Sensitivity-enhanced surface plasmon resonance sensor utilizing a tungsten disulfide (WS_2) nanosheets overlayer

Tungsten disulfide(WS_2), as a representative layered transition metal dichalcogenide(TMDC) material, possesses important potential for applications in highly sensitive sensors. Here, a sensitivity-enhanced surface plasmon resonance(SPR) sensor with a metal film modified by an overlayer of WS_2 nanosheets is proposed and demonstrated. The SPR sensitivity is related to the thickness of the WS_2 overlayer, which can be tailored by coating a WS_2 ethanol suspension with different concentrations or by the number of times of repeated post-coating.Benefitting from its large surface area, high refractive index, and unique optoelectronic properties, the WS_2 nanosheet overlayer coated on the gold film significantly improves the sensing sensitivity. The highest sensitivity(up to 2459.3 nm∕RIU) in the experiment is achieved by coating the WS_2 suspension once. Compared to the case without a WS_2 overlayer, this result shows a sensitivity enhancement of 26.6%. The influence of the WS_2 nanosheet overlayer on the sensing performance improvement is analyzed and discussed. Moreover, the proposed WS_2 SPR sensor has a linear correlation coefficient of 99.76% in refractive index range of 1.333 to 1.360. Besides sensitivity enhancement, the WS_2 nanosheet overlayer is able to show additional advantages, such as protection of metal film from oxidation, tunability of the resonance wavelength region, biocompatibility, capability of vapor,and gas sensing.

Tungsten disulfide: synthesis and applications in electrochemical energy storage and conversion

Recently,two-dimensional transition metal dichalcogenides,particularly WS_(2),raised extensive interest due to its extraordi-nary physicochemical properties.With the merits of low costs and prominent properties such as high anisotropy and distinct crystal structure,WS_(2) is regarded as a competent substitute in the construction of next-generation environmentally benign energy storage and conversion devices.In this review,we begin with the fundamental studies of the structure,properties and preparation of WS_(2),followed by detailed discussions on the development of various WS_(2) and WS_(2)-based composites for electrochemical energy storage and conversion applications.In the end,some prospective prospects and promising develop-ments of WS_(2) in these fields are proposed.

Phosphorus incorporation activates the basal plane of tungsten disulfide for efficient hydrogen evolution catalysis

The basal planes of transition metal dichalcogenides are basically inert for catalysis due to the absence of adsorption and activation sites,which substantially limit their catalytic application.Herein,a facile strategy to activate the basal plane of WS_(2) for hydrogen evolution reaction(HER)catalysis by phosphorous-induced electron density modulation is demonstrated.The optimized P doped WS_(2)(P-WS_(2))nanowires arrays deliver a low overpotential of 88 mV at 10 mA·cm^(-2)with a Tafel slope of 62 mV·dec^(-1)for HER,which is substantially better than the pristine counterpart.X-ray photoelectron spectroscopy confirms the surface electron densities of WS_(2) have been availably manipulated by P doping.Moreover,density functional theory(DFT)studies further prove P doping can redistribute the density of states(DOS)around EF,which endow the inert basal plane of PWS_(2) with edge-like catalytic activity toward hydrogen evolution catalysis.Our work offers a facile and effective approach to modulate the catalytic surface of WS_(2) toward highly efficient HER catalysis.

Tungsten disulfide-based nanomaterials for energy conversion and storage

Energy and environmental issues received widespread attentions due to the fast growth of world population and rapid development of social economy.As a transition metal dichalcogenide,tungsten disulfide(WS_(2))nanomaterials make important research progress in the field of energy conversion and storage.In view of the versatile and rich microstructure of these mate-rials,the modification and controllable synthesis of WS_(2) nanomaterials also inspire a research interest.This review mainly focuses on WS_(2)-based nanomaterials in the application of energy conversion and storage as well as discusses some basic characteristics and modification strategies of them.Finally,the research progress of WS_(2)-based nanomaterials is reviewed and some prospects for future research directions are proposed.This review is expected to be beneficial to the future study of WS_(2) nanomaterials used in the field of energy conversion and storage.

