Nanoscale Determination of the Metal-Insulator Transition in Intercalated Bulk VSe2.

Two-dimensional (2D) materials provide unique opportunities to realize emergent phenomena by reducing their dimensionality. Using scanning tunneling microscopy combined with first-principles calculations, we determine an intriguing case of a metal-insulator transition (MIT) in a bulk compound, (TBA)_0.3VSe₂. Atomic-scale imaging reveals that the initial 4a₀ × 4a₀ charge density wave (CDW) order in 1T-VSe₂ transforms to √7a₀ × √3a₀ ordering upon intercalation, which is associated with an insulating gap with a magnitude of up to approximately 115 meV. Our calculations reveal that this energy gap is highly tunable through electron doping introduced by the intercalant. Moreover, the robustness of the √7a₀ × √3a₀ CDW order against the Lifshitz transition points to the key role of electron-phonon interactions in stabilizing the CDW state. Our work clarifies a rare example of a CDW-driven MIT in quasi-2D materials and establishes cation intercalation as an effective pathway for tuning both the dimensionality and the carrier concentration without inducing strain or disorder.