Negligible Surface Reactivity of Topological Insulators Bi₂Se₃ and Bi₂Te₃ towards Oxygen and Water.

The long-term stability of functional properties of topological insulator materials is crucial for the operation of future topological insulator based devices. Water and oxygen have been reported to be the main sources of surface deterioration by chemical reactions. In the present work, we investigate the behavior of the topological surface states on Bi₂X₃ (X = Se, Te) by valence-band and core level photoemission in a wide range of water and oxygen pressures both in situ (from 10^-8 to 0.1 mbar) and ex situ (at 1 bar). We find that no chemical reactions occur in pure oxygen and in pure water. Water itself does not chemically react with both Bi₂Se₃ and Bi₂Te₃ surfaces and only leads to slight p-doping. In dry air, the oxidation of the Bi₂Te₃ surface occurs on the time scale of months, in the case of Bi₂Se₃ surface of cleaved crystal, not even on the time scale of years. The presence of water, however, promotes the oxidation in air, and we suggest the underlying reactions supported by density functional calculations. All in all, the surface reactivity is found to be negligible, which allows expanding the acceptable ranges of conditions for preparation, handling and operation of future Bi₂X₃-based devices