Structure Of WS2 Research Articles

Few-layer thick WS2 nanosheets produced by intercalation/exfoliation route

Thin two-dimensional (2D) nanostructures of layered materials are of importance for both science and technology due to their unique properties and promising applications. Few-layer tungsten disulfide (WS2) presents some amazing optoelectronic behaviors that have attracted great interest. In this work, WS2 nanosheets with few-layer (<10 layers) thickness have been successfully produced by Li ions intercalation/exfoliation of bulk WS2 via a solvothermal technique. X-ray diffraction indicates that hexagonal crystal structure of bulk WS2 remains the same after the intercalation/exfoliation treatment. Scanning electron microscopy and transmission electron microscopy characterizations reveal that the exfoliated products are single-crystalline thin nanosheets with high crystallinity. Atomic force microscopy reveals that 8-layer thick WS2 nanosheets can be fabricated by the present approach. Raman phonon modes of the prepared WS2 nanosheets present the redshifts due to their decreased thickness. The work provides a facile and affective route to high-quality WS2 thin 2D nanostructures that would have great potentials for optoelectronics.

Direct optical transitions at K- and H-point of Brillouin zone in bulk MoS2, MoSe2, WS2, and WSe2

Modulated reflectance (contactless electroreflectance (CER), photoreflectance (PR), and piezoreflectance (PzR)) has been applied to study direct optical transitions in bulk MoS2, MoSe2, WS2, and WSe2. In order to interpret optical transitions observed in CER, PR, and PzR spectra, the electronic band structure for the four crystals has been calculated from the first principles within the density functional theory for various points of Brillouin zone including K and H points. It is clearly shown that the electronic band structure at H point of Brillouin zone is very symmetric and similar to the electronic band structure at K point, and therefore, direct optical transitions at H point should be expected in modulated reflectance spectra besides the direct optical transitions at the K point of Brillouin zone. This prediction is confirmed by experimental studies of the electronic band structure of MoS2, MoSe2, WS2, and WSe2 crystals by CER, PR, and PzR spectroscopy, i.e., techniques which are very sensitive to critical points of Brillouin zone. For the four crystals besides the A transition at K point, an AH transition at H point has been observed in CER, PR, and PzR spectra a few tens of meV above the A transition. The spectral difference between A and AH transition has been found to be in a very good agreement with theoretical predictions. The second transition at the H point of Brillouin zone (BH transition) overlaps spectrally with the B transition at K point because of small energy differences in the valence (conduction) band positions at H and K points. Therefore, an extra resonance which could be related to the BH transition is not resolved in modulated reflectance spectra at room temperature for the four crystals.

Enabling Quality Interfaces with Mask‐Free Approach to Selective Growth of MoS2/Graphene Stacked Structures

The mask-free patterning technique is employed to fabricate arrays of MoS2 and WS2 structures on silicon and graphene substrates with quality interfaces. By depositing precursor inks with the AFM cantilevers and subsequent heat treatment in the CVD furnace, it is demonstrated that MoS2 and WS2 structures can be formed on graphene surfaces at predefined device architectures. As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re-organized for online delivery, but are not copy-edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

Structure and Properties of Titanium Doped Tungsten Disulfide Thin Films Produced via the Magnetron Co-Sputtering DC Technique

In this work, titanium doped tungsten disulfide thin films (WS2-Ti) were deposited on AISI 304 stainlesssteel substrates using the magnetron co-sputtering DC technique. Films were produced with varying titaniumcathode power from 0 to 25 W. The titanium content of the WS2 structure was determined using energy dispersive spectroscopy (EDS), thereby obtaining a maximum of 10% for the sample grown at 25 W. X-ray diffraction (XRD) results indicated, for the pure sample, the presence of a hexagonal phase of high intensity on the (103) plane. However, with the addition of titanium, the WS2 crystallinity was reduced; nevertheless, when the titanium content was equal to 10%, nanocomposite formation was promoted. This effect was observed in TEM (transmission electron microscopy) images. Finally, the tribological behavior was determined using a ball on disk (BOD) test. With this method, friction coefficient curves were obtained with respect to the number of cycles, allowing the measurement of friction coefficient values of 0.1 for the pure sample and 0.15 for the sample grown at 5 W. Because of the low thickness of the samples, all of them failed during the first cycles.

Pressure coefficients for direct optical transitions in MoS2, MoSe2, WS2, and WSe2 crystals and semiconductor to metal transitions.

