Multilayer Graphene Electrodes Research Articles

Surface modification of multilayer graphene electrodes by local printing of platinum nanoparticles using spark ablation for neural interfacing.

In this paper, we present the surface modification of multilayer graphene electrodes with platinum (Pt) nanoparticles (NPs) using spark ablation. This method yields an individually selective local printing of NPs on an electrode surface at room temperature in a dry process. NP printing is performed as a post-process step to enhance the electrochemical characteristics of graphene electrodes. The NP-printed electrode shows significant improvements in impedance, charge storage capacity (CSC), and charge injection capacity (CIC), versus the equivalent electrodes without NPs. Specifically, electrodes with 40% NP surface density demonstrate 4.5 times lower impedance, 15 times higher CSC, and 4 times better CIC. Electrochemical stability, assessed via continuous cyclic voltammetry (CV) and voltage transient (VT) tests, indicated minimal deviations from the initial performance, while mechanical stability, assessed via ultrasonic vibration, is also improved after the NP printing. Importantly, NP surface densities up to 40% maintain the electrode optical transparency required for compatibility with optical imaging and optogenetics. These results demonstrate selective NP deposition and local modification of electrochemical properties in graphene electrodes for the first time, enabling the cohabitation of graphene electrodes with different electrochemical and optical characteristics on the same substrate for neural interfacing.

Storage of atomic hydrogen in multilayer graphene

The world is moving rapidly to embrace renewable energy sources. One energy carrier, hydrogen, is a growing player in this field. This study was motivated by a desire to find alternative hydrogen storage mechanisms using processes that are cost effective with low environmental impact and good energy storage efficiency. This paper presents initial research findings on the novel approach of employing the patented RMIT Proton Battery to store atomic hydrogen in a multilayer graphene electrode using an acid electrolyte. This is a very different approach to conventional hydrogen energy storage systems. The paper reveals that one supplier's product achieves a 0.35 wt% reversible hydrogen storage in a multilayer graphene material with 0.35 nm layer separation and a specific surface area of 720 m2/g.

Multilayer CVD graphene electrodes using a transfer-free process for the next generation of optically transparent and MRI-compatible neural interfaces.

Multimodal platforms combining electrical neural recording and stimulation, optogenetics, optical imaging, and magnetic resonance (MRI) imaging are emerging as a promising platform to enhance the depth of characterization in neuroscientific research. Electrically conductive, optically transparent, and MRI-compatible electrodes can optimally combine all modalities. Graphene as a suitable electrode candidate material can be grown via chemical vapor deposition (CVD) processes and sandwiched between transparent biocompatible polymers. However, due to the high graphene growth temperature (≥ 900 °C) and the presence of polymers, fabrication is commonly based on a manual transfer process of pre-grown graphene sheets, which causes reliability issues. In this paper, we present CVD-based multilayer graphene electrodes fabricated using a wafer-scale transfer-free process for use in optically transparent and MRI-compatible neural interfaces. Our fabricated electrodes feature very low impedances which are comparable to those of noble metal electrodes of the same size and geometry. They also exhibit the highest charge storage capacity (CSC) reported to date among all previously fabricated CVD graphene electrodes. Our graphene electrodes did not reveal any photo-induced artifact during 10-Hz light pulse illumination. Additionally, we show here, for the first time, that CVD graphene electrodes do not cause any image artifact in a 3T MRI scanner. These results demonstrate that multilayer graphene electrodes are excellent candidates for the next generation of neural interfaces and can substitute the standard conventional metal electrodes. Our fabricated graphene electrodes enable multimodal neural recording, electrical and optogenetic stimulation, while allowing for optical imaging, as well as, artifact-free MRI studies.

