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Reactive chemical doping of the $Bi_2 Se_3$ 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.