WS2 Crystals Research Articles

Liquid Salt Transport Growth of Single Crystals of the Layered Dichalcogenides MoS2 and WS2

The growth of single crystals of MoS2 and WS2 by materials transport through a liquid salt flux made from a low-melting mixture of NaCl and CsCl is presented. The crystals are structurally characte...

Atomically Sharp Dual Grain Boundaries in 2D WS2 Bilayers.

It is shown that tilt grain boundaries (GBs) in bilayer 2D crystals of the transition metal dichalcogenide WS2 can be atomically sharp, where top and bottom layer GBs are located within sub-nanometer distances of each other. This expands the current knowledge of GBs in 2D bilayer crystals, beyond the established large overlapping GB types typically formed in chemical vapor deposition growth, to now include atomically sharp dual bilayer GBs. By using atomic-resolution annular dark-field scanning transmission electron microscopy (ADF-STEM) imaging, different atomic structures in the dual GBs are distinguished considering bilayers with a 3R (AB stacking)/2H (AA' stacking) interface as well as bilayers with 2H/2H boundaries. An in situ heating holder is used in ADF-STEM and the GBs are stable to at least 800 °C, with negligible thermally induced reconstructions observed. Normal dislocation cores are seen in one WS2 layer, but the second WS2 layer has different dislocation structures not seen in freestanding monolayers, which have metal-rich clusters to accommodate the stacking mismatch of the 2H:3R interface. These results reveal the competition between maintaining van der Waals bilayer stacking uniformity and dislocation cores required to stitch tilted bilayer GBs together.

Fabrication of a WS2/p-Si Heterostructure Photocathode Using Direct Hybrid Thermolysis.

P-N heterostructures based on transition-metal dichelcongenides (TMDs) and a conventional semiconductor, such as p-Si, have been considered a promising structure for next-generation electronic devices and applications. However, synthesis of high-quality, wafer-scale TMDs, particularly WS2 on p-Si, is challenging. Herein, we propose an efficient method to directly grow WS2 crystals on p-Si via a hybrid thermolysis process. The WO3 is deposited to prepare the p-Si surface for coating of the (NH4)2WS4 precursor and converted to WS2/p-Si during thermolysis. Moreover, the WS2/p-Si heterojunction photocathode is fabricated and used in solar hydrogen production. The fabricated n-WS2/p-Si heterojunction provided an onset potential of +0.022 V at 10 mA/cm2 and a benchmark current density of -9.8 ± 1.2 mA/cm2 at 0 V. This method reliably and efficiently produced high-quality, wafer-scale WS2 crystals and overcame the challenges associated with previous approaches. The approach developed in this research demonstrates a magnificent progress in the fabrication of 2D material-based electronic devices.

Effect of layer and stacking sequence in simultaneously grown 2H and 3R WS2 atomic layers

In two-dimensional layered materials, layer number and stacking order have strong effects on the optical and electronic properties. Tungsten disulfide (WS2) crystal, as one important member among transition metal dichalcogenides, has been usually prepared in a layered 2H prototype structure with space group P63/mmc () in spite of many other expected ones such as 3R. Here, we report simultaneous growth of 2H and 3R stacked multilayer (ML) WS2 crystals in large scale by chemical vapor deposition and effects of layer number and stacking order on optical and electronic properties. As revealed in Raman and photoluminescence (PL) measurements, with an increase in layer number, 2H and 3R stacked ML WS2 crystals show similar variation of PL and Raman peaks in position and intensity. Compared to 2H stacked ML WS2, however, 3R stacked one always exhibits the larger red (blue) shift of Raman (A1g) peak and the appearance of PL A, B and I peaks at lower energies. Thereby, PL and Raman features depend on not only layer number but also stacking order. In addition, circularly polarized luminescence from two prototype WS2 crystals under circularly polarized excitation has also been investigated, showing obvious spin or valley polarization of these CVD-grown multilayer WS2 crystals.

