Oxygen-Assisted Growth of 2D Vanadium Oxyhalides with Ordered Vanadium Dimerization

Two-dimensional (2D) transition metal halides and oxyhalides offer a versatile platform for investigating emergent quantum phenomena and functionalities. However, achieving structural tunability remains a major challenge. Here, we report an oxygen-assisted chemical vapor deposition (CVD) method for the selective synthesis of vanadium dichloride (VCl₂) and vanadium oxychloride (VOCl). By modulating the oxygen partial pressure within a space-confined growth environment, we achieve precise phase control and synthesize large-area ultrathin crystals with high crystallinity and a well-defined stoichiometry. Atomic-resolution electron microscopy imaging reveals that VOCl exhibits long-range periodic vanadium dimerization, resulting in a symmetry-lowering lattice distortion that enables second-harmonic generation (SHG) in an otherwise centrosymmetric structure. Additionally, VOCl demonstrates pronounced in-plane anisotropy, reflecting the directional phonon responses inherent to its lattice symmetry. These findings not only establish a robust synthetic strategy for 2D transition metal halides and oxyhalides but also identify VOCl as a model system for exploring lattice-driven symmetry breaking and nonlinear optical responses in centrosymmetric systems.