Chirality and vacancy effect on phonon dispersion of MoS2 with strain

Abstract

With the first principles theory calculations, we reveal the strain, chirality and defect effects to the mechanical properties, phonon dispersion, and structural stability of monolayer MoS2. Phonon dispersion relation is calculated based on the lattice dynamics theory. Chirality effect is trivial to the Young's modulus at zero strain but significant to the ultimate strength and Poisson's ratio, regardless of the defects. A phonon band gap of 49 cm−1 exists in the perfect MoS2 lattice, and reduces to 30.6 cm−1 in the MoS2 lattice with two sulfur missing vacancy. The phonon band gap reduces with the increasing of tensile strain, regardless of chirality. The failure mode is investigated near the ultimate tensile strength of both the perfect MoS2 lattice and the MoS2 lattice with two sulfur missing vacancy. Regardless of the defect, elastic instability and structural instability are identified for the MoS2 in zigzag direction tension test and armchair direction tension test, respectively. Based on the weakly linear component in the flextural acoustic mode of phonon dispersion, we evaluate the intrinsic critical compressive strain of buckling to be ϵC=−2.0%±0.5% for the perfect MoS2 lattice and demonstrate it numerically from first principles theory calculation.