Graphene MoS2 Research Articles

Characterization and physical modeling of MOS capacitors in epitaxial graphene monolayers and bilayers on 6H-SiC

Capacitance voltage (CV) measurements are performed on planar MOS capacitors with an Al2O3 dielectric fabricated in hydrogen intercalated monolayer and bilayer graphene grown on 6H-SiC as a function of frequency and temperature. Quantitative models of the CV data are presented in conjunction with the measurements in order to facilitate a physical understanding of graphene MOS systems. An interface state density of order 2 ⋅ 1012 eV−1 cm−2 is found in both material systems. Surface potential fluctuations of order 80-90meV are also assessed in the context of measured data. In bilayer material, a narrow bandgap of 260meV is observed consequent to the spontaneous polarization in the substrate. Supporting measurements of material anisotropy and temperature dependent hysteresis are also presented in the context of the CV data and provide valuable insight into measured and modeled data. The methods outlined in this work should be applicable to most graphene MOS systems.

Narrow‐Gap Quantum Wires Arising from the Edges of Monolayer MoS2Synthesized on Graphene

MoS2/graphene (MoS2/Gr) vertical heterostructures have exhibited great application potentials in high speed electronic and optoelectronic devices, as well as efficient electrocatalysts in hydrogen evolution reaction (HER). The electronic property at the edge of monolayer MoS2is an essential parameter for addressing such applications. Herein, it is reported that, for monolayer MoS2synthesized on Gr/Au foils by chemical vapor deposition, a dramatic decrease of the bandgap from ≈2.20 to ≈0.30 eV occurs at the domain edge within a lateral distance of ≈6 nm, as evidenced by scanning tunneling microscopy/spectroscopy observations. The edges of monolayer MoS2on Gr/Au foils can thus be regarded as narrow‐gap quantum wires considering of their reduced bandgaps. More intriguingly, it is found that this bandgap decrease at the domain edge is closely related to the rather high HER performance for MoS2/Gr/Au foils comparing with that of MoS2/Au foils. Briefly, this work should propel the band structure investigations for MoS2/Gr stacks and their applications in energy related fields.

Facile synthesis of molybdenum disulfide/nitrogen-doped graphene composites for enhanced electrocatalytic hydrogen evolution and electrochemical lithium storage

A facile one-pot hydrothermal route with the mediation of ionic liquid (IL, [BMIM]BF4) is presented to synthesize MoS2/nitrogen-doped graphene (MoS2/NG-IL) composites. It is found that as-prepared MoS2/NG-IL composites exhibit that the de-layered MoS2 sheets with more exposed edge sites and defects are well anchored on the N-doped graphene. Due to the synergism between de-layered MoS2 and N-doped graphene, MoS2/NG-IL composites exhibit better electrochemical performances for hydrogen evolution reaction (HER) and reversible lithium storage in comparison with MoS2. Especially, when 1.0 mL of IL is added in hydrothermal solution, the obtained MoS2/NG-IL10 displays very high electrocatalytic activity for HER with a low onset overpotential of 80 mV and a small Tafel slope of 48.0 mV/dec in 0.5 M H2SO4. As anode of lithium ion battery, MoS2/NG-IL10 delivers a reversible capacity as high as 1169 mAh g−1 at 100 mA g−1 and enhanced rate capability of 782 mAh g−1 at 1000 mA g−1. After 800 cycles, a reversible capacity of about 800 mAh g−1 at 500 mA g−1 can be retained, indicating its excellent cyclic stability.

(Invited) 2D Materials for Wearable Electronics

Tactile sensors, in the form of conformal and embedded devices, have attracted intense research interest because of their diverse applications, from electronic skin (E-skin) for robotics to health care monitoring systems. Much effort has been made to develop large- area and high-performance tactile sensors with good sensitivity and mechanical flexibility. MoS2 semiconductors have recently attracted attention because of their outstanding mechanical and optical transmittance, high gauge factor, and tunable band gap. However, there is no report on multi-cell devices for practical applications that transcend the fabrication of a single cell. In this talk, I present an ultrathin conformal, MoS2-based tactile sensing array covering an area of 2.2 cm ´ 2.2 cm. We integrated the sensor with a graphene electrode and interconnect to achieve good mechanical flexibility and optical transmittance in the visible color range (Figure 1). This sensor shows high sensitivity, good uniformity, and linearity even after 1000 loading cycles. In addition, it provides excellent mechanical flexibility over strain of 1.98% and better than 80% transparency. Figure 1. MoS2 tactile sensor transferred on the fingertip. a-b) Optical and SEM images of the MoS2 tactile sensor on a fingertip. c) Colorized SEM images of the MoS2 strain gauge (left, yellow) and graphene electrode (right, blue). d) Pressure map detected by the MoS2 tactile sensor from contact with pens of different diameters (7 and 5 mm). e) Calculated crosstalk isolation of the MoS2 sensor array with different pen-tip sizes (D : 7,5 and 2 mm). Figure 1

