Large Graphene Sheets Research Articles

Tunable two-photon THz emissions through pair annihilation in graphene with a double gate structure

How far the analogy between massless Dirac fermions in a truly relativistic (1+2)-D spacetime and electrons near the Fermi level in graphene can be seriously taken? A hallmark of relativistic QFT is the multi-photon emission through pair annihilation. In this paper, to address this question we formulate the theoretical basis for a double gate graphene device. In an infinite sheet of pristine graphene the Fermi level is located at the Dirac points, but it can be tunned through gate potentials. This way, electron and hole pockets are induced, forming N and P regions in a large monolayer graphene sheet. The quasi-particles can be accelerated through the source-drain potential to scatter at an intrinsic region, leading to electron-hole annihilation. Feynman amplitudes and emission rates for two-photon emissions arising from electron-hole annihilation in graphene are presented at lowest order, leading to analytical formulae for two photon production, which could experimentally be tested.

Improved-quality graphene films via the synergism of large nanosheet aligning and nanotube bridging for flexible supercapacitors

Scalable production of reduced graphene oxide (rGO) films with high mechanical-electrical properties is desirable as these films are candidates for wearable electronics devices and energy storage applications. Removing structural incompleteness such as wrinkles or voids in the graphene films, which are generated from the assembly process, would greatly optimize their mechanical properties. However, the densely stacked graphene sheets in the films degrade their ionic kinetics and thus limit their development. Here, a horizontal-longitudinal-structure modulating strategy is demonstrated to produce enhanced mechanical, conductive, and capacitive graphene films. Typically, two-dimensional large graphene sheets (LGS) induce regular stacking of graphene oxide (GO) during the assembly process to reduce wrinkles, while one-dimensional single-walled carbon nanotubes (SWCNT) bridge with graphene sheets to strengthen the multidirectional intercalation and reduce GO layer restacking. The simultaneous incorporation of LGS and SWCNT synergistically creates a fine microstructure by improving the alignment of graphene sheets, increasing continuous conductive pathways to facilitate electron transport, and enlarging interlayer spacing to promote electrolyte ion diffusion. As a result, the obtained graphene films are flat and exhibit signally reinforced mechanical properties, electrical conductivity (38727 S m-1), as well as specific capacitance (232 F g-1) as supercapacitor electrodes compared to those of original rGO films. Moreover, owing to the comprehensive improved properties, a flexible gel supercapacitor assembled by the graphene film-based electrodes shows high energy density, good flexibility, and excellent cycling stability (93.8% capacitance retention after 10 000 cycles). This work provides a general strategy to manufacture robust graphene structural materials for energy storage applications in flexible and wearable electronics.

Naturally Occurring Allotropes of Carbon.

Carbon is one of the most important chemical elements, forming a wide range of important allotropes, ranging from diamond over graphite to nanostructural materials such as graphene, fullerenes, and carbon nanotubes (CNTs). Especially these nanomaterials play an important role in technology and are commonly formed in laborious synthetic processes that often are of high energy demand. Recently, fullerenes and their building blocks (buckybowls) have been found in natural fossil materials formed under geological conditions. The question arises of how diverse nature can be in forming different types of natural allotropes of carbon. This is investigated here, using modern analytical methods such as ultrahigh-resolution mass spectrometry and transmission electron microscopy, which facilitate a detailed understanding of the diversity of natural carbon allotropes. Large fullerenes, fullertubes, graphene sheets, and double- and multiwalled CNTs together with single-walled CNTs were detected in natural heavy fossil materials while theoretical calculations on the B3LYP/6-31G(d) level of theory using the ORCA software package support the findings.

Anti-fatigue nanomechanics in the pre-cracked graphene–copper artificial nacre under cyclic tension

The fatigue performance of pre-cracked graphene–copper artificial nacre (GrCu nacre) under cyclic tensile loading is investigated using theoretical analysis and molecular dynamics (MD) simulations. Mechanical models are proposed to describe the fatigue performance of GrCu nacre, which agree well with MD simulations. Compared with nanocomposites consisting of large graphene sheets, graphene fragments cannot improve the ultimate strength of GrCu nacre, but significantly improve its fatigue limit. This anti-fatigue effect is dominated by the graphene–copper interfaces with van der Waals interactions, which can be adjusted by tailoring the architectural parameters of GrCu nacre. The ultimate strength of GrCu nacre decreases as the volume fraction of graphene increases. The fatigue limit of GrCu nacre decreases as the length of graphene fragments increases, and can be approximately described by a quadratic function of interlayer repeat spacing. An anomalous weakening effect occurs when the in-plane distance between graphene fragments is less than 2 nm, while this effect can be ignored when it is higher than 2 nm. The fatigue ratio of common metals is ∼0.4, whereas that of GrCu nacre is generally higher than 0.9. These underlying mechanisms and theoretical models can improve the fundamental understanding of the fatigue performance of composites with incoherent interfaces, as well as the bottom-up design of graphene-metal nanocomposites.

