Tunable Polarity Behavior and Self‐Driven Photoswitching in p‐WSe2/n‐WS2 Heterojunctions

Van der Waals (vdW) p–n heterojunctions consisting of various 2D layer compounds are fascinating new artificial materials that can possess novel physics and functionalities enabling the next‐generation of electronics and optoelectronics devices. Here, it is reported that the WSe₂/WS₂ p–n heterojunctions perform novel electrical transport properties such as distinct rectifying, ambipolar, and hysteresis characteristics. Intriguingly, the novel tunable polarity transition along a route of n‐“anti‐bipolar”–p‐ambipolar is observed in the WSe₂/WS₂ heterojunctions owing to the successive work of conducting channels of junctions, p‐WSe₂ and n‐WS₂ on the electrical transport of the whole systems. The type‐II band alignment obtained from first principle calculations and built‐in potential in this vdW heterojunction can also facilitate the efficient electron–hole separation, thus enabling the significant photovoltaic effect and a much enhanced self‐driven photoswitching response in this system.     

Graphene‐Assisted Antioxidation of Tungsten Disulfide Monolayers: Substrate and Electric‐Field Effect

Transition metal dichalcogenides (TMDs) have emerged as promising materials to complement graphene for advanced optoelectronics. However, irreversible degradation of chemical vapor deposition‐grown monolayer TMDs via oxidation under ambient conditions limits applications of TMD‐based devices. Here, the growth of oxidation‐resistant tungsten disulfide (WS₂) monolayers on graphene is demonstrated, and the mechanism of oxidation of WS₂ on SiO₂, graphene/SiO₂, and on graphene suspended in air is elucidated. While WS₂ on a SiO₂ substrate begins oxidation within weeks, epitaxially grown WS₂ on suspended graphene does not show any sign of oxidation, attributed to the screening effect of surface electric field caused by the substrate. The control of a local oxidation of WS₂ on a SiO₂ substrate by a local electric field created using an atomic force microscope tip is also demonstrated.     

Designing Champion Nanostructures of Tungsten Dichalcogenides for Electrocatalytic Hydrogen Evolution

Fine-tuning strain and vacancies in 2H-phase transition-metal dichalcogenides, although extremely challenging, is crucial for activating the inert basal plane for boosting the hydrogen evolution reaction (HER). Here, atomically curved 2H-WS₂ nanosheets with precisely tunable strain and sulfur vacancies (S-vacancies) along with rich edge sites are synthesized via a one-step approach by harnessing geometric constraints. The approach is based on the confined epitaxy growth of WS₂ in ordered mesoporous graphene derived from nanocrystal superlattices. The spherical curvature imposed by the graphitic mesopores enables the generation of uniform strain and S-vacancies in the as-grown WS₂ nanosheets, and simultaneous manipulation of these two key parameters can be realized by simply adjusting the pore size. In addition, the formation of unique mesoporous WS₂@graphene van der Waals heterostructures ensures the ready access of active sites. Fine-tuning the WS₂ layer number, strain, and S-vacancies enables arguably the best-performing HER 2H-WS₂ electrocatalysts ever reported. Density functional theory calculations indicate that compared with strain, S-vacancies play a more critical role in enhancing the HER activity.     

Wafer‐Scale and Low‐Temperature Growth of 1T‐WS2 Film for Efficient and Stable Hydrogen Evolution Reaction

The metallic 1T phase of WS₂ (1T‐WS₂), which boosts the charge transfer between the electron source and active edge sites, can be used as an efficient electrocatalyst for the hydrogen evolution reaction (HER). As the semiconductor 2H phase of WS₂ (2H‐WS₂) is inherently stable, methods for synthesizing 1T‐WS₂ are limited and complicated. Herein, a uniform wafer‐scale 1T‐WS₂ film is prepared using a plasma‐enhanced chemical vapor deposition (PE‐CVD) system. The growth temperature is maintained at 150 °C enabling the direct synthesis of 1T‐WS₂ films on both rigid dielectric and flexible polymer substrates. Both the crystallinity and number of layers of the as‐grown 1T‐WS₂ are verified by various spectroscopic and microscopic analyses. A distorted 1T structure with a 2a₀ × a₀ superlattice is observed using scanning transmission electron microscopy. An electrochemical analysis of the 1T‐WS₂ film demonstrates its similar catalytic activity and high durability as compared to those of previously reported untreated and planar 1T‐WS₂ films synthesized with CVD and hydrothermal methods. The 1T‐WS₂ does not transform to stable 2H‐WS₂, even after a 700 h exposure to harsh catalytic conditions and 1000 cycles of HERs. This synthetic strategy can provide a facile method to synthesize uniform 1T‐phase 2D materials for electrocatalysis applications.     

