Metal-organic chemical vapor deposition of 2D van der Waals materials-The challenges and the extensive future opportunities

The last decade has witnessed significant progress in two-dimensional van der Waals (2D vdW) materials research; however, a number of challenges remain for their practical applications. The most significant challenge for 2D vdW materials is the control of the early stages of nucleation and growth of the material on preferred surfaces to eventually create large grains with digital thickness controllability, which will enable their incorporation into high-performance electronic and optoelectronic devices. This Perspective discusses the technical challenges to be overcome in the metal-organic chemical vapor deposition (MOCVD) growth of 2D group 6 transition metal dichalcogenide (TMD) atomic crystals and their heterostructures, as well as future research aspects in vdW epitaxy for 2D TMDs via MOCVD. In addition, we encourage the traditional MOCVD community to apply their expertise in the field of "2D vdW materials," which will continue to grow at an exponential rate

Recent advances in metallic transition metal dichalcogenides as electrocatalysts for hydrogen evolution reaction

Layered metallic transition metal dichalcogenides (MTMDs) exhibit distinctive electrical and catalytic properties to drive basal plane activity, and, therefore, they have emerged as promising alternative electrocatalysts for sustainable hydrogen evolution reactions (HERs). A key challenge for realizing-MTMDs-based electrocatalysts is the controllable and scalable synthesis of high-quality MTMDs and the development of engineering strategies that allow tuning their electronic structures. However, the lack of a method for the direct synthesis of MTMDs retaining the structural stability limits optimizing the structural design for the next generation of robust electrocatalysts. In this review, we highlight recent advances in the synthesis of MTMDs comprising groups VB and VIB and various routes for structural engineering to enhance the HER catalytic performance. Furthermore, we provide insight into the potential future directions and the development of MTMDs with high durability as electrocatalysts to generate green hydrogen through water-splitting technology

Electrically Robust Single-Crystalline WTe2 Nanobelts for Nanoscale Electrical Interconnects

As the elements of integrated circuits are downsized to the nanoscale, the current Cu-based interconnects are facing limitations due to increased resistivity and decreased current-carrying capacity because of scaling. Here, the bottom-up synthesis of single-crystalline WTe2 nanobelts and low- and high-field electrical characterization of nanoscale interconnect test structures in various ambient conditions are reported. Unlike exfoliated flakes obtained by the top-down approach, the bottom-up growth mode of WTe2 nanobelts allows systemic characterization of the electrical properties of WTe2 single crystals as a function of channel dimensions. Using a 1D heat transport model and a power law, it is determined that the breakdown of WTe2 devices under vacuum and with AlOx capping layer follows an ideal pattern for Joule heating, far from edge scattering. High-field electrical measurements and self-heating modeling demonstrate that the WTe2 nanobelts have a breakdown current density approaching approximate to 100 MA cm(-2), remarkably higher than those of conventional metals and other transition-metal chalcogenides, and sustain the highest electrical power per channel length (approximate to 16.4 W cm(-1)) among the interconnect candidates. The results suggest superior robustness of WTe2 against high-bias sweep and its possible applicability in future nanoelectronics

?????? ????????? ????????? ?????? ???????????? ?????? 2?????? ?????? ??????

Department of Materials Science and Engineeringope

Atomic transistors based on seamless lateral metal-semiconductor junctions with a sub-1-nm transfer length

