  Gas molecule doping on the topological insulator Bi2 Se3 surface with
existing Se vacancies is investigated using first-principles calculations.
Consistent with experiments, NO2 and O2 are found to occupy the Se vacancy
sites, remove vacancy-doped electrons and restore the band structure of a
perfect surface. In contrast, NO and H2 do not favour passivation of such
vacancies. Interestingly we have revealed a NO2 dissociation process that can
well explain the speculative introduced "photon-doping" effect reported by
recent experiments. Experimental strategies to validate this mechanism are
presented. The choice and the effect of different passivators are discussed.
This step paves the way for the usage of such materials in device applications
utilizing robust topological surface states.