Hybrid graphene–molybdenum disulphide based ring resonator for label-free sensing

In this paper, a novel graphene–MoS2 hybrid structure based surface plasmon resonance sensor is presented for label-free analysis. The structure consists of a silicon nitride (Si3N4) dielectric layer vertically coupled to a thin layer of metallic strip made of silver (Ag) on top. A hybrid graphene–MoS2 layer is added on top of the metallic strip to enhance the sensitivity and the quality factor of the sensor. The cladding layer is assumed to be porous alumina (p-Al2O3) increasing the interaction of the surface plasmon mode and the target molecules. Finite difference time domain analysis (FDTD) has been used to design and to analyze the performance of the sensor. It is shown that by addition of hybrid graphene–MoS2 layer to the structure of a surface plasmon resonance based sensor, the refractive index sensitivity and the intrinsic quality factor of the sensor are enhanced simultaneously. It is also shown that addition of the hybrid graphene–MoS2 layer leads to higher values of figure of merit (FOM) for the sensor, and consequently better performance of the sensor. Moreover, the effect of increasing the number of graphene and MoS2 layers is investigated. The proposed sensor is very compact and can be used for lab-on-a-chip sensing applications.

Metal-Insulator-Semiconductor Diode Consisting of Two-Dimensional Nanomaterials.

We present a novel metal-insulator-semiconductor (MIS) diode consisting of graphene, hexagonal BN, and monolayer MoS2 for application in ultrathin nanoelectronics. The MIS heterojunction structure was fabricated by vertically stacking layered materials using a simple wet chemical transfer method. The stacking of each layer was confirmed by confocal scanning Raman spectroscopy and device performance was evaluated using current versus voltage (I-V) and photocurrent measurements. We clearly observed better current rectification and much higher current flow in the MIS diode than in the p-n junction and the metal-semiconductor diodes made of layered materials. The I-V characteristic curve of the MIS diode indicates that current flows mainly across interfaces as a result of carrier tunneling. Moreover, we observed considerably high photocurrent from the MIS diode under visible light illumination.

Graphene and Beyond Graphene MoS2: A New Window in Surface-Plasmon-Resonance-Based Fiber Optic Sensing

We theoretically propose a surface-plasmon-based fiber optic biosensor in metal/graphene/MoS2 configuration with molybdenum disulfide (MoS2) as a bio recognition layer. The proposed configuration works in the visible region of the electromagnetic spectrum with a very high sensitivity. A comparative theoretical study of the sensors with different metallic layers of gold (Au), copper (Cu), and aluminum (Al) has been performed. The sensor has been found to be the most sensitive in both Cu/graphene/MoS2 and Al/graphene/MoS2 configurations with sensitivity of 6.2 μm/RIU. In both of the configurations the thicknesses of Cu and Al layers is 50 nm and the number of layers of graphene is 16 and 27, respectively, while only a single layer of MoS2 has been used. The sensitivity of the sensor in the Au/graphene/MoS2 configuration is 5.0 μm/RIU with comparatively high depth of resonance.

Nanosheets of MoS2 and reduced graphene oxide as hybrid fillers improved the mechanical and tribological properties of bismaleimide composites

The hybrid nanoparticles composed of reduced graphene oxide and MoS2 nanosheets with active amino groups (NH2-rGO/MoS2) were fabricated by a facile and effective method. Then the bismaleimide (BMI) composites filled with the as-prepared NH2-rGO/MoS2 nanoparticles were obtained by a casting method. The microstructure and morphology of NH2-rGO/MoS2 nanoparticles, the mechanical properties, thermal property and tribological properties of NH2-rGO/MoS2/BMI composites were investigated. The results show that the interlaminar distance of MoS2 nanosheets in the hybrid nanoparticles is 0.71 nm, which is much larger than that of bulk MoS2 (0.62 nm); and the NH2-rGO/MoS2/BMI composites possess better friction-reducing and anti-wear performances (the friction coefficient and volume wear rate are about 0.2 and 2.9 × 10−6 mm3/(N·m), respectively) compared to the neat BMI resin. This is mainly attributed to the enhanced toughness of the nanoparticles, good interfacial adhesion between NH2-rGO/MoS2 and BMI matrix as well as the synergistic effect between rGO and MoS2 nanosheets.

