Unraveling the mechanism of vanadium self-intercalation in 1T-VSe₂: atomic-scale evidence for phase transition and superstructure model for intercalation compound

Abstract Self-intercalation is an efficient strategy for tailoring the property of layer structured materials like transition metal dichalcogenides (TMDCs), while the associated kinetics and mechanism remain scarcely explored. In this study, we investigate the atomic-scale dynamics and mechanism of vanadium (V) self-intercalation in multi-layer 1T-VSe₂ using in situ high resolution scanning transmission electron microscopy. The results reveal that the self-intercalation of V induces structural transformation of pristine VSe₂ into three V-enrich intercalated compounds, i.e. V₅Se₈, V₃Se₄ and VSe. The self-intercalated V follows an ordered arrangement of 2×2, 2×1, and 1×1 within the interlayer octahedral sites, corresponding to an intercalation concentration of 25%, 50% and 100% in V₅Se₈, V₃Se₄ and VSe, respectively. The V intercalants induced lattice distortions to the host 1T-VSe₂ such as the dimerization of neighboring lattice V is observed experimentally, which are further supported by density functional theory (DFT) calculations. Finally, a superstructure model generalizing the possible structures of self-intercalated compounds in layered TMDCs is proposed and then validated by the DFT determined formation energy landscape. This study provides comprehensive insights on the kinetics and mechanism of the self-intercalation in layered TMDC materials, contributing to the precise control for the structure and stoichiometry of self-intercalated TMDC compounds.