  Angle-resolved photoelectron spectroscopy is used for a detailed study of the
electronic structure of the topological insulator Bi2Se3. Nominally
stoichiometric and calcium-doped samples were investigated. The pristine
surface shows the topological surface state in the bulk band gap. As time
passes, the Dirac point moves to higher binding energies, indicating an
increasingly strong downward bending of the bands near the surface. This
time-dependent band bending is related to a contamination of the surface and
can be accelerated by intentionally exposing the surface to carbon monoxide and
other species. For a sufficiently strong band bending, additional states appear
at the Fermi level. These are interpreted as quantised conduction band states.
For large band bendings, these states are found to undergo a strong Rashba
splitting. The formation of quantum well states is also observed for the
valence band states. Different interpretations of similar data are also
discussed.
