Vacancy‐Driven Gelation Using Defect‐Rich Nanoassemblies of 2D Transition Metal Dichalcogenides and Polymeric Binder for Biomedical Applications

A new approach of vacancy‐driven gelation to obtain chemically crosslinked hydrogels from defect‐rich 2D molybdenum disulfide (MoS₂) nanoassemblies and polymeric binder is reported. This approach utilizes the planar and edge atomic defects available on the surface of the 2D MoS₂ nanoassemblies to form mechanically resilient and elastomeric nanocomposite hydrogels. The atomic defects present on the lattice plane of 2D MoS₂ nanoassemblies are due to atomic vacancies and can act as an active center for vacancy‐driven gelation with a thiol‐activated terminal such as four‐arm poly(ethylene glycol)–thiol (PEG‐SH) via chemisorption. By modulating the number of vacancies on the 2D MoS₂ nanoassemblies, the physical and chemical properties of the hydrogel network can be controlled. This vacancy‐driven gelation process does not require external stimuli such as UV exposure, chemical initiator, or thermal agitation for crosslinking and thus provides a nontoxic and facile approach to encapsulate cells and proteins. 2D MoS₂ nanoassemblies are cytocompatible, and encapsulated cells in the nanocomposite hydrogels show high viability. Overall, the nanoengineered hydrogel obtained from vacancy‐driven gelation is mechanically resilient and can be used for a range of biomedical applications including tissue engineering, regenerative medicine, and cell and therapeutic delivery.     

Recent Advances in Synthesis and Biomedical Applications of Two‐Dimensional Transition Metal Dichalcogenide Nanosheets

During recent decades, a giant leap in the development of nanotechnology has been witnessed. Numerous nanomaterials with different dimensions and unprecedented features have been developed and provided unimaginably wide scope to solve the challenging problems in biomedicine, such as cancer diagnosis and therapy. Recently, two‐dimensional (2D) transition metal dichalcogenide (TMDC) nanosheets (NSs), including MoS₂, WS₂, and etc., have emerged as novel inorganic graphene analogues and attracted tremendous attention due to their unique structures and distinctive properties, and opened up great opportunities for biomedical applications, including ultrasensitive biosensing, biological imaging, drug delivery, cancer therapy, and antibacterial treatment. A comprehensive overview of different synthetic methods of ultrathin 2D TMDC NSs and their state‐of‐the‐art biomedical applications, especially those that have appeared in the past few years, is presented. At the end of this review, the future opportunities and challenges for 2D TMDC NSs in biomedicine are also discussed.     

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.     

Hierarchical MoS2 Nanosheet@TiO2 Nanotube Array Composites with Enhanced Photocatalytic and Photocurrent Performances

A novel type of hierarchical nanocomposites consisted of MoS₂ nanosheet coating on the self‐ordered TiO₂ nanotube arrays is successfully prepared by a facile combination of anodization and hydrothermal methods. The MoS₂ nanosheets are uniformly decorated on the tube top surface and the intertubular voids with film appearance changing from brown to black color. Anatase TiO₂ nanotube arrays (NTAs) with clean top surfaces and the appropriate amount of MoS₂ precursors are key to the growth of perfect compositing TiO₂@MoS₂ hybrids with significantly enhanced photocatalytic activity and photocurrent response. These results reveal that the strategy provides a flexible and straightforward route for design and preparation nanocomposites based on functional semiconducting nanostructures with 1D self‐ordered TiO₂ NTAs, promising for new opportunities in energy/environment applications, including photocatalysts and other photovoltaic devices.     

