Parallel Graphene Layers Research Articles

Density functional theory study of molecular pillared graphene for High-Performance Sodium-Ion batteries

This study investigates the potential of molecular pillared graphene (MPG) as anode materials for Na-ion batteries using density functional theory (DFT) calculations. Two distinct MPG structures were designed by varying the organic pillar molecule’s chemical composition. The investigated MPGs comprise two parallel graphene layers linked by naphthalene or pyrene via the formation of five-membered boroxine rings. Cohesive energy analysis and ab-initio molecular dynamics simulations confirm the structural stability of these materials. The results demonstrate high Na mobility within the MPG structures due to the low energy barrier for diffusion. Furthermore, the layered structure facilitates Na-ion insertion, leading to exceptional theoretical capacitances calculated to be 832 and 858 mAh/g. The combination of low diffusion barriers and high theoretical capacities suggests MPG as a promising candidate for Na-ion battery anodes. Additionally, Na adsorption induces metallic behavior in the MPG structures, a crucial prerequisite for efficient ion diffusion in anode materials.

Drag resistivity in double-layer gapped graphene structures

We study the frictional drag phenomenon in a double-layer system made of two parallel gapped graphene layers by calculating the Coulomb drag resistivity. The random-phase approximation is employed to determine the polarizability functions of layers and the frequency-dependent dielectric function of the structure. Our computations illustrate that the Coulomb drag resistivity in double-layer gapped graphene systems show some interesting different features, compared to that in other double-layer ones. Coulomb drag resistivity in the system steadily increases as temperature increases but quickly decreases with the increase in interlayer distance. With small interlayer separations, the drag resistivity behaves as a smoothly increasing function of the bandgap. Nevertheless, with sufficiently large separations, the Coulomb drag resistivity increases with small bandgaps but decreases with larger ones. We observe that a finite bandgap has remarkable contributions to the frictional drag phenomenon, therefore it is necessary to take into account this factor in calculations to make better agreements with experimental works.

Capacitance response and concentration fluctuations close to ionic liquid-solvent demixing

We investigate the concentration fluctuations in a simple model of electrolyte (two positively and negatively charged Lennard-Jones spheres in a solution of an uncharged Lennard-Jones liquid) confined between electrodes formed by parallel graphene layers. Using constant potential molecular dynamics simulations and extensive constant charge simulations the effect of the proximity to the demixing transition on the electric double layers is analyzed and compared to the results of our continuum mean field theory. In agreement with our previous theoretical findings, we observe a considerable enhancement of the capacitance when temperature is lowered approaching the demixing transition for dilute ionic solutions. This enhancement is less visible when the ionic concentration is increased. Moreover, we observe the new shape of the capacitance, with a maximum at potential of zero charge, and symmetric maxima for positive and negative voltages.

Investigation of the Morphological, Structural, and Vibrational Behaviour of Graphite Nanoplatelets

This work investigated the morphological, structural, and vibrational properties of graphite nanoplatelets (GNPs) that were produced by ultrasonication. Scanning and transmission electron microscopy, X-ray diffractometry, and Raman spectroscopy identified 120-nm-thick GNP crystallites and 50–2000 μm2 plates with different areas and shapes. Extensive exfoliation was observed by transmission electron microscopy with abundant multi and some monolayer GNPs. X-ray diffractometry confirmed 43 GNP layers along the c -axis. Rietveld X-ray analysis indicated a GNP crystal lattice with stacks of parallel two-dimensional graphene layers and tightly bound hybridized carbon atoms stacked in a translational …ABAB… sequence in hexagonal rings. Raman scattering indicated well-defined GNPs with few defects and no oxide content. All analytical results reveal that GNPs could have significant potential application in electrically conductive reinforcement devices.

