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Synthesis and Characterization of Luminescent Oxide Nanocrystals

Permanent Link: http://ufdc.ufl.edu/UFE0021407/00001

Material Information

Title: Synthesis and Characterization of Luminescent Oxide Nanocrystals
Physical Description: 1 online resource (121 p.)
Language: english
Creator: Seo, Soo Yeon
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2007

Subjects

Subjects / Keywords: biolabeling, colloidal, doping, gd2o3, liquid, luminescent, nanocrystals, oxide, shape, water
Materials Science and Engineering -- Dissertations, Academic -- UF
Genre: Materials Science and Engineering thesis, Ph.D.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: Oxide nanocrystals with controlled geometries exhibit unique shape dependent optical and structural properties. Shape-controlled synthesis of rare earth doped gadolinium oxide (Gd2O3 : Eu3+, Tb3+ or Er3+) and zinc gallate (ZnGa2O4:Eu3+) nanocrystals by non-hydrolytic high temperature (-300 degrees C) methods are reported. Various shapes of Gd2O3 nanocrystals were synthesized, including spheres and plates and advanced shapes such as curved rods and triangles. The nanocrystal shape was shown to be a function of the synthesis parameters, such as metal precursors (acetate, acetyl acetate, chloride or octanoate) and surfactant type ( tri-octyl phosphine oxide-TOPO, or hexadecanediol) and concentration (metal precursor: surfactant molar ratios of 1:2 to 1:5), as well as heating rate (5-25 degrees C/min.) between pre-heat (200 degrees C) and reaction (290 degrees C) temperatures. The effects of these parameters upon nanocrystal shape were explained based on nucleation and growth of oxide nanocrystals. The photoluminescence intensity from Gd2O3:Eu3+ was shown to increase as the concentration of dopant incorporated into the nanocrystals increased (from 0.5 to 10 mol %). The doping efficiency, defined to be the percentage of dopant incorporated into the nanocrystals, ranged from 0.57-6.12% was a function of shape of the Gd2O3 : Eu3 and was discussed in terms of the rate of reaction, product yield and crystal structure. To be used for labeling biomolecules such as DNA, RNA, or proteins, water-soluble luminescent nanocrystals are required. Doped Gd2O3 nanocrystals prepared by the non hydrolytic hot solution method are hydrophobic and are not soluble in water due to organic surfactant encapsulation. A general strategy to convert hydrophobic luminescent nanocrystals (e.g. Gd2O3) to water soluble particles by over-coating the hydrophobic surface with amphiphilic polymers is reported. Specifically, octylamine modified surfaces were coated with poly (acrylic acid) and water dispersions of Gd2O3:Eu3+ were still stable at room temperature after 4months. The non-hydrolytic hot solution synthesis technique was used to grow monodispersed ternary oxide nanospheres (?5nm) of ZnGa2O4: Eu3+ from a variety of metal precursors. Using Gd acetate dehydrate, large (?20nm) complex shaped (triangle and rectangle) ZnGa2O4: Eu3+ nanocrystals were obtained. Based on X-ray diffraction data, the nanocrystals had a cubic spinel structure with no impurity phases. The size of the ZnGa2O4: Eu3+ nanospheres could be varied by changing the molar ratio of Zn to surfactants, with higher concentrations of surfactant resulting in smaller nanocrystals. Analysis of the PL emission suggests that the Eu3+ ions were incorporated into the ZnGa2O4 host.
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 Soo Yeon Seo.
Thesis: Thesis (Ph.D.)--University of Florida, 2007.
Local: Adviser: Holloway, Paul H.

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2007
System ID: UFE0021407:00001

Permanent Link: http://ufdc.ufl.edu/UFE0021407/00001

Material Information

Title: Synthesis and Characterization of Luminescent Oxide Nanocrystals
Physical Description: 1 online resource (121 p.)
Language: english
Creator: Seo, Soo Yeon
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2007

Subjects

Subjects / Keywords: biolabeling, colloidal, doping, gd2o3, liquid, luminescent, nanocrystals, oxide, shape, water
Materials Science and Engineering -- Dissertations, Academic -- UF
Genre: Materials Science and Engineering thesis, Ph.D.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: Oxide nanocrystals with controlled geometries exhibit unique shape dependent optical and structural properties. Shape-controlled synthesis of rare earth doped gadolinium oxide (Gd2O3 : Eu3+, Tb3+ or Er3+) and zinc gallate (ZnGa2O4:Eu3+) nanocrystals by non-hydrolytic high temperature (-300 degrees C) methods are reported. Various shapes of Gd2O3 nanocrystals were synthesized, including spheres and plates and advanced shapes such as curved rods and triangles. The nanocrystal shape was shown to be a function of the synthesis parameters, such as metal precursors (acetate, acetyl acetate, chloride or octanoate) and surfactant type ( tri-octyl phosphine oxide-TOPO, or hexadecanediol) and concentration (metal precursor: surfactant molar ratios of 1:2 to 1:5), as well as heating rate (5-25 degrees C/min.) between pre-heat (200 degrees C) and reaction (290 degrees C) temperatures. The effects of these parameters upon nanocrystal shape were explained based on nucleation and growth of oxide nanocrystals. The photoluminescence intensity from Gd2O3:Eu3+ was shown to increase as the concentration of dopant incorporated into the nanocrystals increased (from 0.5 to 10 mol %). The doping efficiency, defined to be the percentage of dopant incorporated into the nanocrystals, ranged from 0.57-6.12% was a function of shape of the Gd2O3 : Eu3 and was discussed in terms of the rate of reaction, product yield and crystal structure. To be used for labeling biomolecules such as DNA, RNA, or proteins, water-soluble luminescent nanocrystals are required. Doped Gd2O3 nanocrystals prepared by the non hydrolytic hot solution method are hydrophobic and are not soluble in water due to organic surfactant encapsulation. A general strategy to convert hydrophobic luminescent nanocrystals (e.g. Gd2O3) to water soluble particles by over-coating the hydrophobic surface with amphiphilic polymers is reported. Specifically, octylamine modified surfaces were coated with poly (acrylic acid) and water dispersions of Gd2O3:Eu3+ were still stable at room temperature after 4months. The non-hydrolytic hot solution synthesis technique was used to grow monodispersed ternary oxide nanospheres (?5nm) of ZnGa2O4: Eu3+ from a variety of metal precursors. Using Gd acetate dehydrate, large (?20nm) complex shaped (triangle and rectangle) ZnGa2O4: Eu3+ nanocrystals were obtained. Based on X-ray diffraction data, the nanocrystals had a cubic spinel structure with no impurity phases. The size of the ZnGa2O4: Eu3+ nanospheres could be varied by changing the molar ratio of Zn to surfactants, with higher concentrations of surfactant resulting in smaller nanocrystals. Analysis of the PL emission suggests that the Eu3+ ions were incorporated into the ZnGa2O4 host.
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 Soo Yeon Seo.
Thesis: Thesis (Ph.D.)--University of Florida, 2007.
Local: Adviser: Holloway, Paul H.

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2007
System ID: UFE0021407:00001


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SYNTHESIS AND CHARACTERIZATION OF LUMINESCENT OXIDE NANOCRYSTALS


By

SOOYEON SEO

















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 Sooyeon Seo


































To God, my family and friends, this work is dedicated









ACKNOWLEDGMENTS

It is a great relief to come to the end of graduate student life at the University of Florida.

The past years have been full of frustrations, struggles and excitements. I feel very fortunate to

have Prof. Paul H.Holloway as my advisor. His patience and trust have been very important for

me to develop my own thoughts, and his attitude towards science and his open-mindedness have

greatly influenced my perception about scientific research. I am also grateful for the hospitality

and support of Ludie Harmon.

I acknowledge all the members of the Holloway' s group. I enjoyed working with them,

exchanging ideas and learning from each other. The work described in this thesis would not have

been possible without the help of the people in the Department of Materials Science and

Engineering.

I had the honor to work with some wonderful collaborators outside our group. I would

especially like to thank Kerry Siebein in the Maj or Analytical Instrumentation Center (MAIC)

for helping me collect the TEM data. Thanks also to Dr. Kirk Schanze and his group in the

Department of Chemistry for assistance in collecting photoluminescence data.

I am very grateful to my God, my parents, my sister and my brother for their support.

Especially, I thank my sister Youjin for being my town mate. She always did so much to help

and cheer me up. I also wish to thank my other sisters, Sungok and Soyeon. I thank all my

friends for their encouragement and help, both in research and in daily life. I am so lucky to have

them in my life. I wouldn't have gotten this far without them.











TABLE OF CONTENTS


page

ACKNOWLEDGMENT S .............. ...............4.....

LI ST OF T ABLE S ................. ...............8................

LIST OF FIGURES .............. ...............9.....

AB S TRAC T ........._._ ............ ..............._ 14...

CHAPTER

1 INTRODUCTION ................. ...............16......... .....

2 LITERATURE REVIEW ..........._..._ ...............18.......__......


2.1 Fundamentals of Colloidal Nanocrystals ............... .. .... .. ...............18
2.1.1 Chemistry and Physics of Nanocrystals: Size and Shape Issues ......................18
2. 1.2 Synthetic Processes for Colloidal Nanocrystals............... ..............2
2.2 Shape Control of Colloidal Nanocrystals.................. ......... .. .......2
2.2.1 Zero-Dimensional (OD) Spheres and Polyhedrons of Nanocrystals ................. .27
2.2.2 One Dimensional (lD) Rods and Wires of Nanocrystals ................. ...............29
2.2.2.1 1D Semiconductors ...._ ......_____ .......___ ............2
2.2.2.2 1D Metal Oxide Nanocrystals .............. ..... ...............32.
2.2.3 Two Dimensional (2D) Discs and Prisms of Nanocrystals ............... ...............34
2.2.4 Advanced Shapes of Nanocrystal .............. ......... .............3
2.3 Proposed mechanism for shape-control growth of nanocrystals ................. ...............40
2.3.1 Kinetically Induced Anisotropic Control ................. .... .. ....... .................. ..41
2.3.1.1 Cyrstalline Phase Control of Nuleating Seeds by Temperature .........41
2.3.1.2 Surface energy modulation by capping surfactants ........._.................43
2.3.1.3 Growth Regime Control by Monomer Concentration and Temperature45
2.3.2 Oriented Attachment ..........._.... ......_._ ...............47.....
2.4 Application of Nanocrystals in Biomedicine .................. ....__._ ........ .......... ......4
2.4.1 Biocompatible Magnetic Nanocrystals for MR Contrast Effects ......................49
2.4.2 Luminescent Nanocrystals for Fluorescence labels ................. ............... .....51

3 SYNTHESIS AND CHARACTERIZATION OF LUMINESCENT GADOLINIUM
OXIDE NANOCRYSTALS ........._.___..... .__. ...............55....

3.1 Introduction ........._.___..... ._ __ ...............55.....
3.2 Experimental Section .............. ...............55....
3.2.1 M materials ............................. ....... ............5
3.2.2 Synthesis of platelet Gd203: Eu3+ nanocrystals ................. .......................56
3.2.3 Synthesi s of spherical Gd203: Eu3+ nanocrystal s ................ ............ .........5 7
3.2.4 Characterization of Rare Earth Doped Gd203 Nanocrystals. ................... ..........58
3.3 Results and Discussion............... ...............5











3.3.1 Nanoplates and Nanospheres of Gd203: Eu3+ .. .. .. .. .. .. .. .. .. .. .. .. .. .59
3.3.2 Shape control of Gd203: Eu3+ nanocrystals ................. ................ .....__.63
3.3.2.1 Effects of Gd-precursor .............. ...............63....
3.3.2.2 Effects of Surfactants ............... .. ....____ ...............64
3.3.2.3 Effects of the Precursor/Surfactant Ratio .............. .....................6
3.3.2.4 Effects rate of Heating Rate from 2000C to 2900C ..........................67
3.3.3 Crystal Structures of Gd203: Eu3+ Nanocrystals .................. ............. ..........68
3.3.4 Luminescence Properties of Gd203 : Eu3+, Tb3+ and Er3+ Nanocrystals ...........69
3.3.4.1 Eu3+ Fluorescence in Oxides............... .. ...............69

3.3.4.2 Luminescence vs. Gd203 Crystallography............. ...............70
3.3.4.3 Nanocrystals of Gd203: Tb3+............. ....... ...............76
3.3.4.4 Near Infrared Emission From Gd203: Er3+....... ...............77
3.3.5 Eu3+ IHCOrporation into Gd203 Nanocrystals .............. ...............77....
3.3.6 Thermo-gravimetric analysis (TGA) .............. ...............80....
3.3.7 FTIR analysis ................ ...............84........... ....
3.4 Conclusions ................. ...............86.................

4 WATER SOLUBLE SURFACE MODIFICATION OF LUMINESCENT
GADOLINIUM OXIDE NANOCRYSTALS FOR BIOMEDICAL RESEARCH ...............87

4.1 Introduction ................. ...............87.................
4.2 Experimental Sections............... ...............89
4.2.1 M materials .............. ....... ....... ........... ....... .. .. ..........8
4.2.2 Synthesis of Hydrophobically Modified Poly(Acrylic Acid) ................... .........89
4.2.3 Synthesis of Hydrophilic Gd203 : Eu3+ Nanocrystals ..........._...__. .............. ..89
4.2.3 Characterization .............. ...............90....
4.3 Results and Discussion...................... .... .. .............. ........9
4.3.1 Water Soluble Surface Modiaication of Gd203:Eu3+ Nanocrystals .................. .90
4.3.2 Luminescence properties of hydrophilic surface modified Gd203:Eu3+
nanocrystals .............. ........ ...... ............... .. ................... .............9
4.3.3 Dispersion Properties of Hydrophilic Surface Modified Gd203:Eu3+
Nanocrystals ................. ...............94.................
4.4 Conclusions ................. ...............95.................

5 SYNTHESIS AND CHARACTERIZATION OF TERNARY ZnGa204: Eu3+
NANOCRY STALS .............. ...............96....

5.1 Introduction ................. ...............96.................
5.2 Experimental .............. ...............97....
5.2.1 M materials .............. .... ... ....... .... .. .........9
5.2.2 Synthesis of ZnGa204: Eu3+ Nanocrystals ................. ................. ..........97
5.2.3 Characterization of ZnGa204: Eu3+ Nanocrystals ................ ......................98
5.3 Results and Discussion................... ...... ...............9
5.3.1 Shape Control of ZnGa204: Eu3+ Nanocrystals ................. .......................99
5.3.2 Size Control of ZnGa204: Eu3+ Nanocrystals ................ ........._.............101
5.3.3 Crystal Structures of ZnGa204: Eu3+ nanocrystals ......._. ......... .......... .....102
5.3.4 Luminescence properties of ZnGa204: Eu3+ nanocrystals ..........._.._. .............103











5.3.5 Thermo-gravimetric analysis (TGA) .............. ...............105....
5.4 Conclusions ................. ...............106................

6 CONCLUSIONS .............. ...............107....


6.1 Synthesis and Characterization of Luminescent Gadolinium Oxide Nanocrystals ....107
6.2 Water Soluble Surface Modification of Luminescent Gadolinium Oxide
Nanocrystals for Biomedical Research ................ ..... .......... ... .... ......... ........10
6.3 Synthesis and Characterization of Ternary ZnGa204: Eu3+Nanocrystals...................108

7 FUTURE WORK ................. ...............109................

LIST OF REFERENCES ................. ...............110...............


BIOGRAPHICAL SKETCH ................. ...............121......... ......










LIST OF TABLES


Table page

2-1 Various rare earth oxides synthesized by thermolysi s................. ....___ ........._.....3 7

3-1 Nanocrystal shape, normalized PL intensity and Eu concentration in Gd203:Eu3+
nanocrystals versus the Gd precursor. ............. ...............77.....

3-2 Three main factors for doping efficiency of Eu in Gd203 nanocrystals.............................8

3-3 Normalized weight loss based on TGA data............... ...............83..

5-1 Precursors and surfactants used to synthesize ZnGa204: Eu3+ nanospheres ................... ...99










LIST OF FIGURES


Figure page

2-1 Size-tunable fluorescence and spectral range of CdSe QDs with different core size........19

2-2 Energy vs. density of electronic states (DOS) for inorganic crystals of 3, 2, 1, and 0-
dim tensions .............. ...............20....

2-3 Band gap of CdSe quantum rods versus length and width viewed from two different
angles.... ......... .............. 20.......

2-4 TEM images (A-C) and UV-VIS absorption (solid lines) and
photoluminescence(dotted lines) spectra (D- F) of three nanorod CdSe samples. ............21

2-5 Blocking temperature and magnetic coercivity of 4 nm cobalt nanospheres and
nanodiscs ................. ...............22.................

2-6 Crystal-growth diagram. ........... ...............23......

2-7 Illustration of the organometallic precursor method for synthesis of CdSe quantum
dots...... ....... ...............25........

2-8 Geometrical shapes of inorganic nanocrystals. ................ ................. ..............27

2-9 High resolution transmission electron microscope (HRTEM) images of (a) spherical
CoFe204 nanocrystals and (b) multiple and (c) CoFe204 cube nanocrystals.....................28

2-10 HRTEM images (a) y- Fe203 nanocrystals (b) diamond (c) sphere (d) triangle (e,f)
hexagon nanocrystals ........... ...............29......

2-11 CdSe nanorods with different sizes and aspect ratios in different concentrations of
HPA /TOPO surfactants ........._..... ...............3 0....___. ...

2-12 TEM images of (a) starting ZnO nanospheres, and (b) after one day growth of ZnO
nanorods by an oriented attachment process .............. ...............30....

2-13 GaP nanocrystals. HRTEM images of (a) zinc blende nanospheres and (b) wurtzite
nanorods, and absorption and photoluminescence colors from (c) nanospheres and
(d) nanorod s .............. ...............3 1....

2-14 TiO2 nanocrystals formed from single crystal via oriented attachment. ................... .........32

2-15 TEM images of nanorods of (a,b) tungsten oxide (c,d) manganese oxide and (e,f)
titanium dioxide .............. ...............33....

2-16 TEM image of self- assembled 2 nm diameter ZnO nanorods. Inset: higher
magnification image showing the oriented stacks of 1D ZnO............... ..................3










2-17 TEM image of Co nanodisks either self-assembled into ribbons (edge-on view) or
lying flat (right) on the sample support surface ................ ...............34..............

2-18 TEM images (a) Cu2S nanodics], (b) Nis nanoprims............... ...............3

2-19 TEM images and schematic of morphology changes: (a) nanospheres before
irradiation, (b) nanoprisms after 70 hours of irradiation, and (c) the color of light
Rayleigh scattered by nanoprims and nanospheres .............. ...............35....

2-20 Two dimensional (2D) lanthanide oxide nanocrystals: (a) Gd203 nanoplates, (b)
model for the nanoplates assembly, (c) Eu203 nanodisks, (d) Er203nanodisks, and
(e) Pr203 nanoplates. ............. ...............36.....

2-21 Tetrapod shaped nanocrystals of (a) CdSe, (b) MnS, and (c) proposed model of CdTe ...38

2-22 Influence of the shape of CdTe tetrapods on optical absoprtion spectra. (a) tetrapods
having comparable arm diameters but different diameters; (b) tetrapods having
comparable arm diameters but different lengths ................. ...............38...............

2-23 PbS nanocrystals with shapes corresponding to (a) rod-based multipods at 140 OC,
(b) tadpole-shaped monopod, (c) I-shaped bipod, (d) L-shaped bipod, (e) T-shaped
tripod, (f) cross-shaped tetrapod, (g) pentapod, (h) star-shapes at 180 OC, (j)
truncated octahedrons at 250 oC, and (k) conversion of cubes to star-shape to 1D rod-
based multipods by control of the growth parameters. ................ .......... ...............39

2-24 PbSe nanocrystals showing (a) zigzag nanowires packing of octahedral building
blocks,(b) star shaped nanocrystals, (c) radially branched nanowires, and (d)
nanori ngs through ori ented attachm ent ................. ...............40........... ..

2-25 Shape evolution of MnS nanocrystals controlled by the growth temperatures : wires
at 120 oC, (b) spheres at 1800C, and (c) cubes at 250 oC. ............... ...................4

2-26 Variation of the shapes of CdS nanocrystals by changing growth temperature from
(a) 3000C-nanorods, (b) 180 oC-bipods and tripods, and (c) 120 oC-tetrapods..............43

2-27 Temperature-mediated crystalline phase control of (a) MnS and (b) CdS
nanocry stal s........._..._. ...._ ... ...............43.....

2-28 Anisotropic growth along [001] direction of CdSe nanocrystals. The surface growth
rate was effected by selective capping by surfactants to produce (a) short rods (b)
medium rods, and (c) long rods .............. ...............45....

2-29 Disc-shaped nanocrystals of (a) Co and (b) CuS produced by preventing growth
along the [001] direction due to selective capping by surfactants. ................ ................45

2-30 Shape control of Pb S nanocrystals dependent on the growth regime ............... .... .........._.46










2-31 Growth of 1D nanocrystals by the oriented attachment mechanism. (a) aligned TiO2
nanocrystals]; (b) alignment of PbSe nanocrystals [94]; (c) ZnS nanorods containing
some fraction of spherial nanocrystals; (d) ZnS nanorods obtained after the aging;
(e) summary of steps in oriented attachment mechanism for ZnS nanorods ................... ..48

2-32 (a-e ) Nanoscale size effects of iron oxide nanocrystals on magnetism and induced
MR signals. (f) Schematic of DMSA-coated water soluble Fe304 nanocrystals with
multifunctionalities. ............. ...............51.....

2-33 Schematic representation of the use of QDs for bio-labelling. (a) Water soluble QD
comprise a core and hydrophilic shell; (b) QD conjugate with biological molecule
(drawn in red); (c) Bioconjugated QD that binds specifically to designated receptors.....52

2-34 Representative QD core materials scaled as a function of their emission wavelength
over spectrum ................. ...............53.................

2-3 5 (a) Multicolor staining of HeLa cell with red and green QDs. [130] (b) In vivo
labeling of a Xenopus embryo with green-micelle-coated QDs. [122] (c) Image of
QDs targeting prostate cancer in vivo in a mouse bearing a xenograft tumor targeted
using orange-red emitting QD probes. .............. ...............53....

3-1 Synthesis of Gd203: Eu3+ nanospheres by the nonhydrolytic hot solution
route ............... ...............58.................

3-2 HRTEM images of Gd203: Eu3+ nanocrystals (a-b) nanoplates (c) the images
correspond to no tilt and (d) correspond to tilt. Label face as (001) and edges as
(100, 010). .........._. ......_._ ...............60...

3-3 HRTEM images of self-assembled 'stacks' of nanoplates of Gd203: Eu3+ shown at
three different magnificat ions. .............. ...............61....

3-4 HRTEM images of nanospheres of Gd203:Eu3+ from thermal decomposition of
Gd(acac)3 preCUTSor in the presence of (a) hexadecanediol or (b) TOPO. .....................62

3-5 HRTEM images of different shaped Gd203: Eu3+ nanocrystals from (a) Gd(acac)3
(b) Gd-acetate, (c) Gd-chloride, or (d) Gd-octanoate precursors............... ...............6

3-6 HRTEM images of Gd203: Eu3+ nanocrystals synthesized from thermal
decomposition of Gd(acac)3 preCUTSor (a,b) with (a) hexadecanediol (HDD-
nanospheres) and (b) TOPO surfactant (nanospheres and nanoplates), and of Gd-
octanoate precursor (c,d) with (c) TOPO (nanoplates) and (d) octadecene (complex
larger shapes) surfactant. ............. ...............65.....

3-7 HRTEM images of Gd203: Eu3+ nanocrystals synthesized with a Gd(acac)3/TOPO
molar ratio of (a) 1:1 (nanospheres) (b) 1:2 (mixed nanospheres and nanoplates), or
a Gd(acac)3/HDA ratio of (c) 1:2.5 (d) 1:5. ............. ...............66.....










3-8 HRTEM images of Gd203: Eu3+ nanocrystals synthesized with ratios of
Gd(acac)3/HDD of (a) 1:1, (b) 1:3, or (c) 1:5 illustrating oriented attachment of
nanospheres (b-red arrows) to form nanorods in (c) ................. ................. ..........67

3-9 HRTEM images of Gd203: Eu3+ nanocrystals synthesized with a heating rate of (a)
250C/min (mixed curved nanoplates and nanospheres) and (b) 50C/min. ................... ......68

3-10 XRD pattern of Gd203: Eu3+ (a) nanoplates and (b) nanospheres. compared to the
JCDPS patterns for cubic and monoclinic Gd203............. ............6

3-11 Optical transitions from Eu3+ in either a C2 or a. S6 symmetry site in Gd203. ...................70

3-12 Two Gd3+ symmetry sites in Gd203. ............. ...............71

3-13 PL excitation spectra of Gd203:Eu3+ nanocrystals for the emission line at 612 nm of
nanoplates and nanospheres. .............. ...............72....

3-14 Relative PL emission spectra of Gd203 : Eu3+ (10 mol %) of a) nanospheres, b)
nanoplates c) mixed shapes............... ...............75.

3-15 PL emission spectra of mixed shapes of Gd203: Eu3+ (10 mol %) prepared with a)
of Gd(acac)3 / TOPO (1:2), and b) Gd(acac)3 /HDD (1:5). ............... ...................7

3-16 Relative PL emission spectra of Gd203 : Tb3+ (10 mol %) nanocrystals with the
shape of a) spheres (synthesized with Gd(acac)3 and hexadecanediol ), and b)
plates (synthesized with Gd acetate and TOPO) in the presence of oleic acid,
oleylamine and benzyl ether. ............. ...............76.....

3-17 Near infrared (NIR) PL emission spectra from Gd203:Er3+ nanospheres under 488 nm
laser excitation. ............. ...............77.....

3-18 Normalized PL emission intensity at 612 nm, Eu concentration and HRTEM
photomicrographs of Gd203:Eu3+ nanocrystals ................. .............. ...... 79.........

3-19 TGA data from Gd acetate and Gd(acac)3 preCUTSors between RT and 7500C. ................82

3-20 TGA data from nanoplates and nanospheres of Gd203:Eu3+ synthesized ......................83

3-21 FTIR spectra from nanoplates and nanospheres of Gd203: Eu3+ and surfactants..........._...85

4-1 Schematic model of surface modification of Gd203:Eu3+ nanocrystals using
octylamine modified PAA. ............. ...............90.....

4-2 TEM images of Gd203:Eu3+ nanospheres capped with oleic acid and HDD before (a)
and after (b-c) hydrophili c modification ................. ...............91...............

4-3 TEM images of Gd203:Eu3+ nanospheres capped with HAD before (a) and after (b)
hydrophilic modification ................. ...............92.................










4-4 TEM images of Gd203:Eu3+ nanoplates before (a) and after (b) hydrophilic surface
m odification. ............. ...............92.....

4-5 PL emission spectra of oleic acid and HDD capped hydrophobic (before) and PAA
capped hydrophilic (after) Gd203:Eu3+ nanospheres. ................ ................ ........ .93

4-6 PL emission spectra of had capped hydrophobic (before) and hydrophilic (after)
Gd203:Eu3+ nanocrystals ................. ...............94................

4-7 Zeta potential versus aqueous solution pH of hydrophilic PAA surface modified
Gd203:Eu3+ nanocrystals ................. ...............95........... ....

5-1 HRTEM images of ZnGa204: Eu3+ nanospheres prepared with the precursors and
surfactants corresponding to (a-d) in Table 5-1 ................ ...............100.............

5-2 HRTEM images of ZnGa204: Eu3+ nanocrystals prepared with Zn acetate dehydrate
with the ratio of Zn:hexadecanediol of 1:2.5 under standard conditions. ................... .....100

5-3 HRTEM images of ZnGa204: Eu3+ nanocrystals synthesized with a ratio of Zn(acac)2
/ hexadecanediol (a-b) 1:2.5, and (c-d) 1:5; (e) selected area electron diffraction
(SAED) pattern from a cubic spinel crystalline phase ................. .........................101

5-4 The cubic spinel structure of ZnGa204 ..........___.....__ .. ...............10

5-5 XRD pattern of ZnGa204: Eu3+ nanocrystals ................. ...............103........... ..

5-6 PLE spectrum for emission at 612 nm from ZnGa204: Eu3+ nanocrystals. The inset
shows the UV absorption spectrum of undoped ZnGa204 nanocrystals. ................... ......104

5-8 TGA data from larger completed shaped ZnGa204: Eu3+ nanocrystals ..........................106









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

SYNTHESIS AND CHARACTERIZATION OF LUMINESCENT
OXIDE NANOCRYSTALS

By

Sooyeon Seo

August 2007

Chair: Paul H. Holloway
Major: Materials Science and Engineering

Oxide nanocrystals with controlled geometries exhibit unique shape dependent optical and

structural properties. Shape-controlled synthesis of rare earth doped gadolinium oxide (Gd203:

Eu3+, Tb3+ or Er3+) and zinc gallate (ZnGa204:Eu3+) nanocrystals by non-hydrolytic high

temperature (~3000C) methods are reported. Various shapes of Gd203 nanocrystals were

synthesized, including spheres and plates and advanced shapes such as curved rods and triangles.

The nanocrystal shape was shown to be a function of the synthesis parameters, such as metal

precursors (acetate, acetyl acetonate, chloride or octanoate) and surfactant type ( tri-octyl

phosphine oxide-TOPO, or hexadecanediol-HDD) and concentration (metal precursor: surfactant

molar ratios of 1:2 to 1:5), as well as heating rate (5-250C/min.) between pre-heat (2000C) and

reaction (2900C) temperatures. The effects of these parameters upon nanocrystal shape were

explained based on nucleation and growth of oxide nanocrystals. The photoluminescence

intensity from Gd203:Eu3+ WaS shown to increase as the concentration of dopant incorporated

into the nanocrystals increased. The doping efficiency, defined to be the percentage of dopant

incorporated into the nanocrystals, ranged from 0.57-6. 1 mol%, was a function of shape of the

Gd203 : Eu3 and was discussed in terms of the rate of reaction, product yield and crystal

structure.









To be used for labeling biomolecules such as DNA, RNA, or proteins, water soluble

luminescent nanocrystals are required. Doped Gd203 nanocrystals prepared by the non-

hydrolytic hot solution method are hydrophobic and are not soluble in water due to organic

surfactant encapsulation. A general strategy to convert hydrophobic luminescent nanocrystals

(e.g. Gd203) to water soluble particles by over-coating the hydrophobic surface with amphiphilic

polymers is reported. Specifically, octylamine modified surfaces were coated with poly (acrylic

acid) and water dispersions of Gd203:Eu3+ WeTO Still stable at room temperature after four

months.

The non-hydrolytic hot solution synthesis technique was used to grow monodispersed

ternary oxide nanospheres (~5nm) of ZnGa204: Eu3+ from a, variety of metal precursors. Using

Gd acetate dehydrate, large (~20nm) complex shaped (triangle and rectangle) ZnGa204: Eu3+

nanocrystals were obtained. Based on X-ray diffraction data, the nanocrystals had a cubic spinel

structure with no impurity phases. The size of the ZnGa204: Eu3+ nanospheres could be varied by

changing the molar ratio of Zn to surfactants, with higher concentrations of surfactant resulting

in smaller nanocrystals. Analysis of the PL emission suggests that the Eu3+ ionS WeTO

incorporated into the ZnGa204 host.









CHAPTER 1
INTRODUCTION

In the nanoscale regime, the chemical and physical properties of inorganic crystals are

highly dependent on geometrical factors such as size and shape [1, 2]. Precise control of such

factors allows one not only to observe unique properties of the nanocrystals but also to tune their

chemical and physical properties as desired. During the past decades, researchers have explored

efficient synthetic routes to produce well-defined inorganic nanocrystals with a controlled size

and shape. Some advanced nanocrystal structures with interesting geometries including wires [3,

4], tubes [4, 5], ribbons [6, 7], and more complex shapes [7, 8] were produced with gas phase

approaches. On the other hand, the colloidal approach in liquid media provides a convenient and

reproducible route for the fabrication of nanocrystals with controlled size and shape. This

enables the resulting nanocrystals not only to be precisely tuned at the sub-10 nm scale but also

to be easily dispersed in organic or aqueous media for numerous potential applications in

electronic and biological systems. Along with size control, anisotropic shape control of

nanocrystals has been attained through liquid methods [9-17].

The nonhydrolytic molecular precursor decomposition method is an effective route for the

controlled synthesis of both isotropic and anisotropic colloidal nanocrystals [1 l]. Nanocrystals

obtained by such nonhydrolytic synthetic methods in general possess excellent crystallinity and

monodispersity in terms of size and shape. Although there are increasing numbers of examples

of colloidal semiconductor nanocrystals with anisotropic shapes, from simple one-dimensional

rods and wires to advanced multipods and stars, reports of shape-guided rare earth doped oxide

nanocrystals are very limited. To obtain luminescent oxide nanocrystals with the desired shape

and to investigate the relationship of the controlled structures with the luminescent efficiency,









systematic understanding of shape-guiding processes is necessary to provide the insight for

efficient doping.

In this dissertation, Chapter 2 reviews the fundamental physics, synthetic methods, shape

controlled nanocrystals, proposed mechanisms and application in cell biology. In Chapter 3, the

preparation of rare earth doped gadolinium oxide nanocrystals with a nonhydrolytic synthetic

route resulting in various shapes is discussed. The effects of reaction variables on shape-guiding

growth were also studied, as were their crystallographic and luminescent properties. In Chapter

4, a general strategy is introduced to make luminescent Gd203 nanocrystals water soluble by

over-coating hydrophobic surfaces a with amphiphilic polymer. The conversion of hydrophobic

nanocrystals into hydrophilic particles is prerequisite for biological applications. As a labeling

material for biomolecules, especially for the sensitive determination of molecules such as DNA,

RNA, or proteins, well-controlled nanocrystals are required in the nanoscale regime. In Chapter

5, ternary (ZnGa204: Eu3+) oxide nanocrystals have been grown in the shape of spheres and

plates and advanced shapes such as curved rods and triangles. The size of the ZnGa204: Eu3+

nanocrystals can be controlled by varying the concentration of organic surfactants. Finally,

conclusions and future work are summarized in Chapter 6.









CHAPTER 2
LITERATURE REVIEW

2.1 Fundamentals of Colloidal Nanocrystals


2.1.1 Chemistry and Physics of Nanocrystals: Size and Shape Issues

Nanomaterials are small-scaled with at least one dimension between Inm and 100 nm, and

include a range of shapes, such as nanospheres, nanocubes, nanorods, nanosheets and nanotubes

[6, 18-21]. They exhibit novel optical, electronic, magnetic, chemical and mechanical properties

that are different from their bulk counterparts from which they can be derived. Nanocrystals have

attracted broad attention in a variety of fields including catalysis, photovoltaics, and coatings as

well as in the emerging fields of nanomedicine where they can be used as imaging agents and

drug-delivery vectors. Also, inorganic nanomaterials that will be the key component of futuristic

nano-devices have recently emerged as a promising candidate to overcome many of the

limitations of current technologies.

Crystals consist of a periodic array of specific repeating atoms or molecules. The

individual repeating molecules have quantized electronic structures while crystals have

continuous electronic band structures that result from the overlap and combination of atomic

and/or molecular orbitals of the repeating atoms or molecules. Therefore, isolated atoms and

molecules exhibit quantum mechanical properties, while the chemical and physical properties of

bulk crystals obey the laws of classical and quantum mechanics. However, when the crystal size

decrease into the nano-scale regime (1~100 nm), the electronic band of the crystals is further

quantized and the resulting nanocrystals behave as an intermediate between molecules and

crystals [22-24].

In semiconductor quantum dots (QDs), the density of electronic energy levels as a function

of the size varies systematically, which is known as quantum size effects [25-27]. The surface of










nanocrystals plays a key role in their electronic and optical properties due to the high surface to

volume ratio of semiconductor nanocrystals. Also, the melting temperature of nanocrystals

decreases when the nanocrystal size is reduced [28].

These nanoscale phenomena are strongly related to the crucial parameters of size and

shape. Early investigations focused on the nanoscale size effects since the physical properties of

nanocrystals are influenced by the size of nanocrystals [25, 26]. It has been demonstrated both

theoretically and experimentally that the quantized electronic band structure of a crystal is

changed as the crystal size is reduced, resulting in an increase in the band-gap energy [23].

Figure 2-1 demonstrates the size-tunable fluorescence properties and spectral range of six CdSe

quantum dot (QD) nanocrystals with different core sizes [29].

1.0- 10nmABS -.
-* 510 nmn EM
-*-530nm EM
-*-555 nmEM ~CZ
-o- 570 nm EM
-*-590 nm EM rst ore(A
-*-810nm EM I e-~ n
tnxen nn







400 450 500 550 600 650 700
Wavelength (nmn)


Figure 2-1. Size-tunable fluorescence and spectral range of CdSe QDs with different core size.
All samples were excited at 365 nm with a UV source [13].

Similarly, the shape of nanocrystals plays a crucial role in the determination of their

properties [30, 31]. The density of energy states (DOS) for inorganic crystals, which is predicted

by a simple particle in a box type model, evolves from near-continuous energy levels into

quantum states separated by large energies as the dimensionality is decreased from three to zero

(Figure 2-2).















10 00


i:


I~ I t \
L7 -.
II
I' II ` : ` ~ I
Energy


O D


DOS


Figure 2-2.Energy vs. density of electronic states (DOS) for inorganic crystals of 3, 2, 1, and 0-
dimensions [1]

The band-gap energy (E,) of nanocrystals is also influenced by their shape. The band-gap

energy diagram of CdSe nanocrystals with various diameters and lengths, shown in Figure 2-3,

clearly exhibits shape effects [30]. The UV-Vis absorption and photoluminescence spectra on

CdSe rods with different aspect ratio have peak widths comparable to those of spherical quantum

dots (Figure 2-4) [31].









Figue 23.Bnd gp o Cde qantu ros vrsuslenth nd idthvieed rom wo iffren

angles [30] Th mes is th etft oraaarddt)


3D 2D























Fiur -4 EMiags(AC adUVVS bortin(sld ies ndpotlmiecec
(dttdlie) peta D F o hrenaordCdesapls Tenaord hv
diameters~ ~ I of~. madlngh f1(AadD) 0( n ), n 0n n
F ) [31].S

Ote poeris sc a anei poetis o rstl aeasosrogydeedeto
th hp f aorsas 3] nth aeo obl aopersta re~4n ndimtr h












taofashr(Figure 2-5). Enhnce shape A- ani VVSa sortropy (o indues) an prefretialalignment fte




mageti sh pi fnsalongsthelon axis. of the dise fcs. l aopee htae mi imtr h












4~1 dise
10 ii i--"


i: ,n~)~ r"" OI1~C RT



;Lc" O sphere
.-" ~r
-J -_


()t~C

(4bC,
3
E ,,,,
or

~ ra~c,,
t;j
1
~ oct,

~1C(r

DbCU


Figure 2-5.Blocking temperature and magnetic coercivity of 4 nm cobalt nanospheres and
nanodiscs [32].

