF REFERENCES (1). N. J. Turro, Modern Molecular Photochemistry (Univ Science Books, New York, 1991). (2). B. Valeur, Molecular Fluorescence Principles and Applications (Wiley-VCH, Weinheim, 2002). (3). G. S. H. Singhal, Janos; Rabinowitch, Eugene., Excitation-energy migration between chlorophyll and b-carotene, Journal of Chemical Physics (1968) 49, 5206. (4). Govindjee, Excitation Energy Transfer and Ener gy Migration : Some Basics and Background, http://www.life.uiuc.edu/govindj ee/biochem494/foerster.htm Online Class Notes (5). E. G. Rabinowitch, Photosynthesis (John Wiley & Sons, Inc., New York, 1969). (6). G. D. Joly, L. Geiger, S. E. Kooi, T. M. Swager, Highly effective water-soluble fluorescence quenchers of conj ugated polymer thin films in aqueous environments, Macromolecules (2006) 39, 7175. (7). J. H. Wosnick, C. M. Mello, T. M. Swager, Synthesis and application of poly(phenylene ethynylene)s for bioconj ugation: A conjugated polymer-based fluorogenic probe for proteases, Journal of the American Chemical Society (2005) 127, 3400. (8). T. Kippeny, L. A. Swafford, S. J. Ro senthal, Semiconductor na nocrystals: A powerful visual aid for introducing the particle in a box, Journal of Chemical Education (2002) 79, 1094. (9). A. Hagfeldt, M. Gratze l, Light-induced redox reactions in nanocrystalline systems, Chemical Reviews (1995) 95, 49. (10). V. I. Klimov, D. W. McBranch, C. A. Leatherdale, M. G. Bawendi, Electron and hole relaxation pathways in semiconductor quantum dots, Physical Review B (1999) 60, 13740. (11). M. C. Schlamp, X. G. Peng, A. P. A livisatos, Improved efficiencies in light emitting diodes made with CdSe(CdS) core/shell type nanocrystals and a semiconducting polymer, Journal of Applied Physics (1997) 82, 5837. (12). Y. Wang, N. Herron, Nanometer-sized se miconductor clusters Materials synthesis, quantum size effects, an d photophysical properties, Journal of Physical Chemistry (1991) 95, 525.

PAGE 134

134 (13). M. A. El-Sayed, Small is different: Shape-, size-, and composition-dependent properties of some colloidal semiconductor nanocrystals, Accounts of Chemical Research (2004) 37, 326. (14). M. A. Fox, M. T. Dula y, Heterogeneous photocatalysis, Chemical Reviews (1993) 93, 341. (15). L. N. Lewis, Chemical catalysis by colloids and clusters, Chemical Reviews (1993) 93, 2693. (16). J. Aldana, Y.A. Wang, and X.G. Peng, Photochemical instability of CdSe nanocrystals coated by hydrophilic thiols, Journal of the American Chemical Society (2001) 123, 8844. (17). V. I. Klimov, Optical nonlinearities and ultrafast car rier dynamics in semiconductor nanocrystals, Journal of Physical Chemistry B (2000) 104, 6112. (18). V. I. Klimov, D. W. McBranch, Femtos econd 1P-to-1S electron re laxation in strongly confined semiconductor nanocrystals, Physical Review Letters (1998) 80, 4028. (19). N. Le Thomas, E. Herz, O. Schops, U. Woggon, M. V. Artemyev, Exciton fine structure in single CdSe nanorods, Physical Review Letters (2005) 94, 016803. (20). C. A. Leatherdale, W. K. Woo, F. V. Mikulec, M. G. Bawendi, On the absorption cross section of CdSe nanocrystal quantum dots, Journal of Physical Chemistry B (2002) 106, 7619. (21). M. L. Steigerwald, L. E. Brus, Se miconductor crystallites A class of large molecules, Accounts of Chemical Research (1990) 23, 183. (22). A. P. Alivisatos, Perspectives on the physical chemistry of semiconductor nanocrystals, Journal of Physical Chemistry (1996) 100, 13226. (23). A. L. Efros, A. L. Efros, Interband absorption of light in a semiconductor sphere, Soviet Physics Semiconductors-Ussr (1982) 16, 772. (24). A. L. Efros, M. Rosen, The electron ic structure of semiconductor nanocrystals, Annual Review of Materials Science (2000) 30, 475. (25). A. I. Ekimov, F. Hache, M. C. Scha nne-Klein, D. Ricard, C. Flytzanis, I. A. Kudryavtsev, T. V. Yazeva, A. V. Rodina, and Al. L. Efros, Absorption and intensitydependent photoluminescence measurements on CdSe quantum dotsAssingment of the 1st electronic-transitions, Journal of the Optical Society of America B-Optical Physics (1993) 10, 100. (26). L. E. Brus, Electron el ectron and electron-hole interactions in small semiconductor crystallitesThe size dependence of th e lowest excited electronic state Journal of Chemical Physics (1984) 80, 4403.