Fe-induced layer exchange of multilayer graphene for rechargeable battery anodes

Multilayer graphene (MLG) is a promising material for anodes of next-generation thin-film rechargeable batteries. The inverted layer exchange using an Fe catalyst allowed for the low-temperature (600 °C) self-organization of the anode electrode structure, that is, an active material (MLG) on a current collector (Fe). A coin-type cell, fabricated from the MLG electrode and pure Li metal foil, showed distinct peaks, indicating Li intercalation into the MLG in a cyclic voltammogram. After 100 charge/discharge cycles, the capacity was 3.3 μAh cm−2 and coulombic efficiency was 98%. The low-temperature synthesis of a MLG anode structure and its operation were demonstrated.

Memristors based on multilayer graphene electrodes for implementing a low-power neuromorphic electronic synapse

The Ta/Ta2O5/AlN/graphene memristor with silicon-based multilayer graphene films as the bottom electrode has stable electrical characteristics.

Graphene-Boron Nitride-Graphene Cross-Point Memristors with Three Stable Resistive States.

Two-dimensional (2D) material-based memristors have shown several properties that are not shown by traditional ones, such as high transparency, robust mechanical strength and flexibility, superb chemical stability, enhanced thermal heat dissipation, ultralow power consumption, coexistence of bipolar and threshold resistive switching, and ultrastable relaxation when used as electronic synapse (among others). However, several electrical performances often required in memristive applications, such as the generation of multiple stable resistive states for high-density information storage, still have never been demonstrated. Here, we present the first 2D material-based memristors that exhibit three stable and well-distinguishable resistive states. By using a multilayer hexagonal boron nitride (h-BN) stack sandwiched by multilayer graphene (G) electrodes, we fabricate 5 μm × 5 μm cross-point Au/Ti/G/h-BN/G/Au memristors that can switch between each two or three resistive states, depending on the current limitation (CL) and reset voltage used. The use of graphene electrodes plus a small cross-point structure are key elements to observe the tristate operation, which has not been observed in larger (100 μm × 100 μm) devices with an identical Au/Ti/G/h-BN/G/Au structure nor in similar small (5 μm × 5 μm) devices without graphene interfacial layers (i.e., Au/Ti/h-BN/Au). Basically, we generate an intermediate state between the high resistive state and the low resistive state (LRS), named soft-LRS (S-LRS), which may be related to the formation of a narrower conductive nanofilament across the h-BN because of the ability of graphene to limit metal penetration (at low CLs). All the 2D materials have been fabricated using the scalable chemical vapor deposition approach, which is an immediate advantage compared to other works using mechanical exfoliated 2D materials.

Unidirectional Real-Time Photoswitching of Diarylethene Molecular Monolayer Junctions with Multilayer Graphene Electrodes.

We fabricate and characterize vertical molecular junctions consisting of self-assembled monolayers of diarylethene (DAE) contacted by a multilayer graphene (MLG) electrode on the top and gold on the bottom. The DAE molecular junctions show two stable electrical states, a closed state (high conductance) or an open state (low conductance), which are created upon illumination with UV or visible light, respectively. For the Au-DAE-MLG junction structure, we observe that the current levels between the two conductance states are separated by 2 orders of magnitude. However, in a real-time measurement, we observe only unidirectional switching behavior from the open to the closed state.

Multi-layer graphene as a selective detector for future lung cancer biosensing platforms.

