Graphene MoS2 Research Articles

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.

Photoinduced Electron Transfer at the Interface between Heterogeneous Two-Dimensional Layered Materials

We study the optical response of a MoS2–graphene heterostructure by carrying out first-principles calculations of electron dynamics in real time and real space. Our attention is primarily focused on the effect of interfacial contact between MoS2 and graphene. The interfacial contact has a negligible influence on the optical absorption of the MoS2–graphene heterostructure. However, the photoinduced electron dynamics of the heterostructure is crucially different from those of the isolated monolayers of MoS2 and graphene; electronic polarization perpendicular to the MoS2–graphene interface is induced by a laser pulse polarized parallel to the interface. By analyzing the variation in the electron occupation number, we find that electronic transitions in graphene and MoS2 accompany the electron transfer from graphene to MoS2. This is due to the difference in the magnitudes of the density of states of MoS2 and graphene. In other words, the photoinduced electron transfer is governed by the details of the electro...

Promoting sensitivity and selectivity of HCHO sensor based on strained InP3 monolayer: A DFT study

Sensitive materials for formaldehyde (HCHO) sensor need high sensitivity and selectivity. The research on two dimensional (2D) sensitive material is growing, and most studies focus on the pristine or modified graphene. So it is essential to introduce other 2D materials into HCHO gas sensor. In this report, the adsorption behaviors of organic gas molecules including C2H6, C2H4, C2H2, C6H6, C2H5OH and HCHO over indium triphosphide (InP3) monolayer were studied by using first-principle atomistic simulations. The calculation results demonstrate that InP3 monolayer has a high sensitivity and selectivity to HCHO than others. By comparing the structures and adsorption results of InP3 monolayer, graphene and single-layered MoS2, it was found that the polarity bonds and steric effect of the site on monolayer play an important role in the detection of HCHO. The effect of strain on the gas/substrate adsorption systems was also studied, implying that the stained InP3 monolayer could enhance the sensitivity and selectivity to HCHO. This study provides useful insights into the gas-surface interaction that may assist future experimental development of 2D material for HCHO sensing and performance optimization based on strain.

Graphene–MoS2–metal hybrid structures for plasmonic biosensors

Abstract Surface Plasmon Resonance (SPR) biosensors are widely used for real-time label-free detection in medical diagnostics, pharmaceutical researches and food safety. Although there is a growing interest in miniaturization of biosensors for self-detection and diagnostics at out of laboratory, the performance of conventional metal SPR sensors is still limited. In this paper, we propose graphene–MoS 2 – metal hybrid structures based plasmon sensors under the best minimum light intensity approach, which represents the performance analysis in case of the lowest reflected light strength. It is demonstrated that the metal thickness can be reduced from 55 nm to 32 nm and 37 nm meanwhile the performance of the background sensor can be improved by 87% and 13% with the 4 additional MoS2 and graphene layers, respectively. We show that MoS2 based SPR devices provide much better sensitivity performance than graphene based devices. Our results reveal the another promising property of MoS2: The sensitivity of SPR sensors can be greatly increased with a few number of MoS2 within the angular SPR system while reducing the size of the device, especially for particular applications such as detecting a single molecule and biosensing at low biomolecule concentration. Furthermore, we show that the equivalent optical properties of multilayered nanostructures also depend on the layer thickness which is a novel knowledge for the next studies on 2D material based SPR plasmonic devices.

Diverse nitrogen-doped 2D layered mesoporous MoS2/reduced graphene oxide composites with superior structural features for enhancing the performance of lithium ion batteries

In this paper, we report the construction of nitrogen-doped 2D layered mesoporous molybdenum disulfide/reduced graphene oxide (meso-N-MoS2/rGO) hybrids via a nanocasting method using ammonium molybdate as molybdenum source, thiourea as sulfur source, and nitrogen modified mesoporous KIT-6/rGO as template. The as-prepared meso-N-MoS2/rGO is constituted of homogeneously distributed ultrasmall few-layer MoS2 nanoparticles composited with N-modified graphene nanosheets and possesses a large surface area with high porosity. When being used as anode materials for Li ion batteries (LIBs), the obtained meso-N-MoS2/rGO hybrid exhibits prominent electrochemical properties in terms of high specific capacity (1195.1 mA h g−1 at 100 mA g−1) and superior rate performance (606.2 mA h g−1 at 1000 mA g−1). The discharge capacity still maintains 872.8 mA h g−1 even after 50 cycles. Such an intriguing electrochemical performance is mainly attributed to its high porosity, large surface area, strong coupling effect between uniform N-doped MoS2 and N-doped graphene since the N-doping and small size of MoS2 nanoparticles can set up conductive bridges, abundant pyridine nitrogen, and excellent conductivity. In addition, the layered graphene provides conductive networks, which ensure that lithium ions and electrons have a shorter transmission distance, thereby increasing their ion diffusion rate and ease the charge and discharge process in the volume effect. This work provides an effective strategy to synthesize N-doped meso-MoS2/rGO hybrids to achieve significantly enhanced electrochemical performance.

