Two-Dimensional Metallic Vanadium Ditelluride as a High-Performance Electrode Material

Large-Area Epitaxial Growth of Transition Metal Dichalcogenides

Over the past decade, research on atomically thin two-dimensional (2D) transition metal dichalcogenides (TMDs) has expanded rapidly due to their unique properties such as high carrier mobility, significant excitonic effects, and strong spin-orbit couplings. Considerable attention from both scientific and industrial communities has fully fueled the exploration of TMDs toward practical applications. Proposed scenarios, such as ultrascaled transistors, on-chip photonics, flexible optoelectronics, and efficient electrocatalysis, critically depend on the scalable production of large-area TMD films. Correspondingly, substantial efforts have been devoted to refining the synthesizing methodology of 2D TMDs, which brought the field to a stage that necessitates a comprehensive summary. In this Review, we give a systematic overview of the basic designs and significant advancements in large-area epitaxial growth of TMDs. We first sketch out their fundamental structures and diverse properties. Subsequent discussion encompasses the state-of-the-art wafer-scale production designs, single-crystal epitaxial strategies, and techniques for structure modification and postprocessing. Additionally, we highlight the future directions for application-driven material fabrication and persistent challenges, aiming to inspire ongoing exploration along a revolution in the modern semiconductor industry.

Synthesis, Modulation, and Application of Two-Dimensional TMD Heterostructures

Two-dimensional (2D) transition metal dichalcogenide (TMD) heterostructures have attracted a lot of attention due to their rich material diversity and stack geometry, precise controllability of structure and properties, and potential practical applications. These heterostructures not only overcome the inherent limitations of individual materials but also enable the realization of new properties through appropriate combinations, establishing a platform to explore new physical and chemical properties at micro-nano-pico scales. In this review, we systematically summarize the latest research progress in the synthesis, modulation, and application of 2D TMD heterostructures. We first introduce the latest techniques for fabricating 2D TMD heterostructures, examining the rationale, mechanisms, advantages, and disadvantages of each strategy. Furthermore, we emphasize the importance of characteristic modulation in 2D TMD heterostructures and discuss some approaches to achieve novel functionalities. Then, we summarize the representative applications of 2D TMD heterostructures. Finally, we highlight the challenges and future perspectives in the synthesis and device fabrication of 2D TMD heterostructures and provide some feasible solutions.

Large-area and few-layered 1T'-MoTe2thin films grown by cold-wall chemical vapor deposition

This study employs cold-wall chemical vapor deposition to achieve the growth of MoTe2thin films on 4-inch sapphire substrates. A two-step growth process is utilized, incorporating MoO3and Te powder sources under low-pressure conditions to synthesize MoTe2. The resultant MoTe2thin films exhibit a dominant 1T' phase, as evidenced by a prominent Raman peak at 161 cm-1. This preferential 1T' phase formation is attributed to controlled manipulation of the second-step growth temperature, essentially the reaction stage between Te vapor and the pre-deposited MoOxlayer. Under these optimized growth conditions, the thickness of the continuous 1T'-MoTe2films can be precisely tailored within the range of 3.5-5.7 nm (equivalent to 5-8 layers), as determined by atomic force microscopy depth profiling. Hall-effect measurements unveil a typical hole concentration and mobility of 0.2 cm2Vs-1and 7.9 × 1021cm-3, respectively, for the synthesized few-layered 1T'-MoTe2films. Furthermore, Ti/Al bilayer metal contacts deposited on the few-layered 1T'-MoTe2films exhibit low specific contact resistances of approximately 1.0 × 10-4Ω cm2estimated by the transfer length model. This finding suggests a viable approach for achieving low ohmic contact resistance using the 1T'-MoTe2intermediate layer between metallic electrodes and two-dimensional semiconductors.

Controlled Fabrication of Metallic MoO2 Nanosheets towards High-Performance p-Type 2D Transistors

