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Spectroscopoic Studies of Polymers in Transmissive/Absorptive Electrochromic Devices, and Doped Graphite

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Title:
Spectroscopoic Studies of Polymers in Transmissive/Absorptive Electrochromic Devices, and Doped Graphite
Creator:
Nasrollahi, Zahra
Place of Publication:
[Gainesville, Fla.]
Florida
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University of Florida
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english
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1 online resource (11 p.)

Thesis/Dissertation Information

Degree:
Doctorate ( Ph.D.)
Degree Grantor:
University of Florida
Degree Disciplines:
Physics
Committee Chair:
TANNER,DAVID B
Committee Co-Chair:
RINZLER,ANDREW GABRIEL
Committee Members:
MUTTALIB,KHANDKER A
BISWAS,AMLAN
WOODARD,RICHARD P
SO,FRANKY FAT KEI
Graduation Date:
5/3/2014

Subjects

Subjects / Keywords:
Charge carriers ( jstor )
Conductivity ( jstor )
Dielectric materials ( jstor )
Doping ( jstor )
Electronics ( jstor )
Electrons ( jstor )
Graphite ( jstor )
Polymers ( jstor )
Reflectance ( jstor )
Temperature dependence ( jstor )
Physics -- Dissertations, Academic -- UF
electrochromic -- polymer -- transmissive
Genre:
bibliography ( marcgt )
theses ( marcgt )
government publication (state, provincial, terriorial, dependent) ( marcgt )
born-digital ( sobekcm )
Electronic Thesis or Dissertation
Physics thesis, Ph.D.

Notes

Abstract:
Electrochromic polymers (ECPs) exhibit reversible optical modulation in a wide spectral range as a function of an externally applied voltage. In this work, ECPs have been used in absorptive/transmissive electrochromic devices as candidates for smart window applications. The electrochromic devices were fabricated on flexible polyethylene substrates and used ECPs sandwiched between thin films of single-walled carbon nanotubes serving as conductive and flexible electrodes. Unlike ITO, the nanotube films are highly transmissive in the visible and infrared region of the spectrum. The transmission and reflection of the individual device components as well as assembled devices were measured over a wide spectral range (FIR to UV). The devices were switched in situ in the spectrometers. The optical constants of the constituent layers were calculated using the Drude-Lorentz model. The devices demonstrated high transmission contrasts between their colored and bleached states in the VIS, NIR, and MIR spectra, enabling electrically tunable control over the transmission or reflection of both light and heat. This control could lead to reduced heating or cooling costs in real world applications and the flexible nature of the device components allows many applications. ( en )
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.
Thesis:
Thesis (Ph.D.)--University of Florida, 2014.
Local:
Adviser: TANNER,DAVID B.
Local:
Co-adviser: RINZLER,ANDREW GABRIEL.
Electronic Access:
RESTRICTED TO UF STUDENTS, STAFF, FACULTY, AND ON-CAMPUS USE UNTIL 2015-05-31
Statement of Responsibility:
by Zahra Nasrollahi.

Record Information

Source Institution:
UFRGP
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Applicable rights reserved.
Embargo Date:
5/31/2015
Classification:
LD1780 2014 ( lcc )

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1 SPECTROSCOPIC STUDIES OF POLYMERS IN TRANSMISSIVE/ABSORPTIVE ELECTROCHROMIC DEVICES, AND DOPED GRAPHITE By ZAHRA NASROLLAHI A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLME NT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA 201 4

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2 201 4 Zahra Nasrollahi

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3 To my parents and siblings

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4 ACKNOWLEDGMENTS I would like to thank Professor s: David B. Tanner, Richard Woodard, Khandker Muttalib, Andrew Rinzler, Amlan Biswas and Franky So for serving on my supervisory committee. It is my pleasure to thank people who helped me in accomplishing the experiments specifically Dr. Svetlana V asilyeva and Dr. Evan Donoghue from Professor lab and Dr. Fengjun from Professor for providing samples for el e ctrochromic devices and very useful discussions as well Also many thanks to Dr Sef a attin Tongay and my friend Sima Sae i di Varnoo sfaderani for providing doped graphite samples. I am also grateful to my colleagues in Professor Tanner Lab for their collaboration and friendship: Jungseek Hwang, Jeffr e y Hoskins, Richard Ottens, Naween Anand Berik Uzakba iu ly, Chang Long and Luyi Yang I am especially very much grateful to Rich for tremendous help in the lab and to Jeff for being a great listener and helping me with the writing of the dissertation. Thanks to Professor Biswas for helping us with temperature dependent measurement of polya niline sample s Thanks to Professor s : Richard Woodard (the inventor of a very important math function!) Henke Monkhorst, Dmitrii Maslov Khandker Muttalib, Peter Hirschfeld, and Chris Stanton for being wonderful professors and people I enjoyed attending their classes and I learned a lot from them. Their advices were always very helpful and life changing Thanks to machine shop members, Cryogenic lab staff, and electronics shop member s for the hard work and the great help.

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5 Thanks to Darline Latimere, Janne t Germany Pam Marlin Billie Hermansen John Mocko Dr Robert (Bob) Deserio, IT staff members and Kristine Nicola for always being helpful and kind Thanks to Mr Charles Parks and his wife Ms Debbi Parks for inviting me to their home every Christmas an d being very dear friends for me during the difficult times I had at the graduate school. M y very good friend, Fatemeh Ghasemi : I am very lucky to have you in my life I cannot thank you enough. I also thank my other dear friends ( i n no special order!) : Ma ryam Baradaran Haghir, Maryam Mirhadifard, Azam Feiz, Heshamt Saroui, Masoumeh Rajabi, Amin Terouhid, Niusha Nazar Kazemi Sara Bayramzadeh Hura Behforoozi, Sara Ganji, Mansoureh Orouji, Sharareh Sharififar Sahar Mirshamsi, Dani al Sabri Dashti, Ashkan Behnam Sima Saei di Varnoosfaderani, Guita Banan, Nima Rahmatian, Rasool Nas r Isfahani, Shahin Navardi Ali Ashrafi, Negin Jahanmiri Nazli Khodayari Hsing Jung Lin, Sohyun Park Sandipan Dutta Pooja Wadhwa, Maureen Petterson, Katie Leonard, Mona Minkara, Ozlem Demir, and Aysegul Oz Their friendship has been a treasure for me. At the end, the very special t hanks to my beloved mom and dear sibling s for their love, s upport, and sacrifice

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6 TABLE OF CONTENTS page ACKNOWLEDGMENTS ................................ ................................ ................................ .. 4 LIST OF FIGUR ES ................................ ................................ ................................ .......... 8 ABSTRACT ................................ ................................ ................................ ................... 11 CHAPTER 1 INTRODUCTION ................................ ................................ ................................ .... 13 2 CONDUCTIVITY OF CONJUG ATED POLYMERS ................................ ................ 15 Review Of Conjugated Polymers ................................ ................................ ............ 15 Conductivity ................................ ................................ ................................ ............ 18 D oping ................................ ................................ ................................ .............. 18 Photo induced electron transfer ................................ ................................ ....... 19 Disorder ................................ ................................ ................................ ............ 20 Charge stor age in conducting polymers ................................ ........................... 24 3 THIN FILM OPTICS ................................ ................................ ................................ 27 The interaction of electromagnetic waves with matter ................................ ............ 28 Light Propagation through a Single Layer Structure ................................ ............... 32 Kramers Kronig relation or dispersion relation ................................ ........................ 32 Light Propagation through a Multi Layer Structure (Matrix Method) ....................... 33 Microscopic Models ................................ ................................ ................................ 34 Lorentz model ................................ ................................ ................................ ... 34 Drude Lorentz model ................................ ................................ ........................ 36 4 TRANSMISSIVE/ABSORPTIVE ELECTROCHROMIC WINDOWS ....................... 38 General Properties of Electrochromic Devices ................................ ....................... 38 Results and Discussion ................................ ................................ ........................... 40 Conclusion ................................ ................................ ................................ .............. 41 5 ELECTRICAL CONDUCTIVITY IN POLYANILINE THIN FILMS ............................ 48 6 OPTICAL CONDUCTIVITY OF DOPED GRAPHITE ................................ .............. 57 Electrica l Conductivity of GIC in the Basal Plane ................................ .................... 59 Experimental Method ................................ ................................ .............................. 60 Results and Discussion ................................ ................................ ........................... 63 Conclusion ................................ ................................ ................................ .............. 65

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7 7 CONCLUSION ................................ ................................ ................................ ........ 78 LIST OF REFERENCES ................................ ................................ ............................... 79 BIOGRAPHICAL SKETCH ................................ ................................ ............................ 81

