Designer Shape Anisotropy on Transition‐Metal‐Dichalcogenide Nanosheets

MoS₂ and generally speaking, the wide family of transition‐metal dichalcogenides represents a solid nanotechnology platform on which to engineer a wealth of new and outperforming applications involving 2D materials. An even richer flexibility can be gained by extrinsically inducing an in‐plane shape anisotropy of the nanosheets. Here, the synthesis of anisotropic MoS₂ nanosheets is proposed as a prototypical example in this respect starting from a highly conformal chemical vapor deposition on prepatterend substrates and aiming at the more general purpose of tailoring anisotropy of 2D nanosheets by design. This is envisioned to be a suitable configuration for strain engineering as far as strain can be spatially redistributed in morphologically different regions. With a similar approach, both the optical and electronic properties of the 2D transition‐metal dichalcogenides can be tailored over macroscopic sample areas in a self‐organized fashion, thus paving the way for new applications in the field of optical metasurfaces, light harvesting, and catalysis.     

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.     

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.     

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.     

Electrostatic‐Driven Exfoliation and Hybridization of 2D Nanomaterials

Here, direct and effective electrostatic‐driven exfoliation of tungsten trioxide (WO₃) powder into atomically thin WO₃ nanosheets is demonstrated for the first time. Experimental evidence together with theoretical simulations clearly reveal that the strong binding of bovine serum albumin (BSA) on the surface of WO₃ via the protonation of ? NH₂ groups in acidic conditions leads to the effective exfoliation of WO₃ nanosheets under sonication. The exfoliated WO₃ nanosheets have a greatly improved dispersity and stability due to surface‐protective function of BSA, and exhibit a better performance and unique advantages in applications such as visible‐light‐driven photocatalysis, high‐capacity adsorption, and fast electrochromics. Further, simultaneous exfoliation and hybridization of WO₃ and MoS₂ nanosheets are demonstrated to form hybrid WO₃/MoS₂ nanosheets through respective electrostatic and hydrophobic interaction processes. In addition, this electrostatic‐driven exfoliation strategy is applied to exfoliate ultrathin black‐phosphorus nanosheets from its bulk to exhibit a greatly improved stability due to the surface protection by BSA. Overall, the work presented not only presents a facile and effective route to fabricate 2D materials but also brings more opportunities to exploit unusual exotic and synergistic properties in resulting hybrid 2D materials for novel applications.     

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.     

Stabilizing MoS2 Nanosheets through SnO2 Nanocrystal Decoration for High‐Performance Gas Sensing in Air

The unique properties of MoS₂ nanosheets make them a promising candidate for high‐performance room temperature sensing. However, the properties of pristine MoS₂ nanosheets are strongly influenced by the significant adsorption of oxygen in an air environment, which leads to instability of the MoS₂ sensing device, and all sensing results on MoS₂ reported to date were exclusively obtained in an inert atmosphere. This significantly limits the practical sensor application of MoS₂ in an air environment. Herein, a novel nanohybrid of SnO₂ nanocrystal (NC)‐decorated crumpled MoS₂ nanosheet (MoS₂/SnO₂) and its exciting air‐stable property for room temperature sensing of NO₂ are reported. Interestingly, the SnO₂ NCs serve as strong p‐type dopants for MoS₂, leading to p‐type channels in the MoS₂ nanosheets. The SnO₂ NCs also significantly enhance the stability of MoS₂ nanosheets in dry air. As a result, unlike other MoS₂ sensors operated in an inert gas (e.g. N₂), the nanohybrids exhibit high sensitivity, excellent selectivity, and repeatability to NO₂ under a practical dry air environment. This work suggests that NC decoration significantly tunes the properties of MoS₂ nanosheets for various applications.     