The electronic band structure of MoS2, MoSe2, WS2, and WSe2, crystals has been studied at various hydrostatic pressures experimentally by photoreflectance (PR) spectroscopy and theoretically within the density functional theory (DFT). In the PR spectra direct optical transitions (A and B) have been clearly observed and pressure coefficients have been determined for these transitions to be: αA = 2.0 ± 0.1 and αB = 3.6 ± 0.1 meV/kbar for MoS2, αA = 2.3 ± 0.1 and αB = 4.0 ± 0.1 meV/kbar for MoSe2, αA = 2.6 ± 0.1 and αB = 4.1 ± 0.1 meV/kbar for WS2, αA = 3.4 ± 0.1 and αB = 5.0 ± 0.5 meV/kbar for WSe2. It has been found that these coefficients are in an excellent agreement with theoretical predictions. In addition, a comparative study of different computational DFT approaches has been performed and analyzed. For indirect gap the pressure coefficient have been determined theoretically to be −7.9, −5.51, −6.11, and −3.79, meV/kbar for MoS2, MoSe2, WS2, and WSe2, respectively. The negative values of this coefficients imply a narrowing of the fundamental band gap with the increase in hydrostatic pressure and a semiconductor to metal transition for MoS2, MoSe2, WS2, and WSe2, crystals at around 140, 180, 190, and 240 kbar, respectively.

Density functional theory based study of chlorine doped WS2-metal interface

Investigation of a transition metal dichalcogenide (TMD)-metal interface is essential for the effective functioning of monolayer TMD based field effect transistors. In this work, we employ the Density Functional Theory calculations to analyze the modulation of the electronic structure of monolayer WS2 with chlorine doping and the relative changes in the contact properties when interfaced with gold and palladium. We initially examine the atomic and electronic structures of pure and doped monolayer WS2 supercell and explore the formation of midgap states with band splitting near the conduction band edge. Further, we analyze the contact nature of the pure supercell with Au and Pd. We find that while Au is physiosorbed and forms n-type contact, Pd is chemisorped and forms p-type contact with a higher valence electron density. Next, we study the interface formed between the Cl-doped supercell and metals and observe a reduction in the Schottky barrier height (SBH) in comparison to the pure supercell. This reduction found is higher for Pd in comparison to Au, which is further validated by examining the charge transfer occurring at the interface. Our study confirms that Cl doping is an efficient mechanism to reduce the n-SBH for both Au and Pd, which form different types of contact with WS2.

Shear-Induced Isostructural Phase Transition and Metallization of Layered Tungsten Disulfide under Nonhydrostatic Compression

Pressure-induced structural and electronic transformations of tungsten disulfide (WS2) have been studied to 60 GPa, in both hydrostatic and nonhydrostatic conditions, using four-probe electrical resistance measurements, micro-Raman spectroscopy, and synchrotron X-ray diffraction. The results show the evidence for an isostructural phase transition from hexagonal 2Hc phase to hexagonal 2Ha phase, which accompanies the metallization at ∼37 GPa. This isostructural transition occurs displacively over a large pressure range between 15 and 45 GPa and is driven by the presence of strong shear stress developed in the layer structure of WS2 under nonhydrostatic compression. Interestingly, this transition is absent in hydrostatic conditions using He pressure medium, underscoring its strong dependence on the state of stress. We attribute the absence to the incorporation of He atoms between the layers, mitigating the development of shear stress. We also conjecture a possibility of magnetic ordering in WS2 that may occ...

Mixed multilayered vertical heterostructures utilizing strained monolayer WS2.

Creating alternating layers of 2D materials forms vertical heterostructures with diverse electronic and opto-electronic properties. Monolayer WS2 grown by chemical vapour deposition can have inherent strain due to interactions with the substrate. The strain modifies the band structure and properties of monolayer WS2 and can be exploited in a wide range of applications. We demonstrate a non-aqueous transfer method for creating vertical stacks of mixed 2D layers containing a strained monolayer of WS2, with Boron Nitride and Graphene. The 2D materials are all grown by CVD, enabling large area vertical heterostructures to be formed. WS2 monolayers grown by CVD directly on Si substrates with SiO2 surface are easily washed off by water and this makes aqueous based transfer methods challenging for creating vertical stacks on the growth substrate. 2D hexagonal Boron Nitride films are used to provide an insulating layer that limits interactions with a top graphene layer and preserve the strong photoluminescence from the WS2. This transfer method is suitable for layer by layer control of 2D material vertical stacks and is shown to be possible for all CVD grown samples, which opens up pathways for the rapid large scale fabrication of vertical heterostructure systems with atomic thickness depth control and large area coverage.

