Pristine WS2 Research Articles

Epitaxy of NiTe2 on WS2 for the p-Type Schottky Contact and Increased Photoresponse.

Two-dimensional (2D) transition metal dichalcogenides (TMDCs) have great potential applications in the electronic and optoelectronic devices. Nevertheless, due to the difficulty in the efficient doping of atomic-thickness TMDCs or Fermi level pinning (FLP) effects at the metal/semiconductor interface, most TMDC devices exhibit the n-type conduction polarity, which significantly limits their functional applications based on the p-n junction. Here, 2D semi-metal NiTe2 nanosheets were epitaxially grown on the WS2 monolayer by a two-step chemical vapor deposition route. The microstructure and optical characterizations confirm that the vertically stacked NiTe2/WS2 heterostructures are formed by van der Waals epitaxy. Interestingly, p-type WS2 field-effect transistors can be obtained with the hole mobility of ∼4.22 cm2/V·s, when the epitaxial NiTe2 sheets act as the source/drain electrodes. This is attributed to the decreased FLP effect and hence the low potential barrier for holes at the van der Waals contacts. Furthermore, the photodetectors based on the heterostructures show a 2 orders of magnitude increase in the switch ratio, responsivity, and detectivity and a 1 order of magnitude increase in the rise and decay speeds relative to those based on pristine WS2. This work paves the way to realize the p-type contact for monolayer WS2 with significantly enhanced optoelectronic performance.

Influence of varying nitrogen content on the structural, tribomechanical properties and machining performance of CrNx-WS2 nanocomposite coatings

This study reports the reactive co-sputtering of CrN and WS2 in order to develop CrNx-WS2 solid lubricant nanocomposites. The nitrogen flow rate during the deposition process was varied within a range of 10–50 sccm and its influence on crystal structure, composition, surface morphology, mechanical properties and tribological properties were systematically analysed. Grazing Incidence X-Ray Diffraction (GIXRD) studies revealed a change in the preferred orientation (200) with varying nitrogen flow rates along with significant broadening of peaks to demonstrate the formation of CrNx-WS2 nanocomposite thin films. The maximum nanohardness of the CrNx-WS2 nanocomposites was observed to be 29 GPa. Scratch test studies revealed a highest critical load of 222 GPa for CrNx-WS2 films which is three times more than that of the pristine WS2. The frictional coefficient of the CrNx-WS2 nanocomposite thin films was observed to be 0.16 while pristine WS2 had a frictional coefficient of 0.08. The wear studies of CrNx-WS2 nanocomposite thin films showed an improvement by 2.9 times when compared to pristine WS2 and 2.5 times when compared to pristine CrN. Machining studies carried out using the CrNx-WS2 coated tools showed reduction in cutting force by 8–12 % when compared to commercial tools. Tool life studies revealed an increase in tool life by 19 % for CrNx-WS2 coated tools in comparison to commercial tools owing to the reduction in tool-chip contact length.

Activation of nitrogen species mixed with Ar and H2S plasma for directly N-doped TMD films synthesis

Among the transition metal dichalcogenides (TMD), tungsten disulfide (WS2) and molybdenum disulfide (MoS2) are promising sulfides for replacing noble metals in the hydrogen evolution reaction (HER) owing to their abundance and good catalytic activity. However, the catalytic activity is derived from the edge sites of WS2 and MoS2, while their basal planes are inert. We propose a novel process for N-doped TMD synthesis for advanced HER using N2 + Ar + H2S plasma. The high ionization energy of Ar gas enabled nitrogen species activation results in efficient N-doping of TMD (named In situ-MoS2 and In situ-WS2). In situ-MoS2 and WS2 were characterized by various techniques (Raman spectroscopy, XPS, HR-TEM, TOF–SIMS, and OES), confirming nanocrystalline and N-doping. The N-doped TMD were used as electrocatalysts for the HER, with overpotentials of 294 mV (In situ-MoS2) and 298 mV (In situ-WS2) at a current density of 10 mA cm−2, which are lower than those of pristine MoS2 and WS2, respectively. Density functional theory (DFT) calculations were conducted for the hydrogen Gibbs energy (∆GH) to investigate the effect of N doping on the HER activity. Mixed gas plasma proposes a facile and novel fabrication process for direct N doping on TMD as a suitable HER electrocatalyst.

