Single-layer Graphene Electrodes Research Articles

Real-Time Monitoring of Cell Adhesion onto a Soft Substrate by a Graphene Impedance Biosensor.

Soft substrates are interesting for many applications, ranging from mimicking the cellular microenvironment to implants. Conductive electrodes on such substrates allow the realization of flexible, elastic, and transparent sensors. Single-layer graphene as a candidate for such electrodes brings the advantage that the active area of the sensor is transparent and conformal to the underlying substrate. Here, we overcome several challenges facing the routine realization of graphene cell sensors on a canonical soft substrate, namely, poly(dimethylsiloxane) (PDMS). We have systematically studied the effect of surface energy before, during, and after the transfer of graphene. Thus, we have identified a suitable support polymer, optimal substrate (pre)treatment, and an appropriate solvent for the removal of the support. Using this procedure, we can reproducibly obtain stable and intact graphene sensors on a millimeter scale on PDMS, which can withstand continuous measurements in cell culture media for several days. From local nanomechanical measurements, we infer that the softness of the substrate is slightly affected after the graphene transfer. However, we can modulate the stiffness using PDMS with differing compositions. Finally, we show that graphene sensors on PDMS can be successfully used as soft electrodes for real-time monitoring of the cell adhesion kinetics. The routine availability of single-layer graphene electrodes on a soft substrate with tunable stiffness will open a new avenue for studies, where the PDMS-liquid interface is made conducting with minimal alteration of the intrinsic material properties such as softness, flexibility, elasticity, and transparency.

Fast Ion Transfer Associated with Dehydration and Modulation of Hydration Structure in Electric Double-Layer Capacitors Using Molecular Dynamics Simulations and Experiments

Carbon materials, such as graphite and activated carbon, have been widely used as electrodes in batteries and electric double-layer capacitors (EDLCs). Graphene, which has an extremely thin sheet-like structure, is considered as a fundamental carbon material. However, it was less investigated as an electrode material than graphite and activated carbons. This is because graphene is a relatively new material and is difficult to handle. However, using graphene electrodes can enhance the performance of nanodevices. Here, the performance of EDLCs based on single-layer and bilayer graphene electrodes in LiCl, NaCl, and KCl aqueous electrolyte solutions was evaluated using cyclic voltammetry, and the charging mechanism was evaluated using molecular dynamics simulations. KCl aqueous solution provided the highest capacitance compared to LiCl and NaCl aqueous solutions in the case of single-layer graphene electrodes. In contrast, the dependence of the capacitance on the ion species was hardly observed in the case of bilayer graphene. This indicates that Li and Na ions also contributed to the capacitances. The high EDLC performance can be attributed to the fast ion transfer promoted by the dehydration and modification of the second hydration shell on the bilayer graphene because of the relatively strong interaction of ions with the bilayer graphene.

A van der Waals heterojunction strategy to fabricate layer-by-layer single-molecule switch

Single-molecule electronics offer a unique strategy for the miniaturization of electronic devices. However, the existing experiments are limited to the conventional molecular junctions, where a molecule anchors to the electrode pair with linkers. With such a rod-like configuration, the minimum size of the device is defined by the length of the molecule. Here, by incorporating a single molecule with two single-layer graphene electrodes, we fabricated layer-by-layer single-molecule heterojunctions called single-molecule two-dimensional van der Waals heterojunctions (M-2D-vdWHs), of which the sizes are defined by the thickness of the molecule. We controlled the conformation of the M-2D-vdWHs and the cross-plane charge transport through them with the applied electric field and established that they can serve as reversible switches. Our results demonstrate that the M-2D-vdWHs, as stacked from single-layer 2D materials and a single molecule, can respond to electric field stimulus, which promises a diverse class of single-molecule devices with unprecedented size.