The Synthesis and Characterization of Atomically-Thin Pt Monolayer to Multilayer Pt Films on Graphene and MoS2 and Their Application for ORR and HER

This work explores the deposition of Pt monolayers on 3D graphene and 2D MoS2 as catalyst architectures for electrocatalysis. Ni foam was utilized as a starting substrate for the 3D graphene growth before Pt was deposited layer-by-layer in a surface-limited electrochemical scheme to achieve highly wetted and atomically thin growth. A similar surface-limited technique was utilized for the deposition of Pt on both vertically and horizontally aligned MoS2. High resolution TEM, XPS, SEM/EDX, and electroanalytical techniques were used to characterize these samples and reveal the evolution of the atomic/electronic structure as a function of the number of Pt monolayers. Specifically, the ORR and the HER reactions were used to characterize the catalytic activities of the graphene- and the MoS2-templated catalysts, respectively. It is demonstrated that the synthesis scheme outlined here allows for the design of dimensionally-controlled and ligand-flexible catalyst architectures that can have a wide range of chemical applications.

High performance self-gating graphene/MoS2 diode enabled by asymmetric contacts

A graphene-MoS2 (GM) heterostructure based diode is fabricated using asymmetric contacts to MoS2, as well as an asymmetric top gate (ATG). The GM diode exhibits a rectification ratio of 5 from asymmetric contacts, which is improved to 105 after the incorporation of an ATG. This improvement is attributed to the asymmetric modulation of carrier concentration and effective Schottky barrier height (SBH) by the ATG during forward and reverse bias. This is further confirmed from the temperature dependent measurement, where a difference of 0.22 eV is observed between the effective SBH for forward and reverse bias. Moreover, the rectification ratio also depends on carrier concentration in MoS2 and can be varied with the change in temperature as well as back gate voltage. Under laser light illumination, the device demonstrates strong opto-electric response with 100 times improvement in the relative photo current, as well as a responsivity of 1.9 A W−1 and a specific detectivity of 2.4 × 1010 Jones. These devices can also be implemented using other two dimensional (2D) materials and suggest a promising approach to incorporate diverse 2D materials for future nano-electronics and optoelectronics applications.

High Detectivity from a Lateral Graphene–MoS2 Schottky Photodetector Grown by Chemical Vapor Deposition

Abstract2D material–based photodetectors have demonstrated the great potential in future optoelectric applications and the compatibility with the traditional semiconductor technology. However, low detectivity and difficulty of large‐scale fabrication still limit their application. Here, an ultrasensitive in‐plane lateral graphene–MoS2 heterostructure is successfully constructed using one‐step growth by chemical vapor deposition, which is suitable for large‐scale fabrication. The Schottky junction is formed in the channel with the edge contact of graphene and MoS2. It displays good rectification characteristics with an on/off ratio up to 106. As a photodetector, it exhibits excellent detectivity with the specific detectivity D* up to 1.4 × 1014 Jones and the responsivity of 1.1 × 105 A W−1, which benefit from strong absorption, the efficient separation of the photoexcited carriers, and quick charge transport in the Schottky junction device. Moreover, heterostructure photodetector array is demonstrated here which shows the large‐scale fabrication capacity. All of these results prove the potential of 2D material–based junction devices for optoelectronic devices.

Enhanced magnetic properties and tunable Dirac point of graphene/Mn-doped monolayer MoS2 heterostructures

Graphene is one of the most promising spintronic materials due to its high carrier mobility. However, the absence of a band gap and ferromagnetic order in graphene seriously limit its applications in spintronics. How to utilize its high carrier mobility as well as mediate its electronic structure remains a challenge. Herein, we design a novel composite, which is composed of graphene and Mn-doped monolayer MoS2. The magnetic properties and electronic structures of graphene/Mn-doped monolayer MoS2 heterostructures were studied by using density functional theory (DFT) with the van der Waals (vdW) correlations (DFT-D). We found that the heterostructures show increased magnetic moments and more stable ferromagnetic (FM) states compared with that of isolated Mn-doped MoS2 monolayer. Our further studies show that many electrons are transferred to Mn-doped MoS2 monolayer from graphene, which causes the Fermi level to shift down below the Dirac cone about 0.59 eV. The transfered electrons also enhance the FM coupling between Mn ions. Graphene is partially spin polarized because of the magnetic proximity effect, which leads to the spin-dependent gaps for spin-up (16.1 meV) and spin-down (5 meV) at Dirac point, respectively. The introduction of sulfur (S) vacancy to the interface results in a much more stable FM structure and a higher total magnetic moment of the FM state; furthermore, it raises the spin polarization of graphene π orbitals and opens up a small band gap of about 7 meV. These findings propose a new route to facilitate the design of spintronic devices which both need stable ferromagnetism and finite band gap.