Engineering 2D Metal–Organic Framework/MoS2 Interface for Enhanced Alkaline Hydrogen Evolution

2D metal–organic frameworks (MOFs) have been widely investigated for electrocatalysis because of their unique characteristics such as large specific surface area, tunable structures, and enhanced conductivity. However, most of the works are focused on oxygen evolution reaction. There are very limited numbers of reports on MOFs for hydrogen evolution reaction (HER), and generally these reported MOFs suffer from unsatisfactory HER activities. In this contribution, novel 2D Co‐BDC/MoS₂ (BDC stands for 1,4‐benzenedicarboxylate, C₈H₄O₄) hybrid nanosheets are synthesized via a facile sonication‐assisted solution strategy. The introduction of Co‐BDC induces a partial phase transfer from semiconducting 2H‐MoS₂ to metallic 1T‐MoS₂. Compared with 2H‐MoS₂, 1T‐MoS₂ can activate the inert basal plane to provide more catalytic active sites, which contributes significantly to improving HER activity. The well‐designed Co‐BDC/MoS₂ interface is vital for alkaline HER, as Co‐BDC makes it possible to speed up the sluggish water dissociation (rate‐limiting step for alkaline HER), and modified MoS₂ is favorable for the subsequent hydrogen generation step. As expected, the resultant 2D Co‐BDC/MoS₂ hybrid nanosheets demonstrate remarkable catalytic activity and good stability toward alkaline HER, outperforming those of bare Co‐BDC, MoS₂, and almost all the previously reported MOF‐based electrocatalysts.     

Synergetic Exfoliation and Lateral Size Engineering of MoS2 for Enhanced Photocatalytic Hydrogen Generation

Generally, exfoliation is an efficient strategy to create more edge site so as to expose more active sites on molybdenum disulphide (MoS₂). However, the lateral sizes of the resultant MoS₂ monolayers are relatively large (≈50–500 nm), which retain great potential to release more active sites. To further enhance the catalytic performance of MoS₂, a facile cascade centrifugation‐assisted liquid phase exfoliation method is introduced here to fabricate monolayer enriched MoS₂ nanosheets with nanoscale lateral sizes. The as‐prepared MoS₂ revealed a high monolayer yield of 36% and small average lateral sizes ranging from 42 to 9 nm under gradient centrifugations, all exhibiting superior catalytic performances toward photocatalytic H₂ generation. Particularly, the optimized monolayer MoS₂ with an average lateral size of 9 nm achieves an apparent quantum efficiency as high as 77.2% on cadmium sulphide at 420 nm. This work demonstrates that the catalytic performances of MoS₂ could be dramatically enhanced by synergistic exfoliation and lateral size engineering as a result of increased density of active sites and shortened charge diffusion distance, paving a new way for design and fabrication of transition‐metal dichalcogenides‐based materials in the application of hydrogen generation.     

Porous Hybrid Composites of Few‐Layer MoS2 Nanosheets Embedded in a Carbon Matrix with an Excellent Supercapacitor Electrode Performance

Porous hierarchical architectures of few‐layer MoS₂ nanosheets dispersed in carbon matrix are prepared by a microwave‐hydrothermal method followed by annealing treatment via using glucose as C source and structure‐directing agent and (NH₄)₂MoS₄ as both Mo and S sources. It is found that the morphology and size of the secondary building units (SBUs), the size and layer number of MoS₂ nanosheets as well as the distribution of MoS₂ nanosheets in carbon matrix, can be effectively controlled by simply adjusting the molar ratio of (NH₄)₂MoS₄ to glucose, leading to the materials with a low charge–transfer resistance, many electrochemical active sites and a robust structure for an outstanding energy storage performance including a high specific capacitance (589 F g⁻¹ at 0.5 A g⁻¹), a good rate capability (364 F g⁻¹ at 20 A g⁻¹), and an excellent cycling stability (retention 104% after 2000 cycles) for application in supercapacitors. The exceptional rate capability endows the electrode with a high energy density of 72.7 Wh kg⁻¹ and a high power density of 12.0 kW kg⁻¹ simultaneously. This work presents a facile and scalable approach for synthesizing novel heterostructures of MoS₂‐based electrode materials with an enhanced rate capability and cyclability for potential application in supercapacitor.     