Quantum and Classical Characteristics of the Capacitance of Electrode-Electrolyte Interfaces from Joint Density Functional Theory

Quantifying the interfacial response towards addition of charge, the capacitance of electrode-electrolyte interfaces is a key property for the functioning of electrochemical devices. The rich electronic properties of two-dimensional electrode materials strongly affect their electrochemical behavior, giving rise to their so-called quantum capacitance CQ. It is well-accepted that the quantum capacitance is essentially equal to the electronic density of states (DOS) of the electrode. However, the total capacitance Ctot of the electrode-electrolyte interface is also determined by the "classical" double-layer capacitance Cdl. Typically, both contributions are assumed to act in series, resulting in the commonly employed partitioning 1/Ctot = 1/CQ + 1/Cdl. However, in spite of its fundamental relevance, the motivation of this partitioning remained vague to date.I will show how both quantum and classical characteristics of the capacitance of complex electrode-electrolyte interfaces rigorously derive ab initio from joint density functional theory that provides a framework for the combined description of both the electronic density of the electrode and the ionic and dielectric densities of the electrolyte. Whereas the quantum capacitance is determined by the DOS of the Kohn-Sham single-particle orbitals, the double-layer capacitance is determined by the interaction between the local density of states (LDOS) of the electrode and the Fukui function of the electrode-electrolyte system. The derived relation precisely yields the known partitioning of the total interface capacitance, but it also shows that additional capacitive contributions arise, e.g., from the electronic exchange-correlation interaction.This analysis provides a clear perspective on the influence of the electrode material and thickness, as well as temperature, on the charging response of the interface, which is particularly relevant for two-dimensional electrode materials where all capacitive contributions are significant and non-trivial. Moreover, the presented approach makes the transition from thin two-dimensional to thick bulk electrodes mathematically traceable and shows how the classical electrochemical double-layer capacitance is recovered in the limit of a bulk metal electrode. The findings will be exemplified by computational results for a single-layer graphene electrode, and for bulk and single-layer gold electrodes.[1] T. Binninger, Piecewise nonlinearity and capacitance in the joint density functional theory of extended interfaces, Phys. Rev. B 103 (2021), L161403.

Label-Free Probing of Electron Transfer Kinetics of Single Microbial Cells on a Single-Layer Graphene via Structural Color Microscopy.

Studies of electron transfer at the population level veil the nature of the cell itself; however, in situ probing of the electron transfer dynamics of individual cells is still challenging. Here we propose label-free structural color microscopy for this aim. We demonstrate that Shewanella oneidensis MR-1 cells show unique structural color scattering, changing with the redox state of cytochrome complexes in the outer membrane. It enables quantitatively and noninvasive studies of electron transfer in single microbial cells during bioelectrochemical activities, such as extracellular electron transfer (EET) on a transparent single-layer graphene electrode. Increasing the applied potential leads to the associated EET current, accompanied by more oxidized cytochromes. The high spatiotemporal resolution of the proposed method not only demonstrates the large diversity in EET activity among microbial cells but also reveals the subcellular asymmetric distribution of active cytochromes in a single cell. We anticipate that it provides a potential platform for further exploring the electron transfer mechanism of subcellular structure.

Enhanced Thermopower of Saturated Molecules by Noncovalent Anchor-Induced Electron Doping of Single-Layer Graphene Electrode.

Enhancing thermopower is a key goal in organic and molecular thermoelectrics. Herein, it is shown that introducing noncovalent contact with a single-layer graphene (SLG) electrode improves the thermopower of saturated molecules as compared to the traditional gold-thiolate covalent contact. Thermoelectric junction measurements with a liquid-metal technique reveal that the value of Seebeck coefficient in large-area junctions based on n-alkylamine self-assembled monolayers (SAMs) on SLG is increased up to fivefold compared to the analogous junction based on n-alkanethiolate SAMs on gold. Experiments with Raman spectroscopy and field-effect transistor analysis indicate that such enhancements benefit from the creation of new in-gap states and electron doping through noncovalent interaction between the amine anchor and the SLG electrode, which leads to a reduced energy offset between the Fermi level and the transport channel. This work demonstrates that control of interfacial bonding nature in molecular junctions improves the Seebeck effect in saturated molecules.

