Reflectance spectra of polyaniline

We summarize the results of a reflectance study of polyaniline (PANI) in the semiconducting form (emeraldine base) and in the ‘metallic’ form (protonated with camphor sulfonic acid (PANI-CSA) or with sulfuric acid (PANI-H2SO4) measured over a wide spectral range (0.006–6 eV). The emeraldine base spectrum is typical of a semiconductor with an energy gap of 1.3 eV, while both emeraldine salts exhibit spectral features expected for a partially filled conduction band. The reflectance spectra of PANI-CSA and PANI-H2SO4 are analyzed and compared in detail. The PANI-CSA reflectance exhibits metallic signatures arising from the free carriers in the partially filled conduction band, whereas for PANI-H2SO4 the data indicate that the states near the Fermi energy (EF) are localized. This localization arises from the severe disorder present in PANI-H2SO4 (Anderson localization). For PANI-CSA, however, the counterion-induced processability results in significant improvement in material quality, leading to more metallic behavior with extended electronic states near EF. Thus, PANI-CSA is a disordered metal on the metal-insulator boundary, in contrast with PANI-H2SO4 which is characterized as a Fermi glass. The improved sample quality allows a comparison of the spectral features with those expected from the results of band calculations. We find that the optical spectra for PANI-CSA are in full agreement with the polaron lattice band model of the electronic structure.