PAGE 135

135 (27). N. Chestnoy, R. Hull, L. E. Brus, Higher excited electronic states in clusters of ZnSe, CdSe, and ZnS spin-orbit, vibronic, and relaxation phenomena Journal of Chemical Physics (1986) 85, 2237. (28). Y. Kayanuma, Quantum-size effects of interacting elec trons and holes in semiconductor microcrystals with spherical shape, Physical Review B (1988) 38, 9797. (29). M. Nirmal, L. Brus, Luminescence photophysics in semiconductor nanocrystals, Accounts of Chemical Research (1999) 32, 407. (30). A. L. Efros, F. G. Pikus, V. G. Burnett, Density of states of a 2-dimensional electrongas in a long-range random potential, Physical Review B (1993) 47, 2233. (31). A. J. Nozik, Spectroscopy and hot electron relaxation dynamics in semiconductor quantum wells and quantum dots, Annual Review of Physical Chemistry (2001) 52, 193. (32). X. G. Peng, L. Manna, W. D. Yang, J. Wickam, A.Kadavanich, A.P. Alivisatos, Shape control of CdSe nanocrystals, Nature (2000) 404, 59. (33). L. Manna, E. C. Scher, A. P. Alivisat os, Synthesis of soluble and processable rod-, arrow-, teardrop-, and tetrapod-shaped CdSe nanocrystals, Journal of the American Chemical Society (2000) 122, 12700. (34). L. Manna, E. C. Scher, L. S. Li, A. P. Alivisatos, Epitaxial growth and photochemical annealing of graded CdS/ZnS sh ells on colloidal CdSe nanorods, Journal of the American Chemical Society (2002) 124, 7136. (35). C. D. Donega, S. G. Hickey, S. F. Wuister, D. Vanmaekelbergh, A. Meijerink, Single-step synthesis to control the photoluminescen ce quantum yield and size dispersion of CdSe nanocrystals, Journal of Physical Chemistry B (2003) 107, 489. (36). L. A. Swafford, Homogeneously Alloyed CdSxSe1-x Nanocrystals: Synthesis, Characterization, and Compos ition/Size-Dependent Band Gap, Journal of the American Chemical Society (2006). (37). C. B. Murray, D. J. Norris, M. G. Ba wendi, Synthesis and Char acterization of Nearly Monodisperse Cde (E = S, Se, Te) Semiconductor Nanocrystallites, Journal of the American Chemical Society (1993) 115, 8706. (38). T. Mokari, U. Banin, S ynthesis and properties of Cd Se/ZnS core/shell nanorods, Chemistry of Materials (2003) 15, 3955. (39). B. O. Dabbousi, J. Rodr iguezViejo, F.V. Mikulec, J.R. Heine, H. Mattoussi, R. Ober, K.F. Jensen, M.G. Bawendi, (CdSe)ZnS core-shell quantum dots: Synthesis and characterization of a size series of highly luminescent nanocrystallites, Journal of Physical Chemistry B (1997) 101, 9463.

PAGE 136

136 (40). M. A. Hines, P. Guyot-Sionnest, Sy nthesis and characte rization of strongly luminescing ZnS-Capped CdSe nanocrystals, Journal of Physical Chemistry (1996) 100, 468. (41). Y. W. Cao, U. Banin, Synthesis and characterizati on of InAs/InP and InAs/CdSe core/shell nanocrystals, Angewandte Chemie-International Edition (1999) 38, 3692. (42). H. Lee, L. M. Hardison, V.D. Kleima n, P.H. Holloway, H. Yang, Synthesis and characterization of colloidal tern ary ZnCdSe semiconductor nanorods, Journal of Chemical Physics (2006) 125, 029901. (43). M. Dovrat, Y. Goshen, J. Jedrzejewski I. Balberg, A. Sa'ar, Radiative versus nonradiative decay processes in silicon nanocrystals probed by time-resolved photoluminescence spectroscopy, Physical Review B (2004) 69 155311. (44). X. G. Peng, Mechanisms for the Shap e-Control and Shape-E volution of Colloidal Semiconductor Nanocrystals Chem. Euro J (2002) 8, 334. (45). Z. A. Peng, X. G. Peng, Formation of high-quality Cd Te, CdSe, and CdS nanocrystals using CdO as precursor, Journal of the American Chemical Society (2001) 123, 183. (46). Z. A. Peng, X. G. Peng, Nearly monodisperse and shape-controlled CdSe nanocrystals via alternative r outes: Nucleation and growth, Journal of the American Chemical Society (2002) 124, 3343. (47). L. H. Qu, Z. A. Peng, X. G. Peng, Alternative routes toward high quality CdSe nanocrystals, Nano Letters (2001) 1, 333. (48). X. H. Zhong, M. Y. Han, Z. L. Dong, T. J. White, W. Knoll, Composition-tunable ZnxCd1-xSe nanocrystals with high luminescence and stability, Journal of the American Chemical Society (2003) 125, 8589. (49). X. Chen, A. Nazzal, D. Goorsk ey, M. Xiao, Z.A. Peng, X.G. Peng., Polarization spectroscopy of single CdSe quantum rods, Physical Review B (2001) 64, 245304. (50). J. T. Hu, L.S. Li, W. D. Yang, L. Manna, L.W. Wang, A.P.Alivisatos, Linearly polarized emission from colloid al semiconductor quantum rods, Science (2001) 292, 2060. (51). L. Manna, E. C. Scher, A. P. Alivis atos, Shape control of colloidal semiconductor nanocrystals, Journal of Cluster Science (2002) 13, 521. (52). L. S. Li, J. T. Hu, W. D. Yang, A. P. Alivisatos, Band gap variation of sizeand shape-controlled colloidal CdSe quantum rods, Nano Letters (2001) 1, 349. (53). M. B. Mohamed, C. Burda, M. A. El -Sayed, Shape dependent ultrafast relaxation dynamics of CdSe nanocrystal s: Nanorods vs nanodots, Nano Letters (2001) 1, 589.