Two-dimensional (2D) semiconducting transition metal dichalcogenides (TMDCs) are extensively employed as channel materials in advanced electronic devices. The electrical contacts between electrodes and 2D semiconductors play a crucial role in the development of high-performance transistors. While numerous strategies for electrode interface engineering have been proposed to enhance the performance of n-type 2D transistors, upgrading p-type ones in a similar manner remains a challenge. In this work, significant improvements in a p-type WSe2 transistor are demonstrated by utilizing metallic MoO2 nanosheets as the electrode contact, which are controllably fabricated through physical vapor deposition and subsequent annealing. The MoO2 nanosheets exhibit an exceptional electrical conductivity of 8.4 × 104 S m‒1 and a breakdown current density of 3.3 × 106 A cm‒2. The work function of MoO2 nanosheets is determined to be ≈5.1 eV, making them suitable for contacting p-type 2D semiconductors. Employing MoO2 nanosheets as the electrode contact in WSe2 transistors results in a notable increase in the field-effect mobility to 92.0 cm2 V‒1 s‒1, which is one order of magnitude higher than the counterpart devices with conventional electrodes. This study not only introduces an intriguing 2D metal oxide to improve the electrical contact in p-type 2D transistors, but also offers an effective approach to fabricating all-2D devices.

van der Waals Contact for Two-Dimensional Transition Metal Dichalcogenides

Two-dimensional (2D) transition metal dichalcogenides (TMDs) have emerged as highly promising candidates for next-generation electronics owing to their atomically thin structures and surfaces devoid of dangling bonds. However, establishing high-quality metal contacts with TMDs presents a critical challenge, primarily attributed to their ultrathin bodies and delicate lattices. These distinctive characteristics render them susceptible to physical damage and chemical reactions when conventional metallization approaches involving "high-energy" processes are implemented. To tackle this challenge, the concept of van der Waals (vdW) contacts has recently been proposed as a "low-energy" alternative. Within the vdW geometry, metal contacts can be physically laminated or gently deposited onto the 2D channel of TMDs, ensuring the formation of atomically clean and electronically sharp contact interfaces while preserving the inherent properties of the 2D TMDs. Consequently, a considerable number of vdW contact devices have been extensively investigated, revealing unprecedented transport physics or exceptional device performance that was previously unachievable. This review presents recent advancements in vdW contacts for TMD transistors, discussing the merits, limitations, and prospects associated with each device geometry. By doing so, our purpose is to offer a comprehensive understanding of the current research landscape and provide insights into future directions within this rapidly evolving field.

Synchronous Surface-Interface and Crystal-Phase Engineered Multifaceted Hybrid Nanostructure of Fe-(1T)-VSe2 Nanosheet and Fe-CoSe2 Nanorods Doped with P for Rapid HER and OER, Kinetics

Two different nanostructures of two dissimilar highly-potent active electrocatalysts, P-dopped metallic-(1T)-Fe-VSe2 (P,Fe-1T-VSe2 ) nanosheet and P-dopped Fe-CoSe2 (P,Fe-CoSe2 ) nanorods are hybridized and integrated into a single heterostructure (P,Fe-(VCo)Se2 ) on Ni-foam for high-performance water splitting (WS). The catalytic efficiency of VSe2 nanosheets is first enhanced by enriching metallic (1T)-phase, then forming bimetallic Fe-V selenide, and finally by P-doping. Similarly, the catalytic efficiency of CoSe2 nanorods is boosted by first fabricating Fe-Co bimetallic selenide and then P-doping. To develop super-efficient electrocatalysts for WS, two individual electrocatalysts P,Fe-1T-VSe2 nanosheet and P,Fe-CoSe2 are hybridized and integrated to form a heterostructure (P,Fe-(VCo)Se2 ). Metallic (1T)-phase of transition metal dichalcogenides has much higher conductivity than the 2H-phase, while bimetallization and P-doping activate basal planes, develop various active components, and form heterostructures that develop a synergistic interfacial effect, all of which, significantly boost the catalytic efficacy of the P,Fe-(VCo)Se2 . P,Fe-(VCo)Se2 shows excellent performance requiring very low overpotential (ηHER = 50 mV@10 mAcm-2 and ηOER = 230 mV@20 mAcm-2 ). P,Fe-(VCo)Se2 (+, -) device requires a cell potential of 1.48 V to reach 10 mA cm-2 for overall WS.

Recent Strategies for the Synthesis of Phase-Pure Ultrathin 1T/1T' Transition Metal Dichalcogenide Nanosheets

The past few decades have witnessed a notable increase in transition metal dichalcogenide (TMD) related research not only because of the large family of TMD candidates but also because of the various polytypes that arise from the monolayer configuration and layer stacking order. The peculiar physicochemical properties of TMD nanosheets enable an enormous range of applications from fundamental science to industrial technologies based on the preparation of high-quality TMDs. For polymorphic TMDs, the 1T/1T' phase is particularly intriguing because of the enriched density of states, and thus facilitates fruitful chemistry. Herein, we comprehensively discuss the most recent strategies for direct synthesis of phase-pure 1T/1T' TMD nanosheets such as mechanical exfoliation, chemical vapor deposition, wet chemical synthesis, atomic layer deposition, and more. We also review frequently adopted methods for phase engineering in TMD nanosheets ranging from chemical doping and alloying, to charge injection, and irradiation with optical or charged particle beams. Prior to the synthesis methods, we discuss the configuration of TMDs as well as the characterization tools mostly used in experiments. Finally, we discuss the current challenges and opportunities as well as emphasize the promising fields for the future development.

