An Anomalous Formation Pathway for Dislocation-Sulfur Vacancy Complexes in Polycrystalline Monolayer MoS₂

Two-dimensional (2D) molybdenum disulfide (MoS₂) has attracted significant attention recently due to its direct bandgap semiconducting characteristics. Experimental studies on monolayer MoS₂ show that S vacancy concentration varies greatly; while recent theoretical studies show that the formation energy of S vacancy is high and thus its concentration should be low. We perform density functional theory calculations to study the structures and energetics of vacancy and interstitial in both grain boundary (GB) and grain interior (GI) in monolayer MoS₂ and uncover an anomalous formation pathway for dislocation-double S vacancy (V_2S) complexes in MoS₂. In this pathway, a (5|7) defect in an S-polar GB energetically favorably converts to a (4|6) defect, which possesses a duality: dislocation and double S vacancy. Its dislocation character allows it to glide into GI through thermal activation at high temperatures, bringing the double vacancy with it. Our findings here not only explain why V_S is predominant in exfoliated 2D MoS₂ and V_2S is predominant in chemical vapor deposition (CVD)-grown 2D MoS₂ but also reproduce GB patterns in CVD-grown MoS₂. The new pathway for sulfur vacancy formation revealed here provides important insights and guidelines for controlling the quality of monolayer MoS₂.