Highly selective, fast detection of specific lung-cancer biomarkers (CMs) in exhaled human breath is vital to the development of enhanced sensing devices. Today, e-nose is a promising approach for the diagnosis of lung cancer. Nevertheless, considerable challenges to early-stage disease diagnostics still remain: e.g. decrease in sensor sensitivities in the presence of water vapor, sensor drift leading to the inability to calibrate exactly, relatively short sensor lifetimes, and difficulty discriminating between multiple diseases. However, there is a wide scope for breath diagnostics techniques, and all advanced electrodes applicable to e-nose devices will benefit them. Here, we present the promising sensing capabilities of bare multi-layer graphene (MLG) as a proof of concept for advanced e-nose devices and demonstrate its utility for biomolecule discrimination of the most common lung CMs (ethanol, isopropanol, and acetone). We report on a comparative study involving exposure of the three CM solutions on flat MLG (f-MLG) and patterned MLG (p-MLG) electrodes, where the electrical conductivity of p-MLG is significantly increased while applying acetone. Based on sensitivity tests, we demonstrate the ability to monitor the electrical response of graphene electrodes employing graphene of various wettabilities. Specifically, the f-MLG electrode displays almost 2 times higher sheet resistance (30 Ω sq-1) compared to the hydrophilic p-MLG (12 Ω sq-1). We show significant sensitivity to selected specific molecules of pristine f-MLG and p-MLG while applying CM solutions with a 1.4 × 105 ppm concentration. Finally, we show the selectivity of f-MLG and p-MLG-based sensors when exposed to 2.0 × 105 ppm solutions containing different CM combinations. Both sensors were selective in particular to acetone, since the presence of acetone leads to a sheet resistance increase. We demonstrate that an advanced e-nose approach integrated with MLG electrodes has significant potential as a design concept for utilization of molecular detection at variable concentrations such as in early-stage disease diagnosis. This early-stage approach will provide convenient and reusable complex monitoring of CMs compared to typical contact sensors which require target analysis and are limited by disposable measuring. Moreover, further integration of the Internet of Things will introduce advanced e-nose devices as a biotechnological innovation for disease resilience with the potential for commercialization.

Charge injection and transport properties of large area organic junctions based on aryl thin films covalently attached to a multilayer graphene electrode.

The quantum interaction between molecules and electrode materials at molecule/electrode interfaces is a major ingredient in the electron transport properties of organic junctions. Driven by the coupling strength between the two materials, it results mainly in the broadening and energy shift of the interacting molecular orbitals. Using new electrode materials, such as the recently developed semi-conducting two-dimensional nanomaterials, has become a significant advancement in the field of molecular/organic electronics that opens new possibilities for controlling the interfacial electronic properties and thus the charge injection properties. In this article, we report the use of atomically thin two-dimensional multilayer graphene films as the base electrode in organic junctions with a vertical architecture. The interfacial electronic structure dominated by the covalent bonding between bis-thienyl benzene diazonium-based molecules and the multilayer graphene electrode has been probed by ultraviolet photoelectron spectroscopy and the results are compared with those obtained on junctions with standard Au electrodes. Room temperature injection properties of such interfaces have also been explored by electron transport measurements. We find that, despite strong variations of the density of states, the Fermi energy and the injection barriers, both organic junctions with Au base electrodes and multilayer graphene base electrodes show similar electronic responses. We explain this observation by the strong orbital coupling occurring at the bottom electrode/bis-thienyl benzene molecule interface and by the pinning of the hybridized molecular orbitals.

Photocurrent generation of biohybrid systems based on bacterial reaction centers and graphene electrodes

The direct conversion of sunlight into chemical energy via photosynthesis is a unique capability of plants and some bacterial species. Aimed at mimicking this energy conversion process, the combination of inorganic substrates and organic photoactive proteins into an artificial biohybrid system is of a great interest for artificial bio-photovoltaic applications. It also allows to better understand charge transfer processes involved in the photosynthetic chain. In this work, single layer graphene (SLG) and multilayer graphene (MLG) electrodes are used as a platform for the immobilization of reaction centers (RCs) from purple bacteria Rhodobacter sphaeroides, a protein complex responsible for the generation of photo-excited charges. Electrochemical experiments with graphene electrodes and redox molecules reveal fundamental differences in the charge transfer processes for SLG and MLG films. We demonstrate that both graphene-based materials enable the immobilization of RCs without loss of functionality, attested by a photocurrent generation under illumination with IR-light at a wavelength of 870 nm. Furthermore, we report on the dependence of the generated photocurrent on the applied bias voltage, as well as on the presence of charge mediators in the surrounding electrolyte. This work demonstrates that SLG and MLG are a suitable platform for RC immobilization and subsequent photocurrent generation, suggesting a promising potential for graphene-based materials in bio-photovoltaics.