2.1.2 Synthetic Processes for Colloidal Nanocrystals

Synthetic procedures for nanocrystal preparations have been performed by both 'bottom-


up' and 'top-down' approaches, but the 'bottom-up' approach have become favored. In the

bottom-up approach, small building blocks are assembled into larger structures; this approach


includes liquid phase colloidal synthesis in aqueous and nonhydrolytic media. The liquid-phase


colloidal synthetic approach can provide access to extremely fine structure for the convenient


and reproducible shape-controlled synthesis of nanocrystals, and allows the nanocrystals to be


dispersed in either an aqueous [33-39] or a nonhydrolytic media [10, 11, 40-56]. Moreover, these

colloidal nanocrystals are often referred to as 'molecular nanocrystals' and can be modified by


chemical hybridization with other functional materials for applications in electronics and


biological systems.


Liquid phase synthesis relies on chemical reduction of metal salts, electrochemical


pathways, or the controlled decomposition of metastable organometallic compounds by


coprecipitation. Coprecipitaion process for colloidal nanocrystals growth tend to exhibit the

following characteristics [21]:


re -33aOt
Ilii


Osphea

Il-v' O ~:~



.-- dise










(i) The products of the precipitation reactions are sparingly soluble species formed under
conditions of high supersaturation.

(ii) Nucleation is a key step of the precipitation process and conditions are required such
that a large number of nanoparticles are nucleated.

(iii) Secondary processes, such as Ostwald ripening and aggregation, will strongly affect
the size, morphology, and properties of the nanoproducts.

(iv) The surfaces of nanocrystals may be stabilized by a large variety of molecules, For
example, donor ligands, polymers, and surfactants are used to control any growth of
the nanoparticles and to prevent them from agglomerating.

A crystal-growth diagram is illustrated in Figure 2-6. When the monomer concentration

reaches a supersaturation level, nucleation occurs and the monomer is continuously incorporated

onto the seeds resulting in the growth of the nanoparticles and a gradual decrease of monomer

concentration. During these nucleation and growth stages, control of growth parameters and

crystalline phase is critical in determining the final size and shape of the nanocrystals [57].




Nucl eati on Stabilized nanocrysctals

ri~o nuclei




G8Crowth ee

a molecule



tim e

Figure 2-6. Crystal-growth diagram. Csat = saturation monomer concentration above which
nucleation takes place, and Cequil = equilibrium monomer concentration below which
growth ceases [43].

In earlier methods of colloidal nanocrystals synthesis, the nanocrystals were usually grown

at room temperature in hydrolytic (i.e. aqueous) media in the presence of structured micelles [34,










37, 38, 58-61]. Several semiconductor [37, 38, 58, 59, 61] and metal oxide [34, 60, 61]

nanocrystals were grown by this method starting from ionic precursors inside organic micelles.

The disadvantage of these aqueous solution syntheses is that the pH value of the reaction mixture

must be adjusted in both the synthesis and purification steps, and the nanoparticles often exhibit

relatively poor crystallinity and/or polydispersity in their size and shape [36, 38, 62].

As a method to reduce these problems, nonaqueous high temperature thermal reaction

methods with organic surfactants have been developed. Nanocrystals produced by this

nonaqueous colloidal route often exhibit excellent crystallinity and monodispersity. Moreover,

this route has several advantages for the shape-controlled synthesis of nanocrystals including

separation between the nucleation and growth steps, and easier control of growth parameters by

changing variables such as the type of surfactant molecules, monomer concentration, and

temperature.

The work of Steigerwald and co-worker on the use of organometallic precursors in the

nonaqueous solution phase synthesis of nanocrystals provides guidance in nanocrystals synthesis

[38]. Murray and co-workers [63] introduced dimethyl cadmium (Me2Cd) and trioctyl

phosphine selenium (TOP-Se) as precursors to synthesize high quality CdSe quantum dots as

shown in Figure 2-7. In the organometallic precursor method, the coordinating solvent (e.g.

trioctylphosphine oxide-TOPO, TOP and trioctylamine (TOA) provide the crucial environment

for the growth process, stabilizing the resulting colloidal dispersion. The coordinating ligands,

i.e. surfactants, play a crucial role in mediating the growth of the particles. A variety of

organometallic precursors and high boiling point coordinating solvents are potential candidates

for these types of synthesis.















n 250 ~- 320 oC if Se-TOP
injection


Cd source
In TOPO



Figure 2-7.Illustration of the organometallic precursor method for synthesis of CdSe quantum
dots

The traditional organometallic approach has some disadvantages, such as the fact that the

starting materials are generally toxic, expensive, and sometimes unstable and explosive.

Therefore, an inert atmosphere and temperature control during storage and reaction is often

required for the chemical precursors. As an alternative route, the advantages of the traditional

organometallic method may be retained by using less expensive precursors that do not require

sophisticated equipment and procedures. A wide variety of precursors have been identified as

possible candidates, including metal acetates [54] and metal acetylacetonates [40, 64-67]. This

synthetic route provides:

(i) Less expensive and simpler approaches, since this method is a one step synthesis.
Nanocrystals, generally monodispersed and highly crystallized, can be achieved
without further size selection.

(ii) High yield, since in a typical reaction the product yield is higher than 50% for
monodisperse nanocrystals.

(iii) Diversity; this route has been used to grow transition metal (Fe, Mn, and Zn [68, 69] )
and rare earth (Gd [70, 71]) oxide nanoparticles.

(iv) Environmentally friendly; precursors such as metal acetates and metal acetylacetonate
are regarded as "green chemicals", and the by-products of the thermal decomposition
are mostly CO2 and H20 [69, 72]









2.2 Shape Control of Colloidal Nanocrystals

Nanocrystals may be regarded as 'artificial atoms' and are the basic units for nano-scale

devices. The design and operation of these devices will be more easily accomplished using

nanocrystals with distinct sizes and shapes. The shape control of nanobuilding blocks is crucial

for the success of future nano-devices. Inorganic nanocrystals can be classified according to

dimensionality and crystal symmetry (Figure 2-8). Highly symmetric isotropic spheres, cubes,

decahedrons, and tetrahedrons can be classified as zero-dimensional (OD) nanostructures and are

the most familiar shapes in the nano world. Nanospheres of semiconductors, metal oxides, and

metals have been synthesized through a variety of chemical methods. Rods, cylinders and wires

are examples of one-dimensional (lD) nanoblocks. Since CdSe nanorods were first reported by

Alivisatos and co-workers [l l], many studies on the synthesis of 1D nanostructures have been

reported [44, 45, 56, 68, 69, 73, 74]. 1D nanostructures exhibit novel optical and magnetic

properties arising from shape anisotropy as illustrated in Figure 2-2 above.

Disc and plates with polygon shapes belong to 2-dimensional (2D) nanostructures. In

addition to such primitive shapes of inorganic nanocrystals, advanced shapes of nanocrystals

have been developed. Multipod structures of semiconductors including bipods, tripods and

tetrapods, and star-shaped nanocrystals are examples [12, 15, 75, 76].









8-Dj ~11~ ~9 ~





el Mm~


2-D ~-C7 ~j3


Figure 2-8.Geometrical shapes of inorganic nanocrystals [57].
2.2.1 Zero-Dimensional (OD) Spheres and Polyhedrons of Nanocrystals
Zero-dimensional (OD) shapes, including spheres and cubes, have been extensively
studied. Initially, Brus and co-workers successfully synthesized various II-IV semiconductor
nanospheres with high colloidal stability in coordinating solvent, but size tunability and
monodispersity of nanocrystals were poor [25]. Bawendi and co-workers developed more
advanced methodologies to prepare various sized OD CdSe nanocrystals via the method of
inj ecting a precursor solution of dimetyl cadmium into trioctylphosphine oxide (TOPO). The size
of nanocrystals varied from 1.2 to 12 nm with high monodispersity and crystallinity and the
nanocrystals obtained were highly soluble in various organic solvent. Optical spectra exhibit size
dependent quantum confinement effects indicating high monodispersity and high crystallinity of
nanocrystals, as shown above in Figure 2-1 [23].
Controlled growth of isotropic OD spherical or cube nanocrystals of CoFe204 has been
reported [40]. The data indicate that the heating rate and growth temperature control the shape of
CoFe204. A slow heating rate produced a low concentration of metal cations from










decomposition of the precursors, and resulted in growth of cubic CoFe204 nanocrystals having

facets with low surface energy. A faster growth rate at a higher temperature with more metal

cations available resulted in crystal growth being much less selective in directions and hence

produced spherical CoFe204 nanocrystals ( Figure 2-9). The shape of the nanocrystals could be

reversibly changed between spherical and cubic. As illustrated in Figure 2-5 above, the shape of

such nanocrystals strongly affect the magnetic coercivity due to surface anisotropy, and

therefore have tremendous potential for high-density information storage.







Fiur 29Hih esltin rnsisio letrn irocoe(HTE) mge o ()an~8n
spherical~~~~~~~~ Coe0 aorsal n b utpe n c ige~1 mC24cb











hiuexao shapes are olthen 2D projection of 3Dtrunmctres of etaeda (Htrnciated octahera and- n
ioaerrspherctivCoely. aorsasan b utpean c ige-2 mC~24c






















Figure 2-10. HRTEM images (a) ~ 12nm y- Fe203 nanocrystals (b) diamond (c) sphere (d)
triangle (e,f) hexagon nanocrystals These shapes are actually 2D projection of 3D
shapes of inset images respectively [45].

2.2.2 One Dimensional (1D) Rods and Wires of Nanocrystals

One dimensional (lD) rod growth is a fundamental step for anisotropic shape control.

Growth of nanocrystals with more complex structures are possible is based on an understanding

of the mechanisms guiding growth of nanorods. Basically, 1D nanostructures of semiconductors

and metal oxides exhibit unique optical [l l, 12] and magnetic properties [77, 78] due to their

anisotropic shape.

2.2.2.1 1D Semiconductors

Non-hydrolytic synthesis are mainly utilized for high quality nanorod synthesis. Alivisatos

and co-workers first reported CdSe nanorods from a thermal reaction of Me2Cd and TOP-Se in a

hot surfactant mixture of TOPO and hexylphosphonic acid ( HPA) [1 l]. In this synthesis, 1D rod

shaped structures result from preferential growth along the [001] direction of wurtzite CdSe that

is promoted by selective adsorption of HPA molecules on specific faces. With increasing HPA

concentration, the nanocrystal shape evolves from spheres to short rods to long rods with aspect

ratios of 5-20.(Figure 2-1 1).









fiO nm


Figure 2-11. CdSe nanorods with different sizes and aspect ratios in different concentrations of
HPA /TOPO surfactants [1l].

By varying monomer concentration in addition to change of surfactants, this approach

enabled large scale production of CdX (where X= S, Se, Te) nanorods with better control of

their aspect ratio. The synthetic method has been successfully applied for the fabrication of other

semiconductors, including ZnO [48], ZnS [79, 80], ZnSe [75, 79], CdS [13, 80], CdTe [43] and

PbSe [47].

Weller and co-workers have proposed a mechanism of growth of ZnO nanorods via

oriented attachment of dimers and oligomers in a hydrolytic synthesis [48]. Specifically, zinc

acetate produces zinc oxide nanospheres through hydrolysis and aging. The growth of individual

nanorods occurs by oriented and partially fused dimers and oligomers in order to remove high

energy surfaces. The oriented attachment of preformed quasi-spherical ZnO nanoparticles results

in almost perfect rods ( Figure 2-12).










Figure 2-12. TEM images of (a) starting ZnO nanospheres, and (b) after one day growth of
ZnO nanorods by an oriented attachment process [48]










Unlike II-IV semiconductor nanocrystals which have been well studied, study of III-V

semiconductor nanocrystals has been limited. This is most likely due to their greater degree of

covalent bonding and the lack of non-toxic precursors. Moreover, except for column III nitrides,

III-V semiconductors favor isotropic zinc blende crystal structures and thus OD nanocrystal

growth is preferred rather than 1D rod growth. Cheon and co-workers have shown that the

crystalline phase of gallium phosphide nanocrystals can be controlled by adopting suitable

surfactants [81]. Zinc blende GaP is the thermodynamically stable low temperature phase, while

wurtzite GaP is the high temperature stable phase that may be metastable at low temperatures

and has desired electronic properties. When sterically limiting amine surfactants (e.g.

trioctylamine-TOA) are used as capping molecules, formation of nanospheres GaP is favored.

However, when sterically less bulky, linear alkyl amines (e.g. hexadecylamine-HDA) are used,

the staggered conformation is not favored. Wurtzite GaP nanorods were formed when a mixed

surfactants of TOA and HDA was used. The resulting nanocrystals showed unique shape-

dependent spectroscopic features. The absorption spectra exhibited shoulders at 3.48 eV for

spheres and 3.46 eV for rods. The photoluminescence peak was at 2.94 eV for 8 nm GaP

nanospheres, but was red shifted to 2.79 eV for 8 x 45 nm rods. ( Figure 2-13).







Figure~~~~~~~~~~~~~~~~~- 2-3 a aorsas RTMiae f()zn led aopee n b





Figre2-.nanophee nandcytas (d) E nanorod [81].n lnd aoshrs n b









2.2.2.2 1D Metal Oxide Nanocrystals

Nanoscale transition metal oxides have attracted considerable interest due to their optical,

magnetic, electrical and catalytic properties [82]. Penn and Banfield reported naturally aligned

titania nanocrystals grown with a hydrothermal process with an oriented attachment mechanism.

[83-85] In this procedure, titanum alkoxide precursors produced diamond shaped anatase titania

nanocrystals. The nanocrystals were truncated with three different crystalline facets parallel to

{0C1}, {112}, andl {01}\ crystal planes. Bycausep the (00C1) face has the largest~ nulmber and the

(101) face has the lowest number of dangling bonds, the surface energy of the (001) face is

higher than that of the (101) face. When a significant thermal energy was supplied to the system,

oriented attachment occurred most commonly on {112}, occasionally on {001}, and rarely on

{101} faces. The mechamism resulted in a lower total free energy by reducing the surface area

where the crystallites were j oined. This mechanism is distinct from Ostwald ripening, which

involves the dissolution of fine particles and growth of larger particels. Clearly, both

mechanisms can operate in such a titania system.












Figure 2-14. TiO2 nanocrystals formed from single crystal via oriented attachment [85].

Recently, Seo and co-workers have synthesized various 1D nanostructures of transition

metal oxide (e.g. W1sO49, TiO2, Mn304 and V205) using a thermal crystal growth process from a

mixture of metal chloride and surfactants [74, 86]. (Figure 2-15) These metal oxide nanorods

have crystallographically aniotropic structures where surface energy is thought to be a crucial









factor. Both tetragonal TiO2 and Mn304 nanocrystals have a {001} surfaCce with+ higher ennergy,
whereas monoclinic W18O49 nanocrystals possess a {010}C) surface with~ hihe enery. Because theCIIP


growth rate is exponentially proportional to the surface energy under the kinetic growth process-

this depends on the model used to predict growth, the energy difference between the higher

versus lower energy surfaces will promote preferential growth along the <001> directions of

TiO2 and Mn304, and along the <010> direction of W18O49, i.e. rods aligned along the higher

energy crystallographic direction.

















Figure 2-15. TEM images of nanorods of (a,b) tungsten oxide (c,d) manganese oxide and (e,f)
titanium dioxide [74].

O'Brien and co-workers have synthesized other 1D metal oxide nanocrystals using

nonhydrolytic methods. Zinc oxide nanorods with a high degree of crystallinity and a narrow

size distribution were assembled into close-packed "stacks" aligned with the long axis parallel to

each other ( Figure 2-16).






















Figure 2-16. TEM image of self- assembled 2 nm diameter ZnO nanorods. Inset: higher
magnification image showing the oriented stacks of 1D ZnO.

2.2.3 Two Dimensional (2D) Discs and Prisms of Nanocrystals

Synthesis of nanocrystals with controlled shapes has concentrated on the 1D

nanostructures, while studies of 2D nanocrystals have been limited. In a kinetically driven

growth regime, 1D nanorod growth is promoted when preferential growth along a specific

direction is faster. Likewise, when growth along a specific axis is inhibited, the formation of 2D

(e.g. disc shaped) nanocrystals may result. Alivisatos and co-workers reported the formation of

disc shaped cobalt nanocrystals by rapid decomposition of cobalt carbonyl in the presence of

linear amines [51]. The nanodiscs self-assembled into long ribbons by discs stacking face-to-

face, perhaps assisted by the magnetic interaction between individual nanodics. ( Figure 2-17)

Selective adsorption by alkylamines inhibit the growth along the [001] direction while allowing

growth along the perpendicular directions resulted in the growth of nanodiscs.











Figure 2-17. TEM image of Co nanodisks either self-assembled into ribbons (edge-on view) or
lying flat (right) on the sample support surface [51].









Ghezelbash and co-workers demonstrated a solventless thermolysis approach to synthesize

Cu2S [87, 88] and NiS [89] nanodiscs. In the presence of alkanethiol surfactants, preferential

growth along the [100] and [1 10] directions and inhibition of growth along [001] direction

results in the formation of Cu2S nanodiscs (Figure 2-18 (a)). Similarly, rhombohedral NiS

nanoprisms results from inhibition of growth along the [110] direction and fast growth along the

perpendicular <111> directions (Figure 2-18 (b)).










Figure 2-18. TEM images (a) Cu2S nanodics [87], (b) Nis nanoprims [89].

Besides control of surfactant, temperature and time, a photo-induced method for

converting of silver nanospheres into triangular nanoprisms was reported [90]. Photons result in

a colloid with distinctive optical properties that result in the nanoprism shape. Unlike spherical

particles that are derived from Rayleigh light-scatter in the blue, these nanoprisms exhibit

scattering in the red, which could be useful in developing multicolor diagnostic labels. (Figure 2-

19)











Figure 2-19. TEM images and schematic of morphology changes: (a) nanospheres before
irradiation, (b) nanoprisms after 70 hours of irradiation, and (c) the color of light
Rayleigh scattered by nanoprims and nanospheres










Recently, nanoplates of lanthanide oxide nanocrystals have been reported [70, 71]. Square

plate-shaped Gd203 were produced through thermal decomposition of Gd acetate precursor in the

presence of a noncoordinating solvents (octadecene) and oleic acid in coordinating oleylamine

[70]. These nanoplates were highly crystalline with a cubic structure, and the sides of square

nanoplates were parallel to (100) and (010) faces while the top and bottom faces were parallel to

the (001) plane. The nanoplates were only one unit cell of Gd203 thick along the c-axis, with the

top and bottom (001) faces modified by the organic ligands (Figure 2-20 (a,b)).

This important family of rare earth compounds was synthesized by thermolysis of their

benzolacetonate complexes in a oleic acid/oleylamine solvent [52]. Due to selective adsorption

of capping ligands on certain cubic faces during crystal growth, nanocrystals with different

morphologies, such as nanoplates and nanodiscs, were created (Table 2-1 and Figure 2-21 (c-e)).


~B~it~d~b


Figure 2-20. Two dimensional (2D) lanthanide oxide nanocrystals: (a) Gd203 nanoplates, (b)
model for the nanoplates assembly [70], (c) Eu203 nanodisks, (d) Er203nanodisks,
and (e) Pr203 nanoplates [52].


*(lm,
)trm-~l




le I










Table 2-1. Rare earth oxides synthesized by thermolysis of Ln(BA)3(H20)2 or Ce(BA)4 where
Ln=La toY and BA = benzolacetonate, in oleic acid (OA)/Oleylamine (OM) at 250-
330 oC for 20-60 min [52].
OLfoM Tr~q I[mi=] sauctwos Morpho-logy
La,O, 1:7 330 60 103 7 anm nanoplatre
G*C, 0 250 20 Fma3m 2.6 m nrnopolyhedron
PrO, 3:5 310 5o10 20 nm nanoplate
Nd!Or 3:5 310 60 Ta3; 11 nm na~noplate
5m20,rD 3:5 310 60 ra3 11 nm nanorplate
26 nm nanodisk
EuLOr 3:5 3"10 60 @3, 12 nm nanoplate
321 nm nanodisk
Gd;,O, 3:5 310 60 1a3 30 nm nanodisk
Tb,0, 3:5 310 60 to43 34 nm nanodCisk
ErO, 3:5 310 60 13 43 nmr nlandisk
Y,0, 3:.5 310 60 to3 65 nm nanadisk


2.2.4 Advanced Shapes of Nanocrystal

If the assembly of shaped components (OD, 1D and 2D) can be controlled, the

construction of advanced nanostructures can be achieved. [13-15, 76, 91] Alivisatos and co-

workers achieved the formation of arrow, teardrop, tetrapod, and branched tetrapod shaped

nanocrystals of CdSe using mixtures of hexadexylphosphonic acid (HPA) and trioctylphosphine

oxide (TOPO) [14]. The three fundamental parameters that were varied to control the shape of

CdSe nanocrystals were (i) the ratio of the surfactants (HPA/TOPO), (ii) the volume of the initial

injection, and (iii) the time dependence of the monomer concentration. Tetrapods of MnS [13]

and CdTe [15] were also reported This fundamental branched structures results from

nucleation of the cubic zinc blende phase with subsequent aniotropic growth of the hexagonal

wurtzite phase. In case of branched nanostructures, anisotropic colloidal heterostructures are

fabricated by sequential growth of semiconductor dots and rods of different materials, with the

potential for branched connectivity in each generation. Branching is introduced through crystal

phase control, so the large class of semiconductors exhibiting zincblende-wurtzite polytypism

[92] could be incorporated into branched heterostructures by these methods.










a ~b i



10 nm 0.
Fiur 221 Ttrpo sapd ancrstlsof(a C~e () nS ad c)prpoedmoelo




Thgue tetrapod shaped hpoe ntallcy hals very) inteestingS optca propertiessined inaode or



tetrapod nanocrystals, most of the confinement energy is along the diameter of the hexagonal

arms [30]. Tetrapods having comparable arm lengths but different diameters in fact show

remarkable differences in their bandgap energy, whereas spectra of tetrapods with comparable

diameters but different arm lengths are almost identical, as shown in Figure 2-23 [15].

B I8










Figur 2-2 Inlunc othshpofC eterodonpiclaoprinseta()
tetapos avig omprbeamdaeesbtdfeetdaees b erpd





havin comprablearm dametr butdiferetlngts [15].




Cheon and co-workers [93] found that more complex shapes could be synthesized,

including 1D rods, highly faceted stars, truncated octahedrons and cubes. As shown in Figure 2-








24, a variety of rod-based nanocrystals including PbS tadpole shaped monorods, I- shaped

bipods, L-shaped bipods, T-shaped tripods, cross shaped tetrapods, and pentapods were obtained.







Fiur 223 PS ancrstls ihsae orsodn o()rdbsdmliosa 4 C
(b apl-hae ooo, c -hpd io,()L-hpdbpd,()Tsae
triod (f ros-sapd terpd g etpd h tr-hpsa 8 C j rnae
ocahdrn at20oad()cneso fcue osa-hp ol o-ae
mutiod b cnro ofC th rot prmees 8]

Reety naoie sytei sn oine tahmn a sdtopoueP~
nanowire wihcnrlo ie iesosadmrpooy[4.I ddtoosrih






driingfre causin PbS e nanocrystal wt hps toaessemblein to hains. asdmlipd t 4 C
























Figure~ 2-24 Penaorsassoig()zzgnnwire akn fotaerlbidn









Figue22.3 Propnaocrsedl mehawnism a for za shae-ontrol growth of naocrytaherlsbid



The state of mechanistic investigation of nanocrystals formation is primitive. Specifically,

there are no prior kinetic and mechanistic studies of the formation pathway of compositionally

and geometrically well-defined nanocrystals for generalization. Matij evic [3 5] noted that it is not

clear why in some instances the Einal particles are spherical and in others they appear in different

geometric forms, yet are of the same chemical composition.

In the 1950s, Lamer and co-workers studied extensively the formation of colloids and

clusters in homogeneous, initially supersaturated solutions. Their widely cited LaMer

mechanism assumes that homogeneous nucleation occurs forming nuclei of the critical size.

Further growth on the nucleus is spontaneous but diffusion-limited [95]. This mechanism

predicts that as the precursor is consumed, its concentration fall below saturation, and hence no









more nucleation takes place, thereby achieving the needed separation of nucleation and growth in

time that is required for the formation of a monodispersed size distribution. Even though the

LaMer mechanism has been successfully applied in attempts to tune the main variables (i.e.

concentration of reactants), separation of nucleation and growth in reaction time is critical for

synthesis of monodisperse nanocrystals regardless of whether of not the LaMer mechanism is

correct in a given case.

In a separate study, the most preferred classic model for shape control is the Wulff facets

arguments, or Gibbs-Curie-Wulff theorem, which suggests that the shape of a crystal is

determined by the surface energy of each face or facet of the crystal [96]. However, recent

studies reveal that these classic thermodynamic arguments are not sufficient to understand the

shape evolution of nanocrystals, and other factors are influencial.

2.3.1 Kinetically Induced Anisotropic Control

Recent shape control research in nanocrystals illustrate that the kinetically induced

anisotropic growth from molecular precursors is highly effective for producing advanced shapes

of nanocrystals. Important factors for determining the shape of nanocrystals include reaction

temperature and time, the surfactants used for capping, and precursor concentrations during

nucleation and growth.

2.3.1.1 Cyrstalline Phase Control of Nuleating Seeds by Temperature

Nucleating seeds of nanocrystals can potentailly have a variety of crystalline phase that

affect the final shape of nanocrystals. The stable phase of nanocrystals is highly dependent on its

environment, such as the temperautre and the choice of capping molecules. For example, by

adjusting the initial temperature during the nucleation process, the crystalline phase of

nanocrystals can be controlled. An isotropic unit cell structure of the seed generally induces the

isotropic growth of nanocrystals from the seed, and therefore OD nanostructures would be










expected. In contrast, anisotropic unit cell structures of the seed can induce anisotropic growth

along reactive crystallographically directions, and anisotropic shapes of nanocrystals would be

expected.

In case of MnS semiconductor nanocrystals [76], nuclei with the rock-salt phase are more

stable at high temperature (> 200 oC), whereas the wurtzite structure is preferred at temperature

below 200 oC [97]. At high temperature (~- 2000C) the seeds of rock-salt MnS induced isotropic

growth along eight {111} di-rectins, andl 30 nm sizedT narnocubes wereT obtained. In contrast, at


low temperature (~ 120 oC) the nucleation with the hexagonal wurtzite structure resulted in

anisotropic growth along the c-axis of wurtzite and therefore very thin nanowires of 2 nm in

diameter with an aspect ratio of ~ 80 were observed.

Similarly, crystalline phase effects of the seeds can be observed in the case of CdS

nanocrystal growth [13]. CdS has two distinct crystalline phases; an isotropic zinc blende phase

is stable below 2500C, and hexagoanl wurtzite is preferred at high temperature (~ 3000C) At

high temperature, formation of 1D CdS rods is observed from high temperature stable wurtzite

structured seeds, similar to the MnS nanowire growth. However, at lower temperatures, the
formation of tetrahedral shapes of zinc blende seeds truncated with four {111}\ faces is observed.1


The subsequent epitaxial growth of wurtzite pods along the c-axis from the four equivalent


{11} fcesof he ee results in th fraio of CdStetrapods.








Figure 2-25. Shape evolution of MnS nanocrystals controlled by the growth temperatures :
wires at 120 oC, (b) spheres at 1800C, and (c) cubes at 250 oC [97].




























MnS CdS

Crystalline la Cr55talline
phased c~ yphase of
seeds I eed~~


rock ult
cu:-I- m rit ,


and ~~. CdS naorytl [9]


Figure 2-27. Temperature-mediated crystalline phase control of (a) MnS and (b) CdS
nanocrystals [88].

2.3.1.2 Surface energy modulation by capping surfactants

In addition to controlling the crystalline phase of the nucleating seeds, the surface energy

of the nanocrystals can be modulated by introducing surfactants that adsorb onto surfaces of

growing crystallites [1l, 12, 74, 89]. El-sayed's group [98] and Reets's group [99] have

performed pioneering work to understand the growth mechanisms for control of the shape of









transition metal nanocrystals. They report that control of the shape of transition metal

nanocrystals was due to stabilizing reagents bound to the surface of the nanocrystals.

Peng [1l] and Alivisatos [14] explored the growth of CdSe nanocrystals and reported

monomer concentrations in the growth solution was the determining factors in shape control and

shape evolution. The chemical potential of elongated nanocrystals should be slightly higher than

that of a spherical nanocrystal. As a result, the growth of such anisotropic structures should

require a relatively high chemical potential in the solution, i.e. a relatively high monomer

concentration. This condition provides the external environment for the the formation of

elongated [42] or other anisotropic shapes [13, 14]. At high monomer concentrations, the

differences in the growth rate of different faces can lead to anisotropic shapes. The relative

growth rate of the different faces can be controlled by the concentration, size and shape, and

adsorption strength of capping surfactants, such as trioctylphosphine oxide (TOPO) and

hexylphosphonic acid (HPA) [1l]. On the other hand, for a low monomer concentration and low

chemical potential, Ostwald ripening occurs and small nanocrystals dissolve at the expense of

larger ones. Such slow growth conditions favor the formation of a spherical shape. HPA leads to

an increased growth rate of the (001) face of CdSe relative to all other faces since HPA
selectively binds to (100) and {110} surfaCcles andl mhrnedue the growth rateo these~ twom su~rfces.


At low concentration of HPA or in the absence of HPA only spherical nanocrystals are formed.

With higher HPA concentrations, nanorods are obtained with their axis along the [001] direction.

More complicated shape control was subsequently observed with the formation of CdSe

tetrapods [14].















Fiur -28 nsorpcgrwhaon 1 drctiono denaorsas GoThe surface0









Fiue -8. ,nstoi growth ao [0 drci f d n oryt longh srfc
(b edu rd, an [100 andg [11014]






oFigur 2-29. aDis-hpdnncytl f()C nb CuS produr mnepeeenily id t ced~ by f~preventing,, growth

al 11]dietong the [001] direction du t ro sletiv apn ysrfcat 3 7]


2.3.1.3=:, Growth ReieCnro yMnme ocnrainadTeprtr
The~ mooe ocetainan hra eeg k) togyafeth irnal trutur o
th aorstl hoghadlcteblnebewe h kntcad hroyamcgot
reims.Isotropic grwho aorsasi rfre ne h hroyai rwhrgm







regime that. is promoted by cysahg lux ofa mo omers. Afte the inrinscurfced by enerngy gofthe










crystallographic face of the seed is determined, the surface properties can be tailored by the types

and the amounts of adsorbed capping surfactants. The growth processes should be quenched at

appropriate times, since long growth times can result in thermodynamically stable shapes of

nanocrystals.

The architectural features of the PbS nanocrystals are good examples to illustrate the

effects of growth regime. PbS nucleates as tetradecahedron truncated with eight { 111)faces andl

six(100) facesn [93]. Whe~n excess thermal ennerrrgyis suppnlie at a high~ gmrowt temperature


(~3000C), the thermodynamic regime governs the growth process. In this growth regime, nearly

isotropic growth from the seeds is favored, and therefore cube shapes of PbS are obtained.

However, under the conditions of low temperature (~1400C) and in the presence of surfactant

(i.e. dodecanethiol) the growth process shifts into the kinetic growth regime and anisotropic

growth on the high surface energy ( 100}fcesn isl prefermred andlD Irod-based mu~+;nlto stnructure


are obtained, as illustrated in Figure 2-31 [9].











o-o sor

Figre2-0.Shae onro ofPb nnorysal dpe den nte rwhreie[]










For an intermediate growth temperature (~180 oC) star-shaped nanocrystals, as a transient

shape between isotropic and multi-pod shapes, are obtained. The enhanced growth rate on the
{100} facesn in~ndue sh~rinking o the~ sixV {100}) fac~s innt sharp tr~ringular crnenrs whichr finally


results in a star-shaped nanocrystals.

The growth temperature affects the doping of nanocrystals as well as the anisotropic

shapes. The formation of magnetic semiconductor (e.g. Cd i. Mn _Se) nanocrystals with a

homogeneously distributed high level of Mn dopants has been difficult because of surface

segregation of Mn dopants from the host matrix during high temperature thermodynamically

driven synthesis [100]. However, low temperature kinetic growth allows not only the

homogeneous doping of Mn atoms at high levels ( up to~-12% ), but also the 1D growth of

monorod of Cd i. ln _S ( 0.02 < x < 0. 12) [93].

2.3.2 Oriented Attachment

As reported above in section 2.2.2, Penn and Banfield [83, 84, 101]. observed that anatase

and iron oxide nanoparticles with sizes of a few nanometers coalesce under hydrothermal

conditions by a mechanism they call "oriented attachment". The mechanism is based on the fact

that the surface area is reduced by attachment which reduce the total energy of the nanocrystals.

Attachment of nanocrystals were pointed out above for both hydrolytic growth of CdTe [16]

nanowires and nonhydrolytic growth of ZnO [48], ZnS [56], and PbSe [94] nanorods. In this

process, nanocrystals are first formed by at a high chemical potential due to a high monomer

concentration. These nanocrystals form chain-like structure due to induced dipole-dipole

interactions at the early stage of growth. In the final stage of 1D growth via the oriented

attachment process, Ostwald ripening smooths the irregular surface to produce nanocrystals with

a smooth surface. Synthesis by the oriented attachment mechanism can produce nanowires with










controlled wire dimensions and morphology, such as nanorings or straight, zigzag, helical,

branched, or tapered nanowires.









.~5 nm








Fiue2-1 rot f Dnncrsasb th oretd atchetmhni.()algd







Figre -1thewt agng (e) summaryofsteps inth oriented attachment mechanismfo ZnS nanorods


in (c): kinetic formation of primary nanocrystal -interparticle ripening -oriented
attachment Ostwald ripening to form smooth surface nanorods [56].

2.4 Application of Nanocrystals in Biomedicine

Nanocrystals offer some attractive possibilities in biomedicine. First, they have

controllable sizes ranging from a few nanometers up to tens of nanometers, which are smaller

than or comparable to those of a cell ( 10-100 Clm), a virus (20-450 nm ), a protein (5-50 nm) or a

gene (2 nm wide and 10-100 nm long). This means that they can get close to biological

molecules of interest. Indeed, they can be coated with biological molecules to make them interact

with or bind to a biological entity, thereby providing a controllable means of tagging. Second,

inorganic nanocrystals exhibit unique properties and many new nanocrystal labels have been









introduced for biomedical applications. Magnetic separation and fluorescent labeling are the two

most widely used nanotechniques in bioscience. The present work attempts to prepare a new type

of nano-sized hybrid particle that exhibits both a magnetic response and fluorescence for bio-

detection.

Colloidal quantum dots are robust and very stable light emitters that can be broadly tuned

in emission wavelength through size variation. It was quickly realized that colloidal quantum

dots (QDs) were about the size of a typical protein, and thus it was possible to introduce QDs

into cells.

2.4.1 Biocompatible Magnetic Nanocrystals for MR Contrast Effects

Nanocrystal biomedical applications based on magnetic properties include utilization as a

magnetic probe for detection and imaging and as a magnetic vector for cell separation and drug

delivery. In particular, they are employed for magnetic separation, in vivo magnetic drug

targeting, and magnetic resonance imaging (MRI) [102-107] .

Modern MRI is one of the most powerful medical diagnostic tools due to its non-invasive

nature and multi-dimensional tomographic capabilities coupled with high spatial resolution. MRI

relies on the counterbalance between the exceedingly small magnetic moment on a proton, and

the exceedingly large number of protons present in biological tissue, which leads to a measurable

effects in the presence of large magnetic fields. Under an applied magnetic field, induced

magnetic spins in magnetic nanocrystals perturb the nuclear spin relaxation processes of protons

in surrounding water molecules. This effect leads to shorter spin-spin relaxation time (T2) of the

proton, which results in contrast for MR images. When magnetic nanocrystals are conjugated

with biologically active materials, the resulting nanocrystal-biomolecule conjugates have the

multi-functionalities of both MR contrast effect and selective attachment to target molecules.









Santra and Holloway [108] reported highly water-dispersible, multifunctional,

CdS:Mn/ZnS core-shell Qdots using water-in-oil(W/O) microemulsion method. These Qdots are

fluorescent, radio-opaque, paramagnetic, and suitably stable in an in vivo environment [109].

Paramagnetic metal ions (Cr3+, Mn2+, Fe3+ and Gd3+) show suitable effects which depend

on the number of unparied electrons in the ion. Among these ions, the prominent feature of Gd3+

is the high number (seven) of unpaired electrons.. The Gd3+ ion retains a number of unpaired

spins when bound to the organic ligand. The free Gd3+ i08 is extremely toxic, but a large fraction

of the complexes are very stable and thus exhibit much less toxicity. The Gd-DTPA complex has

been approved for clinical use and is now marketed in USA under the name Magnevist" [110-

1 12]. Although many researchers are investigating stability of the series of complexes versus

acute toxicity in vivo, the search for new ligands for complexation is still a hot area for

investigation. Criteria include thermodynamic stability, rates of excretion, toxicity and

biodi stribution.

Iron oxide nanocrystals are the most commonly used superparamagnetic contrast agents.

Various sizes of iron oxide nanocrystals with a narrow size distribution were produced using a

high temperature inj section methods [1 13]. The nanocrystal surface was treated with 2,3-

dimercaptosuccinic acid (DMSA) which makes them stabile in a water dispersion. The water-

soluble iron oxide nanocrystals exhibit excellent size-dependent magnetism and MR contrast

effects. Increasing the size of nanocrystals from 4 to 6, 9, and 12 nm, the mass magnetization at

1.5 T increased from 25 to 43, 80, and 102 emu/(g Fe), respectively, and higher MR contrast

effects can be seen in Figure 2-32.




















Figre2-3. a- ) anscle iz efecs f ronoxde ancrstas n agntim nd ndce
MR sinl.() ceai fD S-otdwae ouleF34nncrsaswt

multifunctionalte [113].
2.4.2 Luminecn aorsasfo loecnelbl
Anohe imorantaplictin o nnocysalsinclboog is thi ueasfuoecec
makestolbe stute and moecle incelsFu resec laelin isuedtvsalz
strctra untstht, uetolac o cntrstorresluio, cnnt e istinused[1-6]
Ths s cieedb atahnga ian t henaoaricelaelad hi onugt bnd it hg

spcfciyt tstretrcporwhccabevsaiebytefu ecneofhelel(e
Fiur 233. hereepormoeul tpialy s n ntboy o te trctretobelaeled
popula reetrpi s vdnadsretvdn an hesrucuet elble sicbtdwt
the~~~~*p bitnltdsrpaii atbd hc ste eogie ytefursec-avdi

construc





iuseful 3. Reetlawie) rangsae oieefaplcatios of or QxDs havben rseenas cel labneis ng [119],cell









tracking [120], cell signaling [121], in-vivo imaging [122] near infra-red imaging [123] and

DNA detection [124].



b 'Ic~




Figure 2-33. Schematic representation of the use of QDs for bio-labelling. (a) Water soluble
QD comprise a core and hydrophilic shell; (b) QD conjugate with biological molecule
(drawn in red); (c) Bioconjugated QD that binds specifically to designated receptors

Luminescent colloidal QDs offer many advantages compared to organic dyes as

fluorescence labels for biological staining and already been used in labelling experiments. QD

properties of interest to biologists include high quantum yield and exceptional resistance to

photochemical degradation and photobleaching. Upon optical excitation, organic fluorophores

can undergo irreversible light-induced reactions such as photo-oxidation. Reacted molecules are

no longer fluorescent, i.e. they have been 'photobleached' The fluoresence emission spectra of

QDs are typically narrow, symmetric and do not exhibit tail to longer wavelength (i.e. red tail),

therefore many different colors from size-tune fluorescent emission can be distinguished without

spectral overlap (see Figure 2-34). For biological flurescence labelling, more colors mean that a

larger number of structures can be simultaneously labelled, each with a different color (see

Figure 2.35) [125-127]. The detection of multiple molecules (markers) in a cell or tissue by QDs

color emission can improve diagnostic efficiency. The decay time of the fluorescence of

nanocrystals is typically longer (ns to Cls) than the decay time of autofluorescence (ps to ns),

therefore time-gated imaging can be used to reduce the autofluorescence background in

fluorescence imaging of cells [128]. Besides spherical nanocrystals, assymmetric nanorods can










also be synthesized as discussed above. Due to their anisotropic shape their emitted fluorescence

light can be polarized which enables detection of the orientation of labelled structures [31i].








