WS2 Materials Research Articles

Performance Enhancement of SnS Solar Cell with Tungsten Disulfide Electron Transport Layer and Molybdenum Trioxide Hole Transport Layer

Herein, a new heterojunction photovoltaic (PV) device is designed by incorporating molybdenum trioxide (MoO3) as a hole transport layer (HTL), tin sulfide (SnS) as an absorber, and tungsten disulfide (WS2) as an electron transport layer (ETL). The PV outputs of the proposed thin‐film solar cell (TFSC) of Ni/MoO3/SnS/WS2/FTO/Al are investigated using the widely used solar cell simulator (SCAPS‐1D). It is found that the SnS TFSC with suitable band alignments at both the SnS/WS2 and MoO3/SnS interfaces gives better photoconversion efficiency than the conventional one. To optimize the material properties, the performance parameters, including open‐circuit voltage (Voc), short‐circuit current density (Jsc), fill factor (FF), and efficiency, have been calculated by varying the influences of the material's thickness, doping concentration, bulk and interface defect densities, operational temperature, and work function of back‐contact. At optimized thicknesses of 0.1 μm for MoO3 HTL and 1.0 μm for SnS absorber, the efficiency is estimated to be 30.42% with Voc of 1.02 V, Jsc of 34.38 mA cm−2, and FF of 87.04% for the suggested TFSC. These outcomes imply that the nontoxic MoO3 and WS2 materials can be applied as HTL and ETL into the inexpensive, highly efficient, and environmentally friendly SnS‐based PV cell.

Investigating the potential of WO3 and WS2 as Cd-free buffer layers in Sb2Se3-Based thin-film solar cells: A numerical study with SCAPS-1D software

In this numerical study, the performance of thin-film solar cells (TFSCs) based on Sb2Se3 with WO3 and WS2 materials as Cd-free buffer layers was investigated using SCAPS-1D software. WO3 and WS2 were chosen for their suitable band gaps, good electrical conductivity, and optical transparency. The proposed solar cell structures, Au/Sb2Se3/WO3/ITO and Au/Sb2Se3/WS2/ITO were theoretically examined, considering various factors such as conduction band offset, absorber and buffer layer thickness, doping concentrations, series and shunt resistances, temperature, activation energy, and C–V characteristic. The conduction band offset at the absorber/buffer interface showed spike-like configurations, with values of 0.38 eV and 0.23 eV for the WO3 and WS2 buffer layers, respectively. The optimized thickness for the absorber layer in both structures was 1 μm, while for WO3 and WS2 buffer layers, it was 0.1 μm and 0.05 μm, respectively. The optimized acceptor and donor doping concentrations were 1 × 1016 cm−3 and 1 × 1018 cm−3, respectively. The efficiency of the proposed photovoltaic devices was found to be 9.538 % (with Voc = 0.561 V, Jsc = 30.856 mA/cm2, FF = 55.84 %) and 10.151 % (with Voc = 0.558 V, Jsc = 29.189 mA/cm2, FF = 62.34 %) for structures with WO3 and WS2 buffer layers, respectively. These results suggest that using non-toxic materials like WO3 and WS2 as buffer layers can be an efficient and environmentally friendly alternative to toxic CdS for fabricating cost-effective and highly efficient Sb2Se3-based TFSCs.

Time-dependent synthesis of tungsten disulfide for flexible supercapacitors

Tungsten disulfide (WS2) with various nanostructures was synthesized for times ranging from 6 to 72 h, using a simple and easy hydrothermal technique, for high-performance supercapacitor applications. The synthesized WS2 for 24 h (WS2–24), among other synthesized WS2, exhibited flower-like structures, composed of numerous nanosheets, demonstrating superior electrochemical properties in terms of electrochemical surface area, charge transfer resistance, specific capacitance, and rate capability. In addition, WS2–24 was coated on a flexible activated carbon cloth substrate to make flexible electrodes and assemble a flexible symmetric supercapacitor with a gel electrolyte. The flexible supercapacitor exhibited a specific capacitance of 262.5 F g−1 at 1 A g−1 with an energy density of 23.6 Wh kg−1 and a power density of 802 W kg−1. It also showed excellent capacitance retention of 94.5 % and a Coulombic efficiency of 86.4 % after 5000 cycles. The remarkable electrochemical properties of the synthesized WS2 materials highlight them as highly effective active materials for high-performance supercapacitor electrodes.

