Single Graphene Sheet Research Articles

Experimental evidence for interlayer decoupling distance of twisted bilayer graphene

Twisted bilayer graphene (tBLG) is two stacked single graphene sheets rotated at a specific angle. The twist angle plays an important role in the physical properties of the tBLG. In this study, we analyzed the interaction distance between two graphene layers in tBLG using Raman spectroscopy and atomic force microscopy. To confirm the interaction of the upper and bottom graphenes in the tBLG, we prepared two tBLGs with twist angles of 5° and 24°. The interlayer distance was controlled by depositing a gold bar between the two graphene layers. We found that the decoupling interlayer distance of the tBLG was different depending on the twist angle (0.74 nm for 5° and 1.66 nm for 24°). This indicates that the interaction between two graphene layers is strongly coupled with small twist angle, as expected.

Effect of single-walled carbon nanotubes on cytochrome P450 activity in human liver microsomes in vitro.

Single-walled carbon nanotubes (SWCNTs) are made from a rolled single sheet of graphene with a diameter in the nanometer range. SWCNTs are potential carriers for drug delivery systems because antibodies or drugs can be loaded on their surface; however, their effect on the activities of cytochrome P450 (CYP) remains unclear. The aim of this study was to investigate the effect of two kinds of SWCNTs with different lengths (FH-P- and SO-SWCNTs) on human CYP activity. In addition, other nano-sized carbon materials, such as carbon black, fullerene-C60 , and fullerene-C70 were also evaluated to compare their effects on CYP activities. Ten CYP substrates (phenacetin, coumarin, bupropion, paclitaxel, tolbutamide, S-mephenytoin, dextromethorphan, chlorzoxazone, midazolam, and testosterone) were used. Testosterone 6β-hydroxylation and midazolam 1'-hydroxylation, which are catalysed by both CYP3A4 and CYP3A5 in liver microsomes, were decreased by 25% and 45%, respectively, in the presence of 0.1mg/ml SO-SWCNT. Dextromethorphan O-demethylation, which is catalysed mainly by CYP2D6, was decreased by 40% in the presence of SO-SWCNT. Other CYP activities, however, were not attenuated by SO-SWCNT. FH-P-SWCNT, carbon black, fullerene-C60 , and fullerene-C70 at 0.1mg/ml had no effect on CYP activities. The Ki values for testosterone 6β-hydroxylation, midazolam 1'-hydroxylation, and dextromethorphan O-demethylation in liver microsomes were 136, 34, and 56μg/ml, respectively. SO-SWCNT was determined to be a competitive inhibitor of CYP3A4, CYP3A5, and CYP2D6. These results suggest that the effect of SO-SWCNT differs among CYP isoforms, and that the inhibition potency depends on the physicochemical properties of the nanocarbons.

Scale Effects on the Ballistic Penetration of Graphene Sheets

Carbon nanostructures are promising ballistic protection materials, due to their low density and excellent mechanical properties. Recent experimental and computational investigations on the behavior of graphene under impact conditions revealed exceptional energy absorption properties as well. However, the reported numerical and experimental values differ by an order of magnitude. In this work, we combined numerical and analytical modeling to address this issue. In the numerical part, we employed reactive molecular dynamics to carry out ballistic tests on single, double, and triple-layered graphene sheets. We used velocity values within the range tested in experiments. Our numerical and the experimental results were used to determine parameters for a scaling law. We find that the specific penetration energy decreases as the number of layers (N) increases, from ∼15 MJ/kg for N = 1 to ∼0.9 MJ/kg for N = 350, for an impact velocity of 900 m/s. These values are in good agreement with simulations and experiments, within the entire range of N values for which data is presently available. Scale effects explain the apparent discrepancy between simulations and experiments.

