Multilayer WS2 Research Articles

Passively Q-switched mid-infrared fluoride fiber laser around 3 µm using a tungsten disulfide (WS2) saturable absorber

In this letter, we demonstrate a passively Q-switched Ho3+/Pr3+ co-doped fluoride fiber laser centered at 2865.7 nm using a tungsten disulfide (WS2) saturable absorber (SA) for the first time, to the best of our knowledge. A multilayer WS2 film was fabricated using the sulfidation grown method and then transferred onto an Au mirror to act as the cavity feedback and SA device in a linear cavity. Under a launched pump power of 318.5 mW, stable Q-switched pulses with an average output power of 48.4 mW were achieved with a pulse duration of 1.73 µs and repetition rate of 131.6 kHz, resulting in a pulse energy of 0.37 µJ. Our experimental results confirm that WS2 can be an effective nonlinear modulator that is suitable for pulse generation at the 3 µm waveband.

Zener Tunneling and Photoresponse of a WS2/Si van der Waals Heterojunction.

Van der Waals heterostructures built from two-dimensional materials on a conventional semiconductor offer novel electronic and optoelectronic properties for next-generation information devices. Here we report that by simply stacking a vapor-phase-synthesized multilayer n-type WS2 film onto a p-type Si substrate, a high-responsivity Zener photodiode can be achieved. We find that above a small reverse threshold voltage of 0.5 V, the fabricated heterojunction exhibits Zener tunneling behavior which was confirmed by its negative temperature coefficient of the breakdown voltage. The WS2/Si heterojunction working in the Zener breakdown regime shows a stable and linear photoresponse, a broadband photoresponse ranging from 340 to 1100 nm with a maximum photoresponsivity of 5.7 A/W at 660 nm and a fast response speed of 670 μs. Such high performance can be attributed to the ultrathin depletion layer involved in the WS2/Si p-n junction, on which a strong electric field can be created even with a small reverse voltage and thereby enabling an efficient separation of the photogenerated electron-hole pairs.

Highly Stable and Tunable Chemical Doping of Multilayer WS2 Field Effect Transistor: Reduction in Contact Resistance.

The development of low resistance contacts to 2D transition-metal dichalcogenides (TMDs) is still a big challenge for the future generation field effect transistors (FETs) and optoelectronic devices. Here, we report a chemical doping technique to achieve low contact resistance by keeping the intrinsic properties of few layers WS2. The transfer length method has been used to investigate the effect of chemical doping on contact resistance. After doping, the contact resistance (Rc) of multilayer (ML) WS2 has been reduced to 0.9 kΩ·μm. The significant reduction of the Rc is mainly due to the high electron doping density, thus a reduction in Schottky barrier height, which limits the device performance. The threshold voltage of ML-WS2 FETs confirms a negative shift upon the chemical doping, as further confirmed from the positions of E(1)2g and A1g peaks in Raman spectra. The n-doped samples possess a high drain current of 65 μA/μm, with an on/off ratio of 1.05 × 10(6) and a field effect mobility of 34.7 cm(2)/(V·s) at room temperature. Furthermore, the photoelectric properties of doped WS2 flakes were also measured under deep ultraviolet light. The potential of using LiF doping in contact engineering of TMDs opens new ways to improve the device performance.

Electronic Properties of MoS2–WS2 Heterostructures Synthesized with Two-Step Lateral Epitaxial Strategy