Electro-Optic Upconversion in van der Waals Heterostructures via Nonequilibrium Photocarrier Tunneling

Ultrafast interlayer charge transfer is one of the most distinct features of van der Waals (vdW) heterostructures. Its dynamics competes with carrier thermalization such that the energy of nonthermalized photocarriers may be harnessed by band engineering. In this study, nonthermalized photocarrier energy is harnessed to achieve near-infrared (NIR) to visible light upconversion in a metal–insulator–semiconductor (MIS) vdW heterostructure tunnel diode consisting of few-layer graphene (FLG), hexagonal boron nitride (hBN), and monolayer tungsten disulfide (WS₂). Photoexcitation of the electrically biased heterostructure with 1.58 eV NIR laser in the linear absorption regime generates emission from the ground exciton state of WS₂, which corresponds to upconversion by ≈370 meV. The upconversion is realized by electrically assisted interlayer transfer of nonthermalized photoexcited holes from FLG to WS₂, followed by formation and radiative recombination of excitons in WS₂. The photocarrier transfer rate can be described by Fowler–Nordheim tunneling mechanism and is electrically tunable by two orders of magnitude by tuning voltage bias applied to the device. This study highlights the prospects for realizing novel electro-optic upconversion devices by exploiting electrically tunable nonthermalized photocarrier relaxation dynamics in vdW heterostructures.     

Efficient Self‐Driven Photodetectors Featuring a Mixed‐Dimensional van der Waals Heterojunction Formed from a CdS Nanowire and a MoTe2 Flake

Heterojunctions formed from low‐dimensional materials can result in photovoltaic and photodetection devices displaying exceptional physical properties and excellent performance. Herein, a mixed‐dimensional van der Waals (vdW) heterojunction comprising a 1D n‐type Ga‐doped CdS nanowire and a 2D p‐type MoTe₂ flake is demonstrated; the corresponding photovoltaic device exhibits an outstanding conversion efficiency of 15.01% under illumination with white light at 650 µW cm^?2. A potential difference of 80 meV measured, using Kelvin probe force microscopy, at the CdS–MoTe₂ interface confirms the separation and accumulation of photoexcited carriers upon illumination. Moreover, the photodetection characteristics of the vdW heterojunction device at zero bias reveal a rapid response time (<50 ms) and a photoresponsivity that are linearly proportional to the power density of the light. Interestingly, the response of the vdW heterojunction device is negligible when illuminated at 580 nm; this exceptional behavior is presumably due to the rapid rate of recombination of the photoexcited carriers of MoTe₂. Such mixed‐dimensional vdW heterojunctions appear to be novel design elements for efficient photovoltaic and self‐driven photodetection devices.     

Universal Imaging of Full Strain Tensor in 2D Crystals with Third‐Harmonic Generation

Quantitatively mapping and monitoring the strain distribution in 2D materials is essential for their physical understanding and function engineering. Optical characterization methods are always appealing due to unique noninvasion and high‐throughput advantages. However, all currently available optical spectroscopic techniques have application limitation, e.g., photoluminescence spectroscopy is for direct‐bandgap semiconducting materials, Raman spectroscopy is for ones with Raman‐active and strain‐sensitive phonon modes, and second‐harmonic generation spectroscopy is only for noncentrosymmetric ones. Here, a universal methodology to measure the full strain tensor in any 2D crystalline material by polarization‐dependent third‐harmonic generation is reported. This technique utilizes the third‐order nonlinear optical response being a universal property in 2D crystals and the nonlinear susceptibility has a one‐to‐one correspondence to strain tensor via a photoelastic tensor. The photoelastic tensor of both a noncentrosymmetric D_3h WS₂ monolayer and a centrosymmetric D_3d WS₂ bilayer is successfully determined, and the strain tensor distribution in homogenously strained and randomly strained monolayer WS₂ is further mapped. In addition, an atlas of photoelastic tensors to monitor the strain distribution in 2D materials belonging to all 32 crystallographic point groups is provided. This universal characterization on strain tensor should facilitate new functionality designs and accelerate device applications in 2D‐materials‐based electronic, optoelectronic, and photovoltaic devices.     

