Topological band-order transition and quantum spin Hall edge engineering in functionalized X-Bi(111) (X = Ga, In, and Tl) bilayer

Growth mechanism and self-polarization of bilayer InSb (111) on Bi (001) substrate

Polarity introduced by inversion symmetry broken along <111> direction has strong impacts on the physical properties and morphological characteristics of III-V component nanostructure. Take III-V component semiconductor InSb as an example, we systematically investigate the growth sequence and morphology evolution of InSb (111) on Bi (001) substrate from adatoms to bilayers. We discovered and verified that the presence of amorphous-like morphology of monolayer InSb was attributed to the strong interaction between mix-polarity InSb and Bi substrate. Further, our comprehensive energy investigations of bilayer InSb reveal that an amorphous first layer will be crystallized and polarized driven by the low surface energy of the reconstructed second layers. Phase diagrams were developed to describe the ongoing polarization process of bilayer InSb under various chemical environments as a function of deposition time. The growth mechanism and polarity phase diagram of bilayer InSb on Bi substrate may advance the progress of polarity controllable growth of low-dimensional InSb nanostructure as well as other polar III-V compound semiconductors.

Spin valley and giant quantum spin Hall gap of hydrofluorinated bismuth nanosheet

Spin-valley and electronic band topological properties have been extensively explored in quantum material science, yet their coexistence has rarely been realized in stoichiometric two-dimensional (2D) materials. We theoretically predict the quantum spin Hall effect (QSHE) in the hydrofluorinated bismuth (Bi₂HF) nanosheet where the hydrogen (H) and fluorine (F) atoms are functionalized on opposite sides of bismuth (Bi) atomic monolayer. Such Bi₂HF nanosheet is found to be a 2D topological insulator with a giant band gap of 0.97 eV which might host room temperature QSHE. The atomistic structure of Bi₂HF nanosheet is noncentrosymmetric and the spontaneous polarization arises from the hydrofluorinated morphology. The phonon spectrum and ab initio molecular dynamic (AIMD) calculations reveal that the proposed Bi₂HF nanosheet is dynamically and thermally stable. The inversion symmetry breaking together with spin-orbit coupling (SOC) leads to the coupling between spin and valley in Bi₂HF nanosheet. The emerging valley-dependent properties and the interplay between intrinsic dipole and SOC are investigated using first-principles calculations combined with an effective Hamiltonian model. The topological invariant of the Bi₂HF nanosheet is confirmed by using Wilson loop method and the calculated helical metallic edge states are shown to host QSHE. The Bi₂HF nanosheet is therefore a promising platform to realize room temperature QSHE and valley spintronics.

Structural and electronic properties of hydrogenated GaBi and InBi honeycomb monolayers with point defects

First-principles calculations are carried out to systematically investigate the structural and electronic properties of point defects in hydrogenated GaBi and InBi monolayers, including vacancies, antisites and Stone-Wales (SW) defects. Our results imply that the perfect H2-Ga(In)Bi is a semiconductor with a bandgap of 0.241 eV (0.265 eV) at the Γ point. The system turns into a metal by introducing a Ga(In) vacancy, substituting a Bi with a Ga(In) atom or substituting an In with a Bi atom. Other defect configurations can tune the bandgap value in the range from 0.09 eV to 0.3 eV. In particular, the exchange of neighboring Ga(In) and Bi increases the bandgap, meanwhile the spin splitting effect is preserved. All SW defects decrease the bandgap. The lowest formation energy of defects occurs when substituting a Ga(In) with a Bi atom and the values of SW defects vary from 0.98 eV to 1.77 eV.