Two-dimensional WS2 Research Articles

Enhanced photoresponsivity of the GOQDs decorated WS2 photodetector

The two-dimensional WS2 material with the capability of achieving high photoelectric characteristics has attracted more and more attention. However, it still suffers from the limited optical absorption and the lower photogenerated carrier lifetime. Here, we report a WS2 photodetector decorated by the environment-friendly and stabilized graphene oxide quantum dots (GOQDs) as hole harvesting media. Through introducing GOQDs by a simple solution method, the photoelectric performances of the photodetector are improved efficiently. The photoresponsivity of the photodetector is increased to 12.5 mA W−1, and the detectivity is also improved to about 2.01 × 1010 Jones. The enhanced photoresponsivity is attributed to the GOQDs-WS2 heterostructure and photogating effect of GOQDs, which can promote the separation of the photogenerated electron-hole pairs and the hole carriers trapped in GOQDs, respectively. These reasons also extend the recombination time between them. This work may provide a new approach for preparing green and environment friendly photodetector with enhanced performance.

CVD growth of monolayer WS2 through controlled seed formation and vapor density

Large area, single layer WS2 has a high potential for use in optoelectrical devices with its high photoluminescence intensity and low response time. In this work, we demonstrate a systematic study of controlled tungsten disulfide (WS2) monolayer growth using chemical vapor deposition (CVD) technique. With a detailed investigation of process parameters such as H2 gas inclusion into the main carrier gas, growth temperature and duration, we have gained insight into two-dimensional (2D) WS2 synthesis through controlling the seed formations and the radical vapor density associated with WO3. We confirm that H2 gas, when included to the carrier gas, is directly involved in WO3 reduction due to its reductive reagent nature, which provides a more effective sulfurization and monolayer formation process. Additionally, by changing the CVD growth configuration, hence, increasing the tungsten related vapor density and confining the reactant radicals, we succeed in realizing larger WS2 monolayers, which is still a technological challenge in order to utilize these structures for practical applications. Further optimization of the growth procedure is demonstrated by tuning the growth duration to prevent the excess seed formations and additional layers which will possibly limit the device performance of the monolayer flakes or films when applied.

Growth of two-dimensional WS2 thin films by pulsed laser deposition technique

Tungsten disulfide (WS2) is a promising material for potential applications in next-generation 2D-electronics. However, for such applications large-area films will be required. Fabrication techniques involving exfoliation give films of high crystal quality but suffer from the problem of extremely small size of the films which makes it very challenging to realize real devices for industrial applications. In this study, we are reporting the growth of large area two-dimensional (2D) films of WS2 using pulsed laser deposition technique. A polycrystalline WS2 target was ablated using a pulsed KrF excimer laser (wavelength: 248 nm; pulse width: 25 ns). The ablated material was deposited over a c-axis aligned single crystal sapphire substrate maintained at 500°C in an ambient vacuum of 10−5 Torr. Thickness of the resulting films was controlled by controlling the number of incident laser pulses. Detailed characterization of the films was carried out using several state-of-the-art techniques including atomic force microscopy, Raman spectroscopy, and ultraviolet–visible spectroscopy. Relationship between the number of monolayers in the WS2 films and the spacing between the A1g(Γ) and E2g1 (Γ) Raman peaks (Δf) was determined. Effect of post annealing on the optical properties of the films was also investigated.

A feasible approach to fabricate two-dimensional WS2 flakes: From monolayer to multilayer

Tungsten disulfide (WS2), with indirect-direct bandgap transition when thinned down to atomically thin flake, provides intriguing applications in next-generation optoelectronics. Here, we develop a feasible approach to fabricate two-dimensional (2D) WS2 flakes by the evaporation and sulfurization of W film using S powder. The layer number and lateral dimension of the as-grown WS2 flakes are confirmed by Raman spectroscopy, photoluminescence (PL), optical microscopy, and atomic force microscopy (AFM). The evolution of the layer number and edge length of the flakes is attributed to the concentration gradient of W vapor on the substrate surface in the atmosphere. Our result opens a novel pathway to fabricate other 2D transition metal dichalcogenides (TMDs) and construct 2D TMDs-based heterostructures.