Self-Assembly-Induced Alternately Stacked Single-Layer MoS2 and N-doped Graphene: A Novel van der Waals Heterostructure for Lithium-Ion Batteries.

In this article, a simple self-assembly strategy for fabricating van der Waals heterostructures from isolated two-dimensional atomic crystals is presented. Specifically, dopamine (DOPA), an excellent self-assembly agent and carbon precursor, was adsorbed on exfoliated MoS2 monolayers through electrostatic interaction, and the surface-modified monolayers self-assembled spontaneously into DOPA-intercalated MoS2. The subsequent in situ conversion of DOPA to highly conductive nitrogen-doped graphene (NDG) in the interlayer space of MoS2 led to the formation of a novel NDG/MoS2 nanocomposite with well-defined alternating structure. The NDG/MoS2 was then studied as an anode for lithium-ion batteries (LIBs). The results show that alternating arrangement of NDG and MoS2 triggers synergistic effect between the two components. The kinetics and cycle life of the anode are greatly improved due to the enhanced electron and Li(+) transport as well as the effective immobilization of soluble polysulfide by NDG. A reversible capacity of more than 460 mAh/g could be delivered even at 5 A/g. Moreover, the abundant voids created at the MoS2-NDG interface also accommodate the volume change during cycling and provide additional active sites for Li(+) storage. These endow the NDG/MoS2 heterostructure with low charge-transfer resistance, high sulfur reservation, and structural robustness, rendering it an advanced anode material for LIBs.

Graphene-mediated highly-dispersed MoS2 nanosheets with enhanced triiodide reduction activity for dye-sensitized solar cells

Novel composites made of two dimensional (2D) MoS2 nanosheets and graphene (G) have been fabricated by a combined approach of chemical vapor deposition and hydrothermal technique. The G film helps to mediate the growth and dispersion of MoS2 nanosheets, of which the unique role of G in the MoS2 growth is revealed by density functional theory study. The results show that the hexagonal lattice carbon of the G film can easily interact with sulfur species derived from reaction precursors, which favors the uniform growth of 2D MoS2. The unique function of the G film demonstrated here can be extended to other carbon substrates for growing 2D MoS2 nanosheets. The G-MoS2 composites consisting of two types of 2D materials are tested as the binder-free counter electrodes (CEs) for dye-sensitized solar cells, showing a high power conversion efficiency of 7.1% that is comparable to the expensive Pt CEs. The 2D G film in the hybrids has two functions: the active sites for dispersing the electrochemically active MoS2 crystals and the high electrical conducting matrix for fast charge transfer. This synergistic effect may help to shed a light on the functionalization of other inorganic materials to G for advanced energy applications.

3D composites of layered MoS2 and graphene nanoribbons for high performance lithium-ion battery anodes

3D composites of layered MoS2 and interconnected graphene nanoribbons (GNRs) synthesized by a facile one-pot hydrothermal method exhibit excellent cycling stability and rate capacity.

Ultra-low voltage electrowetting using graphite surfaces.

The control of wetting behaviour underpins a variety of important applications from lubrication to microdroplet manipulation. Electrowetting is a powerful method to achieve external wetting control, by exploiting the potential-dependence of the liquid contact angle with respect to a solid substrate. Addition of a dielectric film to the surface of the substrate, which insulates the electrode from the liquid thereby suppressing electrolysis, has led to technological advances such as variable focal-length liquid lenses, electronic paper and the actuation of droplets in lab-on-a-chip devices. The presence of the dielectric, however, necessitates the use of large bias voltages (frequently in the 10-100 V range). Here we describe a simple, dielectric-free approach to electrowetting using the basal plane of graphite as the conducting substrate: unprecedented changes in contact angle for ultra-low voltages are seen below the electrolysis threshold (50° with 1 V for a droplet in air, and 100° with 1.5 V for a droplet immersed in hexadecane), which are shown to be reproducible, stable over 100 s of cycles and free of hysteresis. Our results dispel conventional wisdom that reversible, hysteresis-free electrowetting can only be achieved on solid substrates with the use of a dielectric. This work paves the way for the development of a new generation of efficient electrowetting devices using advanced materials such as graphene and monolayer MoS2.