Strong Bending Distortion of a Supercoronene Graphene Model upon Adsorption of Lanthanide Atoms

Numerous applications of graphene involve quasi-infinite sheets, as well as finite structures with edges, pores, graphene quantum dots, etc. In theoretical studies of adsorption of diverse chemical species, including single atoms, molecules, cations, and anions, graphene usually behaves as a very rigid planar structure. However, we found that when adsorbing lanthanide atoms, finite size structures, represented by the widely used supercoronene model, can undergo considerable distortion, and the degree of distortion depends on the number of unpaired electrons, reaching a maximum for Gd (eight unpaired electrons). Lanthanides closely approach the supercoronene surface and increase the interaction energy. Extrapolating to real-world systems, one can expect the existence and magnitude of lanthanide-induced distortion to depend on the size of graphene structures. Quasi-infinite or very large graphene sheets are too rigid to undergo such bending, but it becomes tangible for graphene quantum dots and for atom adsorption closer to graphene edges.

Sulphur-doped graphene based sensor for rapid and efficient gallic acid detection from food related samples

BackgroundNowadays, the food safety issue represents a hot topic. The emerging evidence over the past decade confirms that gallic acid (GA) plays a crucial role, with multiple pharmacological and therapeutic interventions in numerous health complications. MethodsSulphur-doped graphene material (exf-SGR) was obtained from graphite rod electrochemical exfoliation in the presence of inorganic salts mixture such as sodium thiosulfate and ammonium sulphate, Na2S2O3 / (NH4)2SO4 (0.2 M each), at low applied bias (6 V). The complete physico-chemical characterization of the as-prepared material was performed by means of TEM, SEM, STEM-EDS, XRD and XPS analysis. Furthermore, the electrochemical performances of bare glassy carbon (GCE) and modified (exf-SGR/GCE) electrodes for GA assay were evaluated. Significant findingsTEM/SEM images reveal the formation of large dimension graphene sheets having uniformly distributed spherical sulphur crystallites on the surface. XRD demonstrates that the material consists of few-layer S-doped graphenes. From XPS analysis the C/O ratio was determined to be 0.473 while C/S ratio equals 2.741, indicating a high heteroatom doping degree. The exf-SGR/GCE modified electrode shows a superior electrochemical behaviour over a broad GA detection range (0.1–100.0 µM), with a low detection limit (3.03 × 10−8 M). The electrode retains 96.83% of its initial response after 50 consecutive CV measurements indicating an excellent long-term stability and repeatability. The sensor possesses very good anti-interfering capabilities in complex matrix. The developed electrochemical sensor can provide a fast model for GA detection in food related and biological real sample analysis.

Acoustically Induced Giant Synthetic Hall Voltages in Graphene.

Any departure from graphene's flatness leads to the emergence of artificial gauge fields that act on the motion of the Dirac fermions through an associated pseudomagnetic field. Here, we demonstrate the tunability of strong gauge fields in nonlocal experiments using a large planar graphene sheet that conforms to the deformation of a piezoelectric layer by a surface acoustic wave. The acoustic wave induces a longitudinal and a giant synthetic Hall voltage in the absence of external magnetic fields. The superposition of a synthetic Hall potential and a conventional Hall voltage can annihilate the sample's transverse potential at large external magnetic fields. Surface acoustic waves thus provide a promising and facile avenue for the exploitation of gauge fields in large planar graphene systems.

Size effects of graphene sheets on the strengthening mechanism of Al-graphene composites: A molecular dynamics study

Molecular dynamics simulations are performed to investigate the effects of graphene sheet size on the strengthening mechanism in Al-graphene composites during nanoindentation. In all cases, the graphene sheets block the dislocations, and thus the dislocations pile up near the graphene-matrix interface, strengthening the Al matrix. What is of particular interest is that strengthening mechanism is highly dependent on the size of the graphene sheet. If the indenter is not in contact with the graphene sheets, the nanoindentation force tends to increase with a decrease in the graphene sheet size and reaches a maximum value for a graphene sheet size of 30 Å. The trend in force with graphene sheet size is because the larger graphene sheets more effectively dissipate the force and lead to the emission of more dislocations from the Al-graphene interface. On the other hand, when the indenter contacts the graphene sheets, the force decreases with increasing graphene sheet size due to the superior strength of the graphene sheets. The composite containing the graphene sheet with the maximum size that covers the entire matrix shows the highest force that is almost 1.5 times of the value of other Al-graphene composites.

