Atomic Imaging of Phase Transformation and Self-Intercalation of Two-Dimensional CrS ₂ by <i>In Situ</i> TEM

Controlling phase transitions in two-dimensional (2D) materials offers a powerful route for engineering novel electronic and magnetic functionalities. However, atomically resolved visualization of these dynamic processes remains a significant challenge. Herein, we report the synthesis of ultrathin (1.6 nm), single-crystal 1T-CrS₂ nanosheets via atmospheric-pressure chemical vapor deposition (APCVD) and uncover their thermal transformation pathway using <i>in situ</i> heating transmission electron microscopy (TEM). Real-time atomic-scale imaging reveals that upon heating to 500 °C, the material undergoes an irreversible structural and magnetic transformation from a layered, antiferromagnetic 1T-CrS₂ structure into a nonlayered, ferrimagnetic Cr₂S₃ structure. The driving mechanism is identified as a unique self-intercalation process initiated by the thermal depletion of S atoms, which promotes the migration of lattice Cr atoms into van der Waals (vdW) gaps to form new interlayer covalent bonds. This transformation represents a fundamental dimensional crossover from a 2D vdW crystal to a three-dimensionally bonded material at the nanoscale. Our findings elucidate a critical thermal transformation pathway in Cr-based 2D magnets and demonstrate a mechanism for irreversibly switching both the crystal structure and the magnetic order, thereby providing crucial insights for the design of thermally stable phase-change memory devices. a reduction in the slope of the curve was observed compared with the lower-voltage regime.