Photoelectrochemical Enhancement of Graphene@WS2 Nanosheets for Water Splitting Reaction.

Tungsten disulfide nanosheets were successfully prepared by one-step chemical vapor deposition using tungsten oxide and thiourea in an inert gas environment. The size of the obtained nanosheets was subsequently reduced down to below 20 nm in width and 150 nm in length using high-energy ball milling, followed by 0.5 and 1 wt% graphene loading. The corresponding vibrational and structural characterizations are consistent with the fabrication of a pure WS2 structure for neat sampling and the presence of the graphene characteristic vibration modes in graphene@WS2 compounds. Additional morphological and crystal structures were examined and confirmed by high-resolution electron microscopy. Subsequently, the investigations of the optical properties evidenced the high optical absorption (98%) and lower band gap (1.75 eV) for the graphene@WS2 compared to the other samples, with good band-edge alignment to water-splitting reaction. In addition, the photoelectrochemical measurements revealed that the graphene@WS2 (1 wt%) exhibits an excellent photocurrent density (95 μA/cm2 at 1.23 V bias) compared with RHE and higher applied bias potential efficiency under standard simulated solar illumination AM1.5G. Precisely, graphene@WS2 (1 wt%) exhibits 3.3 times higher performance compared to pristine WS2 and higher charge transfer ability, as measured by electrical impedance spectroscopy, suggesting its potential use as an efficient photoanode for hydrogen evolution reaction.

Autonomous scanning probe microscopy investigations over WS2 and Au{111}

Individual atomic defects in 2D materials impact their macroscopic functionality. Correlating the interplay is challenging, however, intelligent hyperspectral scanning tunneling spectroscopy (STS) mapping provides a feasible solution to this technically difficult and time consuming problem. Here, dense spectroscopic volume is collected autonomously via Gaussian process regression, where convolutional neural networks are used in tandem for spectral identification. Acquired data enable defect segmentation, and a workflow is provided for machine-driven decision making during experimentation with capability for user customization. We provide a means towards autonomous experimentation for the benefit of both enhanced reproducibility and user-accessibility. Hyperspectral investigations on WS2 sulfur vacancy sites are explored, which is combined with local density of states confirmation on the Au{111} herringbone reconstruction. Chalcogen vacancies, pristine WS2, Au face-centered cubic, and Au hexagonal close-packed regions are examined and detected by machine learning methods to demonstrate the potential of artificial intelligence for hyperspectral STS mapping.

Combined Biological and Photocatalytic Degradation of Dibutyl Phthalate in a Simulated Wastewater Treatment Plant

The removal of organic pollutant in wastewater has become a major priority in water treatment. In this study, organic pollutant dibutyl phthalate (DBP) has been biologically and photocatalytically degraded in wastewater using modified transition metal dichalcogenides. The as-synthesized nanoparticles were characterized using various characterization techniques, which includes XRD, Raman, FT-IR, SEM, TEM, UV-Vis, XPS, PL, EIS, and photocurrent responses. The nanoparticles synthesized by slightly modified hydrothermal method depicted a hexagonal phase, as evidenced by XRD and Raman analyses. The biological degradation of 69% dibutyl phthalate was achieved. Moreover, the total organic carbon removal efficiency of 70% was further achieved. Incorporating biological and photocatalytic systems significantly improved dibutyl phthalate removal in secondary effluent by three folds when compared to the unilateral operating setup. The optimized parameters such as pH = 7, 5 ppm and DBP concentration with the addition of 10 mg catalysts loading were employed for the photocatalytic degradation of dibutyl phthalate in water. Pristine WS2 exhibited photocatalytic efficiencies of 46% after 60 min illumination. The use of dual system 3% Ce/Gd-WS2 exhibited the highest photodegradation of 85%, with a chemical oxygen demand of 80% and total organic carbon of 77%. The enhanced activity by the composite is attested to the formation of heterojunction exhibiting excellent charge separation and low rate of recombination. The 3% Ce/Gd-WS2 can be used up to seven times and still achieve a degradation of 56%.

