Defect states in WO3studied with photoelectron spectroscopy

The (001) surface of tungsten trioxide has been examined with ultraviolet photoemission spectroscopy. Results are presented for the defect-free surface using photon energies of 16.85, 21.22, and 40.82 eV and compared with x-ray photoemission spectra and a bulk density of states calculated for cubic W${\mathrm{O}}_3$ by Kopp et al. The experimental results show no evidence for the presence of intrinsic surface states in the energy of bulk W${\mathrm{O}}_3$. The calculated density of states can be seen to have both a valence-bandwidth and an energy gap which are too small. Electron irradiation and ion bombardment produced defects in the surface region of W${\mathrm{O}}_3$ which appeared as an increase in the density of states in the energy gap and a shift of the valence band away from the Fermi energy. The increase of the density of states in the band gap can be explained by the creation of oxygen vacancies and the valence-band shift by a band-bending effect. However, an alternative explanation that the shift is due to the creation of a layer of disordered W${\mathrm{O}}_3$ at the surface cannot be ruled out. Five distinct surfaces with different electronic structure were observed and are discussed.