Effects of Nitrogen Exposure on the Bismuth Selenide Density of States

Bi₂Se₃ is a topological insulator widely used for scientific studies due in part to the ease with which it can be cleaved, exposing a clean surface for study. Typically the materials exhibit n-type doping attributed to selenium vacancies which results in a shift of the Dirac point to more than 100 meV below the Fermi level. Using cryogenic scanning tunneling microscopy (STM), we observed a shift in the expected density of states spectra when the crystals were cleaved in a pure nitrogen gas environment. The shift tends to restore the Dirac point to the Fermi level. These results are compared against crystals cleaved in a pure helium gas environment which reproduces the expected pristine spectra. We will present density functional theory calculations supporting the picture that nitrogen can bind to the selenium vacancies and shift the density of states. Furthermore, we will present data showing an upper-bound on the level of gas exposure necessary for saturated adsorption.