Strong Coupling of MoS2 Nanosheets and Nitrogen‐Doped Graphene for High‐Performance Pseudocapacitance Lithium Storage

Layered material MoS₂ is widely applied as a promising anode for lithium‐ion batteries (LIBs). Herein, a scalable and facile dopamine‐assisted hydrothermal technique for the preparation of strongly coupled MoS₂ nanosheets and nitrogen‐doped graphene (MoS₂/N‐G) composite is developed. In this composite, the interconnected MoS₂ nanosheets are well wrapped onto the surface of graphene, forming a unique veil‐like architecture. Experimental results indicate that dopamine plays multiple roles in the synthesis: a binding agent to anchor and uniformly disperse MoS₂ nanosheets, a morphology promoter, and the precursor for in situ nitrogen doping during the self‐polymerization process. Density functional theory calculations further reveal that a strong interaction exists at the interface of MoS₂ nanosheets and nitrogen‐doped graphene, which facilitates the charge transfer in the hybrid system. When used as the anode for LIBs, the resulting MoS₂/N‐G composite electrode exhibits much higher and more stable Li‐ion storage capacity (e.g., 1102 mAh g^?1 at 100 mA g^?1) than that of MoS₂/G electrode without employing the dopamine linker. Significantly, it is also identified that the thin MoS₂ nanosheets display outstanding high‐rate capability due to surface‐dominated pseudocapacitance contribution.     

Glucose‐Induced Synthesis of 1T‐MoS2/C Hybrid for High‐Rate Lithium‐Ion Batteries

1T phase MoS₂ possesses higher conductivity than the 2H phase, which is a key parameter of electrochemical performance for lithium ion batteries (LIBs). Herein, a 1T‐MoS₂/C hybrid is successfully synthesized through facile hydrothermal method with a proper glucose additive. The synthesized hybrid material is composed of smaller and fewer‐layer 1T‐MoS₂ nanosheets covered by thin carbon layers with an enlarged interlayer spacing of 0.94 nm. When it is used as an anode material for LIBs, the enlarged interlayer spacing facilitates rapid intercalating and deintercalating of lithium ions and accommodates volume change during cycling. The high intrinsic conductivity of 1T‐MoS₂ also contributes to a faster transfer of lithium ions and electrons. Moreover, much smaller and fewer‐layer nanosheets can shorten the diffusion path of lithium ions and accelerate reaction kinetics, leading to an improved electrochemical performance. It delivers a high initial capacity of 920.6 mAh g^?1 at 1 A g^?1 and the capacity can maintain 870 mAh g^?1 even after 300 cycles, showing a superior cycling stability. The electrode presents a high rate performance as well with a reversible capacity of 600 mAh g^?1 at 10 A g^?1. These results show that the 1T‐MoS₂/C hybrid shows potential for use in high‐performance lithium‐ion batteries.     

Nickel@Nitrogen‐Doped Carbon@MoS2 Nanosheets: An Efficient Electrocatalyst for Hydrogen Evolution Reaction

Developing cheap, abundant, and easily available electrocatalysts to drive the hydrogen evolution reaction (HER) at small overpotentials is an urgent demand of hydrogen production from water splitting. Molybdenum disulfide (MoS₂) based composites have emerged as competitive electrocatalysts for HER in recent years. Herein, nickel@nitrogen‐doped carbon@MoS₂ nanosheets (Ni@NC@MoS₂) hybrid sub‐microspheres are presented as HER catalyst. MoS₂ nanosheets with expanded interlayer spacings are vertically grown on nickel@nitrogen‐doped carbon (Ni@NC) substrate to form Ni@NC@MoS₂ hierarchical sub‐microspheres by a simple hydrothermal process. The formed Ni@NC@MoS₂ composites display excellent electrocatalytic activity for HER with an onset overpotential of 18 mV, a low overpotential of 82 mV at 10 mA cm^?2, a small Tafel slope of 47.5 mV dec^?1, and high durability in 0.5 H₂SO₄ solution. The outstanding HER performance of the Ni@NC@MoS₂ catalyst can be ascribed to the synergistic effect of dense catalytic sites on MoS₂ nanosheets with exposed edges and expanded interlayer spacings, and the rapid electron transfer from Ni@NC substrate to MoS₂ nanosheets. The excellent Ni@NC@MoS₂ electrocatalyst promises potential application in practical hydrogen production, and the strategy reported here can also be extended to grow MoS₂ on other nitrogen‐doped carbon encapsulated metal species for various applications.     

