Nonlinear Saturable Absorption of Liquid‐Exfoliated Molybdenum/Tungsten Ditelluride Nanosheets

Molybdenum disulfide (MoS₂) and tungsten disulfide (WS₂), two representative transition metal dichalcogenide materials, have captured tremendous interest for their unique electronic, optical, and chemical properties. Compared with MoS₂ and WS₂, molybdenum ditelluride (MoTe₂) and tungsten ditelluride (WTe₂) possess similar lattice structures while having smaller bandgaps (less than 1 eV), which is particularly interesting for applications in the near‐infrared wavelength regime. Here, few‐layer MoTe₂/WTe₂ nanosheets are fabricated by a liquid exfoliation method using sodium deoxycholate bile salt as surfactant, and the nonlinear optical properties of the nanosheets are investigated. The results demonstrate that MoTe₂/WTe₂ nanosheets exhibit nonlinear saturable absorption property at 1.55 μm. Soliton mode‐locking operations are realized separately in erbium‐doped fiber lasers utilizing two types of MoTe₂/WTe₂‐based saturable absorbers, one of which is prepared by depositing the nanosheets on side polished fibers, while the other is fabricated by mixing the nanosheets with polyvinyl alcohol and then evaporating them on substrates. Numerous applications may benefit from the nonlinear saturable absorption features of MoTe₂/WTe₂ nanosheets, such as visible/near‐infrared pulsed laser, materials processing, optical sensors, and modulators.     

Design of XS2 (X = W or Mo)-Decorated VS2 Hybrid Nano-Architectures with Abundant Active Edge Sites for High-Rate Asymmetric Supercapacitors and Hydrogen Evolution Reactions

Two-dimensional layered transition metal dichalcogenides have emerged as promising materials for supercapacitors and hydrogen evolution reaction (HER) applications. Herein, the molybdenum sulfide (MoS₂)@vanadium sulfide (VS₂) and tungsten sulfide (WS₂)@VS₂ hybrid nano-architectures prepared via a facile one-step hydrothermal approach is reported. Hierarchical hybrids lead to rich exposed active edge sites, tuned porous nanopetals-decorated morphologies, and high intrinsic activity owing to the strong interfacial interaction between the two materials. Fabricated supercapacitors using MoS₂@VS₂ and WS₂@VS₂ electrodes exhibit high specific capacitances of 513 and 615 F g⁻¹, respectively, at an applied current of 2.5 A g⁻¹ by the three-electrode configuration. The asymmetric device fabricated using WS₂@VS₂ electrode exhibits a high specific capacitance of 222 F g⁻¹ at an applied current of 2.5 A g⁻¹ with the specific energy of 52 Wh kg⁻¹ at a specific power of 1 kW kg⁻¹. For HER, the WS₂@VS₂ catalyst shows noble characteristics with an overpotential of 56 mV to yield 10 mA cm⁻², a Tafel slope of 39 mV dec⁻¹, and an exchange current density of 1.73 mA cm⁻². In addition, density functional theory calculations are used to evaluate the durable heterostructure formation and adsorption of hydrogen atom on the various accessible sites of MoS₂@VS₂ and WS₂@VS₂ heterostructures.     

Effect of microwaves on synthesis of MoS2 and WS2

The present work was aimed on utilizing the solid state microwave synthetic method for the growth of molybdenum disulphide (MoS₂) and tungsten disulphide (WS₂) in powder as well as in the form of thin films. It was observed that the microwave exposure of simple powder mixture of Mo (or W) and S could not lead to the formation of MoS₂ (or WS₂).Therefore the work was pursued by the study of the possibility to use this technique to grow thin films. Either Mo or W in the form of thin foils or Mo layers deposited by sputtering onto glass substrates was used as metal source. These metal samples were introduced with some sulphur into a Pyrex tube and sealed under vacuum. After microwave oven exposure the formation of polycrystalline 2H-WS₂ with well-defined grains was confirmed by X-ray diffraction and scanning electron microscopy, respectively. Mo foil as well as Mo layers deposited on glass showed formation of MoS₂ under the limit of our experimental conditions that is to say homogeneous thin films can be achieved only as small surface films.     

