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 between 100 and 300 meV below the Fermi level. Using room temperature scanning tunneling microscopy (STM), we observed evidence for a shift in the expected density of states spectra when crystals first cleaved in a helium gas environment became exposed to ultra high purity N₂ gas. The shift brings the Dirac point 50 meV closer to the Fermi level. We will also present density functional theory calculations supporting the picture that nitrogen can bind to the selenium vacancies and shift the density of states. Finally, we will present data showing the time scale over which the exposure occurs.