monolayer

The mechanical properties, including the strength and toughness, of two-dimensional transition-metal chalcogenides play a major role in applications such as flexible electronic devices. In this work, we studied atomic vacancies induced by helium and gallium-ion beams and their effects on mechanical properties of suspended MoS₂ monolayers. Atomic-force microscopy (AFM) tests prove that S and MoS_x point defects reduce the stiffness but enhance the fracture toughness of MoS₂ monolayers. The deflection and bifurcation of cracks and the atomic structure near the crack edges are revealed by scanning-transmission-electron-microscope (STEM) images. Molecular-dynamics simulations based on classical force fields reproduce the microscopic features observed in experiments. MD simulations further predict that defective MoS₂ monolayers remain as brittle as pristine ones. The calculated energy release rate is higher in the presence of point defects, which explains the enhancement of fracture toughness as observed in experiments.