Band Structure Engineering of Interfacial Semiconductors Based on Atomically Thin Lead Iodide Crystals

To explore new constituents in two‐dimensional (2D) materials and to combine their best in van der Waals heterostructures is in great demand as being a unique platform to discover new physical phenomena and to design novel functionalities in interface‐based devices. Herein, PbI₂ crystals as thin as a few layers are synthesized, particularly through a facile low‐temperature solution approach with crystals of large size, regular shape, different thicknesses, and high yields. As a prototypical demonstration of band engineering of PbI₂‐based interfacial semiconductors, PbI₂ crystals are assembled with several transition metal dichalcogenide monolayers. The photoluminescence of MoS₂ is enhanced in MoS₂/PbI₂ stacks, while a dramatic photoluminescence quenching of WS₂ and WSe₂ is revealed in WS₂/PbI₂ and WSe₂/PbI₂ stacks. This is attributed to the effective heterojunction formation between PbI₂ and these monolayers; type I band alignment in MoS₂/PbI₂ stacks, where fast‐transferred charge carriers accumulate in MoS₂ with high emission efficiency, results in photoluminescence enhancement, and type II in WS₂/PbI₂ and WSe₂/PbI₂ stacks, with separated electrons and holes suitable for light harvesting, results in photoluminescence quenching. The results demonstrate that MoS₂, WS₂, and WSe₂ monolayers with similar electronic structures show completely distinct light–matter interactions when interfacing with PbI₂, providing unprecedented capabilities to engineer the device performance of 2D heterostructures.     

17% Efficient Organic Solar Cells Based on Liquid Exfoliated WS2 as a Replacement for PEDOT:PSS

The application of liquid‐exfoliated 2D transition metal disulfides (TMDs) as the hole transport layers (HTLs) in nonfullerene‐based organic solar cells is reported. It is shown that solution processing of few‐layer WS₂ or MoS₂ suspensions directly onto transparent indium tin oxide (ITO) electrodes changes their work function without the need for any further treatment. HTLs comprising WS₂ are found to exhibit higher uniformity on ITO than those of MoS₂ and consistently yield solar cells with superior power conversion efficiency (PCE), improved fill factor (FF), enhanced short‐circuit current (J_SC), and lower series resistance than devices based on poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) and MoS₂. Cells based on the ternary bulk‐heterojunction PBDB‐T‐2F:Y6:PC₇₁BM with WS₂ as the HTL exhibit the highest PCE of 17%, with an FF of 78%, open‐circuit voltage of 0.84 V, and a J_SC of 26 mA cm^?2. Analysis of the cells' optical and carrier recombination characteristics indicates that the enhanced performance is most likely attributed to a combination of favorable photonic structure and reduced bimolecular recombination losses in WS₂‐based cells. The achieved PCE is the highest reported to date for organic solar cells comprised of 2D charge transport interlayers and highlights the potential of TMDs as inexpensive HTLs for high‐efficiency organic photovoltaics.     

Synthesis and structure of multi-layered WS<Subscript>2</Subscript>(CoS), MoS<Subscript>2</Subscript>(Mo) nanocapsules and single-layered WS<Subscript>2</Subscript>(W) nanoparticles

Multi-layered MoS₂ (or WS₂) nanocages stuffed with Mo (or CoS/CoO) nanocrystals have been synthesized by using the reaction between metal nanoparticles and sulfur powders. This simple synthesis method, different from the conventional methods for synthesizing pure inorganic fullerenes, is also potentially important for large-scale synthesis of nanoparticles of other metal dichalcogenide. Besides the multi-layered WS₂ nanocapsules, we have successfully fabricated nanocapsules with a single-layered WS₂ sheet encapsulating W by using the arc-discharge method. We discuss possible mechanisms for the formation of the unique core-shell structured nanocapsules.     

Differences in the Toxicological Potential of 2D versus Aggregated Molybdenum Disulfide in the Lung

2D molybdenum disulfide (MoS₂) has distinct optical and electronic properties compared to aggregated MoS₂, enabling wide use of these materials for electronic and biomedical applications. However, the hazard potential of MoS₂ has not been studied extensively. Here, a comprehensive analysis of the pulmonary hazard potential of three aqueous suspended forms of MoS₂—aggregated MoS₂ (Agg‐MoS₂), MoS₂ exfoliated by lithiation (Lit‐MoS₂), and MoS₂ dispersed by Pluronic F87 (PF87‐MoS₂)—is presented. No cytotoxicity is detected in THP‐1 and BEAS‐2B cell lines. However, Agg‐MoS₂ induces strong proinflammatory and profibrogenic responses in vitro. In contrast, Lit‐ and PF87‐MoS₂ have little or no effect. In an acute toxicity study in mice, Agg‐MoS₂ induces acute lung inflammation, while Lit‐MoS₂ and PF87‐MoS₂ have little or no effect. In a subchronic study, there is no evidence of pulmonary fibrosis in response to all forms of MoS₂. These data suggest that exfoliation attenuates the toxicity of Agg‐MoS₂, which is an important consideration toward the safety evaluation and use of nanoscale MoS₂ materials for industrial and biological applications.     