Flexible metal-free supercapacitors based on multilayer graphene electrodes

We report the development of flexible self-supported metal-free electrodes based on non-oxidized graphene multilayer (MLG) paper, utilized to assemble bendable electrochemical supercapacitors. Using a simplified production method various electrodes exhibiting high conductivity were prepared and its structural, morphological, chemical and electrical properties characterized. Measurements in supercapacitors assembled from MLG electrodes and PANI-coated MLG electrodes show excellent electrochemical characteristics, with specific capacitances of 58 mF cm−2 and 451 F g−1 with 84% and 103% of capacitance retention after 1000 cycles, respectively. The energy and power density obtained for the PANI/MLG flexible supercapacitor reached 17.0 Wh g−1 and 3.61 kW kg−1, respectively. Moreover, in bending tests, the supercapacitors displayed an excellent mechanical and electrochemical stability. As an important result of this study, MLG electrodes appear as an interesting material for flexible high performance energy storage devices.

In Situ XPS Reveals Voltage Driven Asymmetric Ion Movement of an Ionic Liquid through the Pores of a Multilayer Graphene Electrode

Under application of a voltage bias, asymmetric ion movement of an ionic liquid (IL) through a multilayered graphene (MLG) electrode has been detected by X-ray photoelectron spectroscopy, via recording the intensity of the two nitrogen peaks. Accordingly, we observe that upon increasing the bias, the two peaks representing the cationic and anionic fragments of the IL start appearing with increasing intensity, together with an asymmetry in their ratio, differing from unity by about 10%. Bias-dependent binding energy shifts followed through atomic features of the IL (F 1s, N 1s, and C 1s) and the graphene electrode (C 1s) indicate that a distinct solid–liquid interface develops throughout the entire intercalation process with an additional and pertinent evidence for finite potential drops across the two electrical double layers. This evidence is bolstered by the fact that the measured binding energy difference between the F 1s of the liquid and C 1s peak of the semisolid MLG electrode is only about half of ...

Improving Graphene-metal Contacts: Thermal Induced Polishing

ABSTRACTGraphene is a very promising material for nanoelectronics applications due to its unique and remarkable electronic and thermal properties. However, when deposited on metallic electrodes the overall thermal conductivity is significantly decreased. This phenomenon has been attributed to the mismatch between the interfaces and contact thermal resistance. Experimentally, one way to improve the graphene/metal contact is through high-temperature annealing, but the detailed mechanisms behind these processes remain unclear. In order to address these questions, we carried out fully atomistic reactive molecular dynamics simulations using the ReaxFF force field to investigate the interactions between multi-layer graphene and metallic electrodes (nickel) under (thermal) annealing. Our results show that the annealing induces an upward-downward movement of the graphene layers, causing a pile-driver-like effect over the metallic surface. This graphene induced movements cause a planarization (thermal polishing-like effect) of the metallic surface, which results in the increase of the effective graphene/metal contact area. This can also explain the experimentally observed improvements of the thermal and electric conductivities.

Interface-Engineered Charge-Transport Properties in Benzenedithiol Molecular Electronic Junctions via Chemically p-Doped Graphene Electrodes.

In this study, we fabricated and characterized vertical molecular junctions consisting of self-assembled monolayers of benzenedithiol (BDT) with a p-doped multilayer graphene electrode. The p-type doping of a graphene film was performed by treating pristine graphene (work function of ∼4.40 eV) with trifluoromethanesulfonic (TFMS) acid, producing a significantly increased work function (∼5.23 eV). The p-doped graphene-electrode molecular junctions statistically showed an order of magnitude higher current density and a lower charge injection barrier height than those of the pristine graphene-electrode molecular junctions, as a result of interface engineering. This enhancement is due to the increased work function of the TFMS-treated p-doped graphene electrode in the highest occupied molecular orbital-mediated tunneling molecular junctions. The validity of these results was proven by a theoretical analysis based on a coherent transport model that considers asymmetric couplings at the electrode-molecule interfaces.