Ambient energy dispersion and long-term stabilisation of large graphene sheets from graphite using a surface energy matched ionic liquid†

Liquid phase exfoliation (LPE) is the most promising method for scalable graphene production, but conventionally requires significant energy input to break apart layers in the bulk material, introducing defects and limiting production of large, few-layer sheets. In this work, we exfoliated graphene under ambient conditions with minimal external energy input using the surface energy matched ionic liquid (IL) 1-ethyl-3-methylimidazolium acetate (EMIm Ac), then isolated and characterised the exfoliated material. This method produced large and thin, graphene sheets with size up to 3 μm. Monolayer sheets were identified in samples that had been left undisturbed for 6 months prior to sampling, meaning the graphene sheets suspended in the ionic liquid are stabilized against restacking; to our knowledge, this is the longest period of time large (non-oxidised) graphene monolayers have been suspended in any liquid. Molecular dynamics simulations show the IL cation is enriched at the graphene surface. We postulate this leads to a positive charge on the graphene sheets in EMIM Ac leading to an electrostatic repulsion which contributes to colloidal stability. Strong adsorption of the EMIM cation to graphene is confirmed by energy dispersive X-ray spectroscopy images that show ionic liquid adsorbed to graphene sheets even after washing. Ambient energy exfoliation using ionic liquids offers simple, energy efficient method for producing large, high quality graphene sheets, with long-term stability in the ionic liquid. [Display omitted]

Development of Highly Sensitive Strain Sensor Using Area-Arrayed Graphene Nanoribbons.

In this study, a basic design of area-arrayed graphene nanoribbon (GNR) strain sensors was proposed to realize the next generation of strain sensors. To fabricate the area-arrayed GNRs, a top-down approach was employed, in which GNRs were cut out from a large graphene sheet using an electron beam lithography technique. GNRs with widths of 400 nm, 300 nm, 200 nm, and 50 nm were fabricated, and their current-voltage characteristics were evaluated. The current values of GNRs with widths of 200 nm and above increased linearly with increasing applied voltage, indicating that these GNRs were metallic conductors and a good ohmic junction was formed between graphene and the electrode. There were two types of GNRs with a width of 50 nm, one with a linear current–voltage relationship and the other with a nonlinear one. We evaluated the strain sensitivity of the 50 nm GNR exhibiting metallic conduction by applying a four-point bending test, and found that the gauge factor of this GNR was about 50. Thus, GNRs with a width of about 50 nm can be used to realize a highly sensitive strain sensor.

Lightweight thermal interface materials based on hierarchically structured graphene paper with superior through-plane thermal conductivity

Graphene-based papers have recently triggered considerable interests in developing the application as thermal interface materials (TIMs) for addressing the interfacial heat transfer issue, but their low through-plane thermal conductivity (κ⊥), resulting from the layer-by-layer stacked architecture, limits the direct use as TIMs. Although various hybrid graphene papers prepared by combining the graphene sheets and the thermally conductive insertions have been proposed to solve this problem, achieving a satisfactory κ⊥ higher than that of commercial TIMs (>5 W m−1 K−1) remains challenging. Here, a strategy aimed at the construction of heat pathways along the through-plane direction inside the graphene paper for achieving a high κ⊥ was demonstrated through the simultaneous filtration of graphene sheets with two different lateral sizes. The as-prepared graphene paper presented a hierarchical structure composed of loosely stacked horizontal layers formed by large graphene sheets, intercalated by a random arrangement of small graphene sheets. Due to the heat pathways formed by small graphene sheets along the through-plane direction, the hierarchically structured graphene paper exhibited an improved κ⊥ as high as 12.6 W m−1 K−1 after a common graphitization post-treatment. In the practical test, our proposed paper as an all-graphene TIM achieved an enhancement in cooling efficiency of ≈ 2.2 times compared to that of the state-of-the-art TIM, demonstrating its superior performance to meet the ever-increasing heat dissipation requirement.

