  We report the existence of metallic two-dimensional electron gases (2DEGs) at
the (001) and (101) surfaces of bulk-insulating TiO2 anatase due to local
chemical doping by oxygen vacancies in the near-surface region. Using
angle-resolved photoemission spectroscopy, we find that the electronic
structure at both surfaces is composed of two occupied subbands of d_xy orbital
character. While the Fermi surface observed at the (001) termination is
isotropic, the 2DEG at the (101) termination is anisotropic and shows a charge
carrier density three times larger than at the (001) surface. Moreover, we
demonstrate that intense UV synchrotron radiation can alter the electronic
structure and stoichiometry of the surface up to the complete disappearance of
the 2DEG. These results open a route for the nano-engineering of confined
electronic states, the control of their metallic or insulating nature, and the
tailoring of their microscopic symmetry, using UV illumination at different
surfaces of anatase.

Materials Science