Oxygen functionalized InSe and TlTe two-dimensional materials: transition from tunable bandgap semiconductors to quantum spin Hall insulators

Abstract

From first-principles calculations, we found that oxygen functionalized InSe and TlTe two-dimensional materials undergo the following changes with the increased concentrations of oxygen coverage, transforming from indirect bandgap semiconductors to direct bandgap semiconductors with tunable bandgap, and finally becoming quantum spin hall insulators. The maximal nontrivial bandgap are 0.121 and 0.169 eV, respectively, which occur at 100% oxygen coverage and are suitable for applications at room temperature. In addition, the topological phases are derived from SOC induced p-p bandgap opening, which can be further determined by Z₂ topological invariants and topologically protected gapless edge states. Significantly, the topological phases can be maintained in excess of 75% oxygen coverage and are robust against external strain, making the quantum spin hall effect easy to achieve experimentally. Thus, the oxygen functionalized InSe and TlTe are fine candidate materials for the design and fabrication of topological devices.

Fig. 1. (a) Top view and side view of the structure of InSe 2D material. The blue and green spheres represent In and Se atoms, respectively. The small square represents one of the primitive cells. (b) The band structure of InSe with the bandgap size.

Fig. 2. Top view and calculated band structure of the oxygen functionalized InSe. (a) InSeO_0.25, and (b) InSeO_0.5. The blue, green and red balls represent In, Se and O atoms respectively.

Fig. 3. Structure and band structure of (a) InSeO_0.75 and (b) InSeO. Orange and dark gray curves represent the band structures without and with SOC, respectively. The blue, green and red balls represent In, Se and O atoms respectively.

Fig. 4. Structure and orbital projected band structure of (a) InSeO_0.75 and (b) InSeO. The left and right sides represent the band structures without and with SOC, respectively. The blue circle represents the p_x,y orbitals, with the size of the circle corresponding to the weight of the projected orbit.

Fig. 5. Phonon spectrum of (a) InSe and (b) InSeO.

Fig. 6. Structure and band structure of (a) TlTe, (b) TlTeO_0.5 and (c) TlTeO. Orange and dark gray curves represent the band structures without and with SOC, respectively. The blue, green and red balls represent Tl, Te and O atoms, respectively.

Fig. 7. Topologically protected edge states of (a) InSeO and (b) TlTeO.

Fig. 8. The bandgaps E_Γ-with and E_g-with of (a) InSeO and (b) TlTeO change with strain. E_Γ-with represents the direct bandgap at the Γ point under SOC and E_g-with represents the global bandgap under SOC. (c) Orbital evolution of InSeO and TlTeO under strain. (d) Band structure of InSeO at 7% tensile strain considering the SOC. (e) Band structure of TlTeO at 6% tensile strain considering the SOC.