Structural Phase Transition Effect on Resistive Switching Behavior of MoS2‐Polyvinylpyrrolidone Nanocomposites Films for Flexible Memory Devices

The 2H phase and 1T phase coexisting in the same molybdenum disulfide (MoS₂) nanosheets can influence the electronic properties of the materials. The 1T phase of MoS₂ is introduced into the 2H‐MoS₂ nanosheets by two‐step hydrothermal synthetic methods. Two types of nonvolatile memory effects, namely write‐once read‐many times memory and rewritable memory effect, are observed in the flexible memory devices with the configuration of Al/1T@2H‐MoS₂‐polyvinylpyrrolidone (PVP)/indium tin oxide (ITO)/polyethylene terephthalate (PET) and Al/2H‐MoS₂‐PVP/ITO/PET, respectively. It is observed that structural phase transition in MoS₂ nanosheets plays an important role on the resistive switching behaviors of the MoS₂‐based device. It is hoped that our results can offer a general route for the preparation of various promising nanocomposites based on 2D nanosheets of layered transition metal dichalcogenides for fabricating the high performance and flexible nonvolatile memory devices through regulating the phase structure in the 2D nanosheets.     

ZnS/C/MoS2 Nanocomposite Derived from Metal–Organic Framework for High‐Performance Photo‐Electrochemical Immunosensing of Carcinoembryonic Antigen

A hexafluorophosphate ionic liquid is used as a functional monomer to prepare a metal–organic framework (Zn‐MOF). Zn‐MOF is used as a template for MoS₂ nanosheets synthesis and further carbonized to yield light‐responsive ZnS/C/MoS₂ nanocomposites. Zn‐MOF, carbonized‐Zn‐MOF, and ZnS/C/MoS₂ nanocomposites are characterized by Fourier transform infrared spectroscopy, transmission electron microscopy, X‐ray diffraction pattern, scanning electron microscopy (SEM), element mapping, Raman spectroscopy, X‐ray photoelectron spectroscopy, fluorescence, and nitrogen‐adsorption analysis. Carcinoembryonic antigen (CEA) is selected as a model to construct an immunosensing platform to evaluate the photo‐electrochemical (PEC) performances of ZnS/C/MoS₂ nanocomposites. A sandwich‐type PEC immunosensor is fabricated by immobilizing CEA antibody (Ab₁) onto the ZnS/C/MoS₂/GCE surface, subsequently binding CEA and the alkaline phosphatase‐gold nanoparticle labeled CEA antibody (ALP‐Au‐Ab₂). The catalytic conversion of vitamin C magnesium phosphate produces ascorbic acid (AA). Upon being illuminated, AA can react with photogenerated holes from ZnS/C/MoS₂ nanocomposites to generate a photocurrent for quantitative assay. Under optimized experimental conditions, the PEC immunosensor exhibits excellent analytical characteristics with a linear range from 2.0 pg mL^?1 to 10.0 ng mL^?1 and a detection limit of 1.30 pg mL^?1 (S/N = 3). The outstanding practicability of this PEC immunosensor is demonstrated by accurate assaying of CEA in clinical serum samples.     

3R MoS2 with Broken Inversion Symmetry: A Promising Ultrathin Nonlinear Optical Device

Nonlinear 2D layered crystals provide ideal platforms for applications and fundamental studies in ultrathin nonlinear optical (NLO) devices. However, the NLO frequency conversion efficiency constrained by lattice symmetry is still limited by layer numbers of 2D crystals. In this work, 3R MoS₂ with broken inversion symmetry structure are grown and proved to be excellent NLO 2D crystals from monolayer (0.65 nm) toward bulk‐like (300 nm) dimension. Thickness and wavelength‐dependent second harmonic generation spectra offer the selection rules of appropriate working conditions. A model comprising of bulk nonlinear contribution and interface interaction is proposed to interpret the observed nonlinear behavior. Polarization enhancement with two petals along staggered stacking direction appears in 3R MoS₂ is first observed and the robust polarization of 3R MoS₂ crystal is caused by the retained broken inversion symmetry. The results provide a new arena for realizing ultrathin NLO devices for 2D layered materials.     

