Enhanced Performance of a Monolayer MoS2/WSe2 Heterojunction as a Photoelectrochemical Cathode

Enhanced photodetection through a perovskite BaTiO3 dielectric in a Si-MoS2 heterojunction

The present investigation deals with the effect of a BaTiO3 (BTO) dielectric layer on the performance of MoS2/p-Si heterojunction photodetectors. The MoS2/p-Si junction demonstrates a responsivity of ∼80 A W-1 and detectivity of ∼1012 Jones. The inclusion of a dielectric BTO layer significantly enhances the performance of MoS2/p-Si photodetectors, leading to a remarkable improvement with a very high responsivity of ∼603 A W-1 and detectivity of ∼1013 Jones. The I-V characteristics of the MoS2/p-Si and MoS2/BTO/p-Si junctions under illumination can be understood by considering their respective energy band diagrams. This addition alters the energy band alignment, leading to higher conduction band offset and valence band offset values. The large photocurrent in forward bias in the MoS2/BTO/p-Si junction may be attributed to the presence of photogenerated electrons in the depletion region of BTO. BTO exhibits characteristics such as a long carrier diffusion length and low recombination rates, contributing to a reduction in carrier recombination within the photodetector for which the photocurrent of the MoS2/BTO/p-Si heterojunction can be improved significantly. The enhanced performance of the MoS2/BTO/p-Si junction, characterized by higher responsivity and detectivity, underscores the potential of this heterojunction for advanced photodetection applications, suggesting promising avenues for further research and development in the field of photodetectors. A comparative study with the available literature reveals that the excellent responsivity of ∼603 A W-1 and detectivity of ∼1013 Jones of the presently studied MoS2/BTO/p-Si heterojunction appear highly promising for various futuristic device applications.

Rational Design of Phase-Engineered WS2/WSe2 Heterostructures by Low-Temperature Plasma-Assisted Sulfurization and Selenization toward Enhanced HER Performance

Efficient hydrogen generation from water splitting underpins chemistry to realize hydrogen economy. The electrocatalytic activity can be effectively modified by two-dimensional (2D) heterostructures, which offer great flexibility. Furthermore, they are useful in enhancing the exposure of the active sites for the hydrogen evolution reaction. Although the 1T-metallic phase of the transition metal dichalcogenides (TMDs) is important for the hydrogen evolution reaction (HER) catalyst, its practical application has not yet been much utilized because of the lack of stability of the 1T phase. Here, we introduce a novel approach to create a 1T-WS2/1T-WSe2 heterostructure using a low-temperature plasma-assisted chemical vapor reaction (PACVR), namely plasma-assisted sulfurization and plasma-assisted selenization processes. This heterostructure exhibits superior electrocatalytic performance due to the presence of the metallic 1T phase and the beneficial synergistic effect at the interface, which is attributed to the transfer of electrons from the underlying WS2 layer to the overlying WSe2 layer. The WS2/WSe2 heterostructure catalyst demonstrates remarkable performance in the HER as evidenced by its small Tafel slope of 57 mV dec-1 and exceptional durability. The usage of plasma helps in replacing the top S atoms with Se atoms, and this ion bombardment also increases the roughness of the thin film, thus adding another factor to enhance the HER performance. This plasma-synthesized low-temperature metallic-phase heterostructure brings out a novel method for the discovery of other catalysts.

AC Characteristics of van der Waals Bipolar Junction Transistors Using an MoS2/WSe2/MoS2 Heterostructure

Two-dimensional layered materials, characterized by their atomically thin thicknesses and surfaces that are free of dangling bonds, hold great promise for fabricating ultrathin, lightweight, and flexible bipolar junction transistors (BJTs). In this paper, a van der Waals (vdW) BJT was fabricated by vertically stacking MoS2, WSe2, and MoS2 flakes in sequence. The AC characteristics of the vdW BJT were studied for the first time, in which a maximum common emitter voltage gain of around 3.5 was observed. By investigating the time domain characteristics of the device under various operating frequencies, the frequency response of the device was summarized, which experimentally proved that the MoS2/WSe2/MoS2 BJT has voltage amplification capability in the 0-200 Hz region. In addition, the phase response of the device was also investigated. A phase inversion was observed in the low-frequency range. As the operating frequency increases, the relative phase between the input and output signals gradually shifts until it is in phase at frequencies exceeding 2.3 kHz. This work demonstrates the signal amplification applications of the vdW BJTs for neuromorphic computing and wearable healthcare devices.

