Single-layer Graphene Electrodes Research Articles

Adsorption and charge transfer of lithium at electrified graphene/electrolyte interface

Energy profiles and charge transfer in Li adsorption/desorption on/from various charged single layer graphene electrodes in various electrolytes are investigated by hybrid first-principles/continuum calculations. The critical values of excess charge density of graphene and Li, at and above the critical value Li adsorption switches to Li desorption, are −0.87 |e|, 0 |e|, and 1.5 |e| per formula unit for pristine graphene (with formula unit Li1C32) in water, monovacancy graphene (MVSLG, Li1C31) in water and in CCl4, respectively. When moving from far away towards the MVSLG in water and CCl4 during adsorption, Li+ ion is gradually reduced to Li atom by charge transfer.

Frequency response of electrolyte-gated graphene electrodes and transistors

The interface between graphene and aqueous electrolytes is of high importance for applications of graphene in the field of biosensors and bioelectronics. The graphene/electrolyte interface is governed by the low density of states of graphene that limits the capacitance near the Dirac point in graphene and the sheet resistance. While several reports have focused on studying the capacitance of graphene as a function of the gate voltage, the frequency response of graphene electrodes and electrolyte-gated transistors has not been discussed so far. Here, we report on the impedance characterization of single layer graphene electrodes and transistors, showing that due to the relatively high sheet resistance of graphene, the frequency response is governed by the distribution of resistive and capacitive circuit elements along the graphene/electrolyte interface. Based on an analytical solution for the impedance of the distributed circuit elements, we model the graphene/electrolyte interface both for the electrode and the transistor configurations. Using this model, we can extract the relevant material and device parameters such as the voltage-dependent intrinsic sheet and series resistances as well as the interfacial capacitance. The model also provides information about the frequency threshold of electrolyte-gated graphene transistors, above which the device exhibits a non-resistive response, offering an important insight into the suitable frequency range of operation of electrolyte-gated graphene devices.

Efficient organic photovoltaic cells on a single layer graphene transparent conductive electrode using MoOx as an interfacial layer.

The large surface roughness, low work function and high cost of transparent electrodes using multilayer graphene films can limit their application in organic photovoltaic (OPV) cells. Here, we develop single layer graphene (SLG) films as transparent anodes for OPV cells that contain light-absorbing layers comprised of the evaporable molecular organic semiconductor materials, zinc phthalocyanine (ZnPc)/fullerene (C60), as well as a molybdenum oxide (MoOx) interfacial layer. In addition to an increase in the optical transmittance, the SLG anodes had a significant decrease in surface roughness compared to two and four layer graphene (TLG and FLG) anodes fabricated by multiple transfer and stacking of SLGs. Importantly, the introduction of a MoOx interfacial layer not only reduced the energy barrier between the graphene anode and the active layer, but also decreased the resistance of the SLG by nearly ten times. The OPV cells with the structure of polyethylene terephthalate/SLG/MoOx/CuI/ZnPc/C60/bathocuproine/Al were flexible, and had a power conversion efficiency of up to 0.84%, which was only 17.6% lower than the devices with an equivalent structure but prepared on commercial indium tin oxide anodes. Furthermore, the devices with the SLG anode were 50% and 86.7% higher in efficiency than the cells with the TLG and FLG anodes. These results show the potential of SLG electrodes for flexible and wearable OPV cells as well as other organic optoelectronic devices.

Near Room-Temperature Memory Devices Based on Hybrid Spin-Crossover@SiO2 Nanoparticles Coupled to Single-Layer Graphene Nanoelectrodes.

The charge transport properties of SCO [Fe(Htrz)2 (trz)](BF4 ) NPs covered with a silica shell placed in between single-layer graphene electrodes are reported. A reproducible thermal hysteresis loop in the conductance above room-temperature is evidenced. This bistability combined with the versatility of graphene represents a promising scenario for a variety of technological applications but also for future sophisticated fundamental studies.

