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2-D transition metal di-chalcogenides are the promising candidates for ultralow intensity photodetection. However, the performance of these photodetectors is usually limited by ambience induced rapid performance degradation and long-lived charge trapping induced slow response with a large persistent photocurrent when the light source is switched off. Here, we demonstrate an indium tin oxide (ITO)/WSe2/SnSe2-based vertical double heterojunction photoconductive device where the photo-excited hole is confined in the double barrier quantum well, whereas the photo-excited electron can be transferred to either the ITO or the SnSe2 layer in a controlled manner. The intrinsically short transit time of the photoelectrons in the vertical double heterojunction helps us to achieve high responsivity in excess of 1100 A/W and fast transient response time on the order of ${10}~\mu \text{s}$. A large built-in field in the WSe2 sandwich layer results in photodetection at zero external bias allowing a self-powered operation mode. The encapsulation from the top and bottom protects the photo-active WSe2 layer from ambience induced detrimental effects and substrate induced trapping effects helping us to achieve repeatable characteristics over many cycles. [ABSTRACT FROM AUTHOR]

Abstract: Benefiting from the technique of vertically stacking 2D layered materials (2DLMs), an advanced novel device architecture based on a top‐gated MoS2/WSe2 van der Waals (vdWs) heterostructure is designed. By adopting a self‐aligned metal screening layer (Pd) to the WSe2 channel, a fixed p‐doped state of the WSe2 as well as an independent doping control of the MoS2 channel can be achieved, thus guaranteeing an effective energy‐band offset modulation and large through current. In such a device, under specific top‐gate voltages, a sharp PN junction forms at the edge of the Pd layer and can be effectively manipulated. By varying top‐gate voltages, the device can be operated under both quasi‐Esaki diode and unipolar‐Zener diode modes with tunable current modulations. A maximum gate‐coupling efficiency as high as ≈90% and a subthreshold swing smaller than 60 mV dec−1 can be achieved under the band‐to‐band tunneling regime. The superiority of the proposed device architecture is also confirmed by comparison with a traditional heterostructure device. This work demonstrates the feasibility of a new device structure based on vdWs heterostructures and its potential in future low‐power electronic and optoelectronic device applications. [ABSTRACT FROM AUTHOR]

The research field of two dimensional (2D) materials strongly relies on optical microscopy characterization tools to identify atomically thin materials and to determine their number of layers. Moreover, optical microscopy-based techniques opened the door to study the optical properties of these nanomaterials. We presented a comprehensive study of the differential reflectance spectra of 2D semiconducting transition metal dichalcogenides (TMDCs), MoS2, MoSe2, WS2, and WSe2, with thickness ranging from one layer up to six layers. We analyzed the thickness-dependent energy of the different excitonic features, indicating the change in the band structure of the different TMDC materials with the number of layers. Our work provided a route to employ differential reflectance spectroscopy for determining the number of layers of MoS2, MoSe2, WS2, and WSe2. [ABSTRACT FROM AUTHOR]

The research field of two dimensional (2D) materials strongly relies on optical microscopy characterization tools to identify atomically thin materials and to determine their number of layers. Moreover, optical microscopy-based techniques opened the door to study the optical properties of these nanomaterials. We presented a comprehensive study of the differential reflectance spectra of 2D semiconducting transition metal dichalcogenides (TMDCs), MoS2, MoSe2, WS2, and WSe2, with thickness ranging from one layer up to six layers. We analyzed the thickness-dependent energy of the different excitonic features, indicating the change in the band structure of the different TMDC materials with the number of layers. Our work provided a route to employ differential reflectance spectroscopy for determining the number of layers of MoS2, MoSe2, WS2, and WSe2. [ABSTRACT FROM AUTHOR]

A high-performance heterojunction photodetector is formed by combining an n-type Si substrate with p-type monolayer WSe2 obtained using physical vapor deposition. The high quality of the WSe2/Si heterojunction is demonstrated by the suppressed dark current of 1 nA and the extremely high rectification ratio of 107. Under illumination, the heterojunction exhibits a wide photoresponse range from ultraviolet to near-infrared radiation. The introduction of graphene quantum dots (GQDs) greatly elevates the photodetective capabilities of the heterojunction with strong light absorption and long carrier lifetimes. The GQDs/WSe2/Si heterojunction exhibits a high responsivity of ∼ 707 mA·W-1, short response time of 0.2 ms, and good specific detectivity of ∼ 4.51 × 109 Jones. These properties suggest that the GQDs/WSe2/Si heterojunction holds great potential for application in future high-performance photodetectors. [ABSTRACT FROM AUTHOR]

We present quantum yield measurements of single layer WSe2 (1L-WSe2) integrated with high-Q (Q > 106) optical microdisk cavities, using an efficient (η > 90%) near-field coupling scheme based on a tapered optical fiber. Coupling of the excitonic emission is achieved by placing 1L-WSe2 in the evanescent cavity field. This preserves the microresonator high intrinsic quality factor (Q > 106) below the bandgap of 1L-WSe2. The cavity quantum yield is QYc ≈ 10–3, consistent with operation in the broad emitter regime (i.e., the emission lifetime of 1L-WSe2 is significantly shorter than the bare cavity decay time). This scheme can serve as a precise measurement tool for the excitonic emission of layered materials into cavity modes, for both in plane and out of plane excitation. [ABSTRACT FROM AUTHOR]

