Facile synthesis of MoS_2/graphite intercalated composite with enhanced electrochemical performance for sodium ion battery

MoS2 is a promising anode material for sodium ion batteries owing to its two-dimensional layered structure and high specific capacity. But it still exhibits a poor cycle stability and limited rate capability for Na＋ storage because of its poor electrical conductivity and structural instability. In this work, MoS2/graphite composite is fabricated by mechanically delaminated and restacked MoS2 and graphite to form two-dimensional composite layers. The graphite sheets will improve electrical conductivity and prevent the aggregation as well as structure collapse of the MoS2 layers during charge-discharge process. The MoS2/graphite composite exhibits excellent Na＋ storage properties. It delivers a high discharge specific capacity of 358.2 mAh/g at a current density of 100 mA]g in the first discharge process and with capacity retention of 68.1% after 800 cycles （retains 244 mAh/g）. The average discharge specific capacities retain 250.9 and 225.4 mAh/g corresponding to the current densities of 100 and 1000 mA]g, showing excellent rate capability. The improved electrochemical performance is attributed to the improved electrical conductivity and structural stability after composition of graphite sheets. The study demonstrates a new research strategy for improving sodium ion storage properties of Mo52.

Investigation of Single-Wall MoS_2 Monolayer Flakes Grown by Chemical Vapor Deposition

Recently, two-dimensional monolayer molybdenum disulfide(MoS_2), a transition metal dichalcogenide, has received considerable attention due to its direct bandgap, which does not exist in its bulk form, enabling applications in optoelectronics and also thanks to its enhanced catalytic activity which allows it to be used for energy harvesting. However,growth of controllable and high-quality monolayers is still a matter of research and the parameters determining growth mechanism are not completely clear. In this work, chemical vapor deposition is utilized to grow monolayer MoS_2 flakes while deposition duration and temperature effect have been systematically varied to develop a better understanding of the MoS_2 film formation and the influence of these parameters on the quality of the monolayer flakes. Different from previous studies, SEM results show that single-layer MoS_2 flakes do not necessarily grow flat on the surface, but rather they can stay erect and inclined at different angles on the surface, indicating possible gas-phase reactions allowing for monolayer film formation. We have also revealed that process duration influences the amount of MoO_3/MoO_2 within the film network. The homogeneity and the number of layers depend on the change in the desorption–adsorption of radicals together with sulfurization rates, and, inasmuch, a careful optimization of parameters is crucial. Therefore, distinct from the general trend of MoS_2 monolayer formation, our films are rough and heterogeneous with monolayer MoS_2 nanowalls. Despite this roughness and the heterogeneity, we observe a strong photoluminescence located around 675 nm.

Temperature-switching logic in MoS2 single transistors

Due to their unique characteristics,two-dimensional(2D)materials have drawn great attention as promising candidates for the next generation of integrated circuits,which generate a calculation unit with a new working mechanism,called a logic transistor.To figure out the application prospects of logic transistors,exploring the temperature dependence of logic characteristics is important.In this work,we explore the temperature effect on the electrical characteristic of a logic transistor,finding that changes in temperature cause transformation in the calculation:logical output converts from‘AND’at 10 K to‘OR’at 250 K.The transformation phenomenon of temperature regulation in logical output is caused by energy band which decreases with increasing temperature.In the experiment,the indirect band gap of MoS2 shows an obvious decrease from 1.581 eV to 1.535 eV as the temperature increases from 10 K to 250 K.The change of threshold voltage with temperature is consistent with the energy band,which confirms the theoretical analysis.Therefore,as a promising material for future integrated circuits,the demonstrated characteristic of 2D transistors suggests possible application for future functional devices.

2D MoS2 Helical Liquid Crystalline Fibers for Multifunctional Wearable Sensors

Fiber-based material systems are emerging as key elements for next-generation wearable devices due to their remarkable advantages,including large mechanical deformability,breathability,and high durability.Recently,greatly improved mechani-cal stability has been established in functional fiber systems by introducing atomic-thick two-dimensional(2D)materials.Further development of intelligent fibers that can respond to various external stimuli is strongly needed for versatile applica-tions.In this work,helical-shaped semiconductive fibers capable of multifunctional sensing are obtained by wet-spinning MoS2 liquid crystal(LC)dispersions.The mechanical properties of the MoS2 fibers were improved by exploiting high-purity LC dispersions consisting of uniformly-sized MoS2 nanoflakes.Notably,three-dimensional(3D)helical fibers with structural chirality were successfully constructed by controlling the wet-spinning process parameters.The helical fibers exhibited multifunctional sensing characteristics,including(1)photodetection,(2)pH monitoring,(3)gas detection,and(4)3D strain sensing.2D materials with semiconducting properties as well as abundant surface reactive sites enable smart multifunctionalities in one-dimensional(1D)and helical fiber geometry,which is potentially useful for diverse applications such as wearable internet of things(IoT)devices and soft robotics.

Folded MoS2 layers with reduced interlayer coupling

We study molybdenum disulfide （MoS2） structures generated by folding single-layer and bilayer MoS2 flakes. We find that this modified layer stacking leads to a decrease in the interlayer coupling and an enhancement of the photoluminescence emission yield. We additionally find that folded single-layer MoS2 structures show a contribution to photoluminescence spectra of both neutral and charged excitons, which is a characteristic feature of single-layer MoS2 that has not been observed in multilayer MoS2. The results presented here open the door to fabrication of multilayered MoS2 samples with high optical absorption while maintaining the advantageous enhanced photoluminescence emission of single-layer MoS2 by controllably twisting the MoS2 layers.