Computational Chemistry Analysis of Hydrodesulfurization Reactions Catalyzed by Molybdenum Disulfide Nanoparticles

Molybdenum disulfide, MoS2, is a very versatile material used in several applications of science and engineering. In particular, when supported on alumina, it is a widely used catalyst for hydrotreating processes in petroleum refineries. Stringent environmental norms and uncertainty in import of low sulfur crude oil put pressure on refiners worldwide to develop more efficient catalysts to meet the demand of clean fuel. The hydrodesulfurization of thiophene and dibenzothiophene on both unpromoted and nickel-promoted MoS2 catalyst is analyzed using a multiscale approach including classical molecular dynamics (MD) and density functional theory (DFT). MD simulations are performed on a hydrocarbon mixture containing sulfur compounds to determine relative positions of thiophene and dibenzothiophene molecules with respect to the MoS2 catalytic surface previous to possible reactions that are then studied with DFT. The sample box contains a total of 57 515 atoms and is able to represent a realistic size of the nanocatalyst process. Under typical hydrodesulfurization conditions, vacancies are needed for adsorption of sulfur compounds and therefore activation of hydrogen becomes an important step. In addition, complexes of MoS2 with graphene and boron nitride, BN, surfaces are also analyzed. MoS2–graphene complexes show improvement in the efficiency of adsorption of thiophene on the catalyst edge. Both hydrogenation and direct desulfurization pathways are presented on pristine MoS2 clusters as well as under typical hydrodesulfurization conditions.

Hydrothermal synthesis of layered molybdenum sulfide/N-doped graphene hybrid with enhanced supercapacitor performance

Graphene-based composites have been deemed as promising materials in renewable energy-storage applications. Herein, we report a hybrid architecture consisting of layered molybdenum sulfide nanosheets/N-doped graphene (MoS2/NG) synthesized by one-pot hydrothermal method. By adjusting precursor ratios, flower-like MoS2/NG hybrid with nitrogen content of 3.5 at.% on the graphene layers can be obtained. Electrochemical characterizations indicate that the maximum specific capacitance of the MoS2/NG electrodes reaches up to 245F/g at 0.25A/g (and 146F/g at 20A/g). In addition, the electrode exhibits superior cyclic stability with 91.3% capacitance retention after 1000 cycles at 2A/g. The outstanding performance of the MoS2/NG hybrid benefits from the synergistic effect between the layered MoS2 and N-doped graphene.

Half-cell and full-cell investigations of 3D hierarchical MoS2/graphene composite on anode performance in lithium-ion batteries

A novel 3D hierarchical MoS2/graphene (MoS2/GN) composite is designed by a facile one-step hydrothermal co-assembling method without using any templates. SEM and TEM images show that the MoS2/GN flower-like particles are self-assembled by MoS2 nanoflakes and graphene nanosheets. According to the hydrothermal method used and characterization results observed, the formation process is proposed. Electrochemical performances of the MoS2/GN composite as anode active material in Lithium-ion batteries (LIBs) is investigated in both MoS2/GN//Li half-cells and MoS2/GN//LiCoO2 full-cells. The MoS2/GN composite delivers high initial discharge capacities of 1240 mAh g−1 and good capacity retentions (about 80%) after more than 80 cycles in half-cells. Furthermore, the assembled MoS2/GN//LiCoO2 full-cell delivers high initial discharge capacities of 1203 mAh g−1. The excellent lithium storage performances of the obtained MoS2/GN composite can be mainly attributed to the designed novel structure of the composite. The cross-linked graphene nanosheets, the anchoring MoS2 nanoflakes and the synergistic effects between them make the composite good conductivity, enough buffering space for the volume change, and shortened ionic transport length. This work clearly demonstrates that the MoS2/GN composite is a promising alternative material for anode in the LIB applications.