We report high-performance WSe2 phototransistors with two-dimensional (2D) contacts formed between degenerately p-doped WSe2 and undoped WSe2 channel. A photoresponsivity of ∼600 mA/W with a high external quantum efficiency up to 100% and a fast response time (both rise and decay times) shorter than 8 μs have been achieved concurrently. More importantly, our WSe2 phototransistor exhibits a high specific detectivity (∼1013 Jones) in vacuum, comparable or higher than commercial Si- and InGaAs-based photodetectors. Further studies have shown that the high photoresponsivity and short response time of our WSe2 phototransistor are mainly attributed to the lack of Schottky-barriers between degenerately p-doped WSe2 source/drain contacts and undoped WSe2 channel, which can reduce the RC time constant and carrier transit time of a photodetector. Our experimental results provide an accessible strategy to achieve high-performance WSe2 phototransistor architectures by improving their electrical transport and photocurrent generation simultaneously, opening up new avenues for engineering future 2D optoelectronic devices. [ABSTRACT FROM AUTHOR]

In the search for nonprecious metal catalysts for the hydrogen evolution reaction (HER), transition metal dichalcogenides (TMDCs) have been proposed as promising candidates. Here, we present a facile method for significantly decreasing the overpotential required for catalyzing the HER with colloidally synthesized WSe2. Solution phase deposition of 2H WSe2 nanoflowers (NFs) onto carbon fiber electrodes results in low catalytic activity in 0.5 M H2SO4 with an overpotential at −10 mA/cm2 of greater than 600 mV. However, two postdeposition electrode processing steps significantly reduce the overpotential. First, a room-temperature treatment of the prepared electrodes with a dilute solution of the alkylating agent Meerwein’s salt ([Et3O][BF4]) results in a reduction in overpotential by approximately 130 mV at −10 mA/cm2. Second, we observe a decrease in overpotential of approximately 200–300 mV when the TMDC electrode is exposed to H+, Li+, Na+, or K+ ions under a reducing potential. The combined effect of ligand removal and electrochemical activation results in an improvement in overpotential by as much as 400 mV. Notably, the Li+ activated WSe2 NF deposited carbon fiber electrode requires an overpotential of only 243 mV to generate a current density of −10 mA/cm2. Measurement of changes in the material work function and charge transfer resistance ultimately provide rationale for the catalytic improvement. [ABSTRACT FROM AUTHOR]

Emerging two-dimensional (2-D) materials such as transition-metal dichalcogenides show great promise as viable alternatives for semiconductor and optoelectronic devices that progress beyond silicon. Performance variability, reliability, and stochasticity in the measured transport properties represent some of the major challenges in such devices. Native strain arising from interfacial effects due to the presence of a substrate is believed to be a major contributing factor. A full three-dimensional (3-D) mapping of such native nanoscopic strain over micron length scales is highly desirable for gaining a fundamental understanding of interfacial effects but has largely remained elusive. Here, we employ coherent X-ray diffraction imaging to directly image and visualize in 3-D the native strain along the (002) direction in a typical multilayered (∼100–350 layers) 2-D dichalcogenide material (WSe2) on silicon substrate. We observe significant localized strains of ∼0.2% along the out-of-plane direction. Experimentally informed continuum models built from X-ray reconstructions trace the origin of these strains to localized nonuniform contact with the substrate (accentuated by nanometer scale asperities, i.e., surface roughness or contaminants); the mechanically exfoliated stresses and strains are localized to the contact region with the maximum strain near surface asperities being more or less independent of the number of layers. Machine-learned multimillion atomistic models show that the strain effects gain in prominence as we approach a few- to single-monolayer limit. First-principles calculations show a significant band gap shift of up to 125 meV per percent of strain. Finally, we measure the performance of multiple WSe2 transistors fabricated on the same flake; a significant variability in threshold voltage and the “off” current setting is observed among the various devices, which is attributed in part to substrate-induced localized strain. Our integrated approach has broad implications for the direct imaging and quantification of interfacial effects in devices based on layered materials or heterostructures. [ABSTRACT FROM AUTHOR]

Highly linearly symmetric synaptic plasticity can be achieved based on the modulation of carrier types in WSe SB 2 sb with different thicknesses, facilitating a high level of training accuracy for optical neural networks. Keywords: artificial vision system; in-sensor computing; memory; optical neural networks; WSe2 EN artificial vision system in-sensor computing memory optical neural networks WSe2 1 1 1 10/12/23 20231009 NES 231009 B Bioinspired Artificial Visual System b In article number 2303539, Ye Zhou, Su-Ting Han, and co-workers report a 2D WSe SB 2 sb -based retinal perception array that demonstrates an artificial vision system integrating sense and memory visual information. Artificial vision system, in-sensor computing, memory, optical neural networks, WSe2. [Extracted from the article]

TMDC heterostructures provide alternative bases for optoelectronic devices besides conventional Schottky devices. With the continued expansion of the modern semiconductor industry, self-powered devices have emerged as an integral part of electronic and optoelectronic equipments. This work reports a unique seamless nano-hedge (nh) like morphology of MoS 2 deposited on WSe 2 film to form nh-MoS 2 /WSe 2 heterostructure as a self-powered photodetector with a broad spectral range from visible to NIR. The developed heterostructure exhibits a very high responsivity of 900 mAW-1 when illuminated by 860 nm under zero bias (self-powered mode). In addition, the device also shows a high responsivity of 177 mAW-1, 271 mAW-1, and 506 mAW-1 for illumination of 532 nm, 625 nm light, and 950 nm, respectively, at zero applied bias. With low noise equivalent power of 2.6 × 10-14 WHz1/2 and ultra-high responsivity of 2.6 × 104 mAW-1 in photoconductive mode at 860 nm light irradiation, the device exhibits extremely high external quantum efficiency of 6.29 × 103%. Further, the device also displays detectivity of 5 × 1011 Jones, an LDR value of 26 dB, with a fast response time of 15 ms. Developing a broadband photodetector based on nh-MoS 2 /WSe 2 heterostructure opens the route for highly responsive, self-powered future optoelectronic devices. • Development nh-MoS 2 /WSe 2 heterostructure as a self-powered photodetector for broad spectral range from visible to NIR. • A very high responsivity of 900 mAW-1 under 860 nm illumination at zero applied bias. • Very low NEP ∼2.6 × 10-14 WHz1/2, ultra-high responsivity 2.6 × 104 mAW-1 with high external quantum efficiency of 6.29 × 103% under photoconductive mode at 860 nm wavelength is reported. [ABSTRACT FROM AUTHOR]

