Insertion Of Interlayer Research Articles

From layered 2D carbon to 3D tetrahedral allotropes C12 and C18 with physical properties related to diamond: Crystal chemistry and DFT investigations

Mechanisms of changes from 2D to 3D (D = dimensionality) involving 2D C(sp2) trigonal paving to C(sp3) tetrahedral stacking are proposed through puckering of the 2D layers on one hand and interlayer insertion of extra C on the other hand. Such transformations, led to 3D hexagonal C12 and C18 allotropes respectively characterized by lon and bac topologies. Using density functional theory DFT calculations, the two allotropes were found cohesive and stable both mechanically (elastic properties) and dynamically (phonons). Comparisons of the physical properties with known uni C6 were established letting identify ranges of large Vickers hardness: HV (uni C6) = 89 GPa, HV (lon C12) = 97 GPa, and HV (bac C18) = 70 GPa. Whilst C6 was identified with acoustic phonons instability, C12 and C18 were found stable dynamically throughout the acoustic and optic frequency ranges. Furthering on the thermal properties the allotropes were characterized with a temperature dependence curve of the specific heat CV close to experimental data of diamond with best fit for novel C18. The electronic band structures reveal a small band gap of 1 eV for uni C6 and larger direct band gap of 3 eV for the two other 3D allotropes. Such modulations of the electronic and physical properties should open scopes of carbon research.

Nano‐TiO2‐Grafted Carbon Sheet Interlayer for Li–S Battery

Lithium–sulfur (Li–S) battery with high theoretical capacity and high energy density is considered to be a best alternative of Li‐ion battery. However, challenges associated with Li–S batteries are yet to be addressed effectively to enable their commercialization. Uncontrolled shuttling of polysulfides causes rapid capacity decay and low discharge capacity of the battery cell. To address such shuttling effect related to the polysulfide movement, nano‐TiO2‐grafted carbon sheet interlayer is introduced. Proposed material is synthesized by the combined sol–gel and hydrothermal processes. Rietveld analysis is done for the determination of phase(s) present in the material. These are anatase and rutile phases. Scanning and transmission microscopic studies reveal the morphological features of prepared material. Quantitative study of prepared material for its constituents is done by thermogravimetry analysis, which confirms the presence of 83% TiO2. An interlayer of 16 mm diameter is punched out and incorporated in a coin cell assembly. Cell assembled with interlayer shows superior electrochemical performance with initial discharge capacity of 839 mAh g−1 at 0.2 C rate and 63% capacity retention till 100th cycle. Diffusion studies reveal that insertion of interlayer controls the reaction kinetics and supports polar–polar interaction between interlayer and polysulfides.

Increasing the interlayer spacing and generating closed pores to produce petroleum coke-based carbon materials for sodium ion storage

Petroleum coke (PC) is a valuable precursor for sodium-ion battery (SIB) anodes due to its high carbon content and low cost. The regulation of the microcrystalline state and pore structure of the easily-graphitized PC-based carbon is crucial for creating abundant Na+ storage sites. Here we used a precursor transformation strategy to increase the carbon interlayer spacing and generate abundant closed pores in PC-based carbon, significantly increasing its Na+ storage capacity in the plateau region. This was achieved by introducing a large number of oxygen functional groups through mixed acid treatment and then using high-temperature carbonization to decompose the oxygen functional groups and rearrange the carbon microcrystallites, resulting in a transition from open to closed pores. The optimized samples provide a large reversible capacity of 356.0 mAh g−1 at 0.02 A g−1, of which approximately 93% is below 1.0 V. Galvanostatic intermittent titration (GITT) and in-situ X-ray diffraction (XRD) analysis indicate that the sodium storage capacity in the low voltage plateau region involves a joint contribution of interlayer insertion and closed pore filling processes. This study presents a comprehensive method for the development of high-performance carbon anodes using low-cost and highly aromatic precursors.

