Graphene MoS2 Research Articles

Vertically Stacked CVD-Grown 2D Heterostructure for Wafer-Scale Electronics

This paper demonstrates, for the first time, wafer-scale graphene/MoS2 heterostructures prepared by chemical vapor deposition (CVD) and their application in vertical transistors and logic gates. A CVD-grown bulk MoS2 layer is utilized as the vertical channel, whereas CVD-grown monolayer graphene is used as the tunable work-function electrode. The short vertical channel of the transistor is formed by sandwiching bulk MoS2 between the bottom indium tin oxide (ITO, drain electrode) and the top graphene (source electrode). The electron injection barriers at the graphene-MoS2 junction and ITO-MoS2 junction are modulated effectively through variation of the Schottky barrier height and its effective barrier width, respectively, because of the work-function tunability of the graphene electrode. The resulting vertical transistor with the CVD-grown MoS2/graphene heterostructure exhibits a current density exceeding 7 A/cm2, a subthreshold swing of 410 mV/dec, and an on-off current ratio exceeding 103. The large-area synthesis, transfer, and patterning processes of both semiconducting MoS2 and metallic graphene facilitate construction of a wafer-scale array of transistors and logic gates such as NOT, NAND, and NOR.

Mn2+-Assisted DNA Oligonucleotide Adsorption on Ti2C MXene Nanosheets.

As a new type of 2D nanomaterial, MXene (transition metal carbide/nitride) nanosheets are already widely used in catalysis, sensing, and energy research. DNA is a popular sensing molecule. Compared to other 2D materials such as graphene oxide, MoS2, and WS2, few fundamental studies were carried out on DNA adsorption by MXene. Due to its exfoliation and delamination process, the surface of MXene is abundant in -F, -OH, and -O- groups, rendering the surface negatively charged and repelling DNA. In previous studies, surface modification of MXene was performed to promote DNA adsorption. Herein, Mn2+ was discovered to promote DNA adsorption on unmodified Ti2C MXene. Different from Ca2+ and Mg2+, Mn2+ can inverse the ζ-potential of the Ti2C MXene to positive. DNA mainly uses its phosphate backbone for adsorption, while its bases contribute significantly less. In addition, delayed DNA desorption was observed through the addition of inorganic phosphate due to the formation of manganese phosphate to gradually extract Mn2+ from the DNA/MXene complex. Finally, DNA-induced DNA desorption from the Ti2C MXene can hardly distinguish the complementary DNA from a random DNA, which is very different from that for graphene oxide. This difference is likely due to the distinct surface chemistry between the MXene and graphene oxide.

High-performance monolayer MoS2 field-effect transistor with large-scale nitrogen-doped graphene electrodes for Ohmic contact

A finite Schottky barrier and large contact resistance between monolayer MoS2 and electrodes are the major bottlenecks in developing high-performance field-effect transistors (FETs) that hinder the study of intrinsic quantum behaviors such as valley-spin transport at low temperature. A gate-tunable graphene electrode platform has been developed to improve the performance of MoS2 FETs. However, intrinsic misalignment between the work function of pristine graphene and the conduction band of MoS2 results in a large threshold voltage for the FETs, because of which Ohmic contact behaviors are observed only at very high gate voltages and carrier concentrations (∼1013 cm−2). Here, we present high-performance monolayer MoS2 FETs with Ohmic contact at a modest gate voltage by using a chemical-vapor-deposited (CVD) nitrogen-doped graphene with a high intrinsic electron carrier density. The CVD nitrogen-doped graphene and monolayer MoS2 hybrid FETs platform exhibited a large negative shifted threshold voltage of −54.2 V and barrier-free Ohmic contact under zero gate voltage. Transparent contact by nitrogen-doped graphene led to a 214% enhancement in the on-current and a fourfold improvement in the field-effect carrier mobility of monolayer MoS2 FETs compared with those of a pristine graphene electrode platform. The transport measurements, as well as Raman and X-ray photoelectron spectroscopy analyses before and after thermal annealing, reveal that the atomic C-N bonding in the CVD nitrogen-doped graphene is responsible for the dominant effects of electron doping. Large-scale nitrogen-doped graphene electrodes provide a promising device platform for the development of high-performance devices and the study of unique quantum behaviors.