Polarized Light‐Emitting Diodes Based on Patterned MoS2 Nanosheet Hole Transport Layer

Here, this study successfully fabricates few‐layer MoS₂ nanosheets from (NH₄)₂MoS₄ and applies them as the hole transport layer as well as the template for highly polarized organic light‐emitting diodes (OLEDs). The obtained material consists of polycrystalline MoS₂ nanosheets with thicknesses of 2 nm. The MoS₂ nanosheets are patterned by rubbing/ion‐beam treatment. The Raman spectra shows that {poly(9,9‐dioctylfluorene‐alt‐benzothiadiazole), poly[(9,9‐di‐n‐octylfluorenyl‐2,7‐diyl)‐alt‐(benzo[2,1,3]thiadiazol‐4,8‐diyl)]} (F8BT) on patterned MoS₂ exhibits distinctive polarization behavior. It is discovered that patterned MoS₂ not only improves the device efficiency but also changes the polarization behavior of the devices owing to the alignment of F8BT. This work demonstrates a highly efficient polarized OLED with a polarization ratio of 62.5:1 in the emission spectrum (166.7:1 at the peak intensity of 540 nm), which meets the manufacturing requirement. In addition, the use of patterned MoS₂ nanosheets not only tunes the polarization of the OLEDs but also dramatically improves the device performance as compared with that of devices using untreated MoS₂.     

3D 1T‐MoS2/CoS2 Heterostructure via Interface Engineering for Ultrafast Hydrogen Evolution Reaction

Metallic phase (1T) MoS₂ has been regarded as an appealing material for hydrogen evolution reaction. In this work, a novel interface‐induced strategy is reported to achieve stable and high‐percentage 1T MoS₂ through highly active 1T‐MoS₂/CoS₂ hetero‐nanostructure. Herein, a large number of heterointerfaces can be obtained by interlinked 1T‐MoS₂ and CoS₂ nanosheets in situ grown from the molybdate cobalt oxide nanorod under moderate conditions. Owing to the strong interaction between MoS₂ and CoS₂, high‐percentage of metallic‐phase (1T) MoS₂ of 76.6% can be achieved, leading to high electroconductivity and abundant active sites compared to 2H MoS₂. Furthermore, the interlinked MoS₂ and CoS₂ nanosheets can effectively disperse the nanosheets so as to enlarge the exposed active surface area. The near zero free energy of hydrogen adsorption at the heterointerface can also be achieved, indicating the fast kinetics and excellent catalytic activity induced by heterojunction. Therefore, when applied in hydrogen evolution reaction (HER), 1T‐MoS₂/CoS₂ heterostructure delivers low overpotential of 71 and 26 mV at the current density of 10 mA cm^?2 with low Tafel slops of 60 and 43 mV dec^?1, respectively in alkaline and acidic conditions.     

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.     

Sandwich‐like MoS2@SnO2@C with High Capacity and Stability for Sodium/Potassium Ion Batteries

Sandwich‐like MoS₂@SnO₂@C nanosheets are prepared by facile hydrothermal reactions. SnO₂ nanosheets can attach to exfoliated MoS₂ nanosheets to prevent restacking of adjacent MoS₂ nanosheets, and carbon transformed from polyvinylpyrrolidone is coated on MoS₂@SnO₂, forming a sandwich structure to maintain cycling stability. As an anode for sodium‐ion batteries, the electrode greatly deliverers a high initial discharge specific capacity of 530 mA h g^?1 and maintains at 396 mA h g^?1 after 150 cycles at 0.1 A g^?1. Even at a large current density of 1 A g^?1, it can hold 230 mA h g^?1 after 450 cycles. Besides, as an anode for K⁺ storage, the electrode also shows a discharge capacity of 312 mA h g^?1 after 25 cycles at 0.05 A g^?1. This work may provide a new strategy to prepare other composites which can be applied to new kind of rechargeable batteries.     

Hierarchical Octahedra Constructed by Cu2S/MoS2⊂Carbon Framework with Enhanced Sodium Storage

Metal sulfides have aroused considerable attention for efficient sodium storage because of their high capacity and decent redox reversibility. However, the poor rate capability and fast capacity decay greatly hinder their practical application in sodium‐ion batteries. Herein, a self‐template‐based strategy is designed to controllably synthesize hierarchical microoctahedra assembled with Cu₂S/MoS₂ heterojunction nanosheets in the porous carbon framework (Cu₂S/MoS₂?PCF) via a facile coprecipitation method coupled with vulcanization treatment. The Cu₂S/MoS₂?PCF microoctahedra with 2D hybrid nanosubunits reasonably integrate several merits including facilitating the diffusion of electrons and Na⁺ ions, enhancing the electric conductivity, accelerating the ion and charge transfer, and buffering the volume variation. Therefore, the Cu₂S/MoS₂?PCF composite manifests efficient sodium storage performance with high capacity, long cycling life, and excellent rate capability.     