PdO and PtO loaded WS2 boosts NO2 gas sensing characteristics at room temperature

In this work tungsten disulphide nanostructures loaded with platinum-oxide (PtO), or palladium-oxide (PdO) were grown directly onto alumina substrates. This was achieved using a combination of aerosol-assisted chemical vapour deposition (AA-CVD) method with atmospheric pressure CVD technique. At first, tungsten oxide nanowires loaded with either PtO or PdO nanoparticles were successfully co-deposited via AA-CVD followed by sulfurization at 900 °C in the next step. The morphological, structural, and chemical characteristics were investigated using FESEM, TEM, XRD, XPS and Raman spectroscopy. The results confirm the presence of PdO and PtO in the WS2 host matrix. Gas sensing attributes of loaded and pristine WS2 sensors were investigated, at room temperature, towards different analytes (NO2, NH3, H2 etc.). Both pristine and metal-oxide loaded WS2 gas sensors show remarkable responses at room temperature towards NO2 detection. Further, the loaded sensors demonstrated stable, reproducible, ultrasensitive, and enhanced gas sensing response, with a detection limit below 25 ppb. Additionally, the effect of ambient humidity on the sensing response of both loaded and pristine sensors was investigated for NO2 gas. The response of PtO loaded sensor considerably decreased in humid environments, while the response for pristine and PdO loaded sensors increased. However, slightly heating (at 100 °C) the sensors, suppresses the influence of humidity. Finally, the long-term stability of different sensors is investigated, and the results demonstrate high stability with repeatable results after 6 weeks of gas sensing tests. This work exploits an attractive pathway to add functionality in the transition metal dichalcogenide host matrix.

Exploring the sensing ability of B‐ and Si‐doped WS2 monolayer toward CO and NO gas

AbstractSearching for new two‐dimensional (2D) materials to capture toxic gas molecules has drawn great attention from scientific community. In this work, the adsorption of CO and NO molecules on B‐ and Si‐doped WS2 monolayer (B:WS2 and Si:WS2) has been explored using first‐principles calculations. Pristine WS2 monolayer is a non‐magnetic semiconductor with an energy gap of 1.81 eV. It is magnetized upon doping with B atom to form a half‐metallic material, meanwhile the paramagnetic nature is preserved upon Si doping. CO and NO molecules are weakly physisorbed on pristine WS2 monolayer. Doping strategies enhance significantly the adsorption process decreasing drastically the adsorption energy. The adsorbed gas molecules alter considerably the fundamental properties of the doped systems. Specifically, the magnetic properties of B:WS2 system are quenched by both CO and NO molecules to form paramagnetic metallic and semiconductor materials, respectively. CO adsorption increases the energy gap of Si:WS2 system, meanwhile the adsorbed NO molecule induces significant magnetism, leading to the emergence of the half‐metallicity. Results presented herein may recommend B‐ and Si‐doped WS2 monolayer as promising candidates to detect and capture CO and NO molecules. Moreover, doping strategy may be used to functionalize 2D transition metal dichalcogenides for toxic gas sensing.

Two-dimensional tungsten disulfide nanosheets and their application in self-powered photodetectors with ultra-high sensitivity and stability

Two-dimensional (2D) tungsten disulfide nanosheets (WS2) could be a promising candidate for high-performance self-powered photodetectors. The present 2D nanosheets were obtained from liquid exfoliation in a mixture of ethanol, methanol, and isopropanol via a direct dispersion and ultrasonication method. Using the spin-coating technique, a thin film of uniform thickness was formed on the SiO2/Si substrate. Energy-dispersive X-ray analysis showed that the S/W ratio in the fabricated WS2 film was around 1.2 to 1.34, indicating certain deficiencies in the S atoms. These S vacancies induce localized states within the bandgap of pristine WS2, resulting in a higher conductivity in the exfoliated sample. The obtained thin film seems to be highly efficient in photoelectric conversion, with a responsivity of ∼0.12 mA/W at 670 nm under zero bias voltage, with an intensity of 5.2 mW/cm2. Instead, at a bias of 2 V, it exhibits a responsivity of 12.74 mA/W and a detectivity of 1.17 × 1010 cm Hz1/2 W−1 at 4.1 mW/cm2. The present 2D nanosheets exhibit high photon absorption in a wide range of spectra from the near-infrared (IR) to near UV spectrum.