Observation of High-Frequency Raman Modes in FeCl₃- and Zn-Intercalated MoS₂ Flakes.

In place of the widely studied graphene, monolayer or few-layer MoS2 flakes are promising materials for next-generation optoelectronic devices. MoS2 has attracted increasing attention in physics and its applications because of its capacity to undergo indirect-to-direct band gap transition. Raman spectroscopy is a useful and versatile tool to probe the physical properties of pristine and intercalated MoS2. This study investigates for the first time the multiphoton modes of FeCl3- and Zn-intercalated few-layer MoS2 at high frequencies of 1513 and 1732 cm-1 for FeCl3-MoS2 and 1341 and 1604 cm-1 for Zn-MoS2. The substrates interact with MoS2 during intercalation. The Raman peak intensities of the intercalated samples vary with intercalation time while keeping the peak position nearly constant. This finding is interesting and suitable for studying other 2D layered materials.

Single-File Protein Translocations through Graphene–MoS2 Heterostructure Nanopores

Successfully threading unfolded protein molecules through nanopores whose sizes are comparable to that of an amino acid is a prerequisite for the nanopore-based protein sequencing method that promises to be high-throughput and low-cost. While the electric driving method can be effective for a homogeneously charged DNA molecule, it fails to drive an unfolded protein through a nanopore because the net charge of a protein fragment inside of the pore (where the electric field exists) can be positive, negative, or neutral. Here we propose and demonstrate by molecular dynamics simulations protein transport through a nanopore in a quasi-two-dimensional heterostructure stacked together by graphene and molybdenum disulfide (MoS2) nanosheets. Thanks to different van der Waals interactions ( U) between a protein molecule and different 2D surfaces, it is energetically favorable for protein to progressively move from the MoS2 surface to the graphene surface (more negative U) through a nanopore in the heterostructure.

Dye sensitized photoelectrochemical immunosensor for the tumor marker CEA by using a flower-like 3D architecture prepared from graphene oxide and MoS2.

The authors describe a dye-sensitized photoelectrochemical immunoassay for the tumor marker carcinoembryonic antigen (CEA). The method employs the rhodamine dye Rh123 with red color and absorption maximum at 500nm for spectral sensitization, and a 3D nanocomposite prepared from graphene oxide and MoS2 acting as the photoelectric conversion layer. The nanocomposite with flower-like 3D architectures was characterized by transmission electron microscopy, scanning electron microscopy, X-ray powder diffraction, and UV-vis diffuse reflectometry. A photoelectrochemical sandwich immunoassay was developed that is based on the use of the nanocomposite and based on the specific binding of antibody and antigen, and by using a secondary antibody labeled with Rh123 and CdS (Ab2-Rh123@CdS). Under optimal conditions and at a typical working voltage of 0V (vs. Hg/HgCl2), the photocurrent increases linearly 10pgmL-1 to 80ngmL-1 CEA concentration range, with a 3.2pgmL-1 detection limit. Graphical abstract Flower-like GO-MoS2 complex with high efficiency of electron transport was synthesized to construct photoelectrochemical platform. The sandwich-type immunoassay was built on this platform based on specific binding of antigen and antibody. Carcinoembryonic antigen in sample was detected sensitively by using sensitization of rhodamine dye Rh123 as signal amplification strategy.