Double-gate structure enabling remote Coulomb scattering-free transport in atomic-layer-deposited IGO thin-film transistors with HfO2 gate dielectric through insertion of SiO2 interlayer

In this paper, high-performance indium gallium oxide (IGO) thin-film transistor (TFT) with a double-gate (DG) structure was developed using an atomic layer deposition route. The device consisting of 10-nm-thick IGO channel and 2/48-nm-thick SiO2/HfO2 dielectric was designed to be suitable for a display backplane in augmented and virtual reality applications. The fabricated DG TFTs exhibit outstanding device performances with field-effect mobility (μFE) of 65.1 ± 2.3 cm2V−1 s−1, subthreshold swing of 65 ± 1 mVdec−1, and threshold voltage (VTH) of 0.42 ± 0.05 V. Both the (μFE) and SS are considerably improved by more than two-fold in the DG IGO TFTs compared to single-gate (SG) IGO TFTs. Important finding was that the DG mode of IGO TFTs exhibits the nearly temperature independent μFE variations in contrast to the SG mode which suffers from the severe remote Coulomb scattering. The rationale for this disparity is discussed in detail based on the potential distribution along the vertical direction using technology computer-aided design simulation. Furthermore, the DG IGO TFTs exhibit a greatly improved reliability with negligible VTH shift of − 0.22 V under a harsh negative bias thermal and illumination stress condition with an electric field of − 2 MVcm−1 and blue light illumination at 80 °C for 3600 s. It could be attributed to the increased electrostatic potential that results in fast re-trapping of the electrons generated by the light-induced ionization of deep level oxygen vacancy defects.

Reducing contact resistance of MoS2-based field effect transistors through uniform interlayer insertion via atomic layer deposition.

Molybdenum disulfide (MoS2), a semiconducting two-dimensional layered transition metal dichalcogenide (2D TMDC), with attractive properties enables the opening of a new electronics era beyond Si. However, the notoriously high contact resistance (RC) regardless of the electrode metal has been a major challenge in the practical applications of MoS2-based electronics. Moreover, it is difficult to lower RC because the conventional doping technique is unsuitable for MoS2 due to its ultrathin nature. Therefore, the metal-insulator-semiconductor (MIS) architecture has been proposed as a method to fabricate a reliable and stable contact with low RC. Herein, we introduce a strategy to fabricate MIS contact based on atomic layer deposition (ALD) to dramatically reduce the RC of single-layer MoS2 field effect transistors (FETs). We utilize ALD Al2O3 as an interlayer for the MIS contact of bottom-gated MoS2 FETs. Based on the Langmuir isotherm, the uniformity of ALD Al2O3 films on MoS2 can be increased by modulating the precursor injection pressures even at low temperatures of 150 °C. We discovered, for the first time, that film uniformity critically affects RC without altering the film thickness. Additionally, we can add functionality to the uniform interlayer by adopting isopropyl alcohol (IPA) as an oxidant. Tunneling resistance across the MIS contact is lowered by n-type doping of MoS2 induced by IPA as the oxidant in the ALD process. Through a highly uniform interlayer combined with strong doping, the contact resistance is improved by more than two orders of magnitude compared to that of other MoS2 FETs fabricated in this study.

A MXene heterostructure-based piezoionic sensor for wearable sensing applications

Humans rely on ion transport to sense and process tactile information. Piezoionic sensor can directly convert mechanical energy into electrical energy through similar ion migration, showing applications in wearable sensing. However, affected by the electrode materials properties, the relatively low voltage output and poor stability of the piezoionic sensors restrict their development. Here, we designed a MXene/Ag nanoparticles heterostructure electrode-based piezoionic sensor with stable and high-performance voltage output. During ion mediated mechanoelectric conversion, the layered MXene nanosheets in electrodes provide transport channels for movable ions. While the interlayer insertion of Ag nanoparticles increases the interlayer spacing between MXene nanosheets, improving the ion storage and transport. Under a bending deformation of 0.7 % strain, the sensor outputs 11.1 mV voltage. The voltage can maintain stable under cyclic deformation for about 13000 s. Based on the excellent mechanical-to-electric conversion, this sensor can be applied to monitor human body activities. The feasibility of the sensor for guiding high-quality cardiopulmonary resuscitation (CPR) training is also demonstrated. Moreover, it can be adhered on the finger to generate pulse or square wave electrical signals by rhythmic bending the finger, which can transmit information based on the coding principle of Morse code. These results indicate the potential of the self-powered piezoionic sensor in medical rehabilitation and human–computer interaction.