Diode-pumped passively Q-switched Nd:GdYTaO4 laser based on two-dimensional WS2 nanosheet

A diode laser pumped passively Q-switched Nd:Gd0.69Y0.3TaO4 laser based on two-dimensional (2D) layered tungsten disulfide (WS2) saturable absorber was demonstrated for the first time. In the continuous wave (CW) operation, the maximum output power of 5.24 W with slope efficiency of 41% was obtained when the absorbed pump power was 14.18 W. The pulsed Nd:Gd0.69Y0.3TaO4 laser was realized when the 2D material of WS2 saturable absorber was inserted into the cavity. The shortest pulse duration and the highest repetition rate were 640 ns and 231 kHz, respectively. It is believed that a better laser performance can be expected when the quality of 2D material of WS2 and Nd:Gd0.69Y0.3TaO4 mixed crystal is improved.

Material Genome Explorations and New Phases of Two-Dimensional MoS2, WS2, and ReS2 Monolayers

Two-dimensional transition metal dichalcogenides have attracted intense interests in recent years. Existing studies have fully explored the properties of their ground-state structures, but their global energy landscapes are still not well understood. The global energy landscape is important for understanding the experimental synthesis and thermal dynamic properties as well as for discovering new phases. This work uses material genome techniques (huge global search and data mining) to explore the global energy landscapes and new phases of two-dimensional MoS2, WS2, and ReS2 monolayers at ab initio level. Our results show that their energy landscapes have two or three major funnels, each of which consists of a few local minima with hexagonal, octahedral, quadrilateral, nanoribbon structures as well as their distorted and hybrid structures. The global-minimum structures are confined in deep funnels while the higher-energy minima stay in flat funnels and can transform to other minima at high temperatures. A few new phases with high geometry symmetry are found, e.g., quadrilateral phase, nanoribbon phase, distorted hexagonal phase, 4-2-coordination phase, and Pmm2 space group phase. These new phases exhibit novel phonon and electron properties (e.g., direct Γ point band gap and distorted Dirac cone), and could be the new candidates for devices applications. We further demonstrate that these phases could have better stability with charge doping and thus could be fabricated in experiments.

Two-dimensional (2D) few-layers WS2 nanosheets: An ideal nanomaterials with tunable electromagnetic absorption performance

In this study, few-layer (FL) WS2 nanosheets were fabricated through liquid phase exfoliation, and the dielectric and electromagnetic absorption (EA) properties were first reported. The results show that a composite consisting of 30 wt. % FL-WS2 exhibited an effective EA bandwidth of 4.6 GHz at a thickness of 2.5 mm. Meanwhile, the strongest absorption point (RLstr) can reach −63.0 dB. The most noteworthy is that the FL-WS2 based composite can completely absorb the electromagnetic energy from X (8–12 GHz) and Ku (12–18 GHz) bands. These results indicate that FL-WS2 has practical applications against electromagnetic pollution in the future.

Nonlinear optical susceptibility of two-dimensional WS2 measured by hyper Rayleigh scattering: erratum.

This erratum corrects errors in the expressions for ⟨βTMD⟩ and fitted form of IHRS and a consequent data point in Fig.4 of a recent Letter [Opt. Lett.42, 5018 (2017)OPLEDP0146-959210.1364/OL.42.005018]. It also supplies data for the reference compound para-nitroaniline (pNA). The correction to ⟨βTMD⟩ improves experimental agreement from 46% to within 21% of independent scissors-corrected density functional theory (DFT) calculations. Central findings from the original Letter remain intact.

A highly sensitive room temperature humidity sensor based on 2D-WS2 nanosheets

Ambient humidity monitoring is of outmost importance in many technological fields. For this reason, there is still a substantial interest in the development of new simple humidity sensors with high performances. Here it is presented a study to the development of a sensitive, selective and reliable conductometric humidity sensor based on two-dimensional (2D) WS2 nanosheets. The 2D nanomaterial was synthesized by exfoliation of a bulk WS2 with tert–butyllithium, and then deposited onto the interdigitated electrodes of a conductometric ceramic platform. To evaluate the sensing performances, the sensor was exposed to different relative humidity (RH) levels, recording in real time the change of conductivity of the 2D-WS2 layer. The sensor exhibited high sensitivity to humidity at room temperature, showing an increase of the current of about 3 orders of magnitude as RH value was varied between 8% and 85% with fast response and recovery times of 140 s and 30 s, respectively. Field tests, carried out to measure the environmental humidity, demonstrated the good performances of the developed sensor as a highly sensitive and reliable relative humidity probe for environmental applications.

