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Feedback field-effect transistors (FBFETs) have been studied to obtain near-zero subthreshold swings at 300 K with a high on/off current ratio ~1010. However, their structural complexity, such as an epitaxy process after an etch process for a Si channel with a thickness of several nanometers, has limited broader research. We demonstrated a FBFET using in-plane WSe2 p−n homojunction. The WSe2 FBFET exhibited a minimum subthreshold swing of 153 mV/dec with 30 nm gate dielectric. Our modeling-based projection indicates that the swing of this device can be reduced to 14 mV/dec with 1 nm EOT. Also, the gain of the inverter using the WSe2 FBFET can be improved by up to 1.53 times compared to a silicon CMOS inverter, and power consumption can be reduced by up to 11.9%. [ABSTRACT FROM AUTHOR]

Low-dimension materials such as transition metal dichalcogenides (TMDCs) have received extensive research interest and investigation for electronic and optoelectronic applications. Due to their unique widely tunable band structures, they are good candidates for next-generation optoelectronic devices. Particularly, their photoluminescence properties, which are fundamental for optoelectronic applications, are highly sensitive to the nature of the band gap. Monolayer TMDCs in the room temperature range have presented a direct band gap behavior and bright photoluminescence. In this work, we investigate a popular TMDC material WSe2's photoluminescence performance using a Raman spectroscopy laser with temperature dependence. With temperature variation, the lattice constant and the band gap change dramatically, and thus the photoluminescence spectra are changed. By checking the photoluminescence spectra at different temperatures, we are able to reveal the nature of direct-to-indirect band gap in monolayer WSe2. We also implemented density function theory (DFT) simulations to computationally investigate the band gap of WSe2 to provide comprehensive evidence and confirm the experimental results. Our study suggests that monolayer WSe2 is at the transition boundary between the indirect and direct band gap at room temperature. This result provides insights into temperature-dependent optical transition in monolayer WSe2 for quantum control, and is important for cultivating the potential of monolayer WSe2 in thermally tunable optoelectronic devices operating at room temperature. [ABSTRACT FROM AUTHOR]

Two-dimensional (2D) van der Waals layered materials have been explored in depth. They can be vertically stacked into a 2D heterostructure and represent a fundamental way to explore new physical properties and fabricate high-performance nanodevices. However, the controllable and scaled growth of non-layered quasi-2D materials and their heterostructures is still a great challenge. Here, we report a selective two-step growth method for high-quality single crystalline CrTe/WSe2 and CrTe/MoS2 heterostructures by adopting a universal CVD strategy with the assistance of molten salt and mass control. Quasi-2D metallic CrTe was grown on pre-deposited 2D transition metal dichalcogenides (TMDC) under relatively low temperatures. A 2D CrTe/TMDC heterostructure was established to explore the interface's structure using scanning transmission electron microscopy (STEM), and also demonstrate ferromagnetism in a metal–semiconductor CrTe/TMDC heterostructure. [ABSTRACT FROM AUTHOR]

The excellent physicochemical properties of two-dimensional transition-metal dichalcogenides (2D TMDCs) such as WS2 and WSe2 provide potential benefits for biomedical applications, such as drug delivery, photothermal therapy, and bioimaging. WS2 and WSe2 have recently been used as chemosensitizers; however, the detailed molecular basis underlying WS2- and WSe2-induced sensitization remains elusive. Our recent findings showed that 2D TMDCs with different thicknesses and different element compositions induced autophagy in normal human bronchial epithelial cells and mouse alveolar macrophages at sublethal concentrations. Here, we explored the mechanism by which WS2 and WSe2 act as sensitizers to increase lung cancer cell susceptibility to chemotherapeutic agents. The results showed that WS2 and WSe2 enhanced autophagy flux in A549 lung cancer cells at sublethal concentrations without causing significant cell death. Through the autophagy-specific RT2 Profiler PCR Array, we identified the genes significantly affected by WS2 and WSe2 treatment. Furthermore, the key genes that play central roles in regulating autophagy were identified by constructing a molecular interaction network. A mechanism investigation uncovered that WS2 and WSe2 activated autophagy-related signaling pathways by interacting with different cell surface proteins or cytoplasmic proteins. By utilizing this mechanism, the efficacy of the chemotherapeutic agent doxorubicin was enhanced by WS2 and WSe2 pre-treatment in A549 lung cancer cells. This study revealed a feature of WS2 and WSe2 in cancer therapy, in which they eliminate the resistance of A549 lung cancer cells against doxorubicin, at least partially, by inducing autophagy. [ABSTRACT FROM AUTHOR]