Graphene–MoS2 composite: Hydrothermal synthesis and catalytic property in hydrodesulfurization of dibenzothiophene

Graphene–MoS2 composite synthesized by reacting graphene oxide (GO) with MoO3 and NH4SCN at pH<1 produced a physical mixture of graphene and large MoS2 particles. When GO was pre-treated with NaOH solution, smaller MoS2 particles were directly grown on the graphene. The physicochemical properties of the composites were correlated with their catalytic performance in the HDS of DBT. The higher hydrogenating activity obtained over the composite with the larger MoS2 particles was brought about by the formation of two “over-stoichiometric” sulfur: disulfide species S22- bonded to the surface of MoS2 particles and thiol groups attached to the graphene support.

Sensitivity enhancement of surface plasmon resonance sensor based on graphene–MoS2 hybrid structure with TiO2–SiO2 composite layer

In this paper, surface plasmon resonance (SPR) sensor based on graphene–MoS2 hybrid structure with composite layer of TiO2–SiO2 is presented. The angular interrogation method is used for the analysis of reflected light from the sensor. For the calculation of the sensitivity, first of all the thicknesses of TiO2, SiO2 and gold layers are optimized for the monolayer graphene and MoS2. Thereafter, at these optimum thicknesses the reflectance curves are plotted for different sensor structure and comparison of change in resonance angle is made among these structures. It is observed that the sensitivity of the graphene–MoS2-based sensor is enhanced by 9.24 % with respect conventional SPR sensor. The sensitivity is further enhanced by including TiO2–SiO2 composite layer between prism base and metal layer and observed that the enhanced sensitivity for this sensor is 12.82 % with respect to conventional SPR sensor and 3.28 % with respect to graphene–MoS2-based SPR sensor. At the end of this paper, the variation of the sensitivity and minimum reflectance is plotted with respect to sensing layer refractive index at the optimum thickness of all the layers and optimum number of MoS2 and graphene layers. It is also observed that four layers of MoS2 and monolayer graphene are best selection for the maximum enhancement of the sensitivity.

MoS2 graphene fiber based gas sensing devices

A facile method is developed to synthesize a fiber-like reduced graphene oxided (rGO)/molybdenum disulfide (MoS2) composites through a wet-spinning and a hydrothermal method. In the presence of Dodecyl dimethyl benzyl ammonium chloride, graphene provides a substrate for nucleation of MoS2. In this composite, MoS2 nanosheets can be anchored onto the surface of graphene through both physical adsorption and electron transfer by hydrothermal method. This process inhibits both graphene and MoS2 layers from stacking together. The gas sensing properties are evaluated in an intelligent gas sensing analysis system. It is demonstrated that the obtained composite fiber devices show an excellent sensitivity and selectivity to NO2 and NH3 than the individual components in different light illumination conditions and the proper proportion of components was detected.

Performance of graphene–MoS2 based surface plasmon resonance sensor using Silicon layer

In this paper a graphene–MoS2 hybrid structure based surface plasmon resonance biosensor is presented. The performance parameters of the proposed sensor are defined in terms of sensitivity, detection accuracy and quality factor. By the addition of hybrid graphene–MoS2 layer the sensitivity is enhanced but the quality factor and detection accuracy is decreased. Hence to increase the quality factor and detection accuracy a silicon layer is included between metal and MoS2 layer. It is observed that the full width at half maximum of reflectance curve is minimized up to great extent with little decrement in the sensitivity due to the inclusion of silicon layer. Furthermore in this paper, the effect of increasing the number of layers of graphene and MoS2 is also analyzed.

Ultrafast synthesis of MoS2 or WS2-reduced graphene oxide composites via hybrid microwave annealing for anode materials of lithium ion batteries

An ultrafast and simple strategy to synthesize metal sulfides (MoS2 and WS2) anchored on reduced graphene oxide (RGO) composites is reported as anode materials for lithium ion batteries (LIBs). Metal sulfide nanocrystals with homogeneous dispersion onto conducting RGO sheets are obtained in only 45 s by hybrid microwave annealing (HMA) method. The synthesized materials, especially MoS2/RGO composite, exhibit a high Li capacity, an excellent rate capability, and a stable cycling performance, comparable to the reported best MS2/carbon composite electrodes. The results highlight the effectiveness of HMA method to fabricate the metal sulfide/RGO composites with excellent electric properties.