Despite extensive investigations of a wide variety of artificial synapse devices aimed at realizing a neuromorphic hardware system, the identification of a physical parameter that modulates synaptic plasticity is still required. In this context, a novel two-dimensional architecture consisting of a NbSe2/WSe2/Nb2O5 heterostructure placed on an SiO2/p+ Si substrate was designed to overcome the limitations of the conventional silicon-based complementary metal-oxide semiconductor technology. NbSe2, WSe2, and Nb2O5 were used as the metal electrode, active channel, and conductance-modulating layer, respectively. Interestingly, it was found that the post-synaptic current was successfully modulated by the thickness of the interlayer Nb2O5, with a thicker interlayer inducing a higher synapse spike current and a stronger interaction in the sequential pulse mode. Introduction of the Nb2O5 interlayer can facilitate the realization of reliable and controllable synaptic devices for brain-inspired integrated neuromorphic systems.

The electrical and optical properties of semiconducting transition metal dichalcogenides (TMDs) can be tuned by controlling their composition and the number of layers they have. Among various TMDs, the monolayer WSe2 has a direct bandgap of 1.65 eV and exhibits p-type or bipolar behavior, depending on the type of contact metal. Despite these promising properties, a lack of efficient large-area production methods for high-quality, uniform WSe2 hinders its practical device applications. Various methods have been investigated for the synthesis of large-area monolayer WSe2, but the difficulty of precisely controlling solid-state TMD precursors (WO3, MoO3, Se, and S powders) is a major obstacle to the synthesis of uniform TMD layers. In this work, we outline our success in growing large-area, high-quality, monolayered WSe2 by utilizing methane (CH4) gas with precisely controlled pressure as a promoter. When compared to the catalytic growth of monolayered WSe2 without a gas-phase promoter, the catalytic growth of the monolayered WSe2 with a CH4 promoter reduced the nucleation density to 1/1000 and increased the grain size of monolayer WSe2 up to 100 μm. The significant improvement in the optical properties of the resulting WSe2 indicates that CH4 is a suitable candidate as a promoter for the synthesis of TMD materials, because it allows accurate gas control.

Tungsten diselenide (WSe2) was analyzed using x-ray photoelectron spectroscopy. A freshly exfoliated, oxygen-free flake was analyzed. Spectral regions for O 1s, W 4d, W 4f, W 5p, Se 3d and an extended region encompassing Se 3p, Se 3s, W 4d, Se LMM, and C 1s were acquired along with the valence band. Bulk quantitative analyses by x-ray fluorescence and x-ray diffraction indicated the material was stoichiometric and phase pure, respectively. [ABSTRACT FROM AUTHOR]

This report presents a simple and efficient method of layer thinning and p-type doping of WSe2 with vapor XeF2. With this approach, the surface roughness of thinned WSe2 can be controlled to below 0.7 nm at an etched depth of 100 nm. By selecting appropriate vapor XeF2 exposure times, 23-layer and 109-layer WSe2 can be thinned down to monolayer and bilayer, respectively. In addition, the etching rate of WSe2 exhibits a significant dependence on vapor XeF2 exposure pressure and thus can be tuned easily for thinning or patterning applications. From Raman, photoluminescence, X-ray photoelectron spectroscopy (XPS), and electrical characterization, a p-doping effect of WSe2 induced by vapor XeF2 treatment is evident. Based on the surface composition analysis with XPS, the causes of the p-doping effect can be attributed to the presence of substoichiometric WO x ( x < 3) overlayer, trapped reaction product of WF6, and nonstoichiometric WSe x ( x > 2). Furthermore, the p-doping level can be controlled by varying XeF2 exposure time. The thinning and p-doping of WSe2 with vapor XeF2 have the advantages of easy scale-up, high etching selectivity, excellent controllability, and compatibility with conventional complementary metal-oxide-semiconductor fabrication processes, which is promising for applications of building WSe2 devices with versatile functionalities. [ABSTRACT FROM AUTHOR]

This report presents a simple and efficient method of layer thinning and p-type doping of WSe2 with vapor XeF2. With this approach, the surface roughness of thinned WSe2 can be controlled to below 0.7 nm at an etched depth of 100 nm. By selecting appropriate vapor XeF2 exposure times, 23-layer and 109-layer WSe2 can be thinned down to monolayer and bilayer, respectively. In addition, the etching rate of WSe2 exhibits a significant dependence on vapor XeF2 exposure pressure and thus can be tuned easily for thinning or patterning applications. From Raman, photoluminescence, X-ray photoelectron spectroscopy (XPS), and electrical characterization, a p-doping effect of WSe2 induced by vapor XeF2 treatment is evident. Based on the surface composition analysis with XPS, the causes of the p-doping effect can be attributed to the presence of substoichiometric WO x ( x < 3) overlayer, trapped reaction product of WF6, and nonstoichiometric WSe x ( x > 2). Furthermore, the p-doping level can be controlled by varying XeF2 exposure time. The thinning and p-doping of WSe2 with vapor XeF2 have the advantages of easy scale-up, high etching selectivity, excellent controllability, and compatibility with conventional complementary metal-oxide-semiconductor fabrication processes, which is promising for applications of building WSe2 devices with versatile functionalities. [ABSTRACT FROM AUTHOR]

