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 <i>in situ</i> (from 10^–8 to 0.1 mbar) and <i>ex situ</i> (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 <i>p</i>-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

Rapid Surface Oxidation of Sb₂Te₃ as Indication for a Universal Trend in the Chemical Reactivity of Tetradymite Topological Insulators

Within the past few years, topological insulators (TIs) have attracted a lot of interest due to their unique electronic structure with spin-polarized topological surface states (TSSs), which may pave the way for these materials to have a great potential in multiple applications. However, to enable consideration of TIs as building blocks for novel devices, stability of TSSs toward oxidation should be tested. Among the family of TIs with a tetradymite structure, Sb₂Te₃ is of <i>p</i>-type and appears to be the least explored material since its TSS is unoccupied in the ground state, a property that allows the use of optical excitations to generate spin currents relevant for spintronics. Here, we report relatively fast surface oxidation of Sb₂Te₃ under ambient conditions. We show that the clean surface reacts rapidly with molecular oxygen and slowly with water, and that humidity plays an important role during oxide layer growth. In humid air, we show that Sb₂Te₃ oxidizes on a time scale of minutes to hours, and much faster than other tetradymite TIs. The high surface reactivity revealed by our experiments is of critical importance and must be taken into account for the production and exploitation of novel TI-based devices using Sb₂Te₃ as a working material. Our results contribute to the comprehensive understanding of the universal trend underlying the chemical reactivity of tetradymite TIs