Empirical potential for molecular simulation of graphene nanoplatelets

A new empirical potential for layered graphitic materials is reported. Interatomic interactions within a single graphene sheet are modeled using a Stillinger-Weber potential. Interatomic interactions between atoms in different sheets of graphene in the nanoplatelet are modeled using a Lennard-Jones interaction potential. The potential is validated by comparing molecular dynamics simulations of tensile deformation with the reported elastic constants for graphite. The graphite is found to fracture into graphene nanoplatelets when subjected to ∼15% tensile strain normal to the basal surface of the graphene stack, with an ultimate stress of 2.0 GPa and toughness of 0.33 GPa. This force field is useful to model molecular interactions in an important class of composite systems comprising 2D materials like graphene and multi-layer graphene nanoplatelets.

Complementary metal‐SU8‐graphene method for making integrated graphene nanocircuits

This work reports a complementary metal-SU8-graphene (MSG) method for constructing integrated graphene nanocircuits with excellent air and temperature stability. In this approach, electron beam (e-beam) lithography was first used to selectively pattern micro/nanoscale meshes on pristine graphene to form p-type regions. Then SU-8 thin films were spun and e-beam patterned to convert the target region of p-type graphene into n-type. The SU-8 was utilised as a doping source and an encapsulating layer for n-type graphene. To demonstrate the proposed MSG method, a graphene-based voltage inverter is constructed by complementary doping p- and n-type field effect transistors on a single graphene sheet. The electronic and thermal characteristics of the fabricated device were also studied.

The forward rainbow scattering of low energy protons by a graphene sheet

Abstract This article studies the rainbow scattering of 5-keV protons by the single sheet of free-standing graphene and its possible use as a tool for investigation of the ion-graphene interaction. The proton-graphene interaction potential was constructed by using the Doyle-Turner, ZBL, and Moliere proton-carbon interaction potentials. The thermal motion of carbon atoms was included by averaging the potentials according to the Debye model. Proton trajectories were obtained by numerical solution of the corresponding Newton equations of motion. They were used to obtain the mapping of the proton initial positions to their scattering angles. Morphological properties of the introduced mapping including its multiplicity and the rainbow singularities were used to explain important features of the obtained angular distributions of transmitted protons.

Supercapacitor Electrodes from the in Situ Reaction between Two-Dimensional Sheets of Black Phosphorus and Graphene Oxide.

Two-dimensional materials show considerable promise as high surface area electrodes for energy-storage applications such as supercapacitors. A single sheet of graphene possesses a large specific surface area because of its atomically thin thickness. However, to package this area efficiently in a device, it must be confined within a finite three-dimensional volume without restacking of the sheet faces. Herein, we present a method of maintaining the high surface area through the use of a hybrid thin film in which few-layer-exfoliated black phosphorus (BP) reduces graphene oxide (GO) flakes. When the film is exposed to moisture, a redox reaction between the BP and the GO forms an interpenetrating network of reduced GO (RGO) and a liquid electrolyte of intermediate phosphorus acids H xPO y. The presence of the liquid H xPO y electrolyte in the RGO/H xPO y film stabilizes and preserves an open-channel structure enabling rapid ion diffusion, leading to an excellent charging rate capability (up to 500 mV s-1 and retaining 62.3% of initial capacitance at a large current density of 50 A g-1) when used as electrodes in supercapacitors.

Tunable Plasmonic Absorber Based on TiN-Nanosphere Liquid Crystal Hybrid in Visible and Near-Infrared Regions

In this paper, a tunable plasmonic absorber based on TiN-nanosphere/liquid crystal (LC) nanocomposite in visible and near-infrared regions is proposed. TiN-nanosphere/LC nanocomposite is a combination of titanium nitride (TiN) nanospheres dispersed in a host of LC and plays the main role in post fabrication tunability. The proposed absorber has three more than 90% absorption peaks and the absorption tunability of about 76 nm. It is shown that TiN-nanospheres are able to support localized surface plasmon resonance (LSPR). The Maxwell-Garnett theory is utilized to approximate the permittivity of the composite structure. Also, the effect of geometric parameters on the absorption is studied. Moreover, a single sheet of graphene is utilized to compensate the decrement of the absorption caused by the geometric parameters.