Highly ordered arrays and characterization of WS2 flakes grown by low pressure chemical vapour deposition

Highly ordered arrays of WS2 crystals are grown on the SiO2/Si substrate using Pt/Ti metal as seed materials by a low pressure chemical vapor deposition method. High magnification SEM images and Raman spectrum reveal that the WS2 flakes are induced and grown around the Pt/Ti metal, and the diameter of a single unit is up to 100 μm, which have contribute to the direct construction of functional devices on the substrate. HRTEM measurements further confirm that the obtained flakes are the WS2 crystal with good crystallinity. The results show that highly ordered arrays of WS2 crystals will have potential applications in electronic and optoelectronic devices.

Chemical vapor deposition of monolayer-thin WS2 crystals from the WF6 and H2S precursors at low deposition temperature.

Monolayer-thin WS2 with (0002) texture grows by chemical vapor deposition (CVD) from gas-phase precursors WF6 and H2S at a deposition temperature of 450 °C on 300 mm Si wafers covered with an amorphous Al2O3 starting surface. We investigate the growth and nucleation mechanism during the CVD process by analyzing the morphology of the WS2 crystals. The CVD process consists of two distinct growth regimes. During (i) the initial growth regime, a fast and self-limiting reaction of the CVD precursors with the Al2O3 starting surface forms predominantly monolayer-thin WS2 crystals and AlF3 crystals that completely cover the starting surface. During (ii) the steady-state growth regime, a much slower, anisotropic reaction on the bottom, first WS2 layer proceeds with the next WS2 layer growing preferentially in the lateral dimensions. We propose that the precursor adsorption reaction rate strongly diminishes when the precursors have no more access to the Al2O3 surface as soon as the WS2 layer completely covers the Al2O3 surface and that the WS2 crystal basal planes and AlF3 crystals have a low reactivity for WF6 adsorption at 450 °C. Nonetheless, a second layer of WS2 starts to form before the first WS2 layer completely covers the starting surface, albeit the surface coverage of the second layer is low (<20%, after 25 min of CVD reaction). During the steady-state growth regime, predominantly the WS2 crystals in the second monolayer continue to grow in lateral dimensions up to ∼40 nm. These crystals reach larger lateral dimensions compared to the crystals in the bottom, first layer due to low reactivity for WF6 adsorption on the WS2 basal plane compared to Al2O3. Presumably, they grow laterally by precursor species that adsorb on and diffuse across the WS2 surface, before being incorporated at the more reactive edges of the WS2 crystals in the second layer. Such a process proceeds slowly with only up to 40% surface coverage of the second WS2 layer after 150 min of CVD reaction. The CVD reaction is mediated by the starting surface: WF6 precursor preferentially adsorbs on Al2O3, whereas adsorption is not observed on SiO2. Nevertheless, WS2 grows on SiO2 in close proximity to Al2O3 in 90 nm pitch Al2O3/SiO2 line patterns. Hence, functionalization of the starting surface (e.g., SiO2 with Al2O3) can provide opportunities to grow monolayer-thin WS2 crystals at predetermined locations by selective, lateral growth with tunable crystal size, even at low deposition temperatures.

Reversible electron doping in monolayer WS2 via a chemical strategy

Controlled doping, as a cornerstone of the semiconductor industry, becomes one of the most important topics for two-dimensional (2D) semiconductors such as tungsten disulfide (WS2), with intriguing physical properties. Here, we present a facile, controllable and reversible strategy for surface charge transfer doping in the monolayer WS2 crystals. After triethylamine treatment, the field effect transistors (FETs) based on monolayer WS2 exhibit enhanced mobility up to 28.6 cm2 V−1 s−1, which is much larger than that of the pristine one (9.8 cm2 V−1 s−1). In addition, the n-doping via triethylamine treatment can also improve the photoresponsivity of the device, i.e. from pristine 6.4 × 10−3 AW−1 to the doped 21 AW−1. Interestingly, the electron transport properties of the doped WS2 can be recovered to be close to that of the intrinsic WS2 via acetone immersion. Moreover, the electron doping and reversibility are evidently observed in Raman, photoluminscence, and x-ray photoelectron spectroscopy (XPS). The reversibility of electrical and optical modification holds potential for functional diversification of 2D devices.

