Intrinsic Fermi Level Pinning and Reversible Depinning at Ultralow-Defect Epitaxial MoSe2/Au(111) Interfaces.

Fermi level pinning (FLP) fundamentally limits contact engineering in two-dimensional semiconductors, yet its intrinsic origin remains obscured by the presence of interfacial disorder and defect-induced gap states. Although theoretical models attribute FLP to metal-induced gap states (MIGSs) and interface dipoles, direct experimental validation at an ideal van der Waals interface has remained elusive. Here, we establish an ultralow-defect epitaxial monolayer MoSe2/Au(111) interface (defect density ∼ 1.7 × 1010 cm-2) to directly probe its electronic structure using scanning tunneling spectroscopy and field-emission resonance. Despite the nearly pristine interface, we observe symmetric electron and hole Schottky barriers alongside pronounced work function lowering, demonstrating that FLP persists even without significant interfacial disorder. These measurements quantitatively support the MIGS-induced interface dipole model. Remarkably, selenium intercalation reversibly suppresses the interfacial dipole, restores the work function, and shifts the Fermi level, resulting in substantial depinning and establishing a strategy for band alignment engineering at van der Waals interfaces.