Atomistic Kinetics of Dislocation-Mediated Grain Growth in Monolayer MoS₂

Grain boundaries (GBs) in 2D materials are arrays of dislocations that strongly influence electronic, optical, mechanical, and transport properties. In monolayer MoS₂, Mo 5|7 and S 5|7 dislocation cores are structurally established, yet how their distinct mobilities govern GB migration and grain growth has remained unresolved. Here, we directly image and quantify GB mobility in polycrystalline monolayer MoS₂ at the level of individual dislocation cores. An atomic-scale in-situ heating study reveals that mobile GBs consist solely of Mo 5|7 dislocations, whereas immobile GBs contain S 5|7 kink sites that suppress GB motion. Dark-field TEM grain-growth measurements show that mobile GBs drive rapid coarsening, while S 5|7-pinned immobile GBs arrest microstructural evolution. Molecular dynamics simulations correlate this mobility contrast with distinct local mechanical and energetic environments around Mo 5|7 and S 5|7 cores, providing an atomistic origin for their different migration kinetics. These findings establish a direct structure-kinetics-growth relationship for GB evolution in monolayer MoS₂ and demonstrate a broadly applicable in-situ electron-microscopy platform for probing defect-driven microstructural dynamics in 2D materials.