Defect engineering and hydrogen-induced reversibility in metallic states of MoS₂ grain boundaries

One-dimensional metallic states along grain boundaries (GBs) in two-dimensional (2D) semiconducting materials offer unique opportunities for electronic and quantum device applications. However, the stability and tunability of these metallic states remain poorly understood. Here, we systematically investigate how point defects and hydrogenation affect the electronic properties of representative GBs in monolayer MoS₂ using density functional theory. We identify two classes of GBs based on their symmetry response to point defects: defect-sensitive boundaries, which lose metallic states due to symmetry breaking, and defect-robust boundaries, which preserve metallic conduction owing to symmetry retention. Remarkably, hydrogenation can reverse the effects of point defects, restoring metallic states in defect-sensitive GBs and opening band gaps in defect-robust ones. These findings reveal a reversible and controllable mechanism for tuning grain boundary conduction through defect engineering and chemical functionalization, offering new pathways for nanoscale interconnects and reconfigurable 2D electronic devices.