Orientation Of Graphene Research Articles

Towards Wafer-Scale Monocrystalline Graphene Growth and Characterization.

Since its discovery in 2004, graphene has boosted numerous fundamental sciences and technological applications due to its massless Dirac particle-like linear band dispersion, that causes unprecedented physical properties. Among the various methods for synthesizing graphene, chemical vapor deposition is the most suitable approach for scalable production on a wafer scale, which is a critical step for practical applications. Graphene grain boundaries (GGBs), consisting of nonhexagonal carbon rings and therefore modulating the properties of graphene films, are inevitably formed via the merging of adjacent graphene domains with different orientations. Large-area monocrystalline graphene synthesis without forming GGBs has been challenging, let alone observing such boundaries. Here, an up-to-date review is presented of how to grow wafer-scale monocrystalline graphene without GGBs. One approach is to make single domain sizes as large as possible by reducing or passivating the number of nucleation sites. Another approach is to align graphene domains in identical orientations, and then merge them atomically. The recently developed methods for observing graphene orientation and GGBs both at the atomic and macro-scales are also presented. Finally, perspectives for future research in graphene growth are discussed.

Quantitative determination of the spatial orientation of graphene by polarized Raman spectroscopy

Polarized Raman spectroscopy has been employed to characterize transverse sections of graphene monolayers upon both a copper substrate and a polyester film. Well-defined Raman spectra can be obtained from the one atom thick transverse sections of graphene because of the strong resonance Raman scattering. The intensity of Raman 2D band (I2D) is independent of the axis of laser polarization when the laser beam is perpendicular to the surface of the graphene monolayer but I2D is found to vary as approximately the 4th power of the cosine of the angle between the axis of laser polarization and the plane of graphene when the direction of laser propagation is parallel to the graphene sheet. It is demonstrated that a generalized spherical expanded harmonics orientation distribution function (ODF) can be used to quantify the spatial orientation of the graphene. The roughness of the graphene, evaluated using atomic force microscopy, shows a good correlation with the ODF determined using polarized Raman spectroscopy, showing how the Raman technique may be employed to quantify the spatial orientation of graphene. It is also demonstrated how the technique can be used to quantify the orientation of graphene in high-ordered pyrolytic graphite and graphene paper.

Synthesis and characterization of a poly-tetraaniline-urethane/Ag-nanowire or/graphene conductive elastomer

Tetraaniline-containing poly(urethane-urea) (TAPU), a new material, was synthesized in this study and blended with Ag nanowire (≤ 3 wt.%), denoted as TAPU/A, or graphene (≤ 3 wt.%), denoted as TAPU/G, to form a conductive elastomer. The conductivities can be improved from less than 10−10 S/cm, for pure PU, to 6 × 10−2 S/cm, for the TAPU/A 3 %. The tensile strength and modulus were increased twofold and 20- to 60-fold, respectively, for TAPU/A 3 % or TAPU/G 3 %. The viscoelastic creep was simulated effectively using a Burgers model. Additionally, TAPU/A and TAPU/G demonstrate a higher viscosity, a higher retardation time, and a lower compliance D 1 than regular TAPU does. TAPU/Ag and TAPU/G exhibit less elasticity but more insignificant permanent deformation than TAPU does, because the additives function as a chain holder. The stretching tension induces the orientation of silver nanowire or graphene. Such orientation is believed to affect electrical conductivity positively. The solid form of the elastomer, TAPU/A or TAPU/G, can be heated and dissolved in a solvent to be reshaped, indicating that this compound is uniquely reworkable.

Correlation between micrometer-scale ripple alignment and atomic-scale crystallographic orientation of monolayer graphene.

Deformation normal to the surface is intrinsic in two-dimensional materials due to phononic thermal fluctuations at finite temperatures. Graphene's negative thermal expansion coefficient is generally explained by such an intrinsic property. Recently, friction measurements on graphene exfoliated on a silicon oxide surface revealed an anomalous anisotropy whose origin was believed to be the formation of ripple domains. Here, we uncover the atomistic origin of the observed friction domains using a cantilever torsion microscopy in conjunction with angle-resolved photoemission spectroscopy. We experimentally demonstrate that ripples on graphene are formed along the zigzag direction of the hexagonal lattice. The formation of zigzag directional ripple is consistent with our theoretical model that takes account of the atomic-scale bending stiffness of carbon-carbon bonds and the interaction of graphene with the substrate. The correlation between micrometer-scale ripple alignment and atomic-scale arrangement of exfoliated monolayer graphene is first discovered and suggests a practical tool for measuring lattice orientation of graphene.

Formation of Graphene Grain Boundaries on Cu(100) Surface and a Route Towards Their Elimination in Chemical Vapor Deposition Growth

Grain boundaries (GBs) in graphene prepared by chemical vapor deposition (CVD) greatly degrade the electrical and mechanical properties of graphene and thus hinder the applications of graphene in electronic devices. The seamless stitching of graphene flakes can avoid GBs, wherein the identical orientation of graphene domain is required. In this letter, the graphene orientation on one of the most used catalyst surface — Cu(100) surface, is explored by density functional theory (DFT) calculations. Our calculation demonstrates that a zigzag edged hexagonal graphene domain on a Cu(100) surface has two equivalent energetically preferred orientations, which are 30 degree away from each other. Therefore, the fusion of graphene domains on Cu(100) surface during CVD growth will inevitably lead to densely distributed GBs in the synthesized graphene. Aiming to solve this problem, a simple route, that applies external strain to break the symmetry of the Cu(100) surface, was proposed and proved efficient.

Tunable MoS2 bