Nanoscale Dimensional Changes and Optical Properties of Polyaniline Measured by In Situ Spectroscopic Ellipsometry

The thickness and optical properties of polyaniline films deposited on gold substrates by electropolymerization were determined using in situ spectroscopic ellipsometry. The polymer films were deposited by potential cycling in 0.1 M aniline/ 1 M . For the simultaneous determination of optical properties and thickness, two approaches were used: ( i ) The evolution of the ellipsometric parameters Δ and Ψ at a constant wavelength was evaluated assuming a homogeneous layer growth resulting in a parabolic growth curve with a film thickness of ca. 65 nm (reduced state) after 40 cycles. ( ii ) The ellipsometric spectra at various potentials were evaluated using the Arwin and Aspnes (AA) approach, minimizing spectral features in the calculated optical properties of the film originating from the Au substrate by variation of the film thickness. The two approaches gave identical results for the reduced film which can be described as dielectric for photon energies in the range 1.5 to 3.0 eV. The volume changes during oxidation and subsequent reduction exhibit a significant hysteresis. Using the Arwin and Aspnes approach we were able to follow the film thickness as a function of electrode potential monitoring swelling and shrinking of the film during oxidation and reduction. A thickness increase of roughly 30%, from 65 to 85 nm, was observed during oxidation of the film. Using the thicknesses, the optical properties were calculated in the range 1.5 to 4.5 eV. In the reduced state, one absorption band is observed at ca. 4.0 eV. In the oxidized state, two main absorption bands are present, one below 1.5 eV and another at ca. 3 eV. The evolution of the spectra with potential is discussed.