Simultaneous quantization of bulk conduction and valence states through   adsorption of nonmagnetic impurities on Bi2Se3
  Exposing the (111) surface of the topological insulator Bi2Se3 to carbon monoxide results in strong shifts of the features observed in angle-resolved photoemission. The behavior is very similar to an often reported `aging' effect of the surface and it is concluded that this aging is most likely due to the adsorption of rest gas molecules. The spectral changes are also similar to those recently reported in connection with the adsorption of the magnetic adatom Fe. All spectral changes can be explained by a simultaneous confinement of the conduction band and valence band states. This is only possible because of the unusual bulk electronic structure of Bi2Se3. The valence band quantization leads to spectral features which resemble those of a band gap opening at the Dirac point. 
Marco Bianchi, Richard C. Hatch, Jianli Mi, Bo Brummerstedt Iversen,   Philip Hofmann
cond-mat.mtrl-sci cond-mat.mes-hall