WS2 Materials Research Articles

Preparation of Few-Layered WS2 and Its Thermal Catalysis for Dihydroxylammonium-5,5′-Bistetrazole-1,1′-Diolate

In this study, few-layered tungsten disulfide (WS2) was prepared using a liquid phase exfoliation (LPE) method, and its thermal catalytic effects on an important kind of energetic salts, dihydroxylammonium-5,5′-bistetrazole-1,1′-diolate (TKX-50), were investigated. Few-layered WS2 nanosheets were obtained successfully from LPE process. And the effects of the catalytic activity of the bulk and few-layered WS2 on the thermal decomposition behavior of TKX-50 were studied by using synchronous thermal analysis (STA). Moreover, the thermal analysis data was analyzed furtherly by using the thermokinetic software AKTS. The results showed the WS2 materials had an intrinsic thermal catalysis performance for TKX-50 thermal decomposition. With the few-layered WS2 added, the initial decomposition temperature and activation energy (Ea) of TKX-50 had been decreased more efficiently. A possible thermal catalysis decomposition mechanism was proposed based on WS2. Two dimensional-layered semiconductor WS2 materials under thermal excitation can promote the primary decomposition of TKX-50 by enhancing the H-transfer progress.

Barrier height control in metal/silicon contacts with atomically thin MoS2 and WS2 interfacial layers

As complementary metal-oxide-semiconductor technology nodes are scaled down, lowering the contact resistance has become a critical problem for continued scaling. In this study, we suggested the reduction method of the Schottky barrier height, one of the main causes of contact resistance, by insertion of atomically thin two-dimensional (2D) materials between the metal and Si interface. Also, we found that the inserted 2D materials could modulate the work function of the metal and mitigate the Fermi level pinning, leading to reduced barrier height and, hence, reduced contact resistance of the metal–semiconductor junction. With the insertion of MoS2 and WS2 materials a two-layer thick, we achieved 160 meV reductions in the Schottky barrier height and increased the current density by 14 times for titanium contact to the n-type silicon. Finally, we suggested a modified band diagram of Ti/n-Si contacts with the 2D interfacial layer. Our results showed that employing 2D materials can be an alternative route for overcoming the contact resistance challenges in modern transistors.

Nanophotonics with 2D transition metal dichalcogenides [Invited

Two-dimensional semiconducting transition metal dichalcogenides (TMDCs) have recently become attractive materials for several optoelectronic applications, such as photodetection, light harvesting, phototransistors, light-emitting diodes, and lasers. Their bandgap lies in the visible and near-IR range, and they possess strong excitonic resonances, high oscillator strengths, and valley-selective response. Coupling these materials to optical nanocavities enhances the quantum yield of exciton emission, enabling advanced quantum optics and nanophotonics devices. Here, we review the state-of-the-art advances of hybrid exciton-polariton structures based on monolayer TMDCs coupled to plasmonic and dielectric nanocavities. We discuss the optical properties of 2D WS2, WSe2, MoS2 and MoSe2 materials, paying special attention to their energy bands, photoluminescence/absorption spectra, excitonic fine structure, and to the dynamics of exciton formation and valley depolarization. We also discuss light-matter interactions in such hybrid exciton-polariton structures. Finally, we focus on weak and strong coupling regimes in monolayer TMDCs-based exciton-polariton systems, envisioning research directions and future opportunities for this material platform.

Growth of Wafer-Scale Standing Layers of WS2 for Self-Biased High-Speed UV-Visible-NIR Optoelectronic Devices.

This work describes the wafer-scale standing growth of (002)-plane-oriented layers of WS2 and their suitability for use in self-biased broad-band high-speed photodetection. The WS2 layers are grown using large-scale sputtering, and the effects of the processing parameters such as the deposition temperature, deposition time, and sputtering power are studied. The structural, physical, chemical, optical, and electrical properties of the WS2 samples are also investigated. On the basis of the broad-band light absorption and high-speed in-plane carrier transport characteristics of the WS2 layers, a self-biased broad-band high-speed photodetector is fabricated by forming a type-II heterojunction. This WS2/Si heterojunction is sensitive to ultraviolet, visible, and near-infrared photons and shows an ultrafast photoresponse (1.1 μs) along with an excellent responsivity (4 mA/W) and a specific detectivity (∼1.5 × 1010 Jones). A comprehensive Mott-Schottky analysis is performed to evaluate the parameters of the device, such as the frequency-dependent flat-band potential and carrier concentration. Further, the photodetection parameters of the device, such as its linear dynamic range, rising time, and falling time, are evaluated to elucidate its spectral and transient characteristics. The device exhibits remarkably improved transient and spectral photodetection performances as compared to those of photodetectors based on atomically thin WS2 and two-dimensional materials. These results suggest that the proposed method is feasible for the manipulation of vertically standing WS2 layers that exhibit high in-plane carrier mobility and allow for high-performance broad-band photodetection and energy device applications.

