Optical generation of free charge carriers in nanocrystalline tin oxide for gas sensor application

Nanocrystalline films of tin oxides SnO2 with nanocrystal sizes of about 100 nm and thickness of 600 nm prepared by thermal oxidation of metallic tin layers were investigated by using the infrared (IR) spectroscopy, Raman scattering and current-voltage measurements in different molecular environments. An increase of the absorption in the middle IR spectral range was detected at room temperature after illumination of the samples by UV light with photon energy of 3.3 eV, which was below the band gap of SnO2. The effect was explained as an evidence of the absorption by non-equilibrium free charge carriers (electrons) in SnO2 nanocrystals. The concentration of photo-generated free charge carriers was calculated from the IR-spectroscopy data according to the Drude model and it was up to 10(18) cm(-3). An effect of the molecular adsorption of iodine on the electrical conductivity of the films was investigated by current-voltage measurements, which revealed a strong increase of the sensor response for nanocrystalline SnO2 under the UV excitation at room temperature. Possible mechanisms of the observed photo-generation of free charge carriers in SnO2 nanocrystals and feasible applications of this effect in gas sensors are discussed. (C) 2011 Elsevier B.V. All rights reserved.