High Mobility WS2 Transistors Realized by Multilayer Graphene Electrodes and Application to High Responsivity Flexible Photodetectors

AbstractThe electrical contact is one of the main issues preventing semiconducting 2D materials to fulfill their potential in electronic and optoelectronic devices. To overcome this problem, a new approach is developed here that uses chemical vapor deposition grown multilayer graphene (MLG) sheets as flexible electrodes for WS2 field‐effect transistors. The gate‐tunable Fermi level, van der Waals interaction with the WS2, and the high electrical conductivity of MLG significantly improve the overall performance of the devices. The carrier mobility of single‐layer WS2 increases about a tenfold (50 cm2 V−1 s−1 at room temperature) by replacing conventional Ti/Au metal electrodes (5 cm2 V−1 s−1) with the MLG electrodes. Further, by replacing the conventional SiO2 substrate with a thin (1 µm) parylene‐C flexible film as insulator, flexible WS2 photodetectors that are able to sustain multiple bending stress tests without significant performance degradation are realized. The flexible photodetectors exhibited extraordinarily high gate‐tunable photoresponsivities, reaching values of 4500 A W−1, and with very short (<2 ms) response time. The work of the heterostacked structure combining WS2, graphene, and the very thin polymer film will find applications in various flexible electronics, such as wearable high‐performance optoelectronics devices.

High Photoresponsivity in Graphene Nanoribbon Field-Effect Transistor Devices Contacted with Graphene Electrodes

Ultranarrow graphene nanoribbons (GNRs) with atomically precise structures are considered a promising class of materials for the realization of optoelectronic and photonic devices with improved functionalities. Here we report the optoelectronic characterization of a field-effect transistor device made of a layer of bottom-up synthesized GNRs contacted with multilayer graphene electrodes, showing high photoresponsivity of 5 × 105 A/W for small incident power in the visible–UV range. Our results show that combining the properties of intrinsic graphene with that of semiconducting GNRs is a viable route to realize novel devices for optoelectronic and sensing applications.

Large-Area CVD-Grown Sub-2 V ReS2 Transistors and Logic Gates.

We demonstrated the fabrication of large-area ReS2 transistors and logic gates composed of a chemical vapor deposition (CVD)-grown multilayer ReS2 semiconductor channel and graphene electrodes. Single-layer graphene was used as the source/drain and coplanar gate electrodes. An ion gel with an ultrahigh capacitance effectively gated the ReS2 channel at a low voltage, below 2 V, through a coplanar gate. The contact resistance of the ion gel-gated ReS2 transistors with graphene electrodes decreased dramatically compared with the SiO2-devices prepared with Cr electrodes. The resulting transistors exhibited good device performances, including a maximum electron mobility of 0.9 cm2/(V s) and an on/off current ratio exceeding 104. NMOS logic devices, such as NOT, NAND, and NOR gates, were assembled using the resulting transistors as a proof of concept demonstration of the applicability of the devices to complex logic circuits. The large-area synthesis of ReS2 semiconductors and graphene electrodes and their applications in logic devices open up new opportunities for realizing future flexible electronics based on 2D nanomaterials.

Multilayer graphene electrodes for one-port surface acoustic wave resonator mass sensor

A one-port surface acoustic wave (SAW) resonator mass sensor composed of multilayer graphene (MLG) electrodes was investigated by the finite element method (FEM) and analyses were carried out to study the enhancement of sensitivity and the secondary effects caused by MLG electrodes on the performance of the resonator. Unlike metal electrodes, MLG electrode offers elastic loading to the contact surface, as evidenced by the increase in the surface velocity of the SAW device. In terms of the sensitivity of the mass sensor, MLG electrode showed the largest center frequency shift in response to a change in mass loading, as well as when used as a gas sensor to detect volatile organic compounds (VOCs). Also, MLG electrodes offered the least triple transit signal (TTS) and bulk acoustic wave (BAW) generations compared with Al and Au–Cr electrodes. Thus, the one-port SAW resonator with graphene electrodes not only possesses excellent performance characteristics but also gives rise to new opportunities in the development of highly sensitive mass sensors.