Figure -34. Reresentaive QD ore matrialssale sa ucio fthi miso
wavelength over spectrum.ll Rersnaie areas ofbooiclitretaeprsne
correpodig tothe prtinnt eissi no highightng [29]


Figure 2-35. (a) Multicolor staining of HeLa cell with red and green QDs. [130] (b) In vivo
labeling of a Xenopus embryo with green-micelle-coated QDs. [122] (c) Image of
QDs targeting prostate cancer in vivo in a mouse bearing a xenograft tumor targeted
using orange-red emitting QD probes [131].

Recently, QDs fluorescencing in the infrared have been demonstrated as a contrast label

for optical detection. QDs with infrared fluorescence are of particular interest if their emission

wavelength is chosen in a spectral window where absorption by the biological molecules (e.g.

water and hemoglobin) is low [132]. Kim and co-workers have demonstrated the use of QDs for









sentinel-lymph-node mapping in pigs, which helps guide surgeons in the removal of tumor cells

[123]. Compared to the currently used visible fluorescence contrast agent, infrared fluorescence

can be imaged through the skin, which allows the surgeon to identify the position of the QDs in

the lymph channels in real time and reduce the size of the incision and allow determination of the

complete removal of the lymph node.

While the above discussion reveals some of the positive attributes for QDs in bio-

applications, there are still many questions about the toxicity of inorganic QDs containing Cd,

Se, Zn, Te, Hg and Pb. These elements can be potent toxins, depending on dosage, that can

accumulate in and damage the tissue. Living cells have been demonstrated to ingest QDs,

allowing their potential use as contrast agents in animals and humans that may remain in the

living tissue for months and presumably even for years [133]. Studies on non-toxic QDs such as

oxide compounds would be desirable. This is one of the objectives of this dissertation.









CHAPTER 3

SYNTHESIS AND CHARACTERIZATION OF LUMINESCENT GADOLINIUM OXIDE
NANOCRYSTALS

3.1 Introduction

Rare earth doped compounds have attracted extensive attention as luminescent components

in many applications. In addition, shape control of colloidal nanocrystals is possible and result in

many advanced shapes of nanocrystals. For example, rare earth doped one dimensional (lD)

structures such as LaPO4: RE ( RE= Eu3+, Tb3+) nanowires [134], Y203:RE nanotubes [135] and

Gd203: Eu3+ nanoplates [71] have been reported. However, the luminescent properties of

nanocrystals with different shapes were not compared.

Eu3+ doped Gd203 nanocrystals have shown red luminescent peaks due to the electric-

dipole 5Do- 7F2 transitions on the trivalent europium ion (Eu3+, 4f6) [136]. Gd203: Eu3+

nanocrystals obtained by nonhydrolytic thermal reactions in the presence of organic surfactants

exhibit excellent crystallinity, monodispersivity and good luminescent efficiency. Moreover, this

synthetic route allows control of the growth and shape of nanocrystals by variables such as the

type of metal precursors, organic surfactants molecules, monomer concentration ratio, and

heating rate. In this chapter, the luminescent properties of Gd203: Eu3+ are reported and

discussed versus the crystal structure, shape of nanocrystals and the quantities of Eu3+ dopant

incorporated into the Gd203.



3.2 Experimental Section

3.2.1 Materials

The following precursor compounds and solvents were purchased from Aldrich: Gd (III)

acetate hydrate Gd(III) acetylacetonate hydrate, Eu(III) acetate hydrate oleic acid (90% ,









tech.), Tb(III) acetate hydrate (90% tech.) Er(III) acetate hydrate (90% tech.), oleic acid

(90% tech.),oleyamine (70%) benzyl ether ( 99%) 1,2-hexadecanediol ( HDD, 97%),

trioctylphosphine oxide (TOPO, 99%) and octadecene (90%). All chemicals were used without

further purification. Absolute ethanol, benzyl ether and hexane were used as received.

3.2.2 Synthesis of platelet Gd203: Eu3 nanocrystals

The general scheme to grow Gd203 nanocrystals is shown in Figure 3.1. In a one pot

reaction to produce Gd203: Eu3+, Gd-acetate (2mmol), Eu-acetate (0.2 mmol), oleic acid

(6mmol), oleylamine (3mmol), benzyl ether (10mmol) and TOPO (4 mmol) were mixed and

magnetically stirred under a flow of nitrogen. In a typical reaction to produce Gd203: Eu3+,

2mmol of Gd-acetate, 0.2 mmol Eu-acetate were mixed with 6mmol oleic acid, 6mmol

oleylamine, 4mmol TOPO and 10mmol benzyl ether in a three-neck reaction flask under

nitrogen. In both cases, the mixture was heated at 2000C for 30 min, resulting in a transparent

brownish solution that was then heated to 2900C with a heating rate of 5-250C /min and

maintained for 2-3h under the N2 blanket.

In a second procedure using two pots, a mixture of 2mmol of Gd-acetate, 0.2mmol Eu-

acetate were mixed with 6mmol oleic acid and heated to 2000C for 30 min in a vial resulting in a

homogeneous brownish solution. At the same time, 10ml of benzyl ether and 4mmol of TOPO

were mixed in a three-neck flask and heated to 2000C for 30 min. After 30 min, the Gd-oleate

solution was rapidly inj ected into the benzyl ether/TOPO mixture and the temperature was raised

to ~ 2900C with a heating rate of 5-250C /min, and kept at that temperature under a N2 blanket

for 150min.

After reflux, the brownish transparent mixtures were cooled to room temperature by

removing the heat source in the case of either the one pot or two pot processes. Under ambient









conditions, ethanol was added to the mixture, and Gd203 nanocrystals were precipitated and

separated via four centrifugations (9000 rpm, 10 min) to remove any residue. The purified

Gd203: Eu3+nanocrystals, capped with organic species, were well dispersed by organic solvents

such as hexane, chloroform and toluene. The properties of the resulting nanocrystals from the

one versus two pot reaction were identical with respect to size, shape and product yield. The data

reported below are all from the one pot approach.

3.2.3 Synthesis of spherical Gd203: Eu3 nanocrystals

To produce spheres of Gd203: Eu3+, Gd(acac)3 (1mmol), Eu-acetate (0.2 mmol), oleic

acid (3mmol), oleylamine (3mmol), benzyl ether (5mmol) and either hexadecanediol (HDD,

2.5mmol) or TOPO (2 mmol) were mixed and magnetically stirred under a flow of nitrogen.

After preheating at 2000C for 30 min, the mixture was heated to ~ 2900C and kept at that

temperature for 150min. Cooling and purification of the nanospheres were performed in the

same way as described above for synthesis of platelet Gd203: Eu3+ nanocrystals. The effects of

other precursors, such as Gd-chloride and Gd-octanoate, and growth solvents, such as

octadecene, on the shape of the rare earth doped Gd203 nanocrystals were investigated as

reported below.

Most batches of Gd203: Eu3+ were processed using two-coordinating solvents (e.g.

TOPO/TOA, TOPO/benzyl ether, or HDD/benzyl ether) with oleic acid for better homogeneous

mixtures of the starting materials. For other dopant elements (Er3+ and Tb3+), the same protocols

were used. For controlling the shape of Gd203 nanocrystals, several critical parameters were

found, including the type of Gd precursor, organic surfactants, the ratio of Gd-

precursor/dominant organic surfactant, and the heating rate as reported below.










Gd(acac), + 1 ,2-RCH (OH)CH20H +RCOO H +R7NH,


S(- 30 min )ZO

in high halin oment

2-Z 3m180- 300 *C










Figure 3-1. Synthesis of Gd203: Eu3+ nanospheres by the nonhydrolytic hot solution route.

3.2.4 Characterization of Rare Earth Doped Gd203 Nanocrystals

Nanocrystals for examination by transmission electron microscopy (TEM) were dispersed in

hexane and drop-cast onto a copper grid with a carbon film containing holes. The solvent was

evaporated in ambient air. For Fourier-transform infrared (FTIR measurements, the powder was

ground together with KBr in a mortar and pestle, then pressed into pellets. For ICP analyses of

concentrations, Gd203: Eu3+ nanocrystals were dissolved in nitric acid. For thermogravimetric

analysis (TGA), Gd203: Eu3+ nanocrystals were heated after ambient drying to remove the

solvent. Photoluminescence (PL) and UV-absorption were measured from Gd203:R

nanocrystals dispersed in hexane contained in a quartz cuvette. For near infrared (NIR) emission,

Gd203 nanocrystals were dried and measured as green powder pellets.

X-ray diffraction (XRD) patterns to determine the crystal structure were obtained using

Philips APD 3720 X-ray diffractometer with Cu K,. radiation source ( h= 0.5418 nm). A JEOL

2010F transmission electron microscope operated at 200 kV was used for collection of images

and determination of the size and shape of the nanocrystals. Absoprtion spectra were collected









with a Shimadzu UV-2401PC spectrophotometer. Photoluminescence (PL) was measured at

room temperature from nanocrystals suspended in hexane using a Flurolog Tau 3

spectrofluorometer (Jobin Yvon Spex Instruments, S.A. Inc) with a 450 W xenon excitation

lamp. Life time was measured with FP-6500 spectrofluorometer ( JASCO). Near-infrared (NIR

emission spectra of Gd203: Er3+ nanocrystals were collected using a 488 nm Ar laser line for

excitation and a Ge detector. Thermo-gravimetric analysis (TGA, Seiko TG/ATD 320U, SSC

5200) was used to investigate the temperatures for decomposition of Gd precursors and to

determine the amount of surfactant bound on the particle surface. For TGA, the samples were

heated up to 8000C in air at a rate of 100C min FTIR spectra from the Gd203 nanocrystals were

recorded with a Thermo Electron Magna 760 FT-IR microscope in order to confirm the nature of

the coating and its bonding to the surface. The concentration of Eu3+ dopants in Gd203

nanocrystals, as prepared, was determined by inductively coupled plasma ( ICP, Perkin-Elmer

Plasma 3200)

3.3 Results and Discussion

3.3.1 Nanoplates and Nanospheres of Gd203: Eu3

As described above, nanoplates of Gd203: Eu3+ WeTO Synthesized using Gd-acetate

as the precursor and using thermal decomposition under nitrogen flow at 290 OC after preheating

at 2000C. Gd-acetate is completely soluble in the oleic acid TOPO and benzyl ether mixture at

2000C. The temperature leads to decomposition of Gd-acetate to a Gd203-organic surfactant

complex with reaction products of CO2 and acetone [72].

High resolution TEM (HRTEM) images in Figure 3.2 (a-b) show that square

nanoplates of Gd203: Eu3+ WeTO formed with 10nm of edge length and Inm thickness. Over

most of the area, the nanoplates were aligned with one edge parallel to the surface and their









surface self-aligned parallel to their neighbor platelets. To verify that the images in Figure 3.2(a)

and (b) were from aligned plates rather than e.g. rods, tilted TEM images were taken in Figure

3.2(c) and (d), and the change in contrast is consistent with plates rather than rods of the same

length. The image shows that the standing plates varied in thickness when tilted in the short

direction and remained of constant edge length when tilted in the long one. The size

distributions of the platelet edge length is more broad (5-10 nm), while the thickness is

monodispersed at Inm.






face

Il~k~1nm















Figure 3-2. HRTEM images of Gd203: Eu3+ nanocrystals (a-b) nanoplates (c) the images
correspond to no tilt and (d) correspond to tilt. Labelled face is (001) and edges are
(100, 010).

As shown in Figures 3-2 and 3-3, the Gd203: Eu3+ nanoplates self-assemble into

"stacks", in which they are aligned with their square planes parallel to each other. The thickness

of and spacing between nanoplates was very uniform at 1 nm and 1.3 nm. This spacing of 1.3nm

is consistent with small angle x-ray diffraction data which reduce to a interplate spacing of 1.2









nm. Face-to-face platelet stacking has also been reported for Cu2S [137] and Co[51] nanodisks.

The existence of a nematic phase due to the entropy of oriented packing was first suggested by

Onsager [138], and has been confirmed by computer simulations of platelets and experimental

data for submicrometer hard disk colloids [139]. The van der Waals attraction is also greater for

nanoplates oriented face-to-face relative to nanoplates oriented edge-to-edge due to the much

greater interfacial contact area provided by the face-to-face configuration [140]. As suggested

by Cao, et al [70], face-to-face stacking for Gd203 might also be assisted by electric dipole
interactions since the faces of the nanoplates are the {001}a \ndl~ theP edges ar {100} alnd {)1 0}


crystal faces.













Figure 3-3. HRTEM images of self-assembled 'stacks' of nanoplates of Gd203: Eu3+ shown at
three different magnifications.

Using the same temperatures and times for reactions but using a precursor of Gd

(acac)3 (1mmol) and hexadecanediol (2.5mmol) as a surfactant, nearly monodisperse 2- 4 nm

diameter nanospheres of Gd203: Eu3+ were grown. Although the mechanism leading to Gd203

nanocrystals is not clear, decomposition of the acetylacetonate ligand in the Gd(acac)3 preCUTSOT

is the only possible oxygen source. Nanospheres of several metal oxides [40, 54, 64-66] have

been prepared by thermal decomposition of metal acetylacetonates under an inert atmosphere,

and several authors have proposed that decomposition of the acac ligand leads to formation of









carbon dioxide that oxidizes the Gd. When TOPO (1mmol) was used as solvent instead of

hexadecanediol (2.5mmol), the same size of Gd203: Eu3+ nanospheres were obtained.












Figure 3-4. HRTEM images of nanospheres of Gd203:Eu3+ from thermal decomposition of
Gd(acac)3 (1mmol) precursor in the presence of (a) hexadecanediol (2.5mmol) or (b)
TOPO (1mmol).

Regardless of the shape of nanocrystals (platelet versus sphere), heating the mixture to

2000C for 30 min before heating to reflux at 2900C is important for monodispersity and high

product yield (~ 80 %). Directly heating the mixture to 2900C from room temperature resulted in

Gd203: Eu3+ nanocrystals with a wide size distribution and relatively low (~ 60 %) product yield.

These observations suggest that the nucleation of Gd203: Eu3+ at the higher temperature is a

slower, continuous process, and subsequent growth leads to a large size distribution.

For many colloidal nanocrystals, particle size can be increased by longer growth times [54]

and larger metal precursor/surfactants ratios [67], often described by the Liftshitz-Slyozov-

Wagner (LSW) model which predicts the time dependence of the particle size. Particle growth is

driven by the dependence of the solubility of a solid phase on the particle size according to the

Gibbs-Thomson relation, assuming that the particles are spherical [141-143]. However, the

particle size of Gd203: Eu3+ did not increase at longer growth times. The independence of the

nanoparticle size from growth time is attributed to the monodispersivity the nanocrystals. At long









times, growth of the nanoparticles would be expected from Ostwald ripening [144]. In addition,

changes in the metal precursor/surfactants ratio changed the shape of particle more than the size.

3.3.2 Shape control of Gd203: Eu3 nanocrystals

3.3.2.1 Effects of Gd-precursor

Thermal decomposition of various Gd precursors in oleic acid, oleylamine, benzyl ether

and TOPO leads to the formation of different morphologies of nanocrystals. Figure 3-5 shows

HRTEM images of Gd203: Eu3+ nanocrystals grown from different Gd precursors at 2900C. As

shown above, Gd203: Eu3+ nanocrystals obtained from Gd(acac)3 are 2- 4 nm nanosphere (Figure

3-5(a)), in contrast to nanoplates of Gd203: Eu3+grOWH fTOm Gd-acetate precursor (Figure 3 -2, 3-

3, and 3-5(b)). Gd-chloride precursors led to growth of 2-4 nm nanospheres (Figure 3-5 (c)), but

the product yield was lower (~ 35 %) than from Gd(acac)3. Interestingly, Gd-octanoate

precursors produced Gd203: Eu3+ nanocrystals with both sphere and plate shape (Figure 3-5 (d)).

The two different shaped nanocrystals separated and the platelet Gd203: Eu3+ Self-aSSembled into

'stacks', similar to those shown in Figures 3-2 and 3-3.

In studies of the shape of nanocrystals, the effects of metal precursors have rarely

been reported. One reason is the difficulty in finding different precursors comparable with the

appropriate organic surfactants. The combined effects of both the precursor and organic

surfactants determine the crystal nucleation, growth and shape. While it is not possible to

suggest a priori specific precursor favorable for aniotropic growth, the reactivity of the precursor

is expected to affect the size and shape of nanocrystals. In case of a highly reactive precusor, a

relatively large number of nuclei could be expected due to fast nucleation and depletion of the

precursor to reduce further growth. On the other hand, a less reactive precursor a should form a

relatively small number of nuclei in the same period of time, and these nuclei will not reduce the

concentration of the precursor monomers as much, allowing growth of elongated shape in some









cases [l l, 42, 145]. In the present study, a Gd-acetate precursor resulted in platelet Gd203: Eu3+

nanocrystals, while Gd(acac)3 resulted in the formation of spherical Gd203: Eu3+ nanocrystals in

the presence of organic surfactant such as TOPO or hexadecanediol. The thermal decomposition

of these two precursors investigated with TGA are reported below and will be related to their

relative reactivity during growth,

a b

















Figure 3-5. HRTEM images of different shaped Gd203: Eu3+ nanocrystals from (a) Gd(acac)3
(b) Gd-acetate, (c) Gd-chloride, or (d) Gd-octanoate precursors

3.3.2.2 Effects of Surfactants

Organic surfactants play an important role in the shape of the nanocrsytals. The reaction

occurred in a mixture of coordinating and noncoordinating organic surfactants. When Gd(acac)3

was used as the precursor, the use of hexadecanediol ( HDD) resulted in spherical Gd203: Eu3+

nanocrystals at a Gd(acac)3/HDD ratio of 1: 2.5, as shown in Figure 3-6(a). In all syntheses,

oleic acid, oleylamine and benzyl ether were co-introduced with the main surfactant (i.e. HDD,

TOPO or octadecene). When HDD was replaced with TOPO, both of nanospheres and

nanoplates were simultaneously formed with a Gd(acac)3/TOPO ratio of 1:1 (Figure 3-6(b)).

The nanoplates were stacked without a preferred direction, but non-stacked individual nanoplates









lying on their faces were rarely observed. Using Gd octanoate precursor and TOPO produced

mixed shapes of spheres and plates of Gd203: Eu3+ nanocrystals (Figure 3-6(c)). Finally, varied

complex shapes of Gd203: Eu3+ nanocrystals (~10 nm spheres, plates, rings and triangles) were

formed from a Gd octanoate precursor when TOPO was replaced with octadecene as the main

organic surfactant (Figure 3-6(d)) and reacted for short times (1 h) at a growth temperature of

2900C after a 2000C nucleation.





















Figure 3-6. HRTEM images of Gd203: Eu3+ nanocrystals synthesized from thermal
decomposition of Gd(acac)3 preCUTSor (a,b) with (a) hexadecanediol (HDD-
nanospheres) and (b) TOPO surfactant (nanospheres and nanoplates), and of Gd-
octanoate precursor (c,d) with (c) TOPO (nanoplates) and (d) octadecene (complex
larger shapes) surfactant.

3.3.2.3 Effects of the Precursor/Surfactant Ratio

The concentration ratio of the Gd precursor and the main organic surfactant (i.e. Gd(acac)3

/TOPO or HDD) was critical to the size and shape of the nanocrystals formed. For a

Gd(acac)3/TOPO molar ratio of 1:1, only nanospheres (<5nm) were observed, as shown in

Figure 3-7(a). For a ratio of 1:2, a mixture of nanospheres and nanoplates was observed (Figure

3-7(b)). Nanospheres of Gd203: Eu3+ were produced with a Gd(acac)3/HDD molar ratio of 1:2.5









(Figure 3-7(c)), but curved worm-like nanocrystals appear along with the dominant formation of

nanospheres at a ratio of 1:5 (Figure 3-7(d)).





















Figure 3-7. HRTEM images of Gd203: Eu3+ nanocrystals synthesized with a Gd(acac)3/TOPO
molar ratio of (a) 1:1 (nanospheres) (b) 1:2 (mixed nanospheres and nanoplates), or
a Gd(acac)3/HDA ratio of (c) 1:2.5 (d) 1:5.

Data in Figures 3-6 and 3-7 show that the type and concentration of surfactants are

important factors for controlling the shape of nanocrystals. Relatively high surfactant

concentrations result in high chemical potentials which are favorable for formation of anisotropic

nanocrystals, regardless of the type of surfactant. In hetero-structured Gd203 nanocrystals

(Figure 3-7(b) and (d)), small (<5 nm) nanospheres are surrounded by anisotropic nanocrystals.

It seems logical to postulate that the small nanospheres play an important role in the growth of

the larger anisotropic nanocrystals, such as plates or curved rods. The co-existence of these

small nanospheres and nanorods is consistent with the "oriented attachment process" discussed

in Chapter 2 [83]. As the molar ratio of Gd(acac)3/HDD was changed from 1:1 to 1:3 to 1:5

(Figure 3-8), quasi-spherical nanocrystals (figure 3-8(a)) apparently attached to one another (red

arrows in Figure 3-8(b)) to form long nanorods (>10nm) as shown in Figure 3-8(c). This shape









change from nanospheres to nanorods was the result of an increased concentration of HDD, and

was not due to an aging process. Upon changing the concentration ratio of Gd(acac)3/TOPO

from 1:1 to 1:2, the shape of the Gd203: Eu3+ changed from nanospheres to mixed spheres and

plates, but did not change further at higher concentrations of the strongly bound TOPO

surfactant. The nanoplates were probably formed due to the high chemical potential at the higher

concentration of TOPO, not from the evolution of nanospheres.













Figure 3-8. HRTEM images of Gd203: Eu3+ nanocrystals synthesized with ratios of
Gd(acac)3/HDD of (a) 1:1, (b) 1:3, or (c) 1:5 illustrating oriented attachment of
nanospheres (b-red arrows) to form nanorods in (c).

3.3.2.4 Effects Rate of Heating from 200oC to 290oC

Different heating rates were investigated using a ratio of Gd(acac)3/HDD of 1:5 with other

reactants oleicc acid, oleylamine and benzyl ether ) after preheating 2000C. If the heating rate is

250C/min from 2000C to the reflux temperature of 2900C, a mixture of nanospheres and a few

curved nanorods were obtained, as shown in Figure 3-9 (a). Slower heating rate of 50C/min led

to a lower density of curved nanocrystals, with the vast maj ority being nanospheres as shown in

Figure 3-9 (b). The result suggest that a rapid growth rate due to larger heating rates result in a

high chemical potential which favors the formation of anisotropic nanocrystals.






















Figure 3-9.HRTEM images of Gd203: Eu3+ nanocrystals synthesized with a heating rate of (a)
250C/min (mixed curved nanoplates and nanospheres) and (b) 50C/min (dominated
by nanospheres).

3.3.3 Crystal Structures of Gd203: Eu3 Nanocrystals

It is known that cubic Gd203 transforms to monoclinic at 1300-1400 oC [146], with the

monoclinic phase being favored at temperatures over 14000C. These different crystallographies

affect the magnetic and optical properties of Gd203 [147]. The crystallographic structure of

Gd203 :Eu3+nanocrystals were examined using Cu K, radiation, and a XRD spectrum is shown in

Figure 3-10. The XRD peaks were broadened due to the small size of nanocrystals comparing to

bulk Gd203: Eu3+, especially for the nanospheres with a smaller size than the nanoplates.

Although the main peak (26 = 32.470) of monoclinic Gd203 (JCPDS file 43-1015) was seen in

both samples, most of the monoclinic peaks overlapped with those from cubic Gd203 (JCPDS

file 43-1014), making it difficult to determine if only a single phase was present. However, a

relatively sharp monoclinic (002) peak was seen at 26 = 32.470 from nanoplates, consistent with

these planes being the face along which the stacks self aligned. It is also possible that nanoplates

contain a larger fraction of the monoclinic (versus cubic) phase compared to Gd203 HRHOSpheres.

Based on this assumption, it could be expected that the emission efficiency of Gd203: Eu3+

nanoplates were less than that of nanospheres because monoclinic crystals show a considerably









lower luminance than cubic crystals.[148]. Interestingly, mixed shaped of Gd203: Eu3+

nanocrystals with larger sizes over 10 nm (Figure 3-6 (d)) have very sharp peak at 26 = 28.50

assigned to (222) plane of the cubic phase (not shown here).



cubic



plates



-I.I~~~ spheres





... I.1 1monoclinic

10 20 30 40 50 60 70
2e

Figure 3-10. XRD pattern of Gd203: Eu3+ (a) nanoplates and (b) nanospheres compared to the
JCDPS patterns for cubic and monoclinic Gd203

3.3.4 Luminescence Properties of Gd203 : Eu +, Tb3 and Er3 Nanocrystals

3.3.4.1 Eu3 Fluorescence in Oxides

Photoluminescence (PL) from Eu3+-doped Gd203 nanocrystals showed a red color attributed to

transitions from the excited 5Do level to the crystal-field-split 7Fj manifolds of the 4f6 electronic

states. The jDo -tF, emission of Eu3+ iS Very sensitive to the crystal field around the Eu3+sites.

Whereas the 5Do 7F13 of Eu3+ is an allowed magnetic-dipole transition, the 5Do 7F24 of Eu3+ is

a forbidden electric-dipole transition (parity selection rule) [136]. However, this selection rule

can be relaxed for Eu3+ in a host lattice lacking inversion symmetry, such as Gd203[148], Y203

[149] and LaPO4 [134].





i


15 192 1 170 cr -


S175 021gg


1 72


c2 s

Figure 3-11. Optical transitions from Eu3+ in either a C2 or a. S6 symmetry site in Gd203 [150].

3.3.4.2 Luminescence vs. Gd203 Crystallography

The two different crystal structures of Gd203 nanocrystals strongly affect the luminescent

properties, with the monoclinic system showing a considerably lower luminance than the cubic

system [151]. Cubic Gd203 is a good host materials for trivalent activators (i.e. Eu3+, Tb3+ and

Er3+), which are believed to substitute on Gd3+ sites. In cubic Gd203: Eu3+, two possible

symmetry sites exist for the Eu3+ ionS, namely S6 and C2 point group symmetry. The crystal

structure of Gd203 is Of the rare-earth sequioxide C-type in which each Gd3+10 i is surrounded by

six oxygen located at the corners of a cube [152]. Two of the corners are vacant and this can

occur along either a body or a face diagonal of the cube, which results in two Gd3+ site

symmetries, namely S6 Of C2, respectively. Figure 3-12 depicts the two types of crystallographic

sites possible in Gd203. The unit cell consists of two types of alternating Gd3+ layers, one

composed of only C2 sites and the other composed of an equal number of S6 and C2 sites; the

ratio of C2 to S6 sites is 3 to 1 [153].













GA z0,(cub ic)


I Oxygen vacancy ......... *
Idiagolnal



Figure 3-12. Two Gd3+ symmetry sites in Gd203.

In fact, electric dipole transitions for Eu3+ ion at S6 site is forbidden because of the strict

inversion symmetry. Therefore, the 5Do7F2,4 transitions originates mostly from the C2 site,

because of its' relatively poor inversion symmetry [136].

Monoclinic Gd203 prOVides three different crystallographic sites Cs for the Eu3+i0H

[154]. These three sites give rise to a maj ority of the 5Do and 7F2 stark levels, which produce

numerous peaks in the range between 600 and 630 nm, even through our measurement was not

sufficient to resolve them.

Figure 3-13 shows the PL excitation (PLE) spectra for emission at 612 nm from Gd203:

Eu3+ nanoplates versus nanospheres at room temperature. For both shapes a broad excitation

band extends from ~250 to ~300 nm, and is thought to result from both host and charge-transfer-

band (CTB) excitation. An electronic excitation transition near 260nm is associated with the 2p

orbital of 02- to the 4f orbital of Eu3+, whose strength is related to the covalency between Ol- and

Eu3+ and the coordination environment around Eu3+ [155]. Another broad band at 280 nm is

related to Gd3+ transitions due to Gd3+- Eu3+ energy transfer. The weak, sharp lines at ~320 and










360-390 nm, shown in the inset, are associated with direct excitation of the f-f shell transitions of

Eu3+ as labeled [150]. While the PLE spectral wavelengths and intensities were not altered, the

shape of the broad CTB were different between nanoplates and nanospheres. When the CTB was

decomposed into two Lorentzian component at 260 and 280 nm (not shown here), their ratio was

different. This is attributed to a variation of the coordination environment around Eu3+ ionS in

these two different samples. In nanocrystals, the coordination environment around the Eu3+ OH

the surface would be different from that around interior Eu3+ since effects such as adsorption,

dangling bonds and defects exist at the surface [156]. In nanoplates versus nanospheres, the ratio

of surface to volume varies which leads to an increase in CT transitions contributed by surface

Eu3+ ionS and a decrease of those contributed by internal Eu3+ ionS.



nanoplates
--- nanospheres





Y~X 50







250 300 350 400
Wavelength ( nm)


Figure 3-13. PL excitation spectra of Gd203:Eu3+ nanocrystals for the emission line at 612 nm
of nanoplates and nanospheres. The inset shows the PLE spectrum from 3 15-400 m at
a higher sensitivity so that the labeled absorption transitions of Eu3+ can be seen.

The emission spectra from Gd203: Eu3+ (10 mol%) nanospheres, nanoplates and mixed

nanocrystals (spheres, platelets, triangles and rings) under 270 nm excitation is shown in Figure









3-14 (a), (b) and (c), respectively. Note that the concentration of Eu3+ that leads to quenching in

the present colloidal nanocrystals is higher (>6 mol%) than that in bulk materials (3-5 mol%)

[148]. This is attributed to reduced energy transfer rates from luminescence centers to quenching

centers due to the interface related size issue, and the fact that the liquid phase synthetic route

appears to sweep dopant ions into the solvent during growth.

The red emssion of Eu3+ is dominated by the 5Do7F2 transition at 612 nm with minor

peaks from 'Do F, ( J = 0,1,3) transitions characteristic of the Gd203 host lattice (see Figure 3-

14). Peaks for J =0 are observed at 578 nm, for J = 1 at 588 and 597 nm and for J = 2 at 612 and

620 nm. Emission peaks near 650 nm ( Do 7F3) and near 700 nm ( Do 7F4) arT alSO shown.

The f-f transitions of Eu3+ are essentially free-ion-like in character and their relative intensities

are sensitive to the crystal environment, therefore the luminescence can be used as the detector of

the local crystal structure. As previous described, several split levels give emission between 600

and 630 nm in monoclinic Gd203, while emssion from 5Do 7F2 at 612 nm is dominant from

cubic Gd203 The low energy peak at 620 nm is stronger than the 612 nm peak from monoclinic

Gd203: Eu3+, and the site symmetry results in a less intense red emission [151, 157, 158]. Both

the nanospheres and nanoplates of Gd203: Eu3+ have a very strong red color and the emission

peak at 612 nm is stronger than that at 620 nm. However the ratio of the peak intensitiess,

1612 1620, is larger for nanosphers versus nanoplates, as shown in Figure 3-14(a) and (b),

suggesting that both cubic and monoclinic Gd203 were present.

Focusing on the shapes of nanocrystals, the emission of 5Do7F2 at 612 nm was stronger in

nanospheres than nanoplates. This could result from several effects. First, the relatively high 612

nm peak suggests that Gd203: Eu3+ nanospheres had a larger fraction of cubic versus monoclinic









phase. The relatively intense peak at 612 nm would originate from the C2 site in the cubic

structure, cosistent with the XRD data discussed above.

Because of their geometry, nanoplates have a higher surface to volume ratio than do

nanospheres. As a result, a larger fraction of Eu3+ ionS are expected to be near the surface with

modified site symmetry, and lower emission intensity might result.

Note that mixed shaped nanocrystals with larger size than those of nanoplates and

nanospheres exhibited relatively poor luminescence (Figure 3-14) even though XRD data

showed a crystalline cubic structure. These mixed shaped nanocrystals were produced by a very

fast reaction of Gd-octanoate and octadecene (<1 h) while nanoplates and nanospheres were

produced by slower reactions (~3 h). In a fast growth process, it is reasonable to postulate that

dopant ions have less time to locate the appropriate sites in the host lattice, and instead remain in

the liquid surfactants. The low intensity from cubic mixed shape nanocrystals could therefore be

attributed to a low doping efficiency, consistent with ICP analysis of the Eu3+COncentration

reported below.

The emission spectra of mixtures of Gd203: Eu3+ nanocrystals (rods + spheres and plates +

spheres) are shown in Figure 3-15. As previously shown in Figure 3-7 (b, d), these mixed

structures were prepared by varying the main organic surfactant. It can be seen that the emission

of mixed nanorods and nanospheres (Figure 3-15(a) grown from Gd(acac)3 and TOPO) was less

intense than from the mixture of nanoplates and nanospheres (Figure 3-15(b) grown from

Gd(acac)3 and HDD). ). It is speculated that the reaction of Gd(acac)3 and hexadecanediol

allowed better incorporation of Eu3+ dopants than that of Gd(acac)3 and TOPO, as discussed

below.

























50 8 00 60 60 66 8 0
waeegh(m

Fiue31. eaieP eiso petao d03 u+(0 o )o ) aopee

(snheiedwt G~ca) adhxaeandolb nnplts snheiedwt
Gdaeat n OPO intepeec foli cd lyain n ezlehr
an )mxe hps(snhsze ihG otnot ,oecai ndotdcn)


580 I00 E20 640
Wevelength (nm)


660 E60 I00


Figure 3-15. PL emission spectra of mixed shapes of Gd203: Eu3+ (10 mol %) prepared with
a) of Gd(acac)3 / TOPO (1:2), and b) Gd(acac)3 /HDD (1:5).









3.3.4.3 Nanocrystals of Gd203: Tb3

The luminescence spectra of Gd203:Tb3+ nanoplates and nanospheres were investigated

using 270 nm excitation. Since the luminescence of terbium originates from a level ( D4)

strongly split by the crystal field, the emission pattern is much more complicated than for Eu3+,

and each peak in the spectrum represents a number of unresolved lines. The main intense

luminescence line is at 542 nm. Like Eu3+ doped Gd203 nanocrystals (Figure 3-14), the transition

energies were the same for nanoplates and nanospheres, but the luminescent intensity of

nanospheres was higher than that of nanoplates (Figure 3-16). The origins) of this effect is

thought to be similar to that for Eu, as discussed above.



"D,-'Fi I -a) spheres
-b) plates













450 500 550 600 650
Wavelength (nm)

Figure 3-16. Relative PL emission spectra of Gd203 : Tb3+ (10 mol %) nanocrystals with the
shape of a) spheres (synthesized with Gd(acac)3 and hexadecanediol ), and b)
plates (synthesized with Gd acetate and TOPO) in the presence of oleic acid,
oleylamine and benzyl ether.









3.3.4.4 Near Infrared Emission From Gd203: Er3

Near infrared (NIR) emission from Er3+ ionS at 1.5 Clm are well known and NIR is

promising for biolabeling [117, 159] since excitation and emission have longer penetration

depths in tissue [132]. Dopant Er3+ ionS were incorporated into the Gd203 nanocrystalline host

and the properties tested. Growth of Gd203: Er3+ nanocrystals with various shapes similar to

those of Gd203: Eu3+ WaS observed. NIR emission was detected from Er3+ doped Gd203

nanospheres produced using the same protocol (section 3.2.3) as for Gd203:Eu3+ as shown in

Figure 3-17 (excited by 488 nm from an Ar laser). The characteristic 4 13/2 4I15/2 and many

sublevel peaks from Er3+ over the range 1450-1650 nm were observed.


Gd~P,: Er, 10mo~l%
Ehwgy told a I







1 210 130 14 5I EA

nm lae exiain
3.3.5~E Eu Inoprtinit G23Nncrytl









thgue luminescent pnroprtes ofR Leisonsetafo Gd203:Eu3+ naorsas h opooy eativpe PLemssionde









intensity at 612 nm and Eu concentration (mol %) in samples with the same solvent volume

prepared from different Gd precursors are reported in Table 3-1. Nanospheres, nanoplates and

mixed samples were prepared by the same route as in Figure 3-14 under standard conditions. Gd

chloride was reacted in the presence of HDD, oleic acid, oleylamine and benzyl ether under

standard condition (see Table 3-1 footnote) and resulted in nanospheres of Gd203:Eu3+. All

samples had the same concentration of Eu3+ in all starting solutions (10 mol%). The results show

that Eu3+ COncentration varies from 0.57 to 6.1 mol % in as-prepared nanocrystals. While the

volume of solvent with dispersed nanocrystals was constant, the amount of nanocrystal varied

from one condition to the next. The concentration of Eu incorporated into the nanocrystals

decreased from Gd(acac)3 (a-HRHOSpheres) to Gd acetate (b-nanoplates) to Gd octanoate (c-

mixed shapes), suggesting a strong correlation between the particle shape and Eu concentration.

Although both Gd chloride (d) and Gd(acac)3 (a) produced nanospheres, the relative PL

intensities were much lower from the chloride versus the nanospheres from Gd(acac)3 ( Table 3-

1). The reaction producing nanospheres from Gd chloride was relatively fast (~30min), the yield

of product and Eu incorporation was low. As depicted, mixed shapes(c) of Gd203:Eu3+

nanocrystals were obtained within 1 h, which is faster reaction than ones (3 h) of nanospheres (a)

and nanoplates (b).

Table 3-1. Nanocrystal shape, normalized PL intensity and Eu concentration in Gd203:Eu3+
nanocrystals versus the Gd prerecursor.
Relative PL intensity Eu3+ COncentration
Gd-precursor morphology Probed at hem=612 ( mol % in final
nm product)
a Gd(acac)3 HRHOSpheres 1 6.1
b Gd acetate nanoplates 0.32 1.9
c Gd octanoate mixed nancrstals 0.21 1.2
d Gd chloride nanospheres 0.0036 0.57
Standard condition: After the solution is preheated at 200 0C for 30 min, it is heated at the rate of 250C/min up
to ~ 2900C under a N2 blanket.