High electromagnetic interference shielding effectiveness in MgO composites reinforced by aligned graphene platelets

ABSTRACTGraphene has been considered as an excellent filler to reinforce ceramics with enhanced properties. However, the uniform dispersion and controlled orientation of graphene sheets in a ceramic matrix have become major challenges toward higher performance. In this paper, we prepared MgO matrix composites with parallel graphene layers through the intercalation of the precursor into expandable graphite. We obtained a high electromagnetic interference (EMI) shielding effectiveness of ~30 dB, due to the multiple reflections and absorptance of electromagnetic waves between the parallel graphene layers. The hardness and strength of the MgO composite were also increased by introducing parallel graphene layers. All these properties suggest that the graphene/MgO composite represents a promising electromagnetic shielding material.

Electromagnetic Energy Surface Modes in Metamaterial-Filled Bi-layer Graphene Structures

This paper presents a theoretical study on the propagation of electromagnetic surface waves supported by the metamaterial-filled bi-layered graphene structure. The effective surface conductivity approach based upon the Kubo’s formalism is used to model the atomically thick graphene layer, while the analytical modeling of the metamaterials used the framework of causality principle-based Kramers-Kroing relations. With respect to the index of refraction, the two configurations of metamaterials have been considered—i.e., double positive (DPS) and double negative (DNG). The impedance boundary conditions (IBCs) have been employed at the interface for the computation of characteristics equations of odd and even surface modes under DPS and DNG configurations. The numerical results regarding the dispersion relations, effective mode index, and phase speed for the odd and even modes of electromagnetic surface waves supported by each configuration of structure have been presented. Further, the influence of chemical potential and thickness of metamaterial filling in parallel graphene layers on the wave propagation characteristics has been analyzed. To check the accuracy of the numerical results, special conditions were employed, which converge to the published results. The proposed work may have potential applications in plasmonic based logic designs, renewable energy devices, THz surface wave guides, and near-field communication devices.

Design of efficient graphene plasmonic coupling circuits for THz applications

PurposeThe coupling characteristics between adjacent circuits are crucial for their efficient design in terms of electromagnetic compatibility features. Specifically, either the wireless power transfer can be enhanced or the interference can be limited. This paper aims to the extraction of the coupling characteristics of surface plasmon polariton waves propagating onto graphene layers to facilitate the telecommunication system design for advanced THz applications.Design/methodology/approachThe surface conductivity of graphene is described at the far-infrared spectrum and modelled accurately by means of a properly modified finite-difference time-domain) scheme. Then, a series of numerical simulations for different coupling setups is conducted to extract an accurate generalised parametric coupling model that is dependent explicitly on the fundamental propagation features of graphene.FindingsThe coupling coefficients of two basic waveguiding setups are examined thoroughly. The initial one includes two parallel graphene layers of infinite dimensions, and it is observed that the coupling is influenced via the ratio between their distances to the confinement of the surface wave. The second scenario is composed of graphene microstrips that are parallel to their small edge, namely, microstrip width. The extracted numerical results indicate that the coupling coefficient depends on the ratio between widths to wavelength.Originality/valueThe accurate extraction of the generalised coupling coefficients for graphene surface wave circuits is conducted in this work via an adjustable numerical technique for a novel family of plasmonic couplers. It is derived that only the fundamental propagation features of graphene, such as the wavelength and the confinement of the surface waves, have an effect on the coupling calculation, thus enabling a consistent electromagnetic compatibility study.

Continuous graphene fibers prepared by liquid crystal spinning as strain sensors for Monitoring Vital Signs

Perfect graphene sheets are zero-gap semiconductors (semi-metals) with extremely high electron mobility and have considerable potential in electronics and optics, high sensitivity sensors, supercapacitors and biodevices. Practically applying the semiconductor properties of graphene has become the focus of research. We prepared a microscopically ordered graphene oxide fiber by liquid crystal spinning. The high concentration (50 mg ml−1) graphene oxide dispersion is favorable for forming highly ordered nematic phase in the spinning process. In a liquid crystal, graphene inside a fiber has an ordered regular structure arranged in parallel along the axial direction. High-strength (432.6 ± 12.7 MPa) and highly conductive (4.9 × 104 S/m) graphene fibers are obtained by chemical reduction, in which a strong π-π bonds forms between parallel graphene layers. Given that strain can change graphene band gap and affect electron transport, it is inherited to the graphene fiber, which is macroscopically conductive. Variations, strain response sensors made from this graphene fiber with high sensitivity, and strain range (2%–8%) can be used in the detection of body health signals (speech and pulse waves). These parameters have potential use in artificial electronics skin and wearable health detection.

