Grain-Size-Controlled Mechanical Properties of Polycrystalline Monolayer MoS₂

Pristine monocrystalline molybdenum disulfide (MoS₂) possesses high mechanical strength comparable to that of stainless steel. Large-area chemical-vapor-deposited monolayer MoS₂ tends to be polycrystalline with intrinsic grain boundaries (GBs). Topological defects and grain size skillfully alter its physical properties in a variety of materials; however, the polycrystallinity and its role played in the mechanical performance of the emerging single-layer MoS₂ remain largely unknown. Here, using large-scale atomistic simulations, GB structures and mechanical characteristics of realistic single-layered polycrystalline MoS₂ of varying grain size prepared by confinement-quenched method are investigated. Depending on misorientation angle, structural energetics of polar-GBs in polycrystals favor diverse dislocation cores, consistent with experimental observations. Polycrystals exhibit grain-size-dependent thermally induced global out-of-plane deformation, although defective GBs in MoS₂ show planar structures that are in contrast to the graphene. Tensile tests show that presence of cohesive GBs pronouncedly deteriorates the in-plane mechanical properties of MoS₂. Both stiffness and strength follow an inverse pseudo Hall-Petch relation to grain size, which is shown to be governed by the weakest link mechanism. Under uniaxial tension, transgranular crack propagates with small deflection, whereas upon biaxial stretching, the crack grows in a kinked manner with large deflection. These findings shed new light in GB-based engineering and control of mechanical properties of MoS₂ crystals toward real-world applications in flexible electronics and nanoelectromechanical systems.