PAGE 137

137 (54). L. Brus, Electronic wa ve-functions in semiconductor clusters Experiment and theory, Journal of Physical Chemistry (1986) 90, 2555. (55). G. Cantele, D. Ninno, G. Iadonisi, Conf ined states in ellipsoidal quantum dots, Journal of Physics-Condensed Matter (2000) 12, 9019. (56). G. Cantele, D. Ninno, G. Iadonisi, Calcul ation of the infrared optical transitions in semiconductor ellipsoidal quantum dots, Nano Letters (2001) 1, 121. (57). Y. Kayanuma, Wannier excitons in low-dimensiona l microstructures Shape dependence of the quantum size effect, Physical Review B (1991) 44, 13085. (58). S. Legoff, B. Stebe, Influence of longit udinal and lateral confinements on excitons in cylindrical quantum dots of semiconductors, Physical Review B (1993) 47, 1383. (59). D. Ninno, G. Iadonisi, F. Buonocore, Carrier localization and photoluminescence in porous silicon, Solid State Communications (1999) 112, 521. (60). A. D. Yoffe, Low-dimensional syst ems Quantum-size effects and electronicproperties of semiconductor microcrystallite s (zero-dimensional systems) and some quasi-2-dimensional systems, Advances in Physics (1993) 42, 173. (61). C. E. Tyner, Application of solar th ermal technology to the destruction of hazardous wastes, Solar Energy Materials (1990) 21, 113. (62). K. Hashizume, M. Vacha, T. Tani, Prep aration and optical prope rties of capped-CdSe nanocrystals, Journal of Luminescence (2000) 87-9, 402. (63). L. Manna, L. W. Wang, R. Cingolani, A. P. Alivisatos, First-principles modeling of unpassivated and surfactant-passivated bulk f acets of wurtzite CdSe. A model system for studying the anisotropic grow th of CdSe nanocrystals, Journal of Physical Chemistry B (2005) 109, 6183. (64). D. J. Norris, A. Sacra, C. B. Murra y, M. G. Bawendi, Measurement of the sizedependent hole spectrum in CdSe quantum dots, Physical Review Letters (1994) 72, 2612. (65). X. G. Peng, M. C. Schlam p, A. V. Kadavanich, A. P. Alivisatos, Epitaxial growth of highly luminescent CdSe/CdS core/shell na nocrystals with photostability and electronic accessibility, Journal of the Americ an Chemical Society (1997) 119, 7019. (66). H. Lee, University of Florida (2005). (67). D. F. Underwood, T. Kippeny, S. J. Ro senthal, Ultrafast carri er dynamics in CdSe nanocrystals determined by femtosecond fluorescence upconversion spectroscopy, Journal of Physical Chemistry B (2001) 105, 436.

PAGE 138

138 (68). T. W. Roberti, N. J. Cherepy, J. Z. Zhang, Nature of the power-dependent ultrafast relaxation process of photoexcited charge carriers in II-VI semiconductor quantum dots: Effects of particle size, su rface, and electronic structure, Journal of Chemical Physics (1998) 108, 2143. (69). B. Oregan, M. Gratzel, A low-cost, high-efficiency sola r-cell based on dye-sensitized colloidal TiO2 films, Nature (1991) 353, 737. (70). D. V. Talapin, A. L. Rogach, A. Kornowski, M. Haase, H. Weller, Highly luminescent monodisperse CdSe and CdSe /ZnS nanocrystals synthesized in a hexadecylamine-trioctylphosphine oxide-trioctylphospine mixture, Nano Letters (2001) 1, 207. (71). J. E. B. Katari, V.L. Colvin, and A. P. Alivisatos, X-Ray P hotoelectron-Spectroscopy of CdSe nanocrystals with applications to studies of the nanocrystal surface, Physical Chemistry (1994) 98, 4109. (72). A. Creti, M. Anni, M.Z. Rossi, G. Lanzani, G. Leo, F. Della Sala, L. Manna, M. Lomascolo., Ultrafast carrier dynamics in core and core/shell CdSe quantum rods: Role of the surface and interface defects, Physical Review B (2005) 72, 125346. (73). P. Reiss, J. Bleuse, A. Pron, Highly luminescent CdSe /ZnSe core/shell nanocrystals of low size dispersion, Nano Letters (2002) 2, 781. (74). Y. W. Cao, U. Banin, Growth and properties of semic onductor core/shell nanocrystals with InAs cores, Journal of the American Chemical Society (2000) 122, 9692. (75). J. Bleuse, S. Carayon, P. Reiss, Optical properties of core/multishell CdSe/Zn(S,Se) nanocrystals, Physica E-Low-Dimensional Systems & Nanostructures (2004) 21, 331. (76). D. C. Pan, Q. Wang, J. B. Pang, S.C. Jiang, X.L. Ji, L.J. An, Semiconductor "nanoonions" with multifold alternating CdS/CdSe or CdSe/CdS structure, Chemistry of Materials (2006) 18, 4253. (77). J. J. Li, J.T. Hu, W.D. Yang, A.P. Alivisatos, Large-scale synthesis of nearly monodisperse CdSe/CdS core/shell nanoc rystals using air-stable reagents via successive ion layer adsorption and reaction, Journal of the American Chemical Society (2003) 125, 12567. (78). L. P. Balet, S. A. Ivanov, A. Piryatin ski, M. Achermann, V. I. Klimov, Inverted core/shell nanocrystals continuously tunabl e between type-I and type-II localization regimes, Nano Letters (2004) 4, 1485. (79). D. V. Talapin, A.L. Rogach, A. Kornowski, H. Weller, CdSe/CdS/ZnS and CdSe/ZnSe/ZnS core-shell-shell nanocrystals, Journal of Physical Chemistry B (2004) 108, 18826.

PAGE 139

139 (80). W. E. Garner, Chemistry of the solid state (Butterworths Scientific Publications, London, 1955). (81). W. E. Martin, Photoluminescence De terminations of Cd Diffusion in ZnSe, Journal of Applied Physics (1973) 44, 5639. (82). P. J. Parbrook, B. Henderson, K. P. Odonnell, P. J. Wright, B. Cockayne, Interdiffusion in wide-bandgap Zn(Cd) S(Se) strained layer superlattices, Semiconductor Science and Technology (1991) 6, 818. (83). A. Rosenauer, T. Reisinger, E. Steink irchner, J. Zweck, W. Gebhardt, High-resolution transmission electron-microscopy determina tion of Cd diffusion in CdSe/ZnSe singlequantum-well structures, Journal of Crystal Growth (1995) 152, 42. (84). M. Strassburg, M. Kuttler, U. W. Pohl D. Bimberg, Diffusi on of Cd, Mg and S in ZnSe-based quantum well structures, Thin Solid Films (1998) 336, 208. (85). W. Chen, J. O. Malm, V. Zwiller, R. Wallenberg, J. O. Bovin, Size dependence of Eu2+ fluorescence in ZnS : Eu2+ nanoparticles, Journal of Applied Physics (2001) 89, 2671. (86). W. Chen, R. Sammynaiken, Y.N. Hu ang, Crystal field, phonon coupling and emission shift of Mn2+ in ZnS : Mn nanoparticles, Journal of Applied Physics (2001) 89, 1120. (87). C. X. Shan, X.W. Fan, J.Y. Zhang, Z. Z. Zhang, B.S. Li, Y.M Lu, Y.C. Liu, D.Z. Shen, X.G. Kong, X.H. Wang, Growth a nd evolution of ZnCdSe quantum dots, Journal of Vacuum Science & Technology B (2002) 20, 1102. (88). X. Y. Wang, J. Y. Zhang, A. Nazzal M. Darragh, M. Xiao, Electronic structure transformation from a quantum-dot to a quantum-wire system: Photoluminescence decay and polarization of colloidal CdSe quantum rods, Applied Physics Letters (2002) 81, 4829. (89). U. Manual, A User's Guide to the Andor iStar (Andor Technology Limited, 2001). (90). Andor, Andor K nowledge Library, http://www.andor.com/library/digital_cameras/ (91). J. Alford, Personal communication concering the iStar CCD system,(2007) (92). S. Cannistra, How to choose a CCD camera, http://www.starrywonders.com/ccdcameraconsiderations.html (93). M. C. Gino, Noise, Noise, Noise, http://www.astrophysassist.com/educate/noise/noise.htm (94). Apogee, CCD University, http://www.ccd.com/ccdu.html