WS2 Nanotubes: Electrical Conduction and Field Emission Under Electron Irradiation and Mechanical Stress

This study reports the electrical transport and the field emission properties of individual multi‐walled tungsten disulphide (WS₂) nanotubes (NTs) under electron beam irradiation and mechanical stress. Electron beam irradiation is used to reduce the nanotube‐electrode contact resistance by one‐order of magnitude. The field emission capability of single WS₂ NTs is investigated, and a field emission current density as high as 600 kA cm^?2 is attained with a turn‐on field of ≈ 100 V μm^?1 and field‐enhancement factor ≈ 50. Moreover, the electrical behavior of individual WS₂ NTs is studied under the application of longitudinal tensile stress. An exponential increase of the nanotube resistivity with tensile strain is demonstrated up to a recorded elongation of 12%, thereby making WS₂ NTs suitable for piezoresistive strain sensor applications.     

Large-Area Synthesis of Ferromagnetic Fe5−xGeTe2/Graphene van der Waals Heterostructures with Curie Temperature above Room Temperature

Van der Waals (vdW) heterostructures combining layered ferromagnets and other 2D crystals are promising building blocks for the realization of ultracompact devices with integrated magnetic, electronic, and optical functionalities. Their implementation in various technologies depends strongly on the development of a bottom-up scalable synthesis approach allowing for realizing highly uniform heterostructures with well-defined interfaces between different 2D-layered materials. It is also required that each material component of the heterostructure remains functional, which ideally includes ferromagnetic order above room temperature for 2D ferromagnets. Here, it is demonstrated that the large-area growth of Fe_5−xGeTe₂/graphene heterostructures is achieved by vdW epitaxy of Fe_5−xGeTe₂ on epitaxial graphene. Structural characterization confirms the realization of a continuous vdW heterostructure film with a sharp interface between Fe_5−xGeTe₂ and graphene. Magnetic and transport studies reveal that the ferromagnetic order persists well above 300 K with a perpendicular magnetic anisotropy. In addition, epitaxial graphene on SiC(0001) continues to exhibit a high electronic quality. These results represent an important advance beyond nonscalable flake exfoliation and stacking methods, thus marking a crucial step toward the implementation of ferromagnetic 2D materials in practical applications.     

Confinement Growth of Layered WS2 in Hollow Beaded Carbon Nanofibers with Synergistic Anchoring Effect to Reinforce Li+/Na+ Storage Performance

Novel nitrogen doped (N‐doped) hollow beaded structural composite carbon nanofibers are successfully applied for lithium‐ion batteries (LIBs) and sodium‐ion batteries (SIBs). Tungsten disulfide (WS₂) nanosheets are confined, through synergistic anchoring, on the surface and inside of hollow beaded carbon nanofibers (HB CNFs) via a hydrothermal reaction method to construct the hierarchical structure HB WS₂@CNFs. Benefiting from this unique advantage, HB WS₂@CNFs exhibits remarkable lithium‐storage performance in terms of high rate capability (≈351 mAh g^?1 at 2 A g^?1) and stable long‐term cycle (≈446 mAh g^?1 at 1 A g^?1 after 100 cycles). Moreover, as an anode material for SIBs, HB WS₂@CNFs obtains excellent long cycle life and rate performance. During the charging/discharging process, the evolution of morphology and composition of the composite are analyzed by a set of ex situ methods. This synergistic anchoring effect between WS₂ nanosheets and HB CNFs is capable of effectively restraining volume expansion from the metal ions intercalation/deintercalation process and improving the cycling stability and rate performance in LIBs and SIBs.     

