Orientation Of Graphene Research Articles

Adsorption of graphene to metal (111) surfaces using the exchange-hole dipole moment model

Graphene has a unique electronic structure and excellent tribological properties. A promising method for graphene production involves depositing vaporized carbon on metal substrates, which can also be used to modify graphene's electronic structure through charge transfer. In this work, graphene adsorption on the (111) surface of seven metals (Al, Cu, Ag, Au, Ni, Pd, and Pt) is investigated computationally using density-functional theory with the exchange-hole dipole moment (XDM) dispersion correction. Two distinct graphene-metal orientations, corresponding to 0∘ and 30∘ relative rotation of the graphene layer, are considered to investigate how lattice mismatch affects adsorption. Our results reproduce reference data from the random-phase approximation more closely than other dispersion-corrected density functionals, confirming that XDM is an excellent method for surface chemistry. The rotational orientation of graphene is found to strongly affect its interaction with the substrate. There is an energetic drive for graphene to align with the metal lattice, particularly for Pd and Pt, which causes the formation of multiple Moiré patterns, in agreement with experimental observations.

Revealing the Crystalline Integrity of Wafer-Scale Graphene on SiO2/Si: An Azimuthal RHEED Approach.

The symmetry of graphene is usually determined by a low-energy electron diffraction (LEED) method when the graphene is on the conductive substrates, but LEED cannot handle graphene transferred to SiO2/Si substrates due to the charging effect. While transmission electron microscopy can generate electron diffraction on post-transferred graphene, this method is too localized. Herein, we employed an azimuthal reflection high-energy electron diffraction (RHEED) method to construct the reciprocal space mapping and determine the symmetry of wafer-size graphene both pre- and post-transfer. In this work, single-crystalline Cu(111) films were prepared on sapphire(0001) and spinel(111) substrates with sputtering. Then the graphene was epitaxially grown on single-crystalline Cu(111) films with a low pressure chemical vapor deposition. The reciprocal space mapping using azimuthal RHEED confirmed that the graphene grown on Cu(111) films was single-crystalline, no matter the form of the monolayer or multilayer structure. While the Cu(111) film grown on sapphire(0001) may occasionally consist of 60° in-plane rotational twinning, the reciprocal space mapping revealed that the in-plane orientation of graphene grown atop was not affected. The proposed method for checking the crystalline integrity of the post-transferred graphene sheets is an important step in the realization of the graphene as a platform to fabricate electronic and optoelectronic devices.

Theory of electrical conductivity and dielectric permittivity of highly aligned graphene-based nanocomposites

Highly aligned graphene-based nanocomposites are of great interest due to their excellent electrical properties along the aligned direction. Graphene fillers in these composites are not necessarily perfectly aligned, but their orientations are highly confined to a certain angle, with 90° giving rise to the randomly oriented state and 0° to the perfectly aligned one. Recent experiments have shown that electrical conductivity and dielectric permittivity of highly aligned graphene-polymer nanocomposites are strongly dependent on this distribution angle, but at present no theory seems to exist to address this issue. In this work we present a new effective-medium theory that is derived from the underlying physical process including the effects of graphene orientation, filler loading, aspect ratio, percolation threshold, interfacial tunneling, and Maxwell–Wagner–Sillars polarization, to determine these two properties. The theory is formulated in the context of preferred orientational average. We highlight this new theory with an application to rGO/epoxy nanocomposites, and demonstrate that the calculated in-plane and out-of-plane conductivity and permittivity are in agreement with the experimental data as the range of graphene orientations changes from the randomly oriented to the highly aligned state. We also show that the percolation thresholds of highly aligned graphene nanocomposites are in general different along the planar and the normal directions, but they converge into a single one when the statistical distribution of graphene fillers is spherically symmetric.

Precise Identification of Graphene's Crystal Structures by Removable Nanowire Epitaxy.

Monitoring crystallographic orientations of graphene is important for the reliable generation of graphene-based nanostructures such as van der Waals heterostructures and graphene nanoribbons because their physical properties are dependent on crystal structures. However, facile and precise identification of graphene's crystallographic orientations is still challenging because the majority of current tools rely on complex atomic-scale imaging. Here, we present an identification method for the crystal orientations and grain boundaries of graphene using the directional alignment between epitaxially grown AuCN nanowires and the underlying graphene. Because the nanowires are visible in scanning electron microscopy, crystal orientations of graphene can be inspected with simple procedures. Kernel density estimation that we used in analyzing the nanowire directions enables precise measurement of graphene's crystal orientations. We also confirm that the imaged nanowires can be simply removed without degrading graphene's quality, thus showing that the present method can be practically used for measuring graphene's crystal structures.