Plasmonic Imaging of Single DNA with Charge Sensitive Monolayer WS2.

Imaging the surface charge of biomolecules such as proteins and DNA, is crucial for comprehending their structure and function. Unfortunately, current methods for label-free, sensitive, and rapid imaging of the surface charge of single DNA molecules are limited. Here, we propose a plasmonic microscopy strategy that utilizes charge-sensitive single-crystal monolayer WS2 materials to image the local charge density of a single λ-DNA molecule. Our study reveals that WS2 is a highly sensitive charge-sensitive material that can accurately measure the local charge density of λ-DNA with high spatial resolution and sensitivity. The consistency of the surface charge density values obtained from the single-crystal monolayer WS2 materials with theoretical simulations demonstrates the reliability of our approach. Our findings suggest that this class of materials has significant implications for the development of label-free, scanning-free, and rapid optical detection and charge imaging of biomolecules.

Near-Field Photoluminescence of WS<sub>2</sub> and MoS<sub>2</sub> Monolayers, Grown by Chemical Vapor Deposition

Monolayer triangular WS2 and MoS2 islands grown by chemical vapor deposition was investigated by near-field photoluminescence (nano-PL) enhanced by the metallized atomic force microscope (AFM) tip. To achieve maximum near-field response from WS2 and MoS2 materials fabricated Au and Ag metallized AFM tips were used. Maximum nano-PL responds from the islands is observed under the resonant conditions when the energy of the localized surface plasmon of the metallized probe coincides with the energy of the exciton luminescence of the WS2 and MoS2 materials. Nano-PL mapping of the exciton response allows visualizing structural defects and determine the local thickness changes of monolayer islands with nanometer spatial resolution.

Adsorption behavior of formaldehyde gas on two-dimensional semiconductor monolayers MS2 (M=W, Mo)

Detecting methanal molecule, an indoor air pollutant and potential carcinogen, is crucial for safeguarding human health, ensuring occupational safety, and maintaining environmental quality. In this study, density functional theory calculations have been performed to explore the adsorption behavior of formaldehyde (methanal) gas on the surface of two-dimensional semiconductor monolayers MS2 (M=W, Mo). Using the Computational DFT-based Nanoscope tool, we compute binding energies and determine configurations at global minimum energy of molecule adsorbed monolayer MS2. Five nonlocal van der Waals functionals; revPBE-vdW, optPBE-vdW, vdW-DF2, optB88-vdW, and optB86b-vdW are used to compute the adsorption energy profiles. The calculated results show that: (i) the optPBE-vdW functional products the largest adsorption energy magnitude, (ii) Methanal molecule exhibits physical adsorption on both MoS2 and WS2 materials (iii) Adsorption of methanal molecules may enhance the electrical conductivity of MoS2 and WS2 upon the electron donation to molecule by substrates. The adsorption energy magnitude, bandgap reduction, and charge transfer of the methanal-MoS2 adsorption system are respectively 1.04, 1.27, and 1.47 times larger than those of the methanal-WS2 adsorption system, while the diffusion barrier energy is 0.25 times smaller. These characteristic adsorption parameters imply that methanal exhibits higher sensitivity to the MoS2 substrate. This study also provides an in-depth discussion regarding the interaction between methanal and the MS2 substrate, focusing on aspects of relaxed geometrical structures, potential energy surface, adsorption energy, response length, recovery time, work function, charge transfer, density of states, and energy band structure.

Layered Transition Metal Sulfides for Supercapacitor Applications

AbstractSupercapacitor (SC) devices holds an important position between traditional capacitors and ion batteries in terms of energy density and power density values. In particular, SC′s greater power density values than Li‐ion batteries make them useful for some specific applications, such as storing energy in hybrid cars while the car slows down. Increasing energy density values is one of the key challenges for the SC community. Transition metal dichalcogenides (TMDCs), are one of the new developing important material systems to have this potential, compared to their counterparts’ transition metal oxides and conductive polymers. This review is about giving insight into the electrochemical performances of two‐dimensional (2D) layered transition metal sulfides (TMS) such as MoS2, WS2, TaS2, NbS2, VS2, TiS2 and ZrS2 materials. On the other hand, the methods mostly used in synthesizing these materials are presented.