Molecular simulation of adsorption behavior for phenol aqueous solution into layered graphene oxides

Graphene oxide (GO) adsorbents possess layered structures with nanoscale interlayer channels, which have been proposed as effective adsorbents for the removal of harmful phenol from aqueous solutions. However, the relevant adsorption process and behavior remain largely unexplored, and, particularly, the interlayer insertion and interaction mechanism need to be clarified. Herein, molecular dynamics simulation was performed to comprehensively investigate the dynamics process and interaction for phenol adsorption into GO interlayers. It is theoretically demonstrated that the adsorption process is thermodynamically spontaneous. The time evolution of molecular insertion amounts and interaction energy were applied to describe the dynamics process, in which different adsorption stages were revealed. Our simulation findings show that the interlayer phenol movement is the rate-limiting step in the adsorption process. Furthermore, by analyzing the interlayer structure and morphology, it is illustrated that the phenol molecules are discretely and disorderly scattered with a lying-down conformation within the GO interlayer. The adsorption interaction is the combination of the π–π and hydrogen-bonding interactions, in which the π-π interaction provides a dominant contribution. The effects of interlayer widths and surface oxidization degrees were thoroughly investigated. Our simulations provide new insight and useful guidance for the development of GO adsorbents.

Improvement of electrical properties by insertion of AlGaN interlayer for N-polar AlGaN/AlN structures on sapphire substrates

In this study, we propose nitrogen-polar (N-polar) Al0.1Ga0.9N/Al0.9Ga0.1N/aluminum nitride (AlN) structures. N-polar Al0.1Ga0.9N/Al0.9Ga0.1N/AlN was grown on a sapphire substrate with a misorientation of 2° with respect to the m-axis using metal–organic vapor deposition. The effects of varying the Al0.9Ga0.1N interlayer thickness from 30 nm to 1 μm using pulsed H2 etching on the planarity and current–voltage characteristics of the samples were investigated. The current first improved upon increasing the interlayer thickness from 30 to 300 nm, owing to the reduction in interfacial impurities between (aluminum) gallium nitride (Al)GaN) and Al0.9Ga0.1N, but subsequently decreased upon further increasing the thickness because of the relaxation growth of the interlayer. Furthermore, pulsed H2 etching of the Al0.9Ga0.1N interlayer suppressed step bunching and improved planarity. Subsequently, the proposed method was employed to fabricate N-polar Al0.1Ga0.9N/Al0.9Ga0.1N/AlN heterostructure FETs, which demonstrated five times higher source-drain current (I DS) than that of conventional structures without an interlayer.

A new Mn-based layered cathode with enlarged interlayer spacing for potassium ion batteries

Layered Mn-based cathode (KxMnO2) has attracted wide attention for potassium ion batteries (PIBs) because of its high specific capacity and energy density. However, the structure and capacity of KxMnO2 cathode are constantly degraded during the cycling due to the strong Jahn-Teller effect of Mn3+ and huge ionic radius of K+. In this work, lithium ion and interlayer water were introduced into Mn layer and K layer in order to suppress the Jahn-Teller effect and expand interlayer spacing, respectively, thus obtaining new types of K0.4Mn1-xLixO2·0.33H2O cathode materials. The interlayer spacing of the K0.4MnO2 increased from 6.34 to 6.93 Å after the interlayer water insertion. X-ray photoelectron spectroscopy studies demonstrated that proper lithium doping can effectively control the ratio of Mn3+/Mn4+ and inhibit the Jahn-Teller effect. In-situ X-ray diffraction exhibited that lithium doping can inhibit the irreversible phase transition and improve the structural stability of materials during cycling. As a result, the optimal K0.4Mn0.9Li0.1O2·0.33H2O not only delivered a higher capacity retention of 84.04 % compared to the value of 28.09 % for K0.4MnO2·0.33H2O, but also maintained a greatly enhanced rate capability. This study provides a new opportunity for designing layered manganese-based cathode materials with high performance for PIBs.

Role of inserting an InGaN strain release interlayer in AlGaN growth

AlGaN grown on a GaN template usually suffers tensile stress, and the accumulated tensile stress may cause formation of cracks. In this paper, a method to reduce the tensile stress during AlGaN growth by inserting an InGaN interlayer is proposed. Although InGaN has a larger in-plane lattice constant, it will not cause the subsequent growth of AlGaN to be subject to greater tensile stress if InGaN is coherently grown on AlGaN. On the contrary, due to the low thermal expansion coefficient of InGaN, the subsequent growth of AlGaN will suffer less tensile stress through an asymmetric temperature varying process. The validity of this method was verified by in-situ curvature monitor, reciprocal space mapping and temperature-dependent Raman spectral observation. In addition, it is found by atomic force microscopy and scanning electron microscopy that the insertion of InGaN interlayer introduces V-shaped pits into the AlGaN structure which can be refilled by later AlGaN growth. It is inferred that they may play additional role in stress release similar to what the patterned substrate usually does.