Band alignment of 2D WS2/HfO2 interfaces from x-ray photoelectron spectroscopy and first-principles calculations

We report on the growth of two-dimensional (2D) WS2 on high-k HfO2/Si substrates by reactive sputtering deposition. Raman, x-ray photoelectron spectroscopy (XPS), and high-resolution transmission electron microscopy characterizations indicate that the 2D WS2 layers exhibit high-quality crystallinity and exact stoichiometry. Through high-resolution XPS valence spectra, we find a type I alignment at the interface of monolayer WS2/HfO2 with a valence band offset (VBO) of 1.95 eV and a conduction band offset (CBO) of 1.57 eV. The VBO and CBO are also found to increase up to 2.24 eV and 2.09 eV, respectively, with increasing WS2 layers. This is consistent with the results obtained from our first-principles calculations. Our theoretical calculations reveal that the remarkable splitting and shift of the W 5dz2 orbital originating from interlayer orbital coupling in thicker WS2 films induce a reduction of its bandgap, leading to an increase in both the VBO and CBO. This observation can be attributed to the asymmetric splitting at different high symmetric k-points caused by the interlayer orbital coupling.

Effects of exciton-plasmon strong coupling on third harmonic generation by two-dimensional WS2 at periodic plasmonic interfaces

We study theoretically the optical response of a WS2 monolayer located near periodic metal nanostructured arrays in two and three dimensions. The emphasis of the simulations is on the strong coupling between excitons supported by WS2 and surface plasmon-polaritons supported by various periodic plasmonic interfaces. It is demonstrated that a monolayer of WS2 placed in close proximity of periodic arrays of either slits or holes results in a Rabi splitting of the corresponding surface plasmon-polariton resonance as revealed in calculated transmission and reflection spectra. The nonlinear regime, at which the few-layer WS2 exhibits experimentally third harmonic generation (THG), is studied in detail. Monolayer transition metal dichalcogenides (TMDs) do not exhibit THG because they are non-centrosymmetric, but here we use the monolayer as an approximation to a thin TMD nanostructure. We show that in the strong coupling regime the third harmonic signal is significantly affected by plasmon-polaritons and the symmetry of hybrid exciton-plasmon modes. It is also shown that the local electromagnetic field induced by plasmons is the major contributor to the enhancement of the third harmonic signal in three dimensions. The local electromagnetic fields resulting from the third harmonic generation are greatly localized and highly sensitive to the environment, thus making it a great tool for nano-probes.

Controlled dynamic screening of excitonic complexes in 2D semiconductors

We report a combined theoretical/experimental study of dynamic screening of excitons in media with frequency-dependent dielectric functions. We develop an analytical model showing that interparticle interactions in an exciton are screened in the range of frequencies from zero to the characteristic binding energy depending on the symmetries and transition energies of that exciton. The problem of the dynamic screening is then reduced to simply solving the Schrodinger equation with an effectively frequency-independent potential. Quantitative predictions of the model are experimentally verified using a test system: neutral, charged and defect-bound excitons in two-dimensional monolayer WS2, screened by metallic, liquid, and semiconducting environments. The screening-induced shifts of the excitonic peaks in photoluminescence spectra are in good agreement with our model.

Nonlinear optical susceptibility of two-dimensional WS2 measured by hyper Rayleigh scattering.

Hyper Rayleigh scattering (HRS) was used to measure the second-order nonlinear susceptibility, χ(2), for liquid exfoliated WS2 monolayers. To the best of our knowledge, it is the first reported application of the HRS technique to assess the bulk-like χ(2) of a two-dimensional (2D) material. The concentration-dependent HRS signal indicated a 4.90±0.30×10-25 esu first hyperpolarizability for 42nm WS2 monolayers under 1064nm laser irradiation using para-nitroaniline as an external reference. The corresponding value of χxxx(2) was calculated to be 460±28 pm V-1. This was within 46% of independent density functional theory predictions. Agreement with theory was improved over related microscopy-based approaches. These results support the use of HRS to evaluate 2D materials for nonlinear frequency mixing applications.