PAGE 140

140 (95). E. C. Scher, L. Manna, A. P. Aliv isatos, Shape control and applications of nanocrystals, Philosophical Transactions of the Royal Society of London Series aMathematical Physical and Engineering Sciences (2003) 361, 241. (96). A. Puzder, A.J. Williamson, N. Zaitseva, G. Galli, L. Manna and A.P. Alivisatos, The effect of organic ligand bi nding on the growth of CdSe nanoparticles probed by Ab initio calculations, Nano Letters (2004) 4, 2361. (97). M. A. Hines, P. Guyot-Sionnest, Br ight UV-blue luminescent colloidal ZnSe nanocrystals, Journal of Physical Chemistry B (1998) 102, 3655. (98). B. D. Cullity, and Stock, S. R., Elements of X-ray Diffraction (Prentice Hall, New York, ed. Third, 2001). (99). M. M. a. F. Rashad, O. A., Synthesi s and characterization of nano-sized nickel ferrites from fly ash for cat alytic oxidation of CO, Materials Chemistry and Physics (2005) 94, 365. (100). R. W. Meulenberg, T. Je nnings, G. F. Strouse, Compre ssive and tensile stress in colloidal CdSe semiconductor quantum dots, Physical Review B (2004) 70 235311. (101). A. V. Baranov, Y.P. Rakovi ch, J.F. Donegan, T.S. Perova R.A. Moore, D.V. Talapin, A.L. Rogach, Y. Masumoto, I. Nabiev, Effect of ZnS shell thickness on the phonon spectra in CdSe quantum dots, Physical Review B (2003) 68, 165306. (102). Y. N. Hwang, S. Shin, H.L. Park, SH Pa rk, U. Kim, H.S. Jeong, E. Shin, D. Kim, Effect of lattice contraction on the Raman shifts of CdSe quantum dots in glass matrices, Physical Review B (1996) 54, 15120. (103). F. Comas, C. Trallero-Giner, N. Studa rt, G. E. Marques, Interface optical phonons in spheroidal dots: Raman selection rules, Physical Review B (2002) 65, 073303. (104). C. Trallero-Giner, A. Debernardi, M. Cardona, E. Me nendez-Proupin, A. I. Ekimov, Optical vibrons in CdSe dots and disp ersion relation of the bulk material, Physical Review B (1998) 57, 4664. (105). R. G. Alonso et al., Raman-spectroscopy of 2 novel semiconductors and related superlattices Cubic Cd1-XMnxSe and Cd1-XZnxSe, Physical Review B (1989) 40, 3720. (106). V. V. Travnikov, V. K. Kaibyshev, Resonance ex citon-phonon spectra in open ZnCdSe/ZnSe nanowires: Raman scattering and hot luminescence, extended and localized exciton states, Physics of the Solid State (2003) 45, 1379. (107). R. Bhushan, V. Prasad, W. Meredith G. Horsburgh, G.D. Brownlie, K.A. Prior, B.C. Cavenett, W. Rothwell, A.J. Dann, Microp robe Raman study of the variation of LO phonon frequency with the Cd concen tration in the ternary compound Zn1-xCdxSe, Journal of Crystal Growth (1996) 159, 103.

PAGE 141

141 (108). C. Ramkumar, K. P. Jain, S. C. Abbi Resonant Raman scatte ring probe of alloying effect in GaAs1-xPx ternary alloy semiconductors, Physical Review B (1996) 54, 7921. (109). C. Ramkumar, K. P. Jain, S. C. Abbi Raman-scattering probe of anharmonic effects due to temperature and compositional disord er in III-V binary and ternary alloy semiconductors, Physical Review B (1996) 53, 13672. (110). T. Kummell et al., Size dependence of strain relaxa tion and lateral quantization in deep etched CdxZn1-xSe/ZnSe quantum wires, Physical Review B (1998) 57, 15439. (111). P. R. Yu, J. M. Nedeljkovic, P. A. Ah renkiel, R. J. Ellingson, A. J. Nozik, Size dependent femtosecond electron cooling dynamics in CdSe quantum rods, Nano Letters (2004) 4, 1089. (112). M. Achermann, J. A. Hollingsworth, V. I. Klimov, Multiexcitons confined within a subexcitonic volume: Spectro scopic and dynamical signature s of neutral and charged biexcitons in ultrasmall semiconductor nanocrystals, Physical Review B (2003) 68, 245302. (113). Y. Kawakami, K. Omae, A. Kaneta, K. Okamoto, Y. Narukawa, T. Mukai, S. Fujita, In inhomogeneity and emission characteristics of InGaN, Journal of PhysicsCondensed Matter (2001) 13, 6993. (114). H. S. Kim, R. A. Mair, J. Li, J. Y. Lin, H. X. Jiang, Time-resolved photoluminescence studies of AlxGa1-xN alloys, Applied Physics Letters (2000) 76, 1252. (115). X. H. Zhong, Y. Y. Feng, W. Knoll, M. Y. Han, Alloyed ZnxCd1-xS nanocrystals with highly narrow luminescen ce spectral width, Journal of the American Chemical Society (2003) 125, 13559. (116). R. F. Mahrt, T. Pauck, U. Lemmer, U. Siegner, M. Hopmeier, R. Hennig, H. Bassler, E.O. Gobel, P.H. Bolivar, G. Wegmann, H. Kuz, U. Scherf, K.Mullen, Dynamics of optical excitations in a ladder-type pi-conj ugated polymer containi ng aggregate states, Physical Review B (1996) 54, 1759. (117). M. Jones, J. Nedeljkovic, R. J. Ellingson, A. J. Nozik, G. Rumbles, Photoenhancement of luminescence in co lloidal CdSe quantum dot solutions, Journal of Physical Chemistry B (2003) 107, 11346. (118). X. Chen, B. Henderson, K. P. Odonnell, Luminescence decay in disordered lowdimensional semiconductors, Applied Physics Letters (1992) 60, 2672. (119). R. Cingolani et al., Exciton spectroscopy in Zn1-XCdxSe/ZnSe quantum-wells, Physical Review B (1995) 51, 5176.

