Electronic structure of puckered group IV-VI two-dimensional monolayer materials

We carry out first-principle calculations on monolayer group IV-VI 2D materials. We study systems consisting of group IV (C, Si, Ge) and group VI elements (O, S, Se, Te) and find that all the materials form buckled puckered geometries. We clarify that VI atoms tend to be located at the lower positions in the buckled structure when the electronegativity of the VI atom is sufficiently larger than that of the IV atom, which is due to the electron transfer from the IV atom to the VI atom. All the calculated bands are doubly degenerated on the first Brillouin zone edge and this degeneracy can be explained based on the group theory.