Erwin and Norris reported the efficiency of Mn doping in II-IV and IV-VI nanocrystals

[160], and conclude that doping efficiency is determined by three main factors: surface

morphology, nanocrystal shape and surfactants in the growth solution. Particularly, (001) facets

of zinc blende nanocrystals play a special role in doping process because energies of these

surfaces are strikingly larger (vs. wurtzite or rock-salt). These surfaces consist of anion dimers

which provide very stable binding sites that are absent from other facets. They suggest that

strong surfactants (such as phosphonic acids) can also bind Mn and compete with surface

adsorption of dopants.

The normalized PL intensity at 612 nm and the Eu concentration from the same volume of

solvent-dispersed nanocrystals from samples a-d (Table 3-1) are shown in Figure 3-18. There is a

strong correlation between the normalized PL intensity and normalized Eu concentration

multiplied by the product yield.


1.0 a -a-P Lintensity
-@--E u concentration







a b c
sample


Fiur 318 Nomaied PL emsso inest t62nE ocnrto n RE

phtmcogah ofG23E3 aorsasgonfo dp eusrad(e
Tabl 3-)









In rare earth doped oxide nanocrystal systems, several factors will control the doping

efficiency, defined to be the concentration of RE incorporated into the nanocrystals. The results

above indicate that the luminescent efficiency of Eu3+ doped Gd203 nanocrystals is not only

dependent on the shape of nanocrystals, but also on the reaction rate and product yield, i.e.

doping efficiency is based on the kinetics of the reaction. In slow reactions, the dopants have

enough time to be incorporated. The incorporated concentration of dopants in as prepared

samples rises monotonically with increasing product yield. Additionally, the crystal str-ucture of

Gd203:Eu3+ nanocrystals can affect the doping efficiency, as it is known that the cubic phase of

Gd203 is a better host than the monoclinic phase. But this should be considered as minor factor

since mixed nanocrystals (c) have lower luminance than samples a and b, even though they are

predominantly cubic phase. The results suggest that doping efficiency is affected by three

factors in Gd203 nanocrystals : rate of reaction, product yield and crystal structure of

nanocrystals as summarized in Table 3-2.

Table 3-2. Three main factors for doping efficiency of Eu in Gd203 nanocrystals (a-d)
Eu3+ COncentration
Reaction Product Crystal structure.
tim ( mol % mn final
time yield (from XRD and PL)
product)
a ~ 3 h ~ 80 % cubic+monoclinic 6.12

b ~ 3 h ~ 80 % cubic+ Monoclinic 1.85

c ~ 1 h ~ 60 % cubic 1.24

d ~ 0.5 h ~ 35 % cubic+monoclinic 0.57
Bold represents the dominant crystal structure in sample b.

3.3.6 Thermo-gravimetric analysis (TGA)

Figure 3-19 shows TGA data from thermal decomposition in air of Gd-acetate and

Gd(acac)3 preCUTSors. The decomposition of Gd-acetate (Gd(CH3COO)3) (red line) takes place in

three steps. A dehydration step observed at ~1000C results in a loss of =3H20 molecules per









monomer The dehydration weight loss of 3H20 was estimated to be ~16.4 % in good agreement

with measured loss of ~17.5 %. Continued decomposition of dehydrated Gd(CH3COO)3

proceeds in two steps, with the first step in the temperature range (300-4000C) corresponding to

formation of Gd202CO3 and volatile products. The second step over the temperature range of

580-6500C occurred by the formation of Gd203 and CO2 [161]. The TGA data from Gd(acac)3

precursor (blue line in Figure 3-19) shows dehydration at ~1000C, followed by decomposition

above ~2000C that proceeds continuously until ~550 oC. The reaction temperatures for the

current solution synthesis scheme for nanocrystals are lower (200-2900C) than the temperatures

for the maj ority of the decomposition, therefore the reactivity of the metal precursors with the

other reactants and the organic surfactants must affect the nucleation of metal oxide phase. The

TGA data in Figure 3-19 show that the decomposition of Gd(acac)3 proceeds gradually, while

the decomposition of Gd acetate occur abruptly at 3500C. The reaction using Gd acetate forms

nuclei in a short period of time which implies that the system would be under kinetic control,

allowing an anisotropic final product (nanoplates). The decomposition of Gd(acac)3 OVeT an

extended temperature range above 2000C results in the formation of a relatively large number of

nuclei and the thermodynamically controlled reaction favors the formation of isotropic

nanospheres.



























100 200 300 400 500 600 700
Temperature (og)

Figure 3-19. TGA data from Gd acetate and Gd(acac)3 preCUTSors between RT and 7500C.

The amount of surfactant coating the surface is a function of the nanoparticle size and

shape. The TGA data from different shapes of the Gd203:Eu3+ nanocrystals coated with organic

surfactants are shown in Figure 3-20. The observed weight loss versus temperature was dramatic

above ~2500C. The small weight loss for temperatures <2500C is attributed to desorption of

physisorbed molecules from the surface. The larger weight loss at >2500C is attributed to

desorption of organic surfactant molecules from the surface, which is complete at ~5000C. Note

that the relative weight loss for T < 2500C was greater (6 versus 4%) and desorption of surfactant

started at a lower temperature (~50 versus ~100 oC) for nanospheres versus nanoplates,

respectively. Data in Figure 3-20 and Table 3-2 show a total weight loss upon heating to >6000C

of 35 % and 42 % for nanoplates (5-7 nm edge and Inm thickness) and nanospheres (2-3 nm

diameters), respectively. These data suggest that the relative amount of surfactant decreased as

the size of the Gd203:Eu3+ nanocrystals increased. The data also suggest that the surfactant is

more strongly bound in stacked nanoplates than for dispersed nanospheres.

























100 200 300 400 500 600
Temperature (OC)

Figure 3-20. TGA data from nanoplates and nanospheres of Gd203:Eu3+ synthesized under
standard conditions listed in Table 3-1.

Table 3-3. Normalized weight loss based on TGA data in Fig 3-20.
Sampe sape Si. First weight loss Second weight loss Total loss
(%) (%) (%)
naopats 5-7 nm, Inm 3931.5 35.4
(edge, thickness)
nanospheres 2-3 nm 6. 1 36. 1 42.2

If we assume a close-packed monolayer of the surfactant on the surface of a Gd203

nanosphere of diameter d, the total weight of the nanoparticle plus the monolayer is (1/6)xnd3p

( nd2/a)(M/No) where d = the diameter of the particle p = the density of the particle, a = the

attachment (head) area per molecule of the surfactant, Mw = the molecular weight of the

surfactant, and No = Avogadro number. Assuming that the TGA heating causes weight loss of

only the surface-bound surfactant, the percentage weight loss from a particle of diameter d can

be calculated using the relation [162]:

Weight loss in % = 100 x [( nd2/a)(Mw/No)] /A (1)









A = (1/6)nd3p + ( xd2/a)(M/No)

In case of Gd203:Eu3+ nanospheres, using d = 25A+ (average diameter), p = 7.407 g cm-3, a = 20

A+2 (fOT Oleic acid) and M, = 282.46 gm moll for oleic acid, the calculated percentage weight

loss using equation (1) is 43 %, in good agreement with the 42 % that was obtained

experimentally. This suggests a monolayer oleic acid coated the nanosphere surface.

3.3.7 FTIR analysis

The capping ligands chemisorbed by the surfaces of the Gd203:Eu3+ nanocrystals can be

identified from the FTIR spectra presented in Figure 3-21. Transmittance data were collected

from the solid sample such as nanoplates and nanospheres of Gd203:Eu3+, TOPO, and

hexadecanediol (HDD), and absorbance were obtained from liquid oleic acid. As reported above,

nanoplates of Gd203: Eu3+ were prepared by Gd(acac)3 preCUTSor with TOPO and oleic acid,

while nanospheres were prepared by Gd(acac)3 with hexadecanediol and oleic acid. The two

absorption bands at 2937 and 2857 cml in all five samples were attributed to the asymmetric

CH2 stretch and the symmetric CH2 stretch, respectively. The C=0 stretch band of the carboxyl

group at 1715 cm-' was detected only from oleic acid, and was absent in the data from the

nanoplates and nanospheres. This suggests that oleic acid molecules are covalently bonded to the

nanocrystals surface and there are no free oleic acid molecules [163]. Two bands at 1430 and

1535 cml from nanoplates and nanospheres probably result from the symmetric and asymmetric

stretching vibrations of carboxylic groups bonded symmetrically or bonded at an angle to the

surface [164]. Absorptions at ~1050 cm-l probably arise from C-O single bond stretching. These

data are consistent with ligands being chemisorbed onto the Gd203:Eu3+ nanocrystals as a

carboxylate.










Nakamoto categorized the interaction between a carboxylate head and a metal atom as

monodentate, bridging bidentatee), chelating bidentatee), or ionic [165]. The wave number shift,

A, between the asymmetric vas(COO-), and symmetric vs(COO-), IR bands can be used to

distinguish the type of the interaction. The largest A (200-320 cm l) corresponds to the

monodentate interaction, the smallest A (< 110 cm-' ) to the chelating bidentate, and a medium

range A (140-190 cm-l ) to the bridging bidentate. In this work, the A ( 1535-1430 = 105 cml )

suggests the chelating bidentate was formed where the interaction between the COO- group and

the Gd atom was covalent.

From pure TOPO, the v (P=0) stretching absorption appears as a sharp peak at 1145 cm ',

and this peak is detected from nanoplates but is absent from nanospheres of Gd203:Eu3+. The

band at 560 cm-l is characteristic of Gd203 [166]. Thus IR adsorption bands show the Gd203:

Eu3+ nanocrystals are coated primarily with oleic acid.





nanolpliere



nanoplates








3000 2500 2000 1500 1000 500
Wavenumber( cm ')


Figure 3-21. FTIR spectra from nanoplates and nanospheres of Gd203: Eu3+ and surfactants
(hexadecanediol, TOPO and oleic acid). The oleic acid is liquid, while the other
samples are solid.









3.4 Conclusions

Well dispersed, crystalline rare earth (Eu3+, Tb3+ and Er3+, 10 mol %) doped Gd203

nanocrystals were synthesized at 290 oC (reaction time, ~ 3 h) by a hot solution nonhydrolytic

route. The Gd203: Eu3+ nanocrystals were changed from nanospheres (<5nm) to nanoplates (Inm

x <10nm2) to mixed shaped nanocrystals (spheres, plates, rods, and triangles) by changes of Gd

precursors, organic surfactants, concentrations and heating rate (between 200 and 2900C reaction

temperatures). The mechanisms leading to isotropic versus anisotropic growth of Gd203: Eu3+

nanocrystals were discussed. The PL emission and excitation properties of nanospheres,

nanoplates and mixed shape nanocrystals were reported. The intensity of the Eu3+ 5Do 7F2

emission at 612nm from nanospheres was more intense than that from nanoplates or mixed shape

nanocrystals due to a higher concentration (6. 12 %) of Eu in nanospheres versus nanoplates(1.85

%) or mixed shape nanocrystals (1.24 %). Three main factors for doping efficiency are suggested

in Gd203 nanocrystals : rate of reaction, product yield and crystal structure of nanocrystals

which dominantly affect the doping efficiency in order. It is note that Gd(acac)3 is Slowly reacted

(~ 3 h) with oleic acid and HDD surfactants and induce, well doped, efficiently luminescent

Gd203: Eu3+ nanospheres with high product yield (~ 80 %). In this way, this experimental

advances in rare earth doped oxide nanocrystals in controlling shape and crystal structure of

Gd203: Eu3+ nanocrystals can also be important in optimizing the luminescent efficiency from

rare earth doping.









CHAPTER 4
WATER SOLUBLE SURFACE MODIFICATION OF LUMINESCENT GADOLINIUM
OXIDE NANOCRYSTALS FOR BIOMEDICAL RESEARCH

4.1 Introduction

Thermal decomposition of metal precursors in stabilizing organic surfactants has been

proven to produce monodispersed luminescent nanocrystals. This synthetic procedure also

allows control over the shape and size of nanocrystals (see Chapter 3) with the desired optical

properties, e.g. high photoluminescence yield, narrow emission peaks and stability against

photobleaching [37, 38, 59, 61, 71]. Nanocrystals synthesized by this method are hydrophobic

because of the coordinating agent, and are therefore insoluble in aqueous solutions making them

incompatible with biological systems. To disperse them in aqueous solutions, a polar surface

must be created to render them water soluble.

Several methods have been developed to modify the surface of nanocrystals for water

solubility [117, 118, 167-170]. In addition to making the nanocrystals hydrophilic, the treatment

should also a) prevent nanocrystals from flocculating during long-term storage, b) maintain or

improve the nanocrystal's fluorescence quantum yield, and c) maintain the sub-10 nm

nanocrystal size [168]. Most surface modification methods rely on the placement of the

hydrophobic surfactant coatings by ligand molecules that are reactive towards the nanocrystals

surface on one end, and has a hydrophilic groups on the other end. Alivisatos, et al and Nie, et al

first used such chemical exchange reactions to modify the surface chemistry of quantum dots

(QDs) [117, 118]. In had method, a bifunctional molecule, such as mercaptoacetic acid (MAA),

competed with TOPO (or another organic stabilizer) for binding to a metal atom on the Qdots

surface. With excess bifunctional molecules in solution, the thiol (-SH) functional groups on the

MAA displaces the phosphonic oxide (from the TOPO) that was initially bound to the metal

atoms. If the bifunctional molecules contains a polar functional group opposite to the thiol (-SH)









functional group, the Qdots become polar and are soluble in water solutions. Some disadvantages

of using MAA was rapid flocculation of the hydrophilic nanocrystals and a significant decrease

in quantum yield. Particle aggregation was attributed to a weak bond between the thiol (-SH)

group and the Qdots surface which allowed the hydrophilic ligand shell around the nanocrystal to

disintegration [167, 169].

Another approach to make the nanocrystals water soluble is to grow a hydrophilic silica

shell through surface silanization [170, 171]. At first a ligand exchange is used to substitute the

original hydrophobic surfactant with another surfactant shell (e.g.

mercaptopropyltrimethoxysilane) which is cross-linked for improved stability. To make this

silica shell hydrophilic, molecules with methoxysilane groups at one end and hydrophilic groups

at the other end are attached by siloxane bonds, resulting in a multi-layer shell. Silanized

nanocrystals are extremely stable in solution, but the silanization process is laborious and the

resulting shell often is inhomogeneous.

Recently, methods to coat hydrophobic nanocrystals with amphiphilic polymers has been

reported [119, 125, 133, 172]. In these approaches, hydrophobic tails of the amphiphilic polymer

intercalate the hydrophobic surfactant molecules on the nanocrystals surface and thus form an

additional coating layer. The water solubility of polymer coated nanocrystals is ensured by

hydrophilic groups that self-assemble on the polymer shell and the are cross-linked for better

stability. This method is not based on ligand exchange, i.e. not based on replacing the original

hydrophobic surfactant with hydrophilic molecules, but rather depends on the whole nanocrystals

being covered with a cross-linked hydrophilic polymer shell. There is no direct interaction

between the polymer and the nanocrystals surface atoms, and therefore the original luminescent

efficiency should be preserved. In addition, the large number of hydrophobic side chains on the









amphiphilic polymer strengthens the hydrophobic structure at the surface, resulting in more

stable water-soluble nanocrystals. The data indicate that this amphiphilic shell is thinner and

more homogeneous than silica shells, although the multiple shells around the inorganic core

make the overall diameter larger than nanocrystals made hydrophilic by surfactant exchange.

In the present study, the amphiphilic polymer coating method was applied to make

luminescent Gd203: Eu3+ nanocrystals hydrophilic. The optical properties and size was

monitored

4.2 Experimental Sections

4.2.1 Materials

Octylamine, anhydrous N, N-dimethylformamide (DMF), 1-[3 -(dimethyl-amino)-propyl]-

ethylcarbodiimide hydrochloride (EDC) and poly(acrylic acid) (PAA, molecular weight 2000)

were purchased from Aldrich and were used as received without further purification.

4.2.2 Synthesis of Hydrophobically Modified Poly(Acrylic Acid)

Octylamine (3.5g) in DMF solution was transferred dropwise into a stirred anhydrous

DMF solution containing dry PAA powder (4g) and 1-[3-(dimethyl-amino)-propyl]-

ethylcarbodiimide hydro chloride (1.2g). The reaction was allowed to continue for 24 h at room

temperature. After 24h an excess of H120 was added to the solution, resulting in the precipitation

of the polymer. The precipitated polymer was dissolved in methanol, which was then evaporated

to obtain the final purified amine modified PAA polymer product.

4.2.3 Synthesis of Hydrophilic Gd203 : Eu3 Nanocrystals

The Gd203 : Eu3+ nanoplates and nanospheres were prepared by the non hydrolytic hot

solution synthesis route as described in section 3.2.2 and 3.2.3. Under ambient conditions,

hydrophobic Gd203 nanocrystals (20mg) were dispersed in 15ml hexane. The suspension was

added dropwise while stirring into a hexane solution of the amine modified PAA (60 mg /15ml).









The hexane was removed by evaporation to yield a thin film of polymer/nanocrystals composite

on the wall of the flask. The dry film was redispersed in distilled water with stirring, and

aggregated material and excess polymer was removed by filtration (0.2 Clm, syringe filter).

4.2.3 Characterization

The morphology of modified Gd203:Eu3+ nanocrystals were imaged using high resolution

TEM (JEOL 2010F, 200kV). Photoluminescence (PL) was measured at room temperature from

nanocrystals suspended in hexane using a Flurolog Tau 3 spectrofluorometer (Jobin Yvon Spex

Instruments, S.A. Inc) with a 450 W xenon excitation lamp. For confirmation of the

dispersibility in water, the Zeta potential was measured with a colloidal dynamics acoustosizer.

4.3 Results and Discussion

4.3.1 Water Soluble Surface Modification of Gd203:Eu3 Nanocrystals

A schematic model structure, after surface modification of Gd203:Eu3+ nanocrystals, is

shown in Figure 4-1. In this model, the surfactant chains for hydrophobically capped Gd203:Eu3

nanocrystals are pointing away from the nanocrystals surface, in a brush-like arrangement (left

side of Figure 4-1). A plausible configuration for the amphiphilic polymer coating is shown on

the right of Figure 4-1 where the hydrophobic alkyl chains of the octylamine-modified PAA

intercalate with the surfactant coating, and the outer surface is covered by the hydrophilic ends.








Figure~ ~ ~ ~ ~ ~ ~ ~~~~~~n 4-. Shmtcmdlo uraemdfcto fG20:u+nncytl sn
octylaminer moife PAA









TEM images of Gd203:Eu3+ nanospheres before and after modification are shown in Figure

4-2. The original Gd203:Eu3+ nanospheres were prepared using hexadecanediol oleic acid,

oleylamine and benzyl ether. Although the strong affinity of surfactants such as oleic acid

increase the difficulty of modifying these hydrophobic nanocrystals to make them dispersible in

aqueous solutions [173], the TEM images showed a lack of aggregation of nanoparticles. The

aqueous dispersed Gd203:Eu3+ nanospheres (Figure 4-2 (b-c)) were stable suspensions even

after 3 months in lab ambient. The TEM images show that the average sizes of Gd203:Eu3+

nanospheres are about 2-3 nm and 3-4 nm before and after modification, respectively.















Figure 4-2. TEM images of Gd203:Eu3+ nanospheres capped with oleic acid and HDD before
(a) and after (b-c) hydrophilic modification.

An amine group coordinated to the Gd atoms was also prepared to investigate the effect of

the low bond strength on surface modification. These Gd203:Eu3+ nanocrystals were synthesized

with hexadecylamine (HDA) instead of HDD and oleic acid in a relatively fast reaction (<1 h).

HDA has been successfully used for the synthesis of IIVI semiconductor nanocrystals.

However, HDA capped Gd203:Eu3+ nanocrystals exhibited poor crystallinity and were

polydispersed with relatively low product yield (see Figure 4-3(a)). Although it was expected

that the weak coordination of HDA to the nanocrystals would result in easier and more effective









surface modification, the poor crystallinity of unmodified nanocrystals results in poor surface

modification and therefore particle agglomeration, as shown in Figure 4-3(b).

la I IC-IL1lb


Figure 4-3. TEM images of Gd203:Eu3+ nanospheres capped with HAD before (a) and after
(b) hydrophilic modification.

Using the same approach, modification of the surface of Gd203:Eu3+ nanoplates was

attempted. As shown previously in Figure 3-3 and Figure 3-6(b), nanoplates self-assembled into

closely stacked arrays. The modified hydrophilic Gd203:Eu3+ nanoplates were aggregated,

consistent with the expectation that the surfaces of the assembled nanocrystals were difficult to

completely modify and produce a stable dispersion.


TEM images of Gd203:Eu3+ nanoplates before (a) and after (b) hydrophilic
surface modification.


Figure 4-4.










4.3.2 Luminescence properties of hydrophilic surface modified Gd203:Eu3 nanocrystals

Surface modified hydrophilic nanocrystals should at least retain the optical properties of

hydrophobic capped nanocrystals. The photoluminescent spectra from Gd203:Eu3+ nanospheres

prepared with oleic acid and HDD before and after hydrophilic modification are compared in

Figure 4-5. The spectral distribution did not change but the emission intensity at 612 nm

decreased to 47 % of the initial intensity after the hydrophilic modification.



before
1 -- after (47%6)











560 560 600 620 610 660 600 100
Wavelength(nrn)


Figure 4-5. PL emission spectra of oleic acid and HDD capped hydrophobic (before) and
PAA capped hydrophilic (after) Gd203:Eu3+ nanospheres.

HDA capped hydrophobic Gd203:Eu3+ nanocrystals can also be surface modified with

PAA hydrophilic polymer (see Figure 4-3), and again the PL spectra are the same but the

intensity after modification is only 26% of the original PL emission yield at 612 nm (Figure 4-6).

Both nanocrystals of Gd203:Eu3+ synthesized with HDD and HDA are well dispersed in

water after the hydrophilic surface modification, but both also decrease the 612 nm PL emission

are observed in Figure 4-5 and Figure 4-6. These decreases are likely the result of differences in

the type and binding strength of capping surfactants, such as oleic acid and amines. The largest

drop in PL emission was from Gd203:Eu3+ nanocrystals prepared with HDA versus those










prepared with oleic acid and HDD. The larger drop for HDA capped nanocrystals is attributed to

the fact that the HDA ligands are labile and these capping groups are easily removed from the

Gd203:Eu3+ surface. As pointed out above, weak encapsulation by HAD results in poor

crystallinity and unstable condition for Eu3+ doping, with or without the PAA surface

modiaicati on.



before
after (26%6)














6f0 580 600 620 640 660 680 710
W av ele ngth (nrn)

Figure 4-6. PL emission spectra of had capped hydrophobic (before) and hydrophilic (after)
Gd203:Eu3+ nanocrystals.

4.3.3 Dispersion Properties of Hydrophilic Surface Modified Gd203:Eu3 Nanocrystals

The zeta potential of hydrophilic Gd203:Eu3+ (sample in Figure 4-2 (b,c)) after surface

modification is shown versus pH in Figure 4-7. The pH value of the original sample was 7.3 5

that corresponds to a zeta potential of -45 mV, and the dispersion should be stable. For pH values

between 5 and 8, the data indicate that modified Gd203 :Eu3+nanocrystals should be well

Dispersed and aggregation is not expected.



























pH

Figure 4-7. Zeta potential versus aqueous solution pH of hydrophilic PAA surface modified
Gd203:Eu3+ nanocrystals.

4.4 Conclusions

Octylamine-modified poly(acrylic acid) (PAA) has been used to convert the hydrophobic

surface of Gd203:Eu3+ nanocrystals synthesized by a non hydrolytic hot organic solvent

technique to a hydrophilic surface. The surface conversion results in a dispersion of nanocrystals

in aqueous solutions stable in lab ambient for >31 days, especially for nanocrystals synthesized

using oleic acid and hexadecanediol (HDD). The quality of surface modified Gd203:Eu3+

nanocrystals synthesized with hexadecylamine (HDA) was not as good. For both synthesis

routes, the PL spectra was unchanged by the surface modification, but the intensity was

decreased to 47% and 26% of the hydrophobic capped value for synthesis with HDD and HDA,

respectively.









CHAPTER 5
SYNTHESIS AND CHARACTERIZATION OF TERNARY ZNGA204: EU3+
NANOCRYSTALS

5.1 Introduction

An organic phase process known as thermal decomposition synthesis allows precise tuning

of nanocrystals size, shape, and composition, and also allows them to be dispersed in either an

aqueous or a nonhydrolytic media. This technique has been demonstrated for binary

semiconductor (e.g. CdSe [23], ZnSe [174], and PbSe [175] ) and binary transition metal oxide

(e.g. ZnO [68], TiO2 [74], MnO [54], Fe304 [65] and Gd203 [70]) nanocrystals. Recently, this

synthetic route was expanded to synthesis of ternary nanocrystals with controlled stoichiometry .

Zhong and co-workers synthesized ZnxCdl-xS nanocrystals at high temperature by reacting a

mixture of CdO and ZnO oleic acid complexes with sulfur in a noncoordinating solvent

(octadecene) system [176]. They found that the nanocrystals show narrow and composition-

dependent photoluminescence spectra. Lee, et al used hot solution synthesis to grow nanorods of

Cdl-xZnxSe [177]. They showed that the ternary nanorods could be grown from a solution

containing both Cd and Zn precursors, of by solid state diffusion after growth of a CdSe core and

a ZnSe shell [164]. Finally, growth of ternary compounds of MFe204 (M=Fe, Co, Mn, and Mg)

[64, 66] were reported by thermal decomposition synthesis. The composition of the particles was

controlled by the molar ratio of Fe(acac)3 and M(acac)2 reactants, and the shape and size were

changed by varying the reaction conditions. By selecting different elements for M2+, the MFe204

could be molecularly engineered over a wide range of magnetic properties.

However, growth of doped ternary nanocrystals has not been reported, either as II-VI

semiconductors or as oxide compounds. Even the number of reports of doped binary oxide

nanocrystals are few (e.g. Y203: Eu3+ [178] and Gd203: Eu3+ [71]). In the present study, thermal









decomposition synthesis and the resulting photoluminescent properties of Eu3+ doped ternary

ZnGa204 nanocrystals is reported.

Luminescent zinc gallate (ZnGa204) has attracted a great deal of attention due to potential

applications in field emission displays and electroluminescent devices [179, 180]. In principle,

an oxide should show better chemical stability versus competing sulfide phosphors (ZnS: Cu,Cl

and SrGa204:Eu2+), especially at high electron beam currents [181]. ZnGa204 crystallizes as a

cubic spinel with a large band gap of ~4.4 eV, and exhibits an intense green emission when

doped with Mn and blue luminescence without doping via a self-activated center [182]. It has

been suggested that white luminescence could be achieved from ZnGa204 by doping with Mn2+

(green), Eu3+ (red) and Ce3+ (blue) [180].

5.2 Experimental

5.2.1 Materials

Zn (II) acetate hydrate, Zn (II) acetate dehydrate Zn(II) acetylacetonate hydrate, Ga(III)

acetylacetonate hydrate, Ga (III) nitrate hydrate, Eu(III) acetate hydrate oleic acid ( 90% ,

tech.), oleyamine (90%, tech.), benzyl ether ( 99%) 1,2-hexadecanediol ( 97%), and

trioctylphosphine oxide (TOPO, 99%) were purchased from Aldrich and used without further

purification. Absolute ethanol, benzyl ether and hexane were also used as received.

5.2.2 Synthesis of ZnGR204: Eu3 Nanocrystals

Similar procedures were used to prepare all ternary ZnGa204: Eu3+ nanocrystals.

A zinc precursor (e.g. Immol of Zn(II) acetylacetonate hydrate) was mixed with Ga(III)

acetylacetonate hydrate(2mmol), Eu(III) acetate hydrate (0.1 mmol), oleic acid (6mmol),

oleylamine (6mmol), benzyl ether (5 mmol) and hexadecanediol (5 mmol) and magnetically

stirred flowing nitrogen in a three-neck reaction flask. This mixture in the flask was heated to

2000C for 30 min, then heated to 2900C at a heating rate of 250C /min and held for 3h under the









N2 blanket. After reflux, the mixture was cooled to room temperature by removing the heat

source. Under ambient conditions, ethanol was added to the mixture, and ZnGa204: Eu3+ WaS

precipitated and separated via centrifugation (9000 rpm, 10 min). After several purification with

ethanol followed by centrifugation, purified ZnGa204: Eu3+ nanospheres were well dispersed in

organic solvents such as hexane, chloroform or toluene. For other Zn precursors, Zn (II) acetate

hydrate resulted in nanospheres of ZnGa204: Eu3+ with the same concentrations and heating

sequences. When TOPO (2 mmol) was used instead of hexadecanediol (5 mmol), sphere-like

ZnGa204: Eu3+ nanocrystals were again produced. However, Zn (II) acetate dehydrate reacting

with Ga(III) acetylacetonate hydrate (2mmol), Eu (III) acetate hydrate (0.1 mmol), oleic acid

(6mmol), oleylamine (6mmol), benzyl ether (5 mmol) and hexadecanediol (2.5 mmol) resulted in

mixed-shaped large ZnGa204: Eu3+ nanocrystals.

5.2.3 Characterization of ZnGR204: Eu3 Nanocrystals

X-ray diffraction (XRD) (Philips APD 3720) was used to determine the crystal structure

with Cu Koc. radiation source ( h= 0.5418 nm). A JEOL 2010F transmission electron

microscope (TEM) operated at 200 kV was used to determine the size and shape of the

nanocrystals. Optical absoprtion spectra were collected with a Shimadzu UV-2401PC

spectrophotometer. Photoluminescence (PL) was measured at room temperature from

nanocrystals suspended in hexane using a Flurolog Tau 3 spectrofluorometer ( Jobin Yvon Spex

instruments, S.A. Inc) with a 450 W xenon excitation lamp. Thermo-gravimetric analysis (TGA,

Seiko TG/ATD 320U, SSC 5200) was used to investigate the amount of surfactant bound on the

particle surface. In the TGA, the samples were heated up to 8000C in air at a heating rate of 100C

mmn









5.3 Results and Discussion

5.3.1 Shape Control of ZnGR204: Eu3 Nanocrystals

ZnGa204: Eu3+ nanospheres were prepared with a main surfactant of either hexadecanediol

(HDD) or TOPO and a mixture of oleic acid, oleylamine and benzyl ether with Zn- and Ga-

precursors. The Zn- and Ga-precursors and main surfactant were varied as shown in Table 4-1 to

determine their effects on formation of ternary ZnGa204: Eu3+ nanospheres.

Table 5-1. Precursors and surfactants used to synthesize ZnGa204: Eu3+ nanospheres
Ratio of
Zn Ga Surfactant
Zn/surfactant
a Zn(acac)2 Gd(acac)3 hexadecanediol 1:5
Zn acetate
b Ga nitrate hexadecanediol 1:5
hydrate
Zn acetate
c Gd(acac)3 hexadecanediol 1:5
hydrate
d Zn(acac)2 Gd(acac)3 TOPO 1:2
For synthesis of ZnGa204 by solid state reactions (heating to high temperatures in air),

Zn/Ga ratio much higher than one must be used because of the high vapor pressure of ZnO, but

nonstoichiometric product was still reported [183-185]. In the present case, the Zn:Ga ratio in the

purified ZnxGa2-xO4: Eu3+ nanocrystals was essentially identical to that in the reactant mixtures.

Most samples were prepared with the ratio of Zn:Ga of 1:1. This constant ratio between reactants

and products probably is a result of similar precursors and solubilities of the Zn and Ga

precursors in the nonhydrolytic liquid phase, and results in easy composition adjustments.

High resolution transmission electron microscope (HRTEM) images of ZnGa204: Eu3+

nanospheres (from combinations a-d listed in Table 5-1 and discussed in section 5.2.2) are shown

in Figure 5-1. The ZnGa204: Eu3+ nanocrystals are mainly monodisperse, crystalline spherical

particles, with a diameter of <5 nm. The average diameter of ZnGa204: Eu3+ nanospheres in

sample (a) are larger (~ 3-4 nm) than samples (b-d) (~2-3 nm).































Figure 5-1.HRTEM images of ZnGa204: Eu3+ nanospheres prepared with the precursors and
surfactants corresponding to (a-d) in Table 5-1.

When the Zn precursor was changed to Zn acetate dehydrates with a ratio of

Zn/hexadecanediol (1:2.5), complex shaped triangular and rectangular ZnGa204: Eu3+

nanocrystals were observed as shown in Figure 5-2. Because of the poor contrast in the TEM

images, these nanocrystals could be nanoprisms and nanoplates rather than flat triangles and

rectangles.













Figure 5-2.HRTEM images of ZnGa204: Eu3+ nanocrystals prepared with Zn acetate dehydrate
with the ratio of Zn:hexadecanediol of 1:2.5 under standard conditions.









5.3.2 Size Control of ZnGR204: Eu3 Nanocrystals

In most colloidal techniques, particle synthesis from homogeneous solution involves

nucleation and growth [186]. The surfactant ligand is critical to allow particles to grow to a

specific size and then to arrest growth, therefore higher concentration could be expected to better

limit the growth of nanocrystals. With higher concentration of stabilizing surfactants (over a

limited range), smaller nanocrystals were obtained with a Zn(acac)3 / oleylamine system [67]. It

was reported that inj section of additional surfactants, such as thiol, would arrest the growth and

result in formation of smaller ZnO nanocrystals [187].













Or an.







Figure 5-3. HRTEM images of ZnGa204: Eu3+ nanocrystals synthesized with a ratio of
Zn(acac)2 / hexadecanediol (a-b) 1:2.5, and (c-d) 1:5; (e) selected area electron
diffraction (SAED) pattern from a cubic spinel crystalline phase.

HRTEM images of the ZnGa204: Eu3+ nanocrystals using a 1:2.5 molar ratio of the

Zn(acac)2 / hexadecanediol are shown in Figure 5-3 (a-b). Nearly monodispersed, spherical

nanocrystals >4-5 nm in diameter were observed. Smaller nanocrystals of <4 nm in diameter

were obtained as shown in Figure 5-3 (c-d), when a 1:5 molar ratio of Zn(acac)2 /

hexadecanediol was used. The selected area electron diffraction (SAED) pattern of the









nanocrystals show rings from the (220), (311) and (400) planes of cubic spinel ZnGa204 (Figure

5-3(e)).

5.3.3 Crystal Structures of ZnGR204: EU unanocrystals

ZnGa204 has a cubic normal spinel crystal structure with Fd3m space group that can be

thought of as a combination of rock salt and zinc blende structure (see Figure 5-4) [188]. The

oxygen ions are in face-centered cubic close packed configuration. The unit cell contains 8

tetrahedral cations, 16 octahedral cations and 32 oxygen anions. The normal spinel ZnGa204 has

tetrahedrally coordinated Zn sites surrounded by 4 oxygens and octahedrally coordinated Ga

sites surrounded by 6 oxygens.









,-v~7~Octahed~ralinterstice
(32 per unit ell

gZn in tetrhedral site
O Ga in odtahedral shte
oxhygen ar = sar Terrahedra interstice
(64 per unit cell)

Figure 5-4. The cubic spinel structure of ZnGa204 [189].

Figure 5-5 is XRD patterns from ZnGa204: Eu3+ nanocrystals (samples (a-b) in Figure 5-3).

Although peak broadening occurred due to the small size of the nanocrystals, all of the

diffraction peaks could be indexed from the cubic spinel ZnGa204 structure ( JCPDF 38-1240).

The main peak from (3 11) planes of ZnGa204 is Observed at a two-theta of 3 5.60 and no peaks

from Ga203 or ZnO were found, consistent with single phase of ZnGa204: Eu3+ nanocrystals. The









mean crystal size is calculated with the Scherrer' s formula to be 5.0 nm, which is consistent with

the size determined from the HRTEM image (Figure 5-3 (b)).

















10 20 30 40 50 60 70


Figure 5-5.XRD pattern of ZnGa204: Eu3+ nanocrystals (sample (a-b) in Figure 5-3).

5.3.4 Luminescence properties of ZnGR204: Eu3+nanocrystals

The photoluminescence excitation (PLE) spectrum of ZnGa204: Eu3+ nanocrystals (sample

(a-b) in Figure 5-3) are shown in Figure 4-6. The PLE spectrum of Eu3+10 i WaS obtained by

monitoring the Eu3+ 5Do 7F2 luminescence at 612 nm, and it consists of a broad intense band

with a maximum 305 nm and two excitation peaks (stronger one is at 396 nm for 5Fo 5L6) Of

Eu3+. The 305 nm band is due to the charge transfer band (CTB) of 02--Eu3+ together with

absorption of ZnGa204 host lattice, which is shown in the inset of Figure 5-6.

The emission spectrum obtained by excitation into 305 nm is composed of characteristic

emission peaks of Eu3+ ion fTOm 580 to 700 nm, which are associated with the transition from the

excited Do level to 7F, ( J = 1,2,3,4 ) as seen in Figure 5-7. In the spinel Znca204~ structure, Eu3+

ions could occupy at two sites, tetrahedral Zn2+ sites and octahedral Ga3+ sites. It is known that

the relative intensity of SDo- FItransition (magnetic dipole transition) and 5Do- F2 (electric dipole









transition) depends strongly on the local symmetry of Eu3+ ionS. When ions occupy the inversion

center sites, the SDo- FItransition should be relatively strong, while the 5Do- F2 transition is

partly forbidden and should be very weak. Therefore, the ( Do- F2 )/ 5Do- F1) intensity ratio,

known as the asymmetry ratio, is a measure of the degree of distortion from the inversion

symmetry of the local environment of the Eu3+ in the lattice. These ZnGa204: Eu3+ nanocrystals

exhibit high (5Do- F2)/ 5Do- F1) intensity ratios, i.e. large asymmetry ratios, as seen in Figure 5-

7, indicating strong electric fields and low symmetry at the Eu3+ ionS sites. This result suggest

that the Eu3+ ionS occupies tetrahedral Zn2+ sites or distorted octahedral Ga3+ sites with no

inversion symmetry in ZnGa204 nanocrystals. Since the ionic radii of Zn2+ ( 0.6 A+) and Ga3+

(0.62 A+) are so similar and much smaller than that of Eu3+ (0.947 A+), there is no basis for

speculating on a site preference based on ionic size.
















250 300 350 100 450 510
W avele ngth~nm)


Figure 5-6.PLE spectrum for emission at 612 nm from ZnGa204: Eu3+ nanocrystals. The inset
shows the UV absorption spectrum of undoped ZnGa204 nanocrystals.

























515 L00 L25 650 115 100
Wavelength(nm )


Figure 5-7. PL spectrum of ZnGa204: Eu3+ nanospheres excited at 305 nm

5.3.5 Thermo-gravimetric analysis (TGA)

The TGA data from ZnGa204: Eu3+ nanocrystals (sample in Figure 5-2) are shown in

Figure 5-8. These ZnGa204: Eu3+ nanocrystals are relatively large (~20 nm) complex shaped

particles coated with organic surfactants. The weight loss during heating from RT to ~2000C was

~4% which is attributed to desorption of physisorbed molecules from the organic surfactants.

