ArXiv · 2025
Spontaneous formation of charge density wave (CDW) superstructures in monolayers (MLs) of a two-dimensional (2D) crystal lattice is fundamental in understanding its complex quantum states. We report a successful top-down liquid phase exfoliation and stamp transfer process (LPESTP) to create ML VS₂, undergoing a CDW transition at room temperature. Using high-resolution transmission electron microscopy (HRTEM) and electron diffraction (ED), we observed the coexistence of 1T and 2H polymorphic phases in VS₂ at room temperature, and only the 1T phase undergoes CDW transition. We discovered a novel incommensurate CDW superstructure (√7 × √7) R19.1o in ML 1T-VS₂. With an increase in the number of layers, the CDW order changes to a commensurate (2 × 2× 1) superstructure. Using angle-dependent photoelectron spectroscopy and TEM, we have shown that vanadium atoms self-intercalate as V³+ ions in multilayer VS₂ and are responsible for the evolution of the CDW superstructure from the incommensurate (√7 × √7) R 19.1o to the commensurate (2×2×1) order. We also report the observation of novel Moiré superlattices in twisted bilayer 1T-VS₂ flakes with trapped CDW superstructure of the monolayer. The density functional theory (DFT) calculation performed on ML 1T-VS₂ show that the observed (√7 × √7) R 19.1o CDW superstructure has lower energy compared to that of the pristine undistorted ML and the CDW instability is driven by formation of strong soft-phonon modes. Our findings provide an important platform for understanding the evolution of CDW superstructures in 1T-VS₂ with layer numbers and V self-intercalation.
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