Effects of sulfur line vacancy defects on the electronic and optical properties of armchair MoS2 nanoribbon

Based on the density functional theory (DFT) calculations, we study the electronic and optical properties of armchair MoS2 nanoribbons (AMoS2NR) with single and double sulfur line vacancy defects in two different directions, i.e. parallel to the length (longitudinal) and width (transversal) of the structure. The formation energy calculations show that the structures with longitudinal vacancies are easier to create than the transversal ones. Investigating the band structures, it is observed that all the line defects proposed in this study decrease the band gap of the AMoS2NR. In fact, the band gap of 0.68 eV in perfect nanoribbon reduces to the lowest value of 0.11 eV in transversal double vacancy structure (T-DV). Analysis of the total and projected density of states (TDOS and PDOS) of the perfect and proposed defective structures indicates that in the defective structures the contribution of central atoms becomes more dominant especially in the case of the T-DV structure which has the most number of non-terminated bonds. In order to investigate the optical properties of the perfect and defective AMoS(2)NR, we present the real and imaginary parts of dielectric function, absorption coefficient and reflectance spectra for two different polarizations of light, along and perpendicular to the nanoribbon. The results show that all the structures in this study are anisotropic with respect to the type of polarization in the energies below 12 eV and are isotropic above it. It is also depicted that the light polarized along the nanoribbon has higher absorption in the energies below 3 eV. In addition, we show that the proposed line defects cause redshifts in the first peaks of the optical spectra which provides a tunable optical characteristic that can extend the application of AMoS(2)NR in a wide frequency range especially IR and visible.