Preparation and improved photocatalytic activity of mesoporous WS2 using combined hydrothermal-evaporation induced self-assembly method

In this paper, we report mesoporous WS2 nanosheets with a crystalline network that were synthesized using CTAB as a structure-directing agent via self-assembly induced by hydrothermal and thermal evaporation. Powder X-ray diffraction, Raman spectra, and high-resolution X-ray photoelectron spectroscopy results confirmed the formation of WS2 structures. Scanning electron microscopy and transmission electron microscopy were used to observe the as-prepared mesoporous frameworks. The mesoporous WS2 nanosheets have a surface area of 197m2g−1. A possible growth mechanism is reported for these mesoporous WS2 nanosheets. The mesoporous WS2 nanosheets demonstrate high photocatalytic activity. Among different concentrations, 0.1wt% mesoporous WS2 shows superior catalytic activity compared to pristine WS2 nanosheets.

Origin of Hybrid 1T- and 2H-WS2 Ultrathin Layers by Pulsed Laser Deposition

Partial 2H-to-1T phase transition in two-dimensional (2D) WS2 grown by pulsed laser deposition is discovered through X-ray photoelectron spectroscopy. The resulting hybrid structure is observed only on substrates of the noble metals Ag and Au, whereas other metals such as Ni and Al produced only 2H-WS2. The origin of the 1T-phase is attributed to electron doping of the WS2 lattice by Ag or Au atoms, in a mechanism reminiscent of Li in the intercalation of WS2 and the related MoS2 structure. The doping process is viable as both Ag and Au atoms are able to donate one electron each, leading to the formation of a stable d10 shell and a phase transformation in the WS2 structure from 2H-to-1T. In the case of Ni, however, the formation of nickel oxides at the metal surface inhibits its ability to donate electrons to the WS2 lattice, whereas Al is unable to act as electron donor for WS2. Thus, the 1T-phase is absent in either of these substrates.

Carbon and Tungsten Disulfide Nanotubes and Fullerene-like Nanostructures in Thermoset Adhesives: A Critical Review

Thermoset adhesives such as epoxy exhibit high stiffness, strength and adhesion properties, but relatively poor toughness, elongation and conductivity. Nanofillers such as carbon (CNTs) and tungsten disulfide (WS2) nanotubes (INTs) and fullerene-like (IF) nanoparticles (NPs) possess excellent mechanical properties and also high electrical (thermal) conductivity. The quality of dispersion of the nanofillers in the thermoset adhesives has a major effect on the final properties of the nanocomposite adhesive. Dispersion techniques such as solution mixing, ultrasonication, three-roll milling, etc. have been used. Significant enhancements in stiffness, tensile strength, shear and peel strengths, fracture toughness, glass transition temperature, CLTE (coefficient of linear thermal expansion) among others, can be achieved at low CNTs concentrations (less than 1 wt.%), while at higher content of the CNTs, agglomeration reduces the toughness. Also for the WS2 nanoparticles, the lower concentrations (up to 1 wt.%) are preferable in order to improve the mechanical performance of the epoxy-based composites. Enhanced dispersion and interfacial interactions, resulting in improved mechanical performance of the nanocomposites containing CNTs, could be achieved by covalent and noncovalent modifications. The dispersion of WS2 nanostructures in the polymer medium was obtained using simple techniques like shear mixing and sonication without surface modifications. Moreover, it was found that the IF-NPs could covalently interact with the epoxy, without any surface modifications. The toughening mechanism of polymer matrices by the tubular nanofillers is mainly attributed to pull-out and crack bridging of the nanotubes. At the same time, inorganic nanocomposite adhesives exhibit promising results. IF structures of WS2 were found to exhibit crack deflection and bowing mechanisms. Electrically and thermally conductive adhesives have been obtained by adding CNTs, whereas conductive nanocomposites containing INT-WS2 have not been reported. In the case of CNTs the percolation path could be realized already at low levels (0.25 -1 wt.%). Furthermore, as the CNTs content increases, so does the conductivity of the composite material. In addition, the thermal conductivity of CNTs was found to have insignificant effect on the thermal conductivity of the nanoadhesives. Furthermore, the electrical and thermal conductive properties of most surface treated and functionalized CNTs were found to decrease, compared to untreated CNTs. The resulting nanocomposite adhesives containing carbon and tungsten disulfide nanotubes and fullerenes exhibit promising results in various applications.