The 16% weight loss in heating from ~2000C to ~3000C was presumably due to the desorption

of organic surfactants from the particle surface. Decomposition of the surfactants is complete at

4100C and results in a weight loss of 2% between ~3000C and 4100C. The total weight loss from

desorption of surfactants from large completed shaped ZnGa204: Eu3+ nanocrystals is ~20 % ,

while the total loss was 35% for nanoplates and 42% for nanospheres of Gd203:Eu3+, as reported

in Chapter 3. This result is attributed to the previous conclusion that the relative amount of

adsorbed surfactants decreases as the size of the nanocrystals increases.


























100 200 300 400 500 600 700
-Temperature ("C)

Figure 5-8. TGA data from larger completed shaped ZnGa204: Eu3+ nanocrystals (sample in
Figure 4-2).

5.4 Conclusions

Monodispersed Eu3+ doped ternary ZnGa204 HRHOSpheres (~5nm) were prepared from a

variety of metal precursors by a nonhydrolytic thermal decomposition route. Using Gd acetate

dehydrate, large (~20nm) complex shaped (triangle and rectangle) ZnGa204: Eu3+ nanocrystals

were obtained. Based on X-ray diffraction data, the nanocrystals were concluded to have a cubic

spinel structure with no impurity phases. The size of the ZnGa204: Eu3+ nanospheres could be

varied by changing the molar ratio of Zn to surfactants, with higher concentrations of surfactant

(1: 5) resulting in smaller nanocrystals ( < 4 nm). Analysis of the PL emission suggests that the

Eu3+ ionS were incorporated into the ZnGa204 host. It is concluded that the nonhydrolytic thermal

decomposition synthesis route with organic surfactants not only allows the formation of

stoichiometric ternary oxides, but also results in efficient incorporation of rare earth dopants.









CHAPTER 6
CONCLUSIONS

6.1 Synthesis and Characterization of Luminescent Gadolinium Oxide Nanocrystals

1. Well dispersed, crystalline rare earth (Eu3+, Tb3+ and Er3+) doped Gd203 nanocrystals

were synthesized at 290 oC (reaction time, ~ 3 h) by a hot solution nonhydrolytic route.

2. The shape of Gd203: Eu3+ nanocrystals were changed from nanospheres (<5nm) to

nanoplates (Inm x <10nm2) to mixed shaped nanocrystals (spheres, plates, rods, and triangles)

by changes of Gd precursors, organic surfactants, concentrations and heating rate (between 200

and 2900C reaction temperatures).

3. The intensity of the Eu3+ 5Do 7F2 emiSsion at 612nm from nanospheres was more

intense than that from nanoplates or mixed shape nanocrystals due to a higher concentration

(6. 12 %) of Eu in nanospheres versus nanoplates (1.85 %) or mixed shape nanocrystals (1.24 %),

and due to a higher nanocrystal product yield for purified nanospheres.

4. The three main factors leading to larger amounts of dopant in Gd203 nanocrystals are a

slower rate of reaction, a larger product yield and monoclinic crystal structure. Use of Gd(acac)3

precursor with oleic acid and HDD surfactants led to a slow reaction (~ 3 h) with high product

yield (~ 80 %) and 6. 1 % Eu incorporation for bright luminescent Gd203: Eu3+ nanospheres.



6.2 Water Soluble Surface Modification of Luminescent Gadolinium Oxide Nanocrystals
for Biomedical Research

1. Hydrophobic Gd203:Eu3+ nanocrystals were surface modified by Octylamine-modified

poly(acrylic acid) (PAA), amphiphilic polymer. The surface conversion results in a dispersion of

nanocrystals in aqueous solutions stable in lab ambient for >31 days, especially for nanocrystals

synthesized using oleic acid and hexadecanediol (HDD).









2. The PL spectra from Gd203:Eu3+ were unchanged by the PAA surface modification, but

the PL intensity was decreased to 47% and 26% of the hydrophobic capped value for synthesis

with HDD and hexadecylamine (HDA), respectively.



6.3 Synthesis and Characterization of Ternary ZnGR204: Eu3 Nanocrystals

1. Monodispersed Eu3+ doped ternary ZnGa204 HRHOSpheres (~5nm) were prepared from a

variety of metal precursors by a nonhydrolytic thermal decomposition route. Using Gd acetate

dehydrate, large (~20nm) complex shaped (triangle and rectangle) ZnGa204: Eu3+ nanocrystals

were obtained.

2. ZnGa204: Eu3+ nanocrystals had a cubic spinel structure with no impurity phases from

X-ray diffraction and selected area diffraction pattern.

3. The size of the ZnGa204: Eu3+ nanospheres was controlled by changing the molar ratio

of Zn to surfactants from 1:1 to 1:5, with higher concentrations of surfactant (1:5) resulting in

smaller nanocrystals (< 4 nm).

4. Analysis of the PL emission suggests that the Eu3+ ionS were incorporated into the

ZnGa204 host. The nonhydrolytic thermal decomposition synthesis route with organic surfactants

not only allows the formation of stoichiometric ternary oxides, but also results in efficient

incorporation of rare earth dopants.









CHAPTER 7
FUTURE WORK

It is evident that the above approaches can lead to success in shape and size control of

nanocrystals. This would be very useful in incorporating luminescent oxide nanocrystals with

nonhydrolytic liquid phase synthesis. This method can be expanded to other efficient

luminescent oxide systems.

As with a doping issue in this synthetic route, efficiency is not good comparing to the solid

state reaction. Liquid phase synthesis has still a possibility to lost dopant ions into solvent during

synthesis. The synthetic condition could be modified to make the reaction stable for better

incorporation of dopant by trial of various chemical combinations. The fine control of dopants

could allow the co-doped oxide nanocrystals (i.e. Gd203:Er3+, Yb3+ and YVO4: Er3+, b3+, etc) to

be synthesized through this protocol for efficient up-conversion phosphor.









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3735.









BIOGRAPHICAL SKETCH

SooYeon Seo was born in Seoul, South Korea and received her early education in this city.

She was allowed access to the laboratory tools in her home from an early age. This early

influence of science has had a lasting impact on her interest in phenomenon in nature and led to a

foundation of evolutionary thinking in her pursuit of understanding the materials, in the kinds of

research questions that have intrigued her. After she received a M.A in materials science and


engineering, in South Korea, she obtained her Ph.D. degree from the Department of Materials

Science and Engineering, University of Florida in Gainesville, US in 2007.

Sooyeon Seo's general research interests are in the area of luminescent materials in elecro-

and bio- application. The general theme of her current research interests deals with synthesis and

characterization of luminescent oxide nanocrystals related to the candidate for the bio-labeling

application in bio-medicine. She also has other interests and has done research on such topics as

the luminescent nanocrystals as well as general issues related to methodology and optical

analysis in electronic and bio-applications.





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1 SYNTHESIS AND CHARACTERIZATION OF LUMINESCENT OXIDE NANOCRYSTALS By SOOYEON SEO 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

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2 2007 Sooyeon Seo

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3 To God, my family and friends, this work is dedicated

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4 ACKNOWLEDGMENTS It is a great relief to come to the end of graduate student life at the University of Florida. The past years have been full of frustrations, struggles and excitements. I feel very fortunate to have Prof. Paul H.Holloway as my advisor. His patience and trust have been very important for me to develop my own thoughts, and his attitude towards science and his open-mindedness have greatly influenced my perception about scientific research. I am also grateful for the hospitality and support of Ludie Harmon. exchanging ideas and learning from each other. The work described in this thesis would not have been possible without the help of the people in the Department of Materials Science and Engineering. I had the honor to work with some wonderful collaborators outside our group. I would especially like to thank Kerry Siebein in the Major Analytical Instrumentation Center (MAIC) for helping me collect the TEM data. Thanks also to Dr. Kirk Schanze and his group in the Department of Chemistry for assistance in collecting photoluminescence data. I am very grateful to my God, my parents, my sister and my brother for their support. Especially, I thank my sister Youjin for being my town mate. She always did so much to help and cheer me up. I also wish to thank my other sisters, Sungok and Soyeon. I thank all my friends for their encouragement and help, both in research and in daily life. I am so lucky to have

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5 TABLE OF CONTENTS page ACKNOWLEDGMENTS ...............................................................................................................4 LIST OF TABLES ...........................................................................................................................8 LIST OF FIGURES .........................................................................................................................9 ABSTRACT ...................................................................................................................................14 CHAPTER 1 INTRODUCTION ..................................................................................................................16 2 LITERATURE REVIEW .......................................................................................................18 2.1 Fundamentals of Colloidal Nanocrystals ......................................................................18 2.1.1 Chemistry and Physics of Nanocrystals: Size and Shape Issues ......................18 2.1.2 Synthetic Processes for Colloidal Nanocrystals ................................................22 2.2 Shape Control of Colloidal Nanocrystals ......................................................................26 2.2.1 Zero-Dimensional (0D) Spheres and Polyhedrons of Nanocrystals ..................27 2.2.2 One Dimensional (1D) Rods and Wires of Nanocrystals ..................................29 2.2.2.1 1D Semiconductors ............................................................................29 2.2.2.2 1D Metal Oxide Nanocrystals ...........................................................32 2.2.3 Two Dimensional (2D) Discs and Prisms of Nanocrystals ...............................34 2.2.4 Advanced Shapes of Nanocrystal ......................................................................37 2.3 Proposed mechanism for shape-control growth of nanocrystals...................................40 2.3.1 Kinetically Induced Anisotropic Control ...........................................................41 2.3.1.1 Cyrstalline Phase Control of Nuleating Seeds by Temperature .........41 2.3.1.2 Surface energy modulation by capping surfactants ............................43 2.3.1.3 Growth Regime Control by Monomer Concentration and Temperature45 2.3.2 Oriented Attachment ..........................................................................................47 2.4 Application of Nanocrystals in Biomedicine ................................................................48 2.4.1 Biocompatible Magnetic Nanocrystals for MR Contrast Effects ......................49 2.4.2 Luminescent Nanocrystals for Fluorescence labels ...........................................51 3 SYNTHESIS AND CHRACTERIZATION OF LUMINESCENT GADOLINIUM OXIDE NANOCRYSTALS ...................................................................................................55 3.1 Introduction ...................................................................................................................55 3.2 Experimental Section ....................................................................................................55 3.2.1 Materials ............................................................................................................55 3.2.2 Synthesis of platelet Gd2O3: Eu3+ nanocrystals .................................................56 3.2.3 Synthesis of spherical Gd2O3: Eu3+ nanocrystals ...............................................57 3.2.4 Characterization of Rare Earth Doped Gd2O3 Nanocrystals ..............................58 3.3 Results and Discussion ..................................................................................................58

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6 3.3.1 Nanoplates and Nanospheres of Gd2O3: Eu3+ ....................................................59 3.3.2 Shape control of Gd2O3: Eu3+ nanocrystals ........................................................63 3.3.2.1 Effects of Gd-precursor ......................................................................63 3.3.2.2 Effects of Surfactants .........................................................................64 3.3.2.3 Effects of the Precursor/Surfactant Ratio ..........................................65 3.3.2.4 Effects rate of Heating Rate from 200oC to 290oC .............................67 3.3.3 Crystal Structures of Gd2O3: Eu3+ Nanocrystals ................................................68 3.3.4 Luminescence Properties of Gd2O3 : Eu3+, Tb3+ and Er3+ Nanocrystals ...........69 3.3.4.1 Eu3+ Fluorescence in Oxides...............................................................69 3.3.4.2 Luminescence vs. Gd2O3 Crystallography ..........................................70 3.3.4.3 Nanocrystals of Gd2O3: Tb3+ ..............................................................76 3.3.4.4 Near Infrared Emission From Gd2O3: Er3+ .........................................77 3.3.5 Eu3+ Incorporation into Gd2O3 Nanocrystals .....................................................77 3.3.6 Thermo-gravimetric analysis (TGA) .................................................................80 3.3.7 FTIR analysis .....................................................................................................84 3.4 Conclusions ...................................................................................................................86 4 WATER SOLUBLE SURFACE MODIFICATION OF LUMINESCENT GADOLINIUM OXIDE NANOCRYSTALS FOR BIOMEDICAL RESEARCH ...............87 4.1 Introduction ...................................................................................................................87 4.2 Experimental Sections ...................................................................................................89 4.2.1 Materials ............................................................................................................89 4.2.2 Synthesis of Hydrophobically Modified Poly(Acrylic Acid) ............................89 4.2.3 Synthesis of Hydrophilic Gd2O3 : Eu3+ Nanocrystals ........................................89 4.2.3 Characterization .................................................................................................90 4.3 Results and Discussion ................................ ................................ ................................ .. 90 4.3.1 Water Soluble Surface Modification of Gd 2 O 3 :Eu 3+ Nanocrystals ................... 90 4.3.2 Luminescence properties of hydrophilic surface modified Gd2O3:Eu3+ nanocrystals .......................................................................................................93 4.3.3 Dispersion Properties of Hydrophilic Surface Modified Gd2O3:Eu3+ Nanocrystals ......................................................................................................94 4.4 Conclusions ...................................................................................................................95 5 SYNTHESIS AND CHRACTERIZATION OF TERNARY ZnGa2O4: Eu3+ NANOCRYSTALS ................................................................................................................96 5.1 Introduction ...................................................................................................................96 5.2 Experimental .................................................................................................................97 5.2.1 Materials ............................................................................................................97 5.2.2 Synthesis of ZnGa2O4: Eu3+ Nanocrystals .........................................................97 5.2.3 Characterization of ZnGa2O4: Eu3+ Nanocrystals ..............................................98 5.3 Results and Discussion ..................................................................................................99 5.3.1 Shape Control of ZnGa2O4: Eu3+ Nanocrystals ..................................................99 5.3.2 Size Control of ZnGa2O4: Eu3+ Nanocrystals ..................................................101 5.3.3 Crystal Structures of ZnGa2O4: Eu3+ nanocrystals ...........................................102 5.3.4 Luminescence properties of ZnGa2O4: Eu3+ nanocrystals ...............................103

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7 5.3.5 Thermo-gravimetric analysis (TGA) ...............................................................105 5.4 Conclusions .................................................................................................................106 6 CONCLUSIONS ..................................................................................................................107 6.1 Synthesis and Characterization of Luminescent Gadolinium Oxide Nanocrystals ....107 6.2 Water Soluble Surface Modification of Luminescent Gadolinium Oxide Nanocrystals for Biomedical Research .......................................................................107 6.3 Synthesis and Characterization of Ternary ZnGa2O4: Eu3+ Nanocrystals ...................108 7 FUTURE WORK ..................................................................................................................109 LIST OF REFERENCES .............................................................................................................110 BIOGRAPHICAL SKETCH .......................................................................................................121

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8 LIST OF TABLES Table page 2-1 Various rare earth oxides synthesized by thermolysis .......................................................37 3-1 Nanocrystal shape, normalized PL intensity and Eu concentration in Gd2O3:Eu3+ nanocrystals versus the Gd precursor. ...............................................................................77 3-2 Three main factors for doping efficiency of Eu in Gd2O3 nanocrystals............................80 3-3 Normalized weight loss based on TGA data......................................................................83 5-1 Precursors and surfactants used to synthesize ZnGa2O4: Eu3+ nanospheres ......................99

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9 LIST OF FIGURES Figure page 2-1 Size-tunable fluorescence and spectral range of CdSe QDs with different core size. .......19 2-2 Energy vs. density of electronic states (DOS) for inorganic crystals of 3, 2, 1, and 0dimensions .........................................................................................................................20 2-3 Band gap of CdSe quantum rods versus length and width viewed from two different angles ........................................................................................................................20 2-4 TEM images (A-C) and UV-VIS absorption (solid lines) and photoluminescence(dotted lines) spectra (DF) of three nanorod CdSe samples. ............21 2-5 Blocking temperature and magnetic coercivity of 4 nm cobalt nanospheres and nanodiscs ............................................................................................................................22 2-6 Crystal-growth diagram. ..................................................................................................23 2-7 Illustration of the organometallic precursor method for synthesis of CdSe quantum dots .............................................................................................................................25 2-8 Geometrical shapes of inorganic nanocrystals ...................................................................27 2-9 High resolution transmission electron microscope (HRTEM) images of (a) spherical CoFe2O4 nanocrystals and (b) multiple and (c) CoFe2O4 cube nanocrystals.....................28 2-10 Fe2O3 nanocrystals (b) diamond (c) sphere (d) triangle (e,f) hexagon nanocrystals ......................................................................................................29 2-11 CdSe nanorods with different sizes and aspect ratios in different concentrations of HPA /TOPO surfactants .....................................................................................................30 2-12 TEM images of (a) starting ZnO nanospheres, and (b) after one day growth of ZnO nanorods by an oriented attachment process ....................................................................30 2-13 GaP nanocrystals. HRTEM images of (a) zinc blende nanospheres and (b) wurtzite nanorods, and absorption and photoluminescence colors from (c) nanospheres and (d) nanorods .......................................................................................................................31 2-14 TiO2 nanocrystals formed from single crystal via oriented attachment. ............................32 2-15 TEM images of nanorods of (a,b) tungsten oxide (c,d) manganese oxide and (e,f) titanium dioxide .................................................................................................................33 2-16 TEM image of selfassembled 2 nm diameter ZnO nanorods. Inset: higher magnification image showing the oriented stacks of 1D ZnO. ..........................................34

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10 2-17 TEM image of Co nanodisks either self-assembled into ribbons (edge-on view) or lying flat (right) on the sample support surface .................................................................34 2-18 TEM images (a) Cu2S nanodics], (b) NiS nanoprims .....................................................35 2-19 TEM images and schematic of morphology changes: (a) nanospheres before irradiation, (b) nanoprisms after 70 hours of irradiation, and (c) the color of light Rayleigh scattered by nanoprims and nanospheres ...........................................................35 2-20 Two dimensional (2D) lanthanide oxide nanocrystals: (a) Gd2O3 nanoplates, (b) model for the nanoplates assembly, (c) Eu2O3 nanodisks, (d) Er2O3nanodisks, and (e) Pr2O3 nanoplates. ..........................................................................................................36 2-21 Tetrapod shaped nanocrystals of (a) CdSe, (b) MnS, and (c) proposed model of CdTe ...38 2-22 Influence of the shape of CdTe tetrapods on optical absoprtion spectra. (a) tetrapods having comparable arm diameters but different diameters; (b) tetrapods having comparable arm diameters but different lengths. ...............................................................38 2-23 PbS nanocrystals with shapes corresponding to (a) rod-based multipods at 140 C, (b) tadpole-shaped monopod, (c) I-shaped bipod, (d) L-shaped bipod, (e) T-shaped tripod, (f) cross-shaped tetrapod, (g) pentapod, (h) star-shapes at 180 C, (j) truncated octahedrons at 250 C, and (k) conversion of cubes to star-shape to 1D rod-based multipods by control of the growth parameters. ......................................................39 2-24 PbSe nanocrystals showing (a) zigzag nanowires packing of octahedral building blocks,(b) star shaped nanocrystals, (c) radially branched nanowires, and (d) nanorings through oriented attachment ..............................................................................40 2-25 Shape evolution of MnS nanocrystals controlled by the growth temperatures : wires at 120 C, (b) spheres at 180C, and (c) cubes at 250 C. ...............................................42 2-26 Variation of the shapes of CdS nanocrystals by changing growth temperature from (a) 300C-nanorods, (b) 180 C-bipods and tripods, and (c) 120 C-tetrapods..............43 2-27 Temperature-mediated crystalline phase control of (a) MnS and (b) CdS nanocrystals........................................................................................................................43 2-28 Anisotropic growth along [001] direction of CdSe nanocrystals. The surface growth rate was effected by selective capping by surfactants to produce (a) short rods (b) medium rods, and (c) long rods .........................................................................................45 2-29 Disc-shaped nanocrystals of (a) Co and (b) CuS produced by preventing growth along the [001] direction due to selective capping by surfactants. ....................................45 2-30 Shape control of PbS nanocrystals dependent on the growth regime ................................46

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11 2-31 Growth of 1D nanocrystals by the oriented attachment mechanism. (a) aligned TiO2 nanocrystals]; (b) alignment of PbSe nanocrystals [94]; (c) ZnS nanorods containing some fraction of spherial nanocrystals; (d) ZnS nanorods obtained after the aging; (e) summary of steps in oriented attachment mechanism for ZnS nanorods .....................48 2-32 (a-e ) Nanoscale size effects of iron oxide nanocrystals on magnetism and induced MR signals. (f) Schematic of DMSA-coated water soluble Fe3O4 nanocrystals with multifunctionalities. ...........................................................................................................51 2-33 Schematic representation of the use of QDs for bio-labelling. (a) Water soluble QD comprise a core and hydrophilic shell; (b) QD conjugate with biological molecule (drawn in red); (c) Bioconjugated QD that binds specifically to designated receptors .....52 2-34 Representative QD core materials scaled as a function of their emission wavelength over spectrum. ....................................................................................................................53 2-35 (a) Multicolor staining of HeLa cell with red and green QDs. [130] (b) In vivo labeling of a Xenopus embryo with green-micelle-coated QDs. [122] (c) Image of QDs targeting prostate cancer in vivo in a mouse bearing a xenograft tumor targeted using orange-red emitting QD probes. ...............................................................................53 3-1 Synthesis of Gd2O3: Eu3+ nanospheres by the nonhydrolytic hot solution route ..........................................................................................................58 3-2 HRTEM images of Gd2O3: Eu3+ nanocrystals (a-b) nanoplates (c) the images correspond to no tilt and (d) correspond to tilt. Label face as (001) and edges as (100, 010). ..........................................................................................................................60 3-3 HRTEM images of self-2O3: Eu3+ shown at three different magnifications. ...........................................................................................61 3-4 HRTEM images of nanospheres of Gd2O3:Eu3+ from thermal decomposition of Gd(acac)3 precursor in the presence of (a) hexadecanediol or (b) TOPO. .....................62 3-5 HRTEM images of different shaped Gd2O3: Eu3+ nanocrystals from (a) Gd(acac)3 (b) Gd-acetate, (c) Gd-chloride, or (d) Gd-octanoate precursors ......................................64 3-6 HRTEM images of Gd2O3: Eu3+ nanocrystals synthesized from thermal decomposition of Gd(acac)3 precursor (a,b) with (a) hexadecanediol (HDD-nanospheres) and (b) TOPO surfactant (nanospheres and nanoplates), and of Gd-octanoate precursor (c,d) with (c) TOPO (nanoplates) and (d) octadecene (complex larger shapes) surfactant. ...................................................................................................65 3-7 HRTEM images of Gd2O3: Eu3+ nanocrystals synthesized with a Gd(acac)3/TOPO molar ratio of (a) 1:1 (nanospheres) (b) 1:2 (mixed nanospheres and nanoplates), or a Gd(acac)3/HDA ratio of (c) 1:2.5 (d) 1:5. .....................................................................66

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12 3-8 HRTEM images of Gd2O3: Eu3+ nanocrystals synthesized with ratios of Gd(acac)3/HDD of (a) 1:1, (b) 1:3, or (c) 1:5 illustrating oriented attachment of nanospheres (b-red arrows) to form nanorods in (c). .........................................................67 3-9 HRTEM images of Gd2O3: Eu3+ nanocrystals synthesized with a heating rate of (a) 25C/min (mixed curved nanoplates and nanospheres) and (b) 5C/min. .........................68 3-10 XRD pattern of Gd2O3: Eu3+ (a) nanoplates and (b) nanospheres compared to the JCDPS patterns for cubic and monoclinic Gd2O3. .............................................................69 3-11 Optical transitions from Eu3+ in either a C2 or a S6 symmetry site in Gd2O3. ...................70 3-12 Two Gd3+ symmetry sites in Gd2O3. ..................................................................................71 3-13 PL excitation spectra of Gd2O3:Eu3+ nanocrystals for the emission line at 612 nm of nanoplates and nanospheres. ..............................................................................................72 3-14 Relative PL emission spectra of Gd2O3 : Eu3+ (10 mol %) of a) nanospheres, b) nanoplates c) mixed shapes ................................................................................................75 3-15 PL emission spectra of mixed shapes of Gd2O3 : Eu3+ (10 mol %) prepared with a) of Gd(acac)3 / TOPO (1:2), and b) Gd(acac)3 /HDD (1:5). ..............................................75 3-16 Relative PL emission spectra of Gd2O3 : Tb3+ (10 mol %) nanocrystals with the shape of a) spheres (synthesized with Gd(acac)3 and hexadecanediol ), and b) plates (synthesized with Gd acetate and TOPO) in the presence of oleic acid, oleylamine and benzyl ether. .............................................................................................76 3-17 Near infrared (NIR) PL emission spectra from Gd2O3:Er3+ nanospheres under 488 nm laser excitation. ..................................................................................................................77 3-18 Normalized PL emission intensity at 612 nm, Eu concentration and HRTEM photomicrographs of Gd2O3:Eu3+ nanocrystals ..................................................................79 3-19 TGA data from Gd acetate and Gd(acac)3 precursors between RT and 750oC. ................82 3-20 TGA data from nanoplates and nanospheres of Gd2O3:Eu3+ synthesized .........................83 3-21 FTIR spectra from nanoplates and nanospheres of Gd2O3: Eu3+ and surfactants ..............85 4-1 Schematic model of surface modification of Gd2O3:Eu3+ nanocrystals using octylamine modified PAA. ................................................................................................90 4-2 TEM images of Gd2O3:Eu3+ nanospheres capped with oleic acid and HDD before (a) and after (b-c) hydrophilic modification. ...........................................................................91 4-3 TEM images of Gd2O3:Eu3+ nanospheres capped with HAD before (a) and after (b) hydrophilic modification. ...................................................................................................92

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13 4-4 TEM images of Gd2O3:Eu3+ nanoplates before (a) and after (b) hydrophilic surface modification. ......................................................................................................................92 4-5 PL emission spectra of oleic acid and HDD capped hydrophobic (before) and PAA capped hydrophilic (after) Gd2O3:Eu3+ nanospheres. ........................................................93 4-6 PL emission spectra of had capped hydrophobic (before) and hydrophilic (after) Gd2O3:Eu3+ nanocrystals. ...................................................................................................94 4-7 Zeta potential versus aqueous solution pH of hydrophilic PAA surface modified Gd2O3:Eu3+ nanocrystals. ...................................................................................................95 5-1 HRTEM images of ZnGa2O4: Eu3+ nanospheres prepared with the precursors and surfactants corresponding to (a-d) in Table 5-1. ..............................................................100 5-2 HRTEM images of ZnGa2O4: Eu3+ nanocrystals prepared with Zn acetate dehydrate with the ratio of Zn:hexadecanediol of 1:2.5 under standard conditions. ........................100 5-3 HRTEM images of ZnGa2O4: Eu3+ nanocrystals synthesized with a ratio of Zn(acac)2 / hexadecanediol (a-b) 1:2.5, and (c-d) 1:5; (e) selected area electron diffraction (SAED) pattern from a cubic spinel crystalline phase. ....................................................101 5-4 The cubic spinel structure of ZnGa2O4 ...........................................................................102 5-5 XRD pattern of ZnGa2O4: Eu3+ nanocrystals. ..................................................................103 5-6 PLE spectrum for emission at 612 nm from ZnGa2O4: Eu3+ nanocrystals. The inset shows the UV absorption spectrum of undoped ZnGa2O4 nanocrystals. .........................104 5-8 TGA data from larger complexed shaped ZnGa2O4: Eu3+ nanocrystals ..........................106

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14 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 SYNTHESIS AND CHARACTERIZATION OF LUMINESCENT OXIDE NANOCRYSTALS By Sooyeon Seo August 2007 Chair: Paul H. Holloway Major: Materials Science and Engineering Oxide nanocrystals with controlled geometries exhibit unique shape dependent optical and structural properties. Shape-controlled synthesis of rare earth doped gadolinium oxide (Gd2O3: Eu3+, Tb3+ or Er3+) and zinc gallate (ZnGa2O4:Eu3+) nanocrystals by non-hydrolytic high temperature (300oC) methods are reported. Various shapes of Gd2O3 nanocrystals were synthesized, including spheres and plates and advanced shapes such as curved rods and triangles. The nanocrystal shape was shown to be a function of the synthesis parameters, such as metal precursors (acetate, acetyl acetonate, chloride or octanoate) and surfactant type ( tri-octyl phosphine oxide-TOPO, or hexadecanediol-HDD) and concentration (metal precursor: surfactant molar ratios of 1:2 to 1:5), as well as heating rate (5-25oC/min.) between pre-heat (200oC) and reaction (290oC) temperatures. The effects of these parameters upon nanocrystal shape were explained based on nucleation and growth of oxide nanocrystals. The photoluminescence intensity from Gd2O3:Eu3+ was shown to increase as the concentration of dopant incorporated into the nanocrystals increased. The doping efficiency, defined to be the percentage of dopant incorporated into the nanocrystals, ranged from 0.57-6.1 mol%, was a function of shape of the Gd2O3 : Eu3 and was discussed in terms of the rate of reaction, product yield and crystal structure.

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15 To be used for labeling biomolecules such as DNA, RNA, or proteins, water soluble luminescent nanocrystals are required. Doped Gd2O3 nanocrystals prepared by the non-hydrolytic hot solution method are hydrophobic and are not soluble in water due to organic surfactant encapsulation. A general strategy to convert hydrophobic luminescent nanocrystals (e.g. Gd2O3) to water soluble particles by over-coating the hydrophobic surface with amphiphilic polymers is reported. Specifically, octylamine modified surfaces were coated with poly (acrylic acid) and water dispersions of Gd2O3:Eu3+ were still stable at room temperature after four months. The non-hydrolytic hot solution synthesis technique was used to grow monodispersed ternary oxide nanospheres (5nm) of ZnGa2O4: Eu3+ from a variety of metal precursors. Using Gd acetate dehydrate, large (20nm) complex shaped (triangle and rectangle) ZnGa2O4: Eu3+ nanocrystals were obtained. Based on X-ray diffraction data, the nanocrystals had a cubic spinel structure with no impurity phases. The size of the ZnGa2O4: Eu3+ nanospheres could be varied by changing the molar ratio of Zn to surfactants, with higher concentrations of surfactant resulting in smaller nanocrystals. Analysis of the PL emission suggests that the Eu3+ ions were incorporated into the ZnGa2O4 host.

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16 CHAPTER 1 INTRODUCTION In the nanoscale regime, the chemical and physical properties of inorganic crystals are highly dependent on geometrical factors such as size and shape [1, 2]. Precise control of such factors allows one not only to observe unique properties of the nanocrystals but also to tune their chemical and physical properties as desired. During the past decades, researchers have explored efficient synthetic routes to produce well-defined inorganic nanocrystals with a controlled size and shape. Some advanced nanocrystal structures with interesting geometries including wires [3, 4], tubes [4, 5], ribbons [6, 7], and more complex shapes [7, 8] were produced with gas phase approaches. On the other hand, the colloidal approach in liquid media provides a convenient and reproducible route for the fabrication of nanocrystals with controlled size and shape. This enables the resulting nanocrystals not only to be precisely tuned at the sub-10 nm scale but also to be easily dispersed in organic or aqueous media for numerous potential applications in electronic and biological systems. Along with size control, anisotropic shape control of nanocrystals has been attained through liquid methods [9-17]. The nonhydrolytic molecular precursor decomposition method is an effective route for the controlled synthesis of both isotropic and anisotropic colloidal nanocrystals [11]. Nanocrystals obtained by such nonhydrolytic synthetic methods in general possess excellent crystallinity and monodispersity in terms of size and shape. Although there are increasing numbers of examples of colloidal semiconductor nanocrystals with anisotropic shapes, from simple one-dimensional rods and wires to advanced multipods and stars, reports of shape-guided rare earth doped oxide nanocrystals are very limited. To obtain luminescent oxide nanocrystals with the desired shape and to investigate the relationship of the controlled structures with the luminescent efficiency,

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17 systematic understanding of shape-guiding processes is necessary to provide the insight for efficient doping. In this dissertation, Chapter 2 reviews the fundamental physics, synthetic methods, shape controlled nanocrystals, proposed mechanisms and application in cell biology. In Chapter 3, the preparation of rare earth doped gadolinium oxide nanocrystals with a nonhydrolytic synthetic route resulting in various shapes is discussed. The effects of reaction variables on shape-guiding growth were also studied, as were their crystallographic and luminescent properties. In Chapter 4, a general strategy is introduced to make luminescent Gd2O3 nanocrystals water soluble by over-coating hydrophobic surfaces a with amphiphilic polymer. The conversion of hydrophobic nanocrystals into hydrophilic particles is prerequisite for biological applications. As a labeling material for biomolecules, especially for the sensitive determination of molecules such as DNA, RNA, or proteins, well-controlled nanocrystals are required in the nanoscale regime. In Chapter 5, ternary (ZnGa2O4: Eu3+) oxide nanocrystals have been grown in the shape of spheres and plates and advanced shapes such as curved rods and triangles. The size of the ZnGa2O4: Eu3+ nanocrystals can be controlled by varying the concentration of organic surfactants. Finally, conclusions and future work are summarized in Chapter 6.

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18 CHAPTER 2 LITERATURE REVIEW 2.1 Fundamentals of Colloidal Nanocrystals 2.1.1 Chemistry and Physics of Nanocrystals: Size and Shape Issues Nanomaterials are small-scaled with at least one dimension between 1nm and 100 nm, and include a range of shapes, such as nanospheres, nanocubes, nanorods, nanosheets and nanotubes [6, 18-21]. They exhibit novel optical, electronic, magnetic, chemical and mechanical properties that are different from their bulk counterparts from which they can be derived. Nanocrystals have attracted broad attention in a variety of fields including catalysis, photovoltaics, and coatings as well as in the emerging fields of nanomedicine where they can be used as imaging agents and drug-delivery vectors. Also, inorganic nanomaterials that will be the key component of futuristic nano-devices have recently emerged as a promising candidate to overcome many of the limitations of current technologies. Crystals consist of a periodic array of specific repeating atoms or molecules. The individual repeating molecules have quantized electronic structures while crystals have continuous electronic band structures that result from the overlap and combination of atomic and/or molecular orbitals of the repeating atoms or molecules. Therefore, isolated atoms and molecules exhibit quantum mechanical properties, while the chemical and physical properties of bulk crystals obey the laws of classical and quantum mechanics. However, when the crystal size decrease into the nano-scale regime (1~100 nm), the electronic band of the crystals is further quantized and the resulting nanocrystals behave as an intermediate between molecules and crystals [22-24]. In semiconductor quantum dots (QDs), the density of electronic energy levels as a function of the size varies systematically, which is known as quantum size effects [25-27]. The surface of

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19 nanocrystals plays a key role in their electronic and optical properties due to the high surface to volume ratio of semiconductor nanocrystals. Also, the melting temperature of nanocrystals decreases when the nanocrystal size is reduced [28]. These nanoscale phenomena are strongly related to the crucial parameters of size and shape. Early investigations focused on the nanoscale size effects since the physical properties of nanocrystals are influenced by the size of nanocrystals [25, 26]. It has been demonstrated both theoretically and experimentally that the quantized electronic band structure of a crystal is changed as the crystal size is reduced, resulting in an increase in the band-gap energy [23]. Figure 2-1 demonstrates the size-tunable fluorescence properties and spectral range of six CdSe quantum dot (QD) nanocrystals with different core sizes [29]. Figure 2-1. Size-tunable fluorescence and spectral range of CdSe QDs with different core size. All samples were excited at 365 nm with a UV source [13]. Similarly, the shape of nanocrystals plays a crucial role in the determination of their properties [30, 31]. The density of energy states (DOS) for inorganic crystals, which is predicted by a simple particle in a box type model, evolves from near-continuous energy levels into quantum states separated by large energies as the dimensionality is decreased from three to zero (Figure 2-2).

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20 Figure 2-2. Energy vs. density of electronic states (DOS) for inorganic crystals of 3, 2, 1, and 0dimensions [1] The band-gap energy (Eg) of nanocrystals is also influenced by their shape. The band-gap energy diagram of CdSe nanocrystals with various diameters and lengths, shown in Figure 2-3, clearly exhibits shape effects [30]. The UV-Vis absorption and photoluminescence spectra on CdSe rods with different aspect ratio have peak widths comparable to those of spherical quantum dots (Figure 2-4) [31]. Figure 2-3. Band gap of CdSe quantum rods versus length and width viewed from two different angles [30]. The mesh is the best fit to real data (red dots).

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21 Figure 2-4. TEM images (A-C) and UV-VIS absorption (solid lines) and photoluminescence (dotted lines) spectra (DF) of three nanorod CdSe samples. The nanorods have diameters of ~4.2 nm and lengths of 11( A and D), 20 (B and E) and 40 nm ( C and F ) [31]. Other properties, such as magnetic properties, of crystals are also strongly dependent on the shape of nanocrystals [32]. In the case of cobalt nanospheres that are ~ 4 nm in diameter, the blocking temperature of the nanospheres is 20 K. However, when nanocrystals with a nanodisc shape (height ~ 4 nm) are formed, a huge increase in the blocking temperature is observed due to enhanced anisotropy. Similarly, the magnetic coercivity of the discs has a higher value relative to that of a sphere (Figure 2-5). Enhanced shape anisotropy induces a preferential alignment of the magnetic spins along the long axis of the discs.

