  Two-dimensional (2D) semiconducting transition metal dichalcogenides (TMDs)
can withstand a large deformation without fracture or inelastic relaxation,
making them attractive for application in novel strain-engineered and flexible
electronic and optoelectronic devices. In this study, we characterize the
mechanical response of monolayer group VI TMDs to large elastic deformation
using first-principles density functional theory calculations. We find that the
ultimate strength and the overall stress response of these 2D materials is
strongly influenced by their chemical composition and loading direction. We
demonstrate that differences in the observed mechanical behavior can be
attributed to the spatial redistribution of the occupied hybridized electronic
states in the region between the transition metal atom and the chalcogens. In
spite of the strong covalent bonding between the transition metal and the
chalcogens, we find that a simple linear relationship can be established to
describe the dependence of the mechanical strength on the charge transfer from
the transition metal atom to the chalcogens.