MoSe2-WS2 Nanostructure for an Efficient Hydrogen Generation under White Light LED Irradiation.

In this work, MoSe2-WS2 nanocomposites consisting of WS2 nanoparticles covered with few MoSe2 nanosheets were successfully developed via an easy hydrothermal synthesis method. Their nanostructure and photocatalytic hydrogen evolution (PHE) performance are investigated by a series of characterization techniques. The PHE rate of MoSe2-WS2 is evaluated under the white light LED irradiation. Under LED illumination, the highest PHE of MoSe2-WS2 nanocomposite is 1600.2 µmol g−1 h−1. When compared with pristine WS2, the MoSe2-WS2 nanostructures demonstrated improved PHE rate, which is 10-fold higher than that of the pristine one. This work suggests that MoSe2-WS2 could be a promising photocatalyst candidate and might stimulate the further studies of other layered materials for energy conversion and storage.

ZIF-8 channeled and coordination-bridging two-dimensional WS2 membrane for efficient organic solvent nanofiltration

Zeolitic imidazolate framework-8 (ZIF-8)/WS2 composite 2D membranes were constructed by in-situ growth of ZIF-8 in the interlayer of WS2 laminar membrane. The growth of ZIF-8 nanoparticles in WS2 membrane expanded the interlayer spacing, and endowed WS2 membrane with fixed interlayer spacing by coordination-bridges between WS2 and ZIF-8, which made ZIF-8/WS2 membrane possess not only high solvent permeance but also excellent stability in organic solvents. Isopropanol (IPA) permeance through ZIF-8/WS2 membrane reached 272 L/m2·h·bar, which was about 1.6 times higher than that through pristine WS2 membrane, and the rejection rates for Direct Black 38 (DB38), Congo Red (CR) and Rose Bengal (RosB) were 99%, 99% and 30% respectively. Furthermore, the IPA permeance through ZIF-8/WS2 membrane was almost unchanged after soaking in IPA for 192 h, and IPA permeance and rejection rate for DB 38 changed slightly during 100 h of cross-flow filtration, demonstrating remarkable stability of ZIF-8/WS2 membranes in organic solvents and a promising application potential in organic solvent nanofiltration.

Layer and material-type dependent photoresponse in WSe2/WS2 vertical heterostructures

Transition metal dichalcogenide (TMD) heterostructures are promising for a variety of applications in photovoltaics and photosensing. Successfully exploiting these heterostructures will require an understanding of their layer-dependent electronic structures. However, there is no experimental data demonstrating the layer-number dependence of photovoltaic effects (PVEs) in vertical TMD heterojunctions. Here, by combining scanning electrochemical cell microscopy (SECCM) with optical probes, we report the first layer-dependence of photocurrents in WSe2/WS2 vertical heterostructures as well as in pristine WS2 and WSe2 layers. For WS2, we find that photocurrents increase with increasing layer thickness, whereas for WSe2 the layer dependence is more complex and depends on both the layer number and applied bias (Vb ). We further find that photocurrents in the WSe2/WS2 heterostructures exhibit anomalous layer and material-type dependent behaviors. Our results advance the understanding of photoresponse in atomically thin WSe2/WS2 heterostructures and pave the way to novel nanoelectronic and optoelectronic devices.