Emission of plasmons by drifting Dirac electrons: A hallmark of hydrodynamic transport

Direct current in clean semiconductors and metals was recently shown to obey the laws of hydrodynamics in a broad range of temperatures and sample dimensions. However, the determination of frequency window for hydrodynamic phenomena remains challenging. Here, we reveal a phenomenon being a hallmark of high-frequency hydrodynamic transport, the Cerenkov emission of plasmons by drifting Dirac electrons. The effect appears in hydrodynamic regime only due to reduction of plasmon velocity by electron-electron collisions below the velocity of carrier drift. To characterize the Cerenkov effect quantitatively, we analytically find the high-frequency non-local conductivity of drifting Dirac electrons across the hydrodynamic-to-ballistic crossover. We find the growth rates of hydrodynamic plasmon instabilities in two experimentally relevant setups: parallel graphene layers and graphene covered by subwavelength grating, further showing their absence in ballistic regime. We argue that the possibility of Cerenkov emission is linked to singular structure of non-local conductivity of Dirac materials and is independent on specific dielectric environment.

Interlayer fractional quantum Hall effect in a coupled graphene double layer

In two-dimensional (2D) electron systems under strong magnetic fields, interactions can cause fractional quantum Hall (FQH) effects. Bringing two 2D conductors to proximity, a new set of correlated states can emerge due to interactions between electrons in the same and opposite layers. Here we report interlayer correlated FQH states in a system of two parallel graphene layers separated by a thin insulator. Current flow in one layer generates different quantized Hall signals in the two layers. This result is interpreted by composite fermion (CF) theory with different intralayer and interlayer Chern-Simons gauge-field coupling. We observe FQH states corresponding to integer values of CF Landau level (LL) filling in both layers, as well as "semi-quantized" states, where a full CF LL couples to a continuously varying partially filled CF LL. Remarkably, we also recognize a quantized state between two coupled half-filled CF LLs, attributable to an interlayer CF exciton condensate.

Characteristics of dispersion modes supported by Graphene Chiral Graphene waveguide

A theoretical formulation is obtained for a chiral filled waveguide formed by two parallel graphene layers. The thin graphene layers are extended infinitesimally and Kubo formula is used for surface conductivity. The fields expressions in the model are obtained using Maxwell’s equations. It is shown that a chiral filled graphene parallel-plate waveguide can guide hybrid mode instead of quasi-transverse electromagnetic modes. The influence of chirality and or chemical potential on the properties of electromagnetic wave propagation in a planar graphene-chiral-graphene waveguide is discovered under realistic material parameters. A dispersion expression is derived, which covers the achiral graphene-chiral-graphene case. It is observed that when the chirality is raised to certain value the propagating mode is vanished. It is also revealed that chirality and chemical potential might modulate the propagation mode in the waveguide. This work may enrich the electromagnetic theory which is of great importance for Electronic devices. The presented results are compared with published under special cases.

Tunable Low Threshold Optical Tristability at Terahertz Frequencies via a Pair of Parallel Graphene Layers’ Configuration

We investigate theoretically the optical tristability of transmission at a pair of parallel graphene layers system. We discuss the influence of the graphene sheets on the hysteretic response of the TE-polarized transmitted light. It is demonstrated that the optical tristability in this configuration can be realized due to the giant third-order nonlinear conductivity of graphene and appropriate structure parameters. The tristable behavior of the transmitted light can be controlled via suitably varying the Fermi energy of the graphene. Besides, the optical tristable behavior is strongly dependent on the relaxation time of the graphene and the dispersion characteristics of the surrounding dielectrics. Moreover, the threshold of the optical tristability can be reduced by appropriately increasing the number of graphene layers, making this simple structure a good candidate for dynamic tunable and low threshold optical tristable device in the terahertz (THz) frequencies.