Lateral Graphene‐Contacted Vertically Stacked WS2/MoS2 Hybrid Photodetectors with Large Gain

A demonstration is presented of how significant improvements in all‐2D photodetectors can be achieved by exploiting the type‐II band alignment of vertically stacked WS₂/MoS₂ semiconducting heterobilayers and finite density of states of graphene electrodes. The photoresponsivity of WS₂/MoS₂ heterobilayer devices is increased by more than an order of magnitude compared to homobilayer devices and two orders of magnitude compared to monolayer devices of WS₂ and MoS₂, reaching 10³ A W^?1 under an illumination power density of 1.7 × 10² mW cm^?2. The massive improvement in performance is due to the strong Coulomb interaction between WS₂ and MoS₂ layers. The efficient charge transfer at the WS₂/MoS₂ heterointerface and long trapping time of photogenerated charges contribute to the observed large photoconductive gain of ≈3 × 10⁴. Laterally spaced graphene electrodes with vertically stacked 2D van der Waals heterostructures are employed for making high‐performing ultrathin photodetectors.     

Scalable and controllable synthesis of 2D high-proportion 1T-phase MoS2

Two-dimensional molybdenum disulfide (2D MoS₂) is considered as a promising candidate for many applications due to its unique structure and properties. However, the controllable synthesis of large-scale and high-quality 2D 1T-phase MoS₂ is still a challenge. Herein, we present the scalable and controllable synthesis of 2D MoS₂ from 2H to 1T@2H phase by using K₂SO₄ salt as a simultaneous high-temperature sulfur source and template. The as-synthesized 1T@2H-2D MoS₂ exhibits a high yield and can be easily assembled into freestanding electrode with high specific capacitance of 434 F/g at a scan rate of 1 mV/s in LiClO₄ ethylene carbonate/dimethyl carbonate (EC/DMC). Moreover, various single-crystal 2D transition metal sulfides (WS₂, PbS, MnS and Ni₉S₈) and 2D S-doped carbon can be synthesized using this method. We believe that this study may provide a new sight for scalable and controllable synthesis of other 2D materials beyond 2D MoS₂.

     

In Situ Repair of 2D Chalcogenides under Electron Beam Irradiation

Molybdenum disulfide (MoS₂) and bismuth telluride (Bi₂Te₃) are the two most common types of structures adopted by 2D chalcogenides. In view of their unique physical properties and structure, 2D chalcogenides have potential applications in various fields. However, the excellent properties of these 2D crystals depend critically on their crystal structures, where defects, cracks, holes, or even greater damage can be inevitably introduced during the preparation and transferring processes. Such defects adversely impact the performance of devices made from 2D chalcogenides and, hence, it is important to develop ways to intuitively and precisely repair these 2D crystals on the atomic scale, so as to realize high‐reliability devices from these structures. Here, an in situ study of the repair of the nanopores in MoS₂ and Bi₂Te₃ is carried out under electron beam irradiation by transmission electron microscopy. The experimental conditions allow visualization of the structural dynamics of MoS₂ and Bi₂Te₃ crystals with unprecedented resolution. Real‐time observation of the healing of defects at atomic resolution can potentially help to reproducibly fabricate and simultaneously image single‐crystalline free‐standing 2D chalcogenides. Thus, these findings demonstrate the viability of using an electron beam as an effective tool to precisely engineer materials to suit desired applications in the future.     

Determining the Optimized Interlayer Separation Distance in Vertical Stacked 2D WS2:hBN:MoS2 Heterostructures for Exciton Energy Transfer

The 2D semiconductor monolayer transition metal dichalcogenides, WS₂ and MoS₂, are grown by chemical vapor deposition (CVD) and assembled by sequential transfer into vertical layered heterostructures (VLHs). Insulating hBN, also produced by CVD, is utilized to control the separation between WS₂ and MoS₂ by adjusting the layer number, leading to fine‐scale tuning of the interlayer interactions within the VLHs. The interlayer interactions are studied by photoluminescence (PL) spectroscopy and are demonstrated to be highly sensitive to the input excitation power. For thin hBN separators (one to two layers), the total PL emission switches from quenching to enhancement by increasing the laser power. Femtosecond broadband transient absorption measurements demonstrate that the increase in PL quantum yield results from Förster energy transfer from MoS₂ to WS₂. The PL signal is further enhanced at cryogenic temperatures due to the suppressed nonradiative decay channels. It is shown that (4 ± 1) layers of hBN are optimum for obtaining PL enhancement in the VLHs. Increasing thickness beyond this causes the enhancement factor to diminish, with the WS₂ and MoS₂ then behaving as isolated noninteracting monolayers. These results indicate how controlling the exciton generation rate influences energy transfer and plays an important role in the properties of VLHs.     