Soft X-ray absorption and emission spectra of nanographene prepared from pentacene with hot mesh deposition and soft X-ray irradiation

The molecular orientation and partial density of states were evaluated using NewSUBARU by soft X-ray absorption spectroscopy (XAS) and soft X-ray emission spectroscopy measurements. The degree of molecular alignment was degraded by increasing mesh temperature in hot mesh deposition (HMD), in other words, was changed from pentacene (Pn) to 6,13-dihydropentacene (DHP). At a mesh temperature of 1450 °C, the different XAS was obtained due to the mixing effect of Pn and DHP, and presence of Pn oligomer. The HMD carbon film transformed into the graphite-like film and the graphene on the quartz substrate and the Ni/quartz substrate after soft X-ray irradiation, respectively. The HMD carbon film after soft X-ray irradiation showed the peaks due to terminal carbon such as CH n and COOH in comparison with the reported large graphene sheet. It indicates that the flake size of the graphene on the Ni/quartz substrate was small and had many edges.

Thermal conductivity of hybrid multilayer graphene-fiber carbon membranes

Hybrid graphemne-fiber systems could present an alternative for various industrial applications in need of large area graphene sheets. One way to produce these carbon-based structures is by subjecting an aqueous polyvinyl alcohol solution containing sodium chloride to centrifugal spinning under high humidity conditions. The developed polymer fibers are then subjected to a dehydration and carbonization process to promote the formation of the hybrid carbon structure. Potential applications of this material are highly dependent upon their conducting properties. In this work we analyzed the effect of the NaCl content and humidity conditions during the spinning process and ultimate thermal conductivity of the resultant hybrid graphene-fiber carbon systems. Results show an optimum NaCl added to the carbon precursor solution and spun at a high relative humidity (around 70%) promote the development of veils of graphene oxide multilayer that interconnect with produced fibers. We applied for the first time a thermographic method to determine the thermal conductivity of carbon mats. The thermal conductivity of the hybrid fibers increases as graphene multilayers veils expand between carbon fibers, to reach values up to 28 W m K−1.

Role of surface passivation on the development of camphor based Graphene/SiNWAs schottky diode

In the present work, we have explored the passivation mechanism of Graphene based schottky junction photodiodes. The graphene sheet was developed by low cost camphor as a solid source of carbon to avoid toxic gaseous carbon precursor. Moreover, coupling with Si or SiNWAs makes it a promising candidate for large-scale and cost effective optoelectronic application. The 8–10 μm nanowire arrays in the present case, demonstrates excellent optical light trapping properties with the reflectance of 4% over a wide range of wavelength (200–1200 nm). On the other hand, large scale graphene sheet developed by camphor contributes no defect D band at (∼1360 cm−1) with 2D/G ratio of 2.43. It is found that, the performance of Gr/SiNWAs schottky junction is restricted by strong carrier recombination and relative low schottky barrier height (SBH). We have modified the surfaces of nanowires by H-passivation in order to improve the electrical performance of the device. Passivation of SiNWAs showed the capability to effectively suppress the surface carrier recombination upto certain extent, leading to a more electron transfer and an improvement in the JSC values from 0.3 mA/cm2 to 2.7 mA/cm2 typically at −1 V.

An Atomistic Study of the Stress Corrosion Cracking in Graphene.

Using molecular dynamics (MD) simulations, we study the mechanism of stress corrosion cracking in graphene. Two sets of modelings are conducted. In the first one, large graphene sheets with cracks in the armchair and zigzag directions are exposed to oxygen molecules. The crack growth as a result of chemical reactions between carbon radicals and oxygen molecules at different mechanical tensile stress levels is studied. In the second set of simulations, MD simulations are combined with the density functional-based tight bonding method to enhance the accuracy. This set of modelings focuses on a smaller zone in the vicinity of the crack tip. The impact of initial crack orientation on corrosion is studied by investigating corrosion of cracks in both armchair and zigzag directions. We investigate the subcritical crack propagation occurring as a result of the combined effects of both mechanical loading and chemical reactions. Our results show that cracks in graphene can grow due to chemical reactions with the environmental molecules. The MD modelings also predict that reaction of carbon atoms with oxygen molecules might lead to a stress relaxation at the crack tip, hence preventing further crack propagation. The results show that subcritical crack growth can happen by two mechanisms, which include the failure of C-C bonds or by removing the carbon atoms from graphene sheets in the form of CO or CO2 molecules.

High-contrast optical microscopy of graphene sheets.

In the way of making graphene an industry-friendly material, it must be mass-produced with high-quality and reduced cost over large areas. Assisted by machine-learning techniques, rapid, nondestructive and accurate determination of large graphene sheets on SiO2 /Si substrates has been made possible in recent years by the optical microscopy method. Optimization of the substrate to achieve the maximum contrast can further extend the application of the optical microscopy method for quality control of the mass-produced graphene. Graphene/n2 /n3 three-layer structures, where n2 and n3 are refractive indices, are routinely used for identifying the number of graphene layers by optical reflection microscopy. In this paper, two analytical equations are derived that can be easily used for high-contrast optical imaging of graphene sheets without any need to resort to the cumbersome numerical methods. One of the equations is derived for choosing the best material with refractive index n2 that when coated on a substrate with refractive index n3 , maximizes the optical contrast. The other equation is derived for finding the best thickness of the SiO2 layer in graphene/SiO2 /Si structures, which are in common use for fabrication of graphene-based devices. The results are implemented in a MATLAB GUI, see Supporting Information, to assist the users in using the equations.

