Reactive chemical doping of the $B{i}_{2}S{e}_3$ topological insulator

Using angle resolved photoemission spectroscopy we studied the evolution of the surface electronic structure of the topological insulator ${\text{Bi}}_2{\text{Se}}_3$ as a function of water vapor exposure. We find that a surface reaction with water induces a band bending shifting the Dirac point deep into the occupied states and creating quantum well states with a strong Rashba-type splitting. The surface is thus not chemically inert, but the topological state remains protected. The band bending is traced back to Se-abstraction leaving positively charged vacancies at the surface. Due to the presence of water vapor, a similar effect takes place when ${\text{Bi}}_2{\text{Se}}_3$ crystals are left in vacuum or cleaved in air, which likely explains the aging effect observed in the ${\text{Bi}}_2{\text{Se}}_3$ band structure.