Structure Engineering of MoS2 via Simultaneous Oxygen and Phosphorus Incorporation for Improved Hydrogen Evolution

Oxygen and phosphorus dual‐doped MoS₂ nanosheets (O,P‐MoS₂) with porous structure and continuous conductive network are fabricated using a one‐pot NaH₂PO₂‐assisted hydrothermal approach. By simply changing the precursor solution, the chemical composition and resulting structure can be effectively controlled to obtain desired properties toward the hydrogen evolution reaction (HER). Thanks to the beneficial structure and strong synergistic effects between the incorporated oxygen and phosphorus, the optimal O,P‐MoS₂ exhibit superior electrocatalytic performances compared with those of oxygen single‐doped MoS₂ nanosheets (O‐MoS₂). Specifically, a low HER onset overpotential of 150 mV with a small Tafel slope of 53 mV dec^?1, excellent conductivity, and long‐term durability are achieved by the structural engineering of MoS₂ via O and P co‐doping, making it an efficient HER electrocatalyst for water electrocatalysis. This work provides an alternative strategy to manipulate transition metal dichalcogenides as advanced materials for electrocatalytic and related energy applications.     

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.     

Synthesis of Ultrahigh‐Quality Monolayer Molybdenum Disulfide through In Situ Defect Healing with Thiol Molecules

Monolayer transition metal dichalcogenides are 2D materials with many potential applications. Chemical vapor deposition (CVD) is a promising method to synthesize these materials. However, CVD‐grown materials generally have poorer quality than mechanically exfoliated ones and contain more defects due to the difficulties in controlling precursors' distribution and concentration during growth where solid precursors are used. Here, thiol is proposed to be used as a liquid precursor for CVD growth of high quality and uniform 2D MoS₂. Atomic‐resolved structure characterizations indicate that the concentration of sulfur vacancies in the MoS₂ grown from thiol is the lowest among all reported CVD samples. Low temperature spectroscopic characterization further reveals the ultrahigh optical quality of the grown MoS₂. Density functional theory simulations indicate that thiol molecules could interact with sulfur vacancies in MoS₂ and repair these defects during the growth of MoS₂, resulting in high‐quality MoS₂. This work provides a facile and controllable method for the growth of high‐quality 2D materials with ultralow sulfur vacancies and high optical quality, which will benefit their optoelectronic applications.     

Epitaxial Growth of Rectangle Shape MoS2 with Highly Aligned Orientation on Twofold Symmetry a‐Plane Sapphire

Research on transition metal dichalcogenides (TMDs) has been accelerated by the development of large‐scale synthesis based on chemical vapor deposition (CVD) growth. However, in most cases, CVD‐grown TMDs are composed of randomly oriented grains, and thus contain many distorted grain boundaries (GBs), which seriously degrade their electrical and photoelectrical properties. Here, the epitaxial growth of highly aligned MoS₂ grains is reported on a twofold symmetry a‐plane sapphire substrate. The obtained MoS₂ grains have an unusual rectangle shape with perfect orientation alignment along the [1‐100] crystallographic direction of a‐plane sapphire. It is found that the growth temperature plays a key role in its orientation alignment and morphology evolution, and high temperature is beneficial to the initial MoS₂ seeds rotate to the favorable orientation configurations. In addition, the photoluminescence quenching of the well‐aligned MoS₂ grains indicates a strong MoS₂?substrate interaction which induces the anisotropic growth of MoS₂, and thus brings the formation of rectangle shape grains. Moreover, the well‐aligned MoS₂ grains splice together without GB formation, and thus that has negligible effect on its electrical transport properties. The progress achieved in this work could promote the controlled synthesis of large‐area TMDs single crystal film and the scalable fabrication of high‐performance electronic devices.     

Ruthenium Nanoparticles on Cobalt‐Doped 1T′ Phase MoS2 Nanosheets for Overall Water Splitting

2D MoS₂ nanostructures have recently attracted considerable attention because of their outstanding electrocatalytic properties. The synthesis of unique Co–Ru–MoS₂ hybrid nanosheets with excellent catalytic activity toward overall water splitting in alkaline solution is reported. 1T′ phase MoS₂ nanosheets are doped homogeneously with Co atoms and decorated with Ru nanoparticles. The catalytic performance of hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is characterized by low overpotentials of 52 and 308 mV at 10 mA cm^?2 and Tafel slopes of 55 and 50 mV decade^?1 in 1.0 m KOH, respectively. Analysis of X‐ray photoelectron and absorption spectra of the catalysts show that the MoS₂ well retained its metallic 1T′ phase, which guarantees good electrical conductivity during the reaction. The Gibbs free energy calculation for the reaction pathway in alkaline electrolyte confirms that the Ru nanoparticles on the Co‐doped MoS₂ greatly enhance the HER activity. Water adsorption and dissociation take place favorably on the Ru, and the doped Co further catalyzes HER by making the reaction intermediates more favorable. The high OER performance is attributed to the catalytically active RuO₂ nanoparticles that are produced via oxidation of Ru nanoparticles.     

