Polaron absorption in tungsten oxide nanoparticle aggregates

Small tungsten oxide nanoparticles were prepared from a heated tungsten filament, by gas evaporation in air at pressures between 1 and 9 Torr. Transmission electron microscopy showed that the median particle diameter increased with pressure during evaporation from 7 nm at 1 Torr to 21 nm at 9 Torr. X-ray diffraction measurements were in agreement with data for the monoclinic and triclinic structures of tungsten trioxide. Composition analysis by Rutherford backscattering spectrometry gave evidence for a substoichiometric composition of WO2.7 to WO2.8. Optical reflectance and transmittance of samples deposited on sapphire substrates were measured in the wavelength range of 0.2-10 mum. From these data the absorption coefficient of the particulate samples was obtained. A strong absorption peak in the near infrared was ascribed to polaron absorption. The peak wavelength consistently shifted towards longer wavelengths with increasing evaporation pressure. This is probably a particle size effect and may be connected to an observed bandgap widening with decreasing particle size. (C) 2001 Elsevier Science Ltd. All rights reserved.