Chemical trend of Fermi-level shift in transition metal-doped TiO ₂ films

The effect of doping a wide range of transition metals (TMs) including V, Cr, Mn, Fe, Co, and Ni into rutile TiO₂ films grown on Nb-doped TiO₂(110) single-crystal substrates was investigated by photoemission spectroscopy and X-ray absorption spectroscopy. For all TM-doped TiO ₂ films, the Ti 2p and O 1s core levels were similarly shifted to lower binding energies with increasing film thickness and the shifts were similarly saturated at a film thickness of about 15 nm. These peak shifts could be interpreted in terms of the Fermi level shift, indicating that dopants trap excess electrons from the Ti 3d band like acceptors. The present systematic data revealed that the magnitude of the saturated Fermi level shift correlates well with the effective charge transfer energy.