  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.
