Evidence for the tunneling site on transition-metal oxides: TiO₂(110)

We present atomically resolved scanning tunneling microscopy (STM) images from TiO₂(110) surfaces. After annealing nearly perfect stoichiometric 1 × 1 surfaces to elevated temperatures in ultrahigh vacuum, randomly distributed oxygen vacancies are observed. The apparent shape of these defects provides strong evidence that the STM is imaging undercoordinated Ti atoms, as do first-principles pseudopotential calculations of the electronic states accessible to tunneling. We thus resolve a controversy as to whether STM imaging on this surface is dominated by geometric-structure or electronic effects.