Cutting-Edge Research in Nanoscience and Nanotechnology: Celebrating the 130th Anniversary of Wuhan University

Thanks to the fast-paced progress of microscopic theories and nanotechnologies, a tremendous world of fundamental science and applications has opened up at the nanoscale. Ranging from quantum physics to chemical and biological mechanisms and from device functionality to materials engineering, nanoresearch has become an essential part of various fields. As one of the top universities in China, Wuhan University (WHU) aims to promote cutting-edge nanoresearch in multiple disciplines by leveraging comprehensive academic programs established throughout 130 years of history. As visible in prestigious scientific journals such as ACS Nano, WHU has made impactful advancements in various frontiers, including nanophotonics, functional nanomaterials and devices, biomedical nanomaterials, nanochemistry, and environmental science. In light of these contributions, WHU will be committed to serving talents and scientists wholeheartedly, fully supporting international collaborations and continuously driving innovative research.

VTe2: Broadband Saturable Absorber for Passively Q-Switched Lasers in the Near- and Mid-Infrared Regions

In this study, we demonstrate the fabrication of a novel 2D transition metal dichalcogenide, VTe2, into a saturable absorber (SA) by using the liquid phase exfoliation method. Furthermore, the first-principles calculations were conducted to elucidate the electronic band structures and absorption spectrum. The nonlinear optical absorption properties of VTe2 at 1.0, 2.0, and 3.0 μm were measured using open-aperture Z-scan and P-scan methods, which showed saturation intensities and modulation depths of 95.57 GW/cm2 and 9.24%, 3.11 GW/cm2 and 7.26%, and 15.8 MW/cm2 and 17.1%, respectively. Furthermore, in the realm of practical implementation, the achievement of stable passively Q-switched (PQS) lasers employing SA composed of few-layered VTe2 nanosheets has manifested itself with broadband operating wavelengths from 1.0 to ∼3.0 μm. Specifically, PQS laser operations from near-infrared to mid-infrared with pulse durations of 195 and 563 ns for 1.0 and 2.0 μm solid-state lasers, respectively, and 749 ns for an Er3+-doped fluoride fiber laser at 3.0 μm were obtained. Our experimental results demonstrate that VTe2 is a potential broadband SA device for achieving PQS lasers. To the best of our knowledge, this is the first demonstration of using VTe2 as an SA in PQS lasers in the near- and mid-infrared regions, which highlights the potential of VTe2 for future research and applications in optoelectronic devices.

Ultrathin Rh Nanosheets with Rich Grain Boundaries for Efficient Hydrogen Oxidation Electrocatalysis

Two-dimensional (2D) Pt-group ultrathin nanosheets (NSs) are promising advanced electrocatalysts for energy-related catalytic reactions. However, improving the electrocatalytic activity of 2D Pt-group NSs through the addition of abundant grain boundaries (GBs) and understanding the underlying formation mechanism remain significant challenges. Herein, we report the controllable synthesis of a series of Rh-based nanocrystals (e.g., Rh nanoparticles, Rh NSs, and Rh NSs with GBs) through a CO-mediated kinetic control synthesis route. In light of the 2D NSs' structural advantages and GB modification, the Rh NSs with rich GBs exhibit an enhanced electrocatalytic activity compared to pure Rh NSs and commercial Pt/C toward the hydrogen oxidation reaction (HOR) in alkaline media. Both experimental results and theoretical computations corroborate that the GBs in the Rh NSs have the capacity to ameliorate the adsorption free energy of reaction intermediates during the HOR, thus resulting in outstanding HOR catalytic performance. Our work offers novel perspectives in the realm of developing sophisticated 2D Pt-group metal electrocatalysts with rich GBs for the energy conversion field.

