New two-dimensional allotrope of single layer IV-V semiconductor XBi (X = Si, Ge, Sn)

Using first-principles calculations based on density functional theory, we propose a new two dimensional (2D) tetragonal allotrope of single layer IV-V semiconductor XBi (X = Si, Ge, Sn), which consists of repeated square and octagon rings. The calculated cohesive energy, phonon dispersion and ab-initio molecular dynamics simulations indicate that 2D tetragonal XBi (X = Si, Ge, Sn) are stable at room temperature and have potential to be synthesized in the future. Furthermore, due to the strong spin–orbit coupling (SOC) of Bi atom, the inclusion of SOC alters character of the fundamental band gaps of 2D tetragonal XBi (X = Si, Ge, Sn) and decreases their band gaps. When SOC is include, the electronic band structures show that 2D tetragonal SiBi is a direct band gap semiconductor, while 2D tetragonal GeBi and SnBi are indirect band gap semiconductors. Interestingly, the strain can induce 2D tetragonal XBi (X = Si, Ge, Sn) to undergo a direct to indirect band gap transition or an indirect band gap semiconductor to metal transformation.