Multivalency-Induced Band Gap Opening at MoS2 Edges

Zigzag edges of monolayer MoS2 and other transition-metal (TM) dichalcogenides are experimentally shown to exhibit strong photoluminescence. Atomic models that have been proposed for these edges, however, are all metallic. Here, we address this puzzle by using first-principles calculations. We found that a more generic electron counting model (ECM) can be developed, which, when coupled with the ability of TM atoms at edges to change their valency from 4+ to 5+, can quantitatively account for the band gap opening at the zigzag edges. Due to the ECM, a 3× periodicity along the zigzag edge is necessary to open the band gap. Moreover, consistent with experiment, oxygen adsorption is shown to open even larger band gaps than intrinsic edges.