Molecular beam epitaxy (MBE) provides a simple but powerful way to synthesize large-area high-quality thin films and heterostructures of a wide variety of materials including accomplished group III-V and II-VI semiconductors as well as newly developing oxides and chalcogenides, leading to major discoveries in condensed-matter physics. For two-dimensional (2D) materials, however, main fabrication routes have been mechanical exfoliation and chemical vapor deposition by making good use of weak van der Waals bonding nature between neighboring layers, and MBE growth of 2D materials, in particular on insulating substrates for transport measurements, has been limited despite its fundamental importance for future advanced research. Here, we report layer-by-layer epitaxial growth of scalable transition-metal dichalocogenide (TMDC) thin films on insulating substrates by MBE and demonstrate ambipolar transistor operation. The proposed growth protocol is broadly applicable to other TMDCs, providing a key milestone toward fabrication of van der Waals heterostructures with various 2D materials for novel properties and functionalities. [ABSTRACT FROM AUTHOR]

We demonstrate gate-tunable resonant tunneling and negative differential resistance between two rotationally aligned bilayer graphene sheets separated by bilayer WSe2. We observe large interlayer current densities of 2 and 2.5 μA/μm² and peak-to-valley ratios approaching 4 and 6 at room temperature and 1.5 K, respectively, values that are comparable to epitaxially grown resonant tunneling heterostructures. An excellent agreement between theoretical calculations using a Lorentzian spectral function for the two-dimensional (2D) quasiparticle states, and the experimental data indicates that the interlayer current stems primarily from energy and in-plane momentum conserving 2D-2D tunneling, with minimal contributions from inelastic or non-momentum-conserving tunneling. We demonstrate narrow tunneling resonances with intrinsic half-widths of 4 and 6 meV at 1.5 and 300 K, respectively. [ABSTRACT FROM AUTHOR]

Two-dimensional (2D) molybdenum trioxide (MoO3) with mono- or few-layer thickness can potentially advance many applications, ranging from optoelectronics, catalysis, sensors, and batteries to electrochromic devices. Such ultrathin MoO3 sheets can also be integrated with other 2D materials (e.g., as dopants) to realize new or improved electronic devices. However, there is lack of a rapid and scalable method to controllably grow mono- or few-layer MoO3. Here, we report the first demonstration of using a rapid (<2 min) flame synthesis method to deposit mono- and few-layer MoO3 sheets (several microns in lateral dimension) on a wide variety of layered materials, including mica, MoS2, graphene, and WSe2, based on van der Waals epitaxy. The flame-grown ultrathin MoO3 sheet functions as an efficient hole doping layer for WSe2, enabling WSe2 to reach the lowest sheet and contact resistance reported to date among all the p-type 2D materials (~6.5 kΩ/□ and ~0.8 kΩ·μm, respectively). These results demonstrate that flame synthesis is a rapid and scalable pathway to growing atomically thin 2D metal oxides, opening up new opportunities for advancing 2D electronics. [ABSTRACT FROM AUTHOR]

High-energy ionizing radiation is widely used in medical diagnosis and cancer radiation therapy. However, high-energy radiation can also impose significant damages in healthy tissues during medical treatments via direct DNA damages and indirect damages from production of reactive oxygen species (ROS). Therefore, it is urgent to develop highly effective radioprotectants with low toxicities that can meet the increasing needs for alleviating the adverse effects from cancer radiation therapy and nuclear emergency. In this work, strongly catalytic ultrasmall (sub-5 nm) cysteine-protected WSe2 dots are employed to protect healthy tissues against radiation via diminishing radiation-induced free radicals. The WSe2 dots with high surface activities can recover radiation-induced DNA damages and eliminate the excessive ROS generated from radiation. In vivo experiments confirm that the survival rate of mice treated with WSe2 dots is significantly elevated with radiation damages postponed under exposure to high-dose ionizing radiation. Furthermore, the free radicals in major organs and hematological system can be appreciably omitted, suggesting their unique role as free radical scavengers. These WSe2 dots in ultrasmall size show rapid renal clearance of ≈74% injection dose via urine excretion in 24 h and do not cause any apparent toxicity in vivo for up to 30 d. [ABSTRACT FROM AUTHOR]

We have investigated the electronic properties of WSe2 surfaces covered by fluorinated cobalt phthalocyanine (F16CoPc) using photoemission spectroscopy. We show that a charge transfer occurs at this interface, which results in the creation of holes in the WSe2 surface while the Co center of the phthalocyanine is reduced to Co(I). We observe a potential change in WSe2 approaching the surface as a consequence of the induced positive charges near the surface. In addition, our data allow for a rough estimation of the induced charge density and suggest that the holes might be localized. [ABSTRACT FROM AUTHOR]