Formation of heterojunctions of transition metal dichalcogenides (TMDs) stimulates wide interest in new device physics and technology by tuning optical and electronic properties of TMDs. TMDs heterojunctions are of scientific and technological interest for exploration of next generation flexible electronics. Herein, we report on a two-step epitaxial ambient-pressure CVD technique to construct in-plane MoS2-WS2 heterostructures. The technique has the potential to artificially control the shape and structure of heterostructures or even to be more potentially extendable to growth of TMD superlattice than that of one-step CVD technique. Moreover, the unique MX2 heterostructure with monolayer MoS2 core wrapped by multilayer WS2 is obtained by the technique, which is entirely different from MX2 heterostructures synthesized by existing one-step CVD technique. Transmission electron microscopy, Raman and photoluminescence mapping studies reveal that the obtained heterostructure nanosheets clearly exhibit the modulated structural and optical properties. Electrical transport studies demonstrate that the special MoS2 (monolayer)/WS2 (multilayer) heterojunctions serve as intrinsic lateral p-n diodes and unambiguously show the photovoltaic effect. On the basis of this special heterostructure, depletion-layer width and built-in potential, as well as the built-in electric field distribution, are obtained by KPFM measurement, which are the essential parameters for TMD optoelectronic devices. With further development in future studies, this growth approach is envisaged to bring about a new growth platform for two-dimensional atomic crystals and to create unprecedented architectures therefor.

Pressure-Modulated Conductivity, Carrier Density, and Mobility of Multilayered Tungsten Disulfide.

Tungsten disulfide (WS2) is a layered transition metal dichalcogenide (TMD) that differs from other two-dimensional (2D) compounds such as graphene due to its unique semiconducting, tunable-band-gap nature. Multilayered WS2 exhibits an indirect band gap Eg of ∼1.3 eV, along with a higher load-bearing ability that is promising for strain-tuning device applications, but the electronic properties of multilayered WS2 at higher strain conditions (i.e., static strain >12%) remain an open question. Here we have studied the structural, electronic, electrical, and vibrational properties of multilayered WS2 at hydrostatic pressures up to ∼35 GPa experimentally in a diamond anvil cell and theoretically using first-principles ab initio calculations. Our results show that WS2 undergoes an isostructural semiconductor-to-metallic (S-M) transition at approximately 22 GPa at 280 K, which arises from the overlap of the highest valence and lowest conduction bands. The S-M transition is caused by increased sulfur-sulfur interactions as the interlayer spacing decreases with applied hydrostatic pressure. The metalization in WS2 can be alternatively interpreted as a 2D to 3D (three-dimensional) phase transition that is associated with a substantial modulation of the charge carrier characteristics including a 6-order decrease in resistivity, a 2-order decrease in mobility, and a 4-order increase in carrier concentration. These distinct pressure-tunable characteristics of the dimensionalized WS2 differentiate it from other TMD compounds such as MoS2 and promise future developments in strain-modulated advanced devices.

Preparation and Photoelectrochemical Properties of Multilayered WS2 Coated Titanium Dioxide Nanocomposites

In this paper, nanocomposite titanium dioxide and tungsten sulfide (TiO2-WS2) was prepared through high energy ball milling. The images of scanning electron microscopy (SEM) show that particle size of the as-prepared nanocomposites is approximately ~300 nm. The analysis of X-ray diffraction (XRD) indicates formation of a few layers WS2 anchored on TiO2 nanoparticle surface, which present enhancement (> 90%) of visible light absorption after 32 hours ball milling. The photocurrent of the nanocomposite TiO2-WS2 correspondently increases about 10% under UV illumination.

Direct Observation of Degenerate Two-Photon Absorption and Its Saturation in WS2 and MoS2 Monolayer and Few-Layer Films.

The optical nonlinearity of WS2 and MoS2 monolayer and few-layer films was investigated using the Z-scan technique with femtosecond pulses from the visible to the near-infrared range. The nonlinear absorption of few- and multilayer WS2 and MoS2 films and their dependences on excitation wavelength were studied. WS2 films with 1-3 layers exhibited a giant two-photon absorption (TPA) coefficient as high as (1.0 ± 0.8) × 10(4) cm/GW. TPA saturation was observed for the WS2 film with 1-3 layers and for the MoS2 film with 25-27 layers. The giant nonlinearity of WS2 and MoS2 films is attributed to a two-dimensional confinement, a giant exciton effect, and the band edge resonance of TPA.