Effects of deposition power of IGZO film and graphene layer in IGZO/graphene + Ni/SiO2/Si wafer specimens on the mechanical and electrical properties in tribotests

In the present study, indium gallium zinc oxide (IGZO) films were deposited onto graphene+Ni/SiO2/Si wafer specimens with deposition powers of 60-, 80-, and 100-W, respectively. The effects of deposition power on the microstructure and vacancy defects formed in the graphene+Ni layer are evaluated based on the electrical and mechanical properties of as-received specimens. The role and behavior of the graphene+Ni film on the electrical resistance of the IGZO film with microcracks were evaluated using a tribotester. The variations of the acceleration, ay, in the direction normal to specimen contact surface and the morphology of worn surface and the friction coefficient in a stroke are provided us to identify their wear styles including seizure, stick-slip motion, and running-in instability. After applying a number of swing cycles, the turning angle of the Ni crystalline with respect to the graphene orientation increases with increasing the number of cycles. The mean microcrack length increases with increasing the turning angle. However, the electrical resistance of specimen is not elevated by increasing the microcrack length so long as the connection of the graphene layer with the IGZO film is sustained. Increase in the deposition power promotes the growth of hillocks in the graphene+Ni layer. These hillocks make the graphene+Ni layer convex with an uneven film thickness and lead to void defects in the graphene+Ni layer, they also cause the IGZO film with relatively large mean surface roughness and high friction coefficient.

Harmonic model of corrugations of incommensurate two-dimensional layers

We developed a method to predict the height corrugations of moir\'e structures resulting from varying azimuthal orientations of graphene on hexagonal boron nitride substrates. Our model accounts for the flexural rigidity of graphene and assumes a sinusoidal corrugation of the preferred height of C atoms above the substrate. Using four parameters derived from density functional theory (DFT), the model computes corrugations of incommensurate moir\'e structures currently inaccessible to DFT with an estimated accuracy on the order of 0.01 \AA{}. We predict that azimuthally aligned graphene domains are energetically favored over rotated ones.

DC electric field induced orientation of a graphene in water

Graphene, as a classical two-dimensional material, has various excellent physical properties, which can be further transferred into its nanocomposite. Under external fields, the nonspherical nanoparticles in liquid environment will exhibit various deterministic movements, among them is the orientation behavior. By realizing the orientation control of nanoparticles, we can, on one hand, increase the thermal conductivity of the system along the oriented direction, and on the other hand, fabricate novel nano-devices based on the nanoscale self-assembly, which may become the key components in NEMS and Lab-on-a-chip architectures. However, current studies mainly focus on the orientations of one-dimensional rod-shaped particles, like carbon nanotubes. For a two-dimensional nanoparticle, like graphene, the situation is more complex than the one-dimensional one, because two unit vectors should be defined to monitor the orientation behaviors. As far as we know, this part of research has not been extensively carried out. Thus, in this paper, the molecular dynamics method is used to study the orientation of a single uncharged rectangular graphene in water, induced by DC electric fields. We track the orientations of the normal and long-side vectors of graphene. The results show that at a relatively high electric strength of 1.0 V/nm, the graphene is preferred to orient its normal vector perpendicular and its long-side vector with a small angle(located between 0° and 30°) with respect to the electric direction, respectively. With the increase of the electric field strength, the orientation preference of the normal vector along the electric direction is increased. To explain this phenomenon, we calculate the orientation distribution of water molecules in the first hydration shell. The dipoles tend to be parallel to the electric direction, and the surfaces of water molecules tend to be parallel to the surface of graphene. These two combined effects result in the above orientation behavior of the normal vector. Another interesting phenomenon is that the decrease of the length to width ratio of graphene will cause both the orientation preferences of the normal vector and the long-side vector to decrease. By utilizing the Einstein relation, we can obtain the rotational diffusion coefficients of graphene around the normal vector and long-side vector. The qualitative results show that the orientation orders of the normal vector and long-side vector respectively have negative correlations with the rotational diffusion coefficients of the rotation around the long-side vector and the normal vector. The orientation behavior of the platelike graphene actually comes from the competing effects between its rotational Brownian motion and the external field. Increasing the strength of the external field or reducing the rotational diffusivity will both lead to an increased orientation order of the nonspherical nanoparticle.