Role of Graphene oxide and addition of MoS2 in HDPE matrix for improved tribological properties

The nano filler addition further enhances the properties; especially wear and grating which are very well known. Keeping these points in view, the current work focuses on establishing the tribological behaviour i.e., both slide wear and friction with various load applications for different levels of graphene oxide addition (0.5% to 2%) and MoS2 addition at 0.8%.The specimens were developed through compression molding technique. The test samples were prepared to the required sizes and subjected to slide wear and friction measurements using pin on disc set up; which is carried out as per ASTM guidelines. The weight change measurements were done by noting the initial and final weight of the test samples to get the slide wear loss. The hardness esteems were obtained by utilizing Shore D Hardness analyzer according to ASTM guidelines. The weight change measurements to get slide wear loses were done. The coefficient of friction was computed by dividing frictional load with normal load. It is very well seen from the test data that the slide wear resistance increments with increment in graphene oxide content (0.5% to 2%) in steps of 0.5%; as the hardness level showed an increase with a similar level of increment in graphene oxide content. The co-efficient of friction has demonstrated a declining pattern with increment in graphene content for the load applications of various loads. The slide wear and friction results have been interpreted based on logical thinking involving literature reports and supporting tests. Thus, there are positive responses observed in respect of the graphene oxide addition to HDPE matrix. This makes the material suitable for wear resistant applications in automotive industries.

Wireless, Smart, Human Motion Monitoring Using Solution Processed Fabrication of Graphene–MoS2 Transistors on Paper

AbstractCombining 2D graphene, which has zero bandgap yet high carrier mobility, with a permanent bandgap of 2D MoS2 opens up numerous possibilities for development of future electronic devices and sensors. This study reports for the first time the fabrication of flexible all paper based few layer graphene (Gr)–MoS2 field effect transistor (FET) using biodegradable cellulose paper as a substrate for growing Gr/MoS2 as well as dielectric and pencil graphite as gate. The as fabricated Gr/MoS2 transistor exhibits on/off ratio of ≈99 with carrier mobility of 18.7 cm2 V−1 s−1, which is remarkable considering the simplicity and low cost of the device fabrication. The Gr/MoS2 transistor is utilized as strain sensor, which could detect negligible strains as low as 0.5%. Detailed mechanism studies in both resistor and FET configurations with Gr/MoS2 as a channel are presented to better understand the sensing behavior under both tensile and compressive strain conditions. Further, by interfacing the sensor with the microcontroller, the data are acquired and transferred to the smartphone through bluetooth communication thus enabling human motion monitoring. This low cost, large area Gr/MoS2 FET based strain sensor has huge potential in wireless wearable electronics, such as artificial electronic skin, medical diagnostics, human machine interface, etc.

Spontaneous Transport of Single-Stranded DNA through Graphene-MoS2 Heterostructure Nanopores.

The effective transport of a single-stranded DNA (ssDNA) molecule through a solid-state nanopore is essential to the future success of high-throughput and low-cost DNA sequencing. Compatible with current electric sensing technologies, here, we propose and demonstrate by molecular dynamics simulations the ssDNA transport through a quasi-two-dimensional nanopore in a heterostructure stacked together with different 2D materials, such as graphene and molybdenum disulfide (MoS2). Due to different chemical potentials, U, of DNA bases on different 2D materials, it is energetically favorable for a ssDNA molecule to move from the low- U MoS2 surface to the high- U graphene surface through a nanopore. With the proper attraction between the negatively charged phosphate group in each nucleotide and the positively charged Mo atoms exposed on the pore surface, the ssDNA molecule can be temporarily seized and released thereafter through a thermal activation, that is, a slow and possible nucleotide-by-nucleotide transport. A theoretical formulation is then developed for the free energy of the ssDNA transiting a heterostructure nanopore to properly characterize the non-equilibrium stick-slip-like motion of a ssDNA molecule.

Stable charge retention in graphene-MoS 2 assemblies for resistive switching effect in ultra-thin super-flexible organic memory devices

This study investigated the nonvolatile memory characteristics of devices fabricated with PMMA embedding composite of graphene and molybdenum disulphide, which were employed as charge-trapping centres. X-ray diffraction measurements, Scanning and transmission electron microscope image analysis along with other characterisation techniques were performed for the assessment of the nature of graphene MoS2 hybrid system. Bistable I-V characteristics of the device revealed stable and rewritable memory effect. A significantly high resistance ratio > 104 (Roff/Ron) was observed even after 10 days. The reliability and reproducibility of the devices were tested for 104 cycles. The conduction mechanisms of the fabricated nanocomposite based memory cell were discussed on the basis of experimental data using a charge trapping–detrapping mechanism in the graphene MoS2 hybrid. Owing to the increasing interest in the flexible electronics, bending tests were carried out on the devices fabricated on PET substrates. Bendability test at various bending diameters (40-5 mm) for 100 cycles was carried out to show the mechanical robustness of the device.