Carrier Transport and Photoresponse in GeSe/MoS2 Heterojunction p–n Diodes

Simple stacking of thin van der Waals 2D materials with different physical properties enables one to create heterojunctions (HJs) with novel functionalities and new potential applications. Here, a 2D material p–n HJ of GeSe/MoS₂ is fabricated and its vertical and horizontal carrier transport and photoresponse properties are studied. Substantial rectification with a very high contrast (>10⁴) through the potential barrier in the vertical‐direction tunneling of HJs is observed. The negative differential transconductance with high peak‐to‐valley ratio (>10⁵) due to the series resistance change of GeSe, MoS₂, and HJs at different gate voltages is observed. Moreover, strong and broad‐band photoresponse via the photoconductive effect are also demonstrated. The explored multifunctional properties of the GeSe/MoS₂ HJs are expected to be important for understanding the carrier transport and photoresponse of 2D‐material HJs for achieving their use in various new applications in the electronics and optoelectronics fields.     

Defect-free MoS2 nanosheets: Advanced nanofillers for polymer nanocomposites

The preparation of defect-free MoS₂ nanosheets is a key challenge and essential for practical applications. Herein the dodecanethiol was firstly performed as the antioxidant and surface modifier to produce the defect-free MoS₂ by direct ultrasonication of bulk MoS₂ in N,N-dimethylformamide. Incorporating defect-free MoS₂ into polyethylene obviously improved the properties of PE/MoS₂ nanocomposites. For crystallization under quiescent condition, the half crystallization time (t₀.₅) of nanocomposites containing 0.2 wt% MoS₂ was reduced by 87.0% compared to that of neat PE. A 54.3 °C increase in the temperature of maximum weight loss (T_max) was observed by inclusion of as low as 0.7 wt% defect-free MoS₂ nanosheets. In addition, the uniformly distributed MoS₂ can considerably improve the mechanical properties of composites. These observations suggest that the robust nature, dramatic barrier action of defect-free MoS₂ and the strong nanosheets/matrix interfacial adhesion would be the motivation to improve the performance of the polymeric nanocomposites.     

Electrochemically Exfoliated High‐Quality 2H‐MoS2 for Multiflake Thin Film Flexible Biosensors

2D molybdenum disulfide (MoS₂) gives a new inspiration for the field of nanoelectronics, photovoltaics, and sensorics. However, the most common processing technology, e.g., liquid‐phase based scalable exfoliation used for device fabrication, leads to the number of shortcomings that impede their large area production and integration. Major challenges are associated with the small size and low concentration of MoS₂ flakes, as well as insufficient control over their physical properties, e.g., internal heterogeneity of the metallic and semiconducting phases. Here it is demonstrated that large semiconducting MoS₂ sheets (with dimensions up to 50 µm) can be obtained by a facile cathodic exfoliation approach in nonaqueous electrolyte. The synthetic process avoids surface oxidation thus preserving the MoS₂ sheets with intact crystalline structure. It is further demonstrated at the proof‐of‐concept level, a solution‐processed large area (60 × 60 µm) flexible Ebola biosensor, based on a MoS₂ thin film (6 µm thickness) fabricated via restacking of the multiple flakes on the polyimide substrate. The experimental results reveal a low detection limit (in femtomolar–picomolar range) of the fabricated sensor devices. The presented exfoliation method opens up new opportunities for fabrication of large arrays of multifunctional biomedical devices based on novel 2D materials.     

Layered PtSe2 for Sensing,Photonic, and (Opto‐)Electronic Applications

Since the first experimental discovery of graphene 16 years ago, many other 2D layered nanomaterials have been reported. However, the majority of 2D nanostructures suffer from relatively complicated fabrication processes that have bottlenecked their development and their uptake by industry for practical applications. Here, the recent progress in sensing, photonic, and (opto‐)electronic applications of PtSe₂, a 2D layered material that is likely to be used in industries benefiting from its high air‐stability and semiconductor‐technology‐compatible fabrication methods, is reviewed. The advantages and disadvantages of a range of synthesis methods for PtSe₂ are initially compared, followed by a discussion of its outstanding properties, and industrial and commercial advantages. Research focused on the broadband nonlinear photonic properties of PtSe₂, as well as reports of its use as a saturable absorber in ultrafast lasers, are then reviewed. Additionally, the advances that have been achieved in a range of PtSe₂‐based field‐effect transistors, photodetectors, and sensors are summarized. Finally, a conclusion on these results along with the outlook for the future is presented.     

