Atomically Sharp Crack Tips in Monolayer MoS and Their Enhanced Toughness by Vacancy Defects.

We combine in situ transmission electron microscopy and large-scale molecular dynamics simulations to investigate brittle fracture in 2D monolayer MoS, revealing that cracks propagate with a tip of atomic sharpness through the preferential direction with least energy release. We find that sparse vacancy defects cause crack deflections, while increasing defect density shifts the fracture mechanism from brittle to ductile by the migration of vacancies in the strain fields into networks. The fracture toughness of defective MoS is found to exceed that of graphene due to interactions between the atomically sharp crack tips and vacancy clusters during propagation. These results show that monolayer 2D materials are ideal for revealing fundamental aspects of fracture mechanics not previously possible with thicker materials, similar to studies of dislocation behavior in 2D materials.