The edge-to-edge connected metal-semiconductor junction (MSJ) for two-dimensional (2D) transistors has the potential to reduce the contact length while improving the performance of the devices. However, typical 2D materials are thermally and chemically unstable, which impedes the reproducible achievement of high-quality edge contacts. Here we present a scalable synthetic strategy to fabricate low-resistance edge contacts to atomic transistors using a thermally stable 2D metal, PtTe2. The use of PtTe2 as an epitaxial template enables the lateral growth of monolayer MoS2 to achieve a PtTe2-MoS2 MSJ with the thinnest possible, seamless atomic interface. The synthesized lateral heterojunction enables the reduced dimensions of Schottky barriers and enhanced carrier injection compared to counterparts composed of a vertical 3D metal contact. Furthermore, facile position-selected growth of PtTe2-MoS2 MSJ arrays using conventional lithography can facilitate the design of device layouts with high processability, while providing low contact resistivity and ultrashort transfer length on wafer scales. Edge-to-edge metal-semiconductor junctions have the potential to improve the performance of 2D transistors. Here, the authors report a synthetic strategy to fabricate monolayer MoS2-PtTe2 heterojunction arrays with sub-1-nm transfer length and enhanced carrier injection compared to vertical 3D metallic contacts.11Nsciescopu

Wafer-scale production of patterned transition metal ditelluride layers for two-dimensional metal-semiconductor contacts at the Schottky-Mott limit

A key challenge in the development of two-dimensional (2D) devices is the fabrication of metal-semiconductor junctions with minimal contact resistance and depinned energy levels. An ideal solution for practical applications is to make contacts between 2D van der Waals semiconductors and 2D van der Waals metals. Here we report the wafer-scale production of patterned layers of metallic transition metal ditellurides on different substrates. Our tungsten ditelluride and molybdenum ditelluride layers, which are grown using a tellurization process applied to a precursor transition metal layer, have an electronic performance comparable to that of mechanically exfoliated flakes and can be combined with the 2D semiconductor molybdenum disulfide. The resulting metal-semiconductor junctions are free from significant disorder effects and Fermi-level pinning, and are used to create monolayer molybdenum disulfide field-effect transistors. The Schottky barrier heights of the devices also largely follow the trend of the Schottky-Mott limit. Two-dimensional metallic WTe2 and MoTe2 layers can be combined with a semiconducting MoS2 monolayer to create metal-semiconductor junctions that are free from substantial disorder effects and Fermi-level pinning

Robust 2D layered MXene matrix–boron carbide hybrid films for neutron radiation shielding

Abstract Large-scale fabrication of neutron-shielding films with flexible or complex shapes is challenging. Uniform and high boron carbide (B4C) filler loads with sufficient workability are needed to achieve good neutron-absorption capacity. Here, we show that a two-dimensional (2D) Ti3C2T x MXene hybrid film with homogeneously distributed B4C particles exhibits high mechanical flexibility and anomalous neutron-shielding properties. Layered and solution-processable 2D Ti3C2T x MXene flakes serve as an ideal robust and flexible matrix for high-content B4C fillers (60 wt.%). In addition, the preparation of a scalable neutron shielding MXene/B4C hybrid paint is demonstrated. This composite can be directly integrated with various large-scale surfaces (e.g., stainless steel, glass, and nylon). Because of their low thickness, simple and scalable preparation method, and an absorption capacity of 39.8% for neutrons emitted from a 241Am– 9Be source, the 2D Ti3C2T x MXene hybrid films are promising candidates for use in wearable and lightweight applications

Design of 2D Layered Catalyst by Coherent Heteroepitaxial Conversion for Robust Hydrogen Generation

The structural engineering of 2D layered materials is emerging as a powerful strategy to design catalysts for high-performance hydrogen evolution reaction (HER). However, the ultimate test of this technology under typical operating settings lies in the reduced performance and the shortened lifespan of these catalysts. Here, a novel approach is proposed to design efficient and robust HER catalysts through out-of-plane deformation of 2D heterojunction using metal-organic chemical vapor deposition. High-yield, single-crystalline WTe2 nanobelts are used as an epitaxial template for their coherent conversion to WS2. During the conversion process, the WTe2/WS2 heterostructure containing both lateral and vertical junctions are achieved by coherent heteroepitaxial stacking despite differences in symmetry. The lattice coherency drives out-of-plane deformation of heteroepitaxially grown WS2. The increase in the effective surface area and decrease in the electron-transfer resistance across the 2D heterojunctions in turn enhances the HER performance as well as the long-term durability of these electrocatalysts