Healing Se Vacancies in Bi2Se3 by Ambient Gases

Selenium (Se) vacancies are the most abundant and unavoidable n-type defects in the topological insulator, bismuth selenide (Bi2Se3). A recent study has shown that the surface Se vacancies not only n-dope the system but also result in the splitting of the Dirac cone associated with the surface and the emergence of a nonlinear state pinned at the Fermi level due to the interactions between surface-, defect-, and quantum-well states. In this combined theoretical and experimental work, we show how the defective surfaces of Bi2Se3 slabs can be healed by adsorption of different gases. Depending on the adsorbates, we find that the band structure of Bi2Se3 either reverts back to its pristine form or exhibits localized adsorbate bands near the Fermi level. Notably, our density functional theory calculations show that both atomic and molecular oxygen are isoelectronic to Se, binding strongly to the vacancy position. Along with counterdoping (p-doping) of Bi2Se3 (as reported by earlier studies), oxygen adsorption completely restores the Dirac structure of the surface states. Our experiments confirm that annealing intrinsically n-doped Bi2Se3 samples with oxygen reduces the carrier density by ≈ 6%. This is a reversible process, with the Bi2Se3 slab reverting back to the original carrier concentration on vacuum annealing, thus confirming the healing of vacancies by oxygen. We distinguish the possible features of the adsorbates that can be used to a priori predict their effects on the electronic structure of the Bi2Se3 slab after adsorption. Our results provide a foundation for a general strategy for the in situ engineering of the band structure of the Bi2Se3 family of topological insulators by quenching Se vacancies.