Theoretical study of the interaction of electron donor and acceptor molecules with monolayer WS2

With the aim of understanding recent experimental data concerning molecular doping in WS2-based FET gas sensors, we have investigated the interaction of NH3 and H2O molecules with monolayer WS2, by means of first-principles calculations. The structural relaxations and total energy calculations are performed to determine the preferential binding configurations and it is found that both NH3 and H2O molecules are physisorbed on monolayer WS2. The Bader analysis combined with the plane-averaged differential charge density results indicate that NH3 acts as the electron donor, while H2O acts as the electron acceptor, leading to n- and p-type doping of WS2, respectively. The charge transfer mechanism is discussed in light of the mixing of the molecular highest occupied molecular orbital and lowest unoccupied molecular orbital with the underlying WS2 orbitals. In addition, the modification of the work function is found to be almost linearly dependent on the total charge transfer. The modification of the work function and the carrier concentration can be obtained by tuning the molecule coverages, without destroying the band structure of monolayer WS2. The electrical sensitivities to the gas adsorption make WS2 a gas sensor that promises wide-ranging applications.

Mechanism of charge transfer and its impacts on Fermi-level pinning for gas molecules adsorbed on monolayer WS2.

Density functional theory calculations were performed to assess changes in the geometric and electronic structures of monolayer WS2 upon adsorption of various gas molecules (H2, O2, H2O, NH3, NO, NO2, and CO). The most stable configuration of the adsorbed molecules, the adsorption energy, and the degree of charge transfer between adsorbate and substrate were determined. All evaluated molecules were physisorbed on monolayer WS2 with a low degree of charge transfer and accept charge from the monolayer, except for NH3, which is a charge donor. Band structure calculations showed that the valence and conduction bands of monolayer WS2 are not significantly altered upon adsorption of H2, H2O, NH3, and CO, whereas the lowest unoccupied molecular orbitals of O2, NO, and NO2 are pinned around the Fermi-level when these molecules are adsorbed on monolayer WS2. The phenomenon of Fermi-level pinning was discussed in light of the traditional and orbital mixing charge transfer theories. The impacts of the charge transfer mechanism on Fermi-level pinning were confirmed for the gas molecules adsorbed on monolayer WS2. The proposed mechanism governing Fermi-level pinning is applicable to the systems of adsorbates on recently developed two-dimensional materials, such as graphene and transition metal dichalcogenides.

New Strategy for the Growth of Complex Heterostructures Based on Different 2D Materials

Tungsten disulfide (WS2) monolayers have been synthesized under ultra high vacuum (UHV) conditions on quasi-free-standing hexagonal boron nitride (h-BN) and gold deposited on Ni(111). We find that the synthesis temperature control can be used to tune the WS2 structure. As documented by in situ core level and valence band photoemission and by ex situ Raman spectroscopy, the partially disordered WS2 layer obtained at room temperature transforms to the 2H-WS2 phase at about 400 °C. Low energy electron diffraction confirms the existence of van der Waals epitaxy between WS2 and h-BN and gold substrates. The measured band structure indicates that the WS2 electronic structure is not affected by the interaction with the h-BN and gold substrates.

Fabrication of novel heterostructured few layered WS2-Bi2WO6/Bi3.84W0.16O6.24 composites with enhanced photocatalytic performance

Novel heterostructured composite of few layered WS2-Bi2WO6/Bi3.84W0.16O6.24 has been synthesized via a facile hydrothermal method for the first time. The obtained catalysts were systematically characterized by X-ray diffraction (XRD), SEM, TEM, EDS, Raman spectroscopy, atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS) to certify the existence of WS2 with a few-layered structure among the heterostructured composite, and to illuminate the relationship of its performance and the structural features. Among the as-prepared catalysts, the few layered WS2-Bi2WO6/Bi3.84W0.16O6.24 composite show the best photocatalytic degradation of RhB under visible light irradiation. The enhanced photocatalytic performance of the heterostructured few layered WS2-Bi2WO6/Bi3.84W0.16O6.24 composite is attributed to the two-dimensional structure of WS2 with special properties and the matching energy band structure for efficient separation of photogenerated electron–hole pairs. The formation mechanism and photocatalytic mechanism were proposed. The present work firstly combined layered TMDs with composites to prepare ternary heterostructured catalysts, which provide new ideas for obtaining novel layered TMDs-based composites with excellent performance.