Solvent-free fabrication of broadband WS2 photodetectors on paper

Paper-based devices have attracted extensive attention due to the growing demand for disposable flexible electronics.Herein,we integrate semiconducting devices on cellulose paper substrate through a simple abrasion technique that yields high-performance photodetectors.A solvent-free WS_(2) film deposited on paper favors an effective electron-hole separation and hampers recombination.The as-prepared paper-based WS2 photodetectors exhibit a sensitive photoresponse over a wide spectral range spanning from ultraviolet(365 nm)to near-infrared(940 nm).Their responsivity value reaches up to~270 mA W^(−1) at 35 V under a power density of 35 mW cm^(−2).A high performance photodetector was achieved by controlling the environmental exposure as the ambient oxygen molecules were found to decrease the photoresponse and stability of the WS_(2) photodetector.Furthermore,we have built a spectrometer using such a paperbased WS_(2) device as the photodetecting component to illustrate its potential application.The present work could promote the development of cost-effective disposable photodetection devices.

Improvement of VIS and IR camouflage properties by impregnating cotton fabric with PVB/IF-WS_(2)

In order to examine the possibility to improve its camouflage properties standard cotton fabric with camouflage print was impregnated with poly(vinyl butyral),PVB and fullerene-like nanoparticles of tungsten disulfide,PVB/IF-WS_(2).FTIR analysis excluded any possible chemical interaction of IF-WS_(2) with PVB and the fabric.The camouflage behavior of the impregnated fabric has been examined firstly in the VIS part of the spectrum.Diffuse reflection,specular gloss and color coordinates were measured for three different shades(black,brown and dark green).Thermal imaging was applied to examine the camouflage abilities of this impregnation in IR part of the spectrum.The obtained results show that PVB/IF-WS_(2) impregnation system induced enhacement of the materials camouflage properties,i.e.that IF-WS_(2) have a positive effect on spectrophotometric characteristics of the fabric.

Ultrafast investigation of room temperature valley polarization in“optical bilayer”WS_(2)

Monolayer transition metal dichalcogenides(TMDCs)have become a promising platform in valleytronics due to possessing the regulatable valley degrees of freedom.While,as a result of the rapid intervalley scattering,it is difficult to measure the PL valley polarization of monolayer TMDCs at room temperature,which limits their application in valleytronics devices.Here,we report a room temperature photoluminescence(PL)valley polarization up to 3.73%in an“optical bilayer”WS_(2)formed by transferring monolayer WS_(2)onto flat Ag film.Furthermore,in the transient absorption(TA)measurements,a remarkably long valley depolarization lifetime is found.Thus,we demonstrate the valley properties of such“optical bilayer”WS_(2)resemble actual bilayer WS_(2),in which the robust valley polarization can be attributed to the phonons depletion effect and the blocked interlayer hopping processes.These peculiar valley features in“optical bilayer”WS_(2)provide a particularly simple method to enhance valley control at room temperature.

光谱特征关联性分析用于研究CVD生长的单层二硫化钨非均匀性机制

化学气相沉积(CVD)已被证明是生产二维材料(包括二硫化钨(WS_(2)))最有效的方法.然而,CVD生长的WS_(2)单层中存在的应变、掺杂和缺陷的不均匀性会显著影响材料的光学和电子特性,从而影响其器件应用.在本研究中,我们通过CVD方法在蓝宝石衬底上生长了介观尺寸的三角形单层WS_(2),并利用空间分辨的显微拉曼和光致发光(PL)光谱技术系统地表征了单层WS_(2)中的应变、掺杂和非辐射缺陷中心的不均匀性.我们从全域(整个单层WS_(2)三角形)和分区域(如顶点、边缘、中心)两个维度对其拉曼和PL光谱的相关参数(即峰值位置、强度和半峰宽)随位置的变化进行了相关性分析,发现单层WS_(2)三角形的不同区域往往表现出明显不同的应变、掺杂和缺陷状态.这项研究揭示并解释了单层WS_(2)应变、掺杂和缺陷的不均匀性.此外,我们发现当介观尺寸的三角形结构扩展到连续薄膜时,不均匀性大大降低.我们的工作表明,对光谱特征的相关性分析并辅以对物理机制的了解有助于全面理解材料生长带来的不均匀性,并为优化二维材料的生长过程和器件加工提供指导.