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22 Figure 2-5. Blocking temperature and magnetic coercivity of 4 nm cobalt nanospheres and nanodiscs [32]. 2.1.2 Synthetic Processes for Colloidal Nanocrystals Synthetic procedures --, -bottom-up approach, small building blocks are assembled into larger structures; this approach includes liquid phase colloidal synthesis in aqueous and nonhydrolytic media. The liquid-phase colloidal synthetic approach can provide access to extremely fine structure for the convenient and reproducible shape-controlled synthesis of nanocrystals, and allows the nanocrystals to be dispersed in either an aqueous [33-39] or a nonhydrolytic media [10, 11, 40-56]. Moreover, these chemical hybridization with other functional materials for applications in electronics and biological systems. Liquid phase synthesis relies on chemical reduction of metal salts, electrochemical pathways, or the controlled decomposition of metastable organometallic compounds by coprecipitation. Coprecipitaion process for colloidal nanocrystals growth tend to exhibit the following characteristics [21]:

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23 (i) The products of the precipitation reactions are sparingly soluble species formed under conditions of high supersaturation. (ii) Nucleation is a key step of the precipitation process and conditions are required such that a large number of nanoparticles are nucleated. (iii) Secondary processes, such as Ostwald ripening and aggregation, will strongly affect the size, morphology, and properties of the nanoproducts. (iv) The surfaces of nanocrystals may be stabilized by a large variety of molecules, For example, donor ligands, polymers, and surfactants are used to control any growth of the nanoparticles and to prevent them from agglomerating. A crystal-growth diagram is illustrated in Figure 2-6. When the monomer concentration reaches a supersaturation level, nucleation occurs and the monomer is continuously incorporated onto the seeds resulting in the growth of the nanoparticles and a gradual decrease of monomer concentration. During these nucleation and growth stages, control of growth parameters and crystalline phase is critical in determining the final size and shape of the nanocrystals [57]. Figure 2-6. Crystal-growth diagram. Csat = saturation monomer concentration above which nucleation takes place, and Cequil = equilibrium monomer concentration below which growth ceases [43]. In earlier methods of colloidal nanocrystals synthesis, the nanocrystals were usually grown at room temperature in hydrolytic (i.e. aqueous) media in the presence of structured micelles [34,

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24 37, 38, 58-61]. Several semiconductor [37, 38, 58, 59, 61] and metal oxide [34, 60, 61] nanocrystals were grown by this method starting from ionic precursors inside organic micelles. The disadvantage of these aqueous solution syntheses is that the pH value of the reaction mixture must be adjusted in both the synthesis and purification steps, and the nanoparticles often exhibit relatively poor crystallinity and/or polydispersity in their size and shape [36, 38, 62]. As a method to reduce these problems, nonaqueous high temperature thermal reaction methods with organic surfactants have been developed. Nanocrystals produced by this nonaqueous colloidal route often exhibit excellent crystallinity and monodispersity. Moreover, this route has several advantages for the shape-controlled synthesis of nanocrystals including separation between the nucleation and growth steps, and easier control of growth parameters by changing variables such as the type of surfactant molecules, monomer concentration, and temperature. The work of Steigerwald and co-worker on the use of organometallic precursors in the nonaqueous solution phase synthesis of nanocrystals provides guidance in nanocrystals synthesis [38]. Murray and co-workers [63] introduced dimethyl cadmium (Me2Cd) and trioctyl phosphine selenium (TOP-Se) as precursors to synthesize high quality CdSe quantum dots as shown in Figure 2-7. In the organometallic precursor method, the coordinating solvent (e.g. trioctylphosphine oxide-TOPO, TOP and trioctylamine (TOA) provide the crucial environment for the growth process, stabilizing the resulting colloidal dispersion. The coordinating ligands, i.e. surfactants, play a crucial role in mediating the growth of the particles. A variety of organometallic precursors and high boiling point coordinating solvents are potential candidates for these types of synthesis.

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25 Figure 2-7. Illustration of the organometallic precursor method for synthesis of CdSe quantum dots The traditional organometallic approach has some disadvantages, such as the fact that the starting materials are generally toxic, expensive, and sometimes unstable and explosive. Therefore, an inert atmosphere and temperature control during storage and reaction is often required for the chemical precursors. As an alternative route, the advantages of the traditional organometallic method may be retained by using less expensive precursors that do not require sophisticated equipment and procedures. A wide variety of precursors have been identified as possible candidates, including metal acetates [54] and metal acetylacetonates [40, 64-67]. This synthetic route provides: (i) Less expensive and simpler approaches, since this method is a one step synthesis. Nanocrystals, generally monodispersed and highly crystallized, can be achieved without further size selection. (ii) High yield, since in a typical reaction the product yield is higher than 50% for monodisperse nanocrystals. (iii) Diversity; this route has been used to grow transition metal (Fe, Mn, and Zn [68, 69] ) and rare earth (Gd [70, 71]) oxide nanoparticles. (iv) Environmentally friendly; precursors such as metal acetates and metal acetylacetonate -products of the thermal decomposition are mostly CO2 and H2O [69, 72]

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26 2.2 Shape Control of Colloidal Nanocrystals -scale devices. The design and operation of these devices will be more easily accomplished using nanocrystals with distinct sizes and shapes. The shape control of nanobuilding blocks is crucial for the success of future nano-devices. Inorganic nanocrystals can be classified according to dimensionality and crystal symmetry (Figure 2-8). Highly symmetric isotropic spheres, cubes, decahedrons, and tetrahedrons can be classified as zero-dimensional (0D) nanostructures and are the most familiar shapes in the nano world. Nanospheres of semiconductors, metal oxides, and metals have been synthesized through a variety of chemical methods. Rods, cylinders and wires are examples of one-dimensional (1D) nanoblocks. Since CdSe nanorods were first reported by Alivisatos and co-workers [11], many studies on the synthesis of 1D nanostructures have been reported [44, 45, 56, 68, 69, 73, 74]. 1D nanostructures exhibit novel optical and magnetic properties arising from shape anisotropy as illustrated in Figure 2-2 above. Disc and plates with polygon shapes belong to 2-dimensional (2D) nanostructures. In addition to such primitive shapes of inorganic nanocrystals, advanced shapes of nanocrystals have been developed. Multipod structures of semiconductors including bipods, tripods and tetrapods, and star-shaped nanocrystals are examples [12, 15, 75, 76].

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27 Figure 2-8. Geometrical shapes of inorganic nanocrystals [57]. 2.2.1 Zero-Dimensional (0D) Spheres and Polyhedrons of Nanocrystals Zero-dimensional (0D) shapes, including spheres and cubes, have been extensively studied. Initially, Brus and co-workers successfully synthesized various II-IV semiconductor nanospheres with high colloidal stability in coordinating solvent, but size tunability and monodispersity of nanocrystals were poor [25]. Bawendi and co-workers developed more advanced methodologies to prepare various sized 0D CdSe nanocrystals via the method of injecting a precursor solution of dimetyl cadmium into trioctylphosphine oxide (TOPO). The size of nanocrystals varied from 1.2 to 12 nm with high monodispersity and crystallinity and the nanocrystals obtained were highly soluble in various organic solvent. Optical spectra exhibit size dependent quantum confinement effects indicating high monodispersity and high crystallinity of nanocrystals, as shown above in Figure 2-1 [23]. Controlled growth of isotropic 0D spherical or cube nanocrystals of CoFe2O4 has been reported [40]. The data indicate that the heating rate and growth temperature control the shape of CoFe2O4. A slow heating rate produced a low concentration of metal cations from

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28 decomposition of the precursors, and resulted in growth of cubic CoFe2O4 nanocrystals having facets with low surface energy. A faster growth rate at a higher temperature with more metal cations available resulted in crystal growth being much less selective in directions and hence produced spherical CoFe2O4 nanocrystals ( Figure 2-9). The shape of the nanocrystals could be reversibly changed between spherical and cubic. As illustrated in Figure 2-5 above, the shape of such nanocrystals strongly affect the magnetic coercivity due to surface anisotropy, and therefore have tremendous potential for high-density information storage. Figure 2-9. High resolution transmission electron microscope (HRTEM) images of (a) an ~8 nm spherical CoFe2O4 nanocrystals and (b) multiple and (c) single ~12 nm CoFe2O4 cube nanocrystals [40]. Other 0D shapes of transition-metal oxide nanocrystals have been reported by Cheon and co-workers ( Figure 2-10) [45]. Structurally well-defined iron oxide nanocrystals with shapes consisting of diamonds, triangles, and spheres were obtained from the thermal decomposition of Fe(CO)5 in dodecylamine surfactant and subsequent air oxidation. Triangle, diamond, sphere and hexagon shapes are the 2D projections of 3D structures of tetrahedra, truncated octahedra, and icosahedra, respectively.

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29 Figure 2-10. Fe2O3 nanocrystals (b) diamond (c) sphere (d) triangle (e,f) hexagon nanocrystals These shapes are actually 2D projection of 3D shapes of inset images respectively [45]. 2.2.2 One Dimensional (1D) Rods and Wires of Nanocrystals One dimensional (1D) rod growth is a fundamental step for anisotropic shape control. Growth of nanocrystals with more complex structures are possible is based on an understanding of the mechanisms guiding growth of nanorods. Basically, 1D nanostructures of semiconductors and metal oxides exhibit unique optical [11, 12] and magnetic properties [77, 78] due to their anisotropic shape. 2.2.2.1 1D Semiconductors Non-hydrolytic synthesis are mainly utilized for high quality nanorod synthesis. Alivisatos and co-workers first reported CdSe nanorods from a thermal reacton of Me2Cd and TOP-Se in a hot surfactant mixture of TOPO and hexylphosphonic acid ( HPA) [11]. In this synthesis, 1D rod shaped structures result from preferential growth along the [001] direction of wurtzite CdSe that is promoted by selective adsorption of HPA molecules on specific faces. With increasing HPA concentration, the nanocrystal shape evolves from spheres to short rods to long rods with aspect ratios of 5-20.(Figure 2-11).

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30 Figure 2-11. CdSe nanorods with different sizes and aspect ratios in different concentrations of HPA /TOPO surfactants [11]. By varying monomer concentration in addition to change of surfactants, this approach enabled large scale production of CdX (where X= S, Se, Te) nanorods with better control of their aspect ratio. The synthetic method has been successfully applied for the fabrication of other semiconductors, including ZnO [48], ZnS [79, 80], ZnSe [75, 79], CdS [13, 80], CdTe [43] and PbSe [47]. Weller and co-workers have proposed a mechanism of growth of ZnO nanorods via oriented attachment of dimers and oligomers in a hydrolytic synthesis [48]. Specifically, zinc acetate produces zinc oxide nanospheres through hydrolysis and aging. The growth of individual nanorods occurs by oriented and partially fused dimers and oligomers in order to remove high energy surfaces. The oriented attachment of preformed quasi-spherical ZnO nanoparticles results in almost perfect rods ( Figure 2-12). Figure 2-12. TEM images of (a) starting ZnO nanospheres, and (b) after one day growth of ZnO nanorods by an oriented attachment process [48]

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31 Unlike II-IV semiconductor nanocrystals which have been well studied, study of III-V semiconductor nanocrystals has been limited. This is most likely due to their greater degree of covalent bonding and the lack of non-toxic precursors. Moreover, except for column III nitrides, III-V semiconductors favor isotropic zinc blende crystal structures and thus 0D nanocrystal growth is preferred rather than 1D rod growth. Cheon and co-workers have shown that the crystalline phase of gallium phosphide nanocrystals can be controlled by adopting suitable surfactants [81]. Zinc blende GaP is the thermodynamically stable low temperature phase, while wurtzite GaP is the high temperature stable phase that may be metastable at low temperatures and has desired electronic properties. When sterically limiting amine surfactants (e.g. trioctylamine-TOA) are used as capping molecules, formation of nanospheres GaP is favored. However, when sterically less bulky, linear alkyl amines (e.g. hexadecylamine-HDA) are used, the staggered conformation is not favored. Wurtzite GaP nanorods were formed when a mixed surfactants of TOA and HDA was used. The resulting nanocrystals showed unique shape-dependent spectroscopic features. The absorption spectra exhibited shoulders at 3.48 eV for spheres and 3.46 eV for rods. The photoluminescence peak was at 2.94 eV for 8 nm GaP nanospheres, but was red shifted to 2.79 eV for 8 45 nm rods. ( Figure 2-13). Figure 2-13. GaP nanocrystals. HRTEM images of (a) zinc blende nanospheres and (b) wurtzite nanorods, and absorption and photoluminescence colors from (c) nanospheres and (d) nanorods [81].

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32 2.2.2.2 1D Metal Oxide Nanocrystals Nanoscale transition metal oxides have attracted considerable interest due to their optical, magnetic, electrical and catalytic properties [82]. Penn and Banfield reported naturally aligned titania nanocrystals grown with a hydrothermal process with an oriented attachment mechanism. [83-85] In this procedure, titanum alkoxide precursors produced diamond shaped anatase titania nanocrystals. The nanocrystals were truncated with three different crystalline facets parallel to {001},{112}, and {101} crystal planes. Because the (001) face has the largest number and the (101) face has the lowest number of dangling bonds, the surface energy of the (001) face is higher than that of the (101) face. When a significant thermal energy was supplied to the system, oriented attachment occurred most commonly on {112}, occasionally on {001}, and rarely on {101} faces. The mechamism resulted in a lower total free energy by reducing the surface area where the crystallites were joined. This mechanism is distinct from Ostwald ripening, which involves the dissolution of fine particles and growth of larger particels. Clearly, both mechanisms can operate in such a titania system. Figure 2-14. TiO2 nanocrystals formed from single crystal via oriented attachment [85]. Recently, Seo and co-workers have synthesized various 1D nanostructures of transition metal oxide (e.g. W18O49, TiO2, Mn3O4 and V2O5) using a thermal crystal growth process from a mixture of metal chloride and surfactants [74, 86]. (Figure 2-15) These metal oxide nanorods have crystallographically aniotropic structures where surface energy is thought to be a crucial

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33 factor. Both tetragonal TiO2 and Mn3O4 nanocrystals have a {001} surface with higher energy, whereas monoclinic W18O49 nanocrystals possess a {010} surface with higher enery. Because the growth rate is exponentially proportional to the surface energy under the kinetic growth process-this depends on the model used to predict growth, the energy difference between the higher versus lower energy surfaces will promote preferential growth along the <001> directions of TiO2 and Mn3O4, and along the <010> direction of W18O49, i.e. rods aligned along the higher energy crystallographic direction. Figure 2-15. TEM images of nanorods of (a,b) tungsten oxide (c,d) manganese oxide and (e,f) titanium dioxide [74]. -workers have synthesized other 1D metal oxide nanocrystals using nonhydrolytic methods. Zinc oxide nanorods with a high degree of crystallinity and a narrow size distribution were assembled into close-ng axis parallel to each other ( Figure 2-16).

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34 Figure 2-16. TEM image of selfassembled 2 nm diameter ZnO nanorods. Inset: higher magnification image showing the oriented stacks of 1D ZnO. 2.2.3 Two Dimensional (2D) Discs and Prisms of Nanocrystals Synthesis of nanocrystals with controlled shapes has concentrated on the 1D nanostructures, while studies of 2D nanocrystals have been limited. In a kinetically driven growth regime, 1D nanorod growth is promoted when preferential growth along a specific direction is faster. Likewise, when growth along a specific axis is inhibited, the formation of 2D (e.g. disc shaped) nanocrystals may result. Alivisatos and co-workers reported the formation of disc shaped cobalt nanocrystals by rapid decomposition of cobalt carbonyl in the presence of linear amines [51]. The nanodiscs self-assembled into long ribbons by discs stacking face-to-face, perhaps assisted by the magnetic interaction between individual nanodics. ( Figure 2-17) Selective adsorption by alkylamines inhibit the growth along the [001] direction while allowing growth along the perpendicular directions resulted in the growth of nanodiscs. Figure 2-17. TEM image of Co nanodisks either self-assembled into ribbons (edge-on view) or lying flat (right) on the sample support surface [51].

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35 Ghezelbash and co-workers demonstrated a solventless thermolysis approach to synthesize Cu2S [87, 88] and NiS [89] nanodiscs. In the presence of alkanethiol surfactants, preferential growth along the [100] and [110] directions and inhibition of growth along [001] direction results in the formation of Cu2S nanodiscs (Figure 2-18 (a)). Similarly, rhombohedral NiS nanoprisms results from inhibition of growth along the [110] direction and fast growth along the perpendicular <111> directions (Figure 2-18 (b)). Figure 2-18. TEM images (a) Cu2S nanodics [87], (b) NiS nanoprims [89]. Besides control of surfactant, temperature and time, a photo-induced method for converting of silver nanospheres into triangular nanoprisms was reported [90]. Photons result in a colloid with distinctive optical properties that result in the nanoprism shape. Unlike spherical particles that are derived from Rayleigh light-scatter in the blue, these nanoprisms exhibit scattering in the red, which could be useful in developing multicolor diagnostic labels. (Figure 2-19) Figure 2-19. TEM images and schematic of morphology changes: (a) nanospheres before irradiation, (b) nanoprisms after 70 hours of irradiation, and (c) the color of light Rayleigh scattered by nanoprims and nanospheres

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36 Recently, nanoplates of lanthanide oxide nanocrystals have been reported [70, 71]. Square plate-shaped Gd2O3 were produced through thermal decomposition of Gd acetate precursor in the presence of a noncoordinating solvents (octadecene) and oleic acid in coordinating oleylamine [70]. These nanoplates were highly crystalline with a cubic structure, and the sides of square nanoplates were parallel to (100) and (010) faces while the top and bottom faces were parallel to the (001) plane. The nanoplates were only one unit cell of Gd2O3 thick along the c-axis, with the top and bottom (001) faces modified by the organic ligands (Figure 2-20 (a,b)). This important family of rare earth compounds was synthesized by thermolysis of their benzolacetonate complexes in a oleic acid/oleylamine solvent [52]. Due to selective adsorption of capping ligands on certain cubic faces during crystal growth, nanocrystals with different morphologies, such as nanoplates and nanodiscs, were created (Table 2-1 and Figure 2-21 (c-e)). Figure 2-20. Two dimensional (2D) lanthanide oxide nanocrystals: (a) Gd2O3 nanoplates, (b) model for the nanoplates assembly [70], (c) Eu2O3 nanodisks, (d) Er2O3nanodisks, and (e) Pr2O3 nanoplates [52].

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37 Table 2-1. Rare earth oxides synthesized by thermolysis of Ln(BA)3(H2O)2 or Ce(BA)4 where Ln=La toY and BA = benzolacetonate, in oleic acid (OA)/Oleylamine (OM) at 250-330 C for 20-60 min [52]. 2.2.4 Advanced Shapes of Nanocrystal If the assembly of shaped components (0D, 1D and 2D) can be controlled, the construction of advanced nanostructures can be achieved. [13-15, 76, 91] Alivisatos and co-workers achieved the formation of arrow, teardrop, tetrapod, and branched tetrapod shaped nanocrystals of CdSe using mixtures of hexadexylphosphonic acid (HPA) and trioctylphosphine oxide (TOPO) [14]. The three fundamental parameters that were varied to control the shape of CdSe nanocrystals were (i) the ratio of the surfactants (HPA/TOPO), (ii) the volume of the initial injection, and (iii) the time dependence of the monomer concentration. Tetrapods of MnS [13] and CdTe [15] were also reported This fundamental branched structures results from nucleation of the cubic zinc blende phase with subsequent aniotropic growth of the hexagonal wurtzite phase. In case of branched nanostructures, anisotropic colloidal heterostructures are fabricated by sequential growth of semiconductor dots and rods of different materials, with the potential for branched connectivity in each generation. Branching is introduced through crystal phase control, so the large class of semiconductors exhibiting zincblende-wurtzite polytypism [92] could be incorporated into branched heterostructures by these methods.

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38 Figure 2-21. Tetrapod shaped nanocrystals of (a) CdSe, (b) MnS, and (c) proposed model of CdTe The tetrapod shape potentially has very interesting optical properties, since in a rod or tetrapod nanocrystals, most of the confinement energy is along the diameter of the hexagonal arms [30]. Tetrapods having comparable arm lengths but different diameters in fact show remarkable differences in their bandgap energy, whereas spectra of tetrapods with comparable diameters but different arm lengths are almost identical, as shown in Figure 2-23 [15]. Figure 2-22. Influence of the shape of CdTe tetrapods on optical absoprtion spectra. (a) tetrapods having comparable arm diameters but different diameters; (b) tetrapods having comparable arm diameters but different lengths [15]. Cheon and co-workers [93] found that more complex shapes could be synthesized, including 1D rods, highly faceted stars, truncated octahedrons and cubes. As shown in Figure 2-

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39 24, a variety of rod-based nanocrystals including PbS tadpole shaped monorods, Ishaped bipods, L-shaped bipods, T-shaped tripods, cross shaped tetrapods, and pentapods were obtained. Figure 2-23. PbS nanocrystals with shapes corresponding to (a) rod-based multipods at 140 C, (b) tadpole-shaped monopod, (c) I-shaped bipod, (d) L-shaped bipod, (e) T-shaped tripod, (f) cross-shaped tetrapod, (g) pentapod, (h) star-shapes at 180 C, (j) truncated octahedrons at 250 C, and (k) conversion of cubes to star-shape to 1D rod-based multipods by control of the growth parameters [83]. Recently, nanowires synthesis using oriented attachment was used to produce PbSe nanowires with control of wire dimensions and morphology [94]. In addition to straight nanowires, zigzag, helical, branched, and tapered nanowires as well as single crystal nano rings (Figure 2-25) could all be prepared by adjustment of the reaction conditions. The inherent anisotropy of the crystal structure or crystal surface reactivity was identified as the driving force for the one dimensional growth. Dipolar interaction was concluded to be the most probable driving force causing PbSe nanocrystals to assemble into chains.

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40 Figure 2-24. PbSe nanocrystals showing (a) zigzag nanowires packing of octahedral building blocks,(b) star shaped nanocrystals, (c) radially branched nanowires, and (d) nanorings through oriented attachment [94]. 2.3 Proposed mechanism for shape-control growth of nanocrystals The state of mechanistic investigation of nanocrystals formation is primitive. Specifically, there are no prior kinetic and mechanistic studies of the formation pathway of compositionally and geometrically well-defined nanocrystals for generalization. Matijevic [35] noted that it is not clear why in some instances the final particles are spherical and in others they appear in different geometric forms, yet are of the same chemical composition. In the 1950s, Lamer and co-workers studied extensively the formation of colloids and clusters in homogeneous, initially supersaturated solutions. Their widely cited LaMer mechanism assumes that homogeneous nucleation occurs forming nuclei of the critical size. Further growth on the nucleus is spontaneous but diffusion-limited [95]. This mechanism predicts that as the precursor is consumed, its concentration fall below saturation, and hence no

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41 more nucleation takes place, thereby achieving the needed separation of nucleation and growth in time that is required for the formation of a monodispersed size distribution. Even though the LaMer mechanism has been successfully applied in attempts to tune the main variables (i.e. concentration of reactants), separation of nucleation and growth in reaction time is critical for synthesis of monodisperse nanocrystals regardless of whether of not the LaMer mechanism is correct in a given case. In a separate study, the most preferred classic model for shape control is the Wulff facets arguments, or Gibbs-Curie-Wulff theorem, which suggests that the shape of a crystal is determined by the surface energy of each face or facet of the crystal [96]. However, recent studies reveal that these classic thermodynamic arguments are not sufficient to understand the shape evolution of nanocrystals, and other factors are influencial. 2.3.1 Kinetically Induced Anisotropic Control Recent shape control research in nanocrystals illustrate that the kinetically induced anisotropic growth from molecular precursors is highly effective for producing advanced shapes of nanocrystals. Important factors for determining the shape of nanocrystals include reaction temperature and time, the surfactants used for capping, and precursor concentrations during nucleation and growth. 2.3.1.1 Cyrstalline Phase Control of Nuleating Seeds by Temperature Nucleating seeds of nanocrystals can potentailly have a variety of crystalline phase that affect the final shape of nanocrystals. The stable phase of nanocrystals is highly dependent on its environment, such as the temperautre and the choice of capping molecules. For example, by adjusting the initial temperature during the nucleation process, the crystalline phase of nanocrystals can be controlled. An isotropic unit cell structure of the seed generally induces the isotropic growth of nanocrystals from the seed, and therefore 0D nanostructures would be

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42 expected. In contrast, anisotropic unit cell structures of the seed can induce anisotropic growth along reactive crystallographically directions, and anisotropic shapes of nanocrystals would be expected. In case of MnS semiconductor nanocrystals [76], nuclei with the rock-salt phase are more stable at high temperature (> 200 C), whereas the wurtzite structure is preferred at temperature below 200 C [97]. At high temperature ( ~ 200C) the seeds of rock-salt MnS induced isotropic growth along eight {111} directions, and 30 nm sized nanocubes were obtained. In contrast, at low temperature (~ 120 C) the nucleation with the hexagonal wurtzite structure resulted in anisotropic growth along the c-axis of wurtzite and therefore very thin nanowires of 2 nm in diameter with an aspect ratio of ~ 80 were observed. Similarly, crystalline phase effects of the seeds can be observed in the case of CdS nanocrystal growth [13]. CdS has two distinct crystalline phases; an isotropic zinc blende phase is stable below 250C, and hexagoanl wurtzite is preferred at high temperature (~ 300C) At high temperature, formation of 1D CdS rods is observed from high temperature stable wurtzite structured seeds, similar to the MnS nanowire growth. However, at lower temperatures, the formation of tetrahedral shapes of zinc blende seeds truncated with four {111} faces is observed. The subsequent epitaxial growth of wurtzite pods along the c-axis from the four equivalent {111} faces of the seed results in the formation of CdS tetrapods. Figure 2-25. Shape evolution of MnS nanocrystals controlled by the growth temperatures : wires at 120 C, (b) spheres at 180C, and (c) cubes at 250 C [97].

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43 Figure 2-26. Variation of the shapes of CdS nanocrystals by changing growth temperature from (a) 300C-nanorods, (b) 180 C-bipods and tripods, and (c) 120 C-tetrapods. Figure 2-27 sumarizes the dependence of final shape on the crystalline phase of nuclei in MnS and CdS nanocrystals [9]. Figure 2-27. Temperature-mediated crystalline phase control of (a) MnS and (b) CdS nanocrystals [88]. 2.3.1.2 Surface energy modulation by capping surfactants In aaddition to controlling the crystalline phase of the nucleating seeds, the surface energy of the nanocrystals can be modulated by introducing surfactants that adsorb onto surfaces of growing crystallites [11, 12, 74, 89]. El-[98] [99] have performed pioneering work to understand the growth mechanisms for control of the shape of

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44 transition metal nanocrystals. They report that control of the shape of transition metal nanocrystals was due to stabilizing reagents bound to the surface of the nanocrystals. Peng [11] and Alivisatos [14] explored the growth of CdSe nanocrystals and reported monomer concentrations in the growth solution was the determining factors in shape control and shape evolution. The chemical potential of elongated nanocrystals should be slightly higher than that of a spherical nanocrystal. As a result, the growth of such anisotropic structures should require a relatively high chemical potential in the solution, i.e. a relatively high monomer concentration. This condition provides the external environment for the the formation of elongated [42] or other anisotropic shapes [13, 14]. At high monomer concentrations, the differences in the growth rate of different faces can lead to anisotropic shapes. The relative growth rate of the different faces can be controlled by the concentration, size and shape, and adsorption strength of capping surfactants, such as trioctylphosphine oxide (TOPO) and hexylphosphonic acid (HPA) [11]. On the other hand, for a low monomer concentration and low chemical potential, Ostwald ripening occurs and small nanocrystals dissolve at the expense of larger ones. Such slow growth conditions favor the formation of a spherical shape. HPA leads to an increased growth rate of the (001) face of CdSe relative to all other faces since HPA selectively binds to (100) and {110} surfaces and reduces the growth rate of these two surfaces. At low concentration of HPA or in the absence of HPA only spherical nanocrystals are formed. With higher HPA concentrations, nanorods are obtained with their axis along the [001] direction. More complicated shape control was subsequently observed with the formation of CdSe tetrapods [14].

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45 Figure 2-28. Anisotropic growth along [001] direction of CdSe nanocrystals. The surface growth rate was effected by selective capping by surfactants to produce (a) short rods (b) medium rods, and (c) long rods [14]. There are other examples of preferential ligand binding to the specific facets, as in the case of Co [49] and CuS [87] where amine preferentially binds to the {001} facets to induce [100] and [110] direction leading to disk-shaped growth. Figure 2-29. Disc-shaped nanocrystals of (a) Co and (b) CuS produced by preventing growth along the [001] direction due to selective capping by surfactants [35, 76]. 2.3.1.3 Growth Regime Control by Monomer Concentration and Temperature The monomer concentration and thermal energy (kT) strongly affects the final structure of the nanocrystals through a delicate balance between the kinetic and thermodynamic growth regimes. Isotropic growth of nanocrystals is preferred under the thermodynamic growth regime that is characterized by a sufficient supply of thermal energy (kT) and a low flux of monomers. In contrast, anisotropic growth along a specific direction is facilitated under a kinetic growth regime that is promoted by a high flux of monomers. After the intrinsic surface energy of the

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46 crystallographic face of the seed is determined, the surface properties can be tailored by the types and the amounts of adsorbed capping surfactants. The growth processes should be quenched at appropriate times, since long growth times can result in thermodynamically stable shapes of nanocrystals. The architectural features of the PbS nanocrystals are good examples to illustrate the effects of growth regime. PbS nucleates as tetradecahedron truncated with eight {111}faces and six{100} faces [93]. When excess thermal energy is supplied at a high growth temperature (~300C), the thermodynamic regime governs the growth process. In this growth regime, nearly isotropic growth from the seeds is favored, and therefore cube shapes of PbS are obtained. However, under the conditions of low temperature (~140C) and in the presence of surfactant (i.e. dodecanethiol) the growth process shifts into the kinetic growth regime and anisotropic growth on the high surface energy {100}faces is preferred and 1D rod-based multipod structures are obtained, as illustrated in Figure 2-31 [9]. Figure 2-30. Shape control of PbS nanocrystals dependent on the growth regime [9].

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47 For an intermediate growth temperature (~180 C) star-shaped nanocrystals, as a transient shape between isotropic and multi-pod shapes, are obtained. The enhanced growth rate on the {100} faces induces shrinking of the six {100} faces into sharp triangular corners which finally results in a star-shaped nanocrystals. The growth temperature affects the doping of nanocrystals as well as the anisotropic shapes. The formation of magnetic semiconductor (e.g. Cd1-xMnxSe) nanocrystals with a homogeneously distributed high level of Mn dopants has been difficult because of surface segregation of Mn dopants from the host matrix during high temperature thermodynamically driven synthesis [100]. However, low temperature kinetic growth allows not only the homogeneous doping of Mn atoms at high levels ( up to ~12% ), but also the 1D growth of monorod of Cd1-xMnx [93]. 2.3.2 Oriented Attachment As reported above in section 2.2.2, Penn and Banfield [83, 84, 101]. observed that anatase and iron oxide nanoparticles with sizes of a few nanometers coalesce under hydrothermal that the surface area is reduced by attachment which reduce the total energy of the nanocrystals. Attachment of nanocrystals were pointed out above for both hydrolytic growth of CdTe [16] nanowires and nonhydrolytic growth of ZnO [48], ZnS [56], and PbSe [94] nanorods. In this process, nanocrystals are first formed by at a high chemical potential due to a high monomer concentration. These nanocrystals form chain-like structure due to induced dipole-dipole interactions at the early stage of growth. In the final stage of 1D growth via the oriented attachment process, Ostwald ripening smooths the irregular surface to produce nanocrystals with a smooth surface. Synthesis by the oriented attachment mechanism can produce nanowires with

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48 controlled wire dimensions and morphology, such as nanorings or straight, zigzag, helical, branched, or tapered nanowires. Figure 2-31. Growth of 1D nanocrystals by the oriented attachment mechanism. (a) aligned TiO2 nanocrystals [85]; (b) alignment of PbSe nanocrystals [94]; (c) ZnS nanorods containing some fraction of spherial nanocrystals; (d) ZnS nanorods obtained after the aging; (e) summary of steps in oriented attachment mechanism for ZnS nanorods to form smooth surface nanorods [56]. 2.4 Application of Nanocrystals in Biomedicine Nanocrystals offer some attractive possibilities in biomedicine. First, they have controllable sizes ranging from a few nanometers up to tens of nanometers, which are smaller than or comparable to those of a cell ( 10-100 m), a virus (20-450 nm ), a protein (5-50 nm) or a gene (2 nm wide and 10-100 nm long). This means that they can get close to biological molecules of interest. Indeed, they can be coated with biological molecules to make them interact with or bind to a biological entity, thereby providing a controllable means of tagging. Second, inorganic nanocrystals exhibit unique properties and many new nanocrystal labels have been

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49 introduced for biomedical applications. Magnetic separation and fluorescent labeling are the two most widely used nanotechniques in bioscience. The present work attempts to prepare a new type of nano-sized hybrid particle that exhibits both a magnetic response and fluorescence for bio-detection. Colloidal quantum dots are robust and very stable light emitters that can be broadly tuned in emission wavelength through size variation. It was quickly realized that colloidal quantum dots (QDs) were about the size of a typical protein, and thus it was possible to introduce QDs into cells. 2.4.1 Biocompatible Magnetic Nanocrystals for MR Contrast Effects Nanocrystal biomedical applications based on magnetic properties include utilization as a magnetic probe for detection and imaging and as a magnetic vector for cell separation and drug delivery. In particular, they are employed for magnetic separation, in vivo magnetic drug targeting, and magnetic resonance imaging (MRI) [102-107] Modern MRI is one of the most powerful medical diagnostic tools due to its non-invasive nature and multi-dimensional tomographic capabilities coupled with high spatial resolution. MRI relies on the counterbalance between the exceedingly small magnetic moment on a proton, and the exceedingly large number of protons present in biological tissue, which leads to a measurable effects in the presence of large magnetic fields. Under an applied magnetic field, induced magnetic spins in magnetic nanocrystals perturb the nuclear spin relaxation processes of protons in surrounding water molecules. This effect leads to shorter spin-spin relaxation time (T2) of the proton, which results in contrast for MR images. When magnetic nanocrystals are conjugated with biologically active materials, the resulting nanocrystal-biomolecule conjugates have the multi-functionalities of both MR contrast effect and selective attachment to target molecules.

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50 Santra and Holloway [108] reported highly water-dispersible, multifunctional, CdS:Mn/ZnS core-shell Qdots using water-in-oil(W/O) microemulsion method. These Qdots are fluorescent, radio-opaque, paramagnetic, and suitably stable in an in vivo environment [109]. Paramagnetic metal ions (Cr3+, Mn2+, Fe3+ and Gd3+ ) show suitable effects which depend on the number of unparied electrons in the ion. Among these ions, the prominent feature of Gd3+ is the high number (seven) of unpaired electrons.. The Gd3+ ion retains a number of unpaired spins when bound to the organic ligand. The free Gd3+ ion is extremely toxic, but a large fraction of the complexes are very stable and thus exhibit much less toxicity. The Gd-DTPA complex has [110-112]. Although many researchers are investigating stability of the series of complexes versus acute toxicity in vivo, the search for new ligands for complexation is still a hot area for investigation. Criteria include thermodynamic stability, rates of excretion, toxicity and biodistribution. Iron oxide nanocrystals are the most commonly used superparamagnetic contrast agents. Various sizes of iron oxide nanocrystals with a narrow size distribution were produced using a high temperature injection methods [113]. The nanocrystal surface was treated with 2,3-dimercaptosuccinic acid (DMSA) which makes them stabile in a water dispersion. The water-soluble iron oxide nanocrystals exhibit excellent size-dependent magnetism and MR contrast effects. Increasing the size of nanocrystals from 4 to 6, 9, and 12 nm, the mass magnetization at 1.5 T increased from 25 to 43, 80, and 102 emu/(g Fe), respectively, and higher MR contrast effects can be seen in Figure 2-32.

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51 Figure 2-32. (a-e ) Nanoscale size effects of iron oxide nanocrystals on magnetism and induced MR signals. (f) Schematic of DMSA-coated water soluble Fe3O4 nanocrystals with multifunctionalities [113]. 2.4.2 Luminescent Nanocrystals for Fluorescence labels Another important application of nanocrystals in cell biology is their use as fluorescence markers to label structures and molecules in cells. Fluorescence labelling is used to visualize structural units that, due to lack of contrast or resolution, cannot be distinguished [114-116]. This is achieved by attaching a ligand to the nanoparticle label and this conjugate binds with high specificity to its target receptor, which can be visualized by the fluorescence of the label (see Figure 2-33). The receptor molecule typically is an antibody for the structure to be labelled. A popular receptor pair is avidin and streptavidin, and the structure to be labelled is incubated with the biotinylated streptavidin antibody which is then recognized by the fluorescence-avidin construct Alivisatos [117] and Nie [118] reported the use of colloidal QDs for biological labeling and suggested that the photochemical stability and the versatility could make them extremely useful. Recently, a wide range of applications for QDs have been seen as cell labeling [119], cell

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52 tracking [120], cell signaling [121], in-vivo imaging [122] near infra-red imaging [123] and DNA detection [124]. Figure 2-33. Schematic representation of the use of QDs for bio-labelling. (a) Water soluble QD comprise a core and hydrophilic shell; (b) QD conjugate with biological molecule (drawn in red); (c) Bioconjugated QD that binds specifically to designated receptors Luminescent colloidal QDs offer many advantages compared to organic dyes as fluorescence labels for biological staining and already been used in labelling experiments. QD properties of interest to biologists include high quantum yield and exceptional resistance to photochemical degradation and photobleaching. Upon optical excitation, organic fluorophores can undergo irreversible light-induced reactions such as photo-oxidation. Reacted molecules are ectra of QDs are typically narrow, symmetric and do not exhibit tail to longer wavelength (i.e. red tail), therefore many different colors from size-tune fluorescent emission can be distinguished without spectral overlap (see Figure 2-34). For biological flurescence labelling, more colors mean that a larger number of structures can be simultaneously labelled, each with a different color (see Figure 2.35) [125-127]. The detection of multiple molecules (markers) in a cell or tissue by QDs color emission can improve diagnostic efficiency. The decay time of the fluorescence of nanocrystals is typically longer (ns to s) than the decay time of autofluorescence (ps to ns), therefore time-gated imaging can be used to reduce the autofluorescence background in fluorescence imaging of cells [128]. Besides spherical nanocrystals, assymmetric nanorods can

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53 also be synthesized as discussed above. Due to their anisotropic shape their emitted fluorescence light can be polarized which enables detection of the orientation of labelled structures [31]. Figure 2-34. Representative QD core materials scaled as a function of their emission wavelength over spectrum. Representative areas of biological interest are presented correspoding to the pertinent emission highlighting [129] Figure 2-35. (a) Multicolor staining of HeLa cell with red and green QDs. [130] (b) In vivo labeling of a Xenopus embryo with green-micelle-coated QDs. [122] (c) Image of QDs targeting prostate cancer in vivo in a mouse bearing a xenograft tumor targeted using orange-red emitting QD probes [131]. Recently, QDs fluorescencing in the infrared have been demonstrated as a contrast label for optical detection. QDs with infrared fluorescence are of particular interest if their emission wavelength is chosen in a spectral window where absorption by the biological molecules (e.g. water and hemoglobin) is low [132]. Kim and co-workers have demonstrated the use of QDs for

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54 sentinel-lymph-node mapping in pigs, which helps guide surgeons in the removal of tumor cells [123]. Compared to the currently used visible fluorescence contrast agent, infrared fluorescence can be imaged through the skin, which allows the surgeon to identify the position of the QDs in the lymph channels in real time and reduce the size of the incision and allow determination of the complete removal of the lymph node. While the above discussion reveals some of the positive attributes for QDs in bio-applications, there are still many questions about the toxicity of inorganic QDs containing Cd, Se, Zn, Te, Hg and Pb. These elements can be potent toxins, depending on dosage, that can accumulate in and damage the tissue. Living cells have been demonstrated to ingest QDs, allowing their potential use as contrast agents in animals and humans that may remain in the living tissue for months and presumably even for years [133]. Studies on non-toxic QDs such as oxide compounds would be desirable. This is one of the objectives of this dissertation.