Bundled Defect‐Rich MoS2 for a High‐Rate and Long‐Life Sodium‐Ion Battery: Achieving 3D Diffusion of Sodium Ion by Vacancies to Improve Kinetics

Molybdenum disulfide (MoS₂), a 2D‐layered compound, is regarded as a promising anode for sodium‐ion batteries (SIBs) due to its attractive theoretical capacity and low cost. The main challenges associated with MoS₂ are the low rate capability suffering from the sluggish kinetics of Na⁺ intercalation and the poor cycling stability owning to the stack of MoS₂ sheets. In this work, a unique architecture of bundled defect‐rich MoS₂ (BD‐MoS₂) that consists of MoS₂ with large vacancies bundled by ultrathin MoO₃ is achieved via a facile quenching process. When employed as anode for a SIB, the BD‐MoS₂ electrode exhibits an ultrafast charge/discharge due to the pseudocapacitive‐controlled Na⁺ storage mechanism in it. Further experimental and theoretical calculations show that Na⁺ is able to cross the MoS₂ layer by vacancies, not only limited to diffusion along the layer, thus realizing a 3D Na⁺ diffusion with faster kinetics. Meanwhile, the bundling architecture reduces the stack of sheets with a superior cycle life illustrating the highly reversible capacities of 350 and 272 mAh g^?1 at 2 and 5 A g^?1 after 1000 cycles.     

High‐Performance Photodiode Based on Atomically Thin WSe2/MoS2 Nanoscroll Integration

Self‐assembled structures of 2D materials with novel physical and chemical properties, such as the good electrical and optoelectrical performance in nanoscrolls, have attracted a lot of attention. However, high photoresponse speed as well as high responsivity cannot be achieved simultaneously in the nanoscrolls. Here, a photodiode consisting of single MoS₂ nanoscrolls and a p‐type WSe₂ is demonstrated and shows excellent photovoltaic characteristics with a large open‐circuit voltage of 0.18 V and high current intensity. Benefiting from the heterostructure, the dark current is suppressed resulting in an increased ratio of photocurrent to dark current (two orders of magnitude higher than the single MoS₂ nanoscroll device). Furthermore, it yields high responsivity of 0.3 A W^?1 (corresponding high external quantum efficiency of ≈75%) and fast response time of 5 ms, simultaneously. The response speed is increased by three orders of magnitude over the single MoS₂ nanoscroll device. In addition, broadband photoresponse up to near‐infrared could be achieved. This atomically thin WSe₂/MoS₂ nanoscroll integration not only overcomes the disadvantage of MoS₂ nanoscrolls, but also demonstrates a single nanoscroll‐based heterostructure with high performance, promising its potential in the future optoelectronic applications.     

Roll‐to‐Roll Production of Layer‐Controlled Molybdenum Disulfide: A Platform for 2D Semiconductor‐Based Industrial Applications

A facile methodology for the large‐scale production of layer‐controlled MoS₂ layers on an inexpensive substrate involving a simple coating of single source precursor with subsequent roll‐to‐roll‐based thermal decomposition is developed. The resulting 50 cm long MoS₂ layers synthesized on Ni foils possess excellent long‐range uniformity and optimum stoichiometry. Moreover, this methodology is promising because it enables simple control of the number of MoS₂ layers by simply adjusting the concentration of (NH₄)₂MoS₄. Additionally, the capability of the MoS₂ for practical applications in electronic/optoelectronic devices and catalysts for hydrogen evolution reaction is verified. The MoS₂‐based field effect transistors exhibit unipolar n‐channel transistor behavior with electron mobility of 0.6 cm² V^?1 s^?1 and an on‐off ratio of ≈10³. The MoS₂‐based visible‐light photodetectors are fabricated in order to evaluate their photoelectrical properties, obtaining an 100% yield for active devices with significant photocurrents and extracted photoresponsivity of ≈22 mA W^?1. Moreover, the MoS₂ layers on Ni foils exhibit applicable catalytic activity with observed overpotential of ≈165 mV and a Tafel slope of 133 mV dec^?1. Based on these results, it is envisaged that the cost‐effective methodology will trigger actual industrial applications, as well as novel research related to 2D semiconductor‐based multifaceted applications.     