Ferroelectric semiconductor junctions based on graphene/In2Se3/graphene van der Waals heterostructures

The miniaturization of ferroelectric devices offers prospects for non-volatile memories, low-power electrical switches and emerging technologies beyond existing Si-based integrated circuits. An emerging class of ferroelectrics is based on van der Waals (vdW) two-dimensional materials with potential for nano-ferroelectrics. Here, we report on ferroelectric semiconductor junctions (FSJs) in which the ferroelectric vdW semiconductor α-In2Se3 is embedded between two single-layer graphene electrodes. In these two-terminal devices, the ferroelectric polarization of the nanometre-thick In2Se3 layer modulates the transmission of electrons across the graphene/In2Se3 interface, leading to memristive effects that are controlled by applied voltages and/or by light. The underlying mechanisms of conduction are examined over a range of temperatures and under light excitation revealing thermionic injection, tunnelling and trap-assisted transport. These findings are relevant to future developments of FSJs whose geometry is well suited to miniaturization and low-power electronics, offering opportunities to expand functionalities of ferroelectrics by design of the vdW heterostructure.

Piecewise nonlinearity and capacitance in the joint density functional theory of extended interfaces

The ab initio simulation of charged interfaces in the framework of density functional theory (DFT) is heavily employed for the study of electrochemical energy conversion processes. The capacitance is the primary descriptor for the response of the electrochemical interface. It is essentially equal to the inverse of the energy curvature as a function of electron number, and as such there appears a conflict with the fundamental principle of piecewise linearity in DFT that requires the energy curvature to be zero at fractional electron numbers, i.e. almost everywhere. To resolve this conflict, we derive an exact expression between the energy curvature and the Kohn-Sham density of states, the local density of states, and the Fukui potential. We find that the piecewise linearity requirement does not hold for the volume- or area-specific energy of extended systems and surfaces. Applied to the joint density functional theory of an electrode-electrolyte interface, including the ionic and dielectric response of the electrolyte, the same expression represents a rigorous basis for the partitioning of the total interfacial capacitance into contributions of the quantum capacitance, space-charge capacitance, and electrochemical double-layer capacitance. It provides insight into the influence of the electrode material, thickness, and temperature on the charging characteristics, as demonstrated by results for a bulk gold electrode, a single-layer gold electrode, and a single-layer graphene electrode.

Electrochemical Characterization of Single Layer Graphene/Electrolyte Interface: Effect of Solvent on the Interfacial Capacitance

AbstractThe development of the basic understanding of the charge storage mechanisms in electrodes for energy storage applications needs deep characterization of the electrode/electrolyte interface. In this work, we studied the charge of the double layer capacitance at single layer graphene (SLG) electrode used as a model material, in neat (EMIm‐TFSI) and solvated (with acetonitrile) ionic liquid electrodes. The combination of electrochemical impedance spectroscopy and gravimetric electrochemical quartz crystal microbalance (EQCM) measurements evidence that the presence of solvent drastically increases the charge carrier density at the SLG/ionic liquid interface. The capacitance is thus governed not only by the electronic properties of the graphene, but also by the specific organization of the electrolyte side at the SLG surface originating from the strong interactions existing between the EMIm+ cations and SLG surface. EQCM measurements also show that the carbon structure, with the presence of sp2 carbons, affects the charge storage mechanism by favoring counter‐ion adsorption on SLG electrode versus ion exchange mechanism in amorphous porous carbons.

Electrochemical Characterization of Single Layer Graphene/Electrolyte Interface: Effect of Solvent on the Interfacial Capacitance.

The development of the basic understanding of the charge storage mechanisms in electrodes for energy storage applications needs deep characterization of the electrode/electrolyte interface. In this work, we studied the charge of the double layer capacitance at single layer graphene (SLG) electrode used as a model material, in neat (EMIm‐TFSI) and solvated (with acetonitrile) ionic liquid electrodes. The combination of electrochemical impedance spectroscopy and gravimetric electrochemical quartz crystal microbalance (EQCM) measurements evidence that the presence of solvent drastically increases the charge carrier density at the SLG/ionic liquid interface. The capacitance is thus governed not only by the electronic properties of the graphene, but also by the specific organization of the electrolyte side at the SLG surface originating from the strong interactions existing between the EMIm+ cations and SLG surface. EQCM measurements also show that the carbon structure, with the presence of sp2 carbons, affects the charge storage mechanism by favoring counter‐ion adsorption on SLG electrode versus ion exchange mechanism in amorphous porous carbons.