Effect of lattice defects on electronic structure and thermoelectric properties of two- dimensional WS2 materials

WS2 is a kind of thermoelectric material with great development potential that is used for thermoelectric power generation and refrigeration. In this study, the NEGF-DFT method was used to study the thermoelectric properties of intrinsic 2D-WS2 and the effects of periodic vacancy or substitution atoms on their electronic structure and thermoelectric properties. The results show that the ZT value of p-type 2D-WS2 is better than that of n-type 2D-WS2. The thermoelectric properties of p-type 2D-WS2 and p-type 2D-WS2 with O-atom replacing S-atom are better than those of the corresponding n-type 2D-WS2, while the thermoelectric properties of n-type 2D-WS2 with periodic defects and n-type 2D-WS2 with Se-atom or Te-atom replacement for S-atom are better than those of the corresponding p-type 2D-WS2. The ZT values of n-type 2D-WS2 with periodic defects, n-type 2D-WS2 with Se-atom replacement for S-atom, and n-type 2D-WS2 with Te-atom replacement for S-atom are 2.973, 3.383, and 3.0413, respectively, which indicates 2D-WS2 with Se-atom replacement for S-atom has the best thermoelectric performance.

A first-principles investigation on the adsorption of octanal and nonanal molecules with decorated monolayer WS2 as promising gas sensing platform

With the broad applications of two-dimensional transition, metal dichalcogenides in gas sensing prompt us to investigate the adsorption and gas sensing properties of the bare and metal-decorated tungsten disulfide (WS2) interaction with nonanal and octanal gas molecules by using first-principles methods. In this regard, first-principles calculations based on density functional theory (DFT) have been employed to study the detection of nonanal and octanal as well-known breast cancer biomarkers. The results showed that the pristine WS2 monolayer is not appropriate to develop gas sensors for the target. Based on the detailed DFT calculations, decoration of the WS2 surface with Ni and Pt could enhance gas detection and sensitivity and tune the energy bandgap. The present results suggested that the adsorption energy of nonanal and octanal has been increased to −2.59 and −2.34 eV after interaction with Ni-decorated WS2. In addition, sensitivity has been enhanced significantly, and the recovery time of 7.76 s for Ni-decorated WS2 was achieved after interaction with octanal gas. As a result, the decorated WS2-based nanosensor might be an auspicious platform for gas detection. Undoubtedly, this work can open up a prospect for the use of excellent Ni-/Pt-decorated WS2 materials to achieve high-efficiency detection of nonanal and octanal gas molecules.

WS2 Nanosheet Loaded Silicon-Oxycarbide Electrode for Sodium and Potassium Batteries

Transition metal dichalcogenides (TMDs) such as the WS2 have been widely studied as potential electrode materials for lithium-ion batteries (LIB) owing to TMDs' layered morphology and reversible conversion reaction with the alkali metals between 0 to 2 V (v/s Li/Li+) potentials. However, works involving TMD materials as electrodes for sodium- (NIBs) and potassium-ion batteries (KIBs) are relatively few, mainly due to poor electrode performance arising from significant volume changes and pulverization by the larger size alkali-metal ions. Here, we show that Na+ and K+ cyclability in WS2 TMD is improved by introducing WS2 nanosheets in a chemically and mechanically robust matrix comprising precursor-derived ceramic (PDC) silicon oxycarbide (SiOC) material. The WS2/SiOC composite in fibermat morphology was achieved via electrospinning followed by thermolysis of a polymer solution consisting of a polysiloxane (precursor to SiOC) dispersed with exfoliated WS2 nanosheets. The composite electrode was successfully tested in Na-ion and K-ion half-cells as a working electrode, which rendered the first cycle charge capacity of 474.88 mAh g-1 and 218.91 mAh g-1, respectively. The synergistic effect of the composite electrode leads to higher capacity and improved coulombic efficiency compared to the neat WS2 and neat SiOC materials in these cells.

Nanostructured MoS2 and WS2 Photoresponses under Gas Stimuli.