A chemically active and stable WSe 2 /W structure was prepared by sputtering and selenization annealing in order to replace a high-cost Pt counter electrode in dye-sensitized solar cells. The CV, EIS analysis, and Tafel curve measurements indicated that the WSe 2 /W electrode possesses high conductivity, low charge transfer resistance at the electrolyte-electrode interface, good electrocatalytic activity, and fast reaction kinetics for the a iodide/triiodide redox reaction, which are due to the synergistic effect of W and WSe 2 in combination. The DSSC with a novel WSe 2 /W counter electrode achieved a high power conversion efficiency of 8.22% under standard light illumination, which is comparable to that with a platinum (Pt)-coated FTO electrode (8.20%). [ABSTRACT FROM AUTHOR]

P-type transistors based on high work function transition metal dichalcogenide (TMD) monolayers such as MoS2 are to date difficult to produce, owing to the strong Fermi level pinning at the semiconductor/contact metal interfaces. In this work, the potential of halogenated graphenes is demonstrated as a new class of efficient hole injection layers to TMDs such as MoS2 and WSe2 by taking fluorographene (or GF) as a model buffer layer. Using first-principles computations, two commonly obtained GF stoichiometries, C2F and CF, have been studied as buffer layers between MoS2 and Pt. In particular, for high work function TMDs such as MoS2, it has been shown that C2F forms an ohmic contact, while CF leads to a significant p-SBH value. On the other hand, for low work function TMDs such as WSe2, both C2F and CF lead to p-type ohmic contacts. This analysis shows that the ability of these buffer layers to form p-type contacts depends crucially on the charge redistribution at the GF/metal interface, which is dictated by their chemical interaction and equilibrium geometry. The fundamental electronic structures between the different semiconductor/insulator/metal interfaces which are part of this study have also been investigated. [ABSTRACT FROM AUTHOR]

Optical dipole moment is the key parameter of optical transitions, as it directly determines the strength of light-matter interaction such as intrinsic radiative lifetime. However, experimental determination of these fundamental properties of excitons in monolayer WSe2 is largely limited, because the commonly used measurement, such as (time-resolved) photoluminescence, is inherently difficult to probe the intrinsic properties. For example, dark states below bright exciton can change the photoluminescence emission rate by orders of magnitude and gives an 'effective' radiative lifetime distinctive from the intrinsic one. On the other hand, such 'effective' radiative lifetime becomes important itself because it describes how dark states affect exciton dynamics. Unfortunately, the 'effective' radiative lifetime in monolayer WSe2 is also not determined as it requires photoluminescence measurement with resonant excitation, which is technically difficult. These difficulties are overcome here to obtain both the 'intrinsic' and 'effective' radiative lifetime experimentally. A framework is developed to determine the dipole moment and 'intrinsic' radiative lifetime of delocalized excitons in monolayer WSe2 from the absorption measurements. In addition, the 'effective' radiative lifetime in WSe2 is obtained through time-resolved photoluminescence and absolute quantum-yield measurement at resonant excitation. These results provide helpful information for fundamental understanding of exciton light-matter interaction in WSe2. [ABSTRACT FROM AUTHOR]

Optical dipole moment is the key parameter of optical transitions, as it directly determines the strength of light-matter interaction such as intrinsic radiative lifetime. However, experimental determination of these fundamental properties of excitons in monolayer WSe2 is largely limited, because the commonly used measurement, such as (time-resolved) photoluminescence, is inherently difficult to probe the intrinsic properties. For example, dark states below bright exciton can change the photoluminescence emission rate by orders of magnitude and gives an 'effective' radiative lifetime distinctive from the intrinsic one. On the other hand, such 'effective' radiative lifetime becomes important itself because it describes how dark states affect exciton dynamics. Unfortunately, the 'effective' radiative lifetime in monolayer WSe2 is also not determined as it requires photoluminescence measurement with resonant excitation, which is technically difficult. These difficulties are overcome here to obtain both the 'intrinsic' and 'effective' radiative lifetime experimentally. A framework is developed to determine the dipole moment and 'intrinsic' radiative lifetime of delocalized excitons in monolayer WSe2 from the absorption measurements. In addition, the 'effective' radiative lifetime in WSe2 is obtained through time-resolved photoluminescence and absolute quantum-yield measurement at resonant excitation. These results provide helpful information for fundamental understanding of exciton light-matter interaction in WSe2. [ABSTRACT FROM AUTHOR]

Single layer graphene is an ideal material for the base layer of hot electron transistors (HETs) for potential terahertz (THz) applications. The ultrathin body and exceptionally long mean free path maximizes the probability for ballistic transport across the base of the HET. We demonstrate for the first time the operation of a high-performance HET using a graphene/WSe2 van der Waals (vdW) heterostructure as a base-collector barrier. The resulting device with a GaN/AlN heterojunction as emitter, exhibits a current density of 50 A/cm², direct current gain above 3 and 75% injection efficiency, which are record values among graphene-base HETs. These results not only provide a scheme to overcome the limitations of graphene-base HETs toward THz operation but are also the first demonstration of a GaN/vdW heterostructure in HETs, revealing the potential for novel electronic and optoelectronic applications. [ABSTRACT FROM AUTHOR]

This paper demonstrates the fabrication and operation of a vertically integrated phototransistor based on single‐layer MoS2/multilayer WSe2 van der Waals heterojunction. Under the gate modulation effects, the device which combines advantages of their constituents displays a novel feature in the transfer characteristics where a current peak appears at small gate voltage bias and exhibits excellent optical broadband photodetecting properties. Upon visible light illumination (660 nm), the photodetection capabilities reach the optimal values simultaneously with the photoresponsivity of 17.8 A W−1, photosensivity of 140, and the response speed of <80 ms. The device also shows good photoresponse under near‐infrared light illumination (850 nm) and the obvious dependence on the gate voltage which plays a switching role for near infrared light photodetection. This 2D heterostructured broadband photodetector may provide an available device structure for an essential sensing component in the future wearable electronics applications. [ABSTRACT FROM AUTHOR]

