Novel Superstructure-Phase Two-Dimensional Material 1T-VSe2 at High Pressure

Double Superconducting Dome of Quasi Two-Dimensional TaS2 in Non-Centrosymmetric van der Waals Heterostructure

Two-dimensional van der Waals heterostructures (2D-vdWHs) based on transition metal dichalcogenides (TMDs) provide unparalleled control over electronic properties. However, the interlayer coupling is challenged by the interfacial misalignment and defects, which hinders a comprehensive understanding of the intertwined electronic orders, especially superconductivity and charge density wave (CDW). Here, by using pressure to regulate the interlayer coupling of non-centrosymmetric 6R-TaS2 vdWHs, we observe an unprecedented phase diagram in TMDs. This phase diagram encompasses successive suppression of the original CDW states from alternating H-layer and T-layer configurations, the emergence and disappearance of a new CDW-like state, and a double superconducting dome induced by different interlayer coupling effects. These results not only illuminate the crucial role of interlayer coupling in shaping the complex phase diagram of TMD systems but also pave a new avenue for the creation of a novel family of bulk heterostructures with customized 2D properties.

Electron and ion transport behavior of Vanadium based MXene induced by pressure for Lithium ion intercalated electrodes

Pressure, analogous with temperature and composition, is other meaningful variant for tuning the structure-activity properties of layered materials. In-situ high-pressure electrical results discover that Vanadium based MXene (V₂CT_x MXene) conductivity is increased by one order of magnitude from ambient to 10.4 GPa, and then the conductivity is still fixated on meeting growth as pressure releasing. Increased carrier concentration due to denser compactness is the most important factor in improving sample conductivity. Furthermore, abundant of V₂CT_x samples after preloading different pressures are prepared by the mean of the double-anvil hydraulic press for the first time, and results of increased conductivity were reproduced at ambient conditions. The first-principles calculation of V₂C (non-functional group), V₂CF, V₂CO, and V₂COH explains for the lattice expansion by tracing emotion of different function groups upon decompression. Electrochemical results obtain that once forming V₂CT_x MXene anode rapidly quenched from 2.0 GPa in hydraulic press shows better performance, obviously weakening electric polarization and increasing Li-ion transport rate due to its proper interlaminar densification and improved conductivity. This work opens up a new, simple, and universal approach to develop MXene materials with superior electrical and electrochemical properties, as well as expanding the potential applications for energy storage.

Vanadium-Doped Molybdenum Diselenide Atomic Layers with Room-Temperature Ferromagnetism

Two-dimensional diluted magnetic semiconductors with high Curie temperature are highly sought after because of their potential applications in spintronics. Development of new techniques for preparation of high quanlity diluted magnetic semiconductors is critical for their applications. In this study, vanadium-doped molybdenum selenide, a new diluted magnetic semiconductor, was synthesized by a single-step chemical vapor deposition method. The merit of this method is that the molybdenum and vanadium precursors can be supplied to the growth substrate uniformly. Photoluminescence measurements reveal that the band gap of MoSe 2 decreases after doping, which can be attributed to the formation of impurity energy band caused by p-type doping at the valence band maximum. Thus, the V-doped MoSe 2 still maintains the semiconducting characteristics. Vibrating sample magnetometer studies clearly show the ferromagnetism of V-doped MoSe 2 at room temperature. DFT calculations illustrates the joint contribution of V dopants and nearby atoms to the magnetic moments. This study provides future prospects for the multifunctional application of two-dimensional materials.

Observation of pressure induced charge density wave order and eightfold structure in bulk VSe2

Pressure-induced charge density wave (CDW) state can overcome the low-temperature limitation for practical application, thus seeking its traces in experiments is of great importance. Herein, we provide spectroscopic evidence for the emergence of room temperature CDW order in the narrow pressure range of 10–15 GPa in bulk VSe₂. Moreover, we discovered an 8-coordination structure of VSe₂ with C2/m symmetry in the pressure range of 35–65 GPa by combining the X-ray absorption spectroscopy, X-ray diffraction experiments, and the first-principles calculations. These findings are beneficial for furthering our understanding of the charge modulated structure and its behavior under high pressure.