This study was on the optoelectronic properties of multilayered two-dimensional MoS2 and WS2 materials on a silicon substrate using sputtering physical vapor deposition (PVD) and chemical vapor deposition (CVD) techniques. For the first time, we report ultraviolet (UV) photoresponses under air, CO2, and O2 environments at different flow rates. The electrical Hall effect measurement showed the existence of MoS2 (n-type)/Si (p-type) and WS2 (P-type)/Si (p-type) heterojunctions with a higher sheet carrier concentration of 5.50 × 105 cm−2 for WS2 thin film. The IV electrical results revealed that WS2 is more reactive than MoS2 film under different gas stimuli. WS2 film showed high stability under different bias voltages, even at zero bias voltage, due to the noticeably good carrier mobility of 29.8 × 102 cm2/V. WS2 film indicated a fast rise/decay time of 0.23/0.21 s under air while a faster response of 0.190/0.10 s under a CO2 environment was observed. Additionally, the external quantum efficiency of WS2 revealed a remarkable enhancement in the CO2 environment of 1.62 × 108 compared to MoS2 film with 6.74 × 106. According to our findings, the presence of CO2 on the surface of WS2 improves such optoelectronic properties as photocurrent gain, photoresponsivity, external quantum efficiency, and detectivity. These results indicate potential applications of WS2 as a photodetector under gas stimuli for future optoelectronic applications.

Real-time numerical system convertor via two-dimensional WS2-based memristive device

The intriguing properties of two-dimensional (2D) transition metal dichalcogenides (TMDCs) enable the exploration of new electronic device architectures, particularly the emerging memristive devices for in-memory computing applications. Implementation of arithmetic logic operations taking advantage of the non-linear characteristics of memristor can significantly improve the energy efficiency and simplify the complexity of peripheral circuits. Herein, we demonstrate an arithmetic logic unit function using a lateral volatile memristor based on layered 2D tungsten disulfide (WS2) materials and some combinational logic circuits. Removable oxygen ions were introduced into WS2 materials through oxygen plasma treatment process. The resistive switching of the memristive device caused by the thermophoresis-assisted oxygen ions migration has also been revealed. Based on the characteristics of excitatory postsynaptic current (EPSC), paired-pulse facilitation (PPF), and spike rate dependent plasticity (SRDP), a real-time numerical system convertor was successfully accomplished, which is a significant computing function of arithmetic logic unit. This work paves a new way for developing 2D memristive devices for future arithmetic logic applications.

Rechargeable Magnesium Ion Batteries Based on Nanostructured Tungsten Disulfide Cathodes

Finding effective cathode materials is currently one of the key barriers to the development of magnesium batteries, which offer enticing prospects of larger capacities alongside improved safety relative to Li-ion batteries. Here, we report the hydrothermal synthesis of several types of WS2 nanostructures and their performance as magnesium battery cathodes. The morphology of WS2 materials was controlled through the use of sodium oxalate as a complexing agent and different templating agents, including polyethylene glycol (PEG), polyvinylpyrrolidone (PVP), and hexadecyltrimethyl ammonium bromide (CTAB). A high capacity of 142.7 mAh/g was achieved after 100 cycles at a high current density of 500 mA/g for cathodes based on a nanostructured flower-like WS2. A solution consisting of magnesium (II) bis(trifluoromethanesulfonyl)imide (MgTFSI2) and magnesium (II) chloride (MgCl2) in dimethoxyethane (DME) was used as an effective electrolyte, which contributes to favorable Mg2+ mobility. Weaker ionic bonds and van der Waals forces of WS2 compared with other transition metal oxides/sulfides lay the foundation for fast discharge/charge rate. The enhanced surface area of the nanostructured materials plays a key role in enhancing both the capacity and discharge/charge rate.