The electronic properties tuned by the phase transition between the semiconducting and metallic phase of monolayer MoS2/WS2

Using first-principles methods, we systematically investigate the electronic properties and atomic mechanism of the monolayer MoS2/WS2 homo-junction structure, which contains different phase structures, either the semiconducting hexagonal (H) structure or metallic trigonal (T) structure. Through tuning the size of the lateral homo-junction structure of either MoS2 or WS2, it can produce different boundaries which induce different phase transferred styles. More interestingly, the electronic structures of homo-junction structures can also be tuned by changing the size of the armchair and zigzag shapes of nanoribbons. The homo-junction structure of either MoS2 or WS2 exhibits alterable band structure and band edge position with the changing of the size. The strong dependence of the band offset on the sizes of the homo-junction monolayer also implicates a possible way of patterning quantum structures with size engineering.

Unveiling surface redox charge storage of interacting two-dimensional heteronanosheets in hierarchical architectures.

Two-dimensional (2D) heteronanosheets are currently the focus of intense study due to the unique properties that emerge from the interplay between two low-dimensional nanomaterials with different properties. However, the properties and new phenomena based on the two 2D heteronanosheets interacting in a 3D hierarchical architecture have yet to be explored. Here, we unveil the surface redox charge storage mechanism of surface-exposed WS2 nanosheets assembled in a 3D hierarchical heterostructure using in situ synchrotron X-ray absorption and Raman spectroscopic methods. The surface dominating redox charge storage of WS2 is manifested in a highly reversible and ultrafast capacitive fashion due to the interaction of heteronanosheets and the 3D connectivity of the hierarchical structure. In contrast, compositionally identical 2D WS2 structures fail to provide a fast and high capacitance with different modes of lattice vibration. The distinctive surface capacitive behavior of 3D hierarchically structured heteronanosheets is associated with rapid proton accommodation into the in-plane W-S lattice (with the softening of the E2g bands), the reversible redox transition of the surface-exposed intralayers residing in the electrochemically active 1T phase of WS2 (with the reversible change in the interatomic distance and peak intensity of W-W bonds), and the change in the oxidation state during the proton insertion/deinsertion process. This proposed mechanism agrees with the dramatic improvement in the capacitive performance of the two heteronanosheets coupled in the hierarchical structure.

Band structure and spin–orbit coupling engineering in transition‐metal dichalcogenides

Band structure and spin–orbit coupling engineering in transition‐metal dichalcogenides

Hydrodeoxygenation of p-cresol on unsupported Ni–W–Mo–S catalysts prepared by one step hydrothermal method

Unsupported Ni–Mo–W sulfide catalysts were prepared by one step hydrothermal method using ammonium heptamolybdate and thiocarbamide in water, and characterized by X-ray diffraction (XRD), nitrogen physisorption and transmission electron microscopy (TEM). The effect of W/Mo molar ratio on the catalytic performance was studied using the HDO of p-cresol as a probe. The results showed that the Ni–Mo–W–S catalyst with an appropriate W/Mo molar ratio possesses the short slab length of MoS2 or WS2 structure and many active sites, leading to the high HDO activity and HYD selectivity.

IF-WS2/Nanostructured Carbon Hybrids Generation and Their Characterization

With the aim to develop a new generation of materials that combine either the known energy absorbing properties of carbon nanofibers (CNF), or the carbon-carbon bond strength of graphene sheets (G), with the shock resistance properties reported for Inorganic Fullerene type WS2 structures (IF-WS2), hybrid CNF/IF-WS2 and G/IF-WS2 were generated, characterized and tested. Experimentation revealed that in situ growth of carbon nanostructures with inorganic fullerene tungsten disulfide particulates had to be performed from particular precursors and fabrication conditions to avoid undesirable byproducts that hinder fiber growth or deter graphene generation. The novel protocols that allowed us to integrate the IF-WS2 with the carbon nanostructures, producing dispersions at the nanoscale, are reported. Resulting hybrid CNF/IF-WS2 and G/IF-WS2 products were analyzed by X-ray Diffraction (XRD), Scanning Electron Microscope (SEM) and TEM (Transmission Electron Microscopy). The thermal stability of samples in air was evaluated by Thermogravimetric Analysis (TGA). CNF/IF-WS2 and G/IF-WS2 hybrids were introduced into epoxy matrices, and the mechanical properties of the resulting composites were analyzed using nanoindentation. Epoxy composite samples showed drastic improvements in the Young’s modulus and hardness values by the use of only 1% hybrid weight loadings. The carbon nanofiber inclusions seem to have a much greater impact on the mechanical properties of the composite than the graphene based counterparts.