An approach to high-throughput growth of submillimeter transition metal dichalcogenide single crystals.

High-throughput growth of large size transition metal dichalcogenide (TMD) single crystals is an important challenge for their applications in the next generation electronic and optoelectronic integration devices. Here we report the high-throughput growth of submillimeter monolayer TMD single crystals by two-stage space confined chemical vapor deposition, where the nucleation density of TMD crystals is significantly decreased for the growth of large size monolayer crystals by the space confinement effect. Moreover, high-throughput growth of submillimeter TMD crystals is also achieved by stacking the substrates along the perpendicular direction to the flow of the reaction gases. The mobilities of the TMD materials produced in this way are up to 1.2, 17.0 and 25.0 cm2 (V s)-1 for monolayer WS2, WSe2 and MoS2 single crystals, respectively. The results demonstrate that two-stage space confined growth is a highly promising method for high-throughput fabrication of high-quality submillimeter monolayer TMD single crystals, which will pave a new pathway to large-scale production of TMD-based electronic and optoelectronic devices.

Growth of Monolayer WS2 Single Crystals with Atmospheric Pressure CVD: Role of Temperature

ABTRACTIt has been demonstrated that the introduction of NaCl can significantly improve the quality of monolayer WS2 at the growth temperatures ranging from 700°C to 850°C by atmospheric pressure chemical vapor deposition (APCVD) without the assistant of hydrogen. Here, the influence of NaCl on the nucleation and growth of WS2 has been thoroughly investigated. The morphology and quality of WS2 grown with different temperatures are discussed by optical microscope, Raman and Photoluminescence (PL) spectra. It was found that amount of NaCl can efficiently influence the morphology and quality of WS2 crystals. PL intensity of WS2 crystal increases around three times from the center region to the edge of an individual domain, which may be attributed to the appearance of small triangle hollows formed during the growth at the edge of single crystal WS2.

Tribological behavior of WS2/oil-impregnated porous polyimide solid/liquid composite system

PurposeThis paper aims to study the tribological behavior of the WS2/oil-impregnated porous polyimide (PPI) solid/liquid composite system, in which both PFPE (perfluoropolyether) and SiCH (silahydrocarbons) oils with different hydrocarbon chains were used, respectively. Lubricating mechanism of the composite system was also explored.Design/methodology/approachThe tribological behaviors of the WS2 films against the PPI cylindrical pins before and after immersing oil were evaluated under different loads by a reciprocating-type ball-on-disc tribometer.FindingsThe composite system exhibited the low and stable friction coefficient after the running-in stage, and the lubricant oil played a positive effect. It was found that the WS2/PFPE composite system exhibited more excellent lubricating property, although sole SiCH far exceeds PFPE in lubrication. The abnormal phenomenon mainly resulted from the influence of the oil amount. XRD results on the wear track surfaces indicated that PFPE and SiCH oils with different hydrocarbon chains were likely to preferentially adsorb to the edge plane and basal plane of the WS2 crystals, respectively.Originality/valueIn previous studies, liquid lubricants were directly dripped or spin-coated on the solid lubricant surface. Based on its potential advantage in application, the tribological behavior and mechanism of the solid lubricating film/oil-impregnated PPI composite system were investigated in this study.