Probing Anisotropic Thermal Conductivity of Transition Metal Dichalcogenides MX2 (M = Mo,W and X = S,Se) using Time‐Domain Thermoreflectance

Transition metal dichalcogenides (TMDs) are a group of layered 2D semiconductors that have shown many intriguing electrical and optical properties. However, the thermal transport properties in TMDs are not well understood due to the challenges in characterizing anisotropic thermal conductivity. Here, a variable‐spot‐size time‐domain thermoreflectance approach is developed to simultaneously measure both the in‐plane and the through‐plane thermal conductivity of four kinds of layered TMDs (MoS₂, WS₂, MoSe₂, and WSe₂) over a wide temperature range, 80–300 K. Interestingly, it is found that both the through‐plane thermal conductivity and the Al/TMD interface conductance depend on the modulation frequency of the pump beam for all these four compounds. The frequency‐dependent thermal properties are attributed to the nonequilibrium thermal resistance between the different groups of phonons in the substrate. A two‐channel thermal model is used to analyze the nonequilibrium phonon transport and to derive the intrinsic thermal conductivity at the thermal equilibrium limit. The measurements of the thermal conductivities of bulk TMDs serve as an important benchmark for understanding the thermal conductivity of single‐ and few‐layer TMDs.     

Edge‐Epitaxial Growth of 2D NbS2‐WS2 Lateral Metal‐Semiconductor Heterostructures

2D metal‐semiconductor heterostructures based on transition metal dichalcogenides (TMDs) are considered as intriguing building blocks for various fields, such as contact engineering and high‐frequency devices. Although, a series of p–n junctions utilizing semiconducting TMDs have been constructed hitherto, the realization of such a scheme using 2D metallic analogs has not been reported. Here, the synthesis of uniform monolayer metallic NbS₂ on sapphire substrate with domain size reaching to a millimeter scale via a facile chemical vapor deposition (CVD) route is demonstrated. More importantly, the epitaxial growth of NbS₂‐WS₂ lateral metal‐semiconductor heterostructures via a “two‐step” CVD method is realized. Both the lateral and vertical NbS₂‐WS₂ heterostructures are achieved here. Transmission electron microscopy studies reveal a clear chemical modulation with distinct interfaces. Raman and photoluminescence maps confirm the precisely controlled spatial modulation of the as‐grown NbS₂‐WS₂ heterostructures. The existence of the NbS₂‐WS₂ heterostructures is further manifested by electrical transport measurements. This work broadens the horizon of the in situ synthesis of TMD‐based heterostructures and enlightens the possibility of applications based on 2D metal‐semiconductor heterostructures.     

Efficient Catalysis of Hydrogen Evolution Reaction from WS2(1−x)P2x Nanoribbons

The rational design of Earth abundant electrocatalysts for efficiently catalyzing hydrogen evolution reaction (HER) is believed to lead to the generation of carbon neutral energy carrier. Owing to their fascinating chemical and physical properties, transition metal dichalcogenides (TMDs) are widely studied for this purpose. Of particular note is that doping by foreign atom can bring the advent of electronic perturbation, which affects the intrinsic catalytic property. Hence, through doping, the catalytic activity of such materials could be boosted. A rational synthesis approach that enables phosphorous atom to be doped into WS₂ without inducing phase impurity to form WS_{2(1? x )}P_{2 x} nanoribbon (NRs) is herein reported. It is found that the WS_{2(1? x )}P_{2 x} NRs exhibit considerably enhanced HER performance, requiring only ?98 mV versus reversible hydrogen electrode to achieve a current density of ?10 mA cm^?2. Such a high performance can be attributed to the ease of H‐atom adsorption and desorption due to intrinsically tuned WS₂, and partial formation of NRs, a morphology wherein the exposure of active edges is more pronounced. This finding can provide a fertile ground for subsequent works aiming at tuning intrinsic catalytic activity of TMDs.     

