2D materials and the associated heterostructures define an ideal material platform for investigating physical and chemical properties, and exhibiting new functional applications in (opto)electronic devices, electrocatalysis, and energy storage. 2D transition metal dichalcogenides (2D TMDs), as a member of the 2D materials family including 2D semiconducting TMDs (s-TMDs) and 2D metallic/semimetallic TMDs (m-TMDs) have attracted considerable attention in the scientific community. Over the past decade, the 2D s-TMDs have been extensively researched and reviewed elsewhere. Because of their distinctive physical properties including intrinsic magnetism, chargedensity-wave order and superconductivity, and potential applications, such as high-performance electronic devices, catalysis, and as metal electrode contacts, 2D m-TMDs have grabbed widespread attention in recent years. However, reviews demonstrating the m-TMDs systematically and comprehensively have been rarely reported. Here, the recent advances in 2D m-TMDs in the aspects of their unique structures, synthetic approaches, distinctive physical properties, and functional applications are highlighted. Finally, the current challenges and perspectives are discussed.

Two-dimensional materials (2DMs) with excellent mechanical, thermal, optical, and catalytic properties have attracted a great deal of attention in recent years. Chemical vapor deposition (CVD) is an important method to realize the synthesis of high-quality 2DMs. In the growth of 2DMs through the CVD method, the substrates play an important role and can greatly affect the lateral size, composition, thickness, orientation, and crystal quality of 2DMs. In this review, we first introduce the growth mechanism and the key parameters in the CVD system for the synthesis of 2DMs. Then, the unique physical and chemical properties, advantages, and disadvantages of various substrates used in the CVD technique are summarized. Finally, the opportunities and challenges about the use of the substrate in the CVD process in the future are discussed.

bandgap can overcome the short channel effect, and then can scale down transistors to sub-10 nm [13,14] and even sub-5 nm [10,[15][16][17][18] to extend the Moore's law. Over last decades, typical 2D semiconductors, such as MoS 2 and WSe 2 , have been intensively studied. [19][20][21][22] On the other hand, 2D materials with versatile phase-transition properties offer new opportunities in both fundamental research and future device applications. In the last few years, a lot of exciting experiments have been conducted on this subfamily of 2D materials in the monolayer limit, such as charge density wave (CDW) 1T-TaS 2 , [23][24][25][26] superconducting (SC) 2H-NbSe 2 , [27][28][29] ferromagnetic (FM) CrI 3 , [12,30,31] and Cr 2 Ge 2 Te 6 . [11,32] CDW is a phenomenon of forming a standing-wave-like electron density usually accompanied with a lattice distortion and a bandgap opening due to electronphonon interaction. [33] SC is a phenomenon of zero electrical resistance and expulsion of magnetic flux fields also originated from electron-phonon interaction. [34] Magnetism and electric polarization stem from spontaneous spin/dipole ordering, respectively. [35,36] Parallel and antiparallel spins constitute long-range ferromagnetic (FM) and antiferromagnetic (AFM) ordering, respectively. Similarly, parallel and antiparallel dipoles form ferroelectric (FE) and antiferroelectric (AFE) ordering, respectively. Van der Waals layered materials can show a lot of novel properties, especially in the atomic thickness limit. This includes the follow ing terms: (1) The odd/even layer effect may emerge due to weak interlayer coupling. For example, FM or FE ordering may appear in the odd layers, while AFM or AFE ordering may appear in the even layers; (2) Due to confinement effect in the out-of-plane direction, new ordering structures may appear;(3) The properties can be easily tuned by strain, external fields, interface interaction, and so on. Toward fundamental property measurement and device applications, the challenge remains, such as predicting/discovering new 2D phase-transition materials, controlling the chemical synthesis, overcoming the poor air stability, and so on. Here, a literature survey has been carried out to search possible 2D phase-transition materials. Recent progress on the chemical synthesis and property investigation of some obtained monolayers has been reviewed, including CDW 1T-TaS 2 monolayers, FM CrI 3 mono layers, and so on.Layered materials with phase transitions, such as charge density wave (CDW) and magnetic and dipole ordering, have potential to be exfoliated into monolayers and few-layers and then become a large and important subfamily of two-dimensional (2D) materials. Benefitting from enriched physical properties from the collective interactions, long-range ordering, and related phase transitions, as well as the atomic thickness yet having nondangling bonds on the surface, 2D phase-transition materials have vast potential for use in new-concept and functional devices. Here, potential 2D phase-transit...

Abstract2D metallic vanadium dichalcogenides (VX2) have renewed interest due to their unique physical properties and potential applications as high‐performance functional nanomaterials. Herein, the synthesis and phase transitions of 2D metallic VX2 are investigated. The synthesis is done by chemical vapor deposition using hexagonal boron nitride and mica as substrates. High‐quality VX2 flakes with sub‐10 nm thickness are obtained and characterized by transmission electron microscopy. Temperature‐dependent Raman characterization between 330 and 5 K is conducted and remarkable changes of Raman modes at around 150 K for VS2 flakes and 110 K for VSe2 flakes are revealed, indicating charge density wave phase transitions. Temperature‐dependent electrical measurements have also supported the phase transitions observed in Raman characterization.