Measurement on critical shear stress of circular point contact utilizing Atomic Force Microscope

We propose Bowdon-Johnson-Kendall-Roberts model to measure the critical shear stress of circular point contact, and the critical shear stress map is constructed through the lateral force mode and force map mode of Atomic Force Microscope to research the interface property of nanomaterials. The mean critical shear stresses are measured as 0.24 MPa for monolayer MoS2 and 0.69 MPa for graphene. MoS2 nanosheet and graphene are of the lower critical shear stress and friction coefficient than those of the bulk materials, which are good agreement with the regularities in nanotribology. The critical shear stress can be used to not only quantitatively characterize the frictional properties, but also analyze the friction at different loading stages. The critical shear stress map would play an important role in sliding contact of nanomaterials for the drive part design and nanotribological reliability of nanoelectronic mechanical system.

Vertical crosslinking MoS2/three-dimensional graphene composite towards high performance supercapacitor

Molybdenum disulfide (MoS2) has been stimulated in extensive researches due to its layered structure and the potential as an electrochemical energy material. However, the effects on electrochemical performance of concentration of MoS2 are rarely mentioned. In this paper, the effects of different concentrated layered MoS2 on the morphology and electrochemical properties of the composite of MoS2 and three-dimensional graphene (MoS2/3DG) were discussed. The results show that layered MoS2 was successfully compounded to 3DG and formed a vertical crosslinking structure. It can be observed that MoS2 nanosheets are vertically loaded on the inner and outer surface of graphene when the concentration of MoS2 is 0.20mg/L. The specific capacitance of composite (MoS2 (0.20mg/L)/3DG) reaches 2182.33mF/cm2 at the current density of 1mA/cm2, and the specific capacitance remains 116.83% after 5000 cycles. When the current density increased 100 times (from 1mA/cm2 to 100mA/cm2), the specific capacitance retains 78.9%. Meanwhile, the hybrid energy storage devises can deliver an energy density of 130.34Wh/m2. The superior electrochemical properties are attributed to the synergistic effect of MoS2 and 3DG. Therefore, the material has a potential application on supercapacitor electrode material.

Two-dimensional silicon crystals with sizable band gaps and ultrahigh carrier mobility.

Due to their compatibility in the well-developed Si-based semiconductor industry, exploring two-dimensional (2D) silicon crystals with both sizable band gaps and high carrier mobility is important to develop high-performance electronic and optoelectronic devices on the nanoscale. Here, eleven new 2D silicon crystals are reported based on the strategy of mixing 3-fold and 4-fold coordinated silicon atoms in 2D confined phases and first-principles calculations. We establish that these 2D silicon crystals can be obtained by functionalizing silicene with silicon atoms, dimers, or chains, which exhibit lower formation energy than that of silicene. Their dynamic stability and thermal stability are confirmed by phonon calculations and Born-Oppenheimer molecular dynamic simulation at temperatures up to 700 K. Electronic structure calculations reveal that these 2D silicon crystals are semiconductors with sizable and tunable band gaps, ranging from 1.12 to 1.67 eV, and four of them are direct or quasi-direct band gap semiconductors with strong absorption in the visible-light frequency. The calculated Young's stiffness of 2D silicon crystals ranges from 31 to 88 N m-1, which are comparable to phosphorene, but remarkably smaller than those of MoS2 monolayer and graphene. Remarkably, Cz-P2/c-Si12 possesses a negative Poisson's ratio with a maximum value of -0.055. In particular, 2D silicon crystals possess ultrahigh carrier mobility of up to 1.7 × 105 and 1.3 × 104 cm2 V-1 s-1 at room temperature for electrons and holes, respectively, suitable for high-speed electronic and optoelectronic applications on the nanoscale.

PEO-Linked MoS2–Graphene Nanocomposites with 2D Polar–Nonpolar Amphoteric Surfaces as Sulfur Hosts for High-Performance Li–S Batteries

Layered MoS2–graphene nanocomposites with 2D polar–nonpolar amphoteric surfaces, which were used to confine sulfur for Li–S batteries, have been successfully fabricated through the assembly of polar MoS2 layers and nonopolar graphene with poly(ethylene oxide) cross-linking. Benefiting from the high conductivity of graphene and the strong chemical bonding between polar MoS2 and polysulfides, the MoS2–graphene composites not only ensure unimpeded electrical conducting to the insulating sulfur but also effectively entrap polysulfides therein. The ex situ study further reveals that the MoS2–graphene composites enable spatially regulated Li2S deposition by the preferential deposition of solid Li2S product onto the polar MoS2 layers, making a large amount of fast electron transport paths exposed on graphene for further sulfur reduction. Therefore, the obtained MoS2–sulfur–graphene nanocomposites present excellent rate performance and cycling stability with high reversible capacities of 895 mA h g–1 at 0.2 C aft...