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8 LIST OF FIGURES Figure page 4 1 ECD general scheme [ref. 25]. ................................ ................................ ........... 42 4 2 Black ECP (Ink Black polymer) donor acceptor (DA) copolymer comprising: The electron donor 3,4 propylenedioxythiophene (proDOT), and electron acceptor 2,1,3 benzothiadiazole (BTD). ................................ ............................. 42 4 3 MCCP: Minimally Color changing polymer N alkyl substituted poly (3,4 propylenedioxypyrrole). ................................ ................................ ...................... 42 4 4 Transmission of polyethylene substrate. ................................ ............................ 43 4 5 Transmission of single wall nanotube on polyethylene substrate. ...................... 43 4 6 Transmission of sticky PF on polyethylene substrate. ................................ ........ 44 4 7 Transmission of MCCP/Sticky PF/SWNT/PE. ................................ .................... 44 4 8 Transmission of BP/PE. ................................ ................................ ...................... 45 4 9 Transmission of Black ECP/Sticky PF/SWNT/PE. ................................ .............. 45 4 1 0 Transmission of the working device at different voltages in far infrared. ............ 46 4 1 1 Transmission of the working device at d ifferent voltages in mid infrared. ........... 46 4 1 2 Transmission of the working device at different voltages in near infrared and visible ranges. ................................ ................................ ................................ ..... 47 4 1 3 Transmission of working device at different voltages in the whole frequency range. ................................ ................................ ................................ ................. 47 5 1 Polyaniline reflection, versus frequency at different temperatures using Bruker 113V spe ctrometer. ................................ ................................ ................. 52 5 2 R versus frequency at 300 K, blue: Experimental data, and the red: fit. ............. 52 5 3 R versus frequency at 200 K, blue : Experimental data and the red: fit. .............. 53 5 4 R versus frequency at 100 K, blue: Experimental data, and the red: fit. ............. 53 5 5 R vers us frequency at 33 K, blue: Experimental data and the red: fit. ................ 54 5 6 Optical conductivity versus frequency, at 300K. ................................ ................. 54 5 7 Opti cal conductivity versus frequency, at 200 K. ................................ ................ 55

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9 5 8 Optical conductivity versus frequency, at 100 K. ................................ ................ 55 5 9 Optical conductivity v ersus frequency, at 33 K. ................................ ................. 56 5 10 Comparison of optical and DC conductivity. ................................ ....................... 56 6 1 The picture of the set up for preparation of brominated graphite. ....................... 66 6 2 Reflection of HOPG, 20, and 100 minute doped graphite at room temperature. ................................ ................................ ................................ ....... 66 6 3 Temperature dependen ce of reflectance for HOPG sample ............................... 67 6 4 Reflectance for HOPG sample at 2 extreme temperatures 10 & 300 K, experimental data and the calculated ones from KK or Drude Lorentz model. ... 67 6 5 Temperature dependence of reflectance for 20 minute brominated HOPG sample. ................................ ................................ ................................ ............... 68 6 6 Reflectance for 20 minute brominated HOPG sample at 2 extreme temperatures 10 & 300 K, experimental data and the calculated ones from KK or Drude Lorentz model. ................................ ................................ ............... 68 6 7 Temperature dependence of reflectance for 100 minute brominated HOPG sample. ................................ ................................ ................................ ............... 69 6 8 Reflectance for 100 minute brominated HOPG sample at 2 extreme temperatures 10 & 300 K, experimental data and the calculated ones from KK or Drude Lorentz model. ................................ ................................ ............... 69 6 9 Temperature dependence of optical conductivity for HOPG sample. ................. 70 6 10 Temperature dependence of optical conductivity for 20 minute brominated HOP G sample. ................................ ................................ ................................ ... 70 6 11 Temperature dependence of optical conductivity for 100 minute brominated HOPG sample. ................................ ................................ ................................ ... 71 6 12 Temperature depend ence of real part of dielectric function of HOPG at 10 to 300 K. ................................ ................................ ................................ ................. 71 6 13 Temperature dependence of real part of dielectric function of 20 minute brominated HOPG at 10 to 300 K. ................................ ................................ ...... 72 6 14 Temperature dependence of real part of dielectric function of 100 minute brominated HOPG at 10 to 300 K. ................................ ................................ ...... 72 6 15 Temperature dependence of scattering rate of HOPG at 10 to 300 K. ............... 73

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10 6 16 Temperature dependence of scattering rate of the 20 minute brominated HOPG at 10 to 300 K ................................ ................................ .......................... 73 6 17 Temperature dependence of scattering rate of the 100 minute brominated HOPG at 10 to 300 K. ................................ ................................ ......................... 74 6 18 Temperature dependence of effective carrier density for HOPG at 10 to 300 K. ................................ ................................ ................................ ........................ 74 6 19 Temperature dependence of effective carrier density for 20 minute brominated HOPG at 10 to 300 K. ................................ ................................ ...... 75 6 20 Temperature dep endence of effective carrier density for 100 minute brominated HOPG at 10 to 300 K. ................................ ................................ ...... 75 6 21 Temperature dependence of loss function for HOPG at 10 to 300 K .................. 76 6 22 Temperature dependence of loss function density for 20 minute brominated HOPG at 10 to 300 K. ................................ ................................ ......................... 76 6 23 Temperature dependence of Loss function for 100 minute br ominated HOPG at 10 to 300 K. ................................ ................................ ................................ .... 77

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11 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 SP ECTROSCOPIC STUDIES OF POLYMERS IN TRANSMISSIVE/ABSORPTIVE ELECTROCHROMIC DEVICES, AND DOPED GRAPHITE By Zahra Nasrollahi May 2014 Chair: David Tanner Major: Physics Electrochromic polymers (ECPs) show reversible optical properties in a broad range o f frequency upon the application of an external voltage. In this dissertation ECPs have been utilized in t ransmiss ive /absorptive electrochromic devices to be used in smart window s Th ese devices were made on flexible polyethylene substrates Th in films of single walled carbon nanotubes were used as flexible and conductive electrodes for the devices Unlike indium tin oxide ( ITO ) the nanotube thin films illustrate high transmi ttance in the visible (VIS) and infrared (IR) region s of the electromagnetic spec trum The transmi ttance and reflectance of the individual components of the device were measured as well as of the whole working device over a wide spectral range far infrared (FIR) to ultraviolet (UV) Using the Drude Lorentz model the optical parameters of the component layers were calculated The devices showed high transmittance contrasts between their extreme states in the VIS, near infrared ( NIR ) and mid infrared ( MIR ) frequency rang es This property enables the electrical regulation of transmi ttanc e or reflectance of both heat and light This can give rise to decreased energy costs in the cars and building.

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12 Graphite intercalation compounds (GICs) have impressing thermal, electrical, and magnetic characteristics In this work, highly oriented pyroli tic graphite ( HOPG ) samples were exposed to bromine vapor for different time periods. The reflectance of the samples was measured using Fourier transform infrared ( FTIR ) spectrophotometer, over the FIR and MIR at different temperatures between room tempera ture down to 10 K As the intercalation time is increased the infrared reflectance increases drastically as well. This leads to the increase of optical conductivity of the material calculated by Kramers Kronig (KK) technique. The study of alteration of cha rge carrier density and scattering rate for different bromination times at different temperatures can guide to a better understanding of the significant enhancement of electrical conductivity in the compound

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13 CHAPTER 1 INTRODUCTION The focus of this diss ertation is the sp ectroscopic characterization of conjugated conductive polymers in transmissive/absorptive electrochromic devices and of doped graphite. Electrochromic devices, using single walled carbon nanotubes as the conductive electrode instead of co nventional indium tin oxide (ITO) illustrate a significant enhancement of t ransmittance over a wide electromagnetic frequenc y range from infrared to near ultraviolet C hapter 2 describes typical conjugated polymers and their classification into two groups, different doping mechanisms which change the optical properties, conductivity, and charge carrier concentration in these polymers. Finally, it explains different electronic states of the polymers in different doping levels. Some theoretical models that tr y to explain the characteristics of these polymers are introduced. Chapter 3 describes basic theoretical laws of the propagation of the light through media and interfaces, transmittance and reflectance formulae for single and multilayer samples are explain ed. The relation between different optical parameters and microscopic theories including Drude Lorentz models are presented. Chapter 4 includes the results and discussion of experiments. The e lectrochromic device and its different components are describe d. T he transmission through the individual components as well as the working device as a whole and the result of the Drude Lorentz model fit to the experimental data are presented. Finally, we examine the doping induced electronic states and infrared acti ve vibrational modes of the polymers in the device.

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14 Chapter 5 discusses the results of reflectance measurement s of Polyaniline samples at different temperatures in the IR as well as 4 probe DC measurement s. The resultant conductivit ies from these two metho ds are shown to be consistent. Chapter 6 describes graphite intercalated compounds (GICs) and the spectroscopic studies of reflectance measurements of graphite and brominated graphite samples. The optical conductivity was calculated using KK technique. The results of experiments shows a significant enhancement of optical conductivity in doped graphite compared to the pristine graphite. The other optical parameters such as dielectric function, effective carrier density, scattering rate and loss function were compared for graphite and doped ones. Chapter 7 is the conclusion of the dissertation with final remarks on further studies.