Combination of Electrochemical Surface Plasmon Resonance and Molecular Dynamics Simulation to Evaluate the Quasi-Static Capacitance at the Ionic Liquid/Electrode Interface

1. Introduction Composed of ions solely, ionic liquids (ILs) are low-melting point salts exhibiting excellent properties for electrolytes such as ultra-low volatility, non-flammability, thermal/electrochemical stability. The structure and dynamics of the electric double layer (EDL) are critical in the process of charge/mass transfer. However, different from the conventional electrolytes, ILs exhibit strong Coulomb correlation with high concentration and density, hence Gouy-Champman-Stern model is not applicable to describe the EDL of ILs. For the experimental measurement of the differential capacitance in the EDL, electrochemical impedance spectroscopy (EIS) is the most popular method. But the fast ac potential oscillation in EIS measurements can’t avoid the influence of the slow dynamics of the EDL such as ultraslow relaxation[1] and hysteresis.[2] In the present study, electrochemical surface plasmon resonance (ESPR) is proposed as an effective measurement method of static differential capacitance at the IL/electrode interface. The influences of the slow relaxation are demonstrated at different scan rates of potential. In order to achieve quantitative measurement, molecular dynamics (MD) simulations of the IL between two single-layer graphene electrodes are performed. Moreover, through the data analysis of MD, the variations of ionic layer structure and ionic orientation are revealed and described in detail. 2. Methods 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide ([C4mim+][TFSA−]) was chosen as the IL material for research. In the ESPR experiments, a laser beam was incident through a prism and totally reflected on a 50-nm golden film deposited on a SF15 glass. The golden film was the working electrode, a Pt wire was set as the counter electrode, and an Ag/AgCl wire was inserted into the IL as the quasi-reference electrode. The experiments were performed at different scan rates to determine the influence of slow dynamics.[3] The all-atom MD simulations were performed by using DL_POLY classic. The results in all the charged conditions were simulated from the same initial configuration, which was prepared from IL bulk simulations, vacuum-liquid interface simulations, and uncharged graphene-IL interface simulations.[4] There were 720 ion pairs between two single-layer graphene electrodes within 125 Å. In each charged condition, the charge was allocated evenly to every C atom on the electrodes. The temperature was controlled (NVT ensemble) by using the Berendsen thermostat (0.5 ns as the quick equilibration process) and the Nose-Hoover thermostat (2 ns). Thus, the trajectories in the last 1 ns were chosen as the statistical data for analysis. 3. Results and Discussion By ESPR experiments, the change in SPR angle is recorded simultaneously with cyclic voltammetry, exhibiting a sigmoidal shape with the maximum slope at around −0.6 V (PZC). We find that the variation ranges by the negative scans are larger than those by the positive scans, and the variation range by positive scan increases as the scan rate decreases. These phenomena indicate that the influence of slow dynamics is insignificant for negative scans as was previously observed for a different IL.[1] To study the cause of the sigmoidal change in SPR angle, we propose a mathematical model,[3] by which a linear relationship between SPR angle and the surface charge density at the IL/electrode interface can be established. Consequently, the relative value of the quasi-static differential capacitance is shown in Fig. 1. Comparing with the previous study,[2] we determine the potential of zero charge in this system as −0.61 V, noted as the grey dashed line. This is the first time to obtain the static differential capacitance at the IL/solid electrode, in spite of the relative value. The differential capacitance exhibits a camel-shaped potential dependence, agreeing with the mean-field lattice gas model.[5] For quantitative measurement, MD simulations are performed to produce ionic distributions and surface charge density with respect to potential. By the data fitting of surface charge density and ionic concentration, we estimate the X − 1 as 3.1 mdeg∙cm2∙μC−1. Furthermore, by the observation of the ionic distribution, we confirm the butyl group of C4mim+ in the first ionic layer would provide extra room for the cations in the second layer, resulting in that the state of “ionic crowding” is hardly to occur and the interval of ionic layers varies with the potential on the negatively charged electrode.