Study on Graphene Based Next Generation Flexible Photodetector for Optical Communication

We report on the efficient photodetection (PD) properties of graphene based p-i-n photodetector, where all the three layers are either single or multilayer graphene sheets. We report the bandwidth and responsivity performance of the device. This simple structure paves the way for the next generation flexible wireless communication systems. A theoretical model is used to study the carrier distribution and current in a graphene based p-i-n photodetector system.

Synthesis of Colloidal Graphene by Electrochemical Exfoliation of Graphite in Lithium Sulphate

We report the synthesis of colloidal graphene via electrochemical exfoliation of graphite in Li2SO4 aqueous solution. In this electrochemical exfoliation method, high quality graphene was prepared within short time of 1.5 hr. This electrochemical exfoliation mechanism suggests that SO42- and H2O are intercalated into graphite sheets and as a result single and few-layer graphene sheets are produced with SO2 and O2 gases. The electrochemically exfoliated colloidal graphene was stable in NMP solvent for more than 2 weeks. The colloidal graphene was used for preparing the conducting graphene electrodes by brush painting on A4 paper for flexible supercapacitor applications. Our method exhibits potential for large-scale synthesis of good-quality graphene and its applications in flexible energy storage devices such as supercapacitors.

Optical and Excitonic Properties of Atomically Thin Transition-Metal Dichalcogenides

Starting with the isolation of a single sheet of graphene, the study of layered materials has been one of the most active areas of condensed matter physics, chemistry, and materials science. Single-layer transition-metal dichalcogenides are direct-gap semiconducting analogs of graphene that exhibit novel electronic and optical properties. These features provide exciting opportunities for the discovery of both new fundamental physical phenomena as well as innovative device platforms. Here, we review the progress associated with the creation and use of a simple microscopic framework for describing the optical and excitonic behavior of few-layer transition-metal dichalcogenides, which is based on symmetry, band structure, and the effective interactions between charge carriers in these materials. This approach provides an often quantitative account of experiments that probe the physics associated with strong electron–hole interactions in these quasi two-dimensional systems and has been successfully employed by many groups to both describe and predict emergent excitonic behavior in these layered semiconducting systems.

Large deformation static and dynamic response of carbon nanotubes by mixed atomistic and continuum models

Multiscale modelling based finite element method is employed to investigate the nonlinear static and dynamic response of single walled carbon nanotubes and graphene sheets. The study is carried out using a four noded Kirchhoff rectangular field consistent finite element formulated in cylindrical coordinate system considering geometric and material nonlinearities. The atomic interactions are modelled through the Tersoff–Brenner potential. To avoid membrane locking, the circumferential strain is interpolated consistently using smoothed interpolation functions derived through the least square minimization. The accuracy and efficiency of the present approach are investigated by comparing the multiscale modelling based results with those obtained through molecular mechanics simulation. The quantitative contributions of material nonlinearity and different types of geometric nonlinearities (Green–Lagrange and von–Karman type nonlinearity) on the nonlinear response of carbon nanotubes and graphene sheets subjected to different kinds of load are brought out.

Enhancement of the Seebeck effect in bilayer armchair graphene nanoribbons by tuning the electric fields

The Seebeck coefficient in single and bilayer graphene sheets has been observed to be modest due to the gapless characteristic of these structures. In this work, we demonstrate that this coefficient is significantly high in quasi-1D structures of bilayer armchair graphene nanoribbons (BL-AGNRs) thanks to the open gaps induced by the quantum confinement effect. We show that the Seebeck coefficient of BL-AGNRs is also classified into three groups 3p, 3p + 1, 3p + 2 as the energy gap. And for the semiconducting BL-AGNR of width of 12 dimer lines, the Seebeck coefficient is found as high as 707 μV/K and it increases up to 857 μV/K under the impact of the vertical electric field. While in the semimetallic structure of width of 14 dimer lines, the Seebeck coefficient remarkably enhances 14 times from 40 μV/K to 555 μV/K. Moreover, it unveils an appealing result as the Seebeck coefficient always increases with the increase of the applied potential. Such BL-AGNRs appear to be very promising for the applications of the next generation of both electronic and thermoelectric devices applying electric gates.