The effect of graphene nanoplatelets on the thermal and electrical properties of aluminum nitride ceramics

The thermal conductivity (κ) of AlN (2.9wt.% of Y2O3) is studied as a function of the addition of multilayer graphene (from 0 to 10 vol.%). The κ values of these composites, fabricated by spark plasma sintering (SPS), are independently analyzed for the two characteristic directions defined by the GNPs orientation within the ceramic matrix; that is to say, perpendicular and parallel to the SPS pressing axis. Conversely to other ceramic/graphene systems, AlN composites experience a reduction of κ with the graphene addition for both orientations; actually the decrease of κ for the in-plane graphene orientation results rather unusual. This behavior is conveniently reproduced when an interface thermal resistance is introduced in effective media thermal conductivity models. Also remarkable is the change in the electrical properties of AlN becoming an electrical conductor (200Sm−1) for graphene contents above 5 vol.%.

Structural and electronic properties of two-dimensional stanene and graphene heterostructure.

Structural and electronic properties of two-dimensional stanene and graphene heterostructure (Sn/G) are studied by using first-principles calculations. Various supercell models are constructed in order to reduce the strain induced by the lattice mismatch. The results show that stanene interacts overall weakly with graphene via van der Waals (vdW) interactions. Multiple phases of different crystalline orientation of stanene and graphene could coexist at room temperature. Moreover, interlayer interactions in stanene and graphene heterostructure can induce tunable band gaps at stanene’s Dirac point, and weak p-type and n-type doping of stanene and graphene, respectively, generating a small amount of electron transfer from stanene to graphene. Interestingly, for model mathrm{S}mathrm{n}left(sqrt{7}right)/mathrm{G}(5) , there emerges a band gap about 34 meV overall the band structure, indicating it shows semiconductor feature.

Contrast in nanoscale friction between rotational domains of graphene on Pt(111)

The nanoscale lubrication properties of graphene depend on chemical functionalization, adsorbants, and bonding to the substrate. The dependence of friction on the rotational orientation of graphene on a Pt(111) surface was studied by high-resolution friction force microscopy in ultra-high vacuum and interpreted through complementary simulations. Lateral forces reveal an atomic-scale stick-slip motion with the periodicity of the graphene structure. Additionally, the lateral forces were modulated by a Moiré pattern, which depends on the rotation of the respective domain. Comparing the experimental results to simulations of the friction process based on the Prandtl-Tomlinson model and to atomistic simulations of the tip-sample interactions, it was found that the modulation of lateral forces originated from a structural mismatch between Pt(111) and graphene domains that were locally pinned to the substrate. Domains with preferred orientations, such as the R1° and the R24°, appeared to exhibit lower average friction than those with orientations less frequently observed, such as the R10° domain.

Ultralow Electrical Percolation in Graphene Aerogel/Epoxy Composites

Graphene aerogels (GAs) with a highly aligned, porous structure are prepared using a novel unidirectional freeze casting method, followed by thermal reduction. The unique graphene orientation in a preferred direction is achieved due to the large temperature gradient generated during freeze casting, in which graphene oxide (GO) sheets are expelled by the rapidly advancing ice front to assemble between the aligned ice crystals. The resulting unidirectional GAs (UGAs) possess ultralow densities, high porosities, and large surface areas, as well as excellent electrical conductivities. The solid UGA/epoxy composites fabricated by vacuum-assisted infiltration of liquid epoxy present an extremely low percolation threshold of 0.007 vol %, which is the lowest value for all graphene/polymer composites reported in the literature. Besides, the anisotropic structure of UGAs gives rise to significant anisotropic electrical conductivities of UGA/epoxy composites, a potentially useful attribute for many important applica...

Online Determination of Graphene Lattice Orientation Through Lateral Forces

Rapid progress in graphene engineering has called for a simple and effective method to determine the lattice orientation on graphene before tailoring graphene to the desired edge structures and shapes. In this work, a wavelet transform-based frequency identification method is developed to distinguish the lattice orientation of graphene. The lattice orientation is determined through the different distribution of the frequency power spectrum just from a single scan line. This method is proven both theoretically and experimentally to be useful and controllable. The results at the atomic scale show that the frequencies vary with the lattice orientation of graphene. Thus, an adjusted angle to the desired lattice orientation (zigzag or armchair) can easily be calculated based on the frequency obtained from the single scan line. Ultimately, these results will play a critical role in wafer-size graphene engineering and in the manufacturing of graphene-based nanodevices.Electronic supplementary materialThe online version of this article (doi:10.1186/s11671-016-1553-z) contains supplementary material, which is available to authorized users.