Tungsten Disulfide Heterostructure for Boosting Charge Carriers and for Enhancing Photoanodic Performance in Water Splitting

Photoelectrochemical water splitting (PEC) is a compelling approach to convert solar energy to chemical energy by liberating hydrogen, making it a possible solution to the worldwide energy crisis by eliminating all carbon dioxide emissions. In practice, no single electrode material can meet all PEC water splitting requirements. The major challenges for PEC are electron-hole pair recombination, stability and reduced absorption of visible light. Transition Metal dichalcogenides (TMD) based heterostructures are being envisioned as novel electrode materials because of their exceptional electrical, chemical, mechanical and thermal stability which include high electron mobility, highly exposed active sites, layer dependent band gap, and higher ionic conductivity etc. TMDs have higher electro conductivity resulting in significant enhancement of the electrochemical performance in PEC. Further, it also exhibits desirable electrochemical behaviour through its oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) as a photoelectrode for water splitting. However, due to poor absorbance and significant reflection at the grain boundaries, the light harvesting ability of bulk or nanosized TMD materials is often limited. The charge carriers produced by light illumination will also reach the surface faster than bulk materials in the semiconductor, because of the lesser transport distance.In the present work, heterostructures consisting of Tungsten disulfide (WS2) and cadmium sulphide (CdS) were synthesized using a hydrothermal method and Successive Ionic Layer Adsorption Reaction (SILAR) deposition method. The presence of these nanostructures was confirmed by XRD, Raman, UV-Visible spectroscopy, SEM, TEM and PL spectroscopy. The morphological study clearly indicates a homogeneous distribution of CdS nanospheres on WS2 nanostructures, indicating close contact between them which helps in efficient charge separation and migration. Further, the continuous nature of the heterostructures of the two materials is not limited to any visible discrete entities. The porosity of the pristine WS2 nanostructures allows CdS nanospheres to deposit inside the pores along the entire thickness of the nanosphere, and hence, the highly homogeneous nature of the heterostructures is obtained as indicated by the physical characterization.Due to this unique structure, the obtained heterostructures showed excellent performance as photoanode. The CdS@WS2 photoanode demonstrated a photocurrent density of 0.15 mA cm−2 at 1.23 V vs. RHE, which is over 20 times higher than that of the pure pristine materials. Further, CdS@WS2 heterostructures, as compared to pure materials, have demonstrated longer emission-decay-life times and dramatically quenched the emission of fluorescence compared to pristine materials. In CdS@WS2 heterostructures, the absorption edge of the WS2 nanostructures also exhibit a considerable increase in light absorption in the total visible spectrum. This results in enhanced charge separation with proper band alignment leading to improvements in electrochemical performance of CdS@WS2 photoelectrodes. Further, the low onset potential further eliminates the need for voltage bias in practical applications.

Ultrasensitive chemiresistive humidity sensor based on gold functionalized WS2 nanosheets

An ultrasensitive, low power humidity sensor based on Au nanoparticle-functionalized WS2 nano-sheets (NS) has been demonstrated in this work. Liquid phase exfoliation of bulk WS2 was incorporated to prepare this material, followed by its decoration with Au nanoparticles (NPs) using a wet chemical approach. The compositional and nano-structuralproperties of the as-prepared Au-functionalized WS2 NS have been characterized using microscopic (Transmission Electron Microscopy (TEM)), and spectroscopic (Raman spectroscopy and X-Ray Photoelectron Spectroscopy (XPS)) techniques. The humidity sensing characteristics of Au-functionalized WS2 NS based sensors and pristine WS2 NS based sensors have been studied over an extensive relative humidity range, RH(25 %–75 %) at room temperature. The sensing results of Au-functionalized WS2 NS showed an ultrasensitive response, rapid response/recovery behaviour and excellent repeatability. A giant two orders of magnitude enhancement in humidity sensing response was observed in functionalized nanosheets as compared to the pristine ones (70,000 versus 1000). The sensitivity derived from linear regime of operation in the higher RH% range (55 %–75 %) further suggests two orders of dramatic increase, the values being 3007.9 per RH% for the Au-functionalized device and 44.3 per RH% for pristine WS2 NS based device. An appreciable improvement in the response/recovery time has been observed; 107s and 13s (at 65 % RH) being the response and recovery time respectively for Au-functionalized WS2, whereas the corresponding values for pristine WS2 were found to be 228s and 29s respectively. A probable mechanism for the humidity sensing of Au-functionalized WS2 NS has also been suggested in this work.