Tunable Modal Birefringence in a Low‐Loss Van Der Waals Waveguide

van der Waals (vdW) crystals are promising candidates for integrated phase retardation applications due to their large optical birefringence. Among the two major types of vdW materials, the hyperbolic vdW crystals are inherently inadequate for optical retardation applications since the supported polaritonic modes are exclusively transverse‐magnetic (TM) polarized and relatively lossy. Elliptic vdW crystals, on the other hand, represent a superior choice. For example, molybdenum disulfide (MoS₂) is a natural uniaxial vdW crystal with extreme elliptic anisotropy in the frequency range of optical communication. Both transverse‐electric (TE) polarized ordinary and TM polarized extraordinary waveguide modes can be supported in MoS₂ microcrystals with suitable thicknesses. In this work, low‐loss transmission of these guided modes is demonstrated with nano‐optical imaging at the near‐infrared (NIR) wavelength (1530 nm). More importantly, by combining theoretical calculations and NIR nanoimaging, the modal birefringence between the orthogonally polarized TE and TM modes is shown to be tunable in both sign and magnitude via varying the thickness of the MoS₂ microcrystal. This tunability represents a unique new opportunity to control the polarization behavior of photons with vdW materials.     

Microwave-assisted synthesis of WS2 nanowires through tetrathiotungstate precursors

Abstract

Tungsten disulfide (WS₂) nanowires have been synthesized through a microwave-assisted chemical route that uses tungstic acid, elemental sulfur and monoethanolamine as starting reagents for obtaining a precursor solution of tetrathiotungstate ions. Acidification of the precursor solution yields amorphous precipitates, which lead to the formation of nanowires of WS₂ with thickness of about 5–10 nm when heated at 750 °C under argon atmosphere for 1.5 h. Phase and the microstructure of the prepared powders have been investigated through x-ray powder diffraction and high-resolution transmission electron microscopy, respectively. Optical absorption of the WS₂ powders reveals a red shift of the exciton bands compared to bulk WS₂.     

层状二硫化钼材料的制备和应用进展

二硫化钼(MoS2)是具有天然可调控带隙的二维层状材料,其独特的性质引起了科研人员的广泛关注,在微电子及光电领域具有重要的应用前景。介绍了MoS2的基本性质和常用制备方法,对层状MoS2材料在电子和光电子器件方面的应用进行了总结和展望。     

Metallic MoS2 for High Performance Energy Storage and Energy Conversion

Metallic phase 2D molybdenum disulfide (MoS₂) is an emerging class of materials with remarkably higher electrical conductivity and catalytic activities. The goal of this study is to review the atomic structures and electrochemistry of metallic MoS₂, which is essential for a wide range of existing and new enabling technologies. The scope of this paper ranges from the atomic structure, band structure, electrical and optical properties to fabrication methods, and major emerging applications in electrochemical energy storage and energy conversion. This paper also thoroughly covers the atomic structure–properties–application relationships of metallic MoS₂. Understanding the fundamental properties of these structures is crucial for designing and manufacturing products for emerging applications. Today, a more holistic understanding of the interplay between the structure, chemistry, and performance of metallic MoS₂ is advancing actual applications of this material. This new level of understanding also enables a myriad of new and exciting applications, which motivated this review. There are excellent reviews already on the traditional semiconducting MoS₂, and this review, for the first time, focuses on the uniqueness of conducting metallic MoS₂ for energy applications and offers brand new materials for clean energy application.     