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55 CHAPTER 3 SYNTHESIS AND CHRACTERIZATION OF LUMINESCENT GADOLINIUM OXIDE NANOCRYSTALS 3.1 Introduction Rare earth doped compounds have attracted extensive attention as luminescent components in many applications. In addition, shape control of colloidal nanocrystals is possible and result in many advanced shapes of nanocrystals. For example, rare earth doped one dimensional (1D) structures such as LaPO4: RE ( RE= Eu3+, Tb3+) nanowires [134], Y2O3:RE nanotubes [135] and Gd2O3: Eu3+ nanoplates [71] have been reported. However, the luminescent properties of nanocrystals with different shapes were not compared. Eu3+ doped Gd2O3 nanocrystals have shown red luminescent peaks due to the electric-dipole 5D07F2 transitions on the trivalent europium ion (Eu3+, 4f6) [136]. Gd2O3: Eu3+ nanocrystals obtained by nonhydrolytic thermal reactions in the presence of organic surfactants exhibit excellent crystallinity, monodispersivity and good luminescent efficiency. Moreover, this synthetic route allows control of the growth and shape of nanocrystals by variables such as the type of metal precursors, organic surfactants molecules, monomer concentration ratio, and heating rate. In this chapter, the luminescent properties of Gd2O3: Eu3+ are reported and discussed versus the crystal structure, shape of nanocrystals and the quantities of Eu3+ dopant incorporated into the Gd2O3. 3.2 Experimental Section 3.2.1 Materials The following precursor compounds and solvents were purchased from Aldrich: Gd (III) acetate hydrate Gd(III) acetylacetonate hydrate, Eu(III) acetate hydrate oleic acid (90%

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56 tech.), Tb(III) acetate hydrate (90% tech.) Er(III) acetate hydrate (90% tech.), oleic acid (90% tech.),oleyamine (70%) benzyl ether ( 99%) 1,2-hexadecanediol ( HDD, 97%), trioctylphosphine oxide (TOPO, 99%) and octadecene (90%). All chemicals were used without further purification. Absolute ethanol, benzyl ether and hexane were used as received. 3.2.2 Synthesis of platelet Gd2O3: Eu3+ nanocrystals The general scheme to grow Gd2O3 nanocrystals is shown in Figure 3.1. In a one pot reaction to produce Gd2O3: Eu3+, Gd-acetate (2mmol), Eu-acetate (0.2 mmol), oleic acid (6mmol), oleylamine (3mmol), benzyl ether (10mmol) and TOPO (4 mmol) were mixed and magnetically stirred under a flow of nitrogen. In a typical reaction to produce Gd2O3: Eu3+, 2mmol of Gd-acetate, 0.2 mmol Eu-acetate were mixed with 6mmol oleic acid, 6mmol oleylamine, 4mmol TOPO and 10mmol benzyl ether in a three-neck reaction flask under nitrogen. In both cases, the mixture was heated at 200C for 30 min, resulting in a transparent brownish solution that was then heated to 290C with a heating rate of 5-25C /min and maintained for 2-3h under the N2 blanket. In a second procedure using two pots, a mixture of 2mmol of Gd-acetate, 0.2mmol Eu-acetate were mixed with 6mmol oleic acid and heated to 200C for 30 min in a vial resulting in a homogeneous brownish solution. At the same time, 10ml of benzyl ether and 4mmol of TOPO were mixed in a three-neck flask and heated to 200C for 30 min. After 30 min, the Gd-oleate solution was rapidly injected into the benzyl ether/TOPO mixture and the temperature was raised to ~ 290C with a heating rate of 5-25C /min, and kept at that temperature under a N2 blanket for 150min. After reflux, the brownish transparent mixtures were cooled to room temperature by removing the heat source in the case of either the one pot or two pot processes. Under ambient

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57 conditions, ethanol was added to the mixture, and Gd2O3 nanocrystals were precipitated and separated via four centrifugations (9000 rpm, 10 min) to remove any residue. The purified Gd2O3: Eu3+nanocrystals, capped with organic species, were well dispersed by organic solvents such as hexane, chloroform and toluene. The properties of the resulting nanocrystals from the one versus two pot reaction were identical with respect to size, shape and product yield. The data reported below are all from the one pot approach. 3.2.3 Synthesis of spherical Gd2O3: Eu3+ nanocrystals To produce spheres of Gd2O3: Eu3+, Gd(acac)3 (1mmol), Eu-acetate (0.2 mmol), oleic acid (3mmol), oleylamine (3mmol), benzyl ether (5mmol) and either hexadecanediol (HDD, 2.5mmol) or TOPO (2 mmol) were mixed and magnetically stirred under a flow of nitrogen. After preheating at 200C for 30 min, the mixture was heated to ~ 290C and kept at that temperature for 150min. Cooling and purification of the nanospheres were performed in the same way as described above for synthesis of platelet Gd2O3: Eu3+ nanocrystals. The effects of other precursors, such as Gd-chloride and Gd-octanoate, and growth solvents, such as octadecene, on the shape of the rare earth doped Gd2O3 nanocrystals were investigated as reported below. Most batches of Gd2O3: Eu3+ were processed using two-coordinating solvents (e.g. TOPO/TOA, TOPO/benzyl ether, or HDD/benzyl ether) with oleic acid for better homogeneous mixtures of the starting materials. For other dopant elements (Er3+ and Tb3+), the same protocols were used. For controlling the shape of Gd2O3 nanocrystals, several critical parameters were found, including the type of Gd precursor, organic surfactants, the ratio of Gd-precursor/dominant organic surfactant, and the heating rate as reported below.

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58 Figure 3-1. Synthesis of Gd2O3: Eu3+ nanospheres by the nonhydrolytic hot solution route. 3.2.4 Characterization of Rare Earth Doped Gd2O3 Nanocrystals Nanocrystals for examination by transmission electron microscopy (TEM) were dispersed in hexane and drop-cast onto a copper grid with a carbon film containing holes. The solvent was evaporated in ambient air. For Fourier-transform infrared (FTIR) measurements, the powder was ground together with KBr in a mortar and pestle, then pressed into pellets. For ICP analyses of concentrations, Gd2O3: Eu3+ nanocrystals were dissolved in nitric acid. For thermogravimetric analysis (TGA), Gd2O3: Eu3+ nanocrystals were heated after ambient drying to remove the solvent. Photoluminescence (PL) and UV-absorption were measured from Gd2O3:RE nanocrystals dispersed in hexane contained in a quartz cuvette. For near infrared (NIR) emission, Gd2O3 nanocrystals were dried and measured as green powder pellets. X-ray diffraction (XRD) patterns to determine the crystal structure were obtained using Philips APD 3720 X-ray diffractometer with Cu K. radiation source ( = 0.5418 nm). A JEOL 2010F transmission electron microscope operated at 200 kV was used for collection of images and determination of the size and shape of the nanocrystals. Absoprtion spectra were collected

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59 with a Shimadzu UV-2401PC spectrophotometer. Photoluminescence (PL) was measured at room temperature from nanocrystals suspended in hexane using a Flurolog Tau 3 spectrofluorometer (Jobin Yvon Spex Instruments, S.A. Inc) with a 450 W xenon excitation lamp. Life time was measured with FP-6500 spectrofluorometer ( JASCO). Near-infrared (NIR) emission spectra of Gd2O3: Er3+ nanocrystals were collected using a 488 nm Ar laser line for excitation and a Ge detector. Thermo-gravimetric analysis (TGA, Seiko TG/ATD 320U, SSC 5200) was used to investigate the temperatures for decomposition of Gd precursors and to determine the amount of surfactant bound on the particle surface. For TGA, the samples were heated up to 800C in air at a rate of 10C min-1. FTIR spectra from the Gd2O3 nanocrystals were recorded with a Thermo Electron Magna 760 FT IR m icroscope in order to confirm the nature of the coating and its bonding to the surface. The concent ration of Eu 3+ dopants in Gd 2 O 3 nanocrystals as prepared was determined by inductively coupled plasma ( ICP, Perkin Elmer Plasma 3200) 3.3 Results and Discussion 3.3.1 Nanoplates and Nanospheres of Gd2O3: Eu3+ As described above, nanoplates of Gd2O3: Eu3+ were synthesized using Gd-acetate as the precursor and using thermal decomposition under nitrogen flow at 290 C after preheating at 200C. Gd-acetate is completely soluble in the oleic acid TOPO and benzyl ether mixture at 200C. The temperature leads to decomposition of Gd-acetate to a Gd2O3-organic surfactant complex with reaction products of CO2 and acetone [72]. High resolution TEM (HRTEM) images in Figure 3.2 (a-b) show that square nanoplates of Gd2O3: Eu3+ were formed with 10nm of edge length and 1nm thickness. Over most of the area, the nanoplates were aligned with one edge parallel to the surface and their

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60 suface self-aligned parallel to their neighbor platelets. To verify that the images in Figure 3.2(a) and (b) were from aligned plates rather than e.g. rods, tilted TEM images were taken in Figure 3.2(c) and (d), and the change in contrast is consistent with plates rather than rods of the same length. The image shows that the standing plates varied in thickness when tilted in the short direction and remained of constant edge length when tilted in the long one. The size distributions of the platelet edge length is more broad (5-10 nm), while the thickness is monodispersed at 1nm. Figure 3-2. HRTEM images of Gd2O3: Eu3+ nanocrystals (a-b) nanoplates (c) the images correspond to no tilt and (d) correspond to tilt. Labelled face is (001) and edges are (100, 010). As shown in Figures 3-2 and 3-3, the Gd2O3: Eu3+ nanoplates self-assemble into stacks, in which they are aligned with their square planes parallel to each other. The thickness of and spacing between nanoplates was very uniform at 1 nm and 1.3 nm. This spacing of 1.3nm is consistent with small angle x-ray diffraction data which reduce to a interplate spacing of 1.2

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61 nm. Faceto-face platelet stacking has also been reported for Cu2S [137] and Co[51] nanodisks. The existence of a nematic phase due to the entropy of oriented packing was first suggested by Onsager [138], and has been confirmed by computer simulations of platelets and experimental data for submicrometer hard disk colloids [139]. The van der Waals attraction is also greater for nanoplates oriented face-to-face relative to nanoplates oriented edge-to-edge due to the much greater interfacial contact area provided by the face-to-face configuration [140]. As suggested by Cao, et al [70], face-to-face stacking for Gd2O3 might also be assisted by electric dipole interactions since the faces of the nanoplates are the {001}and the edges are {100} and {010} crystal faces. Figure 3-3. HRTEM images of selfGd2O3: Eu3+ shown at three different magnifications. Using the same temperatures and times for reactions but using a precursor of Gd (acac)3 (1mmol) and hexadecanediol (2.5mmol) as a surfactant, nearly monodisperse 24 nm diameter nanospheres of Gd2O3: Eu3+ were grown. Although the mechanism leading to Gd2O3 nanocrystals is not clear, decomposition of the acetylacetonate ligand in the Gd(acac)3 precursor is the only possible oxygen source. Nanospheres of several metal oxides [40, 54, 64-66] have been prepared by thermal decomposition of metal acetylacetonates under an inert atmosphere, and several authors have proposed that decomposition of the acac ligand leads to formation of

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62 carbon dioxide that oxidizes the Gd. When TOPO (1mmol) was used as solvent instead of hexadecanediol (2.5mmol), the same size of Gd2O3: Eu3+ nanospheres were obtained. Figure 3-4. HRTEM images of nanospheres of Gd2O3:Eu3+ from thermal decomposition of Gd(acac)3 (1mmol) precursor in the presence of (a) hexadecanediol (2.5mmol) or (b) TOPO (1mmol). Regardless of the shape of nanocrystals (platelet versus sphere), heating the mixture to 200C for 30 min before heating to reflux at 290C is important for monodispersity and high product yield (~ 80 %). Directly heating the mixture to 290C from room temperature resulted in Gd2O3: Eu3+ nanocrystals with a wide size distribution and relatively low (~ 60 %) product yield. These observations suggest that the nucleation of Gd2O3: Eu3+ at the higher temperature is a slower, continuous process, and subsequent growth leads to a large size distribution. For many colloidal nanocrystals, particle size can be increased by longer growth times [54] and larger metal precursor/surfactants ratios [67], often described by the Liftshitz-Slyozov-Wagner (LSW) model which predicts the time dependence of the particle size. Particle growth is driven by the dependence of the solubility of a solid phase on the particle size according to the Gibbs-Thomson relation, assuming that the particles are spherical [141-143]. However, the particle size of Gd2O3: Eu3+ did not increase at longer growth times. The independence of the nanoparticle size from growth time is attributed to the monodispersivity the nanocrystals. At long

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63 times, growth of the nanoparticles would be expected from Ostwald ripening [144]. In addition, changes in the metal precursor/surfactants ratio changed the shape of particle more than the size. 3.3.2 Shape control of Gd2O3: Eu3+ nanocrystals 3.3.2.1 Effects of Gd-precursor Thermal decomposition of various Gd precursors in oleic acid, oleylamine, benzyl ether and TOPO leads to the formation of different morphologies of nanocrystals. Figure 3-5 shows HRTEM images of Gd2O3: Eu3+ nanocrystals grown from different Gd precursors at 290C. As shown above, Gd2O3: Eu3+ nanocrystals obtained from Gd(acac)3 are 24 nm nanosphere (Figure 3-5(a)), in contrast to nanoplates of Gd2O3: Eu3+grown from Gd-acetate precursor (Figure 3-2, 3-3, and 3-5(b)). Gd-chloride precursors led to growth of 2-4 nm nanospheres (Figure 3-5 (c)), but the product yield was lower (~ 35 %) than from Gd(acac)3. Interestingly, Gd-octanoate precursors produced Gd2O3: Eu3+ nanocrystals with both sphere and plate shape (Figure 3-5 (d)). The two different shaped nanocrystals separated and the platelet Gd2O3: Eu3+ self-assembled into similar to those shown in Figures 3-2 and 3-3. In studies of the shape of nanocrystals, the effects of metal precursors have rarely been reported. One reason is the difficulty in finding different precursors compatable with the appropriate organic surfactants. The combined effects of both the precursor and organic surfactants determine the crystal nucleation, growth and shape. While it is not possible to suggest a priori specific precursor favorable for aniotropic growth, the reactivity of the precursor is expected to affect the size and shape of nanocrystals. In case of a highly reactive precusor, a relatively large number of nuclei could be expected due to fast nucleation and depletion of the precursor to reduce further growth. On the other hand, a less reactive precursor a should form a relatively small number of nuclei in the same period of time, and these nuclei will not reduce the concentration of the precursor monomers as much, allowing growth of elongated shape in some

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64 cases [11, 42, 145]. In the present study, a Gd-acetate precursor resulted in platelet Gd2O3: Eu3+ nanocrystals, while Gd(acac)3 resuted in the formation of spherical Gd2O3: Eu3+ nanocrystals in the presence of organic surfactant such as TOPO or hexadecanediol. The thermal decomposition of these two precursors investigated with TGA are reported below and will be related to their relative reactivity during growth. Figure 3-5. HRTEM images of different shaped Gd2O3: Eu3+ nanocrystals from (a) Gd(acac)3 (b) Gd-acetate, (c) Gd-chloride, or (d) Gd-octanoate precursors 3.3.2.2 Effects of Surfactants Organic surfactants play an important role in the shape of the nanocrsytals. The reaction occurred in a mixture of coordinating and noncoordinating organic surfactants. When Gd(acac)3 was used as the precursor, the use of hexadecanediol ( HDD) resulted in spherical Gd2O3: Eu3+ nanocrystals at a Gd(acac)3/HDD ratio of 1: 2.5, as shown in Figure 3-6(a). In all syntheses, oleic acid, oleylamine and benzyl ether were co-introduced with the main surfactant (i.e. HDD, TOPO or octadecene). When HDD was replaced with TOPO, both of nanospheres and nanoplates were simultaneously formed with a Gd(acac)3/TOPO ratio of 1:1 (Figure 3-6(b)). The nanoplates were stacked without a preferred direction, but non-stacked individual nanoplates

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65 lying on their faces were rarely observed. Using Gd octanoate precursor and TOPO produced mixed shapes of spheres and plates of Gd2O3: Eu3+ nanocrystals (Figure 3-6(c)). Finally, varied complex shapes of Gd2O3: Eu3+ nanocrystals (10 nm spheres, plates, rings and triangles) were formed from a Gd octanoate precursor when TOPO was replaced with octadecene as the main organic surfactant (Figure 3-6(d)) and reacted for short times (1 h) at a growth temperature of 290C after a 200C nucleation. Figure 3-6. HRTEM images of Gd2O3: Eu3+ nanocrystals synthesized from thermal decomposition of Gd(acac)3 precursor (a,b) with (a) hexadecanediol (HDD-nanospheres) and (b) TOPO surfactant (nanospheres and nanoplates), and of Gd-octanoate precursor (c,d) with (c) TOPO (nanoplates) and (d) octadecene (complex larger shapes) surfactant. 3.3.2.3 Effects of the Precursor/Surfactant Ratio The concentration ratio of the Gd precursor and the main organic surfactant (i.e. Gd(acac)3 /TOPO or HDD) was critical to the size and shape of the nanocrystals formed. For a Gd(acac)3/TOPO molar ratio of 1:1, only nanospheres (<5nm) were observed, as shown in Figure 3-7(a). For a ratio of 1:2, a mixture of nanospheres and nanoplates was observed (Figure 3-7(b)). Nanospheres of Gd2O3: Eu3+ were produced with a Gd(acac)3/HDD molar ratio of 1:2.5

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66 (Figure 3-7(c)), but curved worm-like nanocrystals appear along with the dominant formation of nanospheres at a ratio of 1:5 (Figure 3-7(d)). Figure 3-7. HRTEM images of Gd2O3: Eu3+ nanocrystals synthesized with a Gd(acac)3/TOPO molar ratio of (a) 1:1 (nanospheres) (b) 1:2 (mixed nanospheres and nanoplates), or a Gd(acac)3/HDA ratio of (c) 1:2.5 (d) 1:5. Data in Figures 3-6 and 3-7 show that the type and concentration of surfactants are important factors for controlling the shape of nanocrystals. Relatively high surfactant concentrations result in high chemical potentials which are favorable for formation of anisotropic nanocrystals, regardless of the type of surfactant. In hetero-structured Gd2O3 nanocrystals (Figure 3-7(b) and (d)), small (<5 nm) nanospheres are surrounded by anisotropic nanocrystals. It seems logical to postulate that the small nanospheres play an important role in the growth of the larger anisotropic nanocrystals, such as plates or curved rods. The co-existence of these small nanospheres and nanorods is consistent with the in Chapter 2 [83]. As the molar ratio of Gd(acac)3/HDD was changed from 1:1 to 1:3 to 1:5 (Figure 3-8), quasi-spherical nanocrystals (figure 3-8(a)) apparently attached to one another (red arrows in Figure 3-8(b)) to form long nanorods (>10nm) as shown in Figure 3-8(c). This shape

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67 change from nanospheres to nanorods was the result of an increased concentration of HDD, and was not due to an aging process. Upon changing the concentration ratio of Gd(acac)3/TOPO from 1:1 to 1:2, the shape of the Gd2O3: Eu3+ changed from nanospheres to mixed spheres and plates, but did not change further at higher concentrations of the strongly bound TOPO surfactant. The nanoplates were probably formed due to the high chemical potential at the higher concentration of TOPO, not from the evolution of nanospheres. Figure 3-8. HRTEM images of Gd2O3: Eu3+ nanocrystals synthesized with ratios of Gd(acac)3/HDD of (a) 1:1, (b) 1:3, or (c) 1:5 illustrating oriented attachment of nanospheres (b-red arrows) to form nanorods in (c). 3.3.2.4 Effects Rate of Heating from 200oC to 290oC Different heating rates were investigated using a ratio of Gd(acac)3/HDD of 1:5 with other reactants (oleic acid, oleylamine and benzyl ether ) after preheating 200C. If the heating rate is 25C/min from 200C to the reflux temperature of 290C, a mixture of nanospheres and a few curved nanorods were obtained, as shown in Figure 3-9 (a). Slower heating rate of 5C/min led to a lower density of curved nanocrystals, with the vast majority being nanospheres as shown in Figure 3-9 (b). The result suggest that a rapid growth rate due to larger heating rates result in a high chemical potential which favors the formation of anisotropic nanocrystals.

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68 Figure 3-9. HRTEM images of Gd2O3: Eu3+ nanocrystals synthesized with a heating rate of (a) 25C/min (mixed curved nanoplates and nanospheres) and (b) 5C/min (dominated by nanospheres). 3.3.3 Crystal Structures of Gd2O3: Eu3+ Nanocrystals It is known that cubic Gd2O3 transforms to monoclinic at 1300-1400 C [146], with the monoclinic phase being favored at temperatures over 1400C. These different crystallographies affect the magnetic and optical properties of Gd2O3 [147]. The crystallographic structure of Gd2O3:Eu3+nanocrystals were examined using Cu K radiation, and a XRD spectrum is shown in Figure 3-10. The XRD peaks were broadened due to the small size of nanocrystals comparing to bulk Gd2O3: Eu3+, especially for the nanospheres with a smaller size than the nanoplates. Although the main peak (2 32.47) of monoclinic Gd2O3 (JCPDS file 43-1015) was seen in both samples, most of the monoclinic peaks overlapped with those from cubic Gd2O3 (JCPDS file 43-1014), making it difficult to determine if only a single phase was present. However, a relatively sharp monoclinic (002) peak was seen at 2 32.47 from nanoplates, consistent with these planes being the face along which the stacks self aligned. It is also possible that nanoplates contain a larger fraction of the monoclinic (versus cubic) phase compared to Gd2O3 nanospheres. Based on this assumption, it could be expected that the emission efficiecncy of Gd2O3: Eu3+ nanoplates were less than that of nanospheres because monoclinic crystals show a considerably

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69 lower luminance than cubic crystals.[148]. Interestingly, mixed shaped of Gd2O3: Eu3+ nanocrystals with larger sizes over 10 nm (Figure 3-6 (d)) have very sharp peak at 2 28.5 assigned to (222) plane of the cubic phase (not shown here). Figure 3-10. XRD pattern of Gd2O3: Eu3+ (a) nanoplates and (b) nanospheres compared to the JCDPS patterns for cubic and monoclinic Gd2O3. 3.3.4 Luminescence Properties of Gd2O3 : Eu3+, Tb3+ and Er3+ Nanocrystals 3.3.4.1 Eu3+ Fluorescence in Oxides Photoluminescence (PL) from Eu3+-doped Gd2O3 nanocrystals showed a red color attributed to transitions from the excited 5D0 level to the crystal-field-split 7Fj manifolds of the 4f6 electronic states. The 5D07Fj emission of Eu3+ is very sensitive to the crystal field around the Eu3+sites. Whereas the 5D07F1,3 of Eu3+ is an allowed magnetic-dipole transition, the 5D07F2,4 of Eu3+ is a forbidden electric-dipole transition (parity selection rule) [136]. However, this selection rule can be relaxed for Eu3+ in a host lattice lacking inversion symmetry, such as Gd2O3[148], Y2O3 [149] and LaPO4 [134].

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70 Figure 3-11. Optical transitions from Eu3+ in either a C2 or a S6 symmetry site in Gd2O3 [150]. 3.3.4.2 Luminescence vs. Gd2O3 Crystallography The two different crystal structures of Gd2O3 nanocrystals strongly affect the luminescent properties, with the monoclinic system showing a considerably lower luminance than the cubic system [151]. Cubic Gd2O3 is a good host materials for trivalent activators (i.e. Eu3+, Tb3+ and Er3+), which are believed to substitute on Gd3+ sites. In cubic Gd2O3: Eu3+, two possible symmetry sites exist for the Eu3+ ions, namely S6 and C2 point group symmetry. The crystal structure of Gd2O3 is of the rare-earth sequioxide C-type in which each Gd3+ ion is surrounded by six oxygen located at the corners of a cube [152]. Two of the corners are vacant and this can occur along either a body or a face diagonal of the cube, which results in two Gd3+ site symmetries, namely S6 or C2, respectively. Figure 3-12 depicts the two types of crystallographic sites possible in Gd2O3. The unit cell consists of two types of alternating Gd3+ layers, one composed of only C2 sites and the other composed of an equal number of S6 and C2 sites; the ratio of C2 to S6 sites is 3 to 1 [153].

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71 Figure 3-12 Two Gd3+ symmetry sites in Gd2O3. In fact, electric dipole transitions for Eu3+ ion at S6 site is forbidden because of the strict inversion symmetry. Therefore, the 5D07F2,4 transitions originates mostly from the C2 site, [136]. Monoclinic Gd2O3 provides three different crystallographic sites Cs for the Eu3+ion [154]. These three sites give rise to a majority of the 5D0 and 7F2 stark levels, which produce numerous peaks in the range between 600 and 630 nm, even through our measurement was not sufficient to resolve them. Figure 3-13 shows the PL excitation (PLE) spectra for emission at 612 nm from Gd2O3: Eu3+ nanoplates versus nanospheres at room temperature. For both shapes a broad excitation band extends from 250 to 300 nm, and is thought to result from both host and charge-transfer-band (CTB) excitation. An electronic excitation transition near 260nm is associated with the 2p orbital of O2to the 4f orbital of Eu3+, whose stength is related to the covalency between O2and Eu3+ and the coordination environment around Eu3+ [155]. Another broad band at 280 nm is related to Gd3+ transitions due to Gd3+ Eu3+ energy transfer. The weak, sharp lines at 320 and

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72 360-390 nm, shown in the inset, are associated with direct excitation of the f-f shell transitions of Eu3+ as labeled [150]. While the PLE spectral wavelengths and intensities were not altered, the shape of the broad CTB were different between nanoplates and nanospheres. When the CTB was decomposed into two Lorentzian component at 260 and 280 nm (not shown here), their ratio was different. This is attributed to a variation of the coordination environment around Eu3+ ions in these two different samples. In nanocrystals, the coordination environment around the Eu3+ on the surface would be different from that around interior Eu3+ since effects such as adsorption, dangling bonds and defects exist at the surface [156]. In nanoplates versus nanospheres, the ratio of surface to volume varies which leads to an increase in CT transitions contributed by surface Eu3+ ions and a decrease of those contributed by internal Eu3+ ions. Figure 3-13. PL excitation spectra of Gd2O3:Eu3+ nanocrystals for the emission line at 612 nm of nanoplates and nanospheres. The inset shows the PLE spectrum from 315-400 m at a higher sensitivity so that the labeled absorption transitions of Eu3+ can be seen. The emission spectra from Gd2O3: Eu3+ (10 mol%) nanospheres, nanoplates and mixed nanocrystals (spheres, platelets, triangles and rings) under 270 nm excitation is shown in Figure

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73 3-14 (a), (b) and (c), respectively. Note that the concentration of Eu3+ that leads to quenching in the present colloidal nanocrystals is higher (>6 mol%) than that in bulk materials (3-5 mol%) [148]. This is attributed to reduced energy transfer rates from luminescence centers to quenching centers due to the interface related size issue, and the fact that the liquid phase synthetic route appears to sweep dopant ions into the solvent during growth. The red emssion of Eu3+ is dominated by the 5D07F2 transition at 612 nm with minor peaks from 5D07Fj ( J = 0,1,3) transitions characteristic of the Gd2O3 host lattice (see Figure 3-14). Peaks for J =0 are observed at 578 nm, for J = 1 at 588 and 597 nm and for J = 2 at 612 and 620 nm. Emission peaks near 650 nm (5D07F3 ) and near 700 nm ( 5D07F4 ) are also shown. The f-f transitions of Eu3+ are essentially free-ion-like in character and their relative intensities are sensitive to the crystal environment, therefore the luminescence can be used as the detector of the local crystal structure. As previous described, several split levels give emission between 600 and 630 nm in monoclinic Gd2O3, while emssion from 5D07F2 at 612 nm is dominant from cubic Gd2O3 The low energy peak at 620 nm is stronger than the 612 nm peak from monoclinic Gd2O3: Eu3+, and the site symmetry results in a less intense red emission [151, 157, 158]. Both the nanospheres and nanoplates of Gd2O3: Eu3+ have a very strong red color and the emission peak at 612 nm is stronger than that at 620 nm. However the ratio of the peak intensitiess, I612/I620, is larger for nanosphers versus nanoplates, as shown in Figure 3-14(a) and (b), suggesting that both cubic and monoclinic Gd2O3 were present. Focusing on the shapes of nanocrystals, the emission of 5D07F2 at 612 nm was stronger in nanospheres than nanoplates. This could result from several effects. First, the relatively high 612 nm peak suggests that Gd2O3: Eu3+ nanospheres had a larger fraction of cubic versus monoclinic

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74 phase. The relatively intense peak at 612 nm would originate from the C2 site in the cubic structure, cosistent with the XRD data discussed above. Because of their geometry, nanoplates have a higher surface to volume ratio than do nanospheres. As a result, a larger fraction of Eu3+ ions are expected to be near the surface with modified site symmetry, and lower emission intensity might result. Note that mixed shaped nanocrystals with larger size than those of nanoplates and nanospheres exhibited relatively poor luminescence (Figure 3-14) even though XRD data showed a crystalline cubic structure. These mixed shaped nanocrystals were produced by a very fast reaction of Gd-octanoate and octadecene (<1 h) while nanoplates and nanospheres were produced by slower reactions (~3 h). In a fast growth process, it is reasonable to postulate that dopant ions have less time to locate the appropriate sites in the host lattice, and instead remain in the liquid surfactants. The low intensity from cubic mixed shape nanocrystals could therefore be attributed to a low doping efficiency, consistent with ICP analysis of the Eu3+concentration reported below. The emission spectra of mixtures of Gd2O3: Eu3+ nanocrystals (rods + spheres and plates + spheres) are shown in Figure 3-15. As previously shown in Figure 3-7 (b, d), these mixed structures were prepared by varying the main organic surfactant. It can be seen that the emission of mixed nanorods and nanospheres (Figure 3-15(a) grown from Gd(acac)3 and TOPO) was less intense than from the mixture of nanoplates and nanospheres (Figure 3-15(b) grown from Gd(acac)3 and HDD). ). It is speculated that the reaction of Gd(acac)3 and hexadecanediol allowed better incorporation of Eu3+ dopants than that of Gd(acac)3 and TOPO, as discussed below.

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75 Figure 3-14. Relative PL emission spectra of Gd2O3: Eu3+ (10 mol %) of a) nanospheres (synthesized with Gd(acac)3 and hexadecanediol), b) nanoplates (synthesized with Gd acetate and TOPO) in the presence of oleic acid, oleylamine and benzyl ether, and c) mixed shapes (synthesized with Gd octanoate ,oleic acid and octadecene). Figure 3-15. PL emission spectra of mixed shapes of Gd2O3 : Eu3+ (10 mol %) prepared with a) of Gd(acac)3 / TOPO (1:2), and b) Gd(acac)3 /HDD (1:5).

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76 3.3.4.3 Nanocrystals of Gd2O3: Tb3+ The luminescence spectra of Gd2O3:Tb3+ nanoplates and nanospheres were investigated using 270 nm excitation. Since the luminescence of terbium originates from a level (5D4) strongly split by the crystal field, the emission pattern is much more complicated than for Eu3+, and each peak in the spectrum represents a number of unresolved lines. The main intense luminescence line is at 542 nm. Like Eu3+ doped Gd2O3 nanocrystals (Figure 3-14), the transition energies were the same for nanoplates and nanospheres, but the luminescent intensity of nanospheres was higher than that of nanoplates (Figure 3-16). The origin(s) of this effect is thought to be similar to that for Eu, as discussed above. Figure 3-16. Relative PL emission spectra of Gd2O3 : Tb3+ (10 mol %) nanocrystals with the shape of a) spheres (synthesized with Gd(acac)3 and hexadecanediol ), and b) plates (synthesized with Gd acetate and TOPO) in the presence of oleic acid, oleylamine and benzyl ether.

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77 3.3.4.4 Near Infrared Emission From Gd2O3: Er3+ Near infrared (NIR) emission from Er3+ ions at 1.5 m are well known and NIR is promising for biolabeling [117, 159] since excitation and emission have longer penetration depths in tissue [132]. Dopant Er3+ ions were incorporated into the Gd2O3 nanocrystalline host and the properties tested. Growth of Gd2O3: Er3+ nanocrystals with various shapes similar to those of Gd2O3: Eu3+ was observed. NIR emission was detected from Er3+ doped Gd2O3 nanospheres produced using the same protocol (section 3.2.3) as for Gd2O3:Eu3+ as shown in Figure 3-17 (excited by 488 nm from an Ar laser). The characteristic 4I13/2-4I15/2 and many sublevel peaks from Er3+ over the range 1450-1650 nm were observed. Figure 3-17. Near infrared (NIR) PL emission spectra from Gd2O3:Er3+ nanospheres under 488 nm laser excitation. 3.3.5 Eu3+ Incorporation into Gd2O3 Nanocrystals In Figure 3-14, the relative PL intensities were shown versus the shape of the Gd2O3 nanocrystals. Determination of the concentration of Eu in as-prepared samples by inductively coupled plasma (ICP) is very important since the concentration of Eu3+ ions is directly related to the luminescent properties of Gd2O3:Eu3+ nanocrystals. The morphology, relative PL emission

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78 intensity at 612 nm and Eu concentration (mol %) in samples with the same solvent volume prepared from different Gd precursors are reported in Table 3-1. Nanospheres, nanoplates and mixed samples were prepared by the same route as in Figure 3-14 under standard conditions. Gd chloride was reacted in the presence of HDD, oleic acid, oleylamine and benzyl ether under standard condition (see Table 3-1 footnote) and resulted in nanospheres of Gd2O3:Eu3+. All samples had the same concentration of Eu3+ in all starting solutions (10 mol%). The results show that Eu3+ concentration varies from 0.57 to 6.1 mol % in as-prepared nanocrystals. While the volume of solvent with dispersed nanocrystals was constant, the amount of nanocrystal varied from one condition to the next. The concentration of Eu incorporated into the nanocrystals decreased from Gd(acac)3 (a-nanospheres) to Gd acetate (b-nanoplates) to Gd octanoate (c-mixed shapes), suggesting a strong correlation between the particle shape and Eu concentration. Although both Gd chloride (d) and Gd(acac)3 (a) produced nanospheres, the relative PL intensities were much lower from the chloride versus the nanospheres from Gd(acac)3 ( Table 3-1). The reaction producing nanospheres from Gd chloride was relatively fast (30min), the yield of product and Eu incorporation was low. As depicted, mixed shapes(c) of Gd2O3:Eu3+ nanocrystals were obtained within 1 h, which is faster reaction than ones (3 h) of nanospheres (a) and nanoplates (b). Table 3-1. Nanocrystal shape, normalized PL intensity and Eu concentration in Gd2O3:Eu3+ nanocrystals versus the Gd prerecursor. Gd precursor morphology Relative PL intensity Probed at em =612 nm Eu 3+ concentration ( mol % in final product) a Gd(acac) 3 nanospheres 1 6.1 b Gd acetate nanoplates 0.32 1. 9 c Gd octanoate mixed nanocrystals 0.21 1.2 d Gd chloride nanospheres 0.0036 0.57 Standard condition: After the solution is preheated at 200C for 30 min, it is heated at the rate of 25C/min up to ~ 290C under a N2 blanket.

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79 Erwin and Norris reported the efficiency of Mn doping in II-IV and IV-VI nanocrystals [160], and conclude that doping efficiency is determined by three main factors: surface morphology, nanocrystal shape and surfactants in the growth solution. Particularly, (001) facets of zinc blende nanocrystals play a special role in doping process because energies of these surfaces are strikingly larger (vs. wurtzite or rock-salt). These surfaces consist of anion dimers which provide very stable binding sites that are absent from other facets. They suggest that strong surfactants (such as phosphonic acids) can also bind Mn and compete with surface adsorption of dopants. The normalized PL intensity at 612 nm and the Eu concentration from the same volume of solvent-dispersed nanocrystals from samples a-d (Table 3-1) are shown in Figure 3-18. There is a strong correlation between the normalized PL intensity and normalized Eu concentration multiplied by the product yield. Figure 3-18. Normalized PL emission intensity at 612 nm, Eu concentration and HRTEM photomicrographs of Gd2O3:Eu3+ nanocrystals grown from Gd precursor a-d (see Table 3-1).

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80 In rare earth doped oxide nanocrystal systems, several factors will control the doping efficiency, defined to be the concentration of RE incorporated into the nanocrystals. The results above indicate that the luminescent efficiency of Eu3+ doped Gd2O3 nanocrystals is not only dependent on the shape of nanocrystals, but also on the reaction rate and product yield, i.e. doping efficiency is based on the kinetics of the reaction. In slow reactions, the dopants have enough time to be incorporated. The incorporated concentration of dopants in as prepared samples rises monotonically with increasing product yield. Additionally, the crystal structure of Gd2O3:Eu3+ nanocrystals can affect the doping efficiency, as it is known that the cubic phase of Gd2O3 is a better host than the monoclinic phase. But this should be considered as minor factor since mixed nanocrystals (c) have lower luminance than samples a and b, even though they are predominantly cubic phase. The results suggest that doping efficiency is affected by three factors in Gd2O3 nanocrystals : rate of reaction, product yield and crystal structure of nanocrystals as summarized in Table 3-2. Table 3-2. Three main factors for doping efficiency of Eu in Gd2O3 nanocrystals (a-d) Bold represents the dominant crystal structure in sample b. 3.3.6 Thermo-gravimetric analysis (TGA) Figure 3-19 shows TGA data from thermal decomposition in air of Gd-acetate and Gd(acac)3 precursors. The decomposition of Gd-acetate (Gd(CH3COO)3) (red line) takes place in three steps. A dehydration step observed at 100C results in a loss of 3H2O molecules per

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81 monomer The dehydration weight loss of 3H2O was estimated to be 16.4 % in good agreement with measured loss of 17.5 %. Continued decomposition of dehydrated Gd(CH3COO)3 proceeds in two steps, with the first step in the temperature range (300-400C) corresponding to formation of Gd2O2CO3 and volatile products. The second step over the temperature range of 580-650C occurred by the formation of Gd2O3 and CO2 [161]. The TGA data from Gd(acac)3 precursor (blue line in Figure 3-19) shows dehydration at 100oC, followed by decomposition above 200C that proceeds continuously until 550 oC. The reaction temperatures for the current solution synthesis scheme for nanocrystals are lower (200-290oC) than the temperatures for the majority of the decomposition, therefore the reactivity of the metal precursors with the other reactants and the organic surfactants must affect the nucleation of metal oxide phase. The TGA data in Figure 3-19 show that the decomposition of Gd(acac)3 proceeds gradually, while the decomposition of Gd acetate occur abruptly at 350C. The reaction using Gd acetate forms nuclei in a short period of time which implies that the system would be under kinetic control, allowing an anisotropic final product (nanoplates). The decomposition of Gd(acac)3 over an extended temperature range above 200oC results in the formation of a relatively large number of nuclei and the thermodynamically controlled reaction favors the formation of isotropic nanospheres.

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82 Figure 3-19. TGA data from Gd acetate and Gd(acac)3 precursors between RT and 750oC. The amount of surfactant coating the surface is a function of the nanoparticle size and shape. The TGA data from different shapes of the Gd2O3:Eu3+ nanocrystals coated with organic surfactants are shown in Figure 3-20. The observed weight loss versus temperature was dramatic above 250C. The small weight loss for temperatures <250C is attributed to desorption of physisorbed molecules from the surface. The larger weight loss at >250C is attributed to desorption of organic surfactant molecules from the surface, which is complete at 500C. Note that the relative weight loss for T < 250oC was greater (6 versus 4%) and desorption of surfactant started at a lower temperature (50 versus 100 oC) for nanospheres versus nanoplates, respectively. Data in Figure 3-20 and Table 3-2 show a total weight loss upon heating to >600oC of 35 % and 42 % for nanoplates (5-7 nm edge and 1nm thickness) and nanospheres (2-3 nm diameters), respectively. These data suggest that the relative amount of surfactant decreased as the size of the Gd2O3:Eu3+ nanocrystals increased. The data also suggest that the surfactant is more strongly bound in stacked nanoplates than for dispersed nanospheres.

