The influence of the electric field and electric current on the behavior of oxygen vacancies $\left({\mathrm{V}}_{\mathrm{O}}\mathrm{s}\right)$ in ${\mathrm{TiO}}_2$ anatase was investigated with scanning tunneling microscopy (STM). At the anatase (101) surface ${\mathrm{V}}_{\mathrm{O}}\mathrm{s}$ 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 $\approx +4.3$ V results in surface ${\mathrm{V}}_{\mathrm{O}}\mathrm{s}$ in the scanned area, suggesting that subsurface ${\mathrm{V}}_{\mathrm{O}}\mathrm{s}$ migrate back to the surface. To test this hypothesis, surface ${\mathrm{V}}_{\mathrm{O}}\mathrm{s}$ were first created through bombardment with energetic electrons. The sample was then mildly annealed, which caused the ${\mathrm{V}}_{\mathrm{O}}\mathrm{s}$ to move to the subsurface region, where they formed vacancy clusters. These ${\mathrm{V}}_{\mathrm{O}}$ clusters have various, distinct shapes. Scanning ${\mathrm{V}}_{\mathrm{O}}$ clusters with a high STM bias reproducibly converts them back into groupings of surface ${\mathrm{V}}_{\mathrm{O}}$, with a configuration that is characteristic for each type of cluster. The dependence of the subsurface-to-surface ${\mathrm{V}}_{\mathrm{O}}$ 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.

• Received 2 March 2015

DOI:https://doi.org/10.1103/PhysRevB.91.195403