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A Colloidal Nanoparticle Form of Indium Tin Oxide

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A Colloidal Nanoparticle Form of Indium Tin Oxide A COLLOIDAL NANOPARTICLE FORM OF INDIUM TIN OXIDE: SYSTEM DEVELOPMENT AND CHARACTERIZATION A Dissertation Presented to The Academic Faculty by Richard Allen Gilstrap Jr. In Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy in Materials Science and Engineering Georgia Institute of Technology May 2009 Copyright © Gilstrap 2009 A COLLOIDAL NANOPARTICLE FORM OF INDIUM TIN OXIDE: SYSTEM DEVELOPMENT AND CHARACTERIZATION Approved by: Dr. Christopher J. Summers, Chairman Dr. Rina Tannenbaum School of Materials Science and School of Materials Science and Engineering Engineering Georgia Institute of Technology Georgia Institute of Technology Dr. Ian T. Ferguson Dr. Brent K. Wagner School of Electrical and Computer Georgia Tech Research Institute Engineering Georgia Institute of Technology Georgia Institute of Technology Dr. Hamid Garmestani School of Materials Science and Engineering Georgia Institute of Technology Date Approved: March 5, 2009 This work is dedicated to my late parents, Richard and Ann Gilstrap. ii ACKNOWLEDGEMENTS I would like to express my sincere gratitude to my thesis advisor, Dr. Christopher J. Summers, for his guidance and support throughout the course of my research. His innate curiosity of the physical world around us has been a source of inspiration. I also wish to thank my committee members, Drs. Ian T. Ferguson, Hamid Garmestani, Rina Tannenbaum, and Brent K. Wagner. Each has offered me invaluable guidance of both a theoretical and often, personal nature. Additional thanks to Drs. Meilin Liu, Zhong Lin Wang, and Thomas H. Sanders Jr. for the many discussions about specific topics and the general path of research work. A wealth of assistance were offered me by Drs. Hisham Menkara, Wusheng Tong, and Wounjhang Park over the last several years. The ability to work along side these seasoned scientists during my course of graduate study was indeed a rare opportunity. A special acknowledgement goes to Phillip Graham for the countless discussions about all things materials science. On that note, I would also like to thank the wide variety of Georgia Tech students who have contributed to my educational and research experience. Special thanks to Susan Bowman for calling in a variety of favors on my behalf. Lastly, I must express my most sincere and deep thanks to Tatiana Toteva for her unwavering support, encouragement, and inspiration. iii TABLE OF CONTENTS ACKNOWLEDGEMENTS...………………………………….…………………………iii LIST OF TABLES ……………………………………………………………………... vii LIST OF FIGURES ………………………………………………………….…………viii SUMMARY……………………………………………………………………...………xv CHAPTER 1: INTRODUCTION ………………………………………………………...1 CHAPTER 2: BACKGROUND ……………………………………………………….... 6 2.1 Introduction …………………………………………………………………..… 6 2.2 Transparent Conductivity………………………………………………………..6 2.3 Electron Degeneracy in Indium Tin Oxide …………………………………… 14 2.4 Analysis of Free Electron Concentration in ITO………………..…..…....…....18 2.5 Optical Properties of ITO ………………………………………………...……19 2.5.1 Influence of Light on Electrons.………………………….……………21 2.5.2 Drude Theory Approach to the ITO Reflectance Edge ………………..23 2.5.3 Transparent Region ……..………………………………………..….... 25 2.5.4 Reflective Region……………………………………………………... 26 2.5.5 UV Absorption Edge – Apparent Band Gap of ITO………….………. 28 2.5.6 The Burstein-Moss Shift ……………………………………………….29 2.5.7 Optical Effects of the Semiconductor-to-Metal Transition ……………34 2.5.8 Many-Body Interactions and Band Gap Narrowing …………………..36 2.6 Potential Influence of Quantum Confinement …………..………………..…...36 iv 2.6.1 Fundamental Quantum Confinement Theory …………………………40 2.6.2 Confinement in Colloidal Semiconductors – EMA Theory…………...43 CHAPTER 3: ANALYSIS METHODS …………………………………………….…..48 3.1 Introduction ……………………………………………………………………48 3.2 Fourier Transform Infrared Spectroscopy………..……………………………48 3.3 Inductively Coupled Plasma Mass Spectrometry ……………………………..50 3.4 Thermogravic Analysis ………………………………………………………..51 3.5 X-ray Diffraction Analysis ……………………………………………………51 3.6 Electron Microscopy …………………………………………………………..52 3.7 Four-Point Probe ………………………………………………………………52 3.8 Optical Spectroscopy ………………………………………………………….53 CHAPTER 4: PROCESS DEVELOPMENT AND SYSTEM PROPERTIES …………55 4.1 Introduction ……………………………………………………………………55 4.2 Fundamental Theory of Nucleation and Growth ……………………………..55 4.3 Influence of Impurity Doping …………………………………………………58 4.4 Preparation of Colloidal Semiconductor Nanoparticles ………………………59 4.5 Synthesis of Colloidal Indium Oxide Nanoparticles ………………………….62 4.6 Synthesis of Colloidal Indium Tin Oxide Nanoparticles…………..…………..65 4.7 Verification of Indium Tin Oxide Synthesis………………………..………….70 4.7.1 Solution Body Color………………………………………..