Electrochemically exfoliated WS_(2)in molten salt for sodium-ion battery anode

The poor crystallinity and unstable crystal structure of tungsten disulfide(WS2)limit its application in practice.In this paper,a molten salt electrolysis method is proposed to intercalate metal ions into the interlayers of layered WS2 to obtain few-layer sheetlike structures.The effect of the molten salt system,applied constant current and electrolysis duration on the exfoliation degree of WS2 bulk has been investigated.The results show that the products electrolyzed in molten LiCl-NaCl-KCl and NaClKCl salts under 25 mA were more transparent and thinner flakes sheets due to the uniform intercalation of Li^+and Na^+with smaller size.The exfoliated WS_(2)was used as an anode material for sodium-ion batteries with a potential of 0.01-2.50 V.In comparison,the WS_(2)-NaCl-25 mA electrode displays a high reversible capacity of 373 mAh·g^(-1)at0.1 A·g^(-1)after cycling for 100 cycles at the same time showing great rate and cycle performance.It also presents a high capacitive ratio of 90.65%at 1.0 mV·s^(-1).The molten salt electrolysis provides a new perspective on the exfoliation of layered material,while demonstrating the great potential of WS2 as an anode material for sodium-ion battery.

Conductivity and tribological properties of IL-PANI/WS_(2) composite material in lithium complex grease

An ionic liquid-polyaniline/tungsten disulfide(IL-PANI/WS_(2))composite was synthesized in 1-butyl-3-methylimidazole tetrafluoroborate(LB104)aqueous solution by in-situ polymerization and characterized by Fourier transform infrared spectroscopy.A current-carrying friction and wear tester was used to study the tribological properties of steel-steel and copper-copper friction pairs lubricated by an IL-PANI/WS_(2) lithium complex grease(LCG).After the experiment,scanning electron microscope was used to observe the surface morphology of the wear scar on the steel and copper plates,and X-ray photoelectron spectrometer was used to analyze the elemental composition of the wear scar surface.The results show that compared with greases containing IL-PANI and WS_(2),greases containing IL-PANI/WS_(2) exhibit better antiwear performance when lubricating steel-steel friction pairs and better tribological performance and electrical conductivity when lubricating copper-copper friction pairs.Therefore,it can be concluded that WS_(2) and IL-PANI have a synergistic effect.

Recent advances of phase engineering in group VI transition metal dichalcogenides

As the crystal quality and phase structure of two-dimensional(2D)transition metal dichalcogenides(TMDs)have significant impacts on their properties such as electroconductivity,superconductivity and chemical stability,the precise synthesis,which plays an important role in fundamental researches and industrial applications,is highly required.Group VI TMDs,such as MoS_(2),usually exhibit diverse polymorphs including semiconducting 1H and metallic 1T phases.Even great efforts are devoted to revealing the structure-dependent physicochemical nature of TMDs by modulating their phases from the stable to the metastable at the atomic scale,there are still challenges on the phase-controlled synthesis of Group VI TMDs with metallic or semimetal properties.In this review,methods such as ion intercalation,chemical doping,strain engineering,defect triggering,and electric-field treatment are examined in detail.Finally,challenges and opportunities in this research field are proposed.

Unveiling the origin of anomalous low-frequency Raman mode in CVD-grown monolayer WS_(2)

Substrates provide the necessary support for scientific explorations of numerous promising features and exciting potential applications in two-dimensional (2D) transition metal dichalcogenides (TMDs). To utilize substrate engineering to alter the properties of 2D TMDs and avoid introducing unwanted adverse effects, various experimental techniques, such as high-frequency Raman spectroscopy, have been used to understand the interactions between 2D TMDs and substrates. However, sample-substrate interaction in 2D TMDs is not yet fully understood due to the lack of systematic studies by techniques that are sensitive to 2D TMD-substrate interaction. This work systematically investigates the interaction between tungsten disulfide (WS_(2)) monolayers and substrates by low-frequency Raman spectroscopy, which is very sensitive to WS_(2)-substrate interaction. Strong coupling with substrates is clearly revealed in chemical vapor deposition (CVD)-grown monolayer WS_(2) by its low-wavenumber interface mode. It is demonstrated that the enhanced sample-substrate interaction leads to tensile strain on monolayer WS_(2), which is induced during the cooling process of CVD growth and could be released for monolayer WS_(2) sample after transfer or fabricated by an annealing-free method such as mechanical exfoliation. These results not only suggest the effectiveness of low-frequency Raman spectroscopy for probing sample-substrate interactions in 2D TMDs, but also provide guidance for the design of high-performance devices with the desired sample-substrate coupling strength based on 2D TMDs.

