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

van der Waals Screening by Single-Layer Graphene and Molybdenum Disulfide

A sharp tip of atomic force microscope is employed to probe van der Waals forces of a silicon oxide substrate with adhered graphene. Experimental results obtained in the range of distances from 3 to 20 nm indicate that single-, double-, and triple-layer graphenes screen the van der Waals forces of the substrate. Fluorination of graphene, which makes it electrically insulating, lifts the screening in the single-layer graphene. The van der Waals force from graphene determined per layer decreases with the number of layers. In addition, increased hole doping of graphene increases the force. Finally, we also demonstrate screening of the van der Waals forces of the silicon oxide substrate by single- and double-layer molybdenum disulfide

Nitrogen-Doped Graphene and Twisted Bilayer Graphene <i>via</i> Hyperthermal Ion Implantation with Depth Control

We investigate hyperthermal ion implantation (HyTII) as a means for substitutionally doping layered materials such as graphene. In particular, this systematic study characterizes the efficacy of substitutional N-doping of graphene using HyTII over an N⁺ energy range of 25–100 eV. Scanning tunneling microscopy results establish the incorporation of N substituents into the graphene lattice during HyTII processing. We illustrate the differences in evolution of the characteristic Raman peaks following incremental doses of N⁺. We use the ratios of the integrated D and D′ peaks, <i>I</i>(D)/<i>I</i>(D′) to assess the N⁺ energy-dependent doping efficacy, which shows a strong correlation with previously reported molecular dynamics (MD) simulation results and a peak doping efficiency regime ranging between approximately 30 and 50 eV. We also demonstrate the inherent monolayer depth control of the HyTII process, thereby establishing a unique advantage over other less-specific methods for doping. We achieve this by implementing twisted bilayer graphene (TBG), with one layer of isotopically enriched ¹³C and one layer of natural ¹²C graphene, and modify only the top layer of the TBG sample. By assessing the effects of N-HyTII processing, we uncover dose-dependent shifts in the transfer characteristics consistent with electron doping and we find dose-dependent electronic localization that manifests in low-temperature magnetotransport measurements