Stacked Graphene Research Articles

Highly Q-factor refractive index sensor based on graphene stack

The perfect absorber with high figure of merit (FOM) and sensitivity for mid-infrared application is developed in this study. The structure contains a nanoaperture as a resonator and multi-layer hyperbolic metamaterial of graphene-SiN is place under the nanoaperture to increasing the absorption. The results show how the chemical potential of graphene can improve the absorbance. Finally, a coat of SiO2 is pinpointed over the nanoaperture for enhance the absorption and protecting the surface. The proposed aperture is designed symmetrically based on crossed shape to reduce the polarization effects on sensing. The Q-factor has increased sharply to 3000 for the final device by utilizing the hyperbolic material structure. The left-hand characteristic of the hyperbolic metamaterial provides a condition for concentrating the surface Plasmon’s at the interface of the gold layer with the substrate, leading to the reflection reduction to − 46 dB. This absorber is considered as an optical sensor for various materials under test (MUT) in the optical spectrum and it is simulated by FIT method as a time-domain technique. This sensor has a sensitivity of 400 nm/RIU and the FOM of 89000 RIU−1.

Capacitance of edge-free three-dimensional graphene: New perspectives on the design of carbon structures for supercapacitor applications

The current target for expanding the application scope of supercapacitors is to increase their energy density (E) beyond 20 Wh kg−1. In this regard, edge-free carbon materials show considerable potential because of their high working voltage (U) in organic electrolytes; however, their capacitance (C) remains limited. In this study, we synthesized edge-free three-dimensional (3D) graphene materials with different numbers of graphene stacking layers (nstack). These carbon materials have similar pore morphologies and an edge-free structure because a template method and annealing at 1800 °C were applied, respectively. These features allowed C to remain unaffected by the pore size effect, wettability, parasitic side reactions, and pseudocapacitance. Our results suggested that increasing nstack slightly enhances the areal C; however, such an increase cannot compensate for the decrease in C attributed to the decrease in the specific surface area. We also confirmed that the C of 3D graphene materials has a quantum origin, which results in a “butterfly shaped” cyclic voltammetry curve; we also successfully quantified the quantum capacitance (CQ) for the complete understanding of the origin of C. Based on this knowledge, we estimated that this 3D graphene material can yield a high E of 43 Wh kg−1 once CQ is optimized.

Cardanol surfactant/ultrasound-assisted exfoliation of graphite in a water/ethanol solution

Graphene has high electrical and thermal conductivity, high transparency, and chemical stability that stand out compared to those of other materials. An efficient technique for producing few-layers graphene (<5 layers) is the liquid-phase exfoliation of graphite (LPE). LPE is a simple, cost-effective, ecologically friendly technique that without the use of hazardous solvents can produce graphene without defects in its basal plane. The influence of natural surfactant (cardanol), time and type of ultrasound on the properties of exfoliated graphite was studied. The use of tip ultrasound promotes graphite exfoliation in relatively shorter time (20 min) than ultrasonic bath time (360 min). Another factor that determines the effectiveness of the exfoliation is the concentration of cardanol, which, when used at the critical micelle concentration (CMC) value promotes the formation of micelles that help in the exfoliation and stability of the exfoliated graphite. Few-layers graphene with a concentration of 3.00 ± 0.01 mg mL−1 and an average stacked graphene height of up to 3 nm for 30% of the product were produced by combining the appropriate time of tip ultrasound with the use of cardanol at an equal concentration to its CMC. Furthermore, stable suspension of graphene in aqueous medium was achieved for more than 6 months without significant sedimentation. These results are related to the π-π and hydrophobic-hydrophobic interactions between the cardanol molecules and the graphene surface, which promote the adsorption of cardanol molecules on the graphene surface, and the re-stacking and agglomeration can be reduced due to electrostatic repulsion. Few-layers graphene produced from a new green-surfactant is an innovative technology that transforms low cost products into materials with higher added value for applications in supercapacitors, solar cells, and biosensors.

Hofstadter butterflies in magnetically modulated graphene bilayer: An algebraic approach

It has been shown that Bernal stacked bilayer graphene (BLG) in a uniform magnetic field demonstrates integer quantum Hall effect with a zero Landau-level anomaly \cite{Geimbilayer}. In this article we consider such system in a two dimensional periodic magnetic modulation with square lattice symmetry. It is shown algebraically that the resulting Hofstadter spectrum can be expressed in terms of the corresponding spectrum of monolayer graphene in a similar magnetic modulation. In the weak-field limit, using the tight-binding model, we also derive the Harper-Hofstadter equation for such BLG system in a periodic magnetic modulation. We further demonstrate the topological quantisation of Hall conductivity in such system and point out that the quantised Hall plateaus are equally spaced for all quantum numbers for the quantised Hall conductivity.