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15 CHAPTER 2 CONDUCTIVITY OF CONJUGATED POLYMERS Review of Conjugated Polymers The electronic configuration of the carbon (C) ato m is: 1s 2 2s 2 2p 2 In non conjugated polymers the carbon atom has sp 3 hybridization: f our orbitals with equivalent energies which form four bonds, with the majority of electron density on the bond axes. Non conjugated polymers have only a single bond between neighbors along their backbones. So sometime s, they are called bonded or saturated polymers which have only a single bond and they are chemically stable. The energy gap is large, making non conjugated polymers electronically insulating, and generally, transparent to visible light. In conjugated polymers, sometimes called conjugated polymers carbon atoms in the backbone have s p 2 p z hybridization This hybridization gives rise to three identical bonds and one bond from the remaining p z atomic orbital The bond makes overlap with the p z orbitals of the nearest neighbor carbon atoms on the polymer backbone. Because of the overlap of the atomic p z orbitals, states are delocalized and therefore mobile along the polymer chain 1,2 This delocalization gives rise to several interesting properties of these polymers, such as high conductivity in conducting polymers (CP). The ess ential properties of conjugated polymers, which are different from conventional non conjugated polymers, are as follows: They have a relatively small electronic band gap, E g or gap of about 1 4 eV which makes them behave like semiconductors. They can be easily doped (either oxidized or reduced) usually through insertion of molecular dopant species.

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16 In the doped state, the charge carriers move almost freely along the polymer chain (intrachain interaction) ; h owever, there are weak overlaps between un paired electrons in different polymer chains This is called interchain interaction 3 The strong intrachain interactions and weak interchain interactions make these systems electronically quasi one dimensional, leading to strong coupling of the electronic state s to con formational excitations. T hese excitations are called solit o ns in degenerate ground states, and polarons and bipolarons in non degenerate ground states. For degenerate ground state polymers for example trans polyacetylene the introduction of ele ctrons and /or holes to the polymer chain creates a dom ain wall that separates regions of different bonding structure. These excitations due to the fact that the domain wall has a nonlinear configuration preserving excitation s which pr opagate freely along the polymer chain. Removing the degeneracy of the ground state energy gives rise to significant changes in both the ground state properties and in the creation of the excitations as well A result of the non degeneracy is that solitons are no longer stable excitations. Therefore upon doping the non degenerate ground state polymers, different types of excitations, or charge storage species are formed. Theses excitations are called polarons and bipolarons. According to the intrinsic low d imensional geometry of polymers, such as poly acetylene, early theoretical studies treated them as one dimensional metals, and considered equal C C bond lengths along the chain. However, according to Peierls the ground state of such a one dimensional meta l is unstable with respect to a structural distortion, which leads to the creation of alternating double and single bonds 4 As a result, a spontaneous symmetry breaking happens which opens the energy gap at the Fermi level giving the material semiconduct i ng properties 5

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17 Cha rge carriers are quasi particles, which are a combin ation of a charged particle and lattice distortion instead of free electrons or holes. High conductivity in finite size polymer samples requires a hopping mechanism between polymer cha ins, because polymer materials generally have a modest crystallinity. This also gives rise to an interesting phenomenon: the disorder induced insulator metal transitions (IMT) 6 Upon doping a huge change in conductivity ( ), as much as 10 orders of magnitude or more, can be achieved. A famous example is the conductivity of poly acetylene that can vary from 10 10 1 cm 1 (in cis (CH) n ) or 10 5 1 cm 1 (in trans (CH) n ) to 4x10 +4 1 cm 1 This is comparable with good convent ional metals for example, conductivity of copper is about 5.96X10 5 1 cm 1 After the discover y of conjugated polymers in the late 1970s by S hirakawa, MacDiarmid, and Heeger the study of these materials has become a broad field. It was the discovery that doping could increase the conductivity of polyacetylene by nearly 10 orders of magnitude that provided the initial impetus to the progress in this field. Although most of the initial research on conjugated polymers focused on the development of doped poly mer systems for use as lightweight and flexible conductors, a large portion of current research is trying to utilize their semiconducting nature. Many properties of conjugated polymers make them well suited for applications in display technologies 4 field effect transistors 5 capacitors 6 light emitting devices 7 electrochromic windows 8 and photovoltaic cells 9 An important feature, for electronic applications, is that the optical, electrical, and electrochemical properties of conjugated polymers can be tu ned through modification of the polymer structure. This capability gives rise to using conjugated polymers for several electronic applications 10

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18 Conductivity The conductivity of conducting polymers can change in different ways 1 : Doping The process of addi ng charges to or removing charges from the polymer chain is called doping. This process changes the electronic structure and the electronic response in the polymer, giving rise to several interesting and important properties Doping can be achieved in the following ways: a) Chemical doping by charge transfer Historically doping of conjugated polymer started by charge transfer redox chemistry that is the oxidation (p doping) or reduction (n doping). High doping level leads to metallic evolution of the polymer structure But non crystallinity or disorder can cause disorder induced insulator metal transition (IMT) Thus the electronic structure of doped conjugated polymers is not the same as conventional metals. b) Electrochemical doping Chemical doping is a straig htforward and efficient process, but at the same time it is difficult to control the doping. To solve this problem electrochemical doping is used: Voltage application to the electrodes provides electrons for the polymer, and for electronic charge compensat ion the counterions from the electrolyte diffuse into (or out of) the polymer. Doping level can be controlled quite well by varying the cell voltage c) Photo doping In this method, using photo absorption and charge separation th e polymer is locally oxidized by hole creation and reduced via electron creation. T his electron hole pair is then separated into free charge carriers. The number of free carriers is the result of competition between pump rate and recombination rate. The recombination (decay)

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19 of the ele ctron hole pair to the ground state can be radiative (luminescent) or non radiative. Some luminescent high quantum efficiency conducting polymers are: PPV, and PPP. d) Charge injection at a metal semiconducting polymer (MS) interface At an MS interface, elect ron can be injected to the empty band (LUMO, or conduction band) and holes can be injected to the filled band (HOMO, or valence band). Although there are no counterions, the polymer becomes oxidized or reduced. This makes this method fundamentally different from chemical and electroc hemical doping. Using this method the polymer can be used as an active material for thin film diodes, organic field effect transistors (OFET), and light emitting diodes (LEDs) e) Doping of polyaniline by acid base chemistry Polyaniline provides a distinguishe d chemically doping process: protonation by acid base chemistry that leads to internal redox reaction and evolution from emeraldine base with semiconducting characteristics to emeraldine salt with metallic properties Photo induced electron transfer Upon photo excitation of semiconducting polymers electrons can be excited to band and so the polymer is an electron donor. Combination of this polymer and a molecular electron acceptor such as C 60 and its derivatives leads to long lasting charge separation because the photo induced nonlinear excitations are stable. Conjugated polymers that have higher electron affinities cyano substituted PPV for example, act as an electron acceptor in conjunction with MEH PPV as the electron donor. This phenomenon can be u tilized for manufacturing solar cells with efficiencies compara ble with the amorphous silicon solar cells 1

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20 Disorder It has been shown experimentally that through doping a finite density of states at the Fermi level can be achieved albeit there is a high d ensity of conduction electrons in the Fermi surface in highly doped polymers. Structural disorder has been introduced as the main reason for spatial localization of conduction electron s uch that they can not transport current except via hopping. X ray exper iments have shown that these polymers are not completely crystalline. Some areas are more ordered, while other areas are less ordered 6,11,12,13 Additionally, homogeneous and inhomogeneous disorder can affect transport properties in different ways The co nducting polymer is considered as inhomogeneous if the localization length of electrons ( ) in more disordered regions is comparable to or smaller than the coherence length of the crystalline part ( ). As the disor der decreases, is increased. In this group of polymers, the reason for localization of charge carriers has been suggested to be the quasi one dimensional localization in more disordered regions surrounding the more ordered (crystalli ne) areas T he reason for quasi one dimensional localization is that the polymer length is finite, therefore the charge carrier wave function interferes with its elf ( quantum interference of static back scattering ) unless it diffuses to another chain. In a real material DC conductivity ( ) is limited by the most disordered (least conducting) part of the conduction path, therefore disorder scattering should be considered. Static disorder scattering (scattering time: ) and phonons (scattering time : ) can affect momentum ( is the Fermi wavevector ) and

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21 phonon induced delocalization (scattering time: ) can increase the conductivity wh en the temperature is increased. Real metallic system maintain a finite conductivity as Also the logarithmic temperature derivative of conductivity, has a positive slope at very low temperatures. Inhomogeneous d isorder systems show a change of slope sign for at low In Anderson disorder induced insulator metal transition (IMT) three dimensional (3D) material is considered homogeneously disordered and isotropic. A p erfect crystalline structure with a periodic potential is described by delocalized Bloch wave functions. Defects or impurities can cause significant scattering leading to localization. Anderson showed that if the random part of disorder potential is compar able to the electronic bandwidth it can lead to loc alization of the wave function : (2 1) Mobility edge ( ) is a critical energy that separates localized form extended states. Depending on the value of compared to different transport properties are predicted. If as the temperature goes to zero is finite, and has a positive slope at low s If as the temperature goes to zero the DC conductivity goes to zero as well even though the carrier density ( ) is finite, and also has a negative slope at low When a divergence of o ccurs o n the insulating side of insulating metal transition, where the electronic wave function

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22 becomes delocalized. B ecause of the strong disorder s till the mean free p ath is short According to Ioffe Regel there is a minimum metallic (interatomic distance) that takes place on the insulating side of insulating metal transition. Using this condition Mott proposed a minimum for metallic conductivity in three dimensions: (2 2) w here is the charge of the electron and In the metallic regime, extended states are responsible for charge transport therefore For typical conducting polymers such as doped polyaniline (PANI), result ing in a slowly varying ~ 10 15 s and ~10 With ~ 10 15 s and ~10 the dielectric function is positive even on the metallic side of the insulating metal transition w hile in conventional metals is a large negative number in lo w frequencies. As mentioned above, when as the temperature goes to zero the DC conductivity goes to zero as well. At higher temperatures phonon assisted hopping can take place between localized states. For strongly disordered syste ms in which disorder energy is much larger than the bandwidth Mott variable range hopping (VRH) formula gives the conductivity as: (2 3 ) where is the di mension of the transport system.