Realization of High Current Gain for Van der Waals MoS2/WSe2/MoS2 Bipolar Junction Transistor

Two-dimensional (2D) materials have attracted great attention in the past few years and offer new opportunities for the development of high-performance and multifunctional bipolar junction transistors (BJTs). Here, a van der Waals BJT based on vertically stacked n+-MoS2/WSe2/MoS2 was demonstrated. The electrical performance of the device was investigated under common-base and common-emitter configurations, which show relatively large current gains of α ≈ 0.98 and β ≈ 225. In addition, the breakdown characteristics of the vertically stacked n+-MoS2/WSe2/MoS2 BJT were investigated. An open-emitter base-collector breakdown voltage (BVCBO) of 52.9 V and an open-base collector-emitter breakdown voltage (BVCEO) of 40.3 V were observed under a room-temperature condition. With the increase in the operating temperature, both BVCBO and BVCEO increased. This study demonstrates a promising way to obtain 2D-material-based BJT with high current gains and provides a deep insight into the breakdown characteristics of the device, which may promote the applications of van der Waals BJTs in the fields of integrated circuits.

Impact of Large Gate Voltages and Ultrathin Polymer Electrolytes on Carrier Density in Electric-Double-Layer-Gated Two-Dimensional Crystal Transistors

Electric-double-layer (EDL) gating can induce large capacitance densities (∼1-10 μF cm^-2) in two-dimensional (2D) semiconductors; however, several properties of the electrolyte limit performance. One property is the electrochemical activity which limits the gate voltage (V_G) that can be applied and therefore the maximum extent to which carriers can be modulated. A second property is electrolyte thickness, which sets the response speed of the EDL gate and therefore the time scale over which the channel can be doped. Typical thicknesses are on the order of micrometers, but thinner electrolytes (nanometers) are needed for very-large-scale-integration (VLSI) in terms of both physical thickness and the speed that accompanies scaling. In this study, finite element modeling of an EDL-gated field-effect transistor (FET) is used to self-consistently couple ion transport in the electrolyte to carrier transport in the semiconductor, in which density of states, and therefore quantum capacitance, is included. The model reveals that 50 to 65% of the applied potential drops across the semiconductor, leaving 35 to 50% to drop across the two EDLs. Accounting for the potential drop in the channel suggests that higher carrier densities can be achieved at larger applied V_G without concern for inducing electrochemical reactions. This insight is tested experimentally via Hall measurements of graphene FETs for which V_G is extended from ±3 to ±6 V. Doubling the gate voltage increases the sheet carrier density by an additional 2.3 × 10¹³ cm^-2 for electrons and 1.4 × 10¹³ cm^-2 for holes without inducing electrochemistry. To address the need for thickness scaling, the thickness of the solid polymer electrolyte, poly(ethylene oxide) (PEO):CsClO₄, is decreased from 1 μm to 10 nm and used to EDL gate graphene FETs. Sheet carrier density measurements on graphene Hall bars prove that the carrier densities remain constant throughout the measured thickness range (10 nm-1 μm). The results indicate promise for overcoming the physical and electrical limitations to VLSI while taking advantage of the ultrahigh carrier densities induced by EDL gating.

Controlled Differentiation of Human Neural Progenitor Cells on Molybdenum Disulfide/Graphene Oxide Heterojunction Scaffolds by Photostimulation