Superior Potassium Ion Storage via Vertical MoS2 “Nano‐Rose” with Expanded Interlayers on Graphene

Potassium has its unique advantages over lithium or sodium as a charge carrier in rechargeable batteries. However, progresses in K‐ion battery (KIB) chemistry have so far been hindered by lacking suitable electrode materials to host the relatively large K⁺ ions compared to its Li⁺ and Na⁺ counterparts. Herein, molybdenum disulfide (MoS₂) “roses” grown on reduced graphene oxide sheets (MoS₂@rGO) are synthesized via a two‐step solvothermal route. The as‐synthesized MoS₂@rGO composite, with expanded interlayer spacing of MoS₂, chemically bonded between MoS₂ and rGO, and a unique nano‐architecture, displays the one of the best electrochemical performances to date as an anode material for nonaqueous KIBs. More importantly, a combined K⁺ storage mechanism of intercalation and conversion reaction is also revealed. The findings presented indicate the enormous potential of layered metal dichalcogenides as advanced electrode materials for high‐performance KIBs and also provide new insights and understanding of K⁺ storage mechanism.     

Preparation of MoS2‐Polyvinylpyrrolidone Nanocomposites for Flexible Nonvolatile Rewritable Memory Devices with Reduced Graphene Oxide Electrodes

A facile method for exfoliation and dispersion of molybdenum disulfide (MoS₂) with the aid of polyvinylpyrrolidone (PVP) is proposed. The resultant PVP‐coated MoS₂ nanosheets, i.e., MoS₂‐PVP nanocomposites, are well dispersed in the low‐boiling ethanol solvent, facilitating their thin film preparation and the device fabrication by solution processing technique. As a proof of concept, a flexible memory diode with the configuration of reduced graphene oxide (rGO)/MoS₂‐PVP/Al exhibited a typical bistable electrical switching and nonvolatile rewritable memory effect with the function of flash. These experimental results prove that the electrical transition is due to the charge trapping and detrapping behavior of MoS₂ in the PVP dielectric material. This study paves a way of employing two‐dimensional nanomaterials as both functional materials and conducting electrodes for the future flexible data storage.     

Precisely Aligned Monolayer MoS2 Epitaxially Grown on h‐BN basal Plane

Control of the precise lattice alignment of monolayer molybdenum disulfide (MoS₂) on hexagonal boron nitride (h‐BN) is important for both fundamental and applied studies of this heterostructure but remains elusive. The growth of precisely aligned MoS₂ domains on the basal plane of h‐BN by a low‐pressure chemical vapor deposition technique is reported. Only relative rotation angles of 0° or 60° between MoS₂ and h‐BN basal plane are present. Domains with same orientation stitch and form single‐crystal, domains with different orientations stitch and from mirror grain boundaries. In this way, the grain boundary is minimized and a continuous film stitched by these two types of domains with only mirror grain boundaries is obtained. This growth strategy is also applicable to other 2D materials growth.     

Interface Adhesion between 2D Materials and Elastomers Measured by Buckle Delaminations

2D systems have great promise as next generation electronic materials but require intimate knowledge of their interactions with their neighbors for device fabrication and mechanical manipulation. Although adhesion between 2D materials and stiff substrates such as silicon and copper has been measured, adhesion between 2D materials and soft polymer substrates remains difficult to characterize due to the large deformability of the polymer substrates. In this work, a buckling‐based metrology for measuring the adhesion energy between few layer molybdenum disulfide (MoS₂) and soft elastomeric substrates is proposed and demonstrated. Due to large elastic mismatch, few layer MoS₂ flakes can form spontaneous wrinkles and buckle‐delaminations on elastomer substrates during exfoliation. MoS₂‐elastomer interface toughness can therefore be calculated from the buckle delamination profile measured by atomic force microscopy. The thickness of the MoS₂ flake is obtained by analyzing coexisting wrinkles on the same flake. Using this approach, adhesion of few layer MoS₂ to 10:1 Sylgard 184 polydimethylsiloxane is measured to be 18 ± 2 mJ m⁻², which is about an order of magnitude below graphene‐to‐stiff‐substrate adhesion. Finally, this simple methodology can be generalized to obtain adhesion energies between various combinations of 2D materials and deformable substrates.     

Microwave-assisted synthesis of WS2 nanowires through tetrathiotungstate precursors

Tungsten disulfide (WS₂) nanowires have been synthesized through a microwave-assisted chemical route that uses tungstic acid, elemental sulfur and monoethanolamine as starting reagents for obtaining a precursor solution of tetrathiotungstate ions. Acidification of the precursor solution yields amorphous precipitates, which lead to the formation of nanowires of WS₂ with thickness of about 5–10 nm when heated at 750 °C under argon atmosphere for 1.5 h. Phase and the microstructure of the prepared powders have been investigated through x-ray powder diffraction and high-resolution transmission electron microscopy, respectively. Optical absorption of the WS₂ powders reveals a red shift of the exciton bands compared to bulk WS₂.