Femtosecond Soliton Pulse Generation Using Evanescent Field Interaction Through Tungsten Disulfide (WS <sub>2</sub>) Film

We investigated nonlinear optical characteristics of Tungsten disulfide (WS2) films and experimentally demonstrated their high potential for application as nonlinear saturable absorbers in passively mode-locked fiber lasers. Side polished fiber (SPF) was fabricated and WS2 film was overlaid to provide an efficient evanescent field interaction. The WS2 film was prepared using two methods: liquid phase exfoliation to form few-layer nano-sheets, and chemical vapor deposition (CVD) to grow uniform multilayer WS2 on a SiO2 substrate. Two SPF saturable absorbers were prepared by either spin coating WS2 solution or lifting off the multilayer CVD WS2 on SPF. An all-fiber ring cavity was built and the WS2 film overlaid on SPF was employed as a mode locker along with Er-doped fiber as a gain medium. Using the spin-coated WS2 SPF, stable soliton-like pulses were generated with a spectral width of 5.6 nm and 467 fs pulse duration. The fiber laser cavity containing CVD WS2 SPF generated a transform-limited soliton pulse train with a spectral width of 8.23 nm and a pulse duration of 332 fs. Our study confirmed a high potential of WS2 film as a novel 2-D nonlinear optical material for laser applications.

Preparation and Photoelectrochemical Properties of Multilayered WS2 Coated Titanium Dioxide Nanocomposites

Since TiO2 was discovered for water-splitting to generate hydrogen under UV light, plenty of techniques have been explored to obtain suitable sensitizers for visible-light adsorption in order to increase the activity of water-splitting reaction for hydrogen generation [1-3]. Among them, dispersing a 2D graphene andsimilar layered material on TiO2 nanoparticle exhibits highly photocatalytic activities (i.e. MoS2) [2]. In this paper, graphene-liked WS2 was chosen due to its activity as HER (hydrogen evolution reaction) catalyst and relatively abundant storage [4]. The expected composite was formed through high energy ball milling technique which makes WS2 disperse on the surface of TiO2nanoparticles [5]. The composites consist of 99.5% TiO2 and 0.5% WS2and were prepared by high energy ball milling for five different periods of time-2hours, 4hours, 8hours, 16hours and 32hours. To identify phase evolution of the as-prepared nanocomposites during high energy ball milling procedure, X-ray diffraction (XRD) was performed to examine phase transition, which is shown in Fig.1. It can be seen that, with increasing milling time, the crystallization decrease at first and increase after further milling. To further study the photoelectrochemical (PEC) characterization of composites after ball milling, a three-electrode setup was used, which is shown in inset of Fig. 2. This three-electrode device consists of a reference electrode (Ag/AgCl), a Pt wire counter electrode, a quartz container filled of 0.5M Na2SO4 aqueous solution and a working electrode. Sintered composites on indium-tin-oxide (ITO) glass substrate acts as the working electrode. Fig.2 shows the PEC measurement results of pure TiO2 and milled composites with milling time for 2hours, 8hours and 32hours. As can be seen, composite reaches the best photocurrent response with 8 hours milling. It is obvious that compared to pure TiO2 sample, adding of WS2and high energy ball milling make positive effect to composites. In summary, the structure of TiO2 and WS2 nanocomposites through high energy ball milling has been studied by X-ray diffraction. Photoelectrochemical properties of the TiO2 coated with few layered WS2 shows the optimization photochemical response for 8 hours milled nanocomposites. This work provides a facile and low cost process for large amount production of few layer doped TiO2nanocomposites.

Sodium ion storage properties of WS₂-decorated three-dimensional reduced graphene oxide microspheres.