Graphene-multiferroic interfaces for spintronics applications

Graphene and magnetoelectric multiferroics are promising materials for spintronic devices with high performance and low energy consumption. A very long spin diffusion length and high carrier mobility make graphene attractive for spintronics. The coupling between ferroelectricity and magnetism, which characterises magnetoelectrics, opens the way towards unique device architectures. In this work, we combine the features of both materials by investigating the interface between graphene and BaMnO3, a magnetoelectric multiferroic. We show that electron charge is transferred across the interface and magnetization is induced in the graphene sheet due to the strong interaction between C and Mn. Depending on the relative orientation of graphene and BaMnO3, a quasi-half-metal or a magnetic semiconductor can be obtained. A remarkably large proximity induced spin splitting of the Dirac cones (~300 meV) is achieved. We also show how doping with acceptors can make the high-mobility region of the electronic bands experimentally accessible. This suggests a series of possible applications in spintronics (e.g. spin filters, spin injectors) for hybrid organic-multiferroic materials and reveals hybrid organic-multiferroics as a new class of materials that may exhibit exotic phenomena such as the quantum anomalous Hall effect and a Rashba spin-orbit induced topological gap.

Magnetic assembly of transparent and conducting graphene-based functional composites.

Innovative methods producing transparent and flexible electrodes are highly sought in modern optoelectronic applications to replace metal oxides, but available solutions suffer from drawbacks such as brittleness, unaffordability and inadequate processability. Here we propose a general, simple strategy to produce hierarchical composites of functionalized graphene in polymeric matrices, exhibiting transparency and electron conductivity. These are obtained through protein-assisted functionalization of graphene with magnetic nanoparticles, followed by magnetic-directed assembly of the graphene within polymeric matrices undergoing sol–gel transitions. By applying rotating magnetic fields or magnetic moulds, both graphene orientation and distribution can be controlled within the composite. Importantly, by using magnetic virtual moulds of predefined meshes, graphene assembly is directed into double-percolating networks, reducing the percolation threshold and enabling combined optical transparency and electrical conductivity not accessible in single-network materials. The resulting composites open new possibilities on the quest of transparent electrodes for photovoltaics, organic light-emitting diodes and stretchable optoelectronic devices.

Lattice orientation and crack size effect on the mechanical properties of Graphene

The effect of lattice orientation and crack length on the mechanical properties of Graphene are studied based on molecular dynamics simulations. Bond breaking and crack initiation in an initial edge crack model with 13 different crack lengths, in 10 different lattice orientations of Graphene are examined. In all the lattice orientations, three recurrent fracture patterns are reported. The influence of the lattice orientation and crack length on yield stress and yield strain of Graphene is also investigated. The arm-chair fracture pattern is observed to possess the lowest yield properties. A sudden decrease in yield stress and yield strain can be noticed for crack sizes < 10 nm. However, for larger crack sizes, a linear decrease in yield stress is observed, whereas a constant yield strain of 0.05 is noticed. Therefore, the yield strain of 0.05 can be considered as a critical strain value below which Graphene does not show failure. This information can be utilized as a lower bound for the design of nano-devices for various strain sensor applications. Furthermore, the yield data will be useful while developing the Graphene coating on Silicon surface in order to enhance the mechanical and electrical characteristics of solar cells and to arrest the growth of micro-cracks in Silicon cells.

Vibronic interactions proceeding from combined analytical and numerical considerations: Covalent functionalization of graphene by benzene, distortions, electronic transitions.

Chemically bound states of benzene molecules with graphene are studied both analytically and numerically. The states are formed by switching off intrabonds of π-electrons in C6 rings to interbonds. A number of different undistorted and distorted structures are established both with aligned and with transversal mutual orientation of benzene and graphene. The vibronic interactions causing distortions of bound states are found, by using a combination of analytical and numerical considerations. This allows one to determine all electronic transitions of π-electrons without explicit numerical calculations of excited states, to find the conical intersections of potentials, and to show that the mechanism of distortions is the pseudo-Jahn-Teller effect. It is found that the aligned distorted benzene molecule placed between two graphene sheets makes a chemical bond with both of them, which may be used for fastening of graphene sheets together.