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23 Fo r a 3D sy stem : (2 4 ) w here is a proportionality constant and is Boltzmann constant. This model considers the system as a classical Fermi liquid therefore the electron correlations are n ot taken into account. Efros and Shklovskii showed that localized electron hole interactions are important in the hopping transport, specifically at low temperatures, and so the conduction depends on as: (2 5) where (2 6 ) Very close to insulating metal transition a power law dependence is predicted for the conductivity. At high frequencies for crystalline mate rials is given by the Drude formul a, (2 7 ) where (2 8 ) is the plasma frequency of the free electrons, is the total charge density and is the effective mass of the charge carrier. In 3D material and close to the Anderson insulating metal transition, quantum interference of electronic wave functions decreases

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24 the conductivity compared to This localization correction to Drude formula leads to localization modif ied Drude model (LMDM) The conductivity for this model is given by : (2 9 ) where is a universal constant and is the distance a charge diffuses during one period of electromagnetic oscillation. In this model, for a system near the insulating metal transition localized charge carriers are responsible for the high DC conductivity. A significant result of LMDM is that because of the short scattering time due to strong disorder, becomes positive at very low frequencies in contrast with the negative dielectric function: (2 10) w here is the background dielectric constant. Charge storage in conducting polymers I n c onventional 3D semiconductors, the fourfold (or sixfold, etc.) configuration of every atom to its neighboring ones via covalent bonds renders a rigid structure to the material, therefore the electron and hole are the main electronic excitations. Even thoug h s emiconducting polymers have and orbital bands which can be considered as the valence and conduction bands of a conventional semiconductor, the twofold coordination gives rise to structural distortion in the areas close to the charge The coupling between the electronic excitatio n s and the polymer chain distortion results in nonlinear excitations that are called solitons, polarons, and bipolarons.

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25 Conducting polymers are classified into two groups that show different electronic (spectroscopic) characteristics: (i) Degenerate groun d state polymers (DGSP) and (ii) non degenerate ground state polymers (NDGSP). In DGSPs for example, trans polyacetylene, doping (introduction of electron and/or hole) creates a domain wall that separates regions with different structural alternation. This domain wall can preserve its nonlinear shape and propagate freely along the polymer backbone, so it is called a soliton. Solitons can affect the lattice distortion pattern and change the symmetry of the system, giving rise to a significant change in the i nfrared active vibrational modes (IRAV). Solitons also change the electronic structure of the system creating a single bound electronic state in the middle of band. When this state is occupied by one electron, it is called a neutral soliton and has no charge and spin is . Via doping one electron is added or removed and so a negative or positive soliton can be created with no spin. Therefore there is a reve rse relation between charge and spin. In trans polyacetylene soliton domain s can extend over 14 monomer units of polymer. Theoretical calculations have shown that a charged soliton is the lowest energy configuration for an excess charge on the trans polya cetylene chain This means that E S < where E S is the creation energy of a soliton and is the creation energy of an electron or hole ( ) N eglecting e e coulomb interaction : ( ). Although a single soliton can exist on an imperfect chain, intrinsic excitations (photo or doping produced) occur in the form of bound soliton antisoliton pairs (one charged and the other neutral). These are called charged polarons. Polarons have spin and two bound states that are symmetrically located above a nd below t he center of band gap.

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26 for example, poly(thiophene) an d poly(para phenylene vinylene), solitons are not stable excitations Polarons are formed when the system is slightly and doped while bipolarons are formed when the system is heavily doped These are locally distorted charges over the extended lattice. Bipolaron has charge but no spin. It is a bound state of two charged solitons or two charged polarons with the same charges where the neutral solitons annihilate each other. They have two bands symmetrically placed on both sides of band gap center similar to pol arons but closer to the center. When the states are empty the bipolaron is positive (p type bipolaron) and when they are doubly occupied the bipolaron is negative (n type bipolaron). B ipolarons are delocalized over 6 8 monomer units of polymer backbone. Increasing the doping level gives rise to the evolution of individual bipolaron levels to bipolaronic bands. These bands originate from depletion of electronic states of valence and cond uction band edge. As a results the gap is increased. At high doping level s, bipolaron s can give rise to high conductivity upon voltage application. Most conducting polymer cannot form a negative polaron or bipolaron which is stable. T herefore in many papers only positive polarons and bipolarons have been considered 14,15,16,17

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27 CHAPTER 3 THIN FILM OPTICS In optics, we measure the result of the interaction of light as an electromagnetic wave with matter in three different categories These categories are transmission, reflection, a nd propagation. When light propagat e through one medium and then enters another medium, some part of it transmits through to the second medium while the remainder is reflect ed at the interface between two media. During propagation through the medium, some optical phenomena occur to the electromagnetic waves, such as absorption, scattering, and refraction. Absorption happens when the frequency of the electromagnetic wave is equal to the transition energy of atoms in the medium through which the light is pass ing. In this case, atoms in the medium. This causes t he intensity of the light decrease. The relation between the initial and final intensit ies, I 0 and I respectively, is given by Beer law : (3 1) where is the frequency dependent absorption coefficient Scattering is a nother source of attenuation for light propagation Scattering causes the light to propagate in all directions thus the intensity of the light in the forward direction decreases. Furthermore, a light wave travel ing through a medium has a smaller velocity than when traveling through a vacuum. The velocity varies for different materials as well Th e change of the velocity at an interface leads to refracti on, which is described by the refractive index N The frequency d ependence of refractive index of each material at the interface called dispersion. The refractive index of a material is the ratio of the

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28 velocity of light in a vacuum c = 2.998x10 8 m /s to the velocity of light in th at material, v that is (3 2) The interaction of electromagnetic waves with matter The interaction of electromagnetic radiation with a material is described by these equations are: (3 3) (3 4) (3 5) (3 6) where and , and t are the electric field and magnetic induction in the position at time respectivel y. T he total current density is and the total charge density is I n the presence of a material, electric and magnetic fields give rise to electric dipoles, magnetic moments, polarization charges and induced currents. The external electric field in an isotropic and homogeneous environment tries to align the microscopic di pole moments with it self This leads to a net macroscopic dipole moment in the medium yielding a polarization. Polarization is the net dipole moment per unit volume, and it is parallel with the applied external electric field This can be written as : (3 7)

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29 w here is the electric susceptibility of the material. The electric displacement in the medium is then given by (3 8) Equations (3 7) and ( 3 8) can be combined to give (3 9) w here , (3 10) i s the complex dielectric function. T otal current density, which is the response of a medium to an electric field is defined as (3 11) W here (3 12) is the complex conductivity of the medium. The total current density is the sum of the current densities from bound (localized) charge movement (J bound ), free charge movement ( J cond ), and freee current. This can be written as (3 13) T he relation between the complex conductivity and the complex dielectric function is given by (3 14 )

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30 where is the r eal part of the dielectric function and is the real part of the complex conductivity, also known as the optical conductivity and is the frequency In the case of is referred to as the static dielectric constant, and becomes DC electrical conductivity. T he presence of a magnetic field in an isotropic and homogeneous medium generates a magnetization The strength of the magnetization is proportional to the applied external magnetic field Further M is parallel to H. This is written as (3 15 ) where is the magnetic susceptibility of the material. The magnetic induction is related to the magnetization by (3 1 6 ) Using equations (3 15) and (3 16) we can write (3 1 7 ) W here (3 1 8 ) is the complex magnetic permeability. T he total charge density is the sum of charges induced by polarization, pol and free charges: (3 19) A ssuming w (3 20 )

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31 (3 21 ) (3 2 2 ) (3 23 ) If there are no free charges, equatio ns generate the following wave equations for electric and magnetic fields , (3 2 4 ) (3 2 5 ) Solving these equations for the specific case of plane waves in a n absorbing medium gives (3 26) where and n are extinction coefficient and refractive index, respectively. As is seen in the Eq. (3 2 6 ) t here is an exponential decay of the electric field in the medium. Considering the fact that the intensity of the light is pro portional to the square of electric field , and comparing Eq. (3 2 6 ) with Eq. (3 1 ) it is seen that (3 27 ) where is the wavelength of the light in free space. Further we can re late N, n, and by (3 28 )