PAGE 142

142 (120). S. A. Empedocles, M. G. Bawendi, Quan tum-confined stark effect in single CdSe nanocrystallite quantum dots, Science (1997) 278, 2114. (121). V. Esch, B. Fluegel, G. Khitrova, H.M. Gibbs, J.J. Xu, K.Kang, S.W. Koch, L.C. Liu, S.H. Risbud, N. Peyghambarian, State filli ng, coulomb, and trapping effects in the optical nonlinearity of CdTe quantum dots in glass, Physical Review B (1990) 42, 7450. (122). E. Hendry, M. Koeberg, F. Wang, H. Zhang, C.D. Donega, D. Vanmaekelbergh, M. Bonn, Direct observation of electron-to-hol e energy transfer in CdSe quantum dots, Physical Review Letters (2006) 96, 125201. (123). A. Shabaev, A. L. Efros, 1D exc iton spectroscopy of semiconductor nanorods, Nano Letters (2004) 4, 1821. (124). C. Rulliere, Femtosecond Laser Pulses: Principles and Experiments, 2nd Edition (2004). (125). C. Burda, X. B. Chen, R. Narayanan, M. A. El-Sayed, Chemistry and properties of nanocrystals of different shapes, Chemical Reviews (2005) 105, 1025. (126). K. Brunner, U. Bockelmann, G. Abstreit er, M. Walther, G. Bohm, G. Trankle, G. Weimann, Photoluminescence from a single GaAs/AlGaAs quantum dot Physical Review Letters (1992) 69, 3216. (127). L. W. Wang, M. Califano, A. Zunger, A. Franceschetti Pseudopotential theory of Auger processes in CdSe quantum dots, Physical Review Letters (2003) 91, 56404. (128). A. L. Efros, V. A. Kharchenko, M. Rosen, Breaking the phonon bottleneck in nanometer quantum dots Role of Auger-like processes, Solid State Communications (1995) 93, 281. (129). P. Guyot-Sionnest, M. Shim, C. Matranga M. Hines, Intraband relaxation in CdSe quantum dots, Physical Review B (1999) 60, R2181. (130). J. G. Muller, E. Atas, C. Tan, K. S. Schanze, V. D. Kleiman, The role of exciton hopping and direct energy tr ansfer in the efficien t quenching of conjugated polyelectrolytes, Journal of the American Chemical Society (2006) 128, 4007. (131). V. I. Klimov, A. A. Mikhailovsky, D. W. McBranch, C. A. Leatherdale, M. G. Bawendi, Quantization of multiparticle Auge r rates in semiconductor quantum dots, Science (2000) 287, 1011. (132). J. L. Blackburn, D. C. Selmarten, A. J. Nozik, Electron transfer dynamics in quantum dot/titanium dioxide composites formed by in situ chemical bath deposition, Journal of Physical Chemistry B (2003) 107, 14154.

PAGE 143

143 (133). S. Sauvage, P. Boucaud, Rpsm Lobo, F. Bras, G. Fishman, R. Prazeres, F. Glotin, J.M. Ortega, J.M Gerard, Long polaron lif etime in InAs/GaA s self-assembled quantum dots, Physical Review Letters (2002) 88, 177402. (134). P. C. Sercel, Multiphonon-assisted tu nneling through deep levels A rapid energyrelaxation mechanism in nonideal quantum-dot heterostructures, Physical Review B (1995) 51, 14532. (135). M. Ohishi, K. Tanaka, T. Fujimoto, M. Yoneta, H. Saito, Alloying of CdSe/ZnSe quantum dot grown by an altern ate molecular beam supply, Journal of Crystal Growth (2002) 237, 1320. (136). X. H. Zhong, R. G. Xie, Y. Zhang, T. Basche, W. Knoll, High-quality violetto redemitting ZnSe/CdSe core/shell nanocrystals, Chemistry of Materials (2005) 17, 4038. (137). B. P. Zhang, D. D. Manh, K. Waka tsuki, Y. Segawa, Nanostructures formed on CdSe/ZnSe surfaces, Journal of Crystal Growth (2001) 227, 645. (138). V. V. Nikesh, S. Mahamuni, Highl y photoluminescent ZnSe/ZnS quantum dots, Semiconductor Science and Technology (2001) 16, 687. (139). J. S. Steckel, J.P. Zimmer, S. Coe-Sulliv an, N.E. Stott, V. Bulovic, M.G. Bawendi, Blue luminescence from (CdS)Z nS core-shell nanocrystals, Angewandte ChemieInternational Edition (2004) 43, 2154. (140). Z. H. Yu, L. Guo, H. Du, T. Krauss, J. Silcox, Shell distribution on colloidal CdSe/ZnS quantum dots, Nano Letters (2005) 5, 565. (141). C. Guenaud, E. Deleporte, A. Filoramo P. Lelong, C. Delalande, C. Morhain, E. Tournie, J.P. Faurie, Study of the band a lignment in (Zn, Cd)Se/ZnSe quantum wells by means of photoluminescence excitation spectroscopy, Journal of Applied Physics (2000) 87, 1863. (142). V. I. Kozlovsky, Sadofyev, Yu G., Litvinov, V. G., Crystal and Solid State Physics, Physics of II-VI Compounds, Landolt-Borstein Series (Springer, Berlin, 1982). (143). V. I. Kozlovsky, Y. G. Sadofyev, V. G. Litvinov, Band alignment in ZnCdTe/ZnTe and ZnCdSe/ZnSe SQW structures grown on GaAs(100) by MBE, Nanotechnology (2000) 11, 241. (144). S. H. Wei, S. B. Zhang, A. Zunger, Firs t-principles calculation of band offsets, optical bowings, and defects in CdS, CdSe, CdTe, and their alloys, Journal of Applied Physics (2000) 87, 1304. (145). S. H. Wei, A. Zunger, Calculated natural band offsets of all II-VI and Ill-V semiconductors: Chemical trends and the role of cation d orbitals, Applied Physics Letters (1998) 72, 2011.