Flexible Nonvolatile Transistor Memory with Solution‐Processed Transition Metal Dichalcogenides

Nonvolatile field‐effect transistor (FET) memories containing transition metal dichalcogenide (TMD) nanosheets have been recently developed with great interest by utilizing some of the intriguing photoelectronic properties of TMDs. The TMD nanosheets are, however, employed as semiconducting channels in most of the memories, and only a few works address their function as floating gates. Here, a floating‐gate organic‐FET memory with an all‐in‐one floating‐gate/tunneling layer of the solution‐processed TMD nanosheets is demonstrated. Molybdenum disulfide (MoS₂) is efficiently liquid‐exfoliated by amine‐terminated polystyrene with a controlled amount of MoS₂ nanosheets in an all‐in‐one floating‐gate/tunneling layer, allowing for systematic investigation of concentration‐dependent charge‐trapping and detrapping properties of MoS₂ nanosheets. At an optimized condition, the nonvolatile memory exhibits memory performances with an ON/OFF ratio greater than 10⁴, a program/erase endurance cycle over 400 times, and data retention longer than 7 × 10³ s. All‐in‐one floating‐gate/tunneling layers containing molybdenum diselenide and tungsten disulfide are also developed. Furthermore, a mechanically‐flexible TMD memory on a plastic substrate shows a performance comparable with that on a hard substrate, and the memory properties are rarely altered after outer‐bending events over 500 times at the bending radius of 4.0 mm.     

Layer Thinning and Etching of Mechanically Exfoliated MoS2 Nanosheets by Thermal Annealing in Air

A simple thermal annealing method for layer thinning and etching of mechanically exfoliated MoS₂ nanosheets in air is reported. Using this method, single‐layer (1L) MoS₂ nanosheets are achieved after the thinning of MoS₂ nanosheets from double‐layer (2L) to quadri‐layer (4L) at 330 °C. The as‐prepared 1L MoS₂ nanosheet shows comparable optical and electrical properties with the mechanically exfoliated, pristine one. In addition, for the first time, the MoS₂ mesh with high‐density of triangular pits is also fabricated at 330 °C, which might arise from the anisotropic etching of the active MoS₂ edge sites. As a result of thermal annealing in air, the thinning of MoS₂ nanosheet is possible due to its oxidation to form MoO₃. Importantly, the MoO₃ fragments on the top of thinned MoS₂ layer induces the hole injection, resulting in the p‐type channel in fabricated field‐effect transistors.     

Ganoderma‐Like MoS2/NiS2 with Single Platinum Atoms Doping as an Efficient and Stable Hydrogen Evolution Reaction Catalyst

Herein, a unique ganoderma‐like MoS₂/NiS₂ hetero‐nanostructure with isolated Pt atoms anchored is reported. This novel ganoderma‐like heterostructure can not only efficiently disperse and confine the few‐layer MoS₂ nanosheets to fully expose the edge sites of MoS₂, and provide more opportunity to capture the Pt atoms, but also tune the electronic structure to modify the catalytic activity. Because of the favorable dispersibility and exposed large specific surface area, single Pt atoms can be easily anchored on MoS₂ nanosheets with ultrahigh loading of 1.8 at% (the highest is 1.3 at% to date). Owing to the ganoderma‐like structure and platinum atoms doping, this catalyst shows Pt‐like catalytic activity for the hydrogen evolution reaction with an ultralow overpotential of 34 mV and excellent durability of only 2% increase in overpotential for 72 h under the constant current density of 10 mA cm⁻².     