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32 Light Propagation through a Single Layer Structure When light propagates from one medium with refractive index to another medium with refractive index so me of the light is reflect ed at th e interface and some is transmit ted For simplicity, we assume normal incidence. Using boundary conditions, we find that the reflectance and transmittance amplitude s are : ( 3 2 9 ) and, (3 30 ) If the light is incident from vacuum ( ) on the medium ( ) the reflectance is given by (3 31 ) Kramers Kronig relation or dispersion relation The Kra mers Kronig technique is a causality based integral relation between the real and imaginary part s of a complex function such as the dielectric function, refractive index, or optical conductivity. Consider a generic complex function (3 3 2 ) The Kramers Kronig relations are: (3 31) (3 32)

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33 w here is the principal part of the integral. Sometimes the only information available for a sample is the reflectance measurement data Using Kramers Kronig relations we can calculate the phase , of the reflectance which is seen in (3 33) If we take the logarithm of equation (3 3 4 ) or (3 35 ) To use Kramers Kronig technique we need the reflectance spectrum to cover all f requenc ies, P ractically speaking this is impossible Theref or we must use proper extrapolation s at very low and very high frequencies to ensure results that are more accurate Light Propagation through a Multi Layer Structure (Matrix Method) When light is propagating through a multi layer of absorbing media, the p roblem becomes more complicated. One of the techniques that has been used, based on some approximations, is the matrix method. Using th e boundary conditions for the m th layer, we can write: (3 35) (3 36) where and can be calculated from equations (3 27) and (3 28).

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34 The above equations can be written in a matrix form: (3 37) For a sample with n layers, determining the relat ion between and allows us to calculate the transmission coefficient. Likewise determining the relation between and leads to reflectance coefficient calculation. Micro scopic Models The classical theory of absorption and dispersion is mainly based on Lorentz and Drude models. The Lorentz model is used for insulators and it includes all direct interband transitions. The Drude model is applied to free electron metals, and it involves intraband transitions. Lorentz model In this model, the electrons in the atom are considered bound to the nucleus similar to a small mass that is bound to a big mass by a spring. The motion of a bound electron to the nucleus upon am external el ectric field is described by (3 38) Where m is the mass of the electron and e is the electronic charge. is the local field acting on the electron as the driving force. describes the energy loss of the electron due to damping that arises from various scattering mechanisms in the solids. In this model two approximations have been assumed. The nucleus mass has been considered infinite and the force has been neglect ed. This small force is coming

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35 from the interaction of the electron with the magnetic field of the electromagnetic wave. This approximation is valid because the velocity of the electron is very small compared with If the local elect ric field is considered as an oscillatory function of time the solution to equation (3 38) i s (3 39) and the resulting dipole moment is (3 40) Assuming that the displace ment is small enough there exists a linear relationship between and : (3 41) where is atomic polarizability: (3 42) Because the polarizability is a complex function, the polarization has a different phase from local electric field at all frequencies. Considering N atoms per unit volume, the macroscopic polarization is: (3 43) For simplicity we assu me so Using equation (3 8) (3 9) a nd equation (3 10) w e get

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36 (3 44) Considering atoms with more than one electrons per atom, equation (3 44) i s extended to: (3 45) (3 46) Where is the density of bound electrons with resonance frequency A corresponding quantum mechanical equation can be written (3 47) In this equation is the transition frequency of the electron between two states separated by the energy The parameter is called the oscillator strength, and indicat e s the quantum mechanical probability of a transition, and it satisfies a sum rule that is the quantum analogy to equation (3 46) Equation (3 45) is written for the time electrons are being in the vacuum, but In condensed matter the y are in an ion background so the first term is replace by Drude Lorentz model The Drude model for metals is obtained from Lorentz model by simply equating the restoring force to zero. In addition, the wave function of the free elec trons is distributed almost uniformly, so the electric field is just the average one and there is no need for correction for that.

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37 From equation (3 45) considering we get (3 48) (3 49) So equation (3 45) can be written as (3 50) This is the Drude Lorentz dielectric function model, where is the number density of free electrons, is the effective mass, an d the sum does not include term

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38 CHAPTER 4 TRANSMISSIVE/ABSORPTIVE ELECTROCHROMIC WINDOWS General Properties of Electrochromic Devices In electrochromic (EC) materials, electrochemical redox reactions (electron transfer) or ap plied external voltage lead to reversible and highly stable changes in optical characteristics (optical absorption spectra, transmission, and reflection) and therefor color 7 9, 20 22 Commercial application of these materials for transmissive/ absortptive and reflective electrochromic devices (ECD) needs achievement of certain optical, electrochemical, and mechanical properties. Low cost, solution processability, high transmittance in the bleached states along with high stability (thousands of potential cyc les), large optical contrast, relatively fast switching, color tunability through controlling the polymer structure, and the ability to perform on flexible substrates make them promising materials for various ECD applications such as buildings, providing s ignificant energy saving and color neutral smart windows for cars (rear windows, sunroofs) 2 The main focus of this project is to design of an ECD that can be installed on the windows, with the capability of color tunability, faster response time, and mo re energy saving compared with the existing technologies. The user should be able to control the visible light passing through the window, and also handle IR transmittance adjustment for heat control purposes. The most general configuration of transmissiv e/absortive window ECDs is a sandwich of two facing transparent electrodes coated (spray cast in this work) with electroactive polymers a nd separated by a thin layer of ionic al ly conductive gel electrolyte 25 (figure 4 1) The mobility of the ions determines the time re quired to switch

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39 the ECD between colored (absorptive) and bleached (transmissive) states 23 25 Black ECP (Ink Black polymer) is a donor acceptor (DA) copolymer comprising: The electron donor 3,4 propylenedioxythiophene (proDOT), and electron a cceptor 2,1,3 benzothiadiazole (BTD) Black ECP cathodically coloring (used in working electrode) has a relatively low band gap (1.5 2 eV), that makes it colored in neutral state because of strong absorption, and trasparent in oxidized state as a result of depletion of absorption ( Fig ure 4 2 .); while the counter polymer, an anodically coloring CP, has a high band gap (> 3eV, corresponding to < 410 nm), and so it is highly transparent in i ts neutral state and colored in oxidized state. As an electroactive material for the counter electrode, it supplies the charge balance of the redox reactions taking place on both electrodes and increases lifetime characteristics by suppressing degradation of the functional material. Both polymers are electrochromic (complimentarily cathodically and anodically coloring). Counter electrode has minimum change of color (MCCP) to suppress the residual absorbance in the bleached state and give rise to enhanced tr ansmittance and maximum contrast ( Fig u re 4 3 ). The transparent electrodes are made from single wall carbon nanotube s (SWNTs). The SWNTs compared to the traditional Indium tin oxide (ITO) have some advantages: They are less expensive, more tolerant toward bending on flexible substrates due to their high bending radius, and also they have a low surface resistivity, and high surface area that potentially gives rise to more contact area between nanotubes and EC polymer. Unlike ITO, whose transmittance falls of f in the NIR (near infrared) to become almost opaque for wavelengths longer than 2 m, SWNT films are effectively transparent up to 25 m. A room temperature black body emission spectrum

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40 has its maximum heat emitted at ~1000 cm 1 (10 m) that falls in the MIR (mid infrared) region of electromagnetic spectrum. High transmittance of SWNT in IR region exhibits an ideal view of SWNTs as the sel ected electrodes for this work 26 On the other hand, SWNT has a hydrophobic surface, and it does not adhere easily to m olecules of polymer, so as a solution a pyrene functionalized polyfluorene, designated Sticky PF, was used that strongly binds to SWNT side walls through Van der Waals interactions n contact with the EC polymers Sticky PF, is an alternating co polymer consisting of a fluorene unit substituted with two pyrenes and a fluorene unit substituted with two octyl chains Results and Discussion Before examining the optical properties of working ECD, for better understanding of how the properties of constituent materials affect the performance of the device, transmittance ( T ), and reflectance of different layers were measured over the whole frequency range of FIR to UV. The measurement for NIR UV was done by a Zeiss microscope photometer. Modified Perkin Elmer 16U monoch r om a t o r (1000 40000 cm 1 ) can be used instead of the microscop e. It also has the following abilities that make it advantageous for the future measurements: Working under vac uum condition, and in different temperatures. Fig ures ( 4 4 ) ( 4 9 ) shows the transmission for the layers using Polyethylene (PE) substrate. For m ore clarity the graphs for 30 to 4000cm 1 were drawn separately. The optical parameters of the materials, were calculated using Drude Lorentz model that was mentioned in the previous se ction. In the next step, T was measured for working ECD during its perf ormance. Figure s ( 4 1 0 ) and ( 4 1 3 ) and are the result of transmission measurement and data analysis.

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41 Transmission c ontrast which is defined as the difference between the the colored and bleached states transmission, is about 25 35% in the VIS range while in the NIR region it is about 60%, and in the MIR range it is ~ 40%. B y increasing the voltage from negative to positive the band gap at about 16000 cm 1 is depleted, and two polaron bands appear at around 6000 and 10,000 cm 1 With further increase of voltage the polymer goes to highly doped state in which polaron and completely disappear and bipolarons are formed at about 4000 cm 1 Conclusion The transmissive/ absorptive electrochrom ic devices in this study are transparent in a broader electromagnetic frequency range due to use of single walled carbon nanotube as the transparent conductive electrodes. Previously ind ium tin oxide were used as the electrodes. The devices show a significant transmission change between the colored and bleached state, while the change is not very high in the mid infrared and visible range. From the transmission graphs of the device at dif ferent voltages the evolution of the absorption bands from to polarons and then bipolarons can be observed.