Improved Photo-Excited Carriers Transportation of WS2-O-Doped-Graphene Heterostructures for Solar Steam Generation (Small 19/2023)

Two-dimensional (2D) transition metal dichalcogenides and graphene have revealed promising applications in optoelectronic and energy storage and conversion. However, there are rare reports of modifying the light-to-heat transformation via preparing their heterostructures for solar steam generation. In this work, commercial WS₂ and sucrose are utilized as precursors to produce 2D WS₂-O-doped-graphene heterostructures (WS₂-O-graphene) for solar water evaporation. The WS₂-O-graphene evaporators demonstrate excellent average water evaporation rate (2.11 kg m⁻² h⁻¹) and energy efficiency (82.2%), which are 1.3- and 1.2-fold higher than WS₂ and O-doped graphene-based evaporators, respectively. Furthermore, for the real seawater with different pH values (pH 1 and 12) and rhodamine B pollutants, the WS₂-O-graphene evaporators show great average evaporation rates (≈2.08 and 2.09 kg m⁻² h⁻¹, respectively) for producing freshwater with an extremely low-grade of dye residual and nearly neutral pH values. More interestingly, due to the self-storage water ability of WS₂-O-graphene evaporators, water evaporation can be implemented without the presence of bulk water. As a result, the evaporation rate reaches 3.23 kg m⁻² h⁻¹, which is ≈1.5 times higher than the regular solar water evaporation system. This work provides a new approach for preparing 2D transition metal dichalcogenides and graphene heterostructures for efficient solar water evaporation.     

Discovery of Superconductivity in 2M WS2 with Possible Topological Surface States

Recently the metastable 1T′‐type VIB‐group transition metal dichalcogenides (TMDs) have attracted extensive attention due to their rich and intriguing physical properties, including superconductivity, valleytronics physics, and topological physics. Here, a new layered WS₂ dubbed “2M” WS₂, is constructed from 1T′ WS₂ monolayers, is synthesized. Its phase is defined as 2M based on the number of layers in each unit cell and the subordinate crystallographic system. Intrinsic superconductivity is observed in 2M WS₂ with a transition temperature T_c of 8.8 K, which is the highest among TMDs not subject to any fine‐tuning process. Furthermore, the electronic structure of 2M WS₂ is found by Shubnikov–de Haas oscillations and first‐principles calculations to have a strong anisotropy. In addition, topological surface states with a single Dirac cone, protected by topological invariant Z₂, are predicted through first‐principles calculations. These findings reveal that the new 2M WS₂ might be an interesting topological superconductor candidate from the VIB‐group transition metal dichalcogenides.     

Controlling Photoluminescence Enhancement and Energy Transfer in WS2:hBN:WS2 Vertical Stacks by Precise Interlayer Distances

2D semiconducting transition metal dichalcogenides (TMDs) are endowed with fascinating optical properties especially in their monolayer limit. Insulating hBN films possessing customizable thickness can act as a separation barrier to dictate the interactions between TMDs. In this work, vertical layered heterostructures (VLHs) of WS₂:hBN:WS₂ are fabricated utilizing chemical vapor deposition (CVD)‐grown materials, and the optical performance is evaluated through photoluminescence (PL) spectroscopy. Apart from the prohibited indirect optical transition due to the insertion of hBN spacers, the variation in the doping level of WS₂ drives energy transfer to arise from the layer with lower quantum efficiency to the other layer with higher quantum efficiency, whereby the total PL yield of the heterosystem is increased and the stack exhibits a higher PL intensity compared to the sum of those in the two WS₂ constituents. Such doping effects originate from the interfaces that WS₂ monolayers reside on and interact with. The electron density in the WS₂ is also controlled and subsequent modulation of PL in the heterostructure is demonstrated by applying back‐gated voltages. Other influential factors include the strain in WS₂ and temperature. Being able to tune the energy transfer in the VLHs may expand the development of photonic applications in 2D systems.     

Controlling Structural Anisotropy of Anisotropic 2D Layers in Pseudo‐1D/2D Material Heterojunctions