A New Superconducting 3R-WS2 Phase at High Pressure

High-pressure investigation has been shown to be of paramount significance for changing the conventional lattice or bringing fascinating properties, especially inducing superconducting phases. Here we studied the application of pressure to the recently synthesized 2M-WS₂ with the record T_c (8.8 K) among transition metal dichalcogenides (TMDs) at ambient pressure by electrical resistance investigations, synchrotron X-ray studies, and theoretical calculations. T_c in the initial 2M-WS₂ dropped from the maximum to become undetected, accompanied by a phase transition into a semiconductor, 3R-WS₂, at 15 GPa. The successive metallization and superconducting transitions in 3R-WS₂ were observed at 48.8 GPa with T_c ≈ 2.5 K. This is the first experimental case in which superconductivity has been realized in the 3R phase among TMDs. We propose that the degradation of superconductivity in 2M-WS₂ and the reemergence of superconductivity in 3R-WS₂ are mainly attributable to changes in the density of states near the Fermi surface driven by the interlayer coupling.

2D Materials and Heterostructures at Extreme Pressure

2D materials possess wide-tuning properties ranging from semiconducting and metallization to superconducting, etc., which are determined by their structure, empowering them to be appealing in optoelectronic and photovoltaic applications. Pressure is an effective and clean tool that allows modifications of the electronic structure, crystal structure, morphologies, and compositions of 2D materials through van der Waals (vdW) interaction engineering. This enables an insightful understanding of the variable vdW interaction induced structural changes, structure-property relations as well as contributes to the versatile implications of 2D materials. Here, the recent progress of high-pressure research toward 2D materials and heterostructures, involving graphene, boron nitride, transition metal dichalcogenides, 2D perovskites, black phosphorene, MXene, and covalent-organic frameworks, using diamond anvil cell is summarized. A detailed analysis of pressurized structure, phonon dynamics, superconducting, metallization, doping together with optical property is performed. Further, the pressure-induced optimized properties and potential applications as well as the vision of engineering the vdW interactions in heterostructures are highlighted. Finally, conclusions and outlook are presented on the way forward.

Prediction of topological nontrivial semimetals and pressure-induced Lifshitz transition in 1T'-MoS2 layered bulk polytypes

Recently, bulk MoS2 crystals stacked by 1T'-MoS2 monolayers have been synthesized successfully, but little is known about their stacking sequences and topological properties. Based on first-principles calculations and symmetry-based indicator theory, we discovered that three predicted bulk structures of MoS2 (named 2M-, 1T'- and β-MoS2) stacked by 1T' monolayers are topological insulators and nodal line semimetals with and without spin-orbit coupling. Their stacking stability, electronic structure and the topology origin were systematically investigated. Further research proves that in the absence of SOC the open- and closed-type nodal lines can coexist in the momentum space of 2M-MoS2, which also possesses drumhead-like surface state. Moreover, we predicted a pressure-induced Lifshitz transition at about 1.3 GPa in 2M-MoS2. Our findings greatly enrich the topological phases of MoS2 and probably bring MoS2 to the rapidly growing family of layered topological semimetals.

New Pressure Stabilization Structure in Two-Dimensional PtSe2

The frequency shifts and lattice dynamics to unveil the vibrational properties of platinum diselenide (PtSe₂) are investigated using pressure-dependent polarized Raman scattering at room temperature up to 25 GPa. The two phonon modes E_g and A_1g display similar hardening trends; both the Raman peak positions and full widths at half-maximum have distinct mutation phenomena under high pressure. Especially, the split E_g mode at 4.3 GPa confirms the change of the lattice symmetry. With the aid of the first-principles calculations, a new pressure stabilization structure C2/m of PtSe₂ has been found to be in good agreement with experiments. The band structures calculations reveal that the new phase is a novel type-I Dirac semimetal. The results demonstrate that the pressure-dependent Raman spectra combined with theoretical predictions may open a new window for searching and controlling the phase structure and Dirac cones of two-dimensional materials.