Organic Thin Film Transistors Fabricated by a Solution Process Using Direct Patterned Single-Layer Graphene Electrodes.

We propose the direct transfer method of single-layer graphene (SLG) from metal catalyst Cu-foil to a polymeric insulator and the direct patterning method of the SLG for electrodes of organic thin-film transistors (OTFTs) without contamination using soft-lithography. Through soft-lithography, SLG can be formed in various patterns relatively easily in comparison with the conventional photolithography method that has multiple complex process steps to make graphene patterns. Furthermore, the 6,13-bis(triisopropylsilylethynyl) pentacene OTFTs are fabricated in solution with SLG source and drain electrodes. As a result, the field-effect mobility of OTFTs based on SLG electrodes was enhanced about 4 times in comparison with that of OTFTs using typical metal electrodes due to the decrease in contact resistance.

Intrinsic Tunneling Characteristics of Aryl Alkane Monolayers Sandwiched Between Single-Layer Graphene Electrodes

In this study, we report on charge transport through aryl alkane monolayers sandwiched between single-layer graphene (SLG) electrodes. Raman spectroscopy identified the chemically grafting of aryl diazonium compounds onto bottom SLG electrodes. Current densities of three different aryl alkane monolayers were exponentially deceased with a correct decay coefficient (β) as the length of a tunneling barrier increased. Transition voltage measurements for variable lengths of alkyl chains showed that the SLG/monolayer/SLG arrangement provides the formation of a valid tunneling junction to observe intrinsic charge transport properties of the incorporated molecules. This study demonstrated the possible application of the SLG electrodes to the test beds of molecular junctions.

Electron transfer from FAD-dependent glucose dehydrogenase to single-sheet graphene electrodes

Continuous glucose monitoring (CGM) is an emerging technology that can provide a more complete picture of the diabetes patient’s glucose levels. Amperometric blood glucose tests typically require redox mediators to facilitate charge transfer from the enzyme to the electrode, that are not ideal in CGM settings because of their potential toxicity or long-term stability issues. Direct electron transfer (DET) would eliminate this need and has therefore attracted substantial interest. However, most DET-based glucose biosensor studies so far have used glucose oxidase (GOx) leading to controversial results because the oxygen dependency may be misinterpreted as DET. Here, we overcome this challenge by using an oxygen-insensitive glucose dehydrogenase (GDH). To enable direct electron transfer, the enzyme was immobilized on the surface of high-quality single-layer graphene electrodes via short pyrene linkers (<1 nm). The biosensor strongly responded to glucose even without a redox mediator, implying direct electron transfer. Control measurements on different surfaces further confirm that the response is enzyme-specific. The activity of immobilized enzymes was confirmed by glucose measurements with a conventional ferrocenemethanol mediator as well as relatively unexplored redox mediator - nitrosoaniline. The influence of a most potent interferent in blood, ascorbic acid, was assessed. This is the first demonstration of application of single-layer graphene electrode to obtain DET from an oxygen insensitive enzyme (GDH), highlighting the potential of such devices for applications in CGM.

Observation of Direct Electron Transfer from Glucose Dehydrogenase to Single Sheet Graphene Electrode

Continuous glucose monitoring (CGM) is an emerging technology that can provide a more complete picture of the diabetes patient’s glucose levels. Amperometric blood glucose tests typically require redox mediators to facilitate charge transfer from the enzyme to the electrode, that are not ideal in CGM settings because of their potential toxicity or long-term stability issues. Direct electron transfer (DET) would eliminate this need and has therefore attracted substantial interest. However, most DETs studies so far have used glucose oxidase leading to controversial results because the oxygen dependency may be misinterpreted as DET. Here, we overcome this challenge by using an oxygen-insensitive glucose dehydrogenase. The biosensor strongly responded to glucose even without a redox mediator, implying direct electron transfer (Fig. 1 A). Control measurements on different surfaces further confirm that the response is enzyme-specific. To enable direct electron transfer, the enzyme was immobilized on the surface of high-quality single-layer graphene electrodes via short pyrene linkers (<1 nm) (Fig. 1 B). The activity of immobilized enzymes was confirmed by glucose measurements with relatively unexplored redox mediator - nitrosoaniline, and the influence of a most potent interferent in blood, ascorbic acid, was assessed (Fig. 1 C). This is the first demonstration of DET from an oxygen insensitive enzyme to single-layer graphene, highlighting the potential of such devices for applications in CGM. Figure 1