Atomic-scale mapping of hydrophobic layers on graphene and few-layer MoS2 and WSe2 in water

The structure and the role of the interfacial water in mediating the interactions of extended hydrophobic surfaces are not well understood. Two-dimensional materials provide a variety of large and atomically flat hydrophobic surfaces to facilitate our understanding of hydrophobic interactions. The angstrom resolution capabilities of three-dimensional AFM are exploited to image the interfacial water organization on graphene, few-layer MoS2 and few-layer WSe2. Those interfaces are characterized by the existence of a 2 nm thick region above the solid surface where the liquid density oscillates. The distances between adjacent layers for graphene, few-layer MoS2 and WSe2 are ~0.50 nm. This value is larger than the one predicted and measured for water density oscillations (~0.30 nm). The experiments indicate that on extended hydrophobic surfaces water molecules are expelled from the vicinity of the surface and replaced by several molecular-size hydrophobic layers.

Cyclotron–phonon resonance line-width in monolayer silicene

We theoretically observe the cyclotron–phonon resonance (CPR) in monolayer silicene by considering the magneto-optical absorption power (AP) of the system. The AP is calculated utilising the projection operator technique when a static magnetic field is subjected perpendicularly to the plane of the material and the electron–optical phonon scattering is involved. The absorption spectra show the resonance behaviour where the photon energies at resonant peaks satisfy the cyclotron–phonon (CPR) conditions in which the CPR energy depends linearly on the square root of the magnetic field strength. This dependence is similar to that in graphene and different from that in monolayer MoS2 and conventional low-dimensional semiconductors. The dependence of the full-width at half maximum (FWHM) on the magnetic field strength has the laws similar to those obtained for graphene and monolayer MoS2. However, the FWHM value in silicene is two times smaller than it is in graphene and has the same order as it does in monolayer MoS2. At relatively high temperature (T>300 K), the FWHM for phonon emission differs very little from that for phonon absorption and becomes identical in two cases when T>350 K.

Design and analysis of graphene–MoS2 hybrid layer based SPR biosensor with TiO2–SiO2 nano film for formalin detection: numerical approach

In this paper, a Surface Plasmon Resonance (SPR) biosensor has been numerically developed that used Graphene–MoS2–Au–TiO2–SiO2 hybrid structure for the detection of formalin. This developed sensor has been sensed the presence the formalin based on attenuated total reflection (ATR) method by observing the change of “surface plasmon resonance angle-the change of minimum reflectance attributor” and “the resonance frequency characteristics (RFC)-maximum transmittance attributor”. Chitosan has been used as probe sensing medium to accelerate particular reaction with the formalin. Here, graphene is used as biomolecular recognition element because of its high adsorption ability and optical characteristics which helps to improve sensor sensitivity, MoS2 used for it has larger band gap, high fluroscence quenching ability, higher optical absorption efficiency, TiO2–SiO2 bilayer as the improvement of sensitivity and Gold (Au) as the sharp SPR curve. Numerical results give the impression that the variation of RFC and SPR angle for improper sensing of formalin is quiet negligible that confirms no formalin is detected whereas for proper sensing these change are considerably countable that confirms the detection of formalin. From the sensor sensitivity analysis, owing to add TiO2–SiO2 bilayer with Graphene–MoS2 Hybrid layer, maximum sensitivity of 85.375% has been numerically resulted. This high sensor performance is for taking advantages of Graphene surface high selectively which detect bio molecular compounds through pi-stacking force, larger work function (5.1 eV) of MoS2 which allows the high sensitive detection of bio targets and Rich plasmon happens at the TiO2–SiO2 interface.