Fast growth of large single-crystalline WS_(2) monolayers via chemical vapor deposition

Two-dimensional(2D)tungsten disulfide(WS2)has emerged as a promising ultrathin semiconductor for building high-performance nanoelectronic devices.The controllable synthesis of WS2 monolayers(1L)with both large size and high quality remains as a challenge.Here,we developed a new approach for the chemical vapor deposition(CVD)growth of WS2 monolayers by using K2WS4 as the growth precursor.The simple chemistry involved in our approach allowed for improved controllability and a fast growth rate of~30μm·min−1.We achieved the reliable growth of 1L WS2 flakes with side lengths of up to~500μm and the obtained WS2 flakes were 2D single crystals with low density of defects over a large area as evidenced by various spectroscopic and microscopic characterizations.In addition,the large 1L WS2 single crystals we obtained showed higher electrical performance than their counterparts grown with previous approaches,demonstrating the potential of our approach in producing high quality and large 2D semiconductors for future nanoelectronics.

A facile preparation of WS_(2)nanosheets as a highly effective HER catalyst

Tungsten disulfide(WS_(2))has been considered as a promising hydrogen evolution reaction(HER)candidate due to its high activity,robust chemical stability,and earth-abundant resources.However,the inert basal planes and low electrical conduc-tivity greatly hinder its development in HER.Increasingly,the density of active sites through the structural designing is one of the most effective strategies to enhance the HER performance.Herein,we report a facile one-step hydrothermal method for synthesizing flower-like WS_(2)nanosheets for highly efficient HER.Besides,the effect of preparation temperature is also been discussed.The optimized WS_(2)nanosheets exhibit the enhanced HER activity in strong acidic solutions with a low Tafel slope and a good stability.The improvement of the HER performance can be attributed to sheet-like nanostructures,which greatly increase the edge sites and defects,resulting in a high density of exposed active sites.Besides,these sheet-like nano-structures also can make the acidic electrolyte easily accessible to the surface of WS_(2)and accelerate the electron transfer rate.

Rapid synthesis of transition metal dichalcogenide-carbon aerogel composites for supercapacitor electrodes

Transition metal dichalcogenide(TMD)materials have recently demonstrated exceptional supercapacitor properties after conversion to a metallic phase,which increases the conductivity of the network.However,freestanding,exfoliated transition metal dichalcogenide films exhibit surface areas far below their theoretical maximum(1.2%),can fail during electrochemical operation due to poor mechanical properties,and often require pyrophoric chemicals to process.On the other hand,pyrolyzed carbon aerogels exhibit extraordinary specific surface areas for double layer capacitance,high conductivity,and a strong mechanical network of covalent chemical bonds.In this paper,we demonstrate the scalable,rapid nanomanufacturing of TMD(MoS2 and WS2)and carbon aerogel composites,favoring liquid-phase exfoliation to avoid pyrophoric chemicals.The aerogel matrix support enhances conductivity of the composite and the synthesis can complete in 30 min.We find that the addition of transition metal dichalcogenides does not impact the structure of the aerogel,which maintains a high specific surface area up to 620 m^(2) g−1 with peak pore radii of 10 nm.While supercapacitor tests of the aerogels yield capacitances around 80 F g^(−1) at the lowest applied currents,the aerogels loaded with TMD’s exhibit volumetric capacitances up to 127% greater than the unloaded aerogels.In addition,the WS2 aerogels show excellent cycling stability with no capacitance loss over 2000 cycles,as well as markedly better rate capability and lower charge transfer resistance compared to their MoS2-loaded counterparts.We hypothesize that these differences in performance stem from differences in contact resistance and in the favorability of ion adsorption on the chalcogenides.