Effects of Uniaxial Strain on the Electronic Structure of Transition-Metal Dichalcogenides (TMDs)

Using first-principles calculations, we investigate the effects of uniaxial strain on the electronic structure of monolayer of 1H MX₂ transition-metal dichalcogenides (TMDs) with M= (Mo, W) and X= (S, Se, Te). In the monolayer 1H crystal structure, all these TMDs are semiconductors. Under equilibrium, the fundamental gaps are direct K-K with the indirect gap K-Q being slightly higher for some of the compounds. Under uniaxial strain, along the zig-zag or arm-chair directions, the perfect hexagonal symmetry is broken, the honeycomb Brillouin zone becomes distorted, and the maxima in the valence band and minima in the conduction band at K, Γ, and Q high-symmetry points are shifted. In the range of 0% to 5% uniaxial tensile strain, the TMDs remain direct band-gap semiconductors, and the difference between the indirect and direct gap increases with tensile strain. We also observe that the valence-band and conduction-band edges are slightly displaced from the K point for strains of 2% and larger, changing the degeneracy of the electron and hole valleys, and likely affecting transport, and optical properties. Additionally, we emphasize the importance of considering the splitting of the valence-band maxima resulting from the breaking of the inversion symmetry and the effects of spin-orbit coupling in the hybrid functional calculations.