Structure, defects, and impurities at the rutile TiO2(011)-(2 x 1) surface:: A scanning tunneling microscopy study

The titanium dioxide rutile (011) (equivalent to (101)) surface reconstructs to a stable (2 x 1) structure upon sputtering and annealing in ultrahigh vacuum. A previously proposed model (T.J. Beck, A. Klust, M. Batzill, U. Diebold, C. Di Valentin, A. Selloni, Phys. Rev. Lett. 93 (2004) 036104/1) containing onefold coordinated oxygen atoms (titanyl groups, Ti=O) is supported by Scanning Tunneling Microscopy (STM) measurements. These Ti=O sites are imaged bright in empty-states STM. A few percent of these terminal oxygen atoms are missing at vacuum-annealed surfaces of bulk-reduced samples. These O vacancies are imaged as dark spots. Their number density depends on the reduction state of the bulk. Double vacancies are the most commonly observed defect configuration; single vacancies and vacancies involving several O atoms are present as well. Formation of oxygen vacancies can be suppressed by annealing a sputtered surface first in vacuum and then in oxygen; annealing a sputtered surface in oxygen results in surface restructuring and a (3 x 1) phase. Anti-phase domain boundaries in the (2 x 1) structure are active adsorption sites. Segregation of calcium impurities from the bulk results in an ordered overlayer that exhibits domains with a centered (2 x 1) periodicity in STM. (c) 2006 Elsevier B.V. All rights reserved.