Ultrathin MoS₂-MoO_3-x heterojunction nanosheets with unique features were introduced as biocompatible, non-cytotoxic, and visible light-sensitive stimulator layers for the controlled differentiation of human neural progenitor cells (hNPCs) into nervous lineages. hNPC differentiation was also investigated on reduced graphene oxide (rGO)-containing scaffolds, that is, rGO and rGO/MoS₂-MoO_3-x nanosheets. In darkness, hNPC differentiation into neurons increased on MoS₂-MoO_3-x by a factor of 2.7 due to the excellent biophysical cues and further increased on rGO/MoS₂-MoO_3-x by a factor of 4.4 due to a synergistic effect induced by the rGO. The MoO_3-x domains with antioxidant activity and LSPR absorption induced p-type doping in MoS₂-MoO_3-x. Under photostimulation, the hNPCs on the MoS₂-MoO_3-x exhibited higher differentiation into glial cells by a factor of 1.4, and the decrease in photo-electron current to hNPCs due to the induction of more p-type doping in the MoS₂-MoO_3-x. While the increase in neuronal differentiation of hNPCs on rGO/MoS₂-MoO_3-x by a factor of 1.8 was ascribed to the presence of rGO as an ultrafast electron transferor which quickly transferred photogenerated electrons to hNPCs before their transfer to free radicals, these results demonstrated the promising potential of MoS₂-based scaffolds for applying in the controllable repair and/or regeneration of diseases/disorders related to the nervous system.

Ultrafast Drift Current Terahertz Emission Amplification in the Monolayer WSe2/Si Heterostructure

Two-dimensional transition metal dichalcogenides (TMDs) have great potential application for seamless on-chip integration due to their strong photon-electron-spin-valley coupling. However, the contact-free measurements of the valley-coupled photocurrent in TMDs is still challenging. Here, ultrafast terahertz emission spectroscopy is employed to investigate the photocurrent dynamics in monolayer WSe₂, and an interface-induced drift current amplification is found in the WSe₂/Si heterostructure. The amplification of terahertz emission comes from the photocurrent enlarged by band bending in the WSe₂ and Si junction, and the amplification ratio increase further near the valley resonant transition of WSe₂. In addition, the valley-momentum locked photocurrent in the WSe₂/Si heterostructure reserves the same chirality with monolayer WSe₂ at room temperature. These findings could provide a new method for manipulating valley-momentum locked photocurrent by photon helicity and open new avenues for TMD-based valley-polarized terahertz emission devices.

Exciton tuning in monolayer WSe2 via substrate induced electron doping

We report large exciton tuning in WSe2 monolayers via substrate induced non-degenerate doping. We observe a redshift of ∼62 meV for the A exciton together with a 1-2 orders of magnitude photoluminescence (PL) quenching when the monolayer WSe2 is brought in contact with highly oriented pyrolytic graphite (HOPG) compared to dielectric substrates such as hBN and SiO2. As the evidence of doping from HOPG to WSe2, a drastic increase of the intensity ratio of trions to neutral excitons was observed. Using a systematic PL and Kelvin probe force microscopy (KPFM) investigation on WSe2/HOPG, WSe2/hBN, and WSe2/graphene, we conclude that this unique excitonic behavior is induced by electron doping from the substrate. Our results propose a simple yet efficient way for exciton tuning in monolayer WSe2, which plays a central role in the fundamental understanding and further device development.

Exfoliation of MoS2 Quantum Dots: Recent Progress and Challenges

Although, quantum dots (QDs) of two-dimensional (2D) molybdenum disulfide (MoS₂) have shown great potential for various applications, such as sensing, catalysis, energy storage, and electronics. However, the lack of a simple, scalable, and inexpensive fabrication method for QDs is still a challenge. To overcome this challenge, a lot of attention has been given to the fabrication of QDs, and several fabrication strategies have been established. These exfoliation processes are mainly divided into two categories, the 'top-down' and 'bottom-up' methods. In this review, we have discussed different top-down exfoliation methods used for the fabrication of MoS₂ QDs and the advantages and limitations of these methods. A detailed description of the various properties of QDs is also presented.

Novel Nanoarchitectured Cu2Te as a Photocathodes for Photoelectrochemical Water Splitting Applications

Designing photocathodes with nanostructures has been considered a promising way to improve the photoelectrochemical (PEC) water splitting activity. Cu₂Te is one of the promising semiconducting materials for photoelectrochemical water splitting, the performance of Cu₂Te photocathodes remains poor. In this work, we report the preparation of Cu₂Te nanorods (NRs) and vertical nanosheets (NSs) assembled film on Cu foil through a vapor phase epitaxy (VPE) technique. The obtained nano architectures as photocathodes toward photoelectrochemical (PEC) performance was tested afterwards for the first time. Optimized Cu₂Te NRs and NSs photocathodes showed significant photocurrent density up to 0.53 mA cm^-2 and excellent stability under illumination. Electrochemical impedance spectroscopy and Mott-Schottky analysis were used to analyze in more detail the performance of Cu₂Te NRs and NSs photocathodes. From these analyses, we propose that Cu₂Te NRs and NSs photocathodes are potential candidate materials for use in solar water splitting.