Schottky barrier contrasts in single and bi-layer graphene contacts for MoS2 field-effect transistors

We have investigated single- and bi-layer graphene as source-drain electrodes for n-type MoS2 transistors. Ti-MoS2-graphene heterojunction transistors using both single-layer MoS2 (1M) and 4-layer MoS2 (4M) were fabricated in order to compare graphene electrodes with commonly used Ti electrodes. MoS2-graphene Schottky barrier provided electron injection efficiency up to 130 times higher in the subthreshold regime when compared with MoS2-Ti, which resulted in VDS polarity dependence of device parameters such as threshold voltage (VTH) and subthreshold swing (SS). Comparing single-layer graphene (SG) with bi-layer graphene (BG) in 4M devices, SG electrodes exhibited enhanced device performance with higher on/off ratio and increased field-effect mobility (μFE) due to more sensitive Fermi level shift by gate voltage. Meanwhile, in the strongly accumulated regime, we observed opposing behavior depending on MoS2 thickness for both SG and BG contacts. Differential conductance (σd) of 1M increases with VDS irrespective of VDS polarity, while σd of 4M ceases monotonic growth at positive VDS values transitioning to ohmic-like contact formation. Nevertheless, the low absolute value of σd saturation of the 4M-graphene junction demonstrates that graphene electrode could be unfavorable for high current carrying transistors.

Is single layer graphene a promising anode for sodium-ion batteries?

In an attempt to find an adequate carbon material to achieve a successful reversible adsorption of Na+ ions, single layer graphene, is experimentally investigated in this work, for the first time, as anode for sodium-ion batteries. To this end, single layer graphene that was grown on copper foil by chemical vapor deposition was subjected to extended galvanostatic cycling and to cyclic voltammetry in the potential range of 0-2.8V versus Na/Na+. Regardless of the current density and electrolyte formulation used, the amount of Na+ ions adsorbed/desorbed reversibly per surface area (specific reversible cell capacity) was very modest and comparable to that obtained with bare copper electrodes of reference, thus suggesting that the reversible capacity of the single layer graphene electrode is mostly due to the electrochemical response of the copper substrate. These experimental results clearly agree with recent theoretical calculations showing that the adsorption of Na+ ions on the surface of single layer graphene is energetically unfavourable unless that surface includes significant defects density.

Single-layer graphene as a stable and transparent electrode for nonaqueous radical annihilation electrogenerated chemiluminescence.

We explored the use of single-layer graphene (SLG) obtained by chemical vapor deposition, and transferred to a glass substrate, as a transparent electrode material for use in coupled electrochemical and spectroscopic experiments in nonaqueous media through electrogenerated chemiluminescence (ECL). SLG was used with classical ECL luminophores, rubrene and 9,10-diphenylanthracene, in an inert environment to generate stable electrochemical responses and measure light emission through it. As an electrode material, SLG displayed excellent stability during electrochemical potential stepping and voltammetry in a window that spanned at least from ca. -2.4 to +1.8 V versus SCE in acetonitrile and acetonitrile/benzene. Although the peak splitting between forward and reverse sweeps in voltammetry was larger in comparison to metal electrodes due to in-plane resistance, SLG displayed sufficiently facile electron transfer properties to yield stable voltammetric cycling and ECL. SLG electrodes patterned with poly tetrafluoroethylene permitted the stable generation of radical ions on an SLG microelectrode to be studied through scanning electrochemical microscopy in the generation/collection mode. SLG was able to stably collect radical ions produced by a 50 μm gold tip with up to 96% collection efficiency. The transparency of graphene was used to obtain accurate spectral responses in ECL. While inner filter effects are known to cause a shift in peak emission wavelength of spectroelectrochemical studies, the use of SLG electrodes with detection through the graphene window reduced apparent peak shifts by up to 10 nm in peak wavelength. This work introduces SLG as a virtually transparent, electrochemically active, and chemically stable platform for studying ECL in the radical annihilation mode, where large electrode polarizations could compromise the chemical stability of other existing transparent electrodes.

High performance of low band gap polymer-based ambipolar transistor using single-layer graphene electrodes.