Layered WS2 nanosheet-decorated three-dimensional reduced graphene oxide (3D-RGO) microspheres are prepared as anode materials for sodium ion batteries. WO3 nanocluster-decorated 3D RGO microspheres are transformed into multi-layered WS2-3D RGO microspheres by a simple sulfidation process. The WS2-3D RGO microspheres show Na(+) storage properties superior to those of the WO3-3D RGO microspheres.

Stable, highly-responsive and broadband photodetection based on large-area multilayered WS2 films grown by pulsed-laser deposition.

The progress in the field of graphene has aroused a renaissance of keen research interest in layered transition metal dichalcogenides (TMDs). Tungsten disulfide (WS2), a typical TMD with favorable semiconducting band gap and strong light-matter interaction, exhibits great potential for highly-responsive photodetection. However, WS2-based photodetection is currently unsatisfactory due to the low optical absorption (2%-10%) and poor carrier mobility (0.01-0.91 cm(2) V(-1) s(-1)) of the thin WS2 layers grown by chemical vapor deposition (CVD). Here, we introduce pulsed-laser deposition (PLD) to prepare multilayered WS2 films. Large-area WS2 films of the magnitude of cm(2) are achieved. Comparative measurements of a WS2-based photoresistor demonstrate its stable broadband photoresponse from 370 to 1064 nm, the broadest range demonstrated in WS2 photodetectors. Benefiting from the large optical absorbance (40%-85%) and high carrier mobility (31 cm(2) V(-1) s(-1)), the responsivity of the device approaches a high value of 0.51 A W(-1) in an ambient environment. Such a performance far surpasses the CVD-grown WS2-based photodetectors (μA W(-1)). In a vacuum environment, the responsivity is further enhanced to 0.70 A W(-1) along with an external quantum efficiency of 137% and a photodetectivity of 2.7 × 10(9) cm Hz(1/2) W(-1). These findings stress that the PLD-grown WS2 film may constitute a new paradigm for the next-generation stable, broadband and highly-responsive photodetectors.

The Enhanced Tribological Properties of NiAl Intermetallics: Combined Lubrication of Multilayer Graphene and WS2

In recent years, reducing friction and wear-related adverse impacts on efficiency and durability in moving mechanical systems has gained increased attention. Herein, the search continues for novel materials and lubricants that can potentially reduce friction and wear. As one of the emerging self-lubricating materials, the tribological potential of graphene has been researched deeply. This article was dedicated to explore the combined lubrication of multilayer graphene (MLG) and WS2. The as-prepared sample of NiAl–1.5 wt% MLG–5 wt% WS2 (NB) exhibited excellent tribological properties. During the sliding process, a continuous lubricating film was formed to provide the low-strength junctions at the interface, reducing the friction coefficient and wear rate. Moreover, the MLG played the role of reinforcement particles and improved loading carrying ability.

Novel and Enhanced Optoelectronic Performances of Multilayer MoS2–WS2 Heterostructure Transistors

Van der Waals heterostructures designed by assembling isolated two‐dimensional (2D) crystals have emerged as a new class of artificial materials with interesting and unusual physical properties. Here, the multilayer MoS2–WS2 heterostructures with different configurations are reported and their optoelectronic properties are studied. It is shown that the new heterostructured material possesses new functionalities and superior electrical and optoelectronic properties that far exceed the one for their constituents, MoS2 or WS2. The vertical transistor exhibits a novel rectifying and bipolar behavior, and can also act as photovoltaic cell and self‐driven photodetector with photo‐switching ratio exceeding 103. The planar device also exhibits high field‐effect ON/OFF ratio (>105), high electron mobility of 65 cm2/Vs, and high photo­responsivity of 1.42 A/W compared to that in isolated multilayer MoS2 or WS2 nanoflake transistors. The results suggest that formation of MoS2–WS2 heterostructures could significantly enhance the performance of optoelectronic devices, thus open up possibilities for future nanoelectronic, photovoltaic, and optoelectronic applications.

