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Oxygen molecules adsorbed on TiO2
change their state easily under the influence of electric field.
Using a sharp tip,
we can both image and alter the adsorbed O2 molecules in a well controlled fashion.
Theoretical calculations show that O2 adsorbed on the anatase(101) surface
induces structural changes in the oxide.
In particular, subsurface oxygen vacancies migrate towards the surface.
It happens either spontaneously, or with the help of the tip electric field.
It has been known for about 40 years, that
the field-induced migration of oxygen vacancies
is critical in many applications. However, direct imaging
and detailed understanding of the process is just emerging.