………….70 4.7.2 Composition and Structure …………………………………..………..73 4.7.3 Particle Morphology……………………………………..…………….76 4.7.4 Surface Ligand Character………………………..…………………….79 v 4.7.5 Optical Properties………………………………………..…….……….81 4.8 Summary… ….…………………………………………………..……….……83 CHAPTER 5: INTRINSIC PROPERTY ANALYSIS ……………………..……..…….84 5.1 Introduction ………………………………………………………...………….84 5.2 Colloidal ITO Particle Formation ……….…………………………..….……..84 5.3 Analysis of Conduction Band Filling……………………………….…………88 5.3.1 Analysis Assuming No Quantum Confinement ………………………94 5.3.2 Analysis Including Quantum Confinement……………………………98 5.4 Origin of Excess Electrons in Colloidal ITO………………...………….……103 5.5 Composition-Structure-Property Relations…………………...………………106 5.5.1 Composition-Structure Relations……………………….…………….110 5.5.2 Composition-Property Relations…………………………….………..115 5.6 Summary……………………………………………………………………...123 CHAPTER 6: BATCH-SCALE PROCESSING OF COLLOIDAL ITO ……………..124 6.1 Introduction…………………………………………………………………...124 6.2 Non-Injection Synthesis Route to Colloidal ITO……………………………..125 6.3 Specific Procedures and Reaction Analysis…………………………………..126 6.4 Basic Properties of the Batch-Scale System………………………………….129 6.5 Electrical Analysis of Pressed Pellets…….……………………….………….131 6.6 Summary……………………………………………………………………...136 CHAPTER 7: CONCLUSIONS ……………………………………………………….137 REFERENCES ………………………………………………………………………...142 vi LIST OF TABLES Table 2.1 Families of transparent conductive oxides. 1……….…….….…………….12 Table 2.2 Typical electrical and optical properties of ITO (10% at. Sn) thin films….19 Table 2.3 Density of states for a semiconductor with 3, 2, 1, and 0 degrees of Freedom for the propagation of electrons in its lattice. 2……….………….38 Table 2.4 Roots of the Bessel function χ nl .…………………………………………..45 Table 5.1 Composition and structural properties of colloidal ITO prepared with different initial Sn concentrations. All other process parameters have been held constant………………………………………………………...107 vii LIST OF FIGURES Figure 2.1 Simplified band diagram of an n-type degenerate semiconductor (top) and its associated impurity potentials (bottom). The distance between the electron donor level and conduction band minimum is characterized by the activation energy Ed in electron 2 volts. Degenerate doping has pulled the Fermi level Ef towards the conduction band and created overlap in the impurity potentials (bottom)………...……………………………..9 Figure 2.2 Effects of impurity quenching in an n-type semiconductor. Impurity pairs and/or higher order clusters may serve to limit free electron activity within the host lattice. The lower plot shows a relationship between carrier concentration and impurity dopant concentration often observed in such systems. 2………...…………………...…...13 Figure 2.3 Schematic drawing of the C-type rare-earth (bixbyite) In 2O3 structure. The gray spheres represent oxygen atoms while white spheres denote regular lattice oxygen vacancies which are inherent to the bixbyite structure. Indium atoms are displayed as black spheres………..………………………………………………………...………….15 Figure 2.4 Variation of carrier concentration with the Sn content of sputter targets for two different ITO thin film materials. 3 The trend follows the concentration quenching model previously described in Figure 2.2 for a general case. A similar relationship between Sn content and estimated carrier concentration will be presented for the colloidal ITO developed in this work……………………………………………………………...17 Figure 2.5 Transmission spectra of a typical ITO thin film sample. The material is characterized by an optically transparent window spanning the entire visible range and much of the infrared…………………………………………………..………………….20 Figure 2.6 Reflectivity of an undamped free electron gas as a function of incident light frequency ω and electron plasma ensemble frequency ω p . This behavior in a transparent conductor such as ITO leads to a long wavelength reflection edge at ω ≤ ω p ………....27 Figure 2.7 (a) Schematic band structure of a degenerate semiconductor (at ground state) typified by parabolic conduction and valence bands with band gap E go . (b) Extrinsic donor doping has caused the apparent band gap to expand due to the Burstein- Moss effect. (c) Many body interactions beyond the semiconductor-to-metal transition cause a renormalization effect that partially offsets the expansion………….…………..33 Figure 2.8 Donor impurity level and donor impurity band at low, medium and high doping concentrations for (a) an ordered impurity distribution and (b) a random impurity distribution. 2………………………………………………………………...……………35 viii Figure 2.9 Graphical depiction for the density of states in a semiconductor with 3, 2, 1, and 0 degrees of freedom for the propagation of electrons in its lattice. The colloidal ITO produced in the present work should be represented by the 0-D case with density of states displaying
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