PAGE 144

144 (146). M. D. McGehee, A. J. Heeger, Semi conducting (conjugated) pol ymers as materials for solid-state lasers, Advanced Materials (2000) 12, 1655. (147). R. H. Friend, R. W. Gymer, A.B. Holmes, J.H. Bu rroughes, R.N. Marks, C. Taliani, D.D.C. Bradley, D.A. Dos Santos, J.L. Bredas, M. Logdlund, W.R. Salaneck, Electroluminescence in conjugated polymers, Nature (1999) 397, 121. (148). H. Spanggaard, F. C. Krebs, A brief history of the devel opment of organic and polymeric photovoltaics, Solar Energy Materials and Solar Cells (2004) 83, 125. (149). H. Sirringhaus, N. Tessler, R. H. Friend, Integrated optoelectronic devices based on conjugated polymers, Science (1998) 280, 1741. (150). M. Fakis, Anestopoulos, D ., Giannetas, V., et al, Influe nce of aggregates and solvent aromaticity on the emission of conjugated polymers, Journal of Physical Chemistry B (2006) 49, 24897. (151). J. H. Hsu, W.S. Fann, H.F. Meng, E.S. Chen, E.C. Chang, S.A. Chen, K.W. To, Decay dynamics of interchain excited states in lumine scent conjugated polymer CNPPV, Chemical Physics (2001) 269, 367. (152). F. J. Hua, E. Ruckenstein, Fluorescen ce study of aggregation in water of PEO-grafted polydiphenylamine, Langmuir (2004) 20, 3954. (153). L. O. Palsson et al., Photophysics of a fluorene co-polymer in solution and films, Chemical Physics (2002) 279, 229. (154). P. Wang, C. J. Collison, L. J. Rot hberg, Origins of aggregation quenching in luminescent phenylenevinylene polymers, Journal of Photochemistry and Photobiology a-Chemistry (2001) 144, 63. (155). H. Jiang, X. Y. Zhao, K. S. Sc hanze, Amplified fluorescence quenching of a conjugated polyelectroly te mediated by Ca2+, Langmuir (2006) 22, 5541. (156). M. Fakis, Anestopoulos D., Giannetas, V., et al, Femtosecond time resolved fluorescence dynamics of a cationic wate r soluble poly(fluorenevinylene-cophenylenevinylene), Journal of Physical Chemistry B (2006) 110, 12916. (157). L. H. Chen, D.W. McBranch, H.L. Wa ng, R. Helgeson, F. Wudl, D.G. Whitten, Highly sensitive biological and chemical sensors based on reversible fluorescence quenching in a conjugated polymer, Proceedings of the National Academy of Sciences of the United States of America (1999) 96, 12287. (158). N. GronbechJensen, R. J. Mashl, R. F. Bruinsma, W. M. Gelbar t, Counterion-induced attraction between rigi d polyelectrolytes, Physical Review Letters (1997) 78, 2477.

PAGE 145

145 (159). B. J. Schwartz, Conjugated polymer s as molecular materials : How chain conformation and film morphology influe nce energy transfer and interchain interactions, Annual Review of Physical Chemistry (2003) 54, 141. (160). R. Jakubiak, C. J. Collison, W. C. Wan, L. J. Rothberg, B. R. Hsieh, Aggregation quenching of luminescence in electro luminescent conjugated polymers, Journal of Physical Chemistry A (1999) 103, 2394. (161). S. A. Jenekhe, J. A. Osaheni, Excime rs and exciplexes of conjugated polymers, Science (1994) 265, 765. (162). I. D. W. Samuel, G. Ru mbles, C. J. Collison, Efficient interchain photoluminescence in a high-electron-affi nity conjugated polymer, Physical Review B (1995) 52, 11573. (163). C. J. Collison, L. J. Rothberg, V. Tr eemaneekarn, Y. Li, Conformational effects on the photophysics of conjugated polymers: A two species model for MEH-PPV spectroscopy and dynamics, Macromolecules (2001) 34, 2346. (164). G. H. Gelinck, J. M. Warman, E. G. J. St aring, Polaron pair form ation, migration, and decay on photoexcited poly(phenylenevinylene) chains, Journal of Physical Chemistry (1996) 100, 5485. (165). J. W. Blatchford, S.W. Jessen, L.B. Lin, T.L, Gust afson, D.K. Fu, H.L. Wang, T.M. Swager, A.G. MacDiarmid, A.J. Epstein, Photoluminescence in pyridine-based polymers: Role of aggregates, Physical Review B (1996) 54, 9180. (166). U. Lemmer, S. Heun, R.F. Mahrt, U. Scherf M. Hopmeier, U. Siegner, E.O. Gobel, K. Mullen, H. Bassler, Aggregate Fluo rescence in Conjugated Polymers, Chemical Physics Letters (1995) 240, 373. (167). X. Y. Zhao, M.R. Pinto, L.M. Hardison, J. Mwuara, J. Muller, H. Jiang, D.Witker, V.D. Kleiman, J.R. Reynolds, K.S. Sc hanze,Variable band gap poly(arylene ethynylene) conjugated polyelectrolytes, Macromolecules (2006) 39, 6355. (168). C. Y. Tan, E. Atas, J.G. Muller, M.R. Pinto, V.D. Kleiman, K.S. Schanze Amplified quenching of a conjugated polyelectrolyte by cyanine dyes, Journal of the American Chemical Society (2004) 126, 13685. (169). B. S. Harrison, M. B. Ramey, J. R. Reynolds, K. S. Schanze, Amplified fluorescence quenching in a poly(p-phenylene)-b ased cationic polyelectrolyte, Journal of the American Chemical Society (2000) 122, 8561. (170). G. H. Gelinck, E.G.J. Staring, D.H. Hwang, G.C.W. Spencer, A.B. Holmes, J.M. Warman, The effect of broken conjugation and aggregation on photo-induced charge separation on polyphenylenevinylene chains, Synthetic Metals (1997) 84, 595. (171). C. H. Fan, S.Wang, J.W. Hong, G. C. Bazan, K.W. Plaxco, A.J. Heeger, Beyond superquenching: Hyper-efficient energy tr ansfer from conjugated polymers to gold

