Quantum spin Hall effect in tilted penta silicene and its isoelectronic substitutions.

Silicene, a competitive two-dimensional (2D) material for future electronic devices, has attracted intensive attention in condensed matter physics. Utilizing an adaptive genetic algorithm (AGA), we identify a topological allotrope of silicene, named tilted penta () silicene. Based on first-principles calculations, the geometric and electronic properties of silicene and its isoelectronic substitutions (Ge, Sn) are investigated. Our results indicate that silicene exhibits a semimetallic state with distorted Dirac cones in the absence of spin-orbit coupling (SOC). When SOC is considered, it shows semiconducting behavior with a gap opening of 2.4 meV at the Dirac point. Based on the results of invariant ( = 1) and the helical edge states, we demonstrate that silicene is a topological insulator. Furthermore, the effect of isoelectronic substitutions on silicene is studied. Two stoichiometric phases, , SiGe and SiSn are found to retain the geometric framework of silicene and exhibit high stabilities. Our calculations show that both SiGe and SiSn are QSH insulators with enlarged band gaps of 32.5 meV and 94.3 meV, respectively, at the HSE06 level, offering great potential for practical applications at room temperature.