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83 Figure 3-20. TGA data from nanoplates and nanospheres of Gd2O3:Eu3+ synthesized under standard conditions listed in Table 3-1. Table 3-3. Normalized weight loss based on TGA data in Fig 3-20. If we assume a close-packed monolayer of the surfactant on the surface of a Gd2O3 nanosphere of diameter d, the total weight of the nanoparticle plus the monolayer is (1/6)d3 + ( d2/a)(M/N0) where d = the diameter of the particle = the density of the particle, a = the attachment (head) area per molecule of the surfactant, Mw = the molecular weight of the surfactant, and N0 = Avogadro number. Assuming that the TGA heating causes weight loss of only the surface-bound surfactant, the percentage weight loss from a particle of diameter d can be calculated using the relation [162]: Weight loss in % = 100 [( d2/a)(Mw/N0)] /A (1)

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84 A = (1/6)d3 + ( d2/a)(M/N0) In case of Gd2O3:Eu3+ nanospheres, using d = 25 (average diameter), = 7.407 g cm-3, a = 20 2 (for oleic acid) and Mw = 282.46 gm mol-1 for oleic acid, the calculated percentage weight loss using equation (1) is 43 %, in good agreement with the 42 % that was obtained experimentally. This suggests a monolayer oleic acid coated the nanosphere surface. 3.3.7 FTIR analysis The capping ligands chemisorbed by the surfaces of the Gd2O3:Eu3+ nanocrystals can be identified from the FTIR spectra presented in Figure 3-21. Transmittance data were collected from the solid sample such as nanoplates and nanospheres of Gd2O3:Eu3+, TOPO, and hexadecanediol (HDD), and absorbance were obtained from liquid oleic acid. As reported above, nanoplates of Gd2O3: Eu3+ were prepared by Gd(acac)3 precursor with TOPO and oleic acid, while nanospheres were prepared by Gd(acac)3 with hexadecanediol and oleic acid. The two absorption bands at 2937 and 2857 cm-1 in all five samples were attributed to the asymmetric CH2 stretch and the symmetric CH2 stretch, respectively. The C=O stretch band of the carboxyl group at 1715 cm-1 was detected only from oleic acid, and was absent in the data from the nanoplates and nanospheres. This suggests that oleic acid molecules are covalently bonded to the nanocrystals surface and there are no free oleic acid molecules [163]. Two bands at 1430 and 1535 cm-1 from nanoplates and nanospheres probably result from the symmetric and asymmetric stretching vibrations of carboxylic groups bonded symmetrically or bonded at an angle to the surface [164]. Absorptions at 1050 cm-1 probably arise from C-O single bond stretching. These data are consistent with ligands being chemisorbed onto the Gd2O3:Eu3+ nanocrystals as a carboxylate.

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85 Nakamoto categorized the interaction between a carboxylate head and a metal atom as monodentate, bridging (bidentate), chelating (bidentate), or ionic [165]. The wave number shift, asymmetric as(COO-), and symmetric s(COO-), IR bands can be used to -320 cm-1) corresponds to the cm-1 ) to the chelating bidentate, and a medium -190 cm-1 ) to the b-1430 = 105 cm-1 ) suggests the chelating bidentate was formed where the interaction between the COOgroup and the Gd atom was covalent. From pure TOPO, the (P=O) stretching absorption appears as a sharp peak at 1145 cm-1, and this peak is detected from nanoplates but is absent from nanospheres of Gd2O3:Eu3+. The band at 560 cm-1 is characteristic of Gd2O3 [166]. Thus IR adsorption bands show the Gd2O3: Eu3+ nanocrystals are coated primarily with oleic acid. Figure 3-21. FTIR spectra from nanoplates and nanospheres of Gd2O3: Eu3+ and surfactants (hexadecanediol, TOPO and oleic acid). The oleic acid is liquid, while the other samples are solid.

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86 3.4 Conclusions Well dispersed, crystalline rare earth (Eu3+, Tb3+ and Er3+, 10 mol %) doped Gd2O3 nanocrystals were synthesized at 290 oC (reaction time, ~ 3 h) by a hot solution nonhydrolytic route. The Gd2O3: Eu3+ nanocrystals were changed from nanospheres (<5nm) to nanoplates (1nm x <10nm2) to mixed shaped nanocrystals (spheres, plates, rods, and triangles) by changes of Gd precursors, organic surfactants, concentrations and heating rate (between 200 and 290oC reaction temperatures). The mechanisms leading to isotropic versus anisotropic growth of Gd2O3: Eu3+ nanocrystals were discussed. The PL emission and excitation properties of nanospheres, nanoplates and mixed shape nanocrystals were reported. The intensity of the Eu3+ 5D07F2 emission at 612nm from nanospheres was more intense than that from nanoplates or mixed shape nanocrystals due to a higher concentration (6.12 %) of Eu in nanospheres versus nanoplates(1.85 %) or mixed shape nanocrystals (1.24 %). Three main factors for doping efficiency are suggested in Gd2O3 nanocrystals : rate of reaction, product yield and crystal structure of nanocrystals which dominantly affect the doping efficiency in order. It is note that Gd(acac)3 is slowly reacted (~ 3 h) with oleic acid and HDD surfactants and induce, well doped, efficiently luminescent Gd2O3: Eu3+ nanospheres with high product yield (~ 80 %). In this way, this experimental advances in rare earth doped oxide nanocrystals in controlling shape and crystal structure of Gd2O3: Eu3+ nanocrystals can also be important in optimizing the luminescent efficiency from rare earth doping.

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87 CHAPTER 4 WATER SOLUBLE SURFACE MODIFICATION OF LUMINESCENT GADOLINIUM OXIDE NANOCRYSTALS FOR BIOMEDICAL RESEARCH 4.1 Introduction Thermal decomposition of metal precursors in stabilizing organic surfactants has been proven to produce monodispersed luminescent nanocrystals. This synthetic procedure also allows control over the shape and size of nanocrystals (see Chapter 3) with the desired optical properties, e.g. high photoluminescence yield, narrow emission peaks and stability against photobleaching [37, 38, 59, 61, 71]. Nanocrystals synthesized by this method are hydrophobic because of the coordinating agent, and are therefore insoluble in aqueous solutions making them incompatible with biological systems. To disperse them in aqueous solutions, a polar surface must be created to render them water soluble. Several methods have been developed to modify the surface of nanocrystals for water solubility [117, 118, 167-170]. In addition to making the nanocrystals hydrophilic, the treatment should also a) prevent nanocrystals from flocculating during long-term storage, b) maintain or -10 nm nanocrystal size [168]. Most surface modification methods rely on the placement of the hydrophobic surfactant coatings by ligand molecules that are reactive towards the nanocrystals surface on one end, and has a hydrophilic groups on the other end. Alivisatos, et al and Nie, et al first used such chemical exchange reactions to modify the surface chemistry of quantum dots (QDs) [117, 118]. In had method, a bifunctional molecule, such as mercaptoacetic acid (MAA), competed with TOPO (or another organic stabilizer) for binding to a metal atom on the Qdots surface. With excess bifunctional molecules in solution, the thiol (-SH) functional groups on the MAA displaces the phosphonic oxide (from the TOPO) that was initially bound to the metal atoms. If the bifunctional molecules contains a polar functional group opposite to the thiol (-SH)

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88 functional group, the Qdots become polar and are soluble in water solutions. Some disadvantages of using MAA was rapid flocculation of the hydrophilic nanocrystals and a significant decrease in quantum yield. Particle aggregation was attributed to a weak bond between the thiol (-SH) group and the Qdots surface which allowed the hydrophilic ligand shell around the nanocrystal to disintegration [167, 169]. Another approach to make the nanocrystals water soluble is to grow a hydrophilic silica shell through surface silanization [170, 171]. At first a ligand exchange is used to substitute the original hydrophobic surfactant with another surfactant shell (e.g. mercaptopropyltrimethoxysilane) which is cross-linked for improved stability. To make this silica shell hydrophilic, molecules with methoxysilane groups at one end and hydrophilic groups at the other end are attached by siloxane bonds, resulting in a multi-layer shell. Silanized nanocrystals are extremely stable in solution, but the silanization process is laborious and the resulting shell often is inhomogeneous. Recently, methods to coat hydrophobic nanocrystals with amphiphilic polymers has been reported [119, 125, 133, 172]. In these approaches, hydrophobic tails of the amphiphilic polymer intercalate the hydrophobic surfactant molecules on the nanocrystals surface and thus form an additional coating layer. The water solubility of polymer coated nanocrystals is ensured by hydrophilic groups that self-assemble on the polymer shell and the are cross-linked for better stability. This method is not based on ligand exchange, i.e. not based on replacing the original hydrophobic surfactant with hydrophilic molecules, but rather depends on the whole nanocrystals being covered with a cross-linked hydrophilic polymer shell. There is no direct interaction between the polymer and the nanocrystals surface atoms, and therefore the original luminescent efficiency should be preserved. In addition, the large number of hydrophobic side chains on the

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89 amphiphilic polymer strengthens the hydrophobic structure at the surface, resulting in more stable water-soluble nanocrystals. The data indicate that this amphiphilic shell is thinner and more homogeneous than silica shells, although the multiple shells around the inorganic core make the overall diameter larger than nanocrystals made hydrophilic by surfactant exchange. In the present study, the amphiphilic polymer coating method was applied to make luminescent Gd2O3: Eu3+ nanocrystals hydrophilic. The optical properties and size was monitored 4.2 Experimental Sections 4.2.1 Materials Octylamine, anhydrous N, N-dimethylformamide (DMF), 1-[3-(dimethyl-amino)-propyl]-ethylcarbodiimide hydrochloride (EDC) and poly(acrylic acid) (PAA, molecular weight 2000) were purchased from Aldrich and were used as received without further purification. 4.2.2 Synthesis of Hydrophobically Modified Poly(Acrylic Acid) Octylamine (3.5g) in DMF solution was transferred dropwise into a stirred anhydrous DMF solution containing dry PAA powder (4g) and 1-[3-(dimethyl-amino)-propyl]-ethylcarbodiimide hydro chloride (1.2g). The reaction was allowed to continue for 24 h at room temperature. After 24h an excess of H2O was added to the solution, resulting in the precipitation of the polymer. The precipitated polymer was dissolved in methanol, which was then evaporated to obtain the final purified amine modified PAA polymer product. 4.2.3 Synthesis of Hydrophilic Gd2O3 : Eu3+ Nanocrystals The Gd2O3 : Eu3+ nanoplates and nanospheres were prepared by the non hydrolytic hot solution synthesis route as described in section 3.2.2 and 3.2.3. Under ambient conditions, hydrophobic Gd2O3 nanocrystals (20mg) were dispersed in 15ml hexane. The suspension was added dropwise while stirring into a hexane solution of the amine modified PAA (60 mg /15ml).

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90 The hexane was removed by evaporation to yield a thin film of polymer/nanocrystals composite on the wall of the flask. The dry film was redispersed in distilled water with stirring, and aggregated material and excess polymer was removed by filtration (0.2 m, syringe filter). 4.2.3 Characterization The morphology of modified Gd2O3:Eu3+ nanocrystals were imaged using high resolution TEM (JEOL 2010F, 200kV). Photoluminescence (PL) was measured at room temperature from nanocrystals suspended in hexane using a Flurolog Tau 3 spectrofluorometer (Jobin Yvon Spex Instruments, S.A. Inc) with a 450 W xenon excitation lamp. For confirmation of the dispersibility in water, the Zeta potential was measured with a colloidal dynamics acoustosizer. 4.3 Results and Discussion 4.3.1 Water Soluble Surface Modification of Gd 2 O 3 :Eu 3+ Nanocrystals A schematic model structure, after surface modification of Gd 2 O 3 :Eu 3+ nanocrystals, is shown in Figure 4 1. In this model, the surfactant chains for hydrophobically capped Gd 2 O 3 :Eu 3+ nanocrystals are pointing away from th e nanocrystals surface, in a brush like arrangement (left side of Figure 4 1). A plausible configuration for the amphiphilic polymer coating is shown on the right of Figure 4 1 where the hydrophobic alkyl chains of the octylamine modified PAA intercalate w ith the surfactant coating, and the outer surface is covered by the hydrophilic ends. Figure 4-1. Schematic model of surface modification of Gd2O3:Eu3+ nanocrystals using octylamine modified PAA.

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91 TEM images of Gd2O3:Eu3+ nanospheres before and after modification are shown in Figure 4-2. The original Gd2O3:Eu3+ nanospheres were prepared using hexadecanediol oleic acid, oleylamine and benzyl ether. Although the strong affinity of surfactants such as oleic acid increase the difficulty of modifying these hydrophobic nanocrystals to make them dispersible in aqueous solutions [173], the TEM images showed a lack of aggregation of nanoparticles. The aqueous dispersed Gd2O3:Eu3+ nanospheres (Figure 4-2 (b-c)) were stable suspensions even after 3 months in lab ambient. The TEM images show that the average sizes of Gd2O3:Eu3+ nanospheres are about 2-3 nm and 3-4 nm before and after modification, respectively. Figure 4-2. TEM images of Gd2O3:Eu3+ nanospheres capped with oleic acid and HDD before (a) and after (b-c) hydrophilic modification. An amine group coordinated to the Gd atoms was also prepared to investigate the effect of the low bond strength on surface modification. These Gd2O3:Eu3+ nanocrystals were synthesized with hexadecylamine (HDA) instead of HDD and oleic acid in a relatively fast reaction (<1 h). HDA has been successfully used for the synthesis of II_VI semiconductor nanocrystals. However, HDA capped Gd2O3:Eu3+ nanocrystals exhibited poor crystallinity and were polydispersed with relatively low product yield (see Figure 4-3(a)). Although it was expected that the weak coordination of HDA to the nanocrystals would result in easier and more effective

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92 surface modification, the poor crystallinity of unmodified nanocrystals results in poor surface modification and therefore particle agglomeration, as shown in Figure 4-3(b). Figure 4-3. TEM images of Gd2O3:Eu3+ nanospheres capped with HAD before (a) and after (b) hydrophilic modification. Using the same approach, modification of the surface of Gd2O3:Eu3+ nanoplates was attempted. As shown previously in Figure 3-3 and Figure 3-6(b), nanoplates self-assembled into closely stacked arrays. The modified hydrophilic Gd2O3:Eu3+ nanoplates were aggregated, consistent with the expectation that the surfaces of the assembled nanocrystals were difficult to completely modify and produce a stable dispersion. Figure 4-4. TEM images of Gd2O3:Eu3+ nanoplates before (a) and after (b) hydrophilic surface modification.

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93 4.3.2 Luminescence properties of hydrophilic surface modified Gd2O3:Eu3+ nanocrystals Surface modified hydrophilic nanocrystals should at least retain the optical properties of hydrophobic capped nanocrystals. The photoluminescent spectra from Gd2O3:Eu3+ nanospheres prepared with oleic acid and HDD before and after hydrophilic modification are compared in Figure 4-5. The spectral distribution did not change but the emission intensity at 612 nm decreased to 47 % of the initial intensity after the hydrophilic modification. Figure 4-5. PL emission spectra of oleic acid and HDD capped hydrophobic (before) and PAA capped hydrophilic (after) Gd2O3:Eu3+ nanospheres. HDA capped hydrophobic Gd2O3:Eu3+ nanocrystals can also be surface modified with PAA hydrophilic polymer (see Figure 4-3), and again the PL spectra are the same but the intensity after modification is only 26% of the original PL emission yield at 612 nm (Figure 4-6). Both nanocrystals of Gd2O3:Eu3+ synthesized with HDD and HDA are well dispersed in water after the hydrophilic surface modification, but both also decrease the 612 nm PL emission are observed in Figure 4-5 and Figure 4-6. These decreases are likely the result of differences in the type and binding strength of capping surfactants, such as oleic acid and amines. The largest drop in PL emission was from Gd2O3:Eu3+ nanocrystals prepared with HDA versus those

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94 prepared with oleic acid and HDD. The larger drop for HDA capped nanocrystals is attributed to the fact that the HDA ligands are labile and these capping groups are easily removed from the Gd2O3:Eu3+ surface. As pointed out above, weak encapsulation by HAD results in poor crystallinity and unstable condition for Eu3+ doping, with or without the PAA surface modification. Figure 4-6. PL emission spectra of had capped hydrophobic (before) and hydrophilic (after) Gd2O3:Eu3+ nanocrystals. 4.3.3 Dispersion Properties of Hydrophilic Surface Modified Gd2O3:Eu3+ Nanocrystals The zeta potential of hydrophilic Gd2O3:Eu3+ (sample in Figure 4-2 (b,c)) after surface modification is shown versus pH in Figure 4-7. The pH value of the original sample was 7.35 that corresponds to a zeta potential of -45 mV, and the dispersion should be stable. For pH values between 5 and 8, the data indicate that modified Gd2O3:Eu3+nanocrystals should be well Dispersed and aggregation is not expected.

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95 Figure 4-7. Zeta potential versus aqueous solution pH of hydrophilic PAA surface modified Gd2O3:Eu3+ nanocrystals. 4.4 Conclusions Octylamine-modified poly(acrylic acid) (PAA) has been used to convert the hydrophobic surface of Gd2O3:Eu3+ nanocrystals synthesized by a non hydrolytic hot organic solvent technique to a hydrophilic surface. The surface conversion results in a dispersion of nanocrystals in aqueous solutions stable in lab ambient for >31 days, especially for nanocrystals synthesized using oleic acid and hexadecanediol (HDD). The quality of surface modified Gd2O3:Eu3+ nanocrystals synthesized with hexadecylamine (HDA) was not as good. For both synthesis routes, the PL spectra was unchanged by the surface modification, but the intensity was decreased to 47% and 26% of the hydrophobic capped value for synthesis with HDD and HDA, respectively.

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96 CHAPTER 5 SYNTHESIS AND CHRACTERIZATION OF TERNARY ZNGA2O4: EU3+ NANOCRYSTALS 5.1 Introduction An organic phase process known as thermal decomposition synthesis allows precise tuning of nanocrystals size, shape, and composition, and also allows them to be dispersed in either an aqueous or a nonhydrolytic media. This technique has been demonstrated for binary semiconductor (e.g. CdSe [23], ZnSe [174], and PbSe [175] ) and binary transition metal oxide (e.g. ZnO [68], TiO2 [74], MnO [54], Fe3O4 [65] and Gd2O3 [70]) nanocrystals. Recently, this synthetic route was expanded to synthesis of ternary nanocrystals with controlled stoichiometry Zhong and co-workers synthesized ZnxCd1-xS nanocrystals at high temperature by reacting a mixture of CdO and ZnO oleic acid complexes with sulfur in a noncoordinating solvent (octadecene) system [176]. They found that the nanocrystals show narrow and composition-dependent photoluminescence spectra. Lee, et al used hot solution synthesis to grow nanorods of Cd1-xZnxSe [177]. They showed that the ternary nanorods could be grown from a solution containing both Cd and Zn precursors, of by solid state diffusion after growth of a CdSe core and a ZnSe shell [164]. Finally, growth of ternary compounds of MFe2O4 (M=Fe, Co, Mn, and Mg) [64, 66] were reported by thermal decomposition synthesis. The composition of the particles was controlled by the molar ratio of Fe(acac)3 and M(acac)2 reactants, and the shape and size were changed by varying the reaction conditions. By selecting different elements for M2+, the MFe2O4 could be molecularly engineered over a wide range of magnetic properties. However, growth of doped ternary nanocrystals has not been reported, either as II-VI semiconductors or as oxide compounds. Even the number of reports of doped binary oxide nanocrystals are few (e.g. Y2O3: Eu3+ [178] and Gd2O3: Eu3+ [71]). In the present study, thermal

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97 decomposition synthesis and the resulting photoluminescent properties of Eu3+ doped ternary ZnGa2O4 nanocrystals is reported. Luminescent zinc gallate (ZnGa2O4) has attracted a great deal of attention due to potential applications in field emission displays and electroluminescent devices [179, 180]. In principle, an oxide should show better chemical stability versus competing sulfide phosphors (ZnS: Cu,Cl and SrGa2O4:Eu2+), especially at high electron beam currents [181]. ZnGa2O4 crystallizes as a cubic spinel with a large band gap of 4.4 eV, and exhibits an intense green emission when doped with Mn and blue luminescence without doping via a self-activated center [182]. It has been suggested that white luminescence could be achieved from ZnGa2O4 by doping with Mn2+ (green), Eu3+ (red) and Ce3+ (blue) [180]. 5.2 Experimental 5.2.1 Materials Zn (II) acetate hydrate, Zn (II) acetate dehydrate Zn(II) acetylacetonate hydrate, Ga(III) acetylacetonate hydrate, Ga (III) nitrate hydrate, Eu(III) acetate hydrate oleic acid ( 90% tech.), oleyamine (90%, tech.), benzyl ether ( 99%) 1,2-hexadecanediol ( 97%), and trioctylphosphine oxide (TOPO, 99%) were purchased from Aldrich and used without further purification. Absolute ethanol, benzyl ether and hexane were also used as received. 5.2.2 Synthesis of ZnGa2O4: Eu3+ Nanocrystals Similar procedures were used to prepare all ternary ZnGa2O4: Eu3+ nanocrystals. A zinc precursor (e.g. 1mmol of Zn(II) acetylacetonate hydrate) was mixed with Ga(III) acetylacetonate hydrate(2mmol), Eu(III) acetate hydrate (0.1 mmol), oleic acid (6mmol), oleylamine (6mmol), benzyl ether (5 mmol) and hexadecanediol (5 mmol) and magnetically stirred flowing nitrogen in a three-neck reaction flask. This mixture in the flask was heated to 200C for 30 min, then heated to 290C at a heating rate of 25C /min and held for 3h under the

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98 N2 blanket. After reflux, the mixture was cooled to room temperature by removing the heat source. Under ambient conditions, ethanol was added to the mixture, and ZnGa2O4: Eu3+ was precipitated and separated via centrifugation (9000 rpm, 10 min). After several purification with ethanol followed by centrifugation, purified ZnGa2O4: Eu3+ nanospheres were well dispersed in organic solvents such as hexane, chloroform or toluene. For other Zn precursors, Zn (II) acetate hydrate resulted in nanospheres of ZnGa2O4: Eu3+ with the same concentrations and heating sequences. When TOPO (2 mmol) was used instead of hexadecanediol (5 mmol), sphere-like ZnGa2O4: Eu3+ nanocrystals were again produced. However, Zn (II) acetate dehydrate reacting with Ga(III) acetylacetonate hydrate (2mmol), Eu (III) acetate hydrate (0.1 mmol), oleic acid (6mmol), oleylamine (6mmol), benzyl ether (5 mmol) and hexadecanediol (2.5 mmol) resulted in mixed-shaped large ZnGa2O4: Eu3+ nanocrystals. 5.2.3 Characterization of ZnGa2O4: Eu3+ Nanocrystals X-ray diffraction (XRD) (Philips APD 3720) was used to determine the crystal structure with Cu K. radiation source ( = 0.5418 nm). A JEOL 2010F transmission electron microscope (TEM) operated at 200 kV was used to determine the size and shape of the nanocrystals. Optical absoprtion spectra were collected with a Shimadzu UV-2401PC spectrophotometer. Photoluminescence (PL) was measured at room temperature from nanocrystals suspended in hexane using a Flurolog Tau 3 spectrofluorometer ( Jobin Yvon Spex instruments, S.A. Inc) with a 450 W xenon excitation lamp. Thermo-gravimetric analysis (TGA, Seiko TG/ATD 320U, SSC 5200) was used to investigate the amount of surfactant bound on the particle surface. In the TGA, the samples were heated up to 800C in air at a heating rate of 10C min-1.

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99 5.3 Results and Discussion 5.3.1 Shape Control of ZnGa2O4: Eu3+ Nanocrystals ZnGa2O4: Eu3+ nanospheres were prepared with a main surfactant of either hexadecanediol (HDD) or TOPO and a mixture of oleic acid, oleylamine and benzyl ether with Znand Ga-precursors. The Znand Ga-precursors and main surfactant were varied as shown in Table 4-1 to determine their effects on formation of ternary ZnGa2O4: Eu3+ nanospheres. Table 5-1. Precursors and surfactants used to synthesize ZnGa2O4: Eu3+ nanospheres Zn Ga Surfactant Ratio of Zn/surfactant a Zn(acac) 2 Gd(a cac) 3 hexadecanediol 1:5 b Zn acetate hydrate Ga nitrate hexadecanediol 1:5 c Zn acetate hydrate Gd(acac) 3 hexadecanediol 1:5 d Zn(acac) 2 Gd(acac) 3 TOPO 1:2 For synthesis of ZnGa2O4 by solid state reactions (heating to high temperatures in air), Zn/Ga ratio much higher than one must be used because of the high vapor pressure of ZnO, but nonstoichiometric product was still reported [183-185]. In the present case, the Zn:Ga ratio in the purified ZnxGa2-xO4: Eu3+ nanocrystals was essentially identical to that in the reactant mixtures. Most samples were prepared with the ratio of Zn:Ga of 1:1. This constant ratio between reactants and products probably is a result of similar precursors and solubilities of the Zn and Ga precursors in the nonhydrolytic liquid phase, and results in easy composition adjustments. High resolution transmission electron microscope (HRTEM) images of ZnGa2O4: Eu3+ nanospheres (from combinations a-d listed in Table 5-1 and discussed in section 5.2.2) are shown in Figure 5-1. The ZnGa2O4: Eu3+ nanocrystals are mainly monodisperse, crystalline spherical particles, with a diameter of <5 nm. The average diameter of ZnGa2O4: Eu3+ nanospheres in sample (a) are larger (~ 3-4 nm) than samples (b-d) (~2-3 nm).

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100 Figure 5-1. HRTEM images of ZnGa2O4: Eu3+ nanospheres prepared with the precursors and surfactants corresponding to (a-d) in Table 5-1. When the Zn precursor was changed to Zn acetate dehydrates with a ratio of Zn/hexadecanediol (1:2.5), complex shaped triangular and rectangular ZnGa2O4: Eu3+ nanocrystals were observed as shown in Figure 5-2. Because of the poor contrast in the TEM images, these nanocrystals could be nanoprisms and nanoplates rather than flat triangles and rectangles. Figure 5-2. HRTEM images of ZnGa2O4: Eu3+ nanocrystals prepared with Zn acetate dehydrate with the ratio of Zn:hexadecanediol of 1:2.5 under standard conditions.

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101 5.3.2 Size Control of ZnGa2O4: Eu3+ Nanocrystals In most colloidal techniques, particle synthesis from homogeneous solution involves nucleation and growth [186]. The surfactant ligand is critical to allow particles to grow to a specific size and then to arrest growth, therefore higher concentration could be expected to better limit the growth of nanocrystals. With higher concentration of stabilizing surfactants (over a limited range), smaller nanocrystals were obtained with a Zn(acac)3 / oleylamine system [67]. It was reported that injection of additional surfactants, such as thiol, would arrest the growth and result in formation of smaller ZnO nanocrystals [187]. Figure 5-3. HRTEM images of ZnGa2O4: Eu3+ nanocrystals synthesized with a ratio of Zn(acac)2 / hexadecanediol (a-b) 1:2.5, and (c-d) 1:5; (e) selected area electron diffraction (SAED) pattern from a cubic spinel crystalline phase. HRTEM images of the ZnGa2O4: Eu3+ nanocrystals using a 1:2.5 molar ratio of the Zn(acac)2 / hexadecanediol are shown in Figure 5-3 (a-b). Nearly monodispersed, spherical nanocrystals >4-5 nm in diameter were observed. Smaller nanocrystals of <4 nm in diameter were obtained as shown in Figure 5-3 (c-d), when a 1:5 molar ratio of Zn(acac)2 / hexadecanediol was used. The selected area electron diffraction (SAED) pattern of the

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102 nanocrystals show rings from the (220), (311) and (400) planes of cubic spinel ZnGa2O4 (Figure 5-3(e)). 5.3.3 Crystal Structures of ZnGa2O4: Eu3+ nanocrystals ZnGa2O4 has a cubic normal spinel crystal structure with Fd3m space group that can be thought of as a combination of rock salt and zinc blende structure (see Figure 5-4) [188]. The oxygen ions are in face-centered cubic close packed configuration. The unit cell contains 8 tetrahedral cations, 16 octahedral cations and 32 oxygen anions. The normal spinel ZnGa2O4 has tetrahedrally coordinated Zn sites surrounded by 4 oxygens and octahedrally coordinated Ga sites surrounded by 6 oxygens. Figure 5-4. The cubic spinel structure of ZnGa2O4 [189]. Figure 5-5 is XRD patterns from ZnGa2O4: Eu3+ nanocrystals (samples (a-b) in Figure 5-3). Although peak broadening occurred due to the small size of the nanocrystals, all of the diffraction peaks could be indexed from the cubic spinel ZnGa2O4 structure ( JCPDF 38-1240). The main peak from (311) planes of ZnGa2O4 is observed at a two-theta of 35.6 and no peaks from Ga2O3 or ZnO were found, consistent with single phase of ZnGa2O4: Eu3+ nanocrystals. The

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103 the size determined from the HRTEM image (Figure 5-3 (b)). Figure 5-5. XRD pattern of ZnGa2O4: Eu3+ nanocrystals (sample (a-b) in Figure 5-3). 5.3.4 Luminescence properties of ZnGa2O4: Eu3+ nanocrystals The photoluminescence excitation (PLE) spectrum of ZnGa2O4: Eu3+ nanocrystals (sample (a-b) in Figure 5-3) are shown in Figure 4-6. The PLE spectrum of Eu3+ ion was obtained by monitoring the Eu3+ 5D07F2 luminescence at 612 nm, and it consists of a broad intense band with a maximum 305 nm and two excitation peaks (stronger one is at 396 nm for 5F05L6 ) of Eu3+. The 305 nm band is due to the charge transfer band (CTB) of O2--Eu3+ together with absorption of ZnGa2O4 host lattice, which is shown in the inset of Figure 5-6. The emission spectrum obtained by excitation into 305 nm is composed of characteristic emission peaks of Eu3+ ion from 580 to 700 nm, which are associated with the transition from the excited 5D0 level to 7Fj ( J = 1,2,3,4 ) as seen in Figure 5-7. In the spinel ZnGa2O4 structure, Eu3+ ions could occupy at two sites, tetrahedral Zn2+ sites and octahedral Ga3+ sites. It is known that the relative intensity of 5D0-7F1 transition (magnetic dipole transition) and 5D0-7F2 (electric dipole

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104 transition) depends strongly on the local symmetry of Eu3+ ions. When ions occupy the inversion center sites, the 5D0-7F1 transition should be relatively strong, while the 5D0-7F2 transition is partly forbidden and should be very weak. Therefore, the ( 5D0-7F2 )/( 5D0-7F1) intensity ratio, known as the asymmetry ratio, is a measure of the degree of distortion from the inversion symmetry of the local environment of the Eu3+ in the lattice. These ZnGa2O4: Eu3+ nanocrystals exhibit high ( 5D0-7F2 )/( 5D0-7F1) intensity ratios, i.e. large asymmetry ratios, as seen in Figure 5-7, indicating strong electric fields and low symmetry at the Eu3+ ions sites. This result suggest that the Eu3+ ions occupies tetrahedral Zn2+ sites or distorted octahedral Ga3+ sites with no inversion symmetry in ZnGa2O4 nanocrystals. Since the ionic radii of Zn2+ ( 0.6 ) and Ga3+ (0.62 ) are so similar and much smaller than that of Eu3+ (0.947 ), there is no basis for speculating on a site preference based on ionic size. Figure 5-6. PLE spectrum for emission at 612 nm from ZnGa2O4: Eu3+ nanocrystals. The inset shows the UV absorption spectrum of undoped ZnGa2O4 nanocrystals.

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105 Figure 5-7. PL spectrum of ZnGa2O4: Eu3+ nanospheres excited at 305 nm 5.3.5 Thermo-gravimetric analysis (TGA) The TGA data from ZnGa2O4: Eu3+ nanocrystals (sample in Figure 5-2) are shown in Figure 5-8. These ZnGa2O4: Eu3+ nanocrystals are relatively large (~20 nm) complex shaped particles coated with organic surfactants. The weight loss during heating from RT to 200C was 4% which is attributed to desorption of physisorbed molecules from the organic surfactants. The 16% weight loss in heating from 200C to 300C was presumably due to the desorption of organic surfactants from the particle surface. Decomposition of the surfactants is complete at 410C and results in a weight loss of 2% between 300C and 410C. The total weight loss from desorption of surfactants from large complexed shaped ZnGa2O4: Eu3+ nanocrystals is 20 % while the total loss was 35% for nanoplates and 42% for nanospheres of Gd2O3:Eu3+, as reported in Chapter 3. This result is attributed to the previous conclusion that the relative amount of adsorbed surfactants decreases as the size of the nanocrystals increases.

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106 Figure 5-8. TGA data from larger complexed shaped ZnGa2O4: Eu3+ nanocrystals (sample in Figure 4-2). 5.4 Conclusions Monodispersed Eu3+ doped ternary ZnGa2O4 nanospheres (5nm) were prepared from a variety of metal precursors by a nonhydrolytic thermal decomposition route. Using Gd acetate dehydrate, large (20nm) complex shaped (triangle and rectangle) ZnGa2O4: Eu3+ nanocrystals were obtained. Based on X-ray diffraction data, the nanocrystals were concluded to have a cubic spinel structure with no impurity phases. The size of the ZnGa2O4: Eu3+ nanospheres could be varied by changing the molar ratio of Zn to surfactants, with higher concentrations of surfactant (1: 5) resulting in smaller nanocrystals ( < 4 nm). Analysis of the PL emission suggests that the Eu3+ ions were incorporated into the ZnGa2O4 host. It is concluded that the nonhydrolytic thermal decomposition synthesis route with organic surfactants not only allows the formation of stoichiometric ternary oxides, but also results in efficient incorporation of rare earth dopants.

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107 CHAPTER 6 CONCLUSIONS 6.1 Synthesis and Characterization of Luminescent Gadolinium Oxide Nanocrystals 1. Well dispersed, crystalline rare earth (Eu3+, Tb3+ and Er3+) doped Gd2O3 nanocrystals were synthesized at 290 oC (reaction time, ~ 3 h) by a hot solution nonhydrolytic route. 2. The shape of Gd2O3: Eu3+ nanocrystals were changed from nanospheres (<5nm) to nanoplates (1nm x <10nm2) to mixed shaped nanocrystals (spheres, plates, rods, and triangles) by changes of Gd precursors, organic surfactants, concentrations and heating rate (between 200 and 290oC reaction temperatures). 3. The intensity of the Eu3+ 5D07F2 emission at 612nm from nanospheres was more intense than that from nanoplates or mixed shape nanocrystals due to a higher concentration (6.12 %) of Eu in nanospheres versus nanoplates (1.85 %) or mixed shape nanocrystals (1.24 %), and due to a higher nanocrystal product yield for purified nanospheres. 4. The three main factors leading to larger amounts of dopant in Gd2O3 nanocrystals are a slower rate of reaction, a larger product yield and monoclinic crystal structure. Use of Gd(acac)3 precursor with oleic acid and HDD surfactants led to a slow reaction (~ 3 h) with high product yield (~ 80 %) and 6.1 % Eu incorporation for bright luminescent Gd2O3: Eu3+ nanospheres. 6.2 Water Soluble Surface Modification of Luminescent Gadolinium Oxide Nanocrystals for Biomedical Research 1. Hydrophobic Gd2O3:Eu3+ nanocrystals were surface modified by Octylamine-modified poly(acrylic acid) (PAA), amphiphilic polymer. The surface conversion results in a dispersion of nanocrystals in aqueous solutions stable in lab ambient for >31 days, especially for nanocrystals synthesized using oleic acid and hexadecanediol (HDD).

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108 2. The PL spectra from Gd2O3:Eu3+ were unchanged by the PAA surface modification, but the PL intensity was decreased to 47% and 26% of the hydrophobic capped value for synthesis with HDD and hexadecylamine (HDA), respectively. 6.3 Synthesis and Characterization of Ternary ZnGa2O4: Eu3+ Nanocrystals 1. Monodispersed Eu3+ doped ternary ZnGa2O4 nanospheres (5nm) were prepared from a variety of metal precursors by a nonhydrolytic thermal decomposition route. Using Gd acetate dehydrate, large (20nm) complex shaped (triangle and rectangle) ZnGa2O4: Eu3+ nanocrystals were obtained. 2. ZnGa2O4: Eu3+ nanocrystals had a cubic spinel structure with no impurity phases from X-ray diffraction and selected area diffraction pattern. 3. The size of the ZnGa2O4: Eu3+ nanospheres was controlled by changing the molar ratio of Zn to surfactants from 1:1 to 1:5, with higher concentrations of surfactant (1:5) resulting in smaller nanocrystals ( < 4 nm). 4. Analysis of the PL emission suggests that the Eu3+ ions were incorporated into the ZnGa2O4 host. The nonhydrolytic thermal decomposition synthesis route with organic surfactants not only allows the formation of stoichiometric ternary oxides, but also results in efficient incorporation of rare earth dopants.

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109 CHAPTER 7 FUTURE WORK It is evident that the above approaches can lead to success in shape and size control of nanocrystals. This would be very useful in incorporating luminescent oxide nanocrystals with nonhydrolytic liquid phase synthesis. This method can be expanded to other efficient luminescent oxide systems. As with a doping issue in this synthetic route, efficiency is not good comparing to the solid state reaction. Liquid phase synthesis has still a possibility to lost dopant ions into solvent during synthesis. The synthetic condition could be modified to make the reaction stable for better incorporation of dopant by trial of various chemical combinations. The fine control of dopants could allow the co-doped oxide nanocrystals (i.e. Gd2O3:Er3+, Yb3+ and YVO4: Er3+, Yb3+, etc) to be synthesized through this protocol for efficient up-conversion phosphor.

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121 BIOGRAPHICAL SKETCH SooYeon Seo was born in Seoul, South Korea and received her early education in this city. She was allowed access to the laboratory tools in her home from an early age. This early influence of science has had a lasting impact on her interest in phenomenon in nature and led to a foundation of evolutionary thinking in her pursuit of understanding the materials, in the kinds of research questions that have intrigued her. After she received a M.A in materials s cience and engineering, in South Korea, she obtained her Ph.D. degree from the Department of Materials Science and Engineering, University of Florida in Gainesville, US in 2007. general research interests are in the area of luminescent materials in elecroand bioapplication. The general theme of her current research interests deals with synthesis and characterization of luminescent oxide nanocrystals related to the candidate for the bio-labeling application in bio-medicine. She also has other interests and has done research on such topics as the luminescent nanocrystals as well as general issues related to methodology and optical analysis in electronic and bio-applications.