REAL-TIME SPECTROSCOPIC ELLIPSOMETRY STUDY OF THE ELECTROCHEMICAL DEPOSITION OF POLYPYRROLE THIN-FILMS

The first real-time spectroscopic ellipsometry study of an electropolymerization process is reported. The deposition of nitrate-doped polypyrrole on gold from an aqueous solution of KNO3 has been characterized. Ellipsometric spectra, obtained as a function of time at a fixed potential, provide the bulk dielectric function from 1.5 eV to 3.3 eV and the evolution of the density and microstructure during the preparation of films up to approximately 500-angstrom thick. The polypyrrole dielectric function obtained at 0.6 V (with respect to a saturated calomel electrode), on harmonic oscillator analysis, reveals an interband electronic transition at 3.8 eV and band-to-defect transitions near 1.65 eV and 2.3 eV. The latter have been interpreted within conventional models of dopant-induced bipolaron states in the band gap. Dielectric functions obtained as the potential is reduced show that the relative doping level can be assessed from real-time measurements. A detailed analysis of the ellipsometric spectra vs. time during deposition at 0.6 V suggests that, in the initial 150-angstrom, the film is discontinuous, having originated from a low density of nuclei. Coalescence occurs at a roughly constant thickness of 150-angstrom. For thicker films, the thickness increases linearly with time as the film density gradually increases.