Monolayer Graphene Electrodes Research Articles

Memristors with Monolayer Graphene Electrodes Grown Directly on Sapphire Wafers.

The development of the memristor has generated significant interest due to its non-volatility, simple structure, and low power consumption. Memristors based on graphene offer atomic monolayer thickness, flexibility, and uniformity and have attracted attention as a promising alternative to contemporary field-effect transistor (FET) technology in applications such as logic and memory devices, achieving higher integration density and lower power consumption. The use of graphene as electrodes in memristors could also increase robustness against degradation mechanisms, including oxygen vacancy diffusion to the electrode and unwanted metal ion diffusion. However, to realize this technological transformation, it is necessary to establish a scalable, robust, and cost-effective device fabrication process. Here we report the direct growth of high-quality monolayer graphene on sapphire wafers in a mass-producible, contamination-free, and transfer-free manner, using a commercially available metal-organic chemical vapor deposition (MOCVD) system. By taking advantage of this approach, graphene-electrode based memristors are developed, and all the processes used in the device fabrication incorporating graphene electrodes can be performed at wafer scale. The graphene electrode-based memristor demonstrates promising characteristics in terms of endurance, retention, and ON/OFF ratio. This work presents a possible and viable route to achieving robust graphene-based memristors in a commercially and technologically sustainable manner, paving the way for the realization of more powerful and compact integrated graphene electronics in the future.

Near-infrared wavelength tuning of monolayer graphene on Silicon Nitride waveguides

In this work, we detail the integration of monolayer graphene electrodes in Silicon Nitride waveguides to achieve wavelength tuning. The numerical results show that wavelength shifts of a few hundred to several thousand picometers shift can be obtained depending on the configuration. These results will help guide the design of graphene-based tunable optical devices operating in the near infrared range.

An On-Chip Microscale Vacuum Chamber with High Sealing Performance Using Graphene as Lateral Feedthrough.

On-chip microscale vacuum chambers with high sealing performance and electrical feedthroughs are highly desired for microscale vacuum electronic devices and other MEMS devices. In this paper, we report an on-chip microscale vacuum chamber which achieves a high sealing performance by using monolayer graphene as lateral electrical feedthrough. A vacuum chamber with the dimensions of π × 2 mm × 2 mm × 0.5 mm is fabricated by anodically bonding a glass chip with a through-hole between two Si chips in a vacuum, after monolayer graphene electrodes have been transferred to the surface of one of the Si chips. Benefiting from the atomic thickness of monolayer graphene, the leak rate of Si-glass bonding interface with a monolayer graphene feedthrough is measured at less than 2 × 10-11 Pa·m3/s. The monolayer graphene feedthrough exhibits a minor resistance increase from 22.5 Ω to 31 Ω after anodic bonding, showing good electrical conductance. The pressure of the vacuum chamber is estimated to be 185 Pa by measuring the breakdown voltage. Such a vacuum is found to maintain for more than 50 days without obvious degradation, implying a high sealing performance with a leak rate of less than 1.02 × 10-16 Pa·m3/s.

Electrochemical analysis of ferrocene in bicontinuous microemulsions using β- cyclodextrin modified monolayer graphene electrodes

• The reduction peak of ferrocene increased significantly by modifying β-CD in BME. • Modification density of β-CD on a monolayer graphene exceeds a coverage of 0.5. • Small water domains around β-CD forms a concentration field for ferrocenium ions. A monolayer graphene electrode modified with β-cyclodextrin (β-CD) was produced via a one-step reaction using an originally synthesized pyrene derivative of β-CD (py-β-CD). A py-β-CD-modified graphene electrode with a high modification density exceeding a coverage of 0.5 was obtained. Using this electrode, the electrochemical behaviors of water-insoluble ferrocene (Fc) and of a water-soluble ferrocene derivative in a bicontinuous microemulsion (BME) solution, consisting of a phosphate buffer (pH = 7.4), sodium dodecyl sulfate (SDS), 2-butanol, and toluene, were analyzed to investigate the effects of β-CD modification on the redox reactions of these species. In the reduction reaction of water-insoluble Fc, almost no peak was observed for the unmodified graphene electrode, whereas a high peak current increase was observed for the β-CD-modified electrode. The mechanism of the increase in the peak current of the reduction reaction can be derived from many small water domains formed around the β-CD molecules on the graphene surface. These water domains form many oil–water interfaces on the surface of the graphene electrode, which act as an [Fc] +1 concentration field by interacting with SDS.