(Invited) Controlling Interlayer Stacking Configuration to Optimize Exciton Extraction Pathways in Van Der Waals Materials

Employing ultrafast microscopy methods, we demonstrate how tuning the interlayer coupling by twisting stacking orientations results in different metastable electronic states like Moiré excitons in twisted bilayer graphene (tBLG) and bound triplet pairs (TT) in molecular crystals. Considering first tBLG, we show how stacking-angle tunable absorption resonances form a strongly-bound exciton state due to the interlayer orbitals' symmetric rehybridization. Using two-photon photoluminescence and intraband-transient absorption (TA) microscopies, we have imaged the photoemission and exciton dynamics from single-grains of tBLG. After resonant excitation, our results suggest the formation of strongly bound (up to 690 meV), metastable interlayer exciton states.[1] Our observation of resonant PL emission from twisted bilayer graphene materials is best explained by the theoretically predicted coexistence of strongly bound interlayer excitons and metallic graphene continuum states to form Moiré excitons.Unlike stacked graphene, semiconducting 2D transition metal dichalcogenides (TMDCs) have diffuse interlayer d-orbital overlap. To enhance interlayer electronic coupling in TMDCs, we apply an interlayer-directed E-field, inducing electron-hole dissociation as shown in Fig. 1b. Time-resolved photocurrents show that stacked WSe2 devices shown in Fig. 1a can have both IQE >50% and fast (<60 ps) picosecond electron escape times. Our ultrafast photocurrent rates kinetics give the same E-field-dependent electronic escape and dissociation rates seen from ultrafast optical TA microscopy.[2] To rationalize these fast electronic escape rates, we show the ratio of the electronic rates accurately predicts the actual WSe2 device photocurrent generation efficiency. Lastly, we will show how certain intermolecular twist angle packings of athraditiophene molecular crystals make electron-multiplication by singlet fission of TT states favorable. Singlet fission dynamics are indicated in Fig. 1c by the matching singlet (blue) vs. rising triplet dynamics (red) obtained when the probe polarization aligned along the crystal charge-transfer axis. However, other intermolecular packing angles (at 90o) instead localize and trap excitons as excimers, preventing singlet fission.[3] These interlayer stacked systems collectively demonstrate how remarkably different interlayer electronic states evolve from relatively small changes in interlayer twist angles in van der Waals stacked materials and molecular singlet fission materials.

Influence of layers orientation of graphene stacks in shungite disordered carbon to its integral electrical conductivity

The map of carbon distribution in the sample of graphene-containing shungite is investigated by the electron microscopy. It is found the large quantity of stacks which are formatted by parallel disposed layers of graphene. The model of electrical conductivity of graphene stack is proposed. This model takes into consideration the arbitrary orientation of layers relatively to the direction of current. It is made the numerical calculation of dependence of conductivity from the orientation of graphene layers. It is found the sharp abatement of conductivity by the orientation of graphene layers relatively to the current direction along the corner near the 45 degree. The obtained peculiarities are explained by the influence of geometrical factor.

Composite nanoarchitectonics of ZIF-67 derived CoSe2/rGO with superior charge transfer for oxygen evolution reaction

The poor conductivity and electronic structure of transition metal chalcogenide electrocatalysts turn to be the hurdles as the appropriate successors of noble metal electrocatalysts for oxygen evolution reaction (OER). The conductivity can be adjusted by introducing graphene to construct hetero interface, in which the stacking of graphene remains a challenge. Herein, CoSe2 anchored on reduced graphene oxide (rGO) composites were prepared using zeolitic imidazolate frameworks-67 (ZIF-67) and graphene oxide (GO) as precursors. Without iR-correction, CoSe2 anchored on rGO composites with a superior OER performance just demands a low potential of 296 mV to reach the current density of 10 mA cm−2, which is lower than that of pure CoSe2 (348 mV). The interfacial interaction between CoSe2 and rGO adjusts the electronic structure of cobalt ions and optimizes the filling of eg orbital as well as the bonding strength between cobalt ions and oxygen intermediate species, ameliorating the intrinsic OER activity. Meanwhile, the employment of ZIF-67 with a three-dimensional structure alleviates the coverage of active sites caused by the accumulation of graphene, which is also responsible for the excellent OER activity.