Vacuum Tribological Performance of WS2–MoS2 Composite Film Against Oil-Impregnated Porous Polyimide: Influence of Oil Viscosity

To achieve the desired levels of performance and durability for bearings applied in space, a novel solid–liquid dual lubricating system was established via the WS2–MoS2 composite films sliding against oil-impregnated porous polyimide (PPI), in which polyalphaolefin (PAO) oils with different viscosities were used, respectively. The contact angle measurement indicated that the PAO oil had good wettability on the surfaces of the composite film and the PPI. The vacuum tribological behaviors of single WS2–MoS2 composite film and the dual lubricating system were mainly evaluated under different loads and speeds. In comparison with single WS2–MoS2 composite film, the dual lubricating system exhibited the low friction coefficient and synergistic lubricating effect for the low-viscosity PAO oil under severe sliding conditions. It was concluded that the cleavages of the WS2 and MoS2 crystals along the basal plane were more sufficient and formed small and thin WS2 and MoS2 platelets in the dual lubricating system. Meanwhile, it was found that oil-impregnated PPI readily released PAO lubricating oil with low viscosity, which further decreased friction on the contact interface. The lubricating mechanism of the dual lubricating system was also revealed after correlating the tribological behaviors of the different lubricating systems.

Surface spin accumulation due to the inverse spin Hall effect in WS2 crystals

Applications of semiconductors in spintronics are being developed and revolutionized to fabricate nanostructures in which the spin current is injected, detected and manipulated. The manifestation of the spin Hall effect (SHE) in semiconductors with a simple geometry is still a challenge to implement in spintronics. In this work, we demonstrate the distinct electrical detection of the inverse spin Hall effect (ISHE) in WS2 multilayer crystals at temperatures ranging from 4.2 K to 300 K. The novel idea of this study is the electrical detection of the ISHE in WS2 using an in-plane spin-polarized current, which is realized using non-ferromagnetic electrodes on the top and bottom surfaces of the WS2 channel. The flow alteration of spin-polarized current results in a polarity change in the inverse spin Hall signal. By analyzing the magnitude of the ISHE, we estimated a spin diffusion length of ~250 nm, spin Hall angle of ~0.028, spin polarization of ~0.20 and spin lifetime of ~560 ps in WS2-layered crystals at room temperature. The ISHE in layered, 2D materials, including WS2 crystals, provides a new methodology to develop low-power and fast memory devices to write, read and store information.

Facile access to shape-controlled growth of WS2 monolayer via environment-friendly method

2D materials with tailored morphologies exhibit distinctive shape-dependent properties especially in electrocatalysis and gas sensing, extending their applications in nanoelectronics. Atomic-layer tungsten disulfide (WS2) stands out as a promising candidate, but the controllable synthesis of WS2 still faces unavoidable obstacles, such as strict parameter requirements, low efficiency and serious pollution during the preparation. Here we report an elegant technique for growing homogeneous-luminescence WS2 monolayer with desired shapes and developed electrochemical properties, through the improved chemical vapor deposition method with a semi-closed airflow environment. It is an effective, economic and especially environment-friendly approach with atmospheric pressure and hydrogen-free condition, aiming at synthesizing high-quality and large-scale monolayer WS2 crystals with only one heating zone. Compared with the existing methods, this one significantly minimizes the sulphur contamination to the environment and equipment, enabling the reuse of quartz tube and reducing the cost. More importantly, different shapes of WS2 domains with high crystalline and excellent homogeneity of luminescent intensity are controllable by adjusting the growth temperatures. In addition to the good electrical transport performance of WS2 monolayer, the WS2 domains with different morphologies exhibit modulated properties for catalyzing the electrochemical hydrogen evolution. This systematic study on shape-controlled growth and shape-dependent electrochemical performances of WS2 will further push forward their practical applications.

Two-Dimensional Crystal Grain Size Tuning in WS2 Atomic Layer Deposition: An Insight in the Nucleation Mechanism

When two-dimensional (2D) group-VI transition metal dichalcogenides such as tungsten disulfide (WS2) are grown by atomic layer deposition (ALD) for atomic growth control at low deposition temperatures (≤450 °C), they often suffer from a nanocrystalline grain structure limiting the carrier mobility. The crystallinity and monolayer thickness control during ALD of 2D materials is determined by the nucleation mechanism, which is currently not well understood. Here, we propose a qualitative model for the WS2 nucleation behavior on dielectric surfaces during plasma-enhanced (PE-) ALD using tungsten hexafluoride (WF6), dihydrogen (H2) plasma and dihydrogen sulfide (H2S) based on analyses of the morphology of the WS2 crystals. The WS2 crystal grain size increases from ∼20 to 200 nm by lowering the nucleation density. This is achieved by lowering the precursor adsorption rate on the starting surface using an inherently less reactive starting surface, by decreasing the H2 plasma reactivity, and by enhancing the mob...

