THE NATURE OF ELECTRONIC STATES IN CONDUCTING POLYMER NANO-NETWORKS DISSERTATION Presented in Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy in the Graduate School of The Ohio State University By Oludurotimi O. Adetunji, B.S. * * * * * The Ohio State University 2008 Dissertation Committee: Approved by Professor Arthur J. Epstein, Adviser Professor Micheal Poirier Professor Jay Gupta Adviser Graduate Program in Physics Professor Ciriyam Jayaprakash ABSTRACT The nature of the electronic states of charge carriers and the origin of metal to insulating (MI) transition, in highly interconnected conducting polymer nanostructure network, notably Polyaniline nanofibers (PAN-N), were determined via temperature dependent DC conductivity, optical reflectance (300 cm-1 – 50,000 cm-1), electron paramagnetic resonance (EPR) and X-ray diffraction probes. The nanostructured network has a room temperature (RT) conductivity of ~ 1 S/cm, similar to that of conventional disordered Polyaniline in the Emeraldine Salt (ES) form, but this value is small when compared to the Mott minimum for metallic conductivity (~ 100 S/cm). Therefore, the signature of the temperature dependent DC conductivity (s dc (T ) ) of PAN-N should be dominated by phonon activation, with very little or no metallic contribution. In fact, the signature of the charge transport of PAN-N films shows a large “metallic” contribution from RT to ~ 235 K, with the change in s dc (T ) from RT to the peak of the maximum conductivity ~ 200%. We attribute this large metallic contribution to a “mechanically-induced” crossover from metallic to insulating behavior, due to the “fragile” nature of the conductance at interfiber interfaces. By mechanically modifying the nanostructure morphology via applied pressure, the ii signature of the charge transport resembles that of conventional disordered Polyaniline, having a broad MI peak and moderate change in conductivity( < 15% ) from RT to the peak conductivity. The reduced energy activation W[º d lns dc (T )/ d lnT ] of PAN-N has a negative slope at low temperatures, which suggest that the charge carriers are localized by disorder. Similarly, the dielectric functions for all measured temperature reveal that the charge carriers within the network are localized. EPR measurements show a temperature dependent Pauli susceptibility between 300 K and 130 K, and below 130 K, we see the onset of a Curie-like contribution to the magnetic susceptibility. The nanostructured network has a low magnetic susceptibility, dominated by a weak Curie component, with the density of Curie spins of ~ 1 spin per 200 (2-ring) repeat unit. This suggests that a significant fraction of the spins are paired up as bipolarons, implying that most of the charge carriers are localized. Structural studies indicate that the nanostructure films are ~ 50 % crystalline with a coherence length of ~ 2 nm. This coherence length is similar to the values reported earlier for conventional disordered Polyaniline with higher conductivity. This suggests that the nature of conductance within the interfiber interfaces affects the effective conductivity of the network. The data of other charge dynamics including optical, magnetic, and structural probes suggest that the role of interfiber contacts within the network contributes largely iii to the “metallic” signature of thes dc ()T . We conclude that the MI behavior is due to the “fragile” nature of the conductance at the nanostructure interface, while disorder and localization dominate the charge dynamics. iv Dedicated to my wife and son, Olajumoke and OgoOluwa; and my parents, James and Oladunni v ACKNOWLEDGMENTS I would like to express my sincere gratitude to my advisor, Professor Arthur J. Epstein for his advice, guidance, constant encouragement and invaluable discussions during the course of my research. My appreciation also goes to Dr. Ronald J. Tonucci of the Naval Research Laboratory (NRL) for welcoming me into his lab as an intern and for the great insights I learned from performing research in his laboratory. I also would like to thank Dr. Nan-Rong Chiou for providing high quality nanostructured Polyaniline samples and without whom none of this work would have been possible. I would also like to thank Professor Jay Gupta, Professor Micheal Poirier and Professor Nandini Trivedi for sharing their valuable time as committee members. I gratefully acknowledge the discussions with Dr. V.N. Prigodin, whose theoretical insight was used in the analysis of this study. I am greatly indebted to Dr. Youngmin Kim, Dr. Jung Woo Yoo and Dr. Raju Nandyala for helping with the early stage of my research. My thanks also to the past and current members of Epstein’s laboratory- Drs. Fang Chi Hsu, June-Hyoung Park, Jeremy D. Bergeson, Dr. Derek Lincoln, Mr. Travis