In this work, we present a comprehensive theoretical and computational investigation of exciton fine structures of WSe 2 -monolayers, one of the best-known two-dimensional (2D) transition-metal dichalcogenides (TMDs), in various dielectric-layered environments by solving the first-principles-based Bethe–Salpeter equation. While the physical and electronic properties of atomically thin nanomaterials are normally sensitive to the variation of the surrounding environment, our studies reveal that the influence of the dielectric environment on the exciton fine structures of TMD-MLs is surprisingly limited. We point out that the non-locality of Coulomb screening plays a key role in suppressing the dielectric environment factor and drastically shrinking the fine structure splittings between bright exciton (BX) states and various dark-exciton (DX) states of TMD-MLs. The intriguing non-locality of screening in 2D materials can be manifested by the measurable non-linear correlation between the BX-DX splittings and exciton-binding energies by varying the surrounding dielectric environments. The revealed environment-insensitive exciton fine structures of TMD-ML suggest the robustness of prospective dark-exciton-based optoelectronics against the inevitable variation of the inhomogeneous dielectric environment. [ABSTRACT FROM AUTHOR]

Out-of-plane p-n heterojunctions based on two-dimensional layered materials (2DLMs) with unusual physical characteristics are attracting extensive research attention for their application as photodetectors. However, the present fabrication method based on 2DLMs produces out-of-plane p-n homojunction devices with low photoresponsivity and detectivity. This work reports an ultrasensitive phototransistor based on a laser-induced p-doped WSe2/MoS2 van der Waals heterojunction. The laser treatment is used for p-doping WSe2 nanoflakes using high work function WOx. Then, an n-type MoS2 nanoflake is transferred onto the resulting p-doped WSe2 nanoflake. The built-in electric field of p-doped WSe2/MoS2 is stronger than that of pristine WSe2/MoS2. The p-n junction between p-doped WSe2 and MoS2 can separate more photogenerated electron–hole pairs and inject more electrons into MoS2 under laser illumination than pristine WSe2/MoS2. Thus, a high photoresponsivity (R) of ~1.28 × 105 A·W−1 and high specific detectivity (D*) of ~7.17 × 1013 Jones are achieved under the illumination of a 633 nm laser, which is approximately two orders higher than the best phototransistor based on a WSe2/MoS2 heterojunction. Our work provides an effective and simple method to enhance photoresponsivity and detectivity in two-dimensional (2D) heterojunction phototransistors, indicating the potential applications in fabricating high-performance photodetectors based on 2DLMs. [ABSTRACT FROM AUTHOR]

The increasing interest in two-dimensional materials with unique crystal structures and novel band characteristics has provided numerous new strategies and paradigms in the field of photodetection. However, as the demand for wide-spectrum detection increases, the size of integrated systems and the limitations of mission modules pose significant challenges to existing devices. In this paper, we present a van der Waals heterostructure photodetector based on Ta2NiSe5/WSe2, leveraging the inherent characteristics of heterostructures. Our results demonstrate that this detector exhibits excellent broad-spectrum detection ability from the visible to the infrared bands at room temperature, achieving an extremely high on/off ratio, without the need for an external bias voltage. Furthermore, compared to a pure material detector, it exhibits a fast response and low dark currents (~3.6 pA), with rise and fall times of 278 μs and 283 μs for the response rate, respectively. Our findings provide a promising method for wide-spectrum detection and enrich the diversity of room-temperature photoelectric detection. [ABSTRACT FROM AUTHOR]

