Direct Observation of Self-Intercalation Driven 2D-to-3D Phase Transition in VSe2.

Precise control of phase transitions is essential for tuning properties of two-dimensional (2D) materials. Self-intercalation can modulate structural and electronic states in layered systems, yet its microscopic mechanism remains unclear owing to scarce atomic-scale in situ evidence. Using atomic-resolution scanning transmission electron microscopy (STEM) with energy-dispersive X-ray spectroscopy (EDS), we directly visualize the self-intercalation-driven conversion from the 2D 1T phase to a three-dimensional (3D) self-intercalated phase in VSe2. In situ manipulation reveals atomic structural evolution as vanadium ions migrate into van der Waals (vdW) gaps during the 2D-to-3D transition. Density functional theory (DFT) calculations confirm the stability and intrinsic ferromagnetism of the 3D phase. This work establishes a structural evolution model for the 2D-to-3D transition in VSe2, elucidates the atomic mechanism of self-intercalation-induced phase transitions in transition metal dichalcogenides (TMDs), and provides a mechanistic foundation for rational phase engineering of low-dimensional magnetic materials.