Hollow MoS2/rGO composites as high-performance anode materials for lithium-ion batteries

As a representative of transition metal sulfides, MoS2 has a graphene-like structure and has recently been widely used as an anode in lithium ion batteries. However, the disadvantages of poor electrical conductivity and cycle performance limit its application. Here, hollow MoS2/reduced graphene oxide (rGO) composites were successfully synthesized by a hydrothermal method using polystyrene microspheres as hard templates. The hollow MoS2 microspheres with a size of about 180 nm are distributed on the surface of graphene sheets. The hollow MoS2/rGO composites exhibit superior electrochemical properties as anode materials for lithium-ion batteries compared with the hollow MoS2 microspheres. They show a high reversible capacity of 752.4 mAh g−1 at 500 mA g−1 after 100 cycles with negligible decay as well as a high-rate performance (620.8 mA h g−1, 1 A g−1). The excellent lithium storage performance of the hollow MoS2/rGO composites can be attributed to the synergistic effect of the hollow MoS2 microspheres and graphene sheets. Graphene effectively suppresses the agglomeration of the MoS2 and maintain the structural stability of the material.

Performance enhancement of passively Q-switched Nd:YVO4 laser using graphene–molybdenum disulphide heterojunction as a saturable absorber

Two-dimensional (2D) materials, such as graphene and molybdenum disulphide (MoS2), are attractive for pulse laser applications owing to their exceptional nonlinear optical properties. In this paper, we fabricated three different saturable absorbers (SAs) based on plasma-enhanced chemical vapour deposition (PE-CVD) and liquid-phase exfoliation. The transmissivities of fabricated graphene, MoS2 and graphene–MoS2 SAs were 88.76%, 91.88% and 82.23% at 1064 nm, respectively, which were measured by a UV–Vis-NIR spectrophotometer. Their modulation depths on SiO2 substrate were measured to be 7.1%, 7.8% and 15.01% by the balanced synchronous twin-detector measurement system. Subsequently, these SAs (graphene SA, MoS2 SA, graphene and MoS2 SA, and graphene-MoS2 heterojunction SA) were placed in the same linear laser resonator, and their pulse output characteristics were systematically analysed and compared. When the absorbed pump power was 6.56 W, the heterojunction-based passively Q-switched Nd:YVO4 laser generated stable laser pulses with a pulse width of 180 ns and corresponding repetition rate of 640 kHz, which showed strongly enhanced saturable absorption properties than individual graphene (320 ns, 980 kHz), individual MoS2 (280 ns, 630 kHz), or graphene and MoS2 (248 ns, 760 kHz). And we attributed this mainly to rapid carrier relaxation induced by interlayer charge transfer in heterojunction because of type II band alignment. These results open up a possibility of actively regulating the saturable absorption properties of SAs by constructing heterojunctions with 2D materials.

Seaweed‐like 2D‐2D Architecture of MoS2/rGO Composites for Enhanced Selective Aerobic Oxidative Coupling of Amines

AbstractTo provide a solution for pursuing high redox reactivity beyond the compromise between the numbers of sulfur vacancies and electron transfer ability, 2D‐2D architecture of MoS2/rGO composite was fabricated with 2D graphene‐like transition metal dichalcogenides and graphene. By employing this material as heterogeneous catalyst, imines, also known as Schiff base, was synthesized by direct aerobic oxidative coupling of amines. Taking advantage of few‐layered MoS2 nanosheets and graphene, catalytic performance was significantly enhanced, exhibiting both excellent conversion and remarkable selectivity. Due to the good oil‐absorbing capability of this open porous 2D‐2D architecture, the hydrolysis of NH‐imines by water was inhibited and thus aldehydes byproducts were diminished. A second order kinetic behavior was observed in the imines formation, indicating the bimolecular condensation might be the rate determining step in such coupling reaction.

Boron Doped Graphene Quantum Structure and MoS2 Nanohybrid as Anode Materials for Highly Reversible Lithium Storage.