Monte Carlo simulations of effective electrical conductivity of graphene/poly(methyl methacrylate) nanocomposite: Landauer-Buttiker approach

Three-dimensional Monte Carlo simulation was used to investigate the percolation phenomenon and electrical conductivity of graphene/poly(methyl methacrylate) (PMMA) nanocomposite. The electrical conductivity of this nanocomposite was investigated based on three-dimensional resistor network. By employing Landauer-Buttiker (L-B) formula for calculating tunneling resistance of a graphene-polymer-graphene junction and the intrinsic resistance of graphene sheets, transmission probabilities were estimated by scattering theory and Wentzel-Kramers-Brillouin approximation. Effects of cubic representative volume element size, tunneling distance, graphene diameter, orientation of graphene, and image potential on conductivities of graphene/PMMA nanocomposite were studied using this model. The simulation results showed that, as tunneling distance increased, volume fraction of graphenes required for creating an electrically conductive model was decreased. Also, the nanocomposite with a larger diameter of graphene had lower percolation threshold and lower conductivity. Graphene disperse in polymer with normal vector parallel to the electric field had minimum conductivity and maximum percolation threshold. Taking into account the effect of image potential increased the conductivity of graphene/PMMA and decreased percolation threshold.

Morphology Effect of Vertical Graphene on the High Performance of Supercapacitor Electrode.

Graphene and its composites are widely investigated as supercapacitor electrodes due to their large specific surface area. However, the severe aggregation and disordered alignment of graphene sheets hamper the maximum utilization of its surface area. Here we report an optimized structure for supercapacitor electrode, i.e., the vertical graphene sheets, which have a vertical structure and open architecture for ion transport pathway. The effect of morphology and orientation of vertical graphene on the performance of supercapacitor is examined using a combination of model calculation and experimental study. Both results consistently demonstrate that the vertical graphene electrode has a much superior performance than that of lateral graphene electrode. Typically, the areal capacitances of a vertical graphene electrode reach 8.4 mF/cm(2) at scan rate of 100 mV/s; this is about 38% higher than that of a lateral graphene electrode and about 6 times higher than that of graphite paper. To further improve its performance, a MnO2 nanoflake layer is coated on the surface of graphene to provide a high pseudocapacitive contribution to the overall areal capacitance which increases to 500 mF/cm(2) at scan rate of 5 mV/s. The reasons for these significant improvements are studied in detail and are attributed to the fast ion diffusion and enhanced charge storage capacity. The microscopic manipulation of graphene electrode configuration could greatly improve its specific capacitance, and furthermore, boost the energy density of supercapacitor. Our results demonstrate that the vertical graphene electrode is more efficient and practical for the high performance energy storage device with high power and energy densities.

Microstructural analysis of aluminum nitride – GPLs composites

The study concerns a new material with an aluminum nitride matrix and a graphene additive. The material was obtained by the hot-pressing method. The material prepared in this way shows the anisotropy of the microstructure. This study focuses mainly on the microstructure of the material. The research contains the microstructure observation of sintered bodies, examination of the phase composition and identification of phases in the composites using the EDS method. Additional SEM observations of generated material fractures were made. Microstructural images were taken using computer-aided analysis, which gave the following parameters: coefficient of particle orientation, phases content, particle size. Such investigation was fulfilled by ultrasonic measurement of an anisotropy of longitudinal wave velocity, which gave an answer to the graphene orientation in the composites.

Thermal properties of pressure sintered alumina–graphene composites

The work concerns the alumina-graphene ma- terials sintered by two different pressure methods. The different particle sizes of graphene were used. The prepa- ration route of the matrix-graphene mixture was discussed in the paper. The so-prepared compositions with different amount of graphene were hot-pressed and spark plasma sintered. The influence on uniaxial pressure during the sintering process on the microstructure was presented by the SEM microstructural observations and ultrasonic measurements. The material with unidirectional oriented graphene particles was prepared, and the anisotropy was even higher than 30 % for 10 mass% of graphene additive. The influence of graphene orientation as an effect of pressing process on the thermal properties was analysed. The anisotropy of thermal conductivity was 90 % for 10 mass% of graphene. The thermal diffusivity and ther- mal conductivity of composites manufactured by hot- pressing and spark plasma sintering method were com- pared. The experiment-based calculation of the specific heat versus temperature was presented in the paper. The thermal expansion coefficient was determined by dilato- metric method. The thermal stability was analysed by thermogravimetric method, and it showed that composites with up to 2 mass% of graphene can work at temperatures higher than 700 � C.