Nano-Physical Characterization of Chemical Vapor Deposition-Grown Monolayer Graphene for High Performance Electrode: Raman, Surface-Enhanced Raman Spectroscopy, and Electrostatic Force Microscopy Studies.

To achieve high-quality chemical vapor deposition of monolayer graphene electrodes (CVD-MG), appropriate characterization at each fabrication step is essential. In this article, (1) Raman spectroscopy/microscopy are employed to unravel the contact effect between the CVD-MG and Cu foil in suspended/supported formation. (2) The Surface-Enhanced Raman spectroscopy (SERS) system is described, unveiling the presence of a z-directional radial breathing-like mode (RBLM) around 150 cm−1, which matches the Raman shift of the radial breathing mode (RBM) from single-walled carbon nanotubes (SWCNTs) around 150 cm−1. This result indicates the CVD-MG located between the Au NPs and Au film is not flat but comprises heterogeneous protrusions of some domains along the z-axis. Consequently, the degree of carrier mobility can be influenced, as the protruding domains result in lower carrier mobility due to flexural phonon–electron scattering. A strongly enhanced G-peak domain, ascribed to the presence of scrolled graphene nanoribbons (sGNRs), was observed, and there remains the possibility for the fabrication of sGNRs as sources of open bandgap devices. (3) Electrostatic force microscopy (EFM) is used for the measurement of surface charge distribution of graphene at the nanoscale and is crucial in substantiating the electrical performance of CVD-MG, which was influenced by the surface structure of the Cu foil. The ripple (RP) structures were determined using EFM correlated with Raman spectroscopy, exhibiting a higher tapping amplitude which was observed with structurally stable and hydrophobic RPs with a threading type than surrounding RPs. (4) To reduce the RP density and height, a plausible fabrication could be developed that controls the electrical properties of the CVD-MG by tuning the cooling rate.

Twisted monolayer and bilayer graphene for vertical tunneling transistors

We prepare twist-controlled resonant tunneling transistors consisting of monolayer and Bernal bilayer graphene electrodes separated by a thin layer of hexagonal boron nitride. The resonant conditions are achieved by closely aligning the crystallographic orientation of graphene electrodes, which leads to momentum conservation for tunneling electrons at certain bias voltages. Under such conditions, negative differential conductance can be achieved. Application of in-plane magnetic field leads to electrons acquiring additional momentum during the tunneling process, which allows control over the resonant conditions.

Pushing the conductance and transparency limit of monolayer graphene electrodes for flexible organic light-emitting diodes

Graphene has emerged as an attractive candidate for flexible transparent electrode (FTE) for a new generation of flexible optoelectronics. Despite tremendous potential and broad earlier interest, the promise of graphene FTE has been plagued by the intrinsic trade-off between electrical conductance and transparency with a figure of merit (σDC/σOp) considerably lower than that of the state-of-the-art ITO electrodes (σDC/σOp <123 for graphene vs. ∼240 for ITO). Here we report a synergistic electrical/optical modulation strategy to simultaneously boost the conductance and transparency. We show that a tetrakis(pentafluorophenyl)boric acid (HTB) coating can function as highly effective hole doping layer to increase the conductance of monolayer graphene by sevenfold and at the same time as an anti-reflective layer to boost the visible transmittance to 98.8%. Such simultaneous improvement in conductance and transparency breaks previous limit in graphene FTEs and yields an unprecedented figure of merit (σDC/σOp ∼323) that rivals the best commercial ITO electrode. Using the tailored monolayer graphene as the flexible anode, we further demonstrate high-performance green organic light-emitting diodes (OLEDs) with the maximum current, power and external quantum efficiencies (111.4 cd A-1, 124.9 lm W-1 and 29.7%) outperforming all comparable flexible OLEDs and surpassing that with standard rigid ITO by 43%. This study defines a straightforward pathway to tailor optoelectronic properties of monolayer graphene and to fully capture their potential as a generational FTE for flexible optoelectronics.

