Reflectance of conducting polyaniline near the metal-insulator transition: disorder induced localization

Abstract

Summary form only given. We present the reflectance spectra of conducting polyaniline film protonated with sulfuric acid (PANI-H/sub 2/SO/sub 4/) and of newly processed film protonated with camphor sulfonic acid (PANI-CSA) over a wide spectral range (0.006 eV-6 eV) at room temperature. The reflectance of PANI-H/sub 2/SO/sub 4/reveals the signatures of the intraband excitations in the infrared (IR), including increasing reflectance approaching to far-IR and a reflectance minimum around 1.7 eV. However, the corresponding optical conductivity (/spl sigma/(/spl omega/)) and the real part of dielectric function (/spl epsiv//sub 1/(/spl omega/)) are far from those of a metal. Instead, the optical spectra indicate that the charge carriers are localized with considerable oscillator strength within the gap. On the other hand, the optical spectra of PANI- CSA are characterized by distinct metal-like signatures arising from the combination of the Drude-like free carrier excitations in the far-IR and the existence of plasma resonance around 1.2 eV. These results indicate that the new processing of polyaniline with functionalized sulfonic acid (with surfactants as counterions) has greatly improved the homogeneity of the material and has resulted in a significant decrease in the extent of molecular scale disorder, thereby leading to the delocalization of the charge carriers in PANI-CSA. However, the deviation of /spl sigma/(/spl omega/) and /spl epsiv//sub 1/(/spl omega/) from the normal Drude metallic behaviors below 0.2 eV indicates that disorder induced localization still plays an important role in PANI-CSA. We present a quantitative analysis of /spl sigma/(/spl omega/) and /spl spl epsiv//sub 1/(/spl omega/) of PANI-CSA in terms of the localization modified Drude model, implying that PANI-CSA is a disordered metal on the metal-insulator boundary.