Optical reflectance studies of conducting polymers on the metal-insulator boundary

Abstract

Recent progress in the processing of polyaniline protonated with camphor sulfonic acid (PANI-CSA) and polypyrrole doped with PFg (PPy-PF 6) has provided opportunities to investigate the nature of the metallic state and associated metal-insulator (M-I) transition in conducting polymers. We present the reflectance spectra of such materials over a wide spectral range from 0.002 eV to 6eV, yielding a wealth of information on the electronic states near the Fermi Energy (E F) and the electronic structure over a wide energy range. A quantitative analysis of spectra in terms of the localization modified Drude model leads to the conclusion that heavily doped conducting polymers are disordered metals near the M-I boundary. The optical spectra of such 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 E F, the optical spectra of the conventional materials indicate that the states near E F are localized due to severe disorder in the context of Anderson localization ( Fermi glass). The two categories of samples show different charge dynamics in the far-infrared, consistent with theoretical predictions.