Mapping the 3D surface potential in Bi₂Se₃.

Bi₂Se₃ initially emerged as a particularly promising host of topological physics. However, in actual materials, several issues have been uncovered including strong surface band bending and potential fluctuations. To investigate these concerns, we study nominally stoichiometric Bi₂Se₃ using scanning tunnelling microscopy. Here we identify two distinct distributions of BiSe antisites that act as nanometer-scale sensors for the surface band-bending field. To confirm this, we examine bulk Cu-doped Bi₂Se₃ and demonstrate a significantly reduced surface band-bending field. In addition, we find that in the case of unintentionally doped Bi₂Se₃, lateral fluctuations of the Dirac point can be directly correlated with specific near-surface point defects, namely Se vacancies.