A Study Of Stress And Doping Effects On WSe₂ Properties

In recent years, Transition-Metal Dichalcogenide Compounds (TMDCs), which is similar to Graphene layer structure and belong to natural semiconductor materials, attract the attention of scholars in the field of semiconductor materials and devices and become one of the research areas. In this paper, the first principles based on the density functional theory were performed to discuss the outstanding representatives in TMDCs, WSe2.We investigate the stress, doping and WS2(1-x)Se2x alloys effects on stability and electronic structure. The conclusion is as follows:1. External stress has a great effect on the crystal structure, electronic structure and optical properties of WSe2. Studies found that the lattice constant a, c and volume decreases with the increase of external stress, WSe2 more likely to be compressed along the c direction, with the increase of stress the band gap of WSe2 decreases and when the stress is 35GPa, WSe2 becomes metal. Studies of external stress effects on WSe2 optical properties found with stress increases from OGPa to 60GPa, refractive index increases from 3.8 to 4.5, extinction coefficient is always 0, the real part of dielectric function increases from 14.5 to 20, imaginary part is always 0. From the absorption spectrum we find WSe2 showed a strong absorptive capacity from visible light to ultraviolet area. Under the action of external pressure, its absorptive capacity is further strengthened and move to the deep ultraviolet absorption range.2. Mo, Se doped 2H-WSe2 has three kinds of configurations (R.Mo-wse2, 1Mo-wse2 and Rs-wse2), the investigation in three doped systems shows of c parameters in RMo-wse2 and Rs-wse2 are more than pure WSe2, but in IMo-wse2 system the bond of Mo and adjacent Se atoms makes c parameter smaller. The calculation results show IMo-wse2 with a Mo atom embedded into the interlayer, its formation energy is maximum and stability is worse than RMo-wse2 with one of W atoms replaced by a Mo atom, which implies it is more energetically favorable that one W atom is substituted than that one Mo atom is embedded into the interlayer of the 2H-WSe2 supercell. The calculation results of electronic structure show the band gap of RMo-wse2 and Rs-wse2 is wider than pure system, but the band gap of IMo-wse2 is 0.071eV and become narrow, it has the trend of being metal. The results of the total charge density on the (110) surface for the two doping systems show the doped V atom and adjacent Se atoms form coordinate bonds from shared electrons.3. WS2(1-x)Se2x(x=1/3,1/2,2/3) solid solutions are formed by Se atoms replace S atoms. In WS2(i-x)Se2x systems, Se atoms as solute atoms into WS2 bring small lattice distortion degree and easy to form a stabile structure. The analysis based on the optimization of geometric structure shows with the increase of concentration of Se doping, the lattice constant a increased gradually, leading to cell volume also increases with the increase of concentration. To study the structure of the solid solution, we find its surface different from pure WS2 or WSe2 is not smooth, the surface atoms are no longer in the same plane. The calculation results of electronic structure show the band gap of WSSe is narrower than the gap of WS4/3Se2/3 and WS2/3Se4/3, but the band gap of all WS2(i-x)Se2x(x=1/3,1/2,2/3) solid solutions is wider than WSe2, narrower than WS2. It can be seen WS2(1-x)Se2x(x=1/3,1/2,2/3) solid solution without changing the character of the direct band gap of pure WSe2 and WS2, can effectively adjust the band gap width.