Mesoporous Nitrogen-Doped Carbon Nanospheres as Sulfur Matrix and a Novel Chelate-Modified Separator for High-Performance Room-Temperature Na-S Batteries.

Room-temperature sodium-sulfur (RT/Na-S) batteries are considered among the most promising next-generation energy storage and conversion systems because of the earth-abundant reserves of sodium and sulfur. These batteries also possess the advantages of high theoretical gravimetric capacity, high energy density, and low cost. Herein, highly uniform Fe3+ /polyacrylamide nanospheres (FPNs) are fabricated on a large-scale by a facile, low-cost approach. Subsequently, mesoporous nitrogen-doped carbon nanospheres (PNC-Ns), obtained by carbonizing FPNs, are applied as a sulfur matrix to improve the utilization of sulfur, enhance the overall conductivity of the cathode, and inhibit the shuttling of sodium polysulfides (SPSs). In addition, graphene and FPNs are simultaneously coated onto the side of the separator to form a FPNs-graphene-functionalized separator (FPNs-G/separator); here, the mesoporous FPNs effectively anchor and block the SPSs, while the large specific area graphene sheets eliminate the intrinsic mechanical brittleness of the FPNs and improve the overall conductivity of RT/Na-S batteries. When S/PNC-Ns as a cathode and FPNs-G/separator are assembled into an RT/Na-S battery, it delivers a high discharge capacity (639 mAh g-1 at 0.1 C after 400 cycles), stable cycle life (396 mAh g-1 at 0.5 C after 800 cycles), and good rate performance (228 mAh g-1 at 2 C).

Chemically synthesized graphene as a precursor to Prussian blue-based nanocomposite: A multifunctional material for transparent aqueous K-ion battery or electrochromic device

This work describes the preparation of a novel graphene/Prussian blue nanocomposite thin and transparent film and its application as both electrochromic material and cathode for transparent K-ion battery operating in aqueous electrolyte. Large graphene sheets were chemically synthesized from benzene, at a water/benzene liquid/liquid interface, using solid iron chloride as both oxidizing and catalyst, yielding a transparent film stabilized at the liquid/liquid interface that can be easily transferred to desirable substrates. We took advantage of the different iron species remaining as a side-product of the synthesis to promote the Prussian blue growing, through a heterogeneous electrochemical reaction between those iron species in the film and ferrocyanide ions in the electrolyte aqueous solution. Nanocubes of Prussian blue grow over the graphene and cover the entire substrate, yielding a bluish, homogeneous and transparent graphene/Prussian blue multifunctional nanocomposite, which was characterized by different techniques. Finally, we demonstrate two possibilities of a real application of this nanocomposite thin film, as cathodic material for aqueous K+-ion transparent batteries, presenting capacities up to 141 mAh g−1 at 0.093 A g−1, and as electrode in an electrochromic device paired with WO3 and using an aqueous KCl solution as electrolyte, presenting very stable optical variation through 200 electrochromic cycles.

Inducing and controlling magnetism in the honeycomb lattice through a harmonic trapping potential

We study strongly interacting ultracold spin-1/2 fermions in a honeycomb lattice in the presence of a harmonic trap. Tuning the strength of the harmonic trap we show that it is possible to confine the fermions in artificial structures reminiscent of graphene nanoflakes in solid state. The confinement on small structures induces magnetic effects which are absent in a large graphene sheet. Increasing the strength of the harmonic potential we are able to induce different magnetic states, such as a N\'eel-like antiferromagnetic or ferromagnetic states, as well as mixtures of these basic states. The realization of different magnetic patterns is associated with the terminations of the artificial structures, in turn controlled by the confining potential.

Handling and Risk Mitigation of Nanoscale Graphene and Related Materials: Some Considerations and Recommendations

The purpose of this communication is to put forward some considerations and recommendations while handling nanomaterials, especially graphene and its derivatives. A large graphene sheet is generally stable and inert; thus, graphene and its derivatives are not considered hazardous, but good laboratory practices should be taken seriously for the safe handling and use of such materials. This article provides some insights about nanoscale graphene handling and some important considerations.