Novel Structural Design and Adsorption/Insertion Coordinating Quasi-Metallic Na Storage Mechanism toward High-performance Hard Carbon Anode Derived from Carboxymethyl Cellulose.

Hard Carbon have become the most promising anode candidates for sodium-ion batteries, but the poor rate performance and cycle life remain key issues. In this work, N-doped hard carbon with abundant defects and expanded interlayer spacing is constructed by using carboxymethyl cellulose sodium as precursor with the assistance of graphitic carbon nitride. The formation of N-doped nanosheet structure is realized by the CN• or CC• radicals generated through the conversion of nitrile intermediates in the pyrolysis process. This greatly enhances the rate capability (192.8 mAh g-1 at 5.0 A g-1 ) and ultra-long cycle stability (233.3 mAh g-1 after 2000 cycles at 0.5 A g-1 ). In situ Raman spectroscopy, ex situ X-ray diffraction and X-ray photoelectron spectroscopy analysis in combination with comprehensive electrochemical characterizations, reveal that the interlayer insertion coordinated quasi-metallic sodium storage in the low potential plateau region and adsorption storage in the high potential sloping region. The first-principles density functional theory calculations further demonstrate strong coordination effect on nitrogen defect sites to capture sodium, especially with pyrrolic N, uncovering the formation mechanism of quasi-metallic bond in the sodium storage. This work provides new insights into the sodium storage mechanism of high-performance carbonaceous materials, and offers new opportunities for better design of hard carbon anode.

Self-powered photodetectors based on stacked WSe2/graphene/SnS2 p-g-n heterostructures

The insertion of a graphene interlayer between van der Waals (vdW) materials could suppress the interlayer recombination to improve interface quality and facilitate charge transport. However, its effect on photovoltaic-mode detectors has rarely been explored. Here, self-powered photodetectors based on WSe 2 /graphene/SnS 2 vdW heterojunctions were assembled and the impact of adding graphene interlayer on its performance was investigated. The p-g-n heterostructure exhibits a photovoltaic characteristic, V OC = 0.13 V and I SC = 0.12 nA under illumination of 2.98 mW/cm 2 @405 nm, lower than that of a control (WSe 2 /SnS 2 ) counterpart ( V OC = 0.19 V and I SC = 1.15 nA). Yet an FF value up to 48.16 % was obtained under an irradiation power density of 0.332 mW/cm 2 @405 nm, which is visibly higher than that of bare WSe 2 /SnS 2 vdW heterostructure. The insertion of graphene interlayer is deemed to reduce the charge traps between vdW materials that are caused by the large concentration difference of majority carriers. However, in case of vdW heterostructure, the subsequently increased leakage current makes the junction less ideal. Thereby interface engineering methods should be used with caution to tune vdW heterostructures on the nanoscale. The WSe 2 /graphene/SnS 2 p-g-n heterostructure offers a photovoltaic characteristic, V OC = 0.13 V and I SC = 0.12 nA under an illumination of 405 nm. An FF value up to 48.16 % was obtained at 0.332 mW/cm 2 @405 nm, which is slightly higher than that of bare WSe 2 /SnS 2 vdW heterostructure.The addition of graphene interlayer between WSe 2 and SnS 2 . • The WSe 2 /graphene/SnS 2 heterostructure offers a photovoltaic characteristic, V OC = 0.13 V and I SC = 0.12 nA @405 nm. • An FF value up to 48.16 % was obtained at 0.332 mW/cm 2 @405 nm), w. • The addition of graphene interlayer between WSe 2 and SnS 2 makes the junction less ideal.

