Infrared Reflectance Studies of Conducting Polymers

Recent progress in the synthesis and processing of conducting polymers, including polyaniline protonated with camphor sulfonic acid (PANI-CSA) and polypyrrole doped with PF_6 (PPy-PF_6 ), has greatly improved the homogeneity of the material and significantly reduced the degree of structural disorder. These structurally improved materials provide opportunities to study the metal-insulator (M-I) transition and the intrinsic metallic state of conducting polymers. This dissertation studies the optical reflectance spectra of such materials, yielding a wealth of significant information on the metal physics in the infrared (IR), the electronic structure over a wide energy range, and the nature of the M-I transition in heavily doped conducting polymers. In such a case, a quantitative analysis of spectra are given in terms of the localization modified Drude model, leading to the conclusion that heavily doped conducting polymers are disordered metal near the M-I boundary. The optical spectra of structurally improved materials are compared with those obtained from conventionally prepared materials. While the structurally improved materials show distinct metallic signatures indicative of delocalized electronic states at the Fermi level, the optical spectra of the conventional samples indicate that the states near the Fermi level are localized due to severe disorder in the context of Anderson localization. Thus, conventional samples can be characterized as a Fermi glass. In this case, the two categories of samples show different charge dynamics in the far-IR, consistent with theoretical predictions. The improved sample quality provides information on the electronic structure of metallic conducting polymers, allowing a comparison of the spectral features with band calculation results. The data indicate that heavily doped conducting polymers have an electronic structure of a metal with a partially-filled conduction band. The spectral features are in full agreement with the polaron lattice band model. Finally, the possibility of new many-body phenomena, which would be screened in the conventional materials due to severe disorder, has been explored by low temperature measurements on structurally improved materials, especially on metallic PPy(PF_6).