Epitaxial Single‐Layer MoS2 on GaN with Enhanced Valley Helicity

Engineering the substrate of 2D transition metal dichalcogenides can couple the quasiparticle interaction between the 2D material and substrate, providing an additional route to realize conceptual quantum phenomena and novel device functionalities, such as realization of a 12‐time increased valley spitting in single‐layer WSe₂ through the interfacial magnetic exchange field from a ferromagnetic EuS substrate, and band‐to‐band tunnel field‐effect transistors with a subthreshold swing below 60 mV dec⁻¹ at room temperature based on bilayer n‐MoS₂ and heavily doped p‐germanium, etc. Here, it is demonstrated that epitaxially grown single‐layer MoS₂ on a lattice‐matched GaN substrate, possessing a type‐I band alignment, exhibits strong substrate‐induced interactions. The phonons in GaN quickly dissipate the energy of photogenerated carriers through electron–phonon interaction, resulting in a short exciton lifetime in the MoS₂/GaN heterostructure. This interaction enables an enhanced valley helicity at room temperature (0.33 ± 0.05) observed in both steady‐state and time‐resolved circularly polarized photoluminescence measurements. The findings highlight the importance of substrate engineering for modulating the intrinsic valley carriers in ultrathin 2D materials and potentially open new paths for valleytronics and valley‐optoelectronic device applications.     

Size‐Dependent Nonlinear Optical Properties of Atomically Thin Transition Metal Dichalcogenide Nanosheets

Size‐dependent nonlinear optical properties of modification‐free transition metal dichalcogenide (TMD) nanosheets are reported, including MoS₂, WS₂, and NbSe₂. Firstly, a gradient centrifugation method is demonstrated to separate the TMD nanosheets into different sizes. The successful size separation allows the study of size‐dependent nonlinear optical properties of nanoscale TMD materials for the first time. Z‐scan measurements indicate that the dispersion of MoS₂ and WS₂ nanosheets that are 50–60 nm thick leads to reverse saturable absorption (RSA), which is in contrast to the saturable absorption (SA) seen in the thicker samples. Moreover, the NbSe₂ nanosheets show no size‐dependent effects because of their metallic nature. The mechanism behind the size‐dependent nonlinear optical properties of the semiconductive TMD nanosheets is revealed by transient transmission spectra measurements.     

An Electrosynthesized 3D Porous Molybdenum Sulfide/Graphene Film with Enhanced Electrochemical Performance for Lithium Storage

Molybdenum sulfide/graphene composites are promising anode materials for lithium‐ion batteries (LIBs). In this work, MoS_x/graphene composite film with an ideal 3D porous structure is developed via a facile and straightforward electrochemical route. The MoS_x nanoparticles are uniformly anchored on the graphene nanosheets that are randomly arranged, resulting in MoS_x/graphene composites with well‐developed porous structure. Benefiting from such structure and the synergistic effect from two components, this material shows a high specific capacity over 1200 mA h g^?1, an excellent rate performance, and superior cycling stability. The dominating pseudocapacitive behavior in Li storage contributes to the outstanding rate capacity. Importantly, this kind of novel material can be easily produced as 3D microelectrodes for microscaled LIBs that are highly demanded for autonomous microelectronic systems.     

Surface Engineering of MoS2 via Laser‐Induced Exfoliation in Protic Solvents

With excellent performance in the hydrogen evolution reaction (HER), molybdenum disulfide (MoS₂) is considered a promising nonprecious candidate to substitute Pt‐based catalysts. Herein, pulsed laser irradiation in liquid is used to realize one‐step exfoliation of bulk 2H‐MoS₂ to ultrastable few‐layer MoS₂ nanosheets. Such prepared MoS₂ nanosheets are rich in S vacancies and metallic 1T phase, which significantly contribute to the boosted catalytic HER activity. Protic solvents play a pivotal role in the production of S vacancies and 2H‐to‐1T phase transition under laser irradiation. MoS₂ exfoliated in an optimal solvent of formic acid exhibits outstanding HER activity with an overpotential of 180 mV at 10 mA cm^?2 and Tafel slope of 54 mV dec^?1.     

Threshold Voltage Control of Multilayered MoS2 Field‐Effect Transistors via Octadecyltrichlorosilane and their Applications to Active Matrixed Quantum Dot Displays Driven by Enhancement‐Mode Logic Gates