Emerging 2D Materials for Electrocatalytic Applications: Synthesis, Multifaceted Nanostructures, and Catalytic Center Design

Currently, the development of advanced 2D nanomaterials has become an interdisciplinary subject with extensive studies due to their extraordinary physicochemical performances. Beyond graphene, the emerging 2D-material-derived electrocatalysts (2D-ECs) have aroused great attention as one of the best candidates for heterogeneous electrocatalysis. The tunable physicochemical compositions and characteristics of 2D-ECs enable rational structural engineering at the molecular/atomic levels to meet the requirements of different catalytic applications. Due to the lack of instructive and comprehensive reviews, here, the most recent advances in the nanostructure and catalytic center design and the corresponding structure-function relationships of emerging 2D-ECs are systematically summarized. First, the synthetic pathways and state-of-the-art strategies in the multifaceted structural engineering and catalytic center design of 2D-ECs to promote their electrocatalytic activities, such as size and thickness, phase and strain engineering, heterojunctions, heteroatom doping, and defect engineering, are emphasized. Then, the representative applications of 2D-ECs in electrocatalytic fields are depicted and summarized in detail. Finally, the current breakthroughs and primary challenges are highlighted and future directions to guide the perspectives for developing 2D-ECs as highly efficient electrocatalytic nanoplatforms are clarified. This review provides a comprehensive understanding to engineer 2D-ECs and may inspire many novel attempts and new catalytic applications across broad fields.

Study of photogenerated exciton dissociation in transition metal dichalcogenide van der Waals heterojunction A2-MWS4: a first-principles study

In order to explore the photocatalytic hydrogen production efficiency of the MoS₂/WSe₂ heterostructure (A2-MWS₄) as a photocatalyst, it is highly desirable to study the photogenerated exciton dissociation related to photocatalysis. The electronic properties, optical absorption, and lattice dynamic properties of A2-MWS₄ were investigated using a first-principles approach. The results show that the type II energy band alignment of A2-MWS₄ facilitates the dissociation of photogenerated excitons (electrons and holes). The highly localized d-state electrons of A2-MWS₄ induce the formation of internal potentials that promote the dissociation of photogenerated excitons. The hot carrier diffuses its extra energy into the lattice by scattering with phonons and forms a hot spot in the lattice while releasing phonons, which are dragged away from the hot spot by Ridley decay to promote exciton dissociation. These findings could provide insights for research studies on photochemical reactions and photovoltaic devices.

One-Pot Synthesis of Chlorophyll-Assisted Exfoliated MoS2/WS2 Heterostructures via Liquid-Phase Exfoliation Method for Photocatalytic Hydrogen Production

Developing strategies for producing hydrogen economically and in greener ways is still an unaccomplished goal. Photoelectrochemical (PEC) water splitting using photoelectrodes under neutral electrolyte conditions provides possibly one of the greenest routes to produce hydrogen. Here, we demonstrate that chlorophyll extracts can be used as an efficient exfoliant to exfoliate bulk MoS2 and WS2 to form a thin layer of a MoS2/WS2 heterostructure. Thin films of solution-processed MoS2 and WS2 nanosheets display photocurrent densities of -1 and -5 mA/cm2, respectively, and hydrogen evolution under simulated solar irradiation. The exfoliated WS2 is significantly more efficient than the exfoliated MoS2; however, the MoS2/WS2 heterostructure results in a 2500% increase in photocurrent densities compared to the individual constituents and over 12 h of PEC durability under a neutral electrolyte. Surprisingly, in real seawater, the MoS2/WS2 heterostructure exhibits stable hydrogen production after solar illumination for 12 h. The synthesis method showed, for the first time, how the MoS2/WS2 heterostructure can be used to produce hydrogen effectively. Our findings highlight the prospects for this heterostructure, which could be coupled with various processes towards improving PEC efficiency and applications.