Synthesis of BaW2O7-ethylene glycol inorganic–organic hybrid and its topochemical transformation to thin WS2 nanoplates

A novel inorganic–organic hybrid barium tungstate - ethylene glycol Ba(C2H6O2)W2O7 phase has been prepared by non-aqueous precipitation and characterized. According to powder X-ray diffraction, the solid has an orthorhombic lattice (a = b = 6.415Å, c = 13.05Å) and represents a derivative of the H2W2O7 lamellar acid. The Ba(C2H6O2)W2O7 hybrid material is a layered solid and crystallizes as thin plates, which can be further topotacticaly transformed to few-layer WS2 nanoplates. Tungsten sulfide as obtained possesses high specific surface area and increased defectness of layers. Thin-layer WS2 materials as prepared show advantageous properties as hydrogen evolution electrocatalysts, or in combination with TiO2 as co-catalysts for photo catalytic hydrogen production from methanol.

MoS2 materials synthesized on SiO2/Si substrates via MBE

Two-dimensional (2D) MoS2 materials possess indirect-to-direct bandgap tunability, and have enjoyed wide applications in electronics and optoelectronics. Most of investigators have ubiquitously synthesized these materials by using the chemical vaporization deposition (CVD) method. Here we have adopted MoO3 source materials to synthesize MoS2 on 280-nm SiO2/Si substrates via molecular beam epitaxy (MBE). We have obtained triangular nucleation, tens-of-micron domain, and monolayer MoS2. This MBE technique can be applied to synthesizing other members of semiconducting layered transition metal dichalcogenides (TMDCs), such as WS2, MoSe2, and WSe2 materials.

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.

A study of the effect of sonication time on the catalytic performance of layered WS2 from various sources.

WS2 is a transition metal dichalcogenide (TMD) with many potential applications from catalysis to sensing, and is of interest both in its bulk and monolayer forms. There is discrepancy in the literature on the reported electrocatalytic effect of layered WS2. In this study, we examine two issues: the influence of the WS2 source and the effect of a common agitation technique via ultrasonication on the observed electrocatalysis. Bulk WS2 from five different chemical providers demonstrated different HER electrocatalytic performances. Changes to the duration of sonication result in different HER electrocatalytic performances across all WS2 materials. This may affect the efficiency of subsequent modifications from which these TMD materials serve as precursor materials. On the other hand, while WS2 materials from different suppliers showed varying HET performances, changes in sonication time have no significant effect on their HET performances. Both the WS2 source and the duration of sonication have different implications for the electrochemical performance of bulk WS2 and thus represent important variables to consider in research involving WS2.

Effect of the type, size and concentration of solid lubricants on the tribological properties of the polymer PEEK

Poly-ether-ether-ketone (PEEK) is a high-performance, temperature-resistance polymer that is finding an increasing range of applications. In order to even enhance PEEK׳s mechanical and tribological properties particles of different compositions, shapes and sizes are added into its matrix. PEEK has already been combined with many different particles; however, very rarely MoS2 and WS2 – two state-of-the-art solid-lubricants – were added into the PEEK matrix. Furthermore, a comprehensive tribological study combining MoS2 and WS2 particles of different sizes and concentrations has not yet been reported. In this investigation we looked at the effect of micro- and nanosized MoS2 and WS2 particles in PEEK on the dry-sliding tribological behaviour against stainless steel (100Cr6). A non-conventional technique, i.e., the sintering of dry-pressed compacts, was used to prepare the PEEK composites. The results show that all the particles, irrespective of their composition and size, reduce the friction (up to 30%); however, the nanoscale particles require a higher concentration to form an effective low-friction tribofilm. The formation of a tribofilm is necessary to reduce the wear of all the composites (up to 51%); this is strongly promoted by the addition of nano- or microparticles of both the MoS2 and WS2 materials. In addition, the hardness, which is greatly increased by the addition of all the particles, significantly improves the wear behaviour. The results of XPS analyses showed that the oxidation occurs during tribological sliding, which reduces particles beneficial wear behaviour effects.

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.

Comparative study on MoS2 and WS2 for electrocatalytic water splitting

Replacing Pt by earth abundant catalysts is one of the most important tasks toward potential large-scale HER applications. Among many potential candidates, low cost and earth abundant transition metal dichalcogenides such as MoS2 and WS2 have been promising as good H2 evolution electrocatalysts when they are engineered into the structures with active sites. In this work, we have performed systematic studies on the catalytic reactivity of both MoS2 and WS2 materials produced by one-step and scalable thermolysis from (NH4)2WS4 and (NH4)2MoS4 precursors respectively. Structural analysis shows that these materials prepared at a higher thermolysis temperature exhibit higher crystallinity. The H2 evolution electrocatalysts efficiency for the MoS2 prepared at a lower temperature is higher than those at higher temperatures, where amorphous MoS2 or S22− species instead of crystalline MoS2 is the main active site. By contrast, crystalline WS2 prepared at high temperature is identified to be the key reaction site. Both catalysts display excellent efficiency and durability as an electrocatalyst operating in acidic electrolytes. This work provides fundamental insights for further design and preparation of emergent metal dichalcogenide catalysts, beneficial for the development in clean energy.