Fabrication of p-type 2D single-crystalline transistor arrays with Fermi-level-tuned van der Waals semimetal electrodes

High-performance p-type two-dimensional (2D) transistors are fundamental for 2D nanoelectronics. However, the lack of a reliable method for creating high-quality, large-scale p-type 2D semiconductors and a suitable metallization process represents important challenges that need to be addressed for future developments of the field. Here, we report the fabrication of scalable p-type 2D single-crystalline 2H-MoTe2 transistor arrays with Fermi-level-tuned 1T'-phase semimetal contact electrodes. By transforming polycrystalline 1T'-MoTe2 to 2H polymorph via abnormal grain growth, we fabricated 4-inch 2H-MoTe2 wafers with ultra-large single-crystalline domains and spatially-controlled single-crystalline arrays at a low temperature (~500 °C). Furthermore, we demonstrate on-chip transistors by lithographic patterning and layer-by-layer integration of 1T' semimetals and 2H semiconductors. Work function modulation of 1T'-MoTe2 electrodes was achieved by depositing 3D metal (Au) pads, resulting in minimal contact resistance (~0.7 kΩ·μm) and near-zero Schottky barrier height (~14 meV) of the junction interface, and leading to high on-state current (~7.8 μA/μm) and on/off current ratio (~105) in the 2H-MoTe2 transistors.

Controlled Synthesis and Accurate Doping of Wafer-Scale 2D Semiconducting Transition Metal Dichalcogenides

2D semiconducting transition metal dichalcogenide (TMDCs) possess atomically thin thickness, a dangling-bond-free surface, flexible band structure, and silicon-compatible feature, making them one of the most promising channels for constructing state-of-the-art field-effect transistors in the post-Moore's era. However, the existing 2D semiconducting TMDCs fall short of meeting the industry criteria for practical applications in electronics due to their small domain size and the lack of an effective approach to modulate intrinsic physical properties. Therefore, it is crucial to prepare and dope 2D semiconducting TMDCs single crystals with wafer size. In this review, the up-to-date progress regarding the wafer-scale growth of 2D semiconducting TMDC polycrystalline and single-crystal films is systematically summarized. The domain orientation control of 2D TMDCs and the seamless stitching of unidirectionally aligned 2D islands by means of substrate design are proposed. In addition, the accurate and uniform doping of 2D semiconducting TMDCs and the effect on electronic device performances are also discussed. Finally, the dominating challenges pertaining to the enhancement of the electronic device performances of TMDCs are emphasized, and further development directions are put forward. This review provides a systematic and in-depth summary of high-performance device applications of 2D semiconducting TMDCs.

Design, synthesis and application of two-dimensional metal tellurides as high-performance electrode materials

Multifunctional electrode materials with inherent conductivity have attracted extensive attention in recent years. Two-dimensional (2D) metal telluride nanomaterials are more promising owing to their strong metallic properties and unique physical/chemical merits. In this review, recent advancements in the preparation of 2D metal tellurides and their application in electrode materials are presented. First, the most available preparation methods, such as hydro/solvent thermal, chemical vapor deposition, and electrodeposition, are summarized. Then, the unique performance of metal telluride electrodes in capacitors, anode materials of Li/Na ion batteries, electrocatalysis, and lithium-sulfur batteries are discussed. Finally, significant challenges and opportunities in the preparation and application of 2D metal tellurides are proposed.

Stress-driven structural and bond reconstruction in 2D ferromagnetic semiconductor VSe2

Two-dimensional (2D) semiconducting transition metal dichalcogenides can be used to make high-performance electronic, spintronic, and optoelectronic devices. Recently, room-temperature ferromagnetism and semiconduction in 2D VSe₂nanoflakes were attributed to the stable 2H-phase of VSe₂in the 2D limit. Here, our first-principles investigation shows that a metastable semiconducting H' phase can be formed from the H VSe2 monolayer through uniaxial stress or uniaxial strain. The calculated phonon spectra indicate the dynamical stability of the metastable H' VSe₂and the path of phase switching between the H and H' VSe₂phases is calculated. For the uniaxial stress (or strain) scheme, the H' phase can become lower in total energy than the H phase at a transition point. The H' phase has stronger ferromagnetism and its Curier temperature can be enhanced by applying uniaxial stress or strain. Applying uniaxial stress or strain can substantially change spin-resolved electronic structures, energy band edges, and effective carrier masses for both of the H and H' phases, and can cause some flat bands near the band edges in the strained H' phase. Further analysis indicates that one of the Se-Se bonds in the H' phase can be shortened by 19% and the related Se-V-Se bond angles are reduced by 23% with respect to those of the H phase, which is believed to increase the Se-Se covalence feature and reduce the valence of the nearby V atoms. Therefore, structural and bond reconstruction can be realized by applying uniaxial stress in such 2D ferromagnetic semiconductors for potential spintronic and optoelectronic applications.