Herein, the boron-doped graphene quantum structure (BGQS), which contains both the advantages of 0-D graphene quantum dot and 2-D reduced graphene oxide, has been fabricated by top-down hydrothermal method and then mixed with molybdenum sulfide (MoS2) to serve as an active electrode material for the enhanced electrochemical performance of lithium ion battery. Results show that 30 wt% of BGQS/MoS2 nanohybrid delivers the superior electrochemical performance in comparison with other BGQS/MoS2 and bare components. A highly reversible capacity of 3,055 mAh g−1 at a current density of 50 mA g−1 is achieved for the initial discharge and a high reversible capacity of 1,041 mAh g−1 is obtained at 100 mA g−1 after 50 cycles. The improved electrochemical performance in BGQS/MoS2 nanohybrid is attributed to the well exfoliated MoS2 structures and the presence of BGQS, which can provide the vitally nano-dimensional contact for the enhanced electrochemical performance. Results obtained in this study clearly demonstrate that BGQS/MoS2 is a promising material for lithium ion battery and can open a pathway to fabricate novel 2-D nanosheeted nanocomposites for highly reversible Li storage application.

A comparative study of mechanisms of the adsorption of CO2 confined within graphene-MoS2 nanosheets: a DFT trend study.

The space within the interlayer of 2-dimensional (2D) nanosheets provides new and intriguing confined environments for molecular interactions. However, atomic level understanding of the adsorption mechanism of CO2 confined within the interlayer of 2D nanosheets is still limited. Herein, we present a comparative study of the adsorption mechanisms of CO2 confined within graphene–molybdenum disulfide (MoS2) nanosheets using density functional theory (DFT). A comprehensive analysis of CO2 adsorption energies (EAE) at various interlayer spacings of different multilayer structures comprising graphene/graphene (GrapheneB) and MoS2/MoS2 (MoS2B) bilayers as well as graphene/MoS2 (GMoS2) and MoS2/graphene (MoS2G) hybrids is performed to obtain the most stable adsorption configurations. It was found that 7.5 Å and 8.5 Å interlayer spacings are the most stable conformations for CO2 adsorption on the bilayer and hybrid structures, respectively. Adsorption energies of the multilayer structures decreased in the following trend: MoS2B > GrapheneB > MoS2G > GMoS2. By incorporating van der Waals (vdW) interactions between the CO2 molecule and the surfaces, we find that CO2 binds more strongly on these multilayer structures. Furthermore, there is a slight discrepancy in the binding energies of CO2 adsorption on the heterostructures (GMoS2, MoS2G) due to the modality of the atom arrangement (C–Mo–S–O and Mo–S–O–C) in both structures, indicating that conformational anisotropy determines to a certain degree its CO2 adsorption energy. Meanwhile, Bader charge analysis shows that the interaction between CO2 and these surfaces causes charge transfer and redistributions. By contrast, the density of states (DOS) plots show that CO2 physisorption does not have a substantial effect on the electronic properties of graphene and MoS2. In summary, the results obtained in this study could serve as useful guidance in the preparation of graphene–MoS2 nanosheets for the improved adsorption efficiency of CO2.

Free-Standing Molybdenum Disulfide Composite Film Electrode for High Performance and Waterproof Flexible Supercapacitor

Molybdenum disulfide (MoS2) has been studied extensively for energy storage because of its excellent electrical properties and unique structure. In this work, the freestanding flexible composite films were prepared by reduced graphene oxide (RGO), polyaniline (PANI), and MoS2 via a facile method. The all-solid-state flexible supercapacitor based on the ternary composite film shows good electrochemical properties. The special capacitance of supercapacitor is 643.3 mF/cm2 (∼2144.3 F/cm3). And the all-solid-state flexible supercapacitor presents no particularly obvious fading after bending 200 times. Furthermore, the ternary composite film electrode has certain waterproof effect due to the high content of MoS2. The ternary composite film remained high capacitance (614.9 mF/cm2) after soaking in water for 8 h. This work not only develops a facile approach to prepare MoS2 composite film with flexibility, but also provides a guideline for preparing functional supercapacitor by utilizing properties of MoS2 such as photoelectric conversion, catalytic function and so on.

Ultra-sensitive surface plasmon resonance biosensor based on MoS2–graphene hybrid nanostructure with silver metal layer

The optical biosensors based on the plasmonic technology are an important research item in the field of biophotonics. The graphene–molybdenum disulfide (MoS2) based hybrid structures are very effective in designing and fabricating of the sensitive optical biosensors. In this paper, we propose a nanostructure Ag/MoS2/graphene as an optical biosensor with high performance and sensitivity. The proposed configuration for this surface plasmon resonance (SPR) optical biosensor is Kretschmann. Herein, the enhancement of sensitivity for the proposed SPR optical biosensor is investigated in different states. By determining of the numbers of MoS2 layer and the thickness of the metal layer, we increased the sensitivity of the proposed biosensor. The maximum sensitivity ~ 190°/RIU is achieved. For this ultra-sensitive SPR biosensor with maximum sensitivity, the numbers of MoS2 and graphene layer is 2 and the resonance wavelength is determined 680 nm.