Production of plasmons in two layers of graphene with different doping densities traversed by swift electrons

We apply a fully relativistic formulation to calculate the average number of plasmons excited by an energetic electron traversing two parallel graphene layers separated by a given distance d. We use a Drude model in the non-dissipative limit to describe the conductivity of each layer in the THz frequency range, and address the general case in which the layers have different doping densities of charge carriers by defining an asymmetry parameter. With this, we obtain the probability density of exciting the hybridized Dirac Plasmon-Polariton modes that result from the coupling of the layers, and hence the number of plasmons excited by the passing electron along its trajectory. We analyze the effect of different parameters, such as the interlayer distance, the incident particle velocity, and the asymmetry in doping densities. In particular, we find a cusp-like behaviour for nearly equal conductivities, that indicates a high sensitivity of the system to slight variations in doping densities.

3D Vertically Aligned CNT/Graphene Hybrids from Layer‐by‐Layer Transfer for Supercapacitors

The exceptional electrical, optical, thermal and mechanical properties make graphene and carbon nanotubes (CNTs) promising for a large variety of applications, including energy storage. In practice, it is higly important to translator these properties associated with the low‐dimensional carbon nanomaterials into bulk materials/devices. Recent theoretical studies have proven that three‐dimensional (3D) pillared architectures, consisting of parallel graphene layers intercalated by vertically aligned carbon nanotubes (VA‐CNTs) in between, possess desriable transport and mechanical properties in all dimensions while maintaining the excellent properties of their building blocks. However, it remains challenging to experimentally realize such 3D pillared graphene/VA‐CNT hybrids. Here, tunable 3D pillared graphene/VA‐CNT architectures are formed by chemical vapor deposition, and a template‐free contact transfer process is presented, involving the hydrophobic‐hydrophobic interactions between graphene and VA‐CNTs. The resultant 3D graphene/VA‐CNT hybrids are demonstrated to be efficient electrode materials for supercapacitors with good performance. This newly‐developed methodology holds great potential for fabricating various 3D architectures with many other materials for a wide range of multifunctional applications, including energy storage, electrical and thermal managements, and flexible electronics.

The Tunability of Surface Plasmon Polaritons in Graphene Waveguide Structures

The tunability of propagation properties of surface plasmon polariton (SPP) modes in a waveguide formed by two parallel graphene layers separated by a dielectric layer is studied. For this purpose, the dispersion equation of the structure is numerically solved and the effects of applied bias voltage, the role of effective structural parameters, and electron–phonon scattering rate on the propagation of symmetric and antisymmetric SPP waves are investigated. The results of calculations show that considering the electron–phonon scattering rate as a function of Fermi energy and temperature leads to a considerable decrease in the propagation length of SPPs. As the main result of this work, tuning the propagation characteristics of SPPs is possible by varying any of the parameters such as applied voltage, thickness of insulating layer between two graphene layers and permittivities of dielectric layers, and finally the temperature. It is found that antisymmetric mode benefits from a larger propagation length in comparison with that of the symmetric mode.

Synthesis and Tribological Performance of Carbon Nanostructures Formed on AISI 316 Stainless Steel Substrates

Carbon nanostructures were directly grown onto standard AISI 316 stainless steel by spray pyrolysis of α-pinene, a biorenewable material. Two different nanostructures were formed: (1) multi-walled carbon nanotubes when using ferrocene as an external catalyst mixed with α-pinene and (2) ribbon carbon nanofibers when using only α-pinene. In both cases, homogeneous layers of carbon nanostructures randomly distributed and completely covering the metal substrate were observed. Carbon nanotube films were thicker (~230 μm) than carbon nanofiber films (~180 μm). A significant friction reduction was observed for both structures; however, carbon nanofibers displayed a lower friction coefficient (~0.15) than carbon nanotubes (~0.20) at 5 N of load for 200 and 2000 cycles. Scanning electron microscopy and Raman spectroscopy analyses of the wear tracks reveal that, upon rubbing, both carbon nanostructures experienced the removal of stacking faults developing large, parallel and smooth graphene layers with low interfacial shear strength which may account for the friction reduction and wear protection observed.

