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 t;he 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 sigma(omega) and the real part of the dielectric function epsilon(1)(omega) each show different frequency dependences in the far infrared. In the metallic regime sigma(omega)proportional to omega(1/2) for ($) over bar h omega<600 cm(-1) and epsilon(1)(omega)(> 0) increases rapidly as omega-->0, as described by the ''localization-modified Drude model,'' leading to the conclusion that metallic polypyrrole is a disordered metal near the M-I transition. In contrast, the insulating regime is characterized as a Fermi glass as confirmed by sigma(omega) proportional to omega(2) for ($) over bar h omega<600 cm(-1).