Disordered 3 D Multi-layer Graphene Anode Material from CO2 for Sodium-Ion Batteries.

We report the application of disordered 3 D multi-layer graphene, synthesized directly from CO2 gas through a reaction with Li at 550 °C, as an anode for Na-ion batteries (SIBs) toward a sustainable and greener future. The material exhibited a reversible capacity of ∼190 mA h g(-1) with a Coulombic efficiency of 98.5 % at a current density of 15 mA g(-1) . The discharge capacity at higher potentials (>0.2 V vs. Na/Na(+) ) is ascribed to Na-ion adsorption at defect sites, whereas the capacity at low potentials (<0.2 V) is ascribed to intercalation between graphene sheets through electrochemical characterization, Raman spectroscopy, and small-angle X-ray scattering experiments. The disordered multi-layer graphene electrode demonstrated a great rate capability and cyclability. This novel approach to synthesize disordered 3 D multi-layer graphene from CO2 gas makes it attractive not only as an anode material for SIBs but also to mitigate CO2 emission.

Partially Reduced Graphite Oxide: A Close Examination of the Contributions to Its Storage Capacity

Electrical energy storage constitutes an essential component in the development of sustainable energy technologies, given that enables energy to be used on demand. Batteries and supercapacitors have proven to be of crucial importance for advanced and highly efficient energy storage and management. Graphene-derived materials have been used both in batteries and supercaps because of their high electronic conductivity, intercalation properties and large surface area. (1) The main difference between a battery and a supercapacitor lies in their charge-storage mechanism which is related to the nature of their electrochemical processes. By understanding the mechanisms occurring during the electrochemical cycling we might be able to optimize both the energy and power densities in the energy storage devices containing such materials. In the case of batteries, the process would be diffusion-limited and expected to be proportional to the square root of the scan rate while a strictly capacitive contribution (double-layer and pseudocapacitance) would depend linearly on the scan rate and would be related to supercapacitors. (2) Lately, materials exhibiting charge storage using both capacitive and faradaic means have been of great interest. (3,4) Herein, we studied the electrochemical properties of a partially reduced graphite oxide, exhibiting quinone moieties. The identification of both, diffusion-limited and capacitive processes, to the charging storage mechanism in multilayer graphene electrodes in aqueous electrolytes was examined by cyclic voltammetry (Figure 1). This study provides insightful information about the electron-proton transfer reactions in a unique material as it is the partially reduced graphite oxide, where quinone groups and an electron conducting C-C crystalline lattice coexist together in the same unit. The identification and quantification of both pseudocapacitance and double layer contributions to the charging storage mechanism in the different electrodes was also possible. Figure 1. Capacitive and diffusion-controlled contributions to charge storage in a.) Protic and b.) aprotic electrolyte. The analysis of cyclic voltammetry curves at different scan rates allowed determining the fraction of the current arising from diffusion-controlled and capacitive processes. For experiments developed in acid electrolytes, a capacitive contribution of up to 85% was found, whereas only about 22% of the stored charge is capacitive in the case of aprotic electrolytes. From these experiments, it can be inferred that about 63% of the current is primarily pseudocapacitive. The electrochemical evaluation of this material would facilitate the preparation of novel graphene-based electrodes for devices based on the properties desired. References Kaner, R.B., Wassei, J.K., Acc. Chem. Res. 2013, 46(10), 2244-2253.Bard, A.J., Faulkner L.R. Electrochemical Methods: Fundamentals and applications; John Wiley & Sons: New York, 1980.Brezesinski, T., Wang, J., Tolbert, S.H., Dunn B. Nat. Mater. 2010, 9, 146-151.Sathiya, M., Prakash, A.S., Ramesha, K., Tarascon, J.M., Shukla, A.K. J. Am. Chem. Soc 2011, 133, 16291-16299. Figure 1