Single-layer graphene electrode enhanced sensitivity and response speed of β-Ga2O3 solar-blind photodetector

High-sensitivity, rapid response β-Ga2O3 film-based metal-semiconductor-metal (MSM) solar-blind photodetectors were obtained by replacing conventional Au/Ti electrodes with single-layer graphene (SLG) nanosheets. Compared with the photodetector with Au/Ti electrodes, the photodetector using SLG electrodes demonstrates enhanced deep ultraviolet photoresponse properties, including a more rapid response speed (0.078 s), a higher light-to-dark current ratio (~1.41 × 104%), a higher R254/R365 rejection ratio (~1020), and the larger detectivity (~6.29 × 1011 Jones). The integration of SLG with β-Ga2O3 offers a large transparent area to the incident photons, and narrows the barrier at the contact interface upon ultraviolet illumination, strongly suggesting an unlimited potential for deep ultraviolet optoelectronic devices.

Tunneling Across Single-Layer Graphene Electrode Monolayer Junctions

Tunneling Across Single-Layer Graphene Electrode Monolayer Junctions

Structural and Charge Transport Properties of Molecular Tunneling Junctions with Single-Layer Graphene Electrodes

Herein, we present Raman spectroscopy of chemically grafting aryl diazonium molecules onto single-layer graphene (SLG) and charge transport characteristics of the aryl alkane monolayers sandwiched between the SLG electrodes in an electrode/monolayers/electrode motif. Our spectroscopic characterization shows that the aryl component molecules were incorporated as an effective tunnel barrier into the SLG electrodes-based molecular junctions. The temperature-variable and length-dependent transport measurements clearly demonstrate that tunneling occurs through the SLG/monolayers/SLG junctions and the current density can be modulated with a correct decay coefficient (β) by the length of a molecular tunneling barrier.

Overcoming the efficiency limit of organic light-emitting diodes using ultra-thin and transparent graphene electrodes.

We propose an effective way to enhance the out-coupling efficiencies of organic light-emitting diodes (OLEDs) using graphene as a transparent electrode. In this study, we investigated the detrimental adsorption and internal optics occurring in OLEDs with graphene anodes. The optical out-coupling efficiencies of previous OLEDs with transparent graphene electrodes barely exceeded those of OLEDs with conventional transparent electrodes because of the weak microcavity effect. To overcome this issue, we introduced an internal random scattering layer for light extraction and reduced the optical absorption of the graphene by reducing the number of layers in the multilayered graphene film. The efficiencies of the graphene-OLEDs increased significantly with decreasing the number of graphene layers, strongly indicating absorption reduction. The maximum light extraction efficiency was obtained by using a single-layer graphene electrode together with a scattering layer. As a result, a widened angular luminance distribution with a remarkable external quantum efficiency and a luminous efficacy enhancement of 52.8% and 48.5%, respectively, was achieved. Our approach provides a demonstration of graphene-OLED having a performance comparable to that of conventional OLEDs.

Contact-free calibration of an asymmetric multi-layer interferometer for the surface force balance.

The Surface Force Balance (SFB, also known as Surface Force Apparatus, SFA) has provided important insights into many phenomena within the field of colloid and interface science. The technique relies on using white light interferometry to measure the distance between surfaces with sub-nanometer resolution. Up until now, the determination of the distance between the surfaces required a so-called "contact calibration," an invasive procedure during which the surfaces are brought into mechanical contact. This requirement for a contact calibration limits the range of experimental systems that can be investigated with SFB, for example, it precludes experiments with substrates that would be irreversibly modified or damaged by mechanical contact. Here we present a non-invasive method to measure absolute distances without performing a contact calibration. The method can be used for both "symmetric" and "asymmetric" systems. We foresee many applications for this general approach including, most immediately, experiments using single layer graphene electrodes in the SFB which may be damaged when brought into mechanical contact.