Observation of a Novel Charge Density Wave Superstructure in Monolayer 1T-$VS_{2}$ at Room Temperature and its Evolution in Multilayers

TL;DR

Observation of a novel incommensurate CDW superstructure in ML $1T$-VS2 at room temperature and its evolution with layer number and V self-intercalation. The authors develop a top-down $LPESTP$ to produce ML and multilayer VS2 and use $HRTEM$/$ED$ to reveal coexisting $1T$ and $2H$ polymorphs, with a novel $(\sqrt{7} \times \sqrt{7})$ CDW in ML; increasing thickness yields a commensurate $(2\times2\times1)$ CDW driven by V3+ intercalation and interlayer charge transfer. Angle-resolved $XPS$/$UPS$ and $DFT$ calculations confirm V3+ intercalation, V3d–S 3p hybridization, and phonon-driven CDW instabilities, including soft modes that favor the CDW, providing a framework for understanding CDW evolution with layer number and V intercalation in $1T$-VS2. In twisted bilayers, Moiré superlattices coexist with trapped monolayer CDW, highlighting a Moiré-CDW interplay in this system. Overall, the work establishes a platform to study thickness- and intercalation-controlled CDW orders in 1T-VS2 with potential implications for 2D CDW materials and devices.

Abstract

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 VS\textsubscript{2}, 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 VS\textsubscript{2} at room temperature, and only the 1T phase undergoes CDW transition. We discovered a novel incommensurate CDW superstructure ($\sqrt{7} \times \sqrt{7}$) R19.1\textsuperscript{o} in ML 1T-VS\textsubscript{2}. With an increase in the number of layers, the CDW order changes to a commensurate ($2 \times 2\times 1$) superstructure. Using angle-dependent photoelectron spectroscopy and TEM, we have shown that vanadium atoms self-intercalate as V\textsuperscript{3+} ions in multilayer VS\textsubscript{2} and are responsible for the evolution of the CDW superstructure from the incommensurate ($\sqrt{7} \times \sqrt{7}$) R 19.1\textsuperscript{o} to the commensurate ($2\times2\times1$) order. We also report the observation of novel Moiré superlattices in twisted bilayer 1T-VS\textsubscript{2} flakes with trapped CDW superstructure of the monolayer. The density functional theory (DFT) calculation performed on ML 1T-VS\textsubscript{2} show that the observed ($\sqrt{7} \times \sqrt{7}$) R 19.1\textsuperscript{o} 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-VS\textsubscript{2} with layer numbers and V self-intercalation.

Figures (7)

• Figure 1: Schematic of the synthesis process and atomic force microscopy (AFM) topography images of ultrathin VS2 flakes. (a) Schematic of the liquid phase exfoliation and stamp transfer process (LPESTP) of VS2 flakes onto TEM grid. (b) AFM height-trace images of ultrathin VS2 flakes. (c) Height distributions of the ultrathin VS2 flakes in terms of the layer numbers and their number of occurrence in random areas of the SiO2/Si substrate. Thickness of ML VS2 is approximately 0.7 nm.
• Figure 2: Co-existence of 1T and 2H polymorphs of VS2. (a) High resolution transmission electron microscopy (HRTEM) image of an ML VS2 flake at 295 K in real space, which shows the co-existence of 1T and 2H polymorphs as marked. Inset is the fast Fourier transform (FFT) of the above real-space HRTEM image to get the reciprocal-space spots and symmetry. (b) The two polymorphic phases, as marked, and their boundaries are drawn by white dotted lines. Vanadium (larger and brighter spots) and sulfur (smaller and less bright spots) atoms are clearly visible from the atomically resolved Fourier filtered HRTEM image. (c) 1T' and (d) 2H polymorphs are identified from the atomically resolved HRTEM image. Atomic structure models of 1T, 1T', and 2H-VS2 are shown, which confirm their co-existence in ML VS2 flakes.
• Figure 3: Direct observation of CDW order in ML VS2 flakes. (a) HRTEM image (Fourier filtered) of an ML of VS2 flake taken at 295 K. A clearer view of the undistorted Bragg lattice (marked yellow) and CDW super-lattice (marked red) is shown in the top right corner inset. The bottom right inset is the fast Fourier transform (FFT) of the real-space HRTEM image showing distinct spots in the reciprocal space. The undistorted and CDW order spots are marked by yellow and red circles, respectively. The sides of the unit cell of the CDW superstructure are $\sqrt{7}$ times each of the sides of the Bragg-lattice unit cell of VS2 in real space. The unit cells of these two types of lattices make 19.1o angle with each other, (b) Schematics of the unit cells and first Brillouin zones for undistorted ($1\times1$) and ($\sqrt{7} \times \sqrt{7}$) CDW order as marked by yellow and red are shown respectively in reciprocal space along with the CDW modulation positions (red points). (c) Schematic illustration of the ($\sqrt{7} \times \sqrt{7}$) R 19.1o CDW (marked by red) and corresponding undistorted ($1\times1$) Bragg lattice (marked by yellow) unit cells showing the relative angle and surface re-arrangements.
• Figure 4: CDW superlattices in multilayer VS2 flake. (a) Electron diffraction (ED) pattern from a multilayer VS2 flake showing undistorted Bragg spots (marked in yellow) as well as superlattice spots (marked in red). The inset shows a closer view of the diffraction spots near the central beam. ($1\times1$) Bragg spots (yellow) and ($2\times2$) superlattice spots (red) are marked. There is no angle between the Bragg and superlattice spots. (b) HRTEM image of a multilayer VS2 flake showing a clear ($2\times2$) CDW order. The top right inset is the FFT spots corresponding to the HRTEM image of (b), showing clearly the ($2\times2$) CDW order spots and Bragg spots as marked by red and yellow circles, respectively. Bottom right inset is the zoomed-in view of the region marked by the white border area, where undistorted Bragg lattice points (yellow) as well as super-lattice points (red) are marked. (c) and (d) show the schematics of the top and side view of the intercalated V3+ ions in the VS2 lattice, which appear at the ($2\times2$) octahedral void positions between the two VS2 layers, respectively.
• Figure 5: Moiré superlattice in a bilayer VS2 flake. (a) HRTEM image of the Moiré superlattice in real space taken at room temperature (295 K), (b) FFT of the HRTEM image of Figure 5(a) shows the Moiré superlattice spots in reciprocal space. The twist angle between the two layers is estimated to be 20.8o. (c) Simulated Moiré pattern of the twisted bilayer of 1T-VS2 with the twist angle 20.8o matches with the observed pattern as in Figure 5(a).