Electrochemically Enabled Self-Intercalation for Symmetry Breaking and Metallic Ferroelectricity in Nb1+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 Nb1+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 Nb1+xS2. This work establishes an electrochemically oxidized metal precursor-assisted self-intercalation framework for symmetry engineering and multifunctional property modulation in metallic 2D materials.