In situ investigation of 1T'/1H phase transition in colloidal WS₂ monolayers

Controlling the crystal phase of two-dimensional transition metal dichalcogenides (TMDs) is essential for tailoring their electronic and optical properties. Among the polymorphs of WS₂, the metastable 1T' phase exhibits semimetallic or narrow-bandgap character and hosts quantum functionalities distinct from the semiconducting 1H phase. Here, we investigate the temperature-induced 1T'/1H phase transition in colloidally synthesized monolayer WS₂ nanosheets functionalized with organic ligands. The reducing conditions of the synthesis stabilize the 1T' phase via electron doping. Through in situ analyses of both the structural and electronic properties, we monitor the phase evolution during annealing and find that the 1T' phase remains stable up to 300 °C, accompanied by a relative lattice contraction. Between 300 °C and 350 °C, a mixed 1T'/1H regime appears, where the 1H content can be finely tuned by controlling the annealing time. Above 350 °C, a rapid and complete transformation to the 1H phase occurs. We demonstrate that the decomposition of the reducing ligand serves as the primary trigger of the structural transition, revealing a strong interplay among doping, surface chemistry, and lattice structure. Notably, nanosheets with smaller lateral dimensions exhibit slower phase transition kinetics, suggesting that finite size could influence the structural rearrangement underlying the phase transformation.