REAL-TIME SPECTROSCOPIC ELLIPSOMETRY - INSITU CHARACTERIZATION OF PYRROLE ELECTROPOLYMERIZATION

A rapid scanning spectroscopic ellipsometer with optical multichannel detection has been employed for in situ monitoring and analysis of pyrrole electropolymerization on a gold electrode. Spectra were collected over the energy range of 1.5-3.3 eV as a function of time during chronoamperometric deposition (0.6 V vs. SCE) from an aqueous solution of KNO3, and the film growth was monitored to a thickness of approximately 500 angstrom. From the final thick film spectra, both dielectric function and thickness of the semi-transparent polypyrrole are accurately obtained by utilizing an imposed constraint such that spurious artifacts in the calculated polypyrrole optical functions arising from the underlying gold optical response are minimized. A Lorentz harmonic oscillator analysis of the resulting dielectric function reveals an interband electronic transition at 3.8 eV and band-to-defect transitions at 1.65 and 2.3 eV. These values are discussed within the framework of conventional bipolaron models and compared with previous optical and electrochemical studies. A high doping level is indicated for this material in comparison to perchlorate-doped polypyrrole prepared in organic solvent. Using the bulk polypyrrole optical functions, the spectra obtained as a function of time are analyzed by a linear regression algorithm in order to characterize the evolution of film thickness and structure. The algorithm identifies the best fit between experimental and simulated spectra, and the latter are calculated using a variety of possible multilayer models which include features such as film voids and interface roughness. In this way, we deduce regimes of (i) monolayer adsorption and Au interface reordering in the first 10 angstrom, (ii) two dimensional nucleation and then coalescence, which occurs at a roughly constant film thickness of 150 angstrom, and (iii) gradual densification, followed by layer-by-layer growth at a constant rate after 350 angstrom.