We report on all-optical devices prepared from WSe2 combined with drawn tapered fibers as saturable absorbers to achieve ultrashort pulse output. The saturable absorber with a high damage threshold and high saturable absorption characteristics is prepared for application in erbium-doped fiber lasers by the liquid phase exfoliation method for WSe2, and the all-optical device exhibited strong saturable absorption characteristics with a modulation depth of 15% and a saturation intensity of 100.58 W. The net dispersion of the erbium-doped fiber laser cavity is ~−0.1 ps2, and a femtosecond pulse output with a bandwidth of 11.4 nm, a pulse width of 390 fs, and a single-pulse capability of 42 pJ is obtained. Results indicate that the proposed WSe2 saturable absorbers are efficient, photonic devices to realize stable fiber lasers. The results demonstrate that the WSe2 saturable absorber is an effective photonic device for realizing stable fiber lasers, which have a certain significance for the development of potential photonic devices. [ABSTRACT FROM AUTHOR]

Two-dimensional (2D) material-based devices are expected to operate under high temperatures induced by Joule heating and environmental conditions when integrated into compact integrated circuits for practical applications. However, the behavior of these materials at high operating temperatures is obscure as most studies emphasize only room temperature or low-temperature operation. Here, the high-temperature electrical response of the tungsten diselenide (WSe2) field-effect transistor was studied. It is revealed that 350 K is the optimal annealing temperature for the WSe2 transistor, and annealing at this temperature improves on-current, field-effect mobility and on/off ratio around three times. Annealing beyond this temperature (360 K to 670 K) adversely affects the device performance attributed to the partial oxidation of WSe2 at higher temperatures. An increase in hysteresis also confirms the formation of new traps as the device is annealed beyond 350 K. These findings explicate the thermal stability of WSe2 and can help design 2D materials-based durable devices for high-temperature practical applications. [ABSTRACT FROM AUTHOR]

The transition-metal dichalcogenide (TMDC) in the family of MX 2 ( M = Mo , W ; X = S , Se ) and the graphene (Gr) monolayer are an atomically thin semiconductor and a semimetal, respectively. The monolayer MX 2 has been discovered as a new class of semiconductors for electronics and optoelectronics applications. Because of the hexagonal lattice structure of both materials, MX 2 and Gr are often combined with each other to generate van der Waals heterostructures. Here, the MX 2 / Gr heterostructures are investigated theoretically based on density functional theory (DFT). The electronic structure and the optical properties of four different MX 2 / Gr heterostructures are computed. We systematically compare these MX 2 / Gr heterostructures for their complex permittivity, absorption coefficient, reflectivity and refractive index. [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]

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]

We present a series of computer-assisted high-resolution transmission electron (HRTEM) simulations to determine Moiré patters by induced twisting effects between slabs at rotational angles of 3°, 5°, 8°, and 16°, for molybdenum disulfide, graphene, tungsten disulfide, and tungsten selenide layered materials. In order to investigate the electronic structure, a series of numerical simulations using density functional methods (DFT) methods was completed using Cambridge serial total energy package (CASTEP) with a generalized gradient approximation to determine both the band structure and density of states on honeycomb-like new superlattices. Our results indicated metallic transitions when the rotation approached 8° with respect to each other laminates for most of the two-dimensional systems that were analyzed. [ABSTRACT FROM AUTHOR]

We studied the magnetic properties of WSe 2 /MoSe 2 powder. The coercivity field reaches 2600 Oe at 5 K, 4233 Oe at 100 K and 1300 Oe at 300 K. These are the highest values reported for two-dimensional transition metal dichalcogenides. This study is different from the widely reported vacancy and zigzag structure-induced ferromagnetism studies. Importantly, a Raman peak red shift was observed, and that supports the chemical bonding at the interface between WSe 2 and MoSe 2 . The large coercivity field originates from the chemical bonding-induced structural distortion at the interface between WSe 2 and MoSe 2 . [ABSTRACT FROM AUTHOR]

