Engineering of the Topological Surface States and Topological Dangling Bond States in the (0001) Surface of Bi2Se3 via Structural Distortion

Abstract

Topological insulators (TIs) represent a new state of quantum materials that behave as an insulator in bulk but possess metallic surface states on their edges or surfaces. [1] These surface states are protected by time-reversal symmetry, become insensitive to backscattering, and remain preserved under nonmagnetic interactions. [2] The surface states form a single Dirac cone at the timereversal invariant momentum (TRIM) "Γ" point and the helical spin texture in the Bi 2 Se 3 and Bi 2 Te 3 due to strong spin-orbit coupling (SOC) effect. [3][4][5] However, the occurrence of the Dirac cone depends on the thickness and complete set of quintuple layers (QLs), i.e., the QL is bounded by the Se1 terminated (0001) surfaces. [6] The literature reports suggest that the Dirac-like surface states of Bi 2 Se 3 occur for a thickness of 6 QLs and above, whereas for lower thicknesses the hybridization between the top and bottom surfaces creates a finite energy gap. [7,8] Similarly, for different atomic terminations of the surface states other than the complete set of the top QL, the surface band dispersion undergoes significant modifications. The different atomic terminations at various cleaves of the surface of Bi 2 Se 3 are investigated through scanning tunneling microscopy (STM) and firstprinciples electronic structure calculations, which result in a significant enhancement of the degree of spin polarization. [9] The stability of the surface of Bi 2 Se 3 single crystals, cleaved in vacuum, is examined using high-resolution synchrotron-based photoelectron spectroscopy and it has been shown that, even for 5 min exposure to the atmosphere, a Bi2 layer always forms on the surface, which is accompanied by a depletion of Se in the near-surface region and a 1.4eV decrease in the work function. The Bi2 layer remains stable in ultrahigh vacuum, but becomes unstable after prolonged exposer to air. [10] Through low energy ion scattering experiment, after cleaving the Bi 2 Se 3 surface at 80K, it is found that the Bi 2 Se 3 has a complete Bi surface termination. Density functional theory (DFT) calculations also confirm that a Bi layer on top of the bulk-terminated structure is energetically more favorable, which has profound implications on the electrical transport and long-term stability of such materials and devices. [11] The occurrence of Bi-terminated surface on the Bi 2 Se 3 is also observed in the X-ray photoelectron spectroscopy (XPS) because of the different bonding environment for the Bi atoms. [12] Nevertheless, low-temperature removal of protective Se cap was achieved, enabling the sample to be transported in the air for device fabrication. [13] The coexistence of a conducting bilayer ðSe À Bi or Bi À BiÞ on the bare surface of Bi 2 Se 3 is also observed, whose stability is explained through X-ray crystal truncation rod scattering. [14] It is also found that single crystals of Bi 2 Se 3 stored and cleaved in high vacuum prefer Se-terminated surface, whereas samples stored and cleaved in the air have a Bi termination. [12]

Machine-learning guided prediction of thermoelectric properties of topological insulator Bi2Te3−xSex

“…Furthermore, the predicted thermoelectric properties of Bi 2 Te 3− x Se x with x = 1 using the random forest model are compared with the values calculated using the density functional theory (DFT) and Boltzmann transport theory. The electronic band structure of the Bi 2 Te 2 Se shows a band gap of around ∼0.186 eV, relatively larger than the Bi 2 Te 3 band gap of ∼0.14 eV, 55 and smaller than Bi 2 Se 3 (∼0.218 eV), 56 and consistent with the experimental 54 and theoretical 55 values. The Bi 2 Te 2 Se is a dynamically stable compound, which is confirmed by the first Brillouin zone phonon band structure and is in good agreement with the theoretical result.…”

Defect Induced Unconventional Ferromagnetism in Au Intercalated Bi2Se3 Nanocrystals for Spintronic Applications

“…[1,2] The presence of TRS protects the nontrivial surface states of TIs against time-reversal invariant perturbation such as scattering from crystalline defects, non-magnetic impurities, and surface distortions. [3,4] The most interesting property of the topological insulators is the spin-momentum locking phenomena of the surface states where the carriers always move perpendicular to the direction of the momentum. [5] Among the popular 3D TIs, Bi 2 Se 3 is of particular interest as a prototype TI because of having a simple band structure, single Dirac cone at the Γ point, [2,6,7] the direct energy gap of 0.3 eV, [8] helical spin texture [9] etc.…”