Room Temperature Quantum Spin Hall Insulator in Ethynyl-Derivative Functionalized Stanene Films

Abstract

Quantum spin Hall (QSH) insulators feature edge states that topologically protected from backscattering. However, the major obstacles to application for QSH effect are the lack of suitable QSH insulators with a large bulk gap. Based on first-principles calculations, we predict a class of large-gap QSH insulators in ethynyl-derivative functionalized stanene (SnC2X; X = H, F, Cl, Br, I), allowing for viable applications at room temperature. Noticeably, the SnC2Cl, SnC2Br, and SnC2I are QSH insulators with a bulk gap of ~0.2 eV, while the SnC2H and SnC2F can be transformed into QSH insulator under the tensile strains. A single pair of topologically protected helical edge states is established for the edge of these systems with the Dirac point locating at the bulk gap, and their QSH states are confirmed with topological invariant Z2 = 1. The films on BN substrate also maintain a nontrivial large-gap QSH effect, which harbors a Dirac cone lying within the band gap. These findings may shed new light in future design and fabrication of large-gap QSH insulators based on two-dimensional honeycomb lattices in spintronics.

Figure 1

(a) Side and (b) top views of the atomic structures of SnC₂X (X = H, F, Cl, Br, I). Red, blue, and green balls denote X, C, and Sn atoms, respectively. Shadow area in (a) present the unit cell. Phonon band dispersions of (c) denote SnC₂H film.

Figure 2. Orbital-resolved band structures with SOC of SnC₂H (a–c) and SnC₂Br (d–f) under biaxial strain ε = 0%, 4% and −4%, respectively.

The red dots represent the contributions from the s atomic orbital of Sn atom and the blue dots represent contributions from the p_x and p_y atomic orbitals of Sn atom.

Figure 3. The calculated band gaps at Γ point (E_Γ) and the global band gap (E_g) of SnC₂H and SnC₂Br with SOC as a function of biaxail and uniaxial strains.

Notably, the insets in panel show the trend of band gaps of TI phase as a function of external biaxial strain.

Figure 4

(a) Schematic structure of the zigzag-type nanoribbon of SnC₂H and SnC₂Br films. (b) and (c) indicate the band structures of zigzag-type edge states in QSH phase. The helical edge states are indicated by the red lines.

Figure 5. The evolution of atomic s and p_x,y orbitals into the band edges at Γ point of SnC₂X (X = H, F) at (a) equilibrium state, and (b) tensile strain (ε = 2%).

The horizontal red dashed lines in (a,b) indicate the Fermi level.

Figure 6

(a) Top and (b) side views of the schematic illustration of the epitaxial growth SnC₂Br on h-BN (2 × 2) substrate. (c) is orbital-resolved band structure with SOC for SnC₂Br@BN.