Electrochemically Enabled Self-Intercalation for Symmetry Breaking and Metallic Ferroelectricity in Nb_1+xS₂

Two-dimensional (2D) metallic materials that simultaneously exhibit inversion symmetry breaking are highly desirable for nonlinear optics, unconventional ferroelectrics, and symmetry-driven electronic functionalities, yet controllably inducing structural asymmetry without compromising metallicity remains a central challenge. Here, we report a self-intercalation strategy to engineer symmetry breaking in metallic NbS₂ by exploiting the intrinsic oxidation behavior of niobium. Electrochemical anodization precisely tailors the native oxide layer on Nb foil, which serves as a tunable Nb reservoir during space-confined chemical vapor deposition (SCCVD), enabling spontaneous intercalation of excess Nb atoms into the van der Waals gaps of NbS₂. Atomic-resolution characterizations reveal the presence and disordered distribution of intercalated Nb atoms, accompanied by interlayer expansion and superlattice features. Quantitative scanning transmission electron microscopy analysis combined with Energy-dispersive x-ray spectroscopy measurements demonstrates continuously tunable intercalation concentrations in Nb_1+xS₂. Progressive Nb intercalation systematically enhances inversion symmetry breaking, leading to strengthened second-harmonic generation responses. Robust room-temperature ferroelectricity is achieved while preserving metallic transport, demonstrating the coexistence of ferroelectricity and metallicity in self-intercalated Nb_1+xS₂. This work establishes an electrochemically oxidized metal precursor-assisted self-intercalation framework for symmetry engineering and multifunctional property modulation in metallic 2D materials.