Isolation of Monolayer Metastable MoS₂ with Trimerized Molybdenum Atoms

While the Mo-trimerized 2a × 2a superlattice (1T″ phase) in intercalated MoS₂ was identified decades ago, its monolayer form has remained experimentally inaccessible. Here, we uncover a unique layer-dependent phase stability in MoS₂, wherein the √3a × √3a superlattice (1T‴ phase) spontaneously transforms into the 1T″ configuration upon exfoliation to the monolayer limit. Leveraging this phenomenon, we achieve the first successful isolation of high-purity monolayer 1T″-MoS₂. Our comprehensive characterization reveals that this new member of the two-dimensional (2D) materials family exhibits remarkable ambient stability and intrinsic semiconducting properties. Most significantly, electrochemical measurements demonstrate that the basal plane of monolayer 1T″-MoS₂ is intrinsically catalytically active for the hydrogen evolution reaction (HER)-a stark contrast to the edge-dominated activity in stable 2H-MoS₂. Theoretical analysis attributes this high density of basal-plane active sites to specific configurations of surface sulfur atoms. This work not only provides deep insights into phase stability at the atomic scale but also opens a new avenue for the rational design and preparation of metastable 2D polymorphs.