In recent past, for next‐generation device opportunities such as sub‐10 nm channel field‐effect transistors (FETs), tunneling FETs, and high‐end display backplanes, tremendous research on multilayered molybdenum disulfide (MoS₂) among transition metal dichalcogenides has been actively performed. However, nonavailability on a matured threshold voltage control scheme, like a substitutional doping in Si technology, has been plagued for the prosperity of 2D materials in electronics. Herein, an adjustment scheme for threshold voltage of MoS₂ FETs by using self‐assembled monolayer treatment via octadecyltrichlorosilane is proposed and demonstrated to show MoS₂ FETs in an enhancement mode with preservation of electrical parameters such as field‐effect mobility, subthreshold swing, and current on–off ratio. Furthermore, the mechanisms for threshold voltage adjustment are systematically studied by using atomic force microscopy, Raman, temperature‐dependent electrical characterization, etc. For validation of effects of threshold voltage engineering on MoS₂ FETs, full swing inverters, comprising enhancement mode drivers and depletion mode loads are perfectly demonstrated with a maximum gain of 18.2 and a noise margin of ≈45% of 1/2 V_DD. More impressively, quantum dot light‐emitting diodes, driven by enhancement mode MoS₂ FETs, stably demonstrate 120 cd m^?2 at the gate‐to‐source voltage of 5 V, exhibiting promising opportunities for future display application.     

Core–Shell Structure of Hierarchical Quasi‐Hollow MoS2 Microspheres Encapsulated Porous Carbon as Stable Anode for Li‐Ion Batteries

Monodisperse sulfonated polystyrene (SPS) microspheres are employed as both the template and carbon source to prepare MoS₂ quasi‐hollow microspheres‐encapsulated porous carbon. The synthesis procedure involves the hydrothermal growth of MoS₂ ultrathin nanosheets on the surface of SPS microspheres and subsequent annealing to remove SPS core. Incomplete decomposition of SPS during annealing due to the confining effect of MoS₂ shells leaves residual porous carbon in the interior. When being evaluated as the anode materials of Li‐ion batteries, the as‐prepared C@MoS₂ microspheres exhibit excellent cycling stability (95% of capacity retained after 100 cycles) and high rate behavior (560 mAh g^?1 at 5 A g^?1).     

Layered Aggregation with Steric Effect: Morphology‐Homogeneous Semiconductor MoS2 as an Alternative 2D Probe for Visual Immunoassay

Liquid‐phase exfoliation routes unavoidably generate 2D nanostructures with inhomogeneous morphologies. Herein, thickness‐dependent sorting of exfoliated nanostructures is achieved via a treatment of differential‐zone centrifugation in the surfactant aqueous phase. With this approach, homogeneous MoS₂ nanosheets are obtained, and due to the intrinsic semiconducting characteristics, those 2D nanosheets are endowed with desired optical properties, rivaling classic gold nanoparticles in sensing applications. Furthermore, MoS₂ nanosheets with high uniformity and chemical inertness are coupled with proteins, exhibiting high performance in stability and anti‐interferences for bioanalysis. As a consequence of aggregation‐induced steric effect, distinguishing running shifts of antibody‐anchored conjugates in gel electrophoresis are visually responsive to those specific antigens. This assay enables the easy and fast monitoring of tumor biomarkers just according to “naked‐eye” identification of band location in electrophoresis results, which are presented by an alternative visual probe of 2D MoS₂‐protein conjugates. The developed visual immunoassay with the synergistic effect of gel electrophoresis techniques and 2D semiconductors pushes significant progress in “home‐made” tests for disease early diagnosis.     

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.     

High Phase Purity of Large‐Sized 1T′‐MoS2 Monolayers with 2D Superconductivity

The development of transition metal dichalcogenides has greatly accelerated research in the 2D realm, especially for layered MoS₂. Crucially, the metallic MoS₂ monolayer is an ideal platform in which novel topological electronic states can emerge and also exhibits excellent energy conversion and storage properties. However, as its intrinsic metallic phase, little is known about the nature of 2D 1T′‐MoS₂, probably because of limited phase uniformity (<80%) and lateral size (usually <1 µm) in produced materials. Herein, solution processing to realize high phase‐purity 1T′‐MoS₂ monolayers with large lateral size is demonstrated. Direct chemical exfoliation of millimeter‐sized 1T′ crystal is introduced to successfully produce a high‐yield of 1T′‐MoS₂ monolayers with over 97% phase purity and unprecedentedly large size up to tens of micrometers. Furthermore, the large‐sized and high‐quality 1T′‐MoS₂ nanosheets exhibit clear intrinsic superconductivity among all thicknesses down to monolayer, accompanied by a slow drop of transition temperature from 6.1 to 3.0 K. Prominently, unconventional superconducting behavior with upper critical field far beyond the Pauli limit is observed in the centrosymmetric 1T′‐MoS₂ structure. The results open up an ideal approach to explore the properties of 2D metastable polymorphic materials.