Defect-rich MoS2 nanowall catalyst for efficient hydrogen evolution reaction

Designing efficient electrocatalysts for the hydrogen evolution reaction （HER） has attracted substantial attention owing to the urgent demand for clean energy to face the energy crisis and subsequent environmental issues in the near future. Among the large variety of HER catalysts, molybdenum disulfide （MoS2） has been regarded as the most famous catalyst owing to its abundance, low price, high efficiency, and definite catalytic mechanism. In this study, defect-engineered MoS2 nanowall （NW） catalysts with controllable thickness were fabricated and exhibited a significantly enhanced HER performance. Benefiting from the highly exposed active edge sites and the rough surface accompanied by the robust NW structure, the defect-rich MoS2 NW catalyst with an optimized thickness showed an ultralow onset overpotential of 85 mV, a high current density of 310.6 mA·cm^-2 at η = 300 mV, and a low potential of 95 mV to drive a 10 mA·cm^-2 cathodic current. Additionally, excellent electrochemical stability was realized, making this freestanding NW catalyst a promising candidate for practical water splitting and hydrogen production.

Revisiting lithium-storage mechanisms of molybdenum disulfide

Molybdenum disulfide(MoS_(2)),a typical two-dimensional transition metallic layered material,attracts tremendous attentions in the electrochemical energy storage due to its excellent physicochemical properties.However,with the deepening of the research and exploration of the lithium storage mechanism of these advanced MoS_(2)-based anode materials,the complex reaction process influenced by internal and external factors hinders the exhaustive understanding of the lithium storage process.To design stable anode material with high performance,it is urgent to review the mechanisms of reported anode materials and summarize the related factors that influence the reaction processes.This review aims to dissect all possible side reactions during charging and discharging process,uncover internal and external factors inducing various anode reactions and finally put forward strategies of controlling high cycling capacity and super-stable lithium storage capability of MoS_(2).This review will be helpful to the design of MoS_(2)-based lithium-ion batteries(LIBs) with excellent cycle performance to enlarge the application fields of these advanced electrochemical energy storage devices.

Gate-tunable linear magnetoresistance in molybdenum disulfide field-effect transistors with graphene insertion layer

Molybdenum disulfide (MoS2) holds great promise as atomically thin two-dimensional (2D) semiconductor for future electronics and opto-electronics. In this report, we study the magnetoresistance (MR) of MoS2 field-effect transistors (FETs) with graphene insertion layer at the contact interface. Owing to the unique device structure and high-quality contact interface, a gate-tunable linear MR up to 67% is observed at 2 K. By comparing with the MRs of graphene FETs and MoS2 FETs with conventional metal contact, it is found that this unusual MR is most likely to be originated from the contact interfaces between graphene and MoS2, and can be explained by the classical linear MR model caused by spatial fluctuation of carrier mobility. Our study demonstrates large MR responses in MoS2-based systems through heterojunction design, shedding lights for the future magneto-electronics and van der Waals heterostructures.

Rapid synthesis of MoS_(2)-Ag nanocomposites via photoreduction for optical tuning and surface-enhanced Raman scattering applications

Semiconductoremetal nanocomposites have been widely investigated to modify the intrinsic properties of materials used for optoelectronic devices and sensing applications.In this study,a method for rapid synthesis of MoS_(2)-Ag nanocomposites via laser-assisted photoreduction was proposed.For the photoreduction process,we used AgNO_(3)solution as a metal source.Under laser irradiation,Ag ions were easily reduced on MoS_(2)by photo-generated electrons from MoS_(2).The optical properties of MoS_(2)-Ag nanocomposites were easily controlled by simple adjustment of the photoreduction time.To investigate the surface-enhanced Raman scattering(SERS)effect of the MoS_(2)-Ag nanocomposites,the SERS spectra of methylene blue(MB)on MoS_(2)-Ag nanocomposites were measured,and the nanocomposites were found to enhance the Raman scattering intensity of MB up to~106.Therefore,the laser-assisted photoreduction method has great potential for rapid synthesis and optical tuning of semiconductoremetal nanocomposites.

Optoelectronic devices based on two-dimensional transition metal dichalcogenides

In the past few years, two-dimensional （2D） transition metal dichalcogenide （TMDC） materials have attracted increasing attention of the research community, owing to their unique electronic and optical properties, ranging from the valley-spin coupling to the indirect-to-direct bandgap transition when scaling the materials from multi-layer to monolayer. These properties are appealing for the development of novel electronic and optoelectronic devices with important applications in the broad fields of communication, computation, and healthcare. One of the key features of the TMDC family is the indirect-to-direct bandgap transition that occurs when the material thickness decreases from multilayer to monolayer, which is favorable for many photonic applications. TMDCs have also demonstrated unprecedented flexibility and versatility for constructing a wide range of heterostructures with atomic-level control over their layer thickness that is also free of lattice mismatch issues. As a result, layered TMDCs in combination with other 2D materials have the potential for realizing novel high-performance optoelectronic devices over a broad operating spectral range. In this article, we review the recent progress in the synthesis of 2D TMDCs and optoelectronic devices research. We also discuss the challenges facing the scalable applications of the family of 2D materials and provide our perspective on the opportunities offered by these materials for future generations of nanophotonics technology.