Sensitivity enhancement of SPR optical biosensor based on Graphene–MoS2 structure with nanocomposite layer

The optical plasmonic biosensors are an important research item in the field of bio-photonics. The graphene-molybdenum disulfide (MoS2) based hybrid structures are very useful in designing and fabrication of the high sensitive biosensors. In this paper, we designed hetero-structured Air/MoS2/Nanocomposite/MoS2/Graphene as an optical biosensor with high sensitivity. The proposed configuration for this Surface Plasmon Resonance (SPR) biosensor is Otto configuration. Herein, the enhancement sensitivity of this proposed SPR biosensor is investigated in different arrangement of layers. The influence of the refractive index of the coupling prism, the thickness of the nanocomposite layer, the constituent components of the nanocomposite layer and the number of the MoS2 layer is investigated and the optimal values is calculated for the biosensor with maximum sensitivity. The maximum sensitivity ∼200°/RIU is achieved with six layers of MoS2 layer and a nanocomposite layer containing the gold nanoparticles and TiO2 as host dielectric.

Laser-induced improvement in tribological performances of surface coatings with MoS2 nanosheets and graphene

To deal with the corrosive property of bio-oil, laser-induced coatings were synthesized on the typical cylinder liner rubbing surfaces. Two nano-additives including MoS2 nanosheets and graphene were introduced into the coatings. The tribological performances of the coated surfaces were tested on a multifunctional cylinder liner-piston ring tribometer. The surfaces before and after sliding were characterized with modern surface analysis technology including Raman spectra, X-ray diffraction (XRD), scanning electron microscope (SEM), energy dispersive X-ray spectroscopy (EDS) and etc. The results show that the nickel-based coated surfaces can effectively prevent the corrosion of the bio-oil. In addition, the laser-induced coatings had better antifriction and antiwear performances than the corresponding traditional electroless plating coatings. With the introduction of MoS2 nanosheets or graphene, the tribological performances of the coatings were further enhanced. Laser irradiation increased the concentration of the nano-additives in the coatings and the thickness of the tribo-layers, which accounted for their outstanding tribological performances.

Electronic transport and dynamical polarization in bilayer silicene-like systems

Abstract We investigate the semiclassical electronic transport properties of the bilayer silicene-like systems in the presence of charged impurity. The trigonal warping due to the interlayer hopping, and its effect to the band structure of bilayer silicene is discussed. Besides the trigonal warping, the external field also gives rise to the anisotropic effect of the mobility (at finite temperature) which can be explored by the Boltzmann theory. The dynamical polarization as well as the scattering wave vector-dependent screening within random phase approximation are very important in determining the scattering behavior as well as the self-consistent transport. We detailedly discuss the transport behavior under the short- or long-range potential. The phonon scattering with the acoustic phonon mode which dominants at high temperature is also studied within the density functional theory (DFT). Our results are also valid for the bilayer graphene or bilayer MoS2, and other bilayer systems which with strong interlayer coupling.

MoS2-graphene heterostructures as efficient organic compounds sensing 2D materials

In this work, electrical properties and application for volatile organic compounds detection of molybdenum disulfide (MoS2)−graphene (MS/G) heterostructure is investigated. The MS/G heterostructure is synthesized by physical stacking of single-layer (SL) MoS2 over SL graphene. The difference in the work-functions between the MoS2 and graphene leads to electron transfer from MoS2 to graphene, which changes FET charge neutrality point (VCNP) of graphene by as much as 30 V and increases the electron-to-hole ratio in graphene. This charge transport phenomenon is further confirmed by shifting of Raman G peak and quenching of photoluminescence intensity by 50% of MoS2 in the heterostructure. Ultraviolet photoelectron spectroscopy reveals a 0.1 eV upshift of the Fermi level of graphene in MS/G, which is consistent with the electrical double-layer capacitance versus the electrode potential measurement and energy band alignment predicted by first-principle simulations. The heterogenity induced charge transfer in the heterostructure of MS/G results in outstanding performance in chemical sensing. The MS/G FET shows improved stability in dry air with negligible shifting of VCNP, as compared to graphene FET. In the detection of toluene, the MS/G FET-based sensor shows higher sensitivity and superior signal-to-noise ratio compared to MoS2 or graphene individually.