Effect of Cr interlayers on texture and magnetic properties of FeSi films with micrometer thickness

Magnetised soft ferromagnetic films with micrometer thicknesses can be used as π-flippers in spin-echo small-angle neutron scattering instruments. An FeSi film with a total thickness of 2000 nm separated by Cr interlayers (each 10 nm thick) was fabricated using direct-current magnetron sputtering. The thickness of each FeSi layer was 100 nm between adjacent Cr interlayers. Hysteresis loop measurements were performed to characterise the magnetic properties of the FeSi films. X-ray diffraction measurements, high-resolution transmission electron microscopy, and selected-area electron diffraction were used to characterise the texture. Results demonstrated that the saturation magnetisation of the FeSi film separated by Cr interlayers was higher and its coercivity was lower than those of the FeSi film separated by Ta interlayers because of different texture of FeSi layer. The FeSi film separated by Cr interlayers featured FeSi (210) and FeSi (221), and the Cr interlayers presented Cr (110). The FeSi layer between the Cr interlayers demonstrated structural coherency with the upper FeSi layer, and continuity of the lattice fringes from the Cr grains to the FeSi layers was observed. Therefore, degradation in the soft magnetic properties of FeSi films can be prevented by the insertion of Cr interlayers.

Effect of Ta Interlayers on Texture and Magnetic Properties of FeSi Films with Micrometer Thickness.

Magnetized soft ferromagnetic films with micrometer thickness were studied. A FeSi film, with a total thickness of 2000 nm, separated by 10 nm-thick Ta interlayers, was fabricated using the direct-current magnetron sputtering technique. The thickness of each FeSi layer between adjacent Ta layers was 100 nm. Hysteresis loop measurement was used to characterize the magnetic properties of the layer. X-ray diffraction patterns and high-resolution transmission electron microscopy were used to characterize its texture. The experimental results showed that the FeSi film separated by Ta interlayers exhibited a lower saturation magnetization and a higher coercivity than those of the 1140 nm-thick FeSi film. The insertion of Ta interlayers resulted in the disappearance of the crystal plane of FeSi (221), and better texture of the crystal plane of FeSi (210). The FeSi film exhibited a crystal plane of FeSi (210) with a bcc crystalline structure. The Ta interlayers were partially amorphous, exhibiting crystal plane of Ta (002) and TaSi2 (310). The matching of magnetic properties between interlayers and soft magnetic layers played an important role in maintaining its soft magnetic properties.

Effect of Al2O3 interlayers on the microstructure and electrical response of ferroelectric doped HfO2 thin films

Novel devices based on ferroelectric hafnium oxide comply with the increasing demand for highly scalable embedded non-volatile memory devices, especially for in-memory computing applications. However, due to the polycrystalline nature of these hafnium oxide films, highly scaled devices face variability concerns. In order to enable smaller grains to circumvent the current limitations, the introduction of Al2O3 interlayers to interrupt the columnar grain growth is presented herein. Transmission Kikuchi diffraction is utilized to investigate influences of the Al2O3 layer on the microstructure of hafnium oxide. Moreover, electrical analysis indicates how the interlayer affects the wake-up phenomena as well as the electric field distribution within the stack. These results provide evidence on how to control grain size, electric behavior, and crystallization temperature by the insertion of Al2O3 interlayers.

Strategies towards High Performance Lithium‐Sulfur Batteries

AbstractRechargeable Lithium‐sulfur batteries (LSBs) have been considered as a potential candidate for next‐generation energy storage technologies because of ultrahigh‐energy density (2600 Wh kg−1) and being lightweight. However, the practical applications of LSBs are currently limited by lithium polysulfides (LiPSs) shuttle, continuous electrolyte decomposition, and lithium anode corrosion. These challenges are mainly related to the cathode structure framework, the reactive nature of lithium anode, and the redox reactions occurring at the electrode‐electrolyte interfaces. Proper cathode architecture design, development of novel electrolytes, and anode protection have been developed to improve the electrochemical performance of LSBs. In this review, the working principles and challenges of LSBs are briefly introduced. The strategies to overcome the challenges of LSBs, such as electrode design and modification, development of novel electrolytes, separator modification/functional interlayer insertion, and protection of lithium anode are systematically discussed. The advanced in situ/operando characterization techniques deployed to reveal the redox chemistries of LSBs also summarized. Finally, a summary and future perspective for developing electrode structure, electrolyte engineering, functional interlayers/separators, and anode protection for the practical application of LSBs are provided.