Bandwidth broadening of a graphene-based circular polarization converter by phase compensation.

We present a broadband tunable circular polarization converter composed of a single graphene sheet patterned with butterfly-shaped holes, a dielectric spacer, and a 7-layer graphene ground plane. It can convert a linearly polarized wave into a circularly polarized wave in reflection mode. The polarization converter can be dynamically tuned by varying the Fermi energy of the single graphene sheet. Furthermore, the 7-layer graphene acting as a ground plane can modulate the phase of its reflected wave by controlling the Femi energy, which provides constructive interference condition at the surface of the single graphene sheet in a broad bandwidth and therefore significantly broadens the tunable bandwidth of the proposed polarization converter.

Molecular dynamics simulation of the effects of affinity of functional groups and particle-size on the behavior of a graphene sheet in nanofluid

The practical application of thermally enhanced nanofluids in automotive industries for higher energy efficiency requires that nanoparticles be buoyant, leading to Brownian motion in the base fluid without aggregation. Numerous studies have reported the long-term stability of dispersions resulting from steric hindrance and an affinity for the surrounding base fluid via modification of nanoparticles with functional groups. In this study, we performed molecular dynamics simulations of nanofluids containing a single graphene sheet with various functional groups to investigate the influence of affinity, as well as particle-size, on the behavior of the graphene sheet. Using the concept of the speed of nanoparticle, we quantitatively evaluated the dependence of behavior on affinity to investigate whether having more functional groups with more attractive interactions has an impact on the stable dispersion in nanofluid. In addition, the simulation results for the particle-size effect revealed that the larger nanoparticles produced more stable dispersion in nanofluid. We concluded that the behavior of the graphene sheet depends on a combination of two factors: different charge assigned to atoms due to a nitrogen atom, and the difference in mass, which influences the speed of the nanoparticles.

Measuring individual carbon nanotubes and single graphene sheets using atomic force microscope infrared spectroscopy

AstractAtomic force microscope infrared spectroscopy (AFM-IR) combines the spatial resolution of AFM with the chemical specificity of IR spectroscopy. In AFM-IR, sample absorption of pulsed IR light causes rapid thermomechanical expansion, which excites resonance in an AFM cantilever in contact with the sample. The cantilever resonant amplitude is proportional to the local sample IR absorption coefficient. It is difficult to detect thermomechanical expansion in the smallest samples such as 1D and 2D nanomaterials. In this work, we overcome this limitation and use AFM-IR to measure nanometer-scale IR absorption in individual single walled carbon nanotubes and monolayer graphene. By placing a thin layer of polymer beneath the sample, the AFM-IR signal may be increased by up to two orders of magnitude. The polymer beneath the sample thermally insulates the sample and amplifies thermomechanical expansion. Finite element simulations agree with the measurements and provide a general framework for applying this approach to arbitrary samples, including other 1D and 2D materials and thin biological samples.

A simple FDTD approach for the analysis and design of graphene based optical devices

In this paper, a simple finite difference time domain (FDTD) approach is presented for the analysis and design of graphene based optical devices. Here, Maxwell’s curl equations are expressed in normalized flux density. Then, the FDTD method is incorporated with that expression. As the flux density is considered, graphene conductivity which depends on frequency is easily incorporated with the method. The proposed method is used to investigate properties of single and multilayer graphene sheets. To probe the transmission, reflection and surface plasmon polariton (SPP) mode characteristics of graphene in terahertz and far infared frequency regime, numerical examples are demonstrated. Validation of the method has been demonostrated comparing with existing methods and results available in the literature. The accuracy, stability and benefits of proposed method are also presented.

