Electronic properties of group-IV monochalcogenide nanoribbons: Studied from first-principles calculations

The electronic properties of one-dimensional (1D) group-IV monochalcogenide ribbons, denoted as MXNRs (MGe, Sn; XS, Se), are investigated by the density functional theory calculations. It is found from our calculations that all the MXNRs with zigzag edges are metals, because there are two edge bands crossing the Fermi level, which come from the py and pz orbital of edge M atoms and py ones of edge X atoms. In contrast, all the MXNRs with armchair edges (arm-MXNRs) are semiconductors, independent of the ribbon widths. Their band gaps are found to fluctuate, which vary with the ribbon width, finally converging to the values of the corresponding 2D MX compounds. It is important to find that except for the arm-SnSNRs, other three narrow arm-MXNRs exhibit repeatedly band gap transitions between the direct and indirect ones as their widths increase because of the edge symmetry and quantum confinement effect of the 1D ribbons. The calculated binding energies indicate that the arm-MXNRs are generally more stable than the zig-MXNRs. In addition, the hydrogen-terminated GeSNRs are also studied, which exhibit semiconducting properties no matter their edges are zigzag or armchair.