The source of the Ti 3d defect state in the band gap of rutile titania (110) surfaces

The origin of the Ti 3d defect state seen in the band gap for reduced rutile TiO ₂(110) surfaces has been excitingly debated. The probable candidates are bridging O vacancies (V _O) and Ti interstitials (Ti-int) condensed near the surfaces. The aim of this study is to give insights into the source of the gap state via photoelectron spectroscopy combined with ion scattering and elastic recoil detection analyses. We have made three important findings: (i) The intensity of the gap state observed is well correlated with the sheet resistance measured with a 4-point probe, inversely proportional to the density of Ti-int. (ii) Sputterannealing cycles in ultrahigh vacuum (UHV) lead to efficient V _O creation and condensation of Ti-int near the surface, while only annealing below 870 K in UHV condenses subsurface Ti-int but does not create V _O significantly. (iii) The electronic charge to heal a V _O is almost twice that to create an O adatom adsorbed on the 5-fold Ti row. The results obtained here indicate that both the V _O and Ti-interstitials condensed near the surface region contribute to the gap state and the contribution to the gap state from the Ti-int becomes comparable to that from V _O for the substrates with low sheet resistance less than ∼200 .