A new stable, crystalline capping material for topological insulators

To preserve the high quality topological surface state after air exposure without degradation, it is crucial to identify an effective capping layer. In this study, we report an effective capping layer obtained by crystallizing Se. Upon extended exposure to ultrahigh vacuum or humid air, we show by using x-ray photoemission spectroscopy that the stability and resistance to oxidation of crystalline Se capping layers are superior to those of the amorphous Se capping layer, which has been commonly used by current communities. Furthermore, time-dependent Hall measurements showed that crystalline Se capping layers had a much stronger ability to sustain the intrinsic transport properties of Bi2Se3.