Surface Reconstruction-Driven Insulating Behavior in Metallic Charge-Density-Wave 1T-TaSe2

Bulk 1T-TaSe2 is metallic, yet its surface consistently exhibits an insulating gap─a dichotomy long attributed to a surface Mott insulator driven by enhanced electron correlations. Using density functional theory calculations, we show that this insulating surface instead originates from a charge-density-wave (CDW) stacking reconstruction. Whereas the bulk stabilizes a single-layer CDW stacking that supports metallic transport, the surface energetically favors a bilayer stacking in which interlayer hybridization of Ta 5dz2 orbitals opens a ∼0.4 eV gap─a band insulator requiring no on-site Coulomb repulsion. This reconstruction is the thermodynamic ground state for slab thicknesses from two to eight layers, and the calculated surface density of states quantitatively reproduces scanning tunneling spectra of both insulating and metallic domains. CDW surface reconstruction, rather than Mott physics, thus governs the surface electronic structure of 1T-TaSe2, providing a unified explanation for the observed coexistence of metallic and insulating domains.