Effects of Nitrogen Exposure on the Bismuth Selenide Density of States

Bi₂Se₃ is an intensely studied topological insulator; the crystals typically exhibit a Dirac point about 300 meV below the Fermi level, behavior attributed to charged selenium vacancies which act as electron donors. We will present measurements and theoretical calculations of shifting of the Dirac point in response to exposing the surface of the crystals to different gas environments including nitrogen, helium and air. The experiments were conducted on crystals cleaved in a glove-box over-pressured with the desired gas and transferred directly into a chamber pumped to ~10^-6 torr. Tunneling conductance spectra were obtained using scanning tunneling microscopy (STM) at 77K in this vacuum. We find substantial shifts in the Dirac point depending on the gas. To understand experimental observations, we calculated geometric and electronic structural properties of gas-adsorbed systems with and without defects by employing density-functional-theory based method. We will also discuss band structures, projected density of states, and hence the effect of gas adsorption and defects on the Dirac point shift.