The Metallic State of Conducting Polymers: Microwave Dielectric Response and Optical Conductivity
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A.J. EPSTEIN*, J.JOO*, R.S. KOHLMAN*; A.G. MACDIARMID**, J.M. WEISINGER**, Y. MIN**; J.P. POUGET**; AND J. TSUKAMOTO**** *The Ohio State University, Department of Physics, Columbus, OH 43210-1106 "**Universityof Pennsylvania, Department of Chemistry, Philadelphia, PA 19104-6323 ***Universit6 Paris Sud, Laboratoire de Physique des Solides (CNRS-URA2), 91405 Orsay, France ****Toray Industries, Specialty Polymers Laboratory, Shiga 520 Japan
ABSTRACT Recent advances in processing of polyaniline and polyacetylene have resulted in a new generation of conducting polymers with higher dc conductivities. We present the temperature (T) dependent microwave frequency dielectric constant, dc conductivity, and Kramers-Kronig analysis of conducting polyaniline and polyacetylene. The low temperature dielectric constant, F, increases with the square of the x-ray crystalline domain length for preparations of HC1 protonated polyaniline. For the highest crystalline polyaniline samples, c increases dramatically with increasing T, supporting formation of three-dimensional (3-D) coupled "mesoscopic" metallic regions. A "metallic" negative c is observed for d,1-camphor sulfonic acid doped polyaniline prepared in m-cresol. Optical studies show a linear increase in reflectivity below 7000 cm-1. Below 600 cm-1 the reflectance increases rapidly. Kramers-Kronig analysis of the ir-visible results are presented. Highly conducting polyaniline is shown to have two plasma frequencies, one at - 1.1 eV involving all the conduction band electrons, and one at -0.015 eV (120 cm- 1 ) that is suggested to arise from the most delocalized electrons. The concept of inhomogeneous disorder is introduced. The results for polyaniline are compared to those of highly doped polyacetylene which also show metallic negative e demonstrating the intrinsic metallic nature of the new generation of conducting polymers. INTRODUCTION The metallic state of conducting polymers has been widely investigated for the past fifteen years[l] since the report of doped polyacetylene in 1977 [2]. While there have been many studies of electrical conductivity as a function of sample preparation and temperature variation, there are very few studies which examine the conductivity and dielectric response as a function of frequency. Using a combination of temperature dependent dcconductivity, temperature dependent microwave frequency (6.5 x 109 Hz) conductivity and dielectric constant, and conductivity and dielectric constant derived from Kramers-Kronig analysis of reflectance data from 50 cm-1 through to -3.1 eV, a systematic picture emerges of the evolution of the frequency dependent conductivity and dielectric response as a function of improving sample quality. For both the polyaniline and polyacetylene systems we find an evolution from localized charge transport (localized dielectric), to a mesoscopic metallic state, to a 3-dimensional metal with improved processing conditions. We report that for the metallic samples there are two plasma frequencies observed, one at -1.1 eV for polyanilin
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