Graphene Metamaterial Embedded within Bundt Optenna for Ultra-Broadband Infrared Enhanced Absorption.

Graphene is well-known for its extraordinary physical properties such as broadband optical absorption, high electron mobility, and electrical conductivity. All of these make it an excellent candidate for several infrared applications such as photodetection, optical modulation, and optical sensing. However, a standalone monolayer graphene still suffers from a weak infrared absorption, which is ≅2.3%. In this work, a novel configuration of graphene metamaterial embedded inside Bundt optical-antenna (optenna) is demonstrated. It can leverage the graphene absorption up to 57.7% over an ultra-wide wavelength range from 1.26 to 1.68 µm (i.e., Bandwidth ≅ 420 nm). This range covers the entire optical communication bands of O, E, S, C, L, and U. The configuration mainly consists of a Bundt-shaped plasmonic antenna with a graphene metamaterial stack embedded within its nano-wide waveguide that has a 1.5 µm length. The gold average plasmonic loss is ≅25%. This configuration can enhance graphene ultra-broadband absorption through multiple mechanisms. It can nano-focus the infrared radiation down to a 50 nm spot on the graphene metamaterial, thus yielding an 11.5 gain in optical intensity (i.e., 10.6 dB). The metamaterial itself has seven concentric cylindrical graphene layers separated by silicon dioxide thin films, thus each layer contributes to the overall absorption. The focused infrared propagates tangential to the graphene metamaterial layers (i.e., grazing propagation), and thus maximizes the light–graphene interaction length. In addition, each graphene layer experiences a double-face exposure to the nano-focused propagating spot, which increases each layer’s absorption. This configuration is compact and polarization-insensitive. The estimated maximum absorption enhancement compared to the standalone monolayer graphene was 25.1 times (i.e., ≅4 dB). The estimated maximum absorption coefficient of the graphene stack was 5700 cm−1, which is considered as one of the record-high reported coefficients up to date.

Exploring Interfaces Through Synchrotron Radiation Characterization Techniques: A Graphene Case

AbstractRecently, many breakthroughs have been made in graphene research, allowing scientists to explore and understand the material world from a two‐dimensional (2D) perspective. The interface issue of graphene is the most important, because all of its atoms are exposed to the interface for this atomically thin material. The 2D nature necessitates a sensitive and non‐destructive interface probe to detect the structure and properties. Synchrotron radiation (SR) characterization techniques, with the ultra‐high resolution and extremely wide energy range, have been utilized with increasing frequency to explore the challenging interface sciences. In this review, these interface characterization techniques based on SR and how they monitor the structure evolution of graphene in different interfaces such as graphene–substrate and graphene–graphene interface are first introduced. Graphene's layer number, interlayer spacing, and stacking order are governed by these interfaces, determining the final properties. Then, the property detection and modulation in different interfaces of graphene are also discussed. Finally, the current challenges and outlook on the future development for SR techniques to characterize graphene interface are presented.

Effect of External Electric Field Applied to Bilayer Graphene Bandgap: Density Functional Theory Study

Graphene is a prominent material in a wide range of applications, however, the gapless nature of graphene limits its great performance in certain applications, such as in optoelectronic and photocatalytic applications field. Opening the bandgap has become one of the focused studies on graphene material today. Here, we used density functional theory (DFT) calculation to investigate the bandgap opening of bilayer graphene by external electric field applying. The calculation was performed on hexagonal stacked bilayer graphene structure using three different approximations and functional DFT; that is generalized gradient approximation (GGA) PBE and PW-91, as well as Local-density approximations (LDA) -CAPZ. The external electric field varied from 0.0 to 0.5 eV/Å/e in various electric field directions. The calculations results shows that the external E-F applied in perpendicular direction open the bandgap almost 4-times than non-determined the electric field direction. The bandgap opening by LDA functional calculation perform smaller bandgap opening compared to GGA calculation results, while the PBE and PW-91 of GGA produced similar values at low electric field applied. The larger bandgap of 0.80 eV obtained at 0.3 eV/Å/e electric field applied calculated with both GGA functional At 0.5 eV/Å/e electric field applied, bandgap with GGA-PBE almost zero, and the crystal structure changed from cubic to triclinic with layer position become apart.