The Photodetectors Based on Lateral Monolayer MoS2/WS2 Heterojunctions

Monolayer transition metal dichalcogenides (TMDs) show promising potential for next-generation optoelectronics due to excellent light capturing and photodetection capabilities. Photodetectors, as important components of sensing, imaging and communication systems, are able to perceive and convert optical signals to electrical signals. Herein, the large-area and high-quality lateral monolayer MoS2/WS2 heterojunctions were synthesized via the one-step liquid-phase chemical vapor deposition approach. Systematic characterization measurements have verified good uniformity and sharp interfaces of the channel materials. As a result, the photodetectors enhanced by the photogating effect can deliver competitive performance, including responsivity of ~ 567.6 A/W and detectivity of ~ 7.17 × 1011 Jones. In addition, the 1/f noise obtained from the current power spectrum is not conductive to the development of photodetectors, which is considered as originating from charge carrier trapping/detrapping. Therefore, this work may contribute to efficient optoelectronic devices based on lateral monolayer TMD heterostructures.

Coherent Excitation and Control of Plasmons on Gold Using Two-Dimensional Transition Metal Dichalcogenides

The hybrid combination of two-dimensional (2D) transition metal dichalcogenides (TMDs) and plasmonic materials open up novel means of (ultrafast) optoelectronic applications and manipulation of nanoscale light-matter interaction. However, control of the plasmonic excitations by TMDs themselves has not been investigated. Here, we show that the ultrathin 2D WSe₂ crystallites permit nanoscale spatially controlled coherent excitation of surface plasmon polaritons (SPPs) on smooth Au films. The resulting complex plasmonic interference patterns are recorded with nanoscale resolution in a photoemission electron microscope. Modeling shows good agreement with experiments and further indicates how SPPs can be tailored with high spatiotemporal precision using the shape of the 2D TMDs with thicknesses down to single molecular layers. We demonstrate the use of WSe₂ nanocrystals as 2D optical elements for exploring the ultrafast dynamics of SPPs. Using few-femtosecond laser pulse pairs we excite an SPP at the boundary of a WSe₂ crystal and then have a WSe₂ monolayer wedge act as a delay line inducing a spatially varying phase difference down to the attosecond time range. The observed effects are a natural yet unexplored consequence of high dielectric functional values of TMDs in the visible range that should be considered when designing metal-TMD hybrid devices. As the 2D TMD crystals are stable in air, can be defect free, can be synthesized in many shapes, and are reliably positioned on metal surfaces, using them to excite and steer SPPs adds an interesting alternative in designing hybrid structures for plasmonic control.

Rhenium Doping of Layered Transition-Metal Diselenides Triggers Enhancement of Photoelectrochemical Activity

The ever decreasing sources of fossil fuels have launched extensive research of alternative materials that might play a key role in their replacement. Therefore, the scientific community continuously investigates the possibilities of maximizing the working capacity of such materials in order to fulfill energy challenges in the near future. In this context, doping of the semiconducting materials is a versatile strategy to trigger their physicochemical properties as well their electrochemical performance. Herein, the impact of rhenium doping toward photoelectrochemical activity of MoSe₂ and WSe₂ was studied. Our results indicate that rhenium as a dopant contributes to better overall electrochemical performance, that is, an easier electron transfer of these materials compared to pristine compounds. Additionally, the photoelectrochemical measurements revealed that the doping with rhenium generated an enhancement of the photocurrent response of MoSe₂ as well as WSe₂ under UV light illumination.

In situ creation of ZnO@CdS nanoflowers on ITO electrodes for sensitive photoelectrochemical detection of copper ions in blood

A highly selective and sensitive photoelectrochemical (PEC) detection method has been developed for the analysis of copper (Cu2+) ions using nanoflower-like ZnO@CdS heterojunctions, of which ZnO was first in situ grown onto the indium tin oxide electrodes by a hydrothermal method and then coated with CdS through the chemical bath deposition route. It was discovered that the ZnO@CdS heterojunction so formed could serve as a photosensitive catalyst with improved charge separation for visible-light-driven PEC responses. Enhanced visible-light harvesting of nanocomposites could also be expected with CdS as the visible-light sensitizer. Furthermore, the introduction of Cu2+ ions could cause a rational decrease in the photocurrents of nanocomposites through the specific interaction between CdS and Cu2+ ions. A ZnO@CdS heterojunction-based PEC sensor was thereby developed for the detection of Cu2+ ions in blood in the linear concentrations ranging from 0.50 to 80 nM, with a limit of detection of 0.18 nM. Such a heterojunction-based PEC detection platform constructed using two photocatalytic materials with matched band structures are promising for a wide range of applications for sensing Cu2+ ions in clinical diagnostics, food monitoring, and environmental analysis.

