Mechanical properties of 1T-, 1T′-, and 1H-MX2 monolayers and their 1H/1T′-MX2 (M = Mo, W and X = S, Se, Te) heterostructures

Mechanical properties of two-dimensional (2D) transition-metal dichalcogenides (TMDCs) are of vital importance in any practical applications to flexible devices and nano-electromechanical systems. Thus, the mechanical properties of monolayer TMDCs, a stoichiometric formula MX2 in which M = Mo, W and X = S, Se, Te, are investigated by using density functional theory. More importantly, based on the different atomic arrangement, all three chemical isomers, such as 1T, 1T ', and 1H phases, are compared in detail. We found that their 2D Young's moduli and Poisson's ratios display a strong dependence not only on the atomic species but also on the atomic arrangements. For the same structural phase, monolayer TMDCs with the W (S) atom are found to be much stiffer in each chalcogenide (metal) group. Due to the threefold rotation symmetry of the hexagonal lattice, 1T- and 1H-TMDC monolayers belong to the isotropic structures, while the strong anisotropic Young's moduli and Poisson's ratios are observed in the 1T ' phase, i.e., 2D Young's moduli along the armchair direction are nearly 50% larger than those along the zigzag direction for tellurides. Interestingly, 1T-TMDC monolayers show negative Poisson's ratios. Furthermore, their in-plane 1H/1T ' heterostructures could be constructed, and the corresponding mechanical properties are explored. We found that the influence of the 1H/1T ' interface on the mechanical behavior is detrimental, which reduces the in-plane stiffness normal to the 1H/1T ' interface as compared with 1H and 1T ' structures. However, in comparison with the 1T ' phase, a remarkable strength of these novel heterostructures is along the 1H/1T ' interface direction. In brief, the present first-principles results constitute a useful picture for the mechanical properties of 2D TMDCs and their in-plane heterostructures.