Bottom-contact bottom-gate organic field-effect transistors (OFETs) are fabricated using a low band gap pDTTDPP-DT polymer as a channel material and single-layer graphene (SLG) or Au source/drain electrodes. The SLG-based ambipolar OFETs significantly outperform the Au-based ambipolar OFETs, and thermal annealing effectively improves the carrier mobilities of the pDTTDPP-DT films. The difference is attributed to the following facts: (i) the thermally annealed pDTTDPP-DT chains on the SLG assume more crystalline features with an edge-on orientation as compared to the polymer chains on the Au, (ii) the morphological features of the thermally annealed pDTTDPP-DT films on the SLG electrodes are closer to the features of those on the gate dielectric layer, and (iii) the SLG electrode provides a flatter, more hydrophobic surface that is favorable for the polymer crystallization than the Au. In addition, the preferred carrier transport in each electrode-based OFET is associated with the HOMO/LUMO alignment relative to the Fermi level of the employed electrode. All of these experimental results consistently explain why the carrier mobilities of the SLG-based OFET are more than 10 times higher than those of the Au-based OTFT. This work demonstrates the strong dependence of ambipolar carrier transport on the source/drain electrode and annealing temperature.

High-yield fabrication of nm-size gaps in monolayer CVD graphene

Herein we demonstrate the controlled and reproducible fabrication of sub-5 nm wide gaps in single-layer graphene electrodes. The process is implemented for graphene grown via chemical vapor deposition using an electroburning process at room temperature and in vacuum. A yield of over 95% for the gap formation is obtained. This approach allows producing single-layer graphene electrodes for molecular electronics at a large scale. Additionally, from Raman spectroscopy and electroburning carried out simultaneously, we can follow the heating process and infer the temperature at which the gap formation happens.

Electrochemical properties of CVD grown pristine graphene: monolayer- vs. quasi-graphene

We report the electrochemical properties of pristine monolayer, double layer and few-layer (termed quasi-) graphene grown via CVD and transferred using PMMA onto an insulating substrate (silicon dioxide wafers). Characterisation has been performed by Raman spectroscopy, optical spectroscopy, Atomic Force Microscopy and X-ray Photoelectron Spectroscopy, revealing 'true' pristine single-layer graphene (O/C of 0.05) at the former and pristine quasi-graphene at the latter (O/C of 0.07); the term "quasi-graphene" is coined due to the surface comprising on average 4-graphene-layers. The graphene electrodes are electrochemically characterised using both inner-sphere and outer-sphere redox probes with electrochemical performances of the graphene electrodes compared to other available graphitic electrodes, namely that of basal- and edge- plane pyrolytic graphite electrodes constructed from Highly Ordered Pyrolytic Graphite (HOPG), with information on heterogeneous rate constants (k(o)) obtained. The electrochemical rate constants are predominantly influenced by the electronic properties of the graphene surfaces. Monolayer graphene is found to exhibit slow heterogeneous electron transfer (HET) kinetics towards the redox probes studied, with HET rates ca. 2 and 8 times faster at quasi-graphene and HOPG respectively, relative to that of the monolayer graphene electrode. Critically contrasting the performance of monolayer graphene to quasi-graphene and HOPG electrodes reveals that increasing the number of graphene layers results in improved electrochemical properties, where in terms of the electrochemical reversibility of the probes studied: monolayer-graphene < quasi-graphene < HOPG, as governed by the respective HET electrochemical rate constants. Given that edge plane sites are the predominant origin of fast electron transfer kinetics at graphitic materials, the slow HET rates at pristine single-layer graphene electrodes are likely due to graphene's fundamental geometry, which comprises a small edge plane and large basal plane contribution. In the case of quasi-graphene and HOPG, they possess increasing global coverage of electrochemically reactive edge plane sites (respectively) and thus exhibit superior electrochemical performances over that of monolayer graphene. Last, the case of a double-layer graphene electrode is considered, which as a result of its fabrication possesses a large global coverage of edge plane like- sites/defects. In agreement with the former conclusions, the double-layered defect-graphene electrode is found to exhibit fast/favourable electrochemical properties, which is attributed to its large edge plane content (i.e. defect abundant graphene) and thus is further evidence that the electrochemical response is dependent on the density of edge plane sites at graphene based electrodes (influenced by the coverage of graphene-defects and the number of graphene layers).