Electrochemical Characterization of CVD-Grown Graphene for Designing Electrode/Biomolecule Interfaces

In research on enzyme-based biofuel cells, covalent or noncovalent molecular modifications of carbon-based electrode materials are generally used as a method for immobilizing enzymes and/or mediators. However, the influence of these molecular modifications on the electrochemical properties of electrode materials has not been clarified. In this study, we present the electrochemical properties of chemical vapor deposition (CVD)-grown monolayer graphene electrodes before and after molecular modification. The electrochemical properties of graphene electrodes were evaluated by cyclic voltammetry and electrochemical impedance measurements. A covalently modified graphene electrode showed an approximately 25-fold higher charge transfer resistance than before modification. In comparison, the electrochemical properties of a noncovalently modified graphene electrode were not degraded by the modification.

Nonequilibrium Kondo effect in a graphene-coupled quantum dot in the presence of a magnetic field.

Background: Quantum dots connected to larger systems containing a continuum of states like charge reservoirs allow the theoretical study of many-body effects such as the Coulomb blockade and the Kondo effect.Results: Here, we analyze the nonequilibrium Kondo effect and transport phenomena in a quantum dot coupled to pure monolayer graphene electrodes under external magnetic fields for finite on-site Coulomb interaction. The system is described by the pseudogap Anderson Hamiltonian. We use the equation of motion technique to determine the retarded Green’s function of the quantum dot. An analytical formula for the Kondo temperature is derived for electron and hole doping of the graphene leads. The Kondo temperature vanishes in the vicinity of the particle–hole symmetry point and at the Dirac point. In the case of particle–hole asymmetry, the Kondo temperature has a finite value even at the Dirac point. The influence of the on-site Coulomb interaction and the magnetic field on the transport properties of the system shows a tendency similar to the previous results obtained for quantum dots connected to metallic electrodes. Most remarkably, we find that the Kondo resonance does not show up in the density of states and in the differential conductance for zero chemical potential due to the linear energy dispersion of graphene. An analytical method to calculate self-energies is also developed which can be useful in the study of graphene-based systems.Conclusion: Our graphene-based quantum dot system provides a platform for potential applications of nanoelectronics. Furthermore, we also propose an experimental setup for performing measurements in order to verify our model.

Self-Assembled Graphene/MWCNT Bilayers as Platinum-Free Counter Electrode in Dye-Sensitized Solar Cells.

We describe the preparation and properties of bilayers of graphene‐ and multi‐walled carbon nanotubes (MWCNTs) as an alternative to conventionally used platinum‐based counter electrode for dye‐sensitized solar cells (DSSC). The counter electrodes were prepared by a simple and easy‐to‐implement double self‐assembly process. The preparation allows for controlling the surface roughness of electrode in a layer‐by‐layer deposition. Annealing under N2 atmosphere improves the electrode's conductivity and the catalytic activity of graphene and MWCNTs to reduce the I3 − species within the electrolyte of the DSSC. The performance of different counter‐electrodes is compared for ZnO photoanode‐based DSSCs. Bilayer electrodes show higher power conversion efficiencies than monolayer graphene electrodes or monolayer MWCNTs electrodes. The bilayer graphene (bottom)/MWCNTs (top) counter electrode‐based DSSC exhibits a maximum power conversion efficiency of 4.1 % exceeding the efficiency of a reference DSSC with a thin film platinum counter electrode (efficiency of 3.4 %). In addition, the double self‐assembled counter electrodes are mechanically stable, which enables their recycling for DSSCs fabrication without significant loss of the solar cell performance.