Graphene-Based Metasurface for Tunable Absorption and Transmission Characteristics in the Near Mid-Infrared Region

This article reports a graphene-based metasurface to work at terahertz domain as a wideband tunable absorber along with a property of electromagnetic transmittivity. This indeed for the first time claims the widest possible absorption band produced by a graphene metasurface-based rasorber operating in terahertz frequencies (2.24–4.67 THz). Each unit cell comprises stacked graphene, silicon dioxide, and slotted gold layer featuring 99.9% absorptivity as well as 34.01% transmittivity across the respective bands. The other novel feature lies in its ultrathin characteristics ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\lambda _{\mathrm {g}}$ </tex-math></inline-formula> /10.70), which is improved by more than a factor of 4 relative to its closest competitor, and also in the compactness in periodicity ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\lambda _{\mathrm {g}}$ </tex-math></inline-formula> /12.49), which is identically improved by a factor of 6. The design is executed using an extensive analysis of simulated results. The lack of experimental facility at those frequencies accessible to the authors in their country has been compensated by verifying the simulated prediction using a circuit model developed inhouse. A new biasing scheme for achieving tunable absorption has been illustrated. This work also promises some features, i.e., polarization-insensitive transmission and absorption together with angularly stable characteristics of the incident wave up to 40°. The proposed structure should find potential applications in terahertz, including communication security, satellite communication systems, and stealth applications.

Tunable excitons in rhombohedral trilayer graphene

Trilayer graphene is receiving an increasing level of attention due to its stacking--dependent magnetoelectric and optoelectric properties, and its more robust ferromagnetism relative to monolayer and bilayer variants. Additionally, rhombohedral stacked trilayer graphene presents the possibility of easily opening a gap via either an external electric field perpendicular to the layers, or via the application of external strain. In this paper, we consider an external electric field to open a bandgap in rhombohedral trilayer graphene and study the excitonic optical response of the system. This is done via the combination of a tight binding model with the Bethe--Salpeter equation, solved semi--analytically and requiring only a simple numerical quadrature. We then discuss the valley--dependent optical selection rules, followed by the computation of the excitonic linear optical conductivity for the case of a rhombohedral graphene trilayer encapsulated in hexagonal boron nitride. The tunability of the excitonic resonances via an external field is also discussed, together with the increasing localization of the excitonic states as the field increases.

Electrochemical synthesis of few layer graphene in subcritical electrolyte

A novel top-down approach to the graphene synthesis combining electrochemical ion intercalation and supercritical fluid intercalation is proposed. Potentiostatic intercalation of BF4- anion into highly-oriented pyrolytic graphite in a subcritical electrolyte based on a CO2/acetonitrile mixture in a specially designed 3-electrode high-pressure electrochemical cell has been performed. The intercalation stage of the anion inside working electrode made of highly-oriented pyrolytic graphite is studied by means of Raman spectroscopy. The dispersed solid particles contained in the electrolyte have been analyzed after the intercalation and high-pressure cell decompression by means of atomic force and transmission electron microscopies. As a result, few (2−4) layers graphene flakes with partial crystallinity have been detected in the electrolyte. Additionally, the intercalated graphite electrode has been ultrasonicated, which also resulted in the formation of few layer graphene stacks with lower lateral dimensions and higher thickness.

Ultrahigh-Performance ENZ Modulator Based on a Stack of Three-Layer Graphene and ITO

A high-performance electro-absorption optical modulator based on the epsilon-near-zero (ENZ) effect is proposed. The structure consists of a waveguide with a silicon (Si) core over which a stack of graphene/HfO <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$_2$</tex-math></inline-formula> /graphene/ITO/HfO <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$_2$</tex-math></inline-formula> /graphene is grown, covered by a Si cladding. An external voltage is applied across the graphene layers to change the carrier concentration in the indium tin oxide (ITO) layers. Using a self-consistent theory, the required voltage to achieve the ENZ points in the ITO layers is calculated up to 3.42 V for an ITO thickness of 5 nm. The operation of the modulator is investigated using a three-dimensional finite-difference time-domain (FDTD) method, resulting in a modulation depth as high as 5.23 dB/ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\boldsymbol{\mu}$</tex-math></inline-formula> m (5.36 dB/ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\boldsymbol{\mu }$</tex-math></inline-formula> m) at a wavelength of 1.55 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\boldsymbol{\mu }$</tex-math></inline-formula> m for the TE (TM) polarization, which ensures the polarization-insensitivity of our proposed modulator. It is also calculated that the insertion loss of the modulator is in the order of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\boldsymbol{ 2.5 \times 10^{-3}}$</tex-math></inline-formula> dB/ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\boldsymbol{\mu }$</tex-math></inline-formula> m that yields the figure of merit (FOM) of more than 1800. The outstanding features of our proposed modulator are mainly attributed to using the Si cladding layer instead of metal cladding. Furthermore, in contrast to the previously studied structures with metal electrodes, graphene layers significantly reduce the insertion loss.

