Air-Stable Electron Depletion of Bi$_2$Se$_3$ into the Topological Regime using Molybdenum Trioxide*

Bismuth selenide (Bi$_2$Se$_3$) is a three-dimensional strong topological insulator of particular interest due to its relatively large bulk band gap (300 meV) and single set of topologically non-trivial surface states. However, persistent doping makes routine electronic access to the topological regime difficult. Here we explore surface transfer doping via molecular deposition as a route to bring the Fermi level into the topological regime and protect against ambient degradation. Bi$_2$Se$_3$ single crystals are cleaved in ultra-high vacuum and X-ray photoemission spectroscopy is used to measure the shifts in work function, Bi core levels, and charge state of Mo during deposition of MoO$_3$ molecules; the data indicate that MoO$_3$ can lower the Fermi level to within $\sim$ 100 meV of the Dirac point. Thin film transport demonstrates that $\sim$ 10$^{13}$ electrons can be depleted from the Bi$_2$Se$_3$ and that an MoO$_3$ capping layer is stable for days after exposure to ambient.