Electrochemical characterization of mesoporous nanographite films

The mesoporous nanographite (NG) film material consisting of tiny flake-like graphite crystallites of nanometer-scale thickness is deposited onto carbon fibers with diameter of few micrometers using adapted plasma enhanced chemical vapor deposition (CVD) method. Macroscopic morphology parameters of NG material depend on deposition time duration so, that thickness of the NG coating is increased from 0.5 to 2 μm, whereas the average sizes of the individual crystallites and the pores between them are decreased from 0.5 to 0.1 nm, with deposition time extension from 15 to 60 min, respectively. The high resolution electron microscopy examinations indicate that the NG flakes consist of parallel graphene layers separated by about 0.34 nm and those layers are paired at the edges of the flakes producing arced structures. The electrochemical capacitance measurements were served for characterization of the mesoporous nanographite coating; the specific capacitance arising from the electrostatic charge accumulated at the electrode–electrolyte interface was estimated using cyclic voltammetry, galvanostatic charge/discharge measurements, and electrical impedance spectroscopy. The effects of the deposition time duration and other parameters of CVD process on electrochemical performance and structural properties of the prepared NG films are to be discussed.

Commensurability Effects in Viscosity of Nanoconfined Water.

The rate of water flow through hydrophobic nanocapillaries is greatly enhanced as compared to that expected from macroscopic hydrodynamics. This phenomenon is usually described in terms of a relatively large slip length, which is in turn defined by such microscopic properties as the friction between water and capillary surfaces and the viscosity of water. We show that the viscosity of water and, therefore, its flow rate are profoundly affected by the layered structure of confined water if the capillary size becomes less than 2 nm. To this end, we study the structure and dynamics of water confined between two parallel graphene layers using equilibrium molecular dynamics simulations. We find that the shear viscosity is not only greatly enhanced for subnanometer capillaries, but also exhibits large oscillations that originate from commensurability between the capillary size and the size of water molecules. Such oscillating behavior of viscosity and, consequently, the slip length should be taken into account in designing and studying graphene-based and similar membranes for desalination and filtration.

Effect of graphene layer thickness on effective modulus of 3D CNT/Graphene nanostructures

Three-dimensional CNT/Graphene nanostructure is consisted of vertically aligned carbon nanotube pillars grown directly on parallel graphene layers. The effect of graphene layer thickness on mechanical properties of the 3D nanostructure is analyzed. Overall, when the graphene layers experience the out-of-plane loading, the effective properties (Young's modulus, shear modulus, and major Poisson's ratio) of the 3D CNT/Graphene structure are significantly dependent upon the thickness of graphene layers. When the graphene layers experience the in-plane loading, the effective properties of the 3D CNT/Graphene structure depend upon the graphene thickness initially and then remain relatively unchanged as the thickness increases. It is found that the optimal performance of the 3D CNT/Graphene structure requires a minimum of thickness for the graphene layers, g/t > 5.

Chemical and morphological characterization of multi-walled-carbon nanotubes synthesized by carbon deposition from an ethanol–glycerol blend

The results of this work indicated that the carbonaceous material obtained by catalytic chemical vapor deposition (CCVD) method using a glycerol–ethanol mixture as carbon source and LaNiO3 perovskite as catalyst has grown in entangled tubular arrangements and corresponds to multi-walled carbon nanotubes (MWCNTs) which morphology features will depend on temperature conditions. Thus, at 700°C, thinner and well-defined MWCNTs with parallel graphene layers were observed, whereas at higher temperature (800°C and 900°C), bamboo-like MWCNT morphology prevails followed by an increase in their outer diameter. Although the last one is much more susceptible to oxidation in air compared to that obtained at 700°C, the packing of the graphene layers and the crystal size domain (La) along the axial axis of the nanotube corresponds to highly organized material being accord to that observed by transmission electron microscopy (TEM) and Raman spectroscopy. Therefore, the reactivity of bamboo-like MWCNTs to oxidation can only be explained by the presence of local defects associated with the interconnections between compartments.