Fabrication of graphene/MoS2 alternately stacked structure for enhanced lithium storage

Graphene materials have attracted significant research interest as anodes in lithium-ion batteries (LIBs). However, their low volumetric capacity and high working voltage limit the energy density and thus hinder their practical applications. In this research, monolayer graphene and MoS2 were prepared by controlled CVD growth processes. Graphene/MoS2 alternately stacked structure (GMASS) was subsequently fabricated by alternately stacking graphene and MoS2 monolayers layer by layer. When evaluated as an anode for LIB, GMASS demonstrates an obviously reduced working voltage compared to monolayer graphene electrodes (1.31 V vs. 1.46 V). And its volumetric capacity is much higher than that of an average graphite anode (1260 mAh cm−3vs. 461 mAh cm−3). This fundamental research could promote the development of graphene/MoS2 heterostructures for high-energy LIBs.

Monolayer graphene electrodes as alignment layer for ferroelectric liquid crystal devices

Transparent electrodes and alignment layers are two essential parts of the liquid crystal (LC) cell fabrication. Conventionally, indium tin oxide (ITO) acts as transparent electrode and a polyimide film is used as alignment layer. In present study, both of these are replaced by using a monolayer graphene film deposited on glass substrate. The graphene film used to fabricate LC cell show excellent optical transmittance (~95%) over 450–800 nm range and electrical resistance of 328.77 Ω/square. Moreover, the alignment of FLC mesogens over graphene monolayer is achieved owing to the π-π electron stacking between benzene rings of FLC and honeycomb structure of graphene. Polarizing optical microscopy (POM) shows homogeneous planar alignment of filled FLC over graphene under crossed polarizers. The cell is switched between bright and dark states under the application of electric field to demonstrate the working of LC cell. Further, dielectric relaxation spectroscopy is used to measure the dielectric constant and absorption of FLC. This application of graphene would lead to thin and defect-free devices based on LC.

Tunnel spectroscopy of localised electronic states in hexagonal boron nitride

Hexagonal boron nitride is a large band gap layered crystal, frequently incorporated in van der Waals heterostructures as an insulating or tunnel barrier. Localised states with energies within its band gap can emit visible light, relevant to applications in nanophotonics and quantum information processing. However, they also give rise to conducting channels, which can induce electrical breakdown when a large voltage is applied. Here we use gated tunnel transistors to study resonant electron tunnelling through the localised states in few atomic-layer boron nitride barriers sandwiched between two monolayer graphene electrodes. The measurements are used to determine the energy, linewidth, tunnelling transmission probability, and depth within the barrier of more than 50 distinct localised states. A three-step process of electron percolation through two spatially separated localised states is also investigated.

N-Type Ohmic contact and p-type Schottky contact of monolayer InSe transistors.

Owing to their few lateral dangling bonds and enhanced gate electrostatics, two-dimensional semiconductors have attracted much attention for the fabrication of channels in next-generation field-effect transistors (FETs). Herein, combining first-principle band structure calculations with more precise quantum transport simulations, we systematically explore the interface properties between monolayer (ML) indium selenide (InSe) and a sequence of common electrodes in an FET. The ML InSe band structure is damaged by Sc, Au, Cr, Pt, and Pd electrodes but identifiable in contact with Ag, Cu, In, graphene and ML O-terminated Cr2C electrodes. A lateral n-type Schottky contact is generated with Sc, Au, Cr, Pt, Pd, and ML graphene electrodes owing to Fermi level pinning originating from the metal-induced gap states, which feature a pinning factor of 0.32. Luckily, a highly desirable lateral n-type Ohmic contact is generated with the Ag, Cu, and In electrodes. The calculated contact polarity is in agreement with the available experimental results using Au, Cr, ML graphene, Ag, and In as electrodes. Remarkably, a lateral p-type Schottky contact is generated with ML O-terminated Cr2C despite the very high work function of ML InSe. Therefore, this study offers a deeper understanding of ML InSe device interfaces and instructions for the design of ML InSe transistors.