The ability to control the active edge sites of transition metal dichalcogenides (TMDs) is crucial for modulating their chemical activity for various electrochemical applications, including hydrogen evolution reactions. In this study, we demonstrate a colloidal synthetic method to prepare core-shell-like heterostructures composed of MoSe2 and WSe2 via a two-step sequential growth. By overgrowing WSe2 on the surface of preexisting MoSe2 nanosheet edges, MoSe2-core/WSe2-shell heterostructures were successfully obtained. Systematic comparisons of the secondary growth time and sequential order of growth suggest that the low synthetic temperature conditions allow the stable overgrowth of shells rich in WSe2 on top of the core of MoSe2 with low Gibbs formation energy. The electrochemical analysis confirms that the catalytic activity correlates to the core-shell composition variation. Our results propose a new strategy to control the edge site activity of TMD materials prepared by colloidal synthesis, which is applicable to diverse electrochemical applications. [ABSTRACT FROM AUTHOR]

In this study, we investigated the p-doping effects of a fluoropolymer, Cytop, on tungsten diselenides (WSe2). The hole current of the Cytop–WSe2 field-effect transistor (FET) was boosted by the C–F bonds of Cytop having a strong dipole moment, enabling increased hole accumulation. Analysis of the observed p-doping effects using atomic force microscopy (AFM) and Raman spectroscopy shed light on the doping mechanism. Moreover, Cytop reduces the electrical instability by preventing the adsorption of ambient molecules on the WSe2 surface. Annealing Cytop deposited on WSe2 eliminated the possible impurities associated with adsorbates (i.e., moisture and oxygen) that act as traps on the surface of WSe2. After thermal annealing, the Cytop–WSe2 FET afforded higher p-type conductivity and reduced hysteresis. The combination of the Cytop–WSe2 FET with annealing provides a promising method for obtaining high-performance WSe2 p-type transistors. [ABSTRACT FROM AUTHOR]

Although large-scale synthesis of layered two-dimensional (2D) transition metal dichalcogenides (TMDCs) has been made possible, mechanical exfoliation of layered van der Waals crystal is still indispensable as every new material research starts with exfoliated flakes. However, it is often a tedious task to find the flakes with desired thickness and sizes. We propose a method to determine the thickness of few-layer flakes and facilitate the fast searching of flakes with a specific thickness. By using hyperspectral wild field microscopy to acquire differential reflectance and transmittance spectra, we demonstrate unambiguous recognition of typical TMDCs and their thicknesses based on their excitonic resonance features in a single step. Distinct from Raman spectroscopy or atomic force microscopy, our method is non-destructive to the sample. By knowing the contrast between different layers, we developed an algorithm to automatically search for flakes of desired thickness in situ. We extended this method to measure tin dichalcogenides, such as SnS2 and SnSe2, which are indirect bandgap semiconductors regardless of the thickness. We observed distinct spectroscopic behaviors as compared with typical TMDCs. Layer-dependent excitonic features were manifested. Our method is ideal for automatic non-destructive optical inspection in mass production in the semiconductor industry. [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. [ABSTRACT FROM AUTHOR]

Electrochemical CO2 reduction towards value-added chemical feedstocks has been extensively studied in recent years to resolve the energy and environmental problems. The practical application of electrochemical CO2 reduction technology requires a cost-effective, highly efficient, and robust catalyst. To date, vigorous research have been carried out to increase the proficiency of electrocatalysts. In recent years, two-dimensional (2D) graphene and transition metal chalcogenides (TMCs) have displayed excellent activity towards CO2 reduction. This review focuses on the recent progress of 2D graphene and TMCs for selective electrochemical CO2 reduction into CO. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]