ELECTRONIC-STRUCTURE OF POLYPYRROLE FILMS

Chemical and electronic properties of polypyrrole films are investigated for different doping levels using x-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy, and high-resolution electron-energy-loss spectroscopy. The XPS results give evidence for instabilities in perchlorate-doped polypyrrole films, whereas tosylate-doped samples are chemically stable. The valence-band structure of the latter is in line with the theoretically expected features of this one-dimensional polymer. Upon doping, we observe occupied states and electronic transitions in the gap. At low dopant concentrations (4%) these states are polaronic. At around 10% doping they cause a semiconductor-metal transition. The finite density of states at the Fermi energy is in accordance with polaronic bands modulated by Coulomb repulsion, as well as with a disorder-induced destabilization of the Peierls ground state.