Reflectance of conducting polypyrrole: Observation of the metal-insulator transition driven by disorder

By controlling the extent of disorder through electrochemical synthesis at reduced temperatures, conducting polypyrrole (PPy) can be obtained in the metallic regime, in the insulating regime, and in the critical regime of the disorder-induced metal-insulator (M-I) transition. We present the results of reflectance measurements (0.002–6 eV) of PPy carried out at room temperature on the metallic side and on the insulating side of the M-I transition. While the reflectance spectra obtained from samples on both sides of the M-I transition exhibit spectral features expected for a partially filled conduction band, the electronic states near the Fermi energy (E_F) are different in the two regimes. The data obtained from metallic samples indicate delocalized electronic wave functions in the conduction band, whereas the spectral features which characterize the insulating regime indicate that the states near E_F are localized. Consistent with theoretical predictions for the metallic and insulating regimes, the optical conductivity σ(ω) and the real part of the dielectric function ɛ₁(ω) each show different frequency dependences in the far infrared. In the metallic regime σ(ω)∝ω^1/2 for ħω<600 cm^-1 and ɛ₁(ω) (>0) increases rapidly as ω→0, as described by the ‘‘localization-modified Drude model,’’ leading to the conclusion that metallic polypyrrole is a disordered metal near the M-I transition. In contrast, the insulating regime is characterized as a Fermi glass as confirmed by σ(ω)∝ω² for ħω<600 cm^-1.