Two-Dimensional MoxW1\u2212xS2 Graded Alloys: Growth and Optical Properties

Two-dimensional (2D) transition metal dichalcogenides can be alloyed by substitution at the metal atom site with negligible effect on lattice strain, but with significant influence on optical and electrical properties. In this work, we establish the relationship between composition and optical properties of the MoxW1−xS2 alloy by investigating the effect of continuously-varying composition on photoluminescence intensity. We developed a new process for growth of two-dimensional MoxW1−xS2 alloys that span nearly the full composition range along a single crystal, thus avoiding any sample-related heterogeneities. The graded alloy crystals were grown using a diffusion-based chemical vapor deposition (CVD) method that starts by synthesizing a WS2 crystal with a graded point defect distribution, followed by Mo alloying in the second stage. We show that point defects promote the diffusion and alloying, as confirmed by Raman and photoluminescence measurements, density functional theory calculations of the reaction path, and observation that no alloying occurs in CVD-treated exfoliated crystals with low defect density. We observe a significant dependence of the optical quantum yield as a function of the alloy composition reaching the maximum intensity for the equicompositional Mo0.5W0.5S2 alloy. Furthermore, we map the growth-induced strain distribution within the alloyed crystals to decouple composition and strain effects on optical properties: at the same composition, we observe significant decrease in quantum yield with induced strain. Our approach is generally applicable to other 2D materials as well as the optimization of other composition-dependent properties within a single crystal.

Exploring electric field assisted van der Waals weakening of stratified crystals

We report an enhancement in the efficiency of agitative liquid-phase exfoliation of stratified WS2 crystals under the influence of an electric field. We have been successfully able to increase the exfoliation efficiency of WS2 nanoflakes using a co-applied alternating electric field. Loss of centrosymmetry at the facial layers of WS2, together with the matching between the dispersive component of this material and solvent that weakens the van der Waals forces, augment the exfoliation process. However, this is not seen for MoS2 and graphene. The outcomes provide the base for future investigations on the influence of electric field for the exfoliation of layered structures.

Developing seedless growth of atomically thin semiconductor layers: Application to transition metal dichalcogenides

Controlled growth of atomic monolayers of IV-VII transition metal dichalcogenides (TMDs) has provided unprecedented opportunities to fabricate modern optoelectronic nanodevices. However, synthesis of large-area and high quality two-dimensional TMDs is still challenging. We have synthesized WS2 and MoS2 nanosheets by atmospheric pressure chemical vapor deposition (APCVD) at wide-range of processing conditions. The nanostructures were analyzed by optical and confocal microscopy, atomic force microscopy, Raman spectroscopy, and X-ray diffraction to determine the thickness, lateral size and structure of the deposits. Through designing and performing of a set of controlled experiments as well as comparing the attained results with others, we present a general roadmap for APCVD of atomically thin WS2 and MoS2 nanosheets. The appropriate working windows for the controlled synthesis are established. For the controlled growth of 2D WS2 crystals, relatively low growth temperature (750 °C) can be utilized. It is shown that an oxide mass to the gas flow rate of 0.004 favorably results in the formation of 2D WS2. Monolayers and few layers might be processed at higher temperatures (≥ 800 °C) at an oxide mass/gas flow rate ratio of 2–3. The seedless growth of MoS2 nanosheets is more complicated than WS2 possessing with narrower working window. A minimum temperature of 575 °C is required to provide enough oxide vapor pressure for deposition; otherwise, no deposits are formed. Above 650 °C, achieving 2D nanolayers is tricky due to massive deposition of the metal oxide from the vapor phase forming thick layers. In the intermediate range, however, there is a possibility to attain 2D nanostructures through fine controlling of the oxide mas/gas flow rate. Finally, it is shown that through precise control of the processing parameters, a controllable growth of the nanosheets without pre-seeding at lower temperatures and shorter times is feasible.

