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.
We systematically study geometries and band structures of two-dimensional group-V bilayer materials, i.e. phosphorene, arsenene and antimonene. Among the four stacking structures (AA, AB, AC, and AD), the AB stacking structures are found to be the largest band gaps and to be the most energetically stable. We find novel band structures on the whole Brillouin zone edges: four bands have close energies and two of the four bands have the same energy in many cases. We analyze the characteristic features of the band structures based on the group theory and clarify that the features depend on the space group of each stacking structure. We also find that the band splits due to the interlayer interaction is very small and this interaction becomes large as atoms become heavy.
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