First-principle Study Of The Electronic Structures And The Magnetism Of Two-dimensional WS₂

In this paper, using Vienna Ab-initio Simulation Package (VASP) based on density functionaltheory(DFT) with the supercell approach, we have performed a comprehensive study of the electronic andmagnetic properties of transition-metal dichalcogenide WS2and some interesting results have beenobtained. Our work include:1. Tunable electronic structures in WS2We carry out an investigation to explore the effects of thickness, strain and doping on the electronicproperties of transition-metal dichalcogenide WS2. Our results indicate that the band gap of WS2increaseswith the decrease of thickness. When the number of layers reduces to a single layer the indirect gap of bulkbecomes a direct gap. The direct gap of monolayer WS2becomes a indirect gap under the tensile biaxialstrains and the monolayer WS2retains its indirect band gap with increasing strains. The fundamental bandgap of WS2continuously decreases with increasing strains, eventually rendering them metallic by up to21℅. Similarly, the direct gap of monolayer WS2also becomes a indirect gap under the compressive biaxialstrains and the monolayer WS2retains its indirect band gap with increasing strains. the fundamental bandgap of WS2continuously decreases with increasing strains, eventually rendering them metallic by up to16℅. It is also clear that compressive biaxial strains are more effective than tensile biaxial strains inmodulating the band gaps of WS2. The band gap of WS2is not almost affected by Mo doping while Crdoping changes the band gap of WS2greatly. When Cr doping is applied on the monolayer WS2, the directgap becomes an indirect gap and the magnitudes of the band gap begin to decline. By changing dopingconcentration, we can approve that the strain comes from Cr doping turn WS2into indirect gap from direct gap.2. Doping effects on the magnetic properties WS2We studied the magnetism of Cr and Co doping monolayer WS2. Our results demonstrate thatCr-doping and pure WS2have no magnetic moment, and the DOS of spin-up and spin-down aresymmetrical, so they results in magnetic states. Co-doping has a magnetic moment which localizes arounddoping Co atoms. Its DOS of spin-up and spin-down is clearly unsymmetric. There is a impurity level at thebottom of the conduction band, and at the top of the valence band the DOS of spin up is still below theFermi level, while the DOS of the spin-down crosses with the Fermi level. The3d orbital of Co atomsstrong couplings with the3p orbital of S atoms and5d orbital of W atoms by PDOS figures,so the Codoping results in magnetic states. Co doping monolayer WS2having preferably ferromagnetic at roomtemperature, and it is an ideal dilute magnetic semiconductor material for manufacturing electron spindevices. The results provide a theoretical basis for experiments and applications.