Electrodeposition of Au on basal plane graphite and graphene

Electrodeposition is a useful route to nanoparticle formation that allows control over nucleation and growth by tuning the applied potential. Here, electrodeposition is used to create Au structures on highly oriented pyrolytic graphite and monolayer graphene electrodes. It is demonstrated that spontaneous formation of Au occurs immediately on exposure to the solution, which strongly impacts the cyclic voltammetry and obfuscates the underlying deposition mechanism. Voltammetric measurements are combined with ex situ and in situ electrochemical atomic force microscopy to reveal the processes corresponding to Au deposition and stripping. Deposition on spontaneously formed Au dominates the voltammetry, with the deposition on clean areas of the electrode requiring a much greater overpotential. Additionally, comparison of the extent of deposition and the potentials required for different grades of graphite and monolayer graphene are found to be linked to the density of defects of the electrode. Consideration of such factors is important for obtaining small, highly uniform functionalisation of graphene-based materials suitable for applications.

Fast, multicolor photodetection with graphene-contacted p-GaSe/n-InSe van der Waals heterostructures

The integration of different two-dimensional materials within a multilayer van der Waals (vdW) heterostructure offers a promising technology for high performance opto-electronic devices such as photodetectors and light sources. Here we report on the fabrication and electronic properties of vdW heterojunction diodes composed of the direct band gap layered semiconductors InSe and GaSe and transparent monolayer graphene electrodes. We show that the type II band alignment between the two layered materials and their distinctive spectral response, combined with the short channel length and low electrical resistance of graphene electrodes, enable efficient generation and extraction of photoexcited carriers from the heterostructure even when no external voltage is applied. Our devices are fast (∼2 μs), self-driven photodetectors with multicolor photoresponse ranging from the ultraviolet to the near-infrared and offer new routes to miniaturized optoelectronics beyond present semiconductor materials and technologies.

Thickness Dependence of Graphene-Hexagonal Boron Nitride-Graphene Based 2D Capacitor

In this paper, a nanoscale dielectric capacitor was fabricated through wet transfer of CVD growth hexagonal boron nitride hBN as dielectric materials between monolayer graphene electrodes. The specific capacitance, power, and energy the capacitor with different hBN layer number were evaluated through cyclic voltammetry test. Different to traditional capacitor, whose capacitance decrease with dielectric layer distance once electrode is set, the capacitance of graphene-hBN-graphene based nanocapacitor increase with layer number before the thickness goes above 3nm at low scan rate (<50mv/s) due to the strong influence of interface quantum capacitance. The thickness-dependent stacked hBN thin film’ effective permittivity between graphene electrode was analyzed by impedance spectroscopy. we experimentally reveal that the permittivity of hBN thin film first increase then decrease with it layer number increase, which is also observed in other layered structure dielectric materials. The fabricated capacitor also shows promising transparency and flexibility.