Charge and Molecular Transport At Large Area, Single Layer Graphene Electrodes

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Efficient and bright organic light-emitting diodes on single-layer graphene electrodes

Organic light-emitting diodes are emerging as leading technologies for both high quality display and lighting. However, the transparent conductive electrode used in the current organic light-emitting diode technologies increases the overall cost and has limited bendability for future flexible applications. Here we use single-layer graphene as an alternative flexible transparent conductor, yielding white organic light-emitting diodes with brightness and efficiency sufficient for general lighting. The performance improvement is attributed to the device structure, which allows direct hole injection from the single-layer graphene anode into the light-emitting layers, reducing carrier trapping induced efficiency roll-off. By employing a light out-coupling structure, phosphorescent green organic light-emitting diodes exhibit external quantum efficiency >60%, while phosphorescent white organic light-emitting diodes exhibit external quantum efficiency >45% at 10,000 cd m(-2) with colour rendering index of 85. The power efficiency of white organic light-emitting diodes reaches 80 lm W(-1) at 3,000 cd m(-2), comparable to the most efficient lighting technologies.

Kinetics of Interfacial Electron Transfer at Single-Layer Graphene Electrodes in Aqueous and Nonaqueous Solutions

We present a catalog of electron transfer mediators for investigating the heterogeneous electron transfer kinetics of large-area, single-layer graphene electrodes. Scanning electrochemical microscopy (SECM) was used to probe the apparent standard electron transfer rate constant of mediators in aqueous solutions and in acetonitrile and dimethylformamide, allowing for studies of graphene electroactivity at different potentials and in both aqueous and nonaqueous media. In aqueous solution, iron(III) ethylenediaminetetraacetic acid, hexacyanoruthenate(II), hexacyanoferrate(II), hexacyanoferrate(III), octacyanomalybdate(IV), cobalt(III) sepulchrate, and hydroxymethylferrocene exhibited quasi-reversible electron transfer behavior. The electron transfer kinetics of hexaammineruthenium(III), methyl viologen, and tris(2,2'-bipyridyl)ruthenium(II) were found to be reversible in these studies. The electron transfer rate constant of hydroxymethylferrocene and ferrocene, in organic media, was similar to that for hydroxymethylferrocene in water, which, although fast, shows clear kinetic complications that we believe are inherent to graphene. A series of viologens, known to be reversible at metal electrodes, exhibited quasi-reversible electron transfer. For [Co(dapa)(2)](2+), where dapa is 2,6-bis[1-(phenylimino)ethyl]pyridine, in dimethylformamide, the oxidation state of the redox pair investigated affected the observed kinetics. Under similar experimental conditions, the Co(I/II) couple exhibited nearly reversible behavior whereas Co(II/III) had finite kinetics. This behavior was ascribed to the large difference in self-exchange rates for these two processes. To demonstrate the utility of using these mediators for examining graphene electrodes, the kinetics of two mediators with quasi-reversible electron transfer behavior, iron ethylenediaminetetraacetic acid and hexacyanoruthenate, were measured in the presence of a redox-active species [Os(bpy)(2)(dipy)Cl]PF(6), where bpy is 2,2'-bipyridine and dipy is 4,4'-trimethylenedipyridine, adsorbed onto the graphene surface. The kinetics of both mediators were enhanced in the presence of one-hundredth of a monolayer of the osmium complex, showing that even small amounts of impurities on the graphene surface are capable of enhancing the observed kinetics.

