MONOVACANCY IN PHOSPHORENE RIBBONS

We have investigated the reconstruction, energetic and electronic structure of monovacancy in phosphorene [1] ribbons as the monovacancy moves from the ribbon center to the edge. We considered three type of ribbons with symmetric(armchair), asymmetric(zigzag) and cliff geometries, which are depicted in Figure 1. In this work all calculations were performed in Spanish Initiative for Electronic Simulations with Thousands of Atoms (SIESTA). The generalized gradient approximation (GGA) in the form of the Perdew Burke-Ernzerhof (PBE) functional was used. We used 120,110 and 100 atoms for asymmetric, symmetric and cliff geometries. All calculations which we performed in SIESTA are non-spin polarized. We find that monovacancy reconstructs and shows four-coordinated P atoms. The total energy exhibits an alternating behavior when the defect is shifted to next atomic site, revealing that the reconstruction strength depends on the defect orientation with respect to ribbon axis. Approaching the edge, total energy is reduced by ~1.5 eV in symmetric and asymmetric ribbons while energy gain by moving to edge for cliff ribbons is negligible. This is partly because of the structural instability of the cliff shaped ribbons. In symmetric and asymmetric ribbons, the reconstruction is seamless, and the defect state forms a delocalized state with a free electron-like dispersion near the Fermi level. Thus, the ribbon shows a metallic character although pristine ribbon is semiconductor. In cliff shaped ribbons, shown in Figure 1(c), the defect state is a localized state at the Fermi level with little or no dispersion, which depends on defect distance to the edge. Note that pristine cliff geometry is energetically less stable, reflecting the strain in this configuration. Such flat bands near the Fermi level indicate electronic instability that may lead to either spin polarization or symmetry lowering structural transitions. The profile of the ribbon determines if the defect heals seamlessly. The electronic structure is significantly modified by monovacancy.