Engineering Graphene for Neural Sensing and Stimulation Applications

Graphene has recently emerged as an attractive material for neural sensing and stimulation, owing to its flexibility, transparency, excellent electrical conductivity and low noise characteristics. Monolayer graphene and multi-layer porous graphene can be engineered for neural interface applications showing advantages in different aspects. The transparent monolayer graphene electrodes enable simultaneous optical imaging and electrophysiological recordings. We have demonstrated that neural tissue can be imaged through transparent graphene electrodes using multi-photon microscopy without causing any light-induced artifacts in the electrical recordings. Combining optical imaging with electrophysiological recordings enables studies of neural activity with high spatial and temporal resolution simultaneously. Furthermore, graphene electrodes exhibit five to six times less noise compared with Au electrodes of the same size in in vivo recordings. In order to increase the surface area and charge injection capability, porous graphene have been utilized to fabricate neural stimulation electrodes. The 3D structure benefits from graphene’s good conductivity and generates greatly increased capacitance compared to the planar graphene and traditionally used bulk metals. Electrochemical tests were performed on electrodes with various areas. Impedance of the porous electrodes show two orders of magnitude decrease compared to Au electrodes of comparable size. Cyclic voltammetry and pulsed voltammetry results show enhanced charge storage and charge injection capacity for the porous electrodes. 16-electrode and 64-electrode arrays fabricated using standard micro fabrication processes have been used in in vivo neural recording and stimulation experiments. We have demonstrated recordings of evoked potentials from rat somatosensory cortex with high SNR using porous graphene electrodes. Moreover, we have used same arrays for cortical stimulation from the motor cortex of rat to evoke transient ankle and knee flexion in the contralateral leg. In summary, two forms of graphene, namely the monolayer graphene and porous graphene, were engineered for different neural applications. The monolayer graphene electrode shows the capability to record brain activity while simultaneously resolving individual cells and their connections through optical imaging. The porous graphene electrode greatly increases the charge-injection capacity for neural stimulation applications.

Heavily N-doped monolayer graphene electrodes used for high-performance N-channel polymeric thin film transistors

Recently graphene attracted much attention as a promising electrode material for organic field effect transistors (OFETs). However the electrodes used in most of the graphene-based OFETs were prepared from pristine graphene which suffers from relatively low conductivity and poor controlling of work function. In this work, we report the N-doping by Cs2CO3 of the CVD-grown single-layer graphene (SLG) via a facial spin-coating process. The Cs2CO3-engineered SLGs exhibit a heavy and stable N-doping, as well as significantly decreased work function (3.9 eV) compared to pristine graphene. The doped graphene was used as the source/drain electrodes in the bottom-contact top-gated OFETs based on a good electron transporter poly{[N,N′-bis(2-octyldodecyl)-1,4,5,8-naphthalenedicarboximide-2,6-diyl]-alt-5,5′-(2,2′-bithiophene)} (P(NDI2OD-T2)). The polymeric FETs show an enhancement of electron mobility by a factor of 10, as well as a reduction of contact resistance compared to the devices using pristine graphene. It is attributed to a remarkable lowering of electron injection barrier at the polymer/graphene electrode due to the decreased work function of graphene. In addition, the microstructural observations reveal that the face-on molecular packing and morphological feature do not change for the P(NDI2OD-T2) films coated on the doped graphene electrodes compared to those on the SiO2 dielectric, in spite of a highly hydrophobic surface of the graphene.

High-performance organic complementary inverters using monolayer graphene electrodes.

Chemical vapor deposition-grown graphene has been an attractive electrode material for organic electronic devices, such as organic field-effect transistors (OFETs), because it is highly conductive and provides good oxidation and thermal stability properties. However, it still remains a challenge to demonstrate organic complementary circuits using graphene electrodes because of the relatively poor performance of n-type OFETs. Here, we report the development of high-performance organic complementary inverters using graphene as source/drain electrodes and N, N'-ditridecyl-3,4,9,10-perylenetetracarboxylic diimide (PTCDI-C13) and pentacene as n- and p-type organic semiconductors, respectively. Graphene electrodes were n-doped via the formation of NH2-terminated self-assembled monolayers that lowered the work function and the electron injection barrier between the graphene and PTCDI-C13. Thermal annealing improved the molecular packing among PTCDI-C13 groups on the graphene surface, thereby increasing the crystallinity and grain size. The thermally annealed PTCDI-C13 OFETs prepared using n-doped graphene electrodes exhibited a good field-effect mobility of up to 0.43 cm2/(V s), which was comparable to the values obtained from other p-type pentacene OFETs. By integrating p- and n-type OFETs, we successfully fabricated organic complementary inverters that exhibited highly symmetric operation with an excellent voltage gain of up to 124 and good noise margin.