First-principles calculations to explore the oxygen effects on WS2 film in marine environments

WS2 film could become a promising anticorrosion material due to superior mechanical properties and chemical inertness, and thereby it is of significance to investigate oxygen effects so as to explore its corrosion mechanism in marine environments. In this paper, the thermodynamic and kinetic behaviors of oxygen on pristine WS2 and F-modified WS2 were systematically studied by first-principles calculation both in atmospheric and marine environments. The results show that the adsorption energy of oxygen both on pristine WS2 and F-modified WS2 in marine environment were much lower compared to that in atmospheric environment. Especially, the adsorption energy of oxygen on F-modified WS2 in marine environment could possess the lowest value, which was less 0.96 eV than that on pristine WS2 in atmospheric environment. Surprisingly, the diffusion energy barriers of oxygen on F-modified WS2 both in atmospheric and marine environments were much higher than that on WS2 in atmospheric environment. As a result, F-modified WS2 coupled with much higher diffusion energy barrier as well as lowest adsorption energy so that it could exhibit better oxidation resistance to provide superior marine anticorrosion. Indicating that it has a great potential for applications in marine environment.

Fe13+-ion irradiated WS2 with multi-vacancies and Fe dopants for hydrogen evolution reaction

The effect of ion irradiation on the activation and conductivity of the basal plane of 2H-phase WS2 remains elusive for the enhancement of hydrogen evolution reaction (HER) properties. The objective of this study is to increase the number of electrocatalytic active sites by creating multi-vacancies (W vacancy, S vacancy, or the coexistence of W and S vacancies), simultaneously combing with doping strategy to improve the intrinsic conductivity and activate the HER catalytic performance of S sites in the basal plane of pristine WS2 nanospheres. Herein, high-energy Fe13+-ion irradiation technology was used to irradiate the WS2 surface, which not only regulates multi-vacancies, but also introduces Fe dopants with Fe2+ and Fe3+ into WS2. Density functional theory firstly demonstrates that the creation of multi-vacancies coupling with Fe doping could significantly narrow the band gap of pristine WS2, which is beneficial to the acceleration of electron transfer during the HER process. Experimentally, moderate doses of the Fe13+-ion irradiated WS2 electrocatalyst (8*1013) achieved the optimum HER activity with a higher current density, lower onset overpotential and excellent stability in acidic electrolyte. This strategy is a promising perspective for the design of highly efficient electrocatalysts via the Fe13+-ion irradiation pathway.

Ferromagnetic ordering in a THAB exfoliated WS2 nanosheet

Because of the important role of two-dimensional (2D) magnetic semiconductors in low-dimensional spintronic devices, the generation of ferromagnetism within an ultrathin semiconducting sheet of a transition metal dichalcogenide is highly desirable. A pristine WS2 sheet is a diamagnetic semiconducting transition metal dichalcogenide with superior electronic properties. In this study, we synthesised a free-standing WS2 sheet by a chemical route followed by electrochemical exfoliation by a giant molecule. During exfoliation of the WS2 crystal, atomic vacancies were created in the sheet with a lower number of layers. To understand the mechanism of exfoliation, we carried out x-ray diffraction, transmission electron microscopy, atomic force microscopy and Raman measurements. The types of atomic vacancies were realised by energy-dispersive x-ray spectroscopy, high-resolution transmission electron microscopy (fast Fourier transform), and x-ray photoelectron spectroscopy studies. We also observed a ferromagnetic ordering within the exfoliated WS2 sheet, which is explained on the basis of the generation of an atomic vacancy induced spin-moment. The transport study of the exfoliated WS2 sheet suggests that the electro-transport behaviour still remains as a semiconductor even after exfoliation. This ferromagnetic semiconducting system will be applicable in spintronic devices and this technique will enrich the literature, particularly for the preparation of a 2D semiconducting ferromagnet in a facile fashion.