A study of constructing heterojunction between two-dimensional transition metal sulfides (MoS2 and WS2) and (101), (001) faces of TiO2

The present study focuses on anatase TiO2 with various crystal facets, such as {101} and {001} combined with two-dimensional (2D) WS2 nanolayers to form a heterojunction and the related photocatalytic activity. The {101} facets of anatase TiO2 is superior in the compositing of WS2; the composite shows a 90% enhancement compared with pristine TiO2 than the composited WS2 on the {001} facets which only boosted 7%. However, in our previous study, anatase TiO2 with coexposed {101} and {001} facets demonstrated great improvement in photocatalytic activity in the composited MoS2. Results reveal that when anatase TiO2 with coexposed {101} and {001} facets are composited with 2D transition metal sulfide (MoS2 and WS2), dual heterojunction will form. Some {101} facets are co-exposed with {001} facets, and this co-exposure will construct a crystal facet heterojunction. Moreover, a semiconductor heterojunction exists between TiO2 and MoS2 or WS2. This dual heterojunction is found to have a synergistic effect. It is revealed that the position of energy band plays a decision role in the dual heterojunction. This finding offers a deeper understanding in the construction of heterojunction on different crystal facets.

2D materials-based photoelectrochemical cells: Combination of transition metal dichalcogenides and reduced graphene oxide for efficient charge transfer

Transition metal dichalcogenides (TMDs) and their hybrids with reduced graphene oxide (rGO) have attracted great attention for use as electrocatalysts to replace Pt. However, their applications for photoelectrochemical (PEC) cells are not widely known. Herein, we report the fabrication of highly efficient photo-cathodes for PEC cells using only two-dimensional (2D) materials, MoS2, WS2 (with 1T and 2H phases) and rGO sheets. Two types of hybrid films were prepared based on various combinations of the components and optimization of the thickness; multi-layered (1T MoS2/2H MoS2/2H WS2/rGO/ITO) and multi-component (1T MoS2/(2H MoS2+2H WS2+rGO)/ITO) films. The hybrid films with the same component materials but different structures showed high efficiency when used as cathodes in PEC cells in terms of the current density under illumination: the increase by 60 and 67%, respectively, for the multi-layered and the multi-component films. Furthermore, the roles of each component were identified. The 1T MoS2 layer contributes to catalytic activity for hydrogen evolution reaction. The hybrid of 2H MoS2 and 2H WS2 with different band structures facilitates more efficient carrier transport through proper band alignment than the individual MoS2 or WS2 sheets. The rGO sheets act as conductive interconnecting components to prevent the recombination of electrons and holes. Finally, the optimized hybrid film (1T MoS2/(2H MoS2+2H WS2+rGO)/rGO/ITO), based on above roles of each component, showed highly efficient PEC performance. This study points out that the performance of PECs consisting of 2D materials is affected by the combination of 2D materials and the structure of the photoelectrode.

WS2\/Silicon Heterojunction Solar Cells: A CVD Process for the Fabrication of WS2 Films on p-Si Substrates for Photovoltaic and Spectral Responses

With the layer-dependent tunability of their optical band gap in the near-infrared (IR ) to visible spectrum, transition metal dichalcogenides (TMDs), including molybdenum disulfide (MoS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ) and tungsten disulfide (WS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ), exhibit strong light-matter interactions, making them suitable as absorber layers for optoelectronic devices. Currently, the WS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> -based solar cells are fabricated via micromechanical/chemical exfoliation or transfer of two-dimensional (2-D) WS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> layers onto conventional three-dimensional (3-D) bulk semiconductors, which poses a challenge for large-scale integrations and consequent device performances.

Homogeneous time-resolved fluoroimmunoassay of microcystin-LR using layered WS2 nanosheets as a transducer