Platinum nanoflowers decorated three-dimensional graphene–carbon nanotubes hybrid with enhanced electrocatalytic activity

A three-dimensional carbon electrode, in the form of vertically grown multiwalled carbon nanotubes (MWNTs) on a single layer graphene floor (G-MWNTs) is synthesized by one-step chemical vapor deposition (CVD) method, as supporting material for platinum (Pt) electrocatalyst. Nanostructured Pt catalyst is electrodeposited onto this 3D-carbon electrode and their structural and electrochemical properties are investigated by transmission electron microscopy (TEM), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), cyclic voltammetry (CV) and electrochemical impedance spectroscopy. The electrocatalytic performance of the modified electrode (Pt/G-MWNTs) towards methanol oxidation is also investigated by CV. Experimental results demonstrate an enhanced efficiency of the G-MWNTs hybrid film, as Pt catalyst support, for methanol oxidation in comparison to pristine single layer graphene electrode with regard to electroactive surface area, forward anodic peak current density, onset oxidation potential, diffusion efficiency and the ratio of forward to backward anodic peak current density (If/Ib). This enhanced electrochemical and catalytic performance towards methanol oxidation is attributed to the high surface area of three-dimensional G-MWNTs hybrid film due to synergistic combination of both structural and electrical properties of 2D-graphene and 1D-MWNTs, allowing higher dispersion of Pt.

Top illuminated inverted organic ultraviolet photosensors with single layer graphene electrodes

Inverted, top-illuminated organic photodiodes are demonstrated using transparent electrodes including single layer graphene (SLG) and thin gold or silver with poly(3-hexylthiophene) and 1 -(3-methoxycarboyl)-propyl-1-phenyl-(6,6)C61 as active layer. The devices are free of both indium-tin-oxide and poly(3,4-ethylenedioxythiophene):poly(styrene-sulfonate). The maximum solar power conversion efficiencies were 0.3% for SLG due to its series resistance and ∼2% for gold and silver. The organic photodiodes with SLG electrodes had good external quantum efficiency at incident illumination less than 10 mW/cm2 and better performance than gold and silver at wavelengths below 300 nm making them attractive for ultraviolet photosensors.

Electrochemical Double Layer Capacitance of Metallic and Semiconducting SWCNTs and Single Layer Graphene

The specific double layer capacitance that has been achieved with single wall carbon nanotubes (SWCNTs) is below that which is predicted from calculations based on the theoretical surface area of the CNTs and the intrinsic capacitance of the electrolyte. This paper investigates the specific capacitance of metallic (M-) and semiconducting (SC-) CNTs in order to determine if density of states (DOS)/quantum capacitance is a limiting factor in the capacitances achieved. In addition, a model system composed of a large-area single-layer graphene electrode on a nonconducting substrate has been used in order to put a lower bound on the ultimate double layer capacitance that should be achievable with CNT and graphene electrodes. We find that at the specific capacitances achieved here, the quantum capacitance of SWCNTs is not the limiting factor. In addition, the single-sided single-layer graphene electrode indicates that capacitances in excess of 550 F/g should be achievable.

Quantification of the Surface Diffusion of Tripodal Binding Motifs on Graphene Using Scanning Electrochemical Microscopy

The surface diffusion of a cobalt bis-terpyridine, Co(tpy)(2)-containing tripodal compound (1·2PF(6)), designed to noncovalently adsorb to graphene through three pyrene moieties, has been studied by scanning electrochemical microscopy (SECM) on single-layer graphene (SLG). An initial boundary approach was designed in which picoliter droplets (radii ~15-50 μm) of the tripodal compound were deposited on an SLG electrode, yielding microspots in which a monolayer of the tripodal molecules is initially confined. The time evolution of the electrochemical activity of these spots was detected at the aqueous phosphate buffer/SLG interface by SECM, in both generation/collection (G/C) and feedback modes. The tripodal compound microspots exhibit differential reactivity with respect to the underlying graphene substrate in two different electrochemical processes. For example, during the oxygen reduction reaction, adsorbed 1·2PF(6) tripodal molecules generate more H(2)O(2) than the bare graphene surface. This product was detected with spatial and temporal resolution using the SECM tip. The tripodal compound also mediates the oxidation of a Fe(II) species, generated at the SECM tip, under conditions in which SLG shows slow interfacial charge transfer. In each case, SECM images, obtained at increasing times, show a gradual decrease in the electrochemical response due to radial diffusion of the adsorbed molecules outward from the microspots onto the unfunctionalized areas of the SLG surface. This response was fit to a simple surface diffusion model, which yielded excellent agreement between the two experiments for the effective diffusion coefficients: D(eff) = 1.6 (±0.9) × 10(-9) cm(2)/s and D(eff) = 1.5 (±0.6) × 10(-9) cm(2)/s for G/C and feedback modes, respectively. Control experiments ruled out alternative explanations for the observed behavior, such as deactivation of the Co(II/III) species or of the SLG, and verified that the molecules do not diffuse when confined to obstructed areas. The noncovalent nature of the surface functionalization, together with the surface reactivity and mobility of these molecules, provides a means to couple the superior electronic properties of graphene to compounds with enhanced electrochemical performance, a key step toward developing dynamic electrode surfaces for sensing, electrocatalysis, and electronic applications.