二硫化钼纳米片的制备及其光敏和气敏特性研究

利用高温硫化法制备了高纯度MoS_2纳米片。采用X射线衍射仪(XRD)、拉曼光谱仪(Raman spectrometer)、扫描电镜(SEM)和能谱仪(EDS)对MoS_2纳米片的物相、结晶质量以及形貌进行了表征,分析了反应温度对MoS_2纳米片的影响,并对MoS_2纳米片的光敏性和气敏特性进行了研究。结果表明,MoS_2纳米片对绿光响应度更高,红光次之,对甲醇和乙醇两种气体均有较高的灵敏度和响应/恢复速率,并对其气敏机理进行了讨论。     

Novel SiQDs–MoS<Subscript>2</Subscript> heterostructures with increasing solar absorption for the photocatalytic degradation of malachite green

Novel heterostructures based on silicon quantum dots and molybdenum disulfide nanosheets (SiQDs–MoS₂) were synthesized by a hydrothermal method, in which the introduced SiQDs play a determining role in manipulating the morphology, phase and band structure of MoS₂. The resultant SiQDs–MoS₂ is uniform flowerlike 3D microspheres assembled from petallike 2D MoS₂ nanosheets anchored with 0D SiQDs, possessing abundant active sites. Besides, the primary MoS₂ nanosheets consist of both semiconductive 2H and metallic 1T phases accompanied with intralayer mesopores and expanded interlayer spacing, endowing the resulting architectures with effective electron transfer. Significantly, the as-synthesized SiQDs–MoS₂ exhibits intense full solar-spectrum absorption, indicating efficient solar energy harvesting. First-principles calculations simulate similar increased spectral absorption of monolayer MoS₂ adhered with a Si cluster, suggesting the existence of new energy states associated with the integration of SiQDs and MoS₂ nanosheets as evidenced by photoluminescence (PL) spectral analysis. As expected, the current SiQDs–MoS₂ heterostructures demonstrate substantial photocatalytic activity even under visible and near-infrared (NIR) light on degradation of malachite green (MG). The type II electronic structure of SiQDs–MoS₂ was proposed, enabling sufficient photogenerated electrons and holes for the photocatalytic reactions. This study may establish a new frontier on the rational design and feasible development of the hybrid structures with the desirable morphologies, phase compositions and band structures for the catalysis and beyond.     

Lowering the Schottky Barrier Height by Graphene/Ag Electrodes for High‐Mobility MoS2 Field‐Effect Transistors

2D transition metal dichalcogenides (TMDCs) have emerged as promising candidates for post‐silicon nanoelectronics owing to their unique and outstanding semiconducting properties. However, contact engineering for these materials to create high‐performance devices while adapting for large‐area fabrication is still in its nascent stages. In this study, graphene/Ag contacts are introduced into MoS₂ devices, for which a graphene film synthesized by chemical vapor deposition (CVD) is inserted between a CVD‐grown MoS₂ film and a Ag electrode as an interfacial layer. The MoS₂ field‐effect transistors with graphene/Ag contacts show improved electrical and photoelectrical properties, achieving a field‐effect mobility of 35 cm² V^?1 s^?1, an on/off current ratio of 4 × 10⁸, and a photoresponsivity of 2160 A W^?1, compared to those of devices with conventional Ti/Au contacts. These improvements are attributed to the low work function of Ag and the tunability of graphene Fermi level; the n‐doping of Ag in graphene decreases its Fermi level, thereby reducing the Schottky barrier height and contact resistance between the MoS₂ and electrodes. This demonstration of contact interface engineering with CVD‐grown MoS₂ and graphene is a key step toward the practical application of atomically thin TMDC‐based devices with low‐resistance contacts for high‐performance large‐area electronics and optoelectronics.     

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.     

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.