Solvent‐Exchange Strategy toward Aqueous Dispersible MoS2 Nanosheets and Their Nitrogen‐Rich Carbon Sphere Nanocomposites for Efficient Lithium/Sodium Ion Storage

Major challenges in developing 2D transition‐metal disulfides (TMDs) as anode materials for lithium/sodium ion batteries (LIBs/SIBs) lie in rational design and targeted synthesis of TMD‐based nanocomposite structures with precisely controlled ion and electron transport. Herein, a general and scalable solvent‐exchange strategy is presented for uniform dispersion of few‐layer MoS₂ (f‐MoS₂) from high‐boiling‐point solvents (N‐methyl‐2‐pyrrolidone (NMP), N,N‐dimethyl formaldehyde (DMF), etc.) into low‐boiling‐point solvents (water, ethanol, etc.). The solvent‐exchange strategy dramatically simplifies high‐yield production of dispersible MoS₂ nanosheets as well as facilitates subsequent decoration of MoS₂ for various applications. As a demonstration, MoS₂‐decorated nitrogen‐rich carbon spheres (MoS₂‐NCS) are prepared via in situ growth of polypyrrole and subsequent pyrolysis. Benefiting from its ultrathin feature, largely exposed active surface, highly conductive framework and excellent structural integrity, the 2D core–shell architecture of MoS₂‐NCS exhibits an outstanding reversible capacity and excellent cycling performance, achieving high initial discharge capacity of 1087.5 and 508.6 mA h g^?1 at 0.1 A g^?1, capacity retentions of 95.6% and 94.2% after 500 and 250 charge/discharge cycles at 1 A g^?1, for lithium/sodium ion storages, respectively.     

二维纳米片层结构MoS2和其改性材料的合成及在环境领域的应用

类石墨烯的二维纳米片层状MoS_2作为过渡金属硫化物材料家族的典型代表,由于其各方面独特的性能尤其是超薄的厚度和二维结构,已成为近些年材料的研究热点。独特的三明治结构使其具有良好的物理化学性质,MoS_2及其复合材料在催化、传感器、润滑等方面有广阔的前景。总结了国内外MoS_2的合成及其改性方法、性能和应用。针对社会发展过程中日益严重的环境污染问题,重点介绍利用其吸附和催化性能去除环境中污染物,并对其未来发展进行了展望。     

Phosphorus‐Mediated MoS2 Nanowires as a High‐Performance Electrode Material for Quasi‐Solid‐State Sodium‐Ion Intercalation Supercapacitors

Molybdenum disulfide (MoS₂) is a promising electrode material for electrochemical energy storage owing to its high theoretical specific capacity and fascinating 2D layered structure. However, its sluggish kinetics for ionic diffusion and charge transfer limits its practical applications. Here, a promising strategy is reported for enhancing the Na⁺‐ion charge storage kinetics of MoS₂ for supercapacitors. In this strategy, electrical conductivity is enhanced and the diffusion barrier of Na⁺ ion is lowered by a facile phosphorus‐doping treatment. Density functional theory results reveal that the lowest energy barrier of dilute Na‐vacancy diffusion on P‐doped MoS₂ (0.11 eV) is considerably lower than that on pure MoS₂ (0.19 eV), thereby signifying a prominent rate performance at high Na intercalation stages upon P‐doping. Moreover, the Na‐vacancy diffusion coefficient of the P‐doped MoS₂ at room temperatures can be enhanced substantially by approximately two orders of magnitude (10^?6–10^?4 cm² s^?1) compared with pure MoS₂. Finally, the quasi‐solid‐state asymmetrical supercapacitor assembled with P‐doped MoS₂ and MnO₂, as the positive and negative electrode materials, respectively, exhibits an ultrahigh energy density of 67.4 W h kg^?1 at 850 W kg^?1 and excellent cycling stability with 93.4% capacitance retention after 5000 cycles at 8 A g^?1.     

