First principle studies on the structures, electronic properties and Raman spectrums of monolayer WX2 (X = S, Se, Te) under strain condition

The two-dimensional transition-metal dichalcogenides (2D TMDs) WX2 (S, Se, Te) have received extensive attention and research since they have excellent physical properties and have been widely used in the fields of photoelectronics. Monolayer (ML) WX2 has excellent physical properties and can be modified by simple strain. Using the first principles based on density functional theory (DFT), this paper mainly studies the electronic properties of ML WS2, WSe2 and Wte(2). We also study the stabilities of three ML structures, the changes of Raman spectra and the movement of Raman peaks under biaxial tensile and compressive strains. Under the control of strain not only does the bandgap changes, but also the band properties shift between the direct bandgap and the indirect bandgap. With the increase of strain, bond length and bond angle change in the opposite trend. At the same time, we also studied the phonon dispersion relations of WX2 under different strains. We found that three structures showed good thermodynamic stabilities under the tensile strain (1-10%). When the compressive strain is 2%, one of the acoustic modes of WS2 or Wse(2) becomes imaginary at Gamma point, which indicates the structural instability. When tensile strain Raman summit blueshifts and when compressive strains, the redshift occurs.