First-principles probing of Au-doped WS2 monolayer as a superb CO sensor with high selectivity and fast diffusion

Doping with noble metals is considered to be an effective strategy to improve the gas sensitivity of 2D WS2 materials. In this work, the adsorption properties of CO, CO2, NH3 and SO2 on both pure and Au-doped WS2 monolayers are investigated in detail by first-principles calculations. The results show that the adsorption energy of CO on the WS2 monolayer doped with an Au atom is significantly lower than that of the pure WS2 monolayer (−0.15 eV reduced to −0.35 eV). From analysis of the adsorption configuration, it can be found that the CO molecule and WS2/Au monolayer form a new C = S double bond with a bond length of 2.067 Å due to chemisorption, and the calculated results of charge density difference (CDD) and electron localization function (ELF) also show that there exists the charge transfer and electron co-existence occur between them. Further, the density of states (DOS) of the WS2/Au monolayer shifts towards the lower energy state after adsorbing the CO molecule, indicating excellent CO detection sensitivity. On the other hand, both before and after doping with an Au atom, the WS2 monolayer always exhibits weak physical adsorption and insignificant changes in electronic properties for CO2, NH3 and SO2 molecules. Finally, the calculated diffusion coefficients indicate that the CO molecules diffuse more easily on the surface of the WS2/Au monolayer than the other three gases. These findings provide a theoretical basis for the Au-doped WS2 monolayer to improve the detection performance for CO.

Nanoporous Functionalized WS2/MWCNTs Nanocomposite for Trimethylamine Detection Based on Quartz Crystal Microbalance Gas Sensor.

We have developed a tungsten disulfide (WS2)/multiwall carbon nanotubes (MWCNTs) nanocomposite based QCM gas sensor for (trimethylamine) TMA gas sensing of low concentrations. WS2/MWCNTs nanocomposite was synthesized via the hydrothermal method and was characterized for surface morphology, nanostructure, thermal stability, and elementary composition. The TMA-sensing properties of WS2/MWCNTs nanocomposite based QCM sensor were investigated. The composite based QCM sensor showed faster response time, strong response amplitude, good gas capacity, and good selectivity and stability compared with as prepared WS2 and MWCNTs-1 based QCM sensor. The response time of WS2/MWCNTs based QCM sensor was 294.1 and 142.9 s shorter than WS2 and MWCNTs-1 for 500 ppb TMA gas. And the response of the WS2/MWCNTs based QCM sensor was almost stable over 40 days, and the limit of detection (LOD) was 76 ppb calculated by the ICH method. This was ascribed to the fact that MWCNTs provided a skeleton for the growth of WS2 nanosheets and avoided agglomeration. The special structure could not only improve the structure ability but also expose more active adsorption sites. In order to further investigate the adsorption mechanism of the TMA molecule on (pure/functionalized) WS2 materials, density functional theory (DFT) calculations based on first-principle were conducted in the Vienna Ab-initio Simulation Package under ideal conditions.

Li‐Ion Intercalated Exfoliated WS2 Nanosheets with Enhanced Electrocatalytic Hydrogen Evolution Performance

AbstractTungsten disulfide (WS2) as a 2D layered material shows good application in the field of electrocatalysis due to its unique 2D structure, excellent electronic properties, and catalytic activity. However, the electrocatalytic performance of bulk WS2 materials is limited due to the lack of active sites. Hence, a few layers of WS2 nanosheets are prepared by a simple method of lithium (Li)‐ion intercalation exfoliation, and enhanced electrocatalytic hydrogen evolution performance is demonstrated compared to bulk WS2. This method can not only realize the large‐scale preparation of WS2 nanosheets, but also the obtained WS2 nanosheets have a larger lateral size, a good lattice structure, and no chemical impurities remain. Most importantly, it is realized that the reduction in the thickness of WS2 nanosheets will generate additional active sites from the ultrathin planar structure, thereby improving electrocatalytic activity. The exfoliated WS2 nanosheets show a smaller overpotential compared to the bulk WS2, the overpotential is about 320 mV at 10 mA cm–2. This work shows that Li‐ion intercalation exfoliated method is an efficient strategy to prepare 2D nanosheets materials and effectively improve its electrocatalytic performance.