Residual stress tuned magnetic properties of thick CoMnP/Cu multilayers

Electrodeposited hard magnetic thick films have vast applications in the microelectromechanical systems (MEMS). Yet the very large residual stresses (σr) built-up in monolayered thick magnetic films leads to cracks, dimensional changes and deteriorated magnetic properties. Here, we explored quantitatively magnetic properties of CoMnP/Cu multilayers tuned by σr, which in turn are varied by the inserted soft Cu interlayer and thickness of single CoMnP magnetic layers. The configuration of the multilayers is an alternating CoMnP/Cu on Cu-substrate. The thickness of Cu interlayer was 1.4 μm. We kept a sum of all magnetic layers in the multilayers at ∼20 μm to benchmark with a 19.4 μm monolayered CoMnP. The magnetic layers are 94 wt.% Co and possess highly textured (002) hexagonal close packed microstructures. We characterized the apparent crystallite stresses through sin2ψ method by X-ray diffractometer (XRD) and residual film stress by curvature method. The insertion of Cu interlayers effectively reduces σr by 23% through stacking with six single-layered CoMnP. The out-of-plane (OP) anisotropy is slightly reduced. While the maximum energy product in the in-plane (IP) direction can be significantly enhanced by 430% ∼ 690% with increasing the number of the CoMnP single layer in the multilayers. The magneto-elastic behaviors well explain the evolution of the total anisotropy energy of the mono- and multi-layers. By CoMnP/Cu configurations we successfully worked out a strategy to preserve prestigious OP performance while to enhance IP properties by 4 to 6 times to meet ever increasing challenges in MEMS applications.

Enhanced Radiative Efficiency of InGaAs/GaAsP Multiple Quantum Wells by Optimizing the Thickness of Interlayers

InGaAs/GaAsP multiple quantum wells (MQWs) are used in electronic devices because the high radiative efficiency of MQWs improves the device performance. To realize high radiative efficiency, a GaAs interlayer is inserted to decrease the defects at the heterointerface between the well and barrier of the MQWs. However, it is unclear how the interlayer thickness and insertion position affect the radiative efficiency of the MQWs. Herein, the influence of the GaAs interlayer thickness on the radiative efficiency by measuring the photoluminescence of MQWs with different interlayer thicknesses is investigated. A thicker GaAs interlayer below the InGaAs enhances the radiative efficiency. Moreover, the GaAs interlayer above the InGaAs must be thicker than ≈6 nm to enhance the radiative efficiency. The initial growth of GaAs above the InGaAs degrades the surface morphology, which may be associated with the carry‐over of indium on the InGaAs surface and may generate defects between the InGaAs and GaAsP layers. The surface of sufficiently thick GaAs interlayers has a flatter morphology and is free from the degradation of radiative efficiency. The results suggest inserting of interlayers with a smooth surface morphology between strained layers during epitaxial growth is an effective method to suppress nonradiative recombination.

Rapid “self-healing” behavior induced by chloride anions to renew the Fe–Ni(oxy)hydroxide surface for long-term alkaline seawater electrolysis

Due to the surface adsorption and interlayer insertion behavior of chloride anions, the Fe–Ni(oxy)hydroxide catalytic surface is easily destroyed, making it difficult to be used for long-term seawater electrolysis.

In situ measurement and mechanism analysis of the lithium storage behavior of graphene electrodes

Graphene has attracted widespread attention for development of high-performance lithium-ion battery anode materials. In this paper, the lithium storage mechanisms of multilayer graphene, few-layer graphene, and reduced graphene oxide electrodes are investigated, and the influences of layers and defects in the microstructure are analyzed. The evolution of microstructure deformation of different graphene electrodes, combined with the diffusivity and distribution of lithium, is systemically investigated by in-situ Raman spectroscopy during lithiation and delithiation. The results show that graphene electrodes with different structures exhibit different mechanical behaviors and thus different lithium storage capacities. Mechanical analysis indicates that although the surfaces and defects are active, interlayer lithium insertion contributes the most lithium storage capacity. As the number of graphene layers increases and/or defects decrease, more interlayer spaces would be provided. Thus, the lithium storage capacity of multilayer graphene is significantly higher than those of few-layer graphene and reduced graphene oxide. An in-depth understanding of the effect of the microstructure on the lithium storage behavior of graphene electrodes will provide an experimental basis for development of high-performance graphene electrodes.