2D Materials for Gas Sensing Applications: A Review on Graphene Oxide, MoS₂, WS₂ and Phosphorene.

After the synthesis of graphene, in the first year of this century, a wide research field on two-dimensional materials opens. 2D materials are characterized by an intrinsic high surface to volume ratio, due to their heights of few atoms, and, differently from graphene, which is a semimetal with zero or near zero bandgap, they usually have a semiconductive nature. These two characteristics make them promising candidate for a new generation of gas sensing devices. Graphene oxide, being an intermediate product of graphene fabrication, has been the first graphene-like material studied and used to detect target gases, followed by MoS2, in the first years of 2010s. Along with MoS2, which is now experiencing a new birth, after its use as a lubricant, other sulfides and selenides (like WS2, WSe2, MoSe2, etc.) have been used for the fabrication of nanoelectronic devices and for gas sensing applications. All these materials show a bandgap, tunable with the number of layers. On the other hand, 2D materials constituted by one atomic species have been synthetized, like phosphorene (one layer of black phosphorous), germanene (one atom thick layer of germanium) and silicone (one atom thick layer of silicon). In this paper, a comprehensive review of 2D materials-based gas sensor is reported, mainly focused on the recent developments of graphene oxide, exfoliated MoS2 and WS2 and phosphorene, for gas detection applications. We will report on their use as sensitive materials for conductometric, capacitive and optical gas sensors, the state of the art and future perspectives.

MoS2 nanosheets based counter electrodes: An alternative for Pt-free dye-sensitized solar cells

Dye-sensitized solar cells are assembled with MoS2 nanosheets based counter electrodes synthesized via hydrothermal route using ammonium molybdate tetrahydrate and thiourea as precursor materials. Basic and specific studies are followed to characterize MoS2 as counter electrode material by XRD, SEM, TEM, EDX, cyclic voltammetry and electrochemical impedance spectroscopic studies. Dye-sensitized solar cells device is fabricated by using fluorine-doped tin oxide glass plate coated with TiO2 as photoanode, Cis-bis(isothiocyanato)bis(2,2′-bipyridyl-4,4′-dicarboxylato)ruthenium(II) dye as a light harvester, and iodine/iodide as a liquid electrolyte. The fabricated dye-sensitized solar cells device with MoS2 counter electrode showed an open circuit voltage of 795 mV, short circuit current of 19.6 mA/cm2, fill factor of 0.36, and power conversion efficiency of 6.6% upon comparing to Pt counter electrode which showed an open circuit voltage of 827 mV, short circuit current of 14.8 mA/cm2, fill factor of 0.47, and power conversion efficiency of 6.8%. In addition to this, MoS2 coupled reduced graphene oxide counter electrodes are prepared by simply dispersing reduced graphene oxide and MoS2 in isopropyl alcohol which showed the best performance with open circuit voltage of 800 mV, short circuit current of 20.5 mA/cm2, fill factor of 0.42, and power conversion efficiency of 8.1%.

Photodetectors: High Detectivity from a Lateral Graphene–MoS2 Schottky Photodetector Grown by Chemical Vapor Deposition (Adv. Electron. Mater. 9/2018)

High performance of 2D material-based photodetectors is important for their application. An ultrasensitive lateral graphene–MoS2 heterostructure Schottky photodetector is described by Yongzhe Zhang and co-workers in article number 1800069. Moreover, a heterostructure photodetector array is demonstrated, showing the large-scale fabrication capacity. The results prove the potential of 2D materials for future optoelectronics applications.