The influence of the electric field and electric current on the behavior of oxygen vacancies (VOs) in TiO2 anatase was investigated with scanning tunneling microscopy (STM). At the anatase (101) surface VOs are not stable; they migrate into the bulk at temperatures above 200 K. Scanning a clean anatase (101) surface at a sample bias greater than ~+4.3 V results in surface VOs in the scanned area, suggesting that subsurface VOs migrate back to the surface. To test this hypothesis, surface VOs were first created through bombardment with energetic electrons. The sample was then mildly annealed, which caused the VOs to move to the subsurface region, where they formed vacancy clusters. These VO clusters have various, distinct shapes. Scanning VO clusters with a high STM bias reproducibly converts them back into groupings of surface VO, with a configuration that is characteristic for each type of cluster. The dependence of the subsurface-to-surface VO migration on the applied STM bias voltage, tunneling current, and sample temperature was investigated systematically. The results point towards a key role of energetic, "hot" electrons in this process. The findings are closely related to the memristive behavior of oxides and oxygen diffusion in solid-oxide membranes.
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Corresponding author: Ulrike Diebold (diebold).