PAGE 146

146 nanoparticles, Proceedings of the National Academy of Sciences of the United States of America (2003) 100, 6297. (172). B. S. Gaylord, A. J. Heeger, G. C. Bazan, DNA detection using water-soluble conjugated polymers and peptide nucleic acid probes, Proceedings of the National Academy of Sciences of th e United States of America (2002) 99, 10954. (173). M. Stork, B. S. Gaylord, A. J. Heeger, G. C. Bazan, Energy transfer in mixtures of water-soluble oligomers: Effect of charge, aggregation, and surfactant complexation, Advanced Materials (2002) 14, 361. (174). A. Haugeneder, U. Le mmer, U. Scherf, Exciton dissociation dynamics in a conjugated polymer contai ning aggregate states, Chemical Physics Letters (2002) 351, 354. (175). G. Petekidis, G. Fytas, U. Scherf, K. Mullen, G. Fleischer, Dynamics of poly(pphenylene) ladder polymers in solution, Journal of Polymer Science Part B-Polymer Physics (1999) 37, 2211. (176). J. G. Muller, U. Lemmer, G. Raschke, M. Anni, U. Scherf, J.M. Lupton, J. Feldman, Linewidth-limited energy transfer in single conjugated polymer molecules, Physical Review Letters (2003) 91, 267403. (177). B. Schweitzer, G. Wegmann, D. Hertel, R.F. Mahrt, H. Ba ssler, F. Uckert, U. Scherf, K. Mullen, Spontaneous and stimulated emission from a ladder-type conjugated polymer, Physical Review B (1999) 59, 4112. (178). H. P. Gregor, L. B. Luttinger, E. M. Loebl, Metal-polyelectrolyte complexes .4. complexes of polyacrylic acid with magnesi um, calcium, manganese, cobalt and zinc, Journal of Physical Chemistry (1955) 59, 990. (179). I. B. Kim, U. H. F. Bunz, Modulating the sensory response of a conjugated polymer by proteins: An agglutination assa y for mercury ions in water, Journal of the American Chemical Society (2006) 128, 2818. (180). C. Y. Tan, M. R. Pint o, K. S. Schanze, Photophysic s, aggregation and amplified quenching of a water-soluble pol y( phenylene ethynylene), Chemical Communications (2002), 5, 446. (181). X. Y. Zhao, Jiang, H., Schanze, K.S., Water-Soluble Poly(phenylene ethynylene)s of Variable Chain Length: Synthesis, Photophysics and Amplified Quenching, In press (2007). (182). E. Atas, University of Florida (2006). (183). E. Atas, Z. H. Peng, V. D. Kleiman, Energy transfer in unsymmetrical phenylene ethynylene dendrimers, Journal of Physical Chemistry B (2005) 109, 13553.

PAGE 147

147 (184). New Focus, Model 5540 User's Manual: The Berek Polarization Compensator. (185). New Focus, Polarization and Polarization Control. (186). A. Yariv, and Yeh, P., Optical Waves in Crystals: Propagation and Control of Laser Radiation (Wiley-Interscience 2002). (187). R. Rajasekaran, Personal communication for Berek Compensator Crystal Specifications, (2007) (188). M. Dodge, Refractive prope rties of magnesium fluoride, Applied Optics (1984) 23, 1980. (189). J. R. Lakowicz, Principles of Fluorescence Spectroscopy (Kluwer Academic/Plenum Publishers, New York, ed. 2, 1999). (190). R. E. Martin, F. Diederich, Linear monodisperse pi-conjugated oligomers: Model compounds for polymers and more, Angewandte Chemie-International Edition (1999) 38, 1350. (191). U. H. F. Bunz, J.M. Im hof, R.K. Bly, C.G. Bangcuyo, L. Rozanski, D.A.V. Bout, Photophysics of poly [p-(2,5-didodecylphe nylene)ethynylene] in thin films, Macromolecules (2005) 38, 5892. (192). C. E. Halkyard, M. E. Rampey, L. Kl oppenburg, S. L. Studer-Martinez, U. H. F. Bunz, Evidence of aggregate formation fo r 2,5-dialkylpoly(pphenyleneethynylenes) in solution and thin films, Macromolecules (1998) 31, 8655. (193). J. Kim, D. T. McQuade, S. K. McHugh, T. M. Swager Ion-specific aggregation in conjugated polymers: Highly se nsitive and selective fluorescent ion chemosensors, Angewandte Chemie-International Edition (2000) 39, 3868. (194). J. Kim, T. M. Swager, Control of conformational and interpolymer effects in conjugated polymers, Nature (2001) 411, 1030. (195). M. Abramowitz, Johnson I. D., and Davidson, M. W., Fluorescence filter spectral transmission profiles, http://www.olympusmicro.com/primer/java/fluorescence/fluorocubes/index.html, (196). V. F. Kamalov, I. A. Struganova, K. Yoshihara, Temperature dependent radiative lifetime of J-aggregates, Journal of Physical Chemistry (1996) 100, 8640. (197). D. Anestopoulos, Fakis, M etal, Excitation energy transfer in a cationic water-soluble conjugated co-polymer studied by time resolved anisot ropy and fluorescence dynamics, Chemical Physics Letters (2006) 421, 205.