Two-dimensional transition metal dichalcogenides (2D TMDCs) offer several attractive features for use in next-generation electronic and optoelectronic devices. Device applications of TMDCs have gained much research interest, and significant advancement has been recorded. In this review, the overall research advancement in electronic and optoelectronic devices based on TMDCs are summarized and discussed. In particular, we focus on evaluating field effect transistors (FETs), photovoltaic cells, light-emitting diodes (LEDs), photodetectors, lasers, and integrated circuits (ICs) using TMDCs. [ABSTRACT FROM AUTHOR]

Direct observation of grains and boundaries is a vital factor in altering the electrical and optoelectronic properties of transition metal dichalcogenides (TMDs), that is, MoSe2 and WSe2. Here, we report visualization of grains and boundaries of chemical vapor deposition grown MoSe2 and WSe2 on silicon, using optical birefringence of two-dimensional layer covered with nematic liquid crystal (LC). An in-depth study was performed to determine the alignment orientation of LC molecules and their correlation with other grains. Interestingly, we found that alignment of liquid crystal has discrete preferential orientations. From computational simulations, higher adsorption energy for the armchair direction was found to force LC molecules to align on it, compared to that of the zigzag direction. We believe that these TMDs with three-fold symmetric alignment could be utilized for display applications. [ABSTRACT FROM AUTHOR]

The cross-plane thermal conductivity of WSe2 is investigated using reverse nonequilibrium molecular dynamics (RNEMD) and a recently developed Stillinger-Weber potential. It is found that the cross-plane thermal conductivity of WSe2 is strongly size dependent and saturates around 80 layers. Moreover, it is shown that even at 1000 K, ordered crystalline WSe2 does not reach the phonon glass-like limit in the cross-plane direction. [ABSTRACT FROM AUTHOR]

Herein, monolayer and bilayer absorbers were designed to investigate the microwave absorption characteristics of LaCoO 3 and WSe 2 nanomaterials. WSe 2 monolayer sample showed better absorption capabilities as compared to LaCoO 3 single-layer sample. The microwave absorption effectiveness of bilayer absorbents was scrutinized using a simulation approach. The simulation results demonstrated that a bilayer absorber composed of LaCoO 3 as the bottom layer (0.5 mm thickness) and WSe 2 on the top layer (1.5 mm thickness) can obtain a minimum reflection loss (RL mini) of −42.5 dB with an effective absorption bandwidth (EAB) of 2.8 GHz (12.4 GHz–9.6 GHz) at 10.8 GHz. Simulation predictions and experimental findings were juxtaposed; which accord with one another quite well. The findings demonstrate that the exceptional microwave absorption performance of the bilayer absorber may be attributed to its layered architecture. Our study presents a simple synthetic method and new perspectives on engineering a bilayer absorber with simulation and experimental justification. • Bilayer absorbers were designed from LaCoO 3 and WSe 2 nanomaterials. • WSe 2 monolayer sample showed better absorption capabilities as compared to LaCoO 3 sample. • LaCoO 3 as the bottom layer (0.5 mm thickness) and WSe 2 on the top layer (1.5 mm thickness) was the best sample. • Bilayer sample has achieved a minimum reflection loss (RL min) of −42.5 dB with effective absorption Bandwidth (EAB) of 10.8 GHz (9.6–12.4 GHz). • Simulation predictions and experimental findings were juxtaposed; which accord with one another quite well. [ABSTRACT FROM AUTHOR]

Transition metal dichalcogenides (TMDs) have a broad prospect in gas sensors due to their vulnerable sensitivity to gaseous species at room temperature (RT). However, the sensors based on TMDs currently still suffer from the low response and incomplete recovery. Herein, we report an unusual synthesis of WSe 2 concentric nanotriangles grown by chemical vapor deposition (CVD) and their application to highly sensitive sensors. The WSe 2 sensor shows high response to NO 2 and triethylamine (TEA) at RT due to the increased active sites from the concentric nanotriangle structure. Under ultraviolet (UV) illumination the gas sensing performance was further improved, including higher response, complete recovery and ppb-level detection limit. This work shed some light in engineering TMDs nanostructures to achieve improved sensor applications. • Peculiar WSe 2 concentric nanotriangles are grown by CVD method. • The WSe 2 concentric triangles are for the first time applied to gas sensor. • The WSe 2 sensor exhibit significantly enhanced response at RT with complete recovery. [ABSTRACT FROM AUTHOR]

• Ballistic spin-valley transport in bilayer graphene on WSe 2 is studied. • Pure spin, valley currents are perfectly controlled by gate. • Perfect switching of spin-valley-coupled polarization from −100% to + 100% by gate potential is predicted. • The perfect control of spin-valley transport property is due to specific properties from bilayer graphene grown on WSe 2. • The junction is applicable for spin-valley-current-based devices. We investigate the spin-valley dependent transport properties in a gated bilayer graphene (BG) junction placed on top of WSe2. By means of layer-dependent proximity, the spin–orbit interaction (SOI) is induced in the bottom layer, while the top layer is induced into ferromagnetism by a magnetic insulator. As a result of these differing properties, the Fermi level and energy gap become spin-valley dependent, which could lead to controllable spin- and valley-dependent filtering in the BG system. Our findings predict perfect spin-valley polarization control through gate control, given specific energy and electric field conditions, as well as spin-valley-coupled and spin currents when both the electric-field-induced energy gap and the exchange energy are equal to the SOI strength. We predict that there will be very highly sensitive gate control of −100% to + 100% of spin-valley-coupled polarization for large barrier thickness. Additionally, we predict 100% valley polarization can be controlled by the gate when the exchange energy is equal to the SOI. Furthermore, we found that the performance of the junction to control the polarizations can be enhanced by increasing the thickness of the junction. This work reveals the potential of the BG/WSe2 hybrid structure for spin-valley-current-based electronics, such as spin-, valley-, and spin-valley field-effect transistors. [ABSTRACT FROM AUTHOR]