TMDC 2D Materials Synthesis via Two Steps Solution Process at Low Temperature

In nanoelectronics and optoelectronics, two-dimensional transition metal dichalcogenides (2D TMDC) analogous to graphene have drawn considerable attention due to their electrical and innate band gap characteristics. Among them, MoS2 and WS2 show unique properties; direct and indirect band gap of 1.8 eV and 1.2 eV, respectively, depending on the monolayer stacking states. Thin layered WS2 composed of a S-W-S sandwich crystalline with a van der Waals, possessed the coupled spin and valley physics, high photoluminescence efficiency, and composited layered materials with other 2D for many applications. Most studies on 2D TMDC have been focused on the chemical synthesis using chemical vapor deposition (CVD) and mechanical methods like exfoliated flakes by taping or in liquid. In contrast, large scale synthesis of WS2 via solution-process is rare to find. Here, we have prepared new synthetic route combining solution coating and CVD process together. The solution of sodium tungstate soulution and hydrizene hydrate with sodium thiosulphate was coated on the silicon wafer. The coated films from dip coating and spin coating were treated in the CVD with different paramerters to obtain WS2 crystal structures. The temperature and coating conditions were modified to obtain uniform wafer-scale films. The average of 50 um size WS2 crystals with triangle shapes could be obtained at the low temperatures of 700oC and above with sample positions in the CVD.

Reliable Synthesis of Large‐Area Monolayer WS2 Single Crystals, Films, and Heterostructures with Extraordinary Photoluminescence Induced by Water Intercalation

AbstractTwo‐dimensional transition metal dichalcogenides (TMDs) hold great potential for future low‐energy optoelectronics owing to their unique electronic, optical, and mechanical properties. Chemical vapor deposition (CVD) is the technique widely used for the synthesis of large‐area TMDs. However, due to high sensitivity to the growth environment, reliable synthesis of monolayer TMDs via CVD remains challenging. Here a controllable CVD process is developed for large‐area synthesis of monolayer WS2 crystals, films, and in‐plane graphene–WS2 heterostructures by cleaning the reaction tube with hydrochloric acid, sulfuric acid, and aqua regia. The concise cleaning process can remove the residual contaminates attached to the CVD reaction tube and crucibles, reducing the nucleation density but enhancing the diffusion length of WS2 species. The photoluminescence (PL) mappings of a WS2 single crystal and film reveal that the extraordinary PL around the edges of a triangular single crystal is induced by ambient water intercalation at the WS2–sapphire interface. The extraordinary PL can be controlled by the choice of substrates with different wettabilities.

Validation of inter-atomic potential for WS2 and WSe2 crystals through assessment of thermal transport properties

In recent years, transition metal dichalcogenides (TMDs) displaying astonishing properties are emerged as a new class of two-dimensional layered materials. The understanding and characterization of thermal transport in these materials are crucial for efficient engineering of 2D TMD materials for applications such as thermoelectric devices or overcoming general overheating issues. In this work, we obtain accurate Stillinger-Weber type empirical potential parameter sets for single-layer WS2 and WSe2 crystals by utilizing particle swarm optimization, a stochastic search algorithm. For both systems, our results are quite consistent with first-principles calculations in terms of bond distances, lattice parameters, elastic constants and vibrational properties. Using the generated potentials, we investigate the effect of temperature on phonon energies and phonon linewidth by employing spectral energy density analysis. We compare the calculated frequency shift with respect to temperature with corresponding experimental data, clearly demonstrating the accuracy of the generated inter-atomic potentials in this study. Also, we evaluate the lattice thermal conductivities of these materials by means of classical molecular dynamics simulations. The predicted thermal properties are in very good agreement with the ones calculated from first-principles.