Doping-Induced Conductivity Transitions in Molecular Layers of Polyaniline: Optical Studies of Electronic State Changes

The doping-induced conductivity transitions in molecular layers of polyaniline have been studied by monitoring the correlated optical and spectroscopic changes using spectroscopic and single wavelength extinction ellipsometry, also in total internal reflection mode (TIRE), together with reflection spectrometry. The measurements were performed on deposited multilayers as well as on a Langmuir monolayer at the air-water interface, as a function of acidic doping. We found that the characteristic spectroscopic features of conducting and insulating polyaniline persisted down to the single layer, both in the solid state and at the air-water interface. We also investigated in real time the modulation of conductivity induced by the intercalation of Li ions in the polyaniline film, by a combination of time-resolved ellipsometry and reflectivity spectra measurements. In this case, the enhanced sensitivity provided by the TIRE geometry, combined with the relatively fast time scale accessible by the single wavelength ellipsometry, allowed us to follow in detail in real time the doping/dedoping process.