One-pot synthesis of MoS2(1-x)Se2x on N-doped reduced graphene oxide: tailoring chemical and structural properties for photoenhanced hydrogen evolution reaction

In this work we designed a one-pot solvothermal synthesis of MoS_2(1-x)Se_2x nanosheets directly grown on N-doped reduced graphene oxide (hereafter N-rGO). We optimized the synthesis conditions to control the Se : S ratio, with the aim of tailoring the optoelectronic properties of the resulting nanocomposites for their use as electro- and photoelectro-catalysts in the hydrogen evolution reaction (HER). The synthesis protocol made use of ammonium tetrathiomolybdate (ATM) as MoS₂ precursor and dimethyl diselenide (DMDSe) as selenizing agent. By optimizing growth conditions and post-annealing treatments, we produced either partially amorphous or highly crystalline chalcogen-defective electrocatalysts. All samples were tested for the HER in acidic environment, and the best performing among them, for the photoassisted HER. In low crystallinity samples, the introduction of Se is not beneficial for promoting the catalytic activity, and MoS₂/N-rGO was the most active electrocatalyst. On the other hand, after the post-annealing treatment and the consequent crystallization of the materials, the best HER performance was obtained for the sample with x = 0.38, which also showed the highest enhancement upon light irradiation.

Tuning Interface Bridging Between MoSe2 and Three-Dimensional Carbon Framework by Incorporation of MoC Intermediate to Boost Lithium Storage Capability

Highlights

MoSe2/MoC/C multiphase boundaries boost ionic transfer kinetics. MoSe2 (5–10 nm) with rich edge sites is uniformly coated in N-doped framework. The obtained MoSe2 nanodots achieved ultralong cycle performance in LIBs and high capacity retention in full cell.

Abstract

Interface engineering has been widely explored to improve the electrochemical performances of composite electrodes, which governs the interface charge transfer, electron transportation, and structural stability. Herein, MoC is incorporated into MoSe2/C composite as an intermediate phase to alter the bridging between MoSe2- and nitrogen-doped three-dimensional (3D) carbon framework as MoSe2/MoC/N–C connection, which greatly improve the structural stability, electronic conductivity, and interfacial charge transfer. Moreover, the incorporation of MoC into the composites inhibits the overgrowth of MoSe2 nanosheets on the 3D carbon framework, producing much smaller MoSe2 nanodots. The obtained MoSe2 nanodots with fewer layers, rich edge sites, and heteroatom doping ensure the good kinetics to promote pseudo-capacitance contributions. Employing as anode material for lithium-ion batteries, it shows ultralong cycle life (with 90% capacity retention after 5000 cycles at 2 A g−1) and excellent rate capability. Moreover, the constructed LiFePO4//MoSe2/MoC/N–C full cell exhibits over 86% capacity retention at 2 A g−1 after 300 cycles. The results demonstrate the effectiveness of the interface engineering by incorporation of MoC as interface bridging intermediate to boost the lithium storage capability, which can be extended as a potential general strategy for the interface engineering of composite materials.

Electronic supplementary material

The online version of this article (10.1007/s40820-020-00511-4) contains supplementary material, which is available to authorized users.

