Polaron absorption in amorphous tungsten oxide films

Amorphous thin films of tungsten oxide were deposited by sputtering onto gl ass substrates covered by conductive indium-tin oxide. The density and stoi chiometry were determined by Rutherford backscattering spectrometry. Lithiu m ions were intercalated electrochemically into the films. The optical refl ectance and transmittance were measured in the wavelength range from 0.3 to 2.5 mum, at a number of intercalation levels. The polaron absorption peak becomes more symmetric and shifts to higher energies until an intercalation level of 0.25 to 0.3 Li+/W, where a saturation occurs. The shape of the po laron peak is in very good agreement with the theory of Bryksin [Fiz. Tverd . Tela 24, 1110 (1982)]. Within this model, the shift of the absorption pea k is interpreted as an increase in the Fermi level of the material as more Li ions are inserted. (C) 2001 American Institute of Physics.