Effect of adsorption on photocatalytic activity for rhodamine B degradation of copper-doped tungsten disulfide

In this study, the Cu-doped WS2 materials were synthesized by a simple solid-state calcination of mixture of tungstic acid, thiourea and copper (II) acetate monohydrate in Ar gas at 650 oC for 1h, and denoted as xCu-WS2, where x is atomic percentage ratios of Cu/W (x= 1, 3, 5%) and weigh ratio of tungstic acid/thiourea is constant (1:5). The obtained products were characterized by X-ray diffraction, infrared, energy-dispersive X-ray spectroscopy, scan electron microscopy and UV-Vis diffuse reflectance spectroscopy. The photocatalytic performance of the samples was assessed through photodegradation of rhodamine B (RhB). Interestingly, there is a synergistic relationship between adsorption and photocatalysis, in which, a higher relative adsorption might give a better photocatalytic results due to reactive species reacting with absorbed organic matter on the catalyst surface rather than in the bulk of solution. The photodegradation of RhB over the 1Cu-WS2 catalyst was enhanced significantly with the highest efficiency up to 95.35% at pH 8 for 6 hours of visible light irradiation, which is attributed to the high adsorption of RhB cationic dye on the material surface. The photocatalytic mechanism was discussed as well.

Single-step, large-area, variable thickness sputtered WS2 film-based field effect transistors

Single-step, uniform, continuous, large-area WS2 films with variable thickness, controlled by the sputtering time, were grown using a magnetron sputtering method to fabricate and then investigate the characteristics of field-effect transistor (FET) devices. Raman measurements showed that WS2 thin films of different thicknesses all give rise to single-phase WS2 that is free of oxide phases. Raman mapping and optical microscope images taken from the interface between the Si/SiO2 substrate and the WS2 region clearly show the formation of a large-area continuous film. X-ray photoelectron spectroscopy (XPS) measurements revealed the sulfur-deficient formation of WS2 for all of the time-dependent series with S/W atomic ratios of around 1.15–1.30. The FET device fabricated on the WS2 layer grown for 1 s showed dominant p-type channel behavior with off-current values in the pA range and on/off ratios spanning almost four orders of magnitude. On the other hand, ambipolar behavior was realized for FET devices fabricated on WS2 continuous films grown for the 5 s, 10 s, and 30 s. Relatively large mobility values of 16.7 and 15.7 cm2/(V s) were achieved for the FET devices fabricated on the 10 s and 30 s grown WS2 layers, respectively. The present study shows acceptable FET device characteristics for 2D WS2 materials grown in a single step by sputtering.

High-Power Passively Q-Switched Nd:YVO4 Laser Based on WS2 Saturable Absorber

In this paper, we fabricated a novel tungsten disulfide (WS2) saturable absorber (SA) using sol-gel technology and successfully applied it into a Q-switched solid-state laser for the first time. By using sol-gel technology, WS2 materials are completely separated into sol-gel silica glass, which can effectively protect the WS2 materials from oxidation and other damages. Inserting the WS2 SA into the Nd:YVO4 laser cavity and adjusting it, a stable Q-switched pulse was obtained. The average output power was 3.09 W, the repetition rate was 3.38 MHz and the corresponding pulse duration was 99 ns. As far as we know, this is the highest average output power of WS2 SA used in Nd-doped solid state lasers, and the pulse duration is also quite narrow. It shows that sol-gel technology has great potential to make practical two dimensional material absorbers.

Skeleton-Structure WS2@CNT Thin-Film Hybrid Electrodes for High-Performance Quasi-Solid-State Flexible Supercapacitors.

The purpose of this work is to explore the application prospects of WS2 as an active material in flexible electrodes. Since WS2 has similar disadvantages as other two-dimensional layered materials, such as easily stacking, it is essential to develop a three-dimensional structure for its assembly in terms of electrochemical performance. In addition, the low conductivity of WS2 limits its application as flexible electrode material. In order to solve these problems, carbon nanotubes (CNTs) are introduced to improve the conductivity of hybrid WS2 materials and to construct a skeleton structure during WS2 assembly. Compared with pure CNTs and WS2, the WS2@CNT thin-film hybrid with a unique skeleton structure has a high specific area capacitance that reaches a maximum of 752.53 mF/cm2 at a scan rate 20 mV/s. Meanwhile, this hybrid electrode material shows good stability, with only 1.28% loss of its capacitance over 10,000 cycles. In order to prove its feasibility for practical application, a quasi-solid-state flexible supercapacitor is assembled, and its electrochemical characteristics (the specific area capacitance is 574.65 mF/cm2) and bendability (under bending to 135° 10, 000 times, 23.12% loss at a scan rate of 100 mV/s) are further investigated and prove its potential in this field.