A homogeneous time-resolved fluoroimmunoassay method for rapid and sensitive detection of microcystin-LR (MC-LR) in water samples was developed based on the interaction between water-soluble WS2 nanosheets and the conjugate of MC-LR with a luminescent Eu3+ complex BHHBCB-Eu3+ (BHHBCB: 1,2-bis[4′-(1″,1″,1″,2″,2″,3″,3″-heptafluoro-4″,6″-hexanedion-6″-yl)- benzyl]-4-chlorosulfobenzene). The large lateral dimensions and high surface areas of two-dimensional layered WS2 nanosheets enable easy adsorption of the MC-LR-BHHBCB-Eu3+ conjugate, that lead to efficient quenching of the luminescence of Eu3+ complex via energy transfer or electron transfer process. However, the addition of monoclonal anti-MC-LR antibody can induce the formation of MC-LR-BHHBCB-Eu3+/antibody immune complex, which prevents the interaction between WS2 nanosheets and MC-LR-BHHBCB-Eu3+ to result in the restoration of Eu3+ luminescence. This signal transduction mechanism made it possible for analysis of the target MC-LR in a homogeneous system. The present method has advantages of rapidity and simplicity since the B/F (bound reagent/free reagent) separation steps, the solid-phase carrier and antibody labeling or modification process are not necessary. The proposed immunosensing system displayed a wide linear range, good precision and accuracy, and comparable sensitivity with a detection limit of 0.3 μg l−1, which satisfied the World Health Organization (WHO) provisional guideline limit of 1.0 μg l−1 for MC-LR in drinking water.

Ultrasmall WS2 Quantum Dots with Visible Fluorescence for Protection of Cells and Animal Models from Radiation-Induced Damages.

Two-dimensional WS2 materials have attracted wide attention in condensed physics and materials science due to its unique geometric and electronic structures. Particularly, WS2 shows extraordinary catalytic activities when its size decreases to ultrasmall, which provides potential opportunities for medical applications. In this work, WS2 quantum dots with strong catalytic properties were used for in vitro and in vivo protection from ionizing radiation induced cell damages. WS2 quantum dots possess unique optical properties of blue photoluminescence emission and excitation-wavelength dependent emission profiles. In vitro studies showed that cell viability can be considerably improved and cellular reactive oxygen species (ROS) can be removed by WS2 quantum dots. In vivo studies showed WS2 quantum dots can effectively protect the hematopoietic system and DNA from damages caused by high-energy radiation through removing whole-body excessive ROS. Furthermore, WS2 quantum dots showed nearly 80% renal clearance within 24 h post injection and did not cause any obvious toxicities in up to 30 days after treatment.

Correction: On the chemically-assisted excitonic enhancement in environmentally-friendly solution dispersions of two-dimensional MoS2 and WS2

Correction for ‘On the chemically-assisted excitonic enhancement in environmentally-friendly solution dispersions of two-dimensional MoS2 and WS2’ by Dalal Fadil et al., J. Mater. Chem. C, 2017, DOI: 10.1039/c7tc01001j.

The effects of exfoliation, organic solvents and anodic activation on the catalytic hydrogen evolution reaction of tungsten disulfide.

The rational design of transition metal dichalcogenide electrocatalysts for efficiently catalyzing the hydrogen evolution reaction (HER) is believed to lead to the generation of a renewable energy carrier. To this end, our work has made three main contributions. At first, we have demonstrated that exfoliation via ionic liquid assisted grinding combined with gradient centrifugation is an efficient method to exfoliate bulk WS2 to nanosheets with a thickness of a few atomic layers and lateral size dimensions in the range of 100 nm to 2 nm. These WS2 nanosheets decorated with scattered nanodots exhibited highly enhanced catalytic performance for HER with an onset potential of -130 mV vs. RHE, an overpotential of 337 mV at 10 mA cm-2 and a Tafel slope of 80 mV dec-1 in 0.5 M H2SO4. Secondly, we found a strong aging effect on the electrocatalytic performance of WS2 stored in high boiling point organic solvents such as dimethylformamide (DMF). Importantly, the HER ability could be recovered by removing the organic (DMF) residues, which obstructed the electron transport, with acetone. Thirdly, we established that the HER performance of WS2 nanosheets/nanodots could be significantly enhanced by activating the electrode surface at a positive voltage for a very short time (60 s), decreasing the kinetic overpotential by more than 80 mV at 10 mA cm-2. The performance enhancement was found to arise primarily from the ability of a formed proton-intercalated amorphous tungsten trioxide (a-WO3) to provide additional active sites and favourably modify the immediate chemical environment of the WS2 catalyst, rendering it more favorable for local proton delivery and/or transport to the active edge site of WS2. Our results provide new insights into the effects of organic solvents and electrochemical activation on the catalytic performance of two-dimensional WS2 for HER.