Hydrocracking of vacuum residue into lighter fuel oils using nanosheet-structured WS2 catalyst

In this study, we tried hydrocracking of vacuum residue into lighter liquid oils using dispersed colloidal catalysts composed of nanosheet-structured WS2 materials. The vacuum residue of API gravity=2.3° was used as a reactant and hydrocracking reactions were performed in an autoclave batch reactor under 400°C and the initial H2 pressure of 70bar. Both single and multi-layer WS2 nanosheet catalysts were tested and their activities were compared with those of bulk WS2 and MoS2 catalysts. The single-layer WS2, which was the highest in specific surface area (97.6m2/g) due to its smallest particle size, showed the best performances in commercial fuel fraction yield (45.4wt.%), C5-asphaltene conversion (75.3%), API gravity of liquid product (13.8°), and metal removal activity. To characterize the physicochemical properties of catalyst, various characterization techniques were applied, including transmission electron microscope (TEM), X-ray diffraction (XRD) and Brunauer–Emmett–Teller (BET) analysis. In addition, to assess the qualities of hydrocracking products, we carried out API gravity measurement, inductively coupled plasma-mass spectrometry (ICP-MS), and simulated distillation (SIMDIS) analysis.

Photoresponsive and gas sensing field-effect transistors based on multilayer WS₂ nanoflakes.

The photoelectrical properties of multilayer WS2 nanoflakes including field-effect, photosensitive and gas sensing are comprehensively and systematically studied. The transistors perform an n-type behavior with electron mobility of 12 cm2/Vs and exhibit high photosensitive characteristics with response time (τ) of <20 ms, photo-responsivity (Rλ) of 5.7 A/W and external quantum efficiency (EQE) of 1118%. In addition, charge transfer can appear between the multilayer WS2 nanoflakes and the physical-adsorbed gas molecules, greatly influencing the photoelectrical properties of our devices. The ethanol and NH3 molecules can serve as electron donors to enhance the Rλ and EQE significantly. Under the NH3 atmosphere, the maximum Rλ and EQE can even reach 884 A/W and 1.7 × 105%, respectively. This work demonstrates that multilayer WS2 nanoflakes possess important potential for applications in field-effect transistors, highly sensitive photodetectors, and gas sensors, and it will open new way to develop two-dimensional (2D) WS2-based optoelectronics.

Edge-states ferromagnetism of WS2 nanosheets

The multilayer WS2 nanosheets prepared from WO3 nanowires exhibit strong ferromagnetic behavior with saturation magnetization (MS) of 0.0058 emu/g and coercive field (HC) of 92 Oe at room temperature. By decreasing the temperature down to 3 K the Hc is increased up to 1115 Oe, revealing the existence of long-range magnetic ordering. Density functional theory spin-polarized calculations predict that strong ferromagnetic moments in WS2 nanosheets are attributed to the zigzag edge sulphur S and tungsten W atoms. Our findings also suggest that the WS2 nanosheets with a high density of edge spins could be used to fabricate spintronics devices, which are circuits utilizing the spin of the electron to process and store information.

High-performance photocurrent generation from two-dimensional WS2 field-effect transistors

The generation of a photocurrent from two-dimensional tungsten disulfide (WS2) field-effect transistors is examined here, and its dependence on the photon energy is characterized. We found from the WS2 devices that a significant enhancement in the ratio of illuminated current against dark current (Iillum/Idark) of ∼102–103 is attained, even with the application of electric fields of ED = 0.02 and EG = −22 mV/nm, which are much smaller than that of the bulk MoS2 phototransistor. Most importantly, we demonstrate that our multilayer WS2 shows an extremely high external quantum efficiency of ∼7000%, even with the smallest electrical field applied. We also found that photons with an energy near the direct band gap of the bulk WS2, in the range of 1.9–2.34 eV, give rise to a photoresponsivity of ∼0.27 A/W, which exceeds the photoresponsivity of the bulk MoS2 phototransistor. The superior photosensing properties of WS2 demonstrated in this work are expected to be utilized in the development of future high performance two-dimensional optoelectronic devices.