Horizontally Self-Standing Growth of Bi2 O2 Se Achieving Optimal Optoelectric Properties

Air-stable 2D Bi₂ O₂ Se material with high carrier mobility appears as a promising semiconductor platform for future micro/nanoelectronics and optoelectronics. Like most 2D materials, Bi₂ O₂ Se 2D nanostructures normally form on atomically flat mica substrates, in which undesirable defects and structural damage from the subsequent transfer process will largely degrade their photoelectronic performance. Here, a new synthesis route involving successive kinetic and thermodynamic processes is proposed to achieve horizontally self-standing Bi₂ O₂ Se nanostructures on SiO₂ /Si substrates. Fewer defects and avoidance of transfer procedure involving corrosive solvents ensure the integrity of the intrinsic lattice and band structures in Bi₂ O₂ Se nanostructures. In contrast to flat structures grown on mica, it displays reduced dark current and improved photoresponse performance (on-off ratio, photoresponsivity, response time, and detectivity). These results indicate a new potential in high-quality 2D electronic nanostructures with optimal optoelectronic functionality.

Mechanistic insights of molecular metal polyselenides for catalytic hydrogen generation

Molecular metal chalcogenides have attracted great attention as electrocatalysts for the hydrogen evolution reaction (HER). However, efficient utilisation of the active sites and catalytic performance modulation has been challenging. Here we explore the design of immobilized molecular molybdenum polyselenides [Mo₂O₂S₂(Se₂)(Se_x)]^2- that exhibit efficient hydrogen evolution at low overpotential and stability over 1000 cycles. Density functional calculations provide evidence of a unimolecular mechanism in the HER process via the exploration of viable reaction pathways. The discussed findings are of a broad interest in the development of efficient molecular electrocatalytic materials.

Controllable Synthesis Quadratic-Dependent Unsaturated Magnetoresistance of Two-Dimensional Nonlayered Fe7S8 with Robust Environmental Stability

Two-dimensional (2D) iron chalcogenides (FeX, X = S, Se, Te) are emerging as an appealing class of materials for a wide range of research topics, including electronics, spintronics, and catalysis. However, the controlled syntheses and intrinsic property explorations of such fascinating materials still remain daunting challenges, especially for 2D nonlayered Fe₇S₈ with mixed-valence states and high conductivity. Herein, we design a general and temperature-mediated chemical vapor deposition (CVD) approach to synthesize ultrathin and large-domain Fe₇S₈ nanosheets on mica substrates, with the thickness down to ∼4.4 nm (2 unit-cell). Significantly, we uncover a quadratic-dependent unsaturated magnetoresistance (MR) with out-of-plane anisotropy in 2D Fe₇S₈, thanks to its ultrahigh crystalline quality and high conductivity (∼2.7 × 10⁵ S m^-1 at room temperature and ∼1.7 × 10⁶ S m^-1 at 2 K). More interestingly, the CVD-synthesized 2D Fe₇S₈ nanosheets maintain robust environmental stability for more than 8 months. These results hereby lay solid foundations for synthesizing 2D nonlayered iron chalcogenides with mixed-valence states and exploring fascinating quantum phenomena.

Phase-Tunable Synthesis and Etching-Free Transfer of Two-Dimensional Magnetic FeTe

Two-dimensional (2D) Fe-chalcogenides (e.g., FeS, FeSe, and FeTe, etc.) have sparked extensive interest due to their rich phase diagrams including superconductivity, magnetism, and topological state, as well as versatile applications in electronic devices and energy related fields. However, the phase-tunable synthesis and green transfer of such fascinating materials still remain challenging. Herein, we develop a temperature-mediated chemical vapor deposition (CVD) approach to grow ultrathin nonlayered hexagonal and layered tetragonal FeTe nanosheets on mica substrates, with their thicknesses down to ∼2.3 and ∼4.0 nm, respectively. Interestingly, we have observed exciting ferromagnetism with the Curie temperature approaching ∼300 K and high conductivity (∼1.96 × 10⁵ S m^-1) in 2D hexagonal FeTe. More significantly, we have designed a swift, high-efficiency, and etching-free method for the transfer of 2D FeTe nanosheets onto arbitrary substrates, and such a transfer strategy enables the cyclic utilization of growth substrates. These results should propel the further development of phase-tunable synthesis and green transfer of 2D Fe-chalcogenides, as well as their potential applications in spintronic devices.