Using scanning tunneling spectroscopy we have studied the effects of nitrogen gas exposure on the bismuth selenide density of states. We observe a shift in the Dirac point which is qualitatively consistent with theoretical modeling of nitrogen binding to selenium vacancies. In carefully controlled measurements, Bi2Se3 crystals were initially cleaved in a helium gas environment and then exposed to a 22 SCFH flow of ultra-high purity N2 gas. We observe a resulting change in the spectral curves, with the exposure effect saturating after approximately 50 minutes, ultimately bringing the Dirac point about 50 meV closer to the Fermi level. These results are compared to density functional theoretical calculations, which support a picture of N2 molecules physisorbing near Se vacancies and dissociating into individual N atoms which then bind strongly to Se vacancies. In this interpretation, the binding of the N atom to a Se vacancy site removes the surface defect state created by the vacancy and changes the position of the Fermi energy with respect to the Dirac point.
@article{arxiv.2004.01631,
title = {Evidence for nitrogen gas surface doping of the Bi$_2$Se$_3$ topological insulator},
author = {Michael Gottschalk and Mal-Soon Lee and Eric Goodwin and Camille Mikolas and Thomas Chasapis and Duck Young Chung and Mercouri Kanatzidis and S. D. Mahanti and Stuart Tessmer},
journal= {arXiv preprint arXiv:2004.01631},
year = {2023}
}