Chemical vapor deposition and growth dynamics of highly anisotropic 2D lateral heterojunctions between pseudo‐1D ReS₂ and isotropic WS₂ monolayers are reported for the first time. Constituent ReS₂ and WS₂ layers have vastly different atomic structure, crystallizing in anisotropic 1T′ and isotropic 2H phases, respectively. Through high‐resolution scanning transmission electron microscopy, electron energy loss spectroscopy, and angle‐resolved Raman spectroscopy, this study is able to provide the very first atomic look at intimate interfaces between these dissimilar 2D materials. Surprisingly, the results reveal that ReS₂ lateral heterojunctions to WS₂ produce well‐oriented (highly anisotropic) Re‐chains perpendicular to WS₂ edges. When vertically stacked, Re‐chains orient themselves along the WS₂ zigzag direction, and consequently, Re‐chains exhibit six‐fold rotation, resulting in loss of macroscopic scale anisotropy. The degree of anisotropy of ReS₂ on WS₂ largely depends on the domain size, and decreases for increasing domain size due to randomization of Re‐chains and formation of ReS₂ subdomains. Present work establishes the growth dynamics of atomic junctions between novel anisotropic/isotropic 2D materials, and overall results mark the very first demonstration of control over anisotropy direction, which is a significant leap forward for large‐scale nanomanufacturing of anisotropic systems.     

Site‐Selective and van der Waals Epitaxial Growth of Rhenium Disulfide on Graphene

The surface property of growth substrate imposes significant influence in the growth behaviors of 2D materials. Rhenium disulfide (ReS₂) is a new family of 2D transition metal dichalcogenides with unique distorted 1T crystal structure and thickness‐independent direct bandgap. The role of growth substrate is more critical for ReS₂ owing to its weak interlayer coupling property, which leads to preferred growth along the out‐of‐plane direction while suppressing the uniform in‐plane growth. Herein, graphene is introduced as the growth substrate for ReS₂ and the synthesis of graphene/ReS₂ vertical heterostructure is demonstrated via chemical vapor deposition. Compared with the rough surface of SiO₂/Si substrate with dangling bonds which hinders the uniform growth of ReS₂, the inert and smooth surface nature of graphene sheet provides a lower energy barrier for migration of the adatoms, thereby promoting the growth of ReS₂ on the graphene surface along the in‐plane direction. Furthermore, patterning of the graphene/ReS₂ heterostructure is achieved by the selective growth of ReS₂, which is attributed to the strong binding energy between sulfur atoms and graphene surface. The fundamental studies in the role of graphene as the growth template in the formation of van der Waals heterostructures provide better insights into the synthesis of 2D heterostructures.     

Strain-Sensitive Magnetization Reversal of a van der Waals Magnet

By virtue of the layered structure, van der Waals (vdW) magnets are sensitive to the lattice deformation controlled by the external strain, providing an ideal platform to explore the one-step magnetization reversal that is still conceptual in conventional magnets due to the limited strain-tuning range of the coercive field. In this study, a uniaxial tensile strain is applied to thin flakes of the vdW magnet Fe₃GeTe₂ (FGT), and a dramatic increase of the coercive field (H_c) by more than 150% with an applied strain of 0.32% is observed. Moreover, the change of the transition temperatures between the different magnetic phases under strain is investigated, and the phase diagram of FGT in the strain–temperature plane is obtained. Comparing the phase diagram with theoretical results, the strain-tunable magnetism is attributed to the sensitive change of magnetic anisotropy energy. Remarkably, strain allows an ultrasensitive magnetization reversal to be achieved, which may promote the development of novel straintronic device applications.     

Strain Mismatch Induced Tilted Heteroepitaxial (000l) Hexagonal ZnO Films on (001) Cubic Substrates

A novel strain mismatch induced tilted epitaxy method has been demonstrated for producing high quality (000l) hexagonal films on (001) cubic substrates. Highly oriented hexagonal (000l) ZnO films are grown on cubic (001) MgO substrates using Sm_0.28Zr_0.72O_{2? δ} (SZO) as a template. The large lattice mismatch of >13% between the obvious crystallographic matching directions of the template and substrate means that cube‐on‐cube epitaxy is energetically unfavorable, leading to growth instead of two high index, low energy compact planes, close to the {111} orientation. These planes give three different in‐plane orientations resulting from coincidence site lattice matching (12 in‐plane orientations in total) and provide a pseudo‐hexagonal symmetry surface for the ZnO to grow on. The texture of the ensuing (000l) ZnO layer is markedly improved over the template. The work opens up both a new avenue for growing technologically important hexagonal structures on a range of readily available, (001) cubic substrates, as well as showing that there are wide possibilities for heteroepitaxial growth of a range of dissimilar materials.