Microscopic insight into the origin of super-cooled NCCDW state in 1T-TaS₂ nanocrystals.

Tantalum disulfide (1T-TaS₂) is a quasi-two-dimensional transition metal dichalcogenide (TMD) that exhibits a series of charge density wave (CDW) transitions upon cooling and heating. These collective electronic phases can be tuned or disrupted by external stimuli such as pressure, electric fields, or illumination, leading to metastable or hidden metallic states. In nanoscale crystals of 1T-TaS₂, rapid cooling suppresses the insulating commensurate CDW (CCDW) phase and stabilizes a metastable metallic state, known as the super-cooled nearly commensurate CDW (SC-NCCDW) phase. However, the atomic-scale structure and microscopic origin of this state remain elusive. Here, we combine electrical transport measurements with structural characterization to elucidate the nature of the SC-NCCDW phase in 1T-TaS₂ nanocrystals. Temperature-dependent X-ray diffraction reveals that, under gradual cooling, the NCCDW-to-CCDW transition is accompanied by lattice-volume expansion. In contrast, this anomaly is strongly suppressed upon rapid cooling, correlating with the stabilization of the SC-NCCDW state. Complementary high-resolution transmission electron microscopy (HR-TEM) shows that rapid cooling produces a mixed-phase configuration containing structural motifs of both NCCDW and CCDW phases, indicating that the SC-NCCDW represents an intermediate structural configuration frozen by kinetic constraints. These findings provide, to our knowledge, the first direct structural evidence of the SC-NCCDW state and offer a mechanistic understanding of cooling-rate-controlled metastability in layered correlated compounds such as 1T-TaS₂.