Rational Design of Nanoporous MoS2/VS2 Heteroarchitecture for Ultrahigh Performance Ammonia Sensors

2D transition metal dichalcogenides (TMDs) have received widespread interest by virtue of their excellent electrical, optical, and electrochemical characteristics. Recent studies on TMDs have revealed their versatile utilization as electrocatalysts, supercapacitors, battery materials, and sensors, etc. In this study, MoS₂ nanosheets are successfully assembled on the porous VS₂ (P‐VS₂) scaffold to form a MoS₂/VS₂ heterostructure. Their gas‐sensing features, such as sensitivity and selectivity, are investigated by using a quartz crystal microbalance (QCM) technique. The QCM results and density functional theory (DFT) calculations reveal the impressive affinity of the MoS₂/VS₂ heterostructure sensor toward ammonia with a higher adsorption uptake than the pristine MoS₂ or P‐VS₂ sensor. Furthermore, the adsorption kinetics of the MoS₂/VS₂ heterostructure sensor toward ammonia follow the pseudo‐first‐order kinetics model. The excellent sensing features of the MoS₂/VS₂ heterostructure render it attractive for high‐performance ammonia sensors in diverse applications.     

Surface Plasmon‐Enhanced Photodetection in Few Layer MoS2 Phototransistors with Au Nanostructure Arrays

2D Molybdenum disulfide (MoS₂) is a promising candidate material for high‐speed and flexible optoelectronic devices, but only with low photoresponsivity. Here, a large enhancement of photocurrent response is obtained by coupling few‐layer MoS₂ with Au plasmonic nanostructure arrays. Au nanoparticles or nanoplates placed onto few‐layer MoS₂ surface can enhance the local optical field in the MoS₂ layer, due to the localized surface plasmon (LSP) resonance. After depositing 4 nm thick Au nanoparticles sparsely onto few‐layer MoS₂ phototransistors, a doubled increase in the photocurrent response is observed. The photocurrent of few‐layer MoS₂ phototransistors exhibits a threefold enhancement with periodic Au nanoarrays. The simulated optical field distribution confirms that light can be trapped and enhanced near the Au nanoplates. These findings offer an avenue for practical applications of high performance MoS₂‐based optoelectronic devices or systems in the future.     

Contact‐Engineered Electrical Properties of MoS2 Field‐Effect Transistors via Selectively Deposited Thiol‐Molecules

Although 2D molybdenum disulfide (MoS₂) has gained much attention due to its unique electrical and optical properties, the limited electrical contact to 2D semiconductors still impedes the realization of high‐performance 2D MoS₂‐based devices. In this regard, many studies have been conducted to improve the carrier‐injection properties by inserting functional paths, such as graphene or hexagonal boron nitride, between the electrodes and 2D semiconductors. The reported strategies, however, require relatively time‐consuming and low‐yield transfer processes on sub‐micrometer MoS₂ flakes. Here, a simple contact‐engineering method is suggested, introducing chemically adsorbed thiol‐molecules as thin tunneling barriers between the metal electrodes and MoS₂ channels. The selectively deposited thiol‐molecules via the vapor‐deposition process provide additional tunneling paths at the contact regions, improving the carrier‐injection properties with lower activation energies in MoS₂ field‐effect transistors. Additionally, by inserting thiol‐molecules at the only one contact region, asymmetric carrier‐injection is feasible depending on the temperature and gate bias.