Controlled synthesis of graphene oxide/silica hybrid nanocomposites for removal of aromatic pollutants in water

The remarkable characteristics of graphene make it a model candidate for boosting the effectiveness of nano-adsorbents with high potential owing to its large surface area, π–π interaction, and accessible functional groups that interact with an adsorbate. However, the stacking of graphene reduces its influence adsorption characteristics and also its practical application. On the other hand, the widespread use of aromatic compounds in the industry has aggravated the contamination of the water environment, and how to effectively remove them has become a research hotspot. Herein, we develop the functionalization of silica nanoparticles on graphene oxide nanosheet (FGS) by a facile, cheap, and efficient synthesis protocol for adsorption of Trypan Blue (TB) and Bisphenol A (BPA). It was demonstrated that chemical activation with KOH at high autoclaving temperature successfully transformed rice husk ash (RHA) into FGS. The graphene oxide layered interlamination was kept open by using SiO2 to expose the interlayers' strong adsorption sites. XRD, EDX, FTIR, Raman spectroscopy, SEM, HR-TEM, and BET surface area are used to investigate the chemical composition, structure, morphology, and textural nature of the as-produced FGS hybrid nanocomposite. The various oxygen-containing functional groups of the hybrid nanocomposites resulted in a significantly increased adsorption capacity, according to experimental findings. In addition, FGS2, the best composite, has a specific surface area of 1768 m2g−1. Based on Langmuir isotherms, the maximal TB dye and BPA removal capacity attained after 30 min were 455 and 500 mg/g, respectively. The Langmuir isotherm model, a pseudo-second-order kinetic model, and an intraparticle diffusion model have all been used to provide mechanistic insights into the adsorption process. This suggests that BPA and TB adsorption on FGS2 is mostly chemically regulated monolayer adsorption. Due to its unique sp2-hybridized single-atom-layer structure, the exposed graphene oxide nanosheets' extremely hydrophobic effect, hydrogen bonding, and strong—electron donor–acceptor interaction contributed to their improved adsorption of BPA and TB. According to adsorption thermodynamics, FGS2 adsorption of TB and BPA is a spontaneous exothermic reaction that is aided by lowering the temperature. For adsorption-based wastewater cleanup, the produced nanocomposites with a regulated amount of carbon and silica in the form of graphene oxide and silica can be used. These findings suggest that functionalized GO/SiO2 hybrid nanocomposites could be a viable sorbent for the efficient and cost-effective removal of aromatic chemicals from wastewater.

Effect of high rGO ratio on structural properties, photoluminescence and adsorptive & photocatalytic performances under 365 nm-UV and simulated solar lightsofNaTaO3/rGO heterojunction composites

Perovskite NaTaO3 nanocrystals were synthesized using hydrothermal route, and were employed as substrate to deposit several high rGO mass ratios (NaTaO3/rGOx, x = 0, 10, 15 and 20%). X-Ray Diffraction (XRD) proved that NaTaO3 has an orthorhombic perovskite crystalline structure; this phase was conserved after the deposit of rGO, noting a varying of the orientation at the interface. The composites were analyzed by Infrared-spectroscopy (IR), which the absorbed frequencies were all ascribed to different vibrational modes inside the NaTaO3 and rGO structures. Scanning Electron Microscope (SEM) images illustrate the formation of the heterojunction and the mixing of NaTaO3 and rGO sheets with grain seize around 300 nm, and RAMAN-spectroscopy illustrates the obtaining of weak stacked graphene and disordered nano-graphene sheets. In addition, Photoluminescence (PL) reveals large Stocks shift about 60 nm, both XRD and PL reveal that the electronic and structural properties are affected by the deposited rGO. The composites were evaluated for their enhanced adsorptive performance, about 12 mg/g was measured for NaTaO3/rGO20%; and various kinetic models were employed for the adsorption, proving that the adsorption mechanism is governed by film-diffusion and/or chemical reactions process. Finally, the combined process adsorption-photocatalysis for NaTaO3/rGOx illustrate a total elimination of pollutant compared to pristine NaTaO3.