PAGE 148

148 (198). A. A. Mikhailovsky, A. V. Malko, J. A. Hollingsworth, M. G. Bawendi, V. I. Klimov, Multiparticle interactions and stimulated emission in chemically synthesized quantum dots, Applied Physics Letters (2002) 80, 2380. (199). V. I. Klimov, Mechanisms for photogenera tion and recombination of multiexcitons in semiconductor nanocrystals: Implications for lasing and solar energy conversion, Journal of Physical Chemistry B (2006) 110, 16827. (200). D. Katz, T. Wizansky, O. Millo, E. Ro thenber, T. Mokari, U. Banin, Size-dependent tunneling and optical spectroscopy of CdSe quantum rods, Physical Review Letters (2002) 89, 086801. (201). H. Htoon, J. A. Hollingworth, A. V. Ma lko, R. Dickerson, V. I. Klimov, Light amplification in semiconductor nanocrysta ls: Quantum rods versus quantum dots, Applied Physics Letters (2003) 82, 4776. (202). S. A. Ivanov, J. Nanda, A. Piryatinski, M. Achermann, L.P. Balet, I.V. Bezel, P.O. Anikeeva, S. Tretiak, V.I. Klimov, Light amplification using inverted core/shell nanocrystals: Towards lasing in the single-exciton regime, Journal of Physical Chemistry B (2004) 108, 10625. (203). J. Nanda, S.A. Ivanov, H. Htoon, I. B ezel, A Piratinski, S. Tretiak, V.I. Klimov, Absorption cross sections and Auger recombination lifetimes in inverted core-shell nanocrystals: Implications for lasing performance, Journal of Applied Physics (2006) 99, 013707. (204). V. L. Colvin, M. C. Schlamp, A. P. Alivisatos, Light-emit ting-diodes made from cadmium selenide nanocrystals and a semiconducting polymer Nature (1994) 370, 354. (205). B. O. Dabbousi, M. G. Bawendi, O. Onitsuka, M. F. Rubner, Electroluminescence from CdSe quantum-dot polymer composites, Applied Physics Letters (1995) 66, 1316. (206). H. Mattoussi, L.H. Radz ilowski, B.O. Dabbousi, E.L. Thomas, M.G. Bawendi, M.F. Rubner, Electroluminescence from heterost ructures of poly(phenylene vinylene) and inorganic CdSe nanocrystals, Journal of Applied Physics (1998) 83, 7965. (207). H. S. Yang, P. H. Holloway, Electro luminescence from hybrid conjugated polymer CdS : Mn/ZnS core/shell nanocrystals devices, Journal of Physical Chemistry B (2003) 107, 9705. (208). S. Coe, W. K. Woo, M. Bawendi, V. Bulovic, El ectroluminescence from single monolayers of nanocrystals in molecular organic devices, Nature (2002) 420, 800. (209). C. Bonati, M.B. Mohamed, D. Tonti, G. Zgreblic, S. Haacke, F. van Mourik, M. Chergui, Spectral and dynamical characterization of multiexcitons in colloidal CdSe semiconductor quantum dots, Physical Review B (2005) 71, 205317.

PAGE 149

149 (210). C. Burda, M. A. El-Sayed, Highdensity femtosecond transient absorption spectroscopy of semiconductor nanoparticles. A tool to investigate surface quality, Pure and Applied Chemistry (2000) 72, 165. (211). C. Burda, S. Link, M. Mohamed, M. El-Sayed, The relaxation pathways of CdSe nanoparticles monitored with femtosecond timeresolution from the visible to the IR: Assignment of the transient f eatures by carrier quenching, Journal of Physical Chemistry B (2001) 105, 12286. (212). B. P. Zhang, Y.Q. Li, T. Yasuda, Y. Segawa, K.Edamatsu, T. Itoh, Time-resolved photoluminescence of ZnCdSe single quantum dots, Journal of Crystal Growth (2000) 214, 765. (213). N. C. Greenham, X. G. Peng, A. P. Alivisatos, Charge separation and transport in conjugated-polymer/semiconductor-nanocrystal composites studied by photoluminescence quenching and photoconductivity, Physical Review B (1996) 54, 17628. (214). N. C. Greenham, X. G. Peng, A. P. Alivisatos, Charge separation and transport in conjugated polymer cadmium selenide nanocrystal composites studied by photoluminescence quenching and photoconductivity, Synthetic Metals (1997) 84, 545. (215). L. M. Herz, C. Silva, R. T. Phillips, S. Setayesh, K. Mullen, Exciton migration to chain aggregates in conjuga ted polymers: influence of side-chain substitution, Chemical Physics Letters (2001) 347, 318.

PAGE 150

150 BIOGRAPHICAL SKETCH Lindsay Michelle Hardison was born on July 3, 1979, in Torrejon, Spain, where her father, Craig Hardison was stationed as a member of the United States Air Force. She lived there with her father and mother, Susan, until she was 16 m onths old. She then moved to Washington until she started elementary school. Lindsays father was relocated to Hampton, Virginia, where she continued her early academic studies. After attending Hampt on Christian High School for 3 years, Lindsay moved to Melbourne, Florida, in 1997 to begin her undergra duate studies at the Florida Institute of Technology. In 2001, she graduated magna cum laude with a Bachelor of Science degree in research chemistry. She then took a 1-year break from school and worked as an analytical chemist at Midwest Research Institute in Palm Bay. Lindsays drive to continue her education and desire to lear n brought her to the Un iversity of Florida in 2002. She began her doctoral work under the supervision of Professor Valeria D. Kleiman in the area of ultrafast laser spectroscopy of semiconductor na noparticles and conjugated polym ers. Her professional career as a Ph.D. will begin in Hillsboro, Oregon, as a t echnologies development engineer at Intel.