A comparison of single-wall carbon nanotube electrochemical capacitor electrode fabrication methods

Carbon nanotubes (CNTs) are being widely investigated as a replacement for activated carbon in supercapacitors. A wide range of CNT specific capacitances have been reported in the literature based on experiments using different CNT materials, fabrication methods, and characterization routines; making it difficult to draw conclusions about the relative merits of the different fabrication methods. This work systematically compares four solution-based electrode fabrication methods (drop casting, air brushing, filtration, and electrospraying) and, to a lesser extent, some solution preparation techniques to determine if there is an optimum method for fabricating electrochemical capacitor electrodes out of single-wall CNTs (SWCNTs). We have found that it is best to use CNT solutions free from additives that may be difficult to remove from the fabricated electrode. In addition, the CNT solution preparation (e.g., dilution and sonication) had little effect on the resulting specific capacitance. Large differences in performance due to the fabrication methods were not seen, and the differences that were seen could be ascribed to material loss or contamination during the deposition. A single-layer graphene electrode was also fabricated and tested to obtain an estimate of the specific capacitance potentially achievable with SWCNTs, with 550F/g demonstrated using 1 molar (M) sulfuric acid.

The Application of Highly Doped Single-Layer Graphene as the Top Electrodes of Semitransparent Organic Solar Cells

A single-layer graphene film with high conductance and transparency was realized by effective chemical doping. The conductance of single-layer graphene was increased for more than 400% when it was doped with Au nanoparticles and poly(3,4-ethylenedioxythiophene): poly(styrene sulfonic acid). Then semitransparent organic solar cells based on poly(3-hexylthiophene) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM) were fabricated with single-layer graphene and indium tin oxide (ITO) as the top and bottom electrodes, respectively. The performance of the devices was optimized by tuning the active layer thickness and doping the single-layer graphene electrodes. The maximum efficiency of 2.7% was observed in the devices with the area of 20 mm(2) illuminated from graphene electrode under the AM1.5 solar simulator. It is notable that all of the devices showed higher efficiency from the graphene than ITO side, which was attributed to the better transmittance of the graphene electrodes. In addition, the influence of the active area of the organic solar cell on its photovoltaic performance was studied. We found that, when the active areas increased from 6 to 50 mm(2), the power conversion efficiencies decreased from 3% to 2.3% because of the increased series resistances and the decreased edge effect of the devices.

Flexible Graphene-Based Electroluminescent Devices

For the first time, large-area CVD-grown graphene films transferred onto flexible PET substrates were used as transparent conductive electrodes in alternating current electroluminescence (ACEL) devices. The flexible ACEL device based on a single-layer graphene electrode has a turn-on voltage of 80 V; at 480 V (16 kHz), the luminance and luminous efficiency are 1140 cd/m(2) and 5.0 lm/W, respectively. The turn-on voltage increases and the luminance decreases with increasing stacked layers of graphene, which means the single-layer graphene is the best optimal choice as the transparent conductive electrode. Furthermore, it demonstrates that the graphene-based ACEL device is highly flexible and can work very well even under a very large strain of 5.4%, suggesting great potential applications in flexible optoelectronics.