Band structure of Bi surfaces formed on Bi2Se3 upon exposure to air

Bi₂Se₃ has been the focus of intense interest over the past decade due to its topological properties. Bi surfaces are known to form on Bi₂Se₃ upon exposure to atmosphere, but their electronic structure has not been investigated. We report band structure measurements of such Bi surfaces using angle-resolved photoemission spectroscopy. Measured spectra can be well explained by the band structure of a single bilayer of Bi on Bi₂Se₃, and show that Bi surfaces consistently dominate the photoemission signal for air exposure times of at least one hour. These results demonstrate that atmospheric effects should be taken into consideration when identifying two-dimensional transport channels, and when designing surface-sensitive measurements of Bi₂Se₃, ideally limiting air exposure to no more than a few minutes.