Evidence for nitrogen binding to surface defects for topological insulator Bi2Se3

Using scanning tunneling spectroscopy and theoretical simulations we have studied the effects of nitrogen gas exposure on the electronic density of states of Bi2Se3, a well-studied topological insulator. 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 min, ultimately bringing the Dirac point about 50 meV closer to the Fermi level. These results are compared to density functional theory calculations, which support a picture of N2 molecules physisorbing near Se vacancies. Furthermore, ab initio molecular dynamics simulations aided by a Blue Moon ensemble method reveal the dissociative adsorption of N2 molecules which then bind strongly to Se vacancies at the surface. In this scenario, 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.

Automated summary

Using scanning tunneling spectroscopy, the effects of nitrogen gas exposure on the electronic density of states of Bi2Se3 were investigated. Nitrogen gas molecules were observed to physisorb near Se vacancies, resulting in a saturation of the exposure effect after approximately 50 min. Density functional theory calculations and molecular dynamics simulations support the dissociative adsorption of N2 molecules and their strong binding to Se vacancies, which modifies the position of the Fermi energy relative to the Dirac point.