Free Energy Model for Lithium Intercalation in Graphite: Focusing on the Coupling with Graphene Stacking Sequence

During intercalation, some layered intercalation compounds undergo phase separation between several stable phases, known as stages. Of particular interest is the case of graphite, widely used in negative electrodes of batteries. Here, thermodynamics and kinetics of stage formation are studied using a multilayer free energy framework based on mean-field theory, accounting for both intralayer and interlayer interactions. More specifically, an order parameter is introduced to describe the local evolution of the stacking sequence of graphene sheets around lithium islands, enabling the distinction between the liquid-like stage 2L and the stage 2. In particular, we show that an interlayer interaction between the lithium ions and the host structure is necessary to counterbalance the enthalpic term of the lithium intralayer energy and allow the formation of islands with intermediate concentrations, characteristics of the liquid-like phases. Introducing the developed free energy model in Cahn–Hilliard’s and Allen–Cahn’s equations, we study the stages emergence during spinodal decompositions. Staggered domains of lithium surrounded by different graphene stacking arise naturally with characteristics typical of stages 1′, 3L, 2L, 2, and 1.

Compact Interlaminar Lithium Plating Realized by Silver Nanowires Imbedded in a Stacked Graphene Host with a Rational Void Space

Compact Interlaminar Lithium Plating Realized by Silver Nanowires Imbedded in a Stacked Graphene Host with a Rational Void Space

Boosting capacitive performance of manganese oxide nanorods by decorating with three-dimensional crushed graphene

This work reports the rational design of MnOx nanorods on 3D crushed reduced graphene oxide (MnOx/C-rGO) by chemical reduction of Ni-incorporated graphene oxide (GO) followed by chemical etching to remove Ni. The resulting MnOx/C-rGO composite synergistically integrates the electronic properties and geometry structure of MnOx and 3D C-rGO. As a result, MnOx/C-rGO shows a significantly higher specific capacitance (Csp) of 863 F g−1 than MnOx/2D graphene sheets (MnOx/S-rGO) (373 F g−1) and MnOx (200 F g−1) at a current density of 0.2 A g−1. Furthermore, when assembled into symmetric supercapacitors, the MnOx/C-rGO-based device delivers a higher Csp (288 F g−1) than MnOx/S-rGO-based device (75 F g−1) at a current density of 0.3 A g−1. The superior capacitive performance of the MnOx/C-rGO-based symmetric device is attributed to the enlarged accessible surface, reduced lamellar stacking of graphene, and improved ionic transport provided by the 3D architecture of MnOx/C-rGO. In addition, the MnOx/C-rGO-based device exhibits an energy density of 23 Wh kg−1 at a power density of 113 Wkg−1, and long-term cycling stability, demonstrating its promising potential for practical application.Graphical

Intra-device gating effect in graphene electrode-based organic diodes

Current–voltage characteristics of graphene electrode-based thin-film organic diodes show non-saturating high reverse bias currents, unlike standard indium tin oxide-based devices. We show that this deviation is controlled by the intra-device self-gating electric field on graphene anode from the counter cathode through the organic thin-film device layer. This effect contributes to additional charge dynamics due to the quantum capacitance presented by the semi-metallic graphene in series to the semiconductor capacitance of the organic device layer. This leads to a dynamic workfunction change of the graphene electrode as a function of the applied voltage across the diode. The proposed self-gating phenomenon is captured using an analytical model established using the combination of current–voltage and capacitance–voltage relations in these devices based on drift and diffusion equations. The results of the analytical model agree well with the experimental measurements carried out with fabricated P3HT:PCBM based organic thin-film diodes with graphene anode and aluminum cathode. We further support these results with elaborate self-consistent numerical simulations, all showing an increased reverse bias current contributed by the self-gating effects in graphene devices. Our results, being obtained from fundamental device physics, are applicable to a broad range of other devices like light-emitting diodes, perovskite-based devices, and thin-film a-Si devices. • The work function of graphene changes when subject to an external electric field. • The field effects, when employed as an electrode in a thin-film organic device, can impart changes to the behavior of these devices. • A combinational approach contributed by the device layer and the graphene stack can map this behavioral change. • The results showed a non-saturating higher reverse currents in graphene electrode-based organic diodes.