Two-dimensional (2D) self-driven photodetectors have a wide range of applications in wearable, imaging, and flexible electronics. However, the preparation of most self-powered photodetectors is still complex and time-consuming. Simultaneously, the constant work function of a metal, numerous defects, and a large Schottky barrier at the 2D/metal interface hinder the transmission and collection of optical carriers, which will suppress the optical responsivity of the device. This paper proposed a self-driven graphene/WSe 2 /graphene (Gr/WSe 2 /Gr) photodetector with asymmetric Schottky van der Waals (vdWs) contacts. The vdWs contacts are formed by transferring Gr as electrodes using the dry-transfer method, obviating the limitations of defects and Fermi-level pinning at the interface of electrodes made by conventional metal deposition methods to a great extent and resulting in superior dynamic response, which leads to a more efficient and faster collection of photogenerated carriers. This work also demonstrates that the significant surface potential difference of Gr electrodes is a crucial factor to ensure their superior performance. The self-driven Gr/WSe 2 /Gr photodetector exhibits an ultrahigh I light / I dark ratio of 10 6 with a responsivity value of 20.31 mA/W and an open-circuit voltage of 0.37 V at zero bias. The photodetector also has ultrafast response speeds of 42.9 and 56.0 μs. This paper provides a feasible way to develop self-driven optoelectronic devices with a simple structure and excellent performance.

In recent years, there has been an increasing focus on 2D nongraphene materials that range from insulators to semiconductors to metals. As a single-elemental van der Waals semiconductor, tellurium (Te) has captivating anisotropic physical properties. Recent work demonstrated growth of ultrathin Te on WSe 2 with the atomic chains of Te aligned with the armchair directions of the substrate using physical vapor deposition (PVD). In this system, a moiré superlattice is formed where micrometer-scale Te flakes sit on top of the continuous WSe 2 film. Here, we determined the precise orientation of the Te flakes with respect to the substrate and detailed structure of the resulting moiré lattice by combining electron microscopy with image simulations. We directly visualized the moiré lattice using center of mass-differential phase contrast (CoM-DPC). We also investigated the local strain within the Te/WSe 2 layered materials using scanning nanodiffraction techniques. There is a significant tensile strain at the edges of flakes along the direction perpendicular to the Te chain direction, which is an indication of the preferred orientation for the growth of Te on WSe 2 . In addition, we observed local strain relaxation regions within the Te film, specifically attributed to misfit dislocations, which we characterize as having a screw-like nature. The detailed structural analysis gives insight into the growth mechanisms and strain relaxation in this moiré heterostructure.

Bilayer graphene (BLG) was recently shown to host a band-inverted phase with unconventional topology emerging from the Ising-type spin-orbit interaction (SOI) induced by the proximity of transition metal dichalcogenides with large intrinsic SOI. Here, we report the stabilization of this band-inverted phase in BLG symmetrically encapsulated in tungsten diselenide (WSe 2 ) via hydrostatic pressure. Our observations from low temperature transport measurements are consistent with a single particle model with induced Ising SOI of opposite sign on the two graphene layers. To confirm the strengthening of the inverted phase, we present thermal activation measurements and show that the SOI-induced band gap increases by more than 100% due to the applied pressure. Finally, the investigation of Landau level spectra reveals the dependence of the level-crossings on the applied magnetic field, which further confirms the enhancement of SOI with pressure.

n-type field effect transistors (FETs) based on two-dimensional (2D) transition-metal dichalcogenides (TMDs) such as MoS 2 and WS 2 have come close to meeting the requirements set forth in the International Roadmap for Devices and Systems (IRDS). However, p-type 2D FETs are dramatically lagging behind in meeting performance standards. Here, we adopt a three-pronged approach that includes contact engineering, channel length ( L ch ) scaling, and monolayer doping to achieve high performance p-type FETs based on synthetic WSe 2 . Using electrical measurements backed by atomistic imaging and rigorous analysis, Pd was identified as the favorable contact metal for WSe 2 owing to better epitaxy, larger grain size, and higher compressive strain, leading to a lower Schottky barrier height. While the ON-state performance of Pd-contacted WSe 2 FETs was improved by ∼10× by aggressively scaling L ch from 1 μm down to ∼20 nm, ultrascaled FETs were found to be contact limited. To reduce the contact resistance, monolayer tungsten oxyselenide (WO x Se y ) obtained using self-limiting oxidation of bilayer WSe 2 was used as a p-type dopant. This led to ∼5× improvement in the ON-state performance and ∼9× reduction in the contact resistance. We were able to achieve a median ON-state current as high as ∼10 μA/μm for ultrascaled and doped p-type WSe 2 FETs with Pd contacts. We also show the applicability of our monolayer doping strategy to other 2D materials such as MoS 2 , MoTe 2 , and MoSe 2 .