CVD-grown monolayered MoS2 as an effective photosensor operating at low-voltage

We report the fabrication of a photosensor based on as-grown single crystal monolayers of MoS2 synthesized by chemical vapor deposition (CVD). The measurements were performed using Au/Ti leads in a two terminal configuration on CVD-grown MoS2 on a SiO2/Si substrate. The device was operated in air at room temperature at low bias voltages ranging from −2 V to 2 V and its sensing capabilities were tested for two different excitation wavelengths (514.5 nm and 488 nm). The responsivity reached 1.1 mA W−1 when excited with a 514.5 nm laser at a bias of 1.5 V. This responsivity is one order of magnitude larger than that reported from photo devices fabricated using CVD-grown multilayered WS2. A rectifying-effect was observed for the optically excited current, which was four times larger in the direct polarization bias when compared to the reverse bias photocurrent. Such rectifying behavior can be attributed to the asymmetric electrode placement on the triangular MoS2 monocrystal. It is envisioned that these components could eventually be used as efficient and low cost photosensors based on CVD-grown transition metal dichalcogenide monolayers.

Theoretical aspects of WS₂ nanotube chemical unzipping.

Theoretical analysis of experimental data on unzipping multilayered WS₂ nanotubes by consequent intercalation of lithium atoms and 1-octanethiol molecules [C. Nethravathi, et al., ACS Nano, 2013, 7, 7311] is presented. The radial expansion of the tube was described using continuum thin-walled cylinder approximation with parameters evaluated from ab initio calculations. Assuming that the attractive driving force of the 1-octanethiol molecule is its reaction with the intercalated Li ions ab initio calculations of a 1-octanethiol molecule bonding with Li(+) were carried out. In addition, the non-chemical interactions of the 1-octanethiol dipole with an array of positive point charges representing Li(+) were taken into account. Comparing between the energy gain from these interactions and the elastic strain energy of the nanotube allows us to evaluate a value for the tube wall deformation after the implantation of 1-octanethiol molecules. The ab initio molecular dynamics simulation confirmed our estimates and demonstrated that a strained WS₂ nanotube, with a decent concentration of 1-octanethiol molecules, should indeed be unzipped into the WS₂ nanoribbon.

Effect of morphology properties of NiW catalysts on hydrodesulfurization for individual sulfur compounds in fluid catalytic cracking diesel

Three catalysts NiW/Al2O3, NiW–P/Al2O3 and NiW–CA/Al2O3 with different morphologies but similar pore structures and acidity properties were prepared for studying the effect of morphology properties on the hydrodesulfurization (HDS) activity for individual sulfur compounds in fluid catalytic cracking (FCC) diesel. The morphology properties of the selected catalysts in oxidation and sulfidation states were characterized and the contents of individual sulfur compounds were determined by gas chromatography–pulse flame photometric detector (GC–PFPD). The results showed that NiW–P/Al2O3 possessed the highest edge density of WS2 phase and NiW–CA/Al2O3 had the highest stacking degree of multilayered WS2 phase. The HDS assessment showed that NiW–CA/Al2O3 demonstrated positive effects on the HDS of refractory sulfur compounds like 4,6-dimethyldibenzothiophene (4,6-DMDBT) or dibenzothiophene (DBT) with alkyl substitutes containing 3 carbons (C3DBT) but slight negative effects on the HDS of simple sulfur compounds such as benzothiophene (BT) with alkyl substitutes containing 1~5 carbons (C1~C5BT) and DBT with alkyl substitutes containing 1 carbon (C1DBT); while NiW–P/Al2O3 favored for the HDS of simple sulfur compounds. The present results indicated that the morphology of WS2 phase has a great influence on the HDS selectivity for different structured sulfur compounds.