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42 F ig ure 4 1 ECD general scheme Fig ure 4 2 Black ECP (Ink Black polymer ) donor accep tor (DA) copolymer comprising: The electron donor 3,4 propylenedioxythiophene (proDOT), and electron acceptor 2,1,3 benzothiadiazole (BTD) A fter reference 25. Fig ure 4 3 MCCP: Minimally C o lor changing polymer N alkyl substituted poly (3,4 propylenedioxypyrrole) N O O

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43 Fig ure 4 4 Transmission of polyethylene substrate Fig ure 4 5 Tr ansmission of single wall nanotube on polyethylene substrate

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44 Fig ure 4 6 Transmission of sticky PF on polyethylene substrate Fig ure 4 7 Transmission of MCCP/Sticky PF/SWNT/PE.

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45 Fig ure 4 8 Transmission of BP/PE. Fig ure 4 9 Transmis sion of Black ECP/Sticky PF/ SWNT /PE.

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46 Fig ure 4 1 0 Transmission of the working device at different voltages in far infrared Fig ure 4 1 1 Transmission of the working device at different voltages in mid infrared

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47 Fig ure 4 1 2 Transmission of the wor king device at different voltages in near infrared and visible ranges Fig ure 4 1 3 Transmission of working device at different voltages in the whole frequency range

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48 CHAPTER 5 ELECTRICAL CONDUCTIVITY IN POLYANILINE THIN FILMS Electrical conductivity of CPs improves as the degree of chain extension and chain alignment is increased. Within conducting polymers, polyaniline (PANI) has been studied intensively due to its ease of preparation and environmental stability. It is unique among CPs in that its c onductivity can be reversibly controlled, either electrochemically (by oxidation / reduction) or chemically (by protonation / deprotonating). Applications of PANI include electrostatic dissipation, anticorrosion coatings, actives delivery, batteries, and s olar control. Synthesis of polyaniline is commonly performed by chemical oxidative polymerization in an aqueous solution. Material synthesized by this approach is predominately amorphous, intractable, and insoluble in most organic solvents 12, 17 Recently a new polymerization method has been developed 18 which yields a truly soluble, conducting emeraldine salt directly without the need for a postdoping process step. The produced polyaniline has a high molecular weight (>22000) and a moderate conductivity (10 5 1 cm 1 ) and exhibits high solubility in low dielectric constant solvents. The conductivity of thin films of the polymer was enhanced up to 5 orders of magnitude by treatment with quaternary ammonium salts or solvents such as methanol or acetone. The focus of this experiment was to measure electrical conductivity of recently made polyanilin e sample by Crosslink Company. The temperature dependent (33 300 K) reflectance ( R ) measurements were done utilizing unpolarized light at near normal incidence, with a Br uker 113V Fourier Transform interferometer, accompanied by a liquid helium cooled silicon bolometer detector over the frequency spectral range 30 700 cm 1 and from 700 5000 cm 1 with an ambient DTGS detector. Fig ure (5 1 ) shows the result of reflection m easurement versus frequency (wavenumber) at different

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49 temperatures. As shown in the graph, reflection is lower at low frequencies, compared with higher frequencies, which does not exhibit the traditional signature of conventional metals. In thin metal film s (compared to their skin depth), not necessarily a 100% increases as frequency decreases. At each temperature there is a maximum at around 1000 cm 1 (10 m). As the temperature goes down from room temperature to 33 K, reflection decreases by about 10%. The sharp dips are phonon modes, which are not discussed in detail in this work, because we were interested to see how the conductivity varies. It should be noticed that this is not the bulk reflectance of the polymer; it contains the contribution of the glass layer on which the polyaniline has been spray coated. Indeed the reflection of polymer can be influenced by interference and absorption effects. In ord er to extract the optical properties of the polymer thin film, the Drude Lorentz model for multilayered systems should be used to fit the reflectance data. This model has the following form for a single layer 19 : w here and are complex dielectric function and high frequency limit permittivity, and are the oscill ator strength, center frequency and line width of the th damped oscillator respectively and are the Drude contributions in dielectric function. Complex dielectric function is related to complex refractive index through this equation:

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50 and using we can calculate and Therefore we can find different properties such as optical conductivity. For a multi layer structure, the matrix method can be used: For a system with n layers the amplitude and the phase of the reflected light can be calculated from the Fresnel coefficients at each interface and the thicknesses of the films, by simply repeating this process until all the layers have been taken into account (Matrix method). To calculate the conductivity, the thi ckness of the sample was determined to be about 320 nm, using a step height analytic digital AFM. Figures (5 2) (5 5) show the results of the fit of the reflectance data at different temperatures to Drude Lorentz model. The experimental data are in blue co lor and the fits are in red. The scale of 300 K (FIR & MIR) is different from the other temperatures (FIR), and Fig ures (5 6) (5 9) are the calculated correspondent optical conductivities (real part: 1 ) at those temperatures. It should be noted that there are no significant unusual changes in the spectra at different Ts such as presence of new modes or the splitting of existing modes so the same center frequencies were considered for different Ts, and the other parameters were free to change. Again the maximum at ~1000 cm 1 can be seen, correspondent to the maximum at R. The DC conductivity is the Drude contribution of that is calculated by extrapolation of when DC conductivity was also measured using 4 probe technique, in a liquid Helium dewar in the temperature range of 10 to 300 K. Fig ure (5 10) shows the data for both optical calculation and 4 probe measurement (expe riment done in Professor Biswas

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51 lab). As it is seen, there is a very good agreement between two measurements. As T is increased, DC is increased, and its maximum is about 245 1 cm 1 at room temperature. C onclusion For the poly aniline samples, h igh conductivities in the range of 200 to 300 1 cm 1 were achieved, and the results of DC and optical measurements are consistent with each other very well. The result of fitting the data with Variable range hopping model gives rise to d = 2, which means the transport system i n these samples are two dimensional

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52 Fig ure 5 1. Polyaniline reflection, versus frequency at different temperatures using Bruker 113V spectrometer. Fig ure 5 2 R versus frequency at 300 K, blue: Experimental data, and the red: fit

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53 Fig ure 5 3 R ve rsus frequency at 200 K, blue : Experimental data and the red : fit. Figure 5 4 R ver sus frequency at 100 K, blue : Experimental data, and the red: fit

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54 Fig ure 5 5 R versus frequency at 33 K, blue: Experimental data and the red: fit. Fig ure 5 6 Optic al conductivity versus frequency, at 300K

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55 Fig ure 5 7 Optical conductivity versus frequency, at 200 K. Fig ure 5 8 Optical conductivity versus frequency, at 1 00 K

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56 Fig ure 5 9 Optical conductivity versus frequency at 33 K. Fig ure 5 10 Compariso n of optical and DC conductivity

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57 CHAPTER 6 OPTICAL CONDUCTIVITY OF DOPED GRAPHITE Highly anisotropic layered materials in which the interplanar binding forces are small compared to the intraplanar binding forces can form intercalation compounds. This a nisotropy permits the impurity atoms or molecules to be inserted into the space between the layers. The atomic or molecular impurities are called intercalants. Two very well known examples of host materials for intercalation are graphite and transition met al dichalcogenides. A c rystalline graphite lattice consists of almost parallel layers of graphene sheets. This high degree of structural ordering makes it very interesting in terms of its electrical, thermal, magnetic and mechani cal properties. The distance between the layers at room temperature is d 0 = 3.3538 . In every layer, the carbon atoms are arranged in a hexagonal structure with the distance between each atom and its nearest neighbor s being d=1.415 The inter planar distance is almost 2.5 t imes the distance between the nearest neighbor atoms therefore a single layer of graphite can be considered as an independent structural unit of graphe n e lattice. The weak bonding between the layers causes high anisotro py in the properties of graphite N atural gra phite has many imperfections and defects. Several techniques have been developed to manufacture almost perfect graphite 29 The most convenient and effective method is the pyrolysis of hydrocarbons. Pyrolytic graphite is produced by the chemical vapor depositi on of the hydrocarbon gas at very high temper atures (>2500 K) with in a vacuum furnace. Annealing at about 3300 K combined with the application of high pressure leads to highly oriented pyrolytic graphite (HOPG) which is very pure anisotropic and has a density near its theoretical