High-performing 2D electrical and optical devices can be realized by forming an ideal van der Waals (vdW) metal contact with weak interactions and stable interface states. However, the methods for applying metal contacts while avoiding damage from metal deposition present challenges in realizing a uniform, stable vdW interface. To overcome this problem, this study develops a method for forming vdW contacts using a sacrificial Se buffer layer. This study explores this method by investigating the difference in the Schottky barrier height between the vdW metal contact deposited using a buffer layer, a transferred metal contact, and a conventional directly deposited metal contact using rectification and photovoltaic characteristics of a Schottky diode structure with graphite. Evidently, the Se buffer layer method forms the most stable and ideal vdW contact while preventing Fermi-level pinning. A tungsten diselenide Schottky diode fabricated using these vdW contacts with Au and graphite as the top and bottom electrodes, respectively, exhibits excellent operation with an ideality factor of ≈1, an on/off ratio of > 10 7 , and coherent properties. Additionally, when using only the vdW Au contact, the electrical and optical properties of the device can be minutely modulated by changing the structure of the Schottky diode., (© 2023 Wiley-VCH GmbH.)

Transition metal dichalcogenides (TMDs) have been recognised as a candidate material in many electrochemical applications including energy storage, and electrocatalyst. In the TMDs family, the molybdenum disulfide produced via liquid phase exfoliation had received attracted attention for fabricating the electrode. This is due to a clear structural and electrochemical property shown in the literature. To further explore the applications of the electrode made from TMDs materials, the tungsten selenide (WSe 2) with a variety of flake sizes, and its properties were produced through a traditional liquid phase exfoliation using iso-propanol/water as an exfoliation solvent. The cascade centrifugal techniques were then applied to obtain a finely-selected flake size of WSe 2 for electrochemical and physical chacterisation. In this work, we shed more light on the structural and electrocatalyst properties of the size selected WSe 2. A variety of WSe 2 flakes exhibit almost similar structure; however, we have found that the intensity of W L 3 -edge XANES spectra decreases with respect to the increase of flake dimension. This is due to the change in the electronic structure of those materials. The change directly affects the electrocatalytic performance in which, the tiny flake displayed an excellent catalytic property for hydrogen evolution reaction with the Tafel slope of 76.11 mV dec−1 and overpotential of −400 mV vs. RHE (at 10 mA cm−2). This work provides insight into the structural information, which should improve the understanding of TMDs materials in various contexts, especially electrocatalyst and energy storage. • The WSe 2 nanoflakes were produced via liquid phase exfoliation. • The flakes were finely selected using cascade centrifugation. • The Se vacancies affect the hydrogen evolution properties. • The structural properties has been proposed using EXAFS fitting. [ABSTRACT FROM AUTHOR]

Transition metal dichalcogenides (TMDs) have attracted much attention in electrochemistry due to their outstanding properties; however, there is a limit to the production of TMDs samples on a large scale. In this work, we have demonstrated the recycling process of TMDs materials using WSe 2 as a representative of the TMDs family. WSe 2 nanoflakes were synthesised by solvent-assisted liquid phase exfoliation through sonication of "bulk" WSe 2. It has been found that the unexfoliated WSe 2 can be recycled up to 6 times, this improve the production yield. The physical and chemical properties of the exfoliated WSe 2 nanoflakes were investigated during the recycling processes. Interestingly, the recycling processes (number of recycling times) directly affect the flake sizes, and surface chemistry of the exfoliated samples. The obtained flake size decreases from about 1 μm to less than 200 nm (from 1st to 7th exfoliation), the diminished of sample thickness has also been found. Besides, the local structure of WSe 2 remains unchanged, showing the W4+ oxidation state of WSe 2 , which to the best of our knowledge. The applicability of the as-recycled samples were then demonstrated as bi-functional electrode materials: (i) supercapacitor, (ii) and electrocatalyst (for hydrogen evolution reaction). From 1st to 7th exfoliation, the electrochemical performance improves about 1-fold (capacitance increase from 3 to 6 F g−1, and the overpotential at 10 mA cm−2 improves from -0.67 to -0.42 vs. RHE). This study provides an alternative way to improve the synthesis method, which can not only utilise "waste" but also recover the production yield of TMDs leading to the continued development of 2D materials, and related families. [Display omitted] [ABSTRACT FROM AUTHOR]

There is a great attention of scaling down transistor based on 2D materials. Although gate-all-around (GAA) structure based on silicon has been simulated, the GAA structure combined with 2D materials do not have a well understanding. Moreover, the recent proposed edge graphene gated transistor also needs deeper analysis. Here, edge graphene gated 2D-FETs are further investigated and optimized by employing semi-classical numerical method. The physical dimensions can be optimized to obtain performance improvement, and some possible schemes are proposed. We demonstrate fair quantitative comparison for our proposed 0.34 nm graphene edge gated MoS 2 and GAA graphene gated silicon nanowire device structure. It is found that the edge graphene gated MoS 2 FET can achieve superior short channel performance compared to GAA graphene gated silicon nanowire and traditional GAA silicon nanowire FET, due to the generic nature of 2D TMDs, which possess direct narrow bandgap, low dielectric constant and atomic thickness. We have figured out that the 0.34 nm edge graphene gated MoS 2 FET has even better subthreshold swing than the Si-GAA and Graphene-GAA, which shows promising prospect as next generation electronics. [Display omitted] • Propose a GAA graphene gated silicon nanowire device structure • Provide an understanding of the GAA structure combined with 2D materials • Compare the novel structures, showing High horizontal scalability [ABSTRACT FROM AUTHOR]