Perylene tetracarboxylate surfactant assisted liquid phase exfoliation of graphite into graphene nanosheets with facile re-dispersibility in aqueous/organic polar solvents

Liquid phase exfoliation (LPE) is a promising method for graphene production particularly in terms of cost effectiveness and scale up. Nonetheless, it is still challenging to synchronize prime goals of high quality, good yield, large sheet size, stable long term storage and low cost eco-friendly processing. We present a simple and inexpensive green route for large scale production of exfoliated graphene dispersions exploiting the non-covalent surface chemistry between graphene and perylene tetracarboxylate (PTCA) aromatic semiconducting surfactant. Direct sonication of graphite flakes in aqueous PTCA solutions produced high yield of single and few-layer graphene sheets with minimal basal plane defects as revealed by XPS, Raman and FTIR spectroscopy. Uniquely for LPE protocol, the lateral graphene flake dimensions extended upto 10–12 μm range. The exfoliated dispersions exhibited high colloidal stability with shelf-life exceeding a year. Facile re-dispersibility of the dried graphene/PTCA powders was observed in water as well as many polar organic solvents. Significantly, pure aromatic semiconducting nature of surfactant without dielectric moiety ensures tight electrical contact among graphene sheets in thin films. The approach exploiting the simple molecular design of aromatic charged surfactants for graphene exfoliation holds a great prospect for solution processed graphene based nanomaterials and devices.

How the activation process modifies the hydrogen storage behavior of biomass-derived activated carbons

Microporous activated carbons (ACs) derived from biomass residues, by virtue of their low-cost, good thermo-mechanical stability and easy adsorbent regeneration, are widely considered as hydrogen storage materials for near-term applications. The hydrogen uptake performance of activated carbons is known to depend on the pore-textural and surface characteristics, such as size and distribution of micropores and specific surface area. Here, we present a detailed investigation on how the activation processes using KOH, CO2, K2CO3, and H3PO4 modify the microstructure of olive stones-derived ACs and how they affect the ACs’ hydrogen storage behavior. The KOH-activation results in the formation of exfoliated graphene sheets, which are not common in lignocellulose-derived ACs. In addition, the KOH-activation forms supermicropores (1–2 nm) that enhance the hydrogen uptake capacity at high pressures (200 bar). The absolute hydrogen adsorption of KOH-activated sample at 200 bar and 77 K is 6.11 wt%, which is among the highest reported for activated carbon samples. The best hydrogen uptake density per surface area of the carbon we obtained is 2.1 × 10−3 wt% m−2 g which is very close to the theoretical maximum hydrogen uptake density on a single graphene sheet. CO2 and H3PO4 activations are more effective on the creation of ultramicropores (d ≤ 0.7 nm) in the carbon matrix. This order of pore size is useful when hydrogen adsorption is performed at sub-atmospheric pressures. Our study suggests that activated carbons with a homogenous pore size distribution centered at narrow range are not as efficient H2 adsorbents as the ACs with a bimodal PSD.

Graphene Fabry–Perot Cavity Leaky-Wave Antennas: Plasmonic Versus Nonplasmonic Solutions

Tunable THz antennas based on a single unpatterned graphene sheet placed inside a grounded dielectric multilayer are studied with the aim of characterizing their performance in terms of pattern reconfigurability, directivity, and radiation efficiency. The considered structures belong to the class of Fabry–Perot cavity (FPC) antennas, whose radiation mechanism relies on the excitation of cylindrical leaky waves with an ordinary (i.e., nonplasmonic) sinusoidal transverse modal profile. This allows for achieving radiation efficiencies considerably higher than those of alternative graphene-based radiators based on the excitation of surface-plasmon polaritons (SPPs) either in bound or leaky propagation regimes. A customized efficient circuit model has been employed in order to obtain all the radiation characteristics of such graphene FPC antennas, which have also been fully validated by means of a CAD tool. The role of the graphene quality is explicitly taken into account in this comprehensive investigation, proving that it plays a remarkable role in establishing the antenna performance. In particular, it is expected that the standard quality of graphene allows for designing low-efficiency reconfigurable THz antennas based on SPPs and, conversely, high-efficiency FPC antennas with slightly reduced reconfigurability.