Atomic Fracture Mechanism in Suspended 2D Transition Metal Dichalcogenides

A comprehensive understanding of atomic fracture mechanisms in 2D materials is essential for their practical applications, yet this knowledge is currently limited. To address this gap, an aberration-corrected scanning transmission electron microscope (STEM) to induce new cracks in suspended monolayer transition metal dichalcogenides (TMDs) using broad electron beam illumination, is employed. During characterization, a low-dose electron beam to avoid irradiation damage, allowing to observe the atomic fracture behavior in these materials, is utilized. The STEM experiments reveal a novel atomic fracture pattern along the zigzag direction, resulting in a distribution where half of the chalcogen atoms (S or Se) adhered to the molybdenum-terminated (Mo-T) edge and the other half to the chalcogen-terminated (S-T or Se-T) edge. Density functional theory (DFT) calculations suggest that this fracture mode produces a pair of edges with the lowest formation energy. Additionally, molecular dynamics (MD) simulations support the observed fracture behavior under a mixed mechanical loading mode of "I+III" with both in-plane and out-of-plane stress, originating from the ultrathin nature and nonplanar deformation in suspended 2D materials. This research offers new insights for the development of 2D fracture mechanics and is pivotal for designing devices incorporating 2D materials. Non-destructive in situ scanning transmission electron microscopy reveals a novel atomic fracture behavior in suspended monolayer MoS2 and MoSe2, featured by single chalcogen atoms (S or Se) on both crack edges. The out-of-plane deformation, arising from the ultrathin nature of these suspended monolayer films, plays a central role in this fracture process. image