Efficiently enhanced the visible-light absorption of monolayer WS2 by constructing an asymmetric Fabry-Perot cavity

Transition metal dichalcogenides (TMDCs) have been intensively researched during the past decade in consequence of their fascinating properties, such as tunable bandgap, high carrier mobility, and extraordinary luminescent efficiency. But compared with their bulk counterparts, atomically thin two-dimensional (2D) TMDCs have an intrinsic lower light absorption (∼5%), preventing them from further applications in optoelectronic area. Commonly, fabrication of high-performance TMDC nanodevices not only requires complicated or expensive process, but also is hard to simultaneously realize significant and uniform enhancement for large-scale planar integrated device applications. Herein, we provided an asymmetric Fabry–Perot (F-P) cavity method to efficiently enhance the light absorption of the monolayer tungsten disulfide (ML WS2). By constructing the F-P cavity structure of WS2/SiO2/Au with an optimal thickness of SiO2 spacer, the visible-light absorptivity of ML WS2 dramatically increased from 5% to 49%. To further testify the enhancement effect of the F-P cavity, this hybrid structure was integrated into photoconductive device and compared with pristine WS2 device. The results showed that the photoresponsivity of the hybrid F-P cavity structure (8.3 A W−1) was found to be 2 orders higher than that of pristine WS2 (0.028 A W−1). Moreover, this hybrid structure exhibited a more remarkable and uniform light-field enhancement than pristine WS2 under 532-nm excitation, attributing to the formation of the constructive interference in F-P resonance cavity. This novel F-P cavity structure can shed new light on substantially and uniformly improving the low absorptivity of 2D nanomaterials for future developments.

Van der Waals PdSe2/WS2 Heterostructures for Robust High‐Performance Broadband Photodetection from Visible to Infrared Optical Communication Band

AbstractDue to excellent electrical and optoelectronic properties, 2D transition metal dichalcogenides and their van der Waals (vdW) heterostructures have attracted great attention for broadband optoelectronics. Here, an unreported vdW PdSe2/WS2 heterostructure is developed for robust high‐performance broadband photodetection from visible to infrared optical communication band. These heterostructure devices are simply formed by direct selenization of Pd films pre‐deposited on the chemical vapor deposited monolayer WS2, followed by wet‐transfer onto device substrates with pre‐patterned electrodes. Importantly, the obtained heterostructure device exhibits an impressive broadband spectral photoresponse with response times less than 100 ms for different wavelength regions (532 to 1550 nm), where this performance is significantly better than that of pristine monolayer WS2 devices. This performance enhancement is attributed to the type I band alignment of the heterostructure. Under illumination, both intralayer and interlayer excitations are involved to generate carriers in the relevant layer, enabling the broadband photoresponse. Photocarriers would then undergo charge separation in the depletion region with electrons transferred into the charge transport layer of WS2 through the built‐in electric field, followed by the relaxation to valance band via interlayer or intralayer transition. All these findings can indicate the promising potential of vdW PdSe2/WS2 heterostructures for next‐generation high‐performance optoelectronics.

Molybdenum Tungsten Disulfide with a Large Number of Sulfur Vacancies and Electronic Unoccupied States on Silicon Micropillars for Solar Hydrogen Evolution.

Hydrogen energy is a promising alternative for fossil fuels because of its high energy density and carbon-free emission. Si is an ideal light absorber used in solar water splitting to produce H2 gas because of its small band gap, appropriate conduction band position, and high theoretical photocurrent. However, the overpotential required to drive the photoelectrochemical (PEC) hydrogen evolution reaction (HER) on bare Si electrodes is severely high owing to its sluggish kinetics. Herein, a molybdenum tungsten disulfide (MoS2-WS2) composite decorated on a Si photoabsorber is used as a cocatalyst to accelerate HER kinetics and enhance PEC performance. This MoS2-WS2 hybrid showed superior catalytic activity compared with pristine MoS2 or WS2. The optimal MoS2-WS2/Si electrode delivered a photocurrent of -25.9 mA/cm2 at 0 V (vs reversible hydrogen electrode). X-ray absorption spectroscopy demonstrated that MoS2-WS2 possessed a high hole concentration of unoccupied electronic states in the MoS2 component, which could promote to accept large amounts of carriers from the Si photoabsorber. Moreover, a large number of sulfur vacancies are generated in the MoS2 constituent of this hybrid cocatalyst. These sulfur defects served as HER active sites to boost the catalytic efficiency. Besides, the TiO2-protective MoS2-WS2/Si photocathode maintained a current density of -15.0 mA/cm2 after 16 h of the photocatalytic stability measurement.