A Novel Route to Manufacture 2D Layer MoS2 and g-C3N4 by Atmospheric Plasma with Enhanced Visible-Light-Driven Photocatalysis

An atmospheric plasma treatment strategy was developed to prepare two-dimensional (2D) molybdenum disulfide (MoS₂) and graphitic carbon nitride (g-C₃N₄) nanosheets from (NH₄)₂MoS₄ and bulk g-C₃N₄, respectively. The moderate temperature of plasma is beneficial for exfoliating bulk materials to thinner nanosheets. The thicknesses of as-prepared MoS₂ and g-C₃N₄ nanosheets are 2-3 nm and 1.2 nm, respectively. They exhibited excellent photocatalytic activity on account of the nanosheet structure, larger surface area, more flexible photophysical properties, and longer charge carrier average lifetime. Under visible light irradiation, the hydrogen production rates of MoS₂ and g-C₃N₄ by plasma were 3.3 and 1.5 times higher than the corresponding bulk materials, respectively. And g-C₃N₄ by plasma exhibited 2.5 and 1.3 times degradation rates on bulk that for methyl orange and rhodamine B, respectively. The mechanism of plasma preparation was proposed on account of microstructure characterization and online mass spectroscopy, which indicated that gas etching, gas expansion, and the repulsive force of electron play the key roles in the plasma exfoliation. Plasma as an environmentally benign approach provides a general platform for fabricating ultrathin nanosheet materials with prospective applications as photocatalysts for pollutant degradation and water splitting.

Near unity charge separation efficiency leads to pure ultraviolet emission in few layer graphene nanosheets

Two-dimensional materials with van der Waals structure attract intense interest due to their high performance in ultrathin optoelectronic devices. In particular, the high efficiency charge separation between the two-dimensional materials can significantly improve the photo-response of a given device. Here we report the discovery of pure ultraviolet (UV) emission from few layer graphene nanosheets (GNS). Near unity charge separation efficiency is key to pure UV emission. The dynamics of an excited electron were analyzed using femtosecond transient absorption techniques. Electron transfer is observed from surface defect states induced by oxygen-containing functional groups to intrinsic sp² domain states in few layer GNS. Moreover, a solar blind response device based on few layer GNS with a high on-off ratio was successfully fabricated.

MoS2-Based Photodetectors Powered by Asymmetric Contact Structure with Large Work Function Difference

Self-powered devices are widely used in the detection and sensing fields. Asymmetric metal contacts provide an effective way to obtain self-powered devices. Finding two stable metallic electrode materials with large work function differences is the key to obtain highly efficient asymmetric metal contacts structures. However, common metal electrode materials have similar and high work functions, making it difficult to form an asymmetric contacts structure with a large work function difference. Herein, Mo₂C crystals with low work function (3.8 eV) was obtained by chemical vapor deposition (CVD) method. The large work function difference between Mo₂C and Au allowed us to synthesize an efficient Mo₂C/MoS₂/Au photodetector with asymmetric metal contact structure, which enables light detection without external electric power. We believe that this novel device provides a new direction for the design of miniature self-powered photodetectors. These results also highlight the great potential of ultrathin Mo₂C prepared by CVD in heterojunction device applications.

Electrophoretically Deposited MoSe2/WSe2 Heterojunction from Ultrasonically Exfoliated Nanocrystals for Enhanced Electrochemical Photoresponse

The solar response ability and low-cost fabrication of the photoanode are important factors for the effective output of the photoelectrochemical system. Modification of the photoanode by which its ability to absorb irradiation can be manipulated has gained tremendous attention. Here, we demonstrated the MoSe₂, WSe₂, and MoSe₂/WSe₂ nanocrystal thin films prepared by the liquid-phase exfoliated and electrophoresis methods. Atomic force microscopy and high-resolution transmission electron microscopy show that the liquid exfoliated nanocrystals have a few layered dimensions with good crystallinity. Scanning electron microscopy demonstrated uniform distribution and randomly oriented nanocrystals, having a homogeneous shape and size. X-ray diffraction, X-ray photoelectron spectroscopy, and Raman spectra confirm the equal contribution of MoSe₂ and WSe₂ nanocrystals in the formation of the MoSe₂/WSe₂ heterojunction. Because of superior absorption of MoSe₂/WSe₂ heterojunction in the visible region and type-II heterojunction band alignment, in situ measurement of heterojunction electrode shows almost 1.5 times incident photo-to-current conversion efficiency and photoresponsivity in comparison to individual material electrodes. Our result clearly indicates the influence of heterojunction formation between liquid exfoliated nanocrystals on effective separation of photogenerated exciton and enhances charge carrier transfer, which leads to the improvement in photoelectrochemical performance. Liquid exfoliated nanosheet-based heterojunction is attractive as efficient photoanodes for the photoelectrochemical systems.