Abstract
Binary bismuth chalcogenides Bi2Se3, Bi2Te3, and related materials are currently being extensively investigated as the reference topological insulators (TIs) due to their simple surface-state band dispersion (single Dirac cone) and relatively large bulk band gaps. Nanostructures of TIs are of particular interest as an increased surface-to-volume ratio enhances the contribution of surfaces states, meaning they are promising candidates for potential device applications. So far, the vast majority of research efforts have focused on the low-energy (0001) surfaces, which correspond to natural cleavage planes in these layered materials. However, the surfaces of low-dimensional nanostructures (nanoplatelets, nanowires, nanoribbons) inevitably involve higher-index facets. We perform a systematic ab initio investigation of the surfaces of bismuth chalcogenide TI nanostructures characterized by different crystallographic orientations, atomic structures and stoichiometric compositions. We find several stable terminations of high-index surfaces, which can be realized at different values of the chemical potential of one of the constituent elements. For the uniquely defined stoichiometric termination, the topological Dirac fermion states are shown to be strongly anisotropic with a clear dependence of Fermi velocities and spin polarization on the surface orientation. Self-doping effects and the presence of topologically trivial mid-gap states are found to characterize the non-stoichiometric surfaces. The results of our study pave the way towards experimental control of topologically protected surface states in bismuth chalcogenide nanostructures.
Highlights
Bismuth chalcogenides are layered materials composed of covalently bonded sheets termed quintuple layers (QL) (Fig. 1a–c)
The approximation in Equation 1 stems from the omission of a term related to the loss of van der Waals (vdW) energy upon stacking QLs to form the 2D surface
This contribution is assumed to be negligible as the relative magnitude of vdW interactions is small and the leading term to the surface energy is the QL termination energy
Summary
Bismuth chalcogenides are layered materials composed of covalently bonded sheets termed quintuple layers (QL) (Fig. 1a–c). These QL building blocks are held together by weak van der Waals (vdW) interactions, providing natural cleavage planes and giving rise to two equivalent orientations of low-energy surfaces, (0001). The vast majority of investigations of the surface states of bismuth chalcogenide TIs far have focused on the low-energy (0001) surfaces. The properties of topologically protected charge carriers at such surfaces would depend on their crystallographic orientation, atomic structure and chemical composition. The electronic properties of corresponding topological surface-state charge carriers and their dependence on surface orientation and local chemical composition are investigated
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