Anisotropic to Isotropic Transition in Monolayer Group-IV Tellurides

Monolayer group-IV tellurides with phosphorene-derived structures are attracting increasing research interest because of their unique properties. Here, we systematically studied the quasiparticle electronic and optical properties of two-dimensional group-IV tellurides (SiTe, GeTe, SnTe, PbTe) using the GW and Bethe-Salpeter equation method. The calculations revealed that all group-IV tellurides are indirect bandgap semiconductors except for monolayer PbTe with a direct gap of 1.742 eV, while all of them are predicted to have prominent carrier transport ability. We further found that the excitonic effect has a significant impact on the optical properties for monolayer group-IV tellurides, and the predicted exciton binding energy is up to 0.598 eV for SiTe. Interestingly, the physical properties of monolayer group-IV tellurides were subject to an increasingly isotropic trend: from SiTe to PbTe, the differences of the calculated quasiparticle band gap, optical gap, and further exciton binding energy along different directions tended to decrease. We demonstrated that these anisotropic electronic and optical properties originate from the structural anisotropy, which in turn is the result of Coulomb repulsion between non-bonding electron pairs. Our theoretical results provide a deeper understanding of the anisotropic properties of group-IV telluride monolayers.