S. Steinke, Mr. Yurri Bataiev, and Mr. Louis Nemzer, Mr. Austin Carter, Ms. Chia-Yi Chen, Ms. K. Deniz Duman, Mr. Jesse Martin and Mr. Mark Murphey. I would also like to thank Ms. Jenny Finnell for helping with administrative issues. vi The support of the Department of Physics during my time at the Ohio State University has been greatly appreciated along with the financial support of the National Science Foundation (NSF) through the IGERT program and the NSEC program. Finally, I would like to extend a special thanks to my wife, Olajumoke (Queen), our son Ogo-Oluwa, my parents and parents-in-law, and siblings for their loving support and encouragement with which this research can be accomplished. I also thank my friends Emmanuel Olawale and Oluwayomi Fasalojo and their families for their love and prayers throughout my graduate career. vii VITA August 14 1977……..........................................Born Ilorin, Nigeria 2002……………………………………........... B.Sc. Fisk University, Nashville, Tennessee. 2002-2004……………………………………. Graduate Teaching Associate Department of Physics The Ohio State University, Columbus, Ohio. June-December, 2006………………………….Physical Scientist II Naval Research Laboratory, Washington, District of Columbia. 2004-2008 ……………………………………. NSF-IGERT Fellow NSF-NSEC-Polymer Biomed Fellow The Ohio State University PUBLICATIONS Research Publications O. O. Adetunji, U. N. Roy, Y. Cui, J. –O. Ndap, and A. Burger, “Growth of Cr and Co doped CdSe crystals from high temperature selenium solutions,” Journal of Electronic Materials 31 7 (2002). U. N. Roy, B Mekonen, O. O. Adetunji, K. Chattopadhyay, F. Kochari, Y. Cui, A. Burger, and J. T. Goldstein, “ Compositional variations and phase stability during horizonatal Bridgman growth of AgGaTe2 crystals,” Journal of Crystal Growth 241 135 (2002). viii J.-O. Ndap, C. I. Rablau, K. Morrow, O. O Adetunji, V.A. Johnson, K. Chattopadhyay, R. H. Page, and A. Burger, “ Infrared spectroscopy of chromium-doped cadmium selenide,” Journal of Electronic Materials 31 802 (2002). O. O. Adetunji, U. N. Roy, A. Burger, “Study of Comparison between Chromium and Cobalt as a doping efficiency in CdSe”, published in National Conference of Undergraduate Research Proceedings, NCUR proceedings, July 2001. J.-O. Ndap, K. Chattopadhyay, O. O. Adetunji , D. E. Zelmon, and A. Burger “Study of thermal diffusion of Cr2+ in bulk ZnSe,” Journal of Crystal Growth 240 176 (2002). J-O. Ndap, O. O. Adetunji, K. Chattopadhyay, C. I. Rablau, S. U. Egarievwe, X. Ma, S. Morgan and A. Burger, “High-temperature solution growth of Cr 2+: CdSe for tunable mid-IR laser application” Journal of crystal growth 211 290 (2000). S. U. Egarievwe, H. Chen, K. Chattopadhay, J.-O. Ndap, X. Ma, O. O. Adetunji, T. McMillan, A. Burger, and R. B. James, "Study of Au/CdZnTe/CdS m-i-n Detectors Fabricated by Sputtering Technique," Proceedings of SPIE 3768 3768 (1999). FIELD OF STUDY Major Field: Physics · Experimental Condensed Matter Physics · Optical Studies of Nanostructure Conducting Polymer · Electrical and Transport Studies on Nanostructured Conducting Polymers · Electron Paramagnetic Resonance ix TABLE OF CONTENTS Page Abstract................................................................................................................... ii Dedication................................................................................................................v Acknowledgments ................................................................................................. vi Vita....................................................................................................................... viii List of Figures...................................................................................................... xiii List of Tables .................................................................................................... xviii Chapters: 1 Introduction.......................................................................................................1 2. Theoretical Background and Methodology ......................................................7 2.1. Response Functions ......................................................................................7 2.1.1 Drude Model.......................................................................................8 2.1.2 The Lorentz Oscillator Model..........................................................12 2.1.3 Propagation in a medium .................................................................15 2.1.4 Propagation through an interface.....................................................18 2.1.5 Kramer-Kronig Dispersion Relations ..............................................20 2.1.6 The Sum Rule .................................................................................23