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58 value of 2.265 g/cm 3 Real HOPG consists of layered poly crystals. Each poly crystal looks like a mosaic of very small mono crystal grains each having a different size The order of angular spread in the vertical axis (c direction) is 1 degree. T he perfect ion of HOPG is determined by the mosaic spread of the grains This is simply a measure of how parallel the grains are The lower the mosaic spread, the closer HOPG is to perfect Graphite intercalated compounds (GIC s ) can be classified by the number of graphene layers betwe en adjacent intercalants. This number is called the stage index n and it is one of the most important characteristics of GICs. The graphite layers adjacent to the intercalant layer are called graphite bounding layers while other graphite layers are ca lled interior graphite layers The bounding layers are more significantly affected by the intercalation process than the interior layers Electronically pure pristine graphite is a compensated semimetal in which the concentration of electrons and holes are equal. The c oncentration of free carriers is very small being roughly ~10 4 free carrier per atom at room temperature but intercalation with different impurities and at different concentrations leads to a significant variation in the free carrier concent ration This leads to high variability in the electrical, thermal and magnetic properties of the compound with the greatest attention go ing to the electrical conductivity in the plane of the graphite layers. Some GICs can even form s uperconductors C 8 K, and C 4 HgK for example As a result of intercalation electri cal charge is transferred between the graphite layers and the intercalant layer. Depending on the characteristics of this redistribution GIC s can be divided into two main groups: donor type GIC s and acceptor type GIC s

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59 In donor type GIC s the valence electrons of atom s in the intercalant layer become delocalized This results in a drastic increase in the density of free electrons within the graph ite layers T hus t he intercalant acts as a donor provid ing electrons to graphite layers. Donor type GI C s can be prepared using alkali, alkaline earth, and rare earth metals. Some examples of donor type GIC s are C 6 Li, C 8 K, C 8 Rb, C 8 Cs, C 6 Ca, C 6 Ba, C 6 Sr, and C 6 Eu. In acceptor type GIC s electron s are draw n from the graphite layers which then become localized with in the intercalate layers. The free holes which are then created in the carbon layers have a much higher density when compared to their density in undoped graphite. Thus the intercalant acts as an acceptor for th e electrons from the graphite layers Acceptor type GIC s include compounds with inorganic acids, and halogens Electrical Conductivity of GIC in the Basal Plane Two characteristics of the electrical conductivity of GICs are important First is the high in plane conductivity ab Second is the large anisotropy of the in plane to the c axis conductivity Often the latter is expressed as the ratio ab / c Both are affected by intercalation. Pure graphite has a low carrier density (~ 7 x10 18 cm 3 ) and high carrier mobility ~ 1 x 10 4 cm 2 /Vs, at room temperature As a re sult of intercalation the charge density in graphite bounding layers can be increase d by about 2 orders of magnitude while the mobility is only decreased by less than one order of magnitude T he net effect is an increase in the electrical conductivity of graphite layers The enhanced charge density in the se graphite layers decreases rapidly as the distance from the intercalant layer increases The screening length is about the thickness of a single layer of

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60 graphite Contrary to the enhanced current density of the graphite bounding layers the current density in the graphite interior layers remains very low. Further the intercalant layer ha s a small conductivity due to its low free carrier density a nd low carrier mobility Here the assumption is that the Fermi level is between the valence and conduction band s of the intercalant material Therefore the main part of the basal plane conductivity is associated with the bouding graphite layers. The exce ptions to this are alkali metal compounds. In these donor compounds the charge transfer from the intercalant layer to the graphite layer is high However greater coupling between the intercalant layer and graphite layer leads to more charge carrier s being scatter ed thus yielding a lower mobilit y 29 Experimental Method The samples used in this work have a thickness of about 1 2 mm while the surfaces of their ab plane s are approximately 6 mm by 6 mm Bromine intercalation of the samples was performed at room temperature using the following steps T he graphite slice was placed in a small glass petri dish which was nested inside a slightly larger petri dish L iquid Bromine purchased from Fisher Scientific was poured in to a small ceramic dish sitting adjacen t to the graphite sample The l arger petri dish was closed and sealed by para film to make sure that sample is exposed to a high concentration of bromine gas Figure 6 1 shows a picture of this set up following the application of the para film. All steps were performed under a fume hood. I ntercalation time s of 20 minutes and 100 minutes of bromine gas exposure were used The coupling s between individual graphene layers with in the graphite sample are weak enough to permit bromine molecules to separate these layers and sit between them. Application of a liquid intercalant is an easy method for intercalation but it doe s not give rise to a well

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61 staged compound T his was confirmed for our samples via x ray measurement taken by Tongay et al T he molecular intercalants are usually formed as Br though they sometimes appear as (Br 3 ) Also, t here will be expansion i n the c direction of the sample due to the intercalants 30 The reflectance R ( ) of the ab plane of pristine HOPG and the bromine doped samples was measured at near normal incidence using unpolarized light over far infrared (FIR) and mid infrared (MIR) ranges of electromagnetic spe ctrum and at different temperatures from10 to 300 K using a Bruker IFS 113v infrared Fourier transform spectrometer over 40 to 6000 cm 1 (5 750 meV). A Helium cooled bolometer and a room temperature DTGS detector were used in FIR and MIR frequencies respec tively. A room temperature measurement was carried out up to the ultraviolet (UV) using a Zeiss MPM 800 micro spectrophotometer This measurement covered the freque ncy range of 5000 35000 cm 1 or a photon energy range of 0.6 4.4 eV Measurements at low temperature s (from 10 to 300 K) were performed by mounting the sample on a copper sample holder attached to the tip of a continuous flow helium cryostat. L iquid helium was carried from a helium storage tank to the cryostat by a flexible transfer line. Combined application of the liquid helium flow and a silicon diode h eat er affixed to the cryostat allows the temperature to be ad justed with a precision of +/ 5 degrees of Kelvin. The reflectance of an a luminum mirror was measured as a reference spectrum, covering identical frequency ranges and temperatures Because of the errors associated with beam alignment in the spectrometer and temperature dependent drifts

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62 in the transfer line, sample holder, and the spectrometer itself, the following factors are used to correct the measurements : F 1 = (sample spectrum at T / sample spectrum at 300 K) F 2 = (Al mirror spectrum at 300 K / Al mirror spectrum at T) F 3 = (sample spectrum at 300 K / Al mirror spectrum a t 300 K) (with no tran sfer l ine) Now, F = F 1 X F 2 X F 3 To correct for the mirror imperfect reflectance this value was multiplied by F 4 = measu red reflectance data for a luminum taken from the literature there by correct ing for systematic errors associated with the surface of the reference mirror. Therefore R = F 1 X F 2 X F 3 X F 4 Reflectance data are typically measured to within 0.5 % to 1 % accuracy R eflectance data in the FIR w ere corrected to within ~ 1% by comparing it to the reflectance calculat ed at very low frequencies using the Drude model and DC con d uctivity. After calculat ing t he corrected reflectance in both the FIR and MIR regions, the absolute data in the MIR are merged wit h the data in the F IR C onsidering the fact that MIR detectors usually are nonlinear and the absolute value data is slightly higher or lower than the FIR data. The optical parameters such as the complex dielectric function, optical conductivity and effect ive number of charge carrier w ere calculated by Kramers Kronig (KK) analysis In KK method, e xtrapolation of the data at low frequenc ies uses the DC conductivity of the sample and the Drude formula. For the high frequency limit x ray data was used 27, 28 that includes the data from 80,660 cm 1 (10 eV) to 241,980,000 cm 1

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63 (30,000 eV). To get the reflectance data the formula of the compound and volume per formula unit are required. For pure HOPG, 20, and 100 minute brominated HOPG, the values 8.81, 9.81, an d 12.05 A 3 were used respectively. Above the X ray range, as the electrons can be considered as completely free the data are extrapolated as R 4 Results and D iscussion Figure 6 3 shows the near normal incidence reflectance of the HOPG, 20, and 100 minute doped HOPG at ro om temperature. T he reflectance is very high in far infrared region that is the indication of metallic behavior of the samples and so high optical conductivity computed by K ramers K ronig method. Intercalation leads to a significant increas e of the reflectance in mostly MIR region, because FIR reflectance is already almost 1. For HOPG, a t low frequencies the structure attributed to the E 1u IR active phon on mode is observed at ~ 1589 cm 1 In brominated samples this feature develops to a Fano shape kink at the same frequency with a higher reflectance as the bromination time increases. At higher frequencies there is a gradual decrease in HOPG reflectance due to mainly interband transitions. The temperature dependent reflectance measurements for these samples are shown in figure s 6 4, 6 5, and 6 6. Optical parameters such as real part of complex optical conductivity, real part of complex dielectric function, effective carrier density, and scattering rate have been calculated and shown in figures 6 7 to 6 17 for all three samples at all measured temperatures.

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64 The real pa r t of optical conductivity is large and high a t low frequencies. Most of the Drude contribution is at frequencies below the low frequency limit although it show s itself in the real part of dielectric function. Highly negative 1 is consistent with the metallic behavior of the samples and it is observed in all the samples and at all the temperatures Because and from the graphs it can be seen that does not depen d on the temperature and so is not strongly temperature dependent for each sample. At the low frequency ~ 30 cm 1 the values of are as follow : At all temperatures, f or the HOPG sample for the 20 minute doped HOPG and for the 100 minute doped sample. As the time of doping increases the dielectric function becomes more negative (almost threefold!) at very low frequencies, implying a higher value for the plasma frequency and therefore a higher free carrier density in the sample. The s cattering rate ( 1 ) is very small and it shows almost one order of magnitude reduction as the temperature decreases, which is consistent with the simple metallic behavior of materials in which gets longer at low temperatures 1 show s an almost quadratic behavior in HOPG sample that is the characteristics of Fermi liquid materials. However, for the doped samples, it is linearly proportional to frequency ignoring the oscillatio ns due to the small noise in dat a in low frequencies. This change of behavior needs further theoretical investigation.

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65 Conclusion The study of bromine doped graphite shows that the carrier density of the s amples are almost constant with temperature. The density inc reases by increasing the doping time The scattering time or mean free time increases as the temperature decreases. The whole analysis confirms that these samples have simple metallic characteristics

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66 F ig ure 6 1 T he picture of the set up for preparation of brominated graphite Fig ure 6 2 Reflect ance of HOPG 20 and 100 minute doped graphite at room temperature.

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67 F ig ure 6 3 T emperature dependence of the reflectance for the HOPG sample Fig ure 6 4 R eflec tance for the HOPG sample at 2 extreme temperatures 10 & 300 K : experimental data and the calculated reflectance from the Drude Lorentz model

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68 F igure 6 5 T emperature dependence of the reflectance for the 20 minute brominated HOPG sample Fig ure 6 6 R eflec tance for 20 minute brominated HOPG sample at 2 extreme temperatures 10 & 300 K : experimental data and the calculated reflectance from the Drude Lorentz model

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69 F ig ure 6 7 T emperature dependence of the reflectance for the 100 minute brominated HOPG sample F ig ure 6 8 R eflectance for the 100 minute brominated HOPG sample at 2 extreme temperatures 10 & 300 K : experimental data and the calculated reflectance from the Drude Lorentz model

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70 Fig ure 6 9 T emperature dependence of the optical conductivity for the HOPG sample F ig ure 6 1 0 T emperature dependence of the optical conductivity for the 20 minute brominated HOPG sample

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71 F ig ure 6 1 1 T emperature dependence of the optical conductivity for the 100 minute brominated HOPG sample Fig ure 6 1 2 T emperature dependence of the real part of dielectric function of the HOPG at 10 to 300 K

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72 Fig ure 6 1 3 T emperature dep endence of the real part of dielectric function of the 20 minute brominated HOPG at 10 to 300 K Fig ure 6 1 4 T emperature dependence of the real part of dielectric function of the 100 minute brominated HOPG at 10 to 300 K

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73 F ig ure 6 1 5 T emperature dependence of the scattering rate of the HOPG at 10 to 300 K F igure 6 1 6 T emperature dependence of the scattering rate of the 20 minute brominated HOPG at 10 to 300 K

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74 F igure 6 1 7 T emperature dependence of the scattering rate of the 100 minute brominated HOPG at 10 to 300 K F ig ure 6 1 8 T emperature dependence of the effective carrier density for the HOPG at 10 to 300 K

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75 F ig ure 6 19 T emperature dependence of the effective carrier density for the 20 minute brominated HOPG at 10 to 300 K F ig ure 6 2 0 T emperature dependence of the effe ctive carrier density for the 100 minute brominated HOPG at 10 to 300 K

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76 F ig ure 6 2 1 T emperature dependence of the loss function for the HOPG at 10 to 300 K F igure 6 2 2 T emperature dependence of the loss function for the 20 minute brominated HOPG at 10 to 300 K

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77 F igure 6 2 3 T emperature dependence of the l oss function for the 100 minute brominated HOPG at 10 to 300 K

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78 CHAPTER 7 CONCLUSION The optical characterization of two distinct groups of materials were measured in this study. The transmission measurements of transmissive/absorptive electrochromic devices in the far infrared, mid infrared, near infrared and visible ranges of electromagnetic spectrum, and ambient condition were done. The results show that the transmission c ontrast in VIS range is about 25 35 %, in NIR region is about 60%, and in MIR it is about 40%. Therefore for the applications in the visible and mid infrared the device needs to be improved. For further studies it is suggested to m easure the spec t ra in different angles. Furthermore the effect of the thic k ness of active polymers and SWNT on the performance of the device are interesting to be investigated. The second group of materials that were studied, are pristine highly oriented pyrolytic graphite (HOPG) and bromine doped HOPG. These samples were exposed to bromine gas at different periods of time. The reflection of the samples were measured at different temperatures. The result show that these sample have simple metallic characteristics. The scattering rate is almost independent of the tempe rature and it increases by increasing the doping time of the samples. The carrier density of the samples are temperature independent and it increases by increasing the doping time as well.

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79 LIST OF REFERENCES 1 A. J. Heeger, Synthetic Metals 125, 23 (2002 ). 2 Maria Nikolou, Ph.D. thesis, University of Florida, (2005). 3 Jungseek Hwang, Ph.D. thesis, University of Florida, (2001). 4 Peierls, Quantum Theory of Solids (Clarendon Press, Oxford, 1955). 5 A. O. Patil, A. J. Heeger, and F. Wudl, Chem. Rev. 8 8, 183 (1988). 6 R. S. Kohlman and A. J. Epstein. Physical Properties of Polymers Handbook II Eds., J. E. Mark (AIP Press, Woodbury, New York, 1996). 7 Avni Argun, Ph.D. thesis, University of Florida (2004). 8 Irina Schwendeman, Ph.D. thesis, University of Florida (2001). 9 I. Schwendeman, J. Hwang, D. M. Welsh, D. B. Tanner, and J. R. Reynolds, Adv. Mater. 13, 634 (2001). 10 N. C. Heston, Ph.D. thesis, University of Florida, (2009) 11 V. N. Prigodin and A. J. Epstein, Synthetic Metals 125, 43 (2002). 1 2 R. S. Kohlman, A. Zibold, D. B. Tanner, G. G. Ihas, T. Ishiguro, Y. G. Min, A. G. Mac Diarmid, and A. J. Epstein, Phys. Rev. Lett. 78, 3915 (1997). 13 Handbook of Conducting Polymers; Vol.1,2, edited by T.A. Skotheim, R.L. Elsenbaumer, and J.R. Reynold s (Marcel Dekker, Inc., New York, 1998) 14 J. L. Bredas and G. B. Street, Acc. Chem. Res. 18, 309 (1985) 15 A. J. Heeger, S. Kivelson, J. R. Schrieffer, W. P. Su, Rev. Mod. Phys. 60, No. 3, 781 16 A. J. Heeger, R. Pething, Phil. Trans. R. Soc. Lon. A 19 85, 314, 17 35 17 V. N. Prigodin and A. J. Epstein, Synthetic Metals 125, 43 (2002). 18 P.J. Kinlen, J. Liu, Y. Ding, C. R. Graham, and E. E. Remsen, Macromolecules 31, 1735 (1998). 19 F. Wooten, Optical properties of solids (Academic Press, New York, 1 972). 20 S. A. Sapp, G. A. Sotzing, J. L. Reddinger, J. R. Reynolds, Adv. Mater. 8, 808 (1996).

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80 21 I. Schewendeman, R. Hickman, K. Zong, D. M. Welsh, J. R. Reynolds, J. Hwang, and D. B. Tanner, Polymer Materials: Science & Engineering 86, 55 (2002). 22 S. A. Sapp, G. A. Sotzing, J. R. Reynolds, Chem. Mater. 10, 2101 (1998). 23 P. M. Beaujuge, S. Ellinger, and J. R. Reynolds, Nature Materials 7 795 (2008). 24 S. V. Vasilyeva, E. Unur, R. M. Walczak, E. P. Donoghue, A. G. Rinzler, and J. R. Reynolds, ACS App lied Materials & Interfaces 1, 2288 (2009). 25 to Transmissive Window Type Polymer Babiarz, V. W. Ballarotto J. R. Reynolds, (2011); P. Shi, C. M. Amb, E. P. Knott, E. J. Thompson, D. Y. Liu, J. Mei, A. L. Dyer, J. R. Reynolds, Adv. Mater. 22, 4949 4953 (2010). 26 Z. Wu, Z. Chen, X. Du, J. M. Logan, J. Sippel, M. Nikolou, K. Kamars, J. R. Reynolds, D. B. Tanner, A. F. Hebard, A. G. R inzler, Science 305, 1273 (2004). 27 http://henke.lbl.gov/optical_constants/ 28 ray Interactions: Photoabsor ption, Scattering, Transmission and Reflectio n at E=50 30,000 eV, Z=1 Atomic Data and Nuclear Data t ables 54 181 342 (1993) 29 M. S. Dresselhaus and G. Dresselhaus, Adv. Phys. 51 1 (2002) 30 S. Tongay, J. Hwang, D. B. Tanner, H. K. Pal, D. Maslov, and A. F. Hebard, Phys. Rev. B 81, 115428 (201 0)

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81 BIOGRAPHICAL SKETCH Zahra Nasrollahi came to UF in fall 2007. She enjoyed taking courses at the Physics department on different subjects with several professors : Maslov, Monkhorst, Khandker Muttalib, Chris Stanton Peter H irschfeld and Richard Woodard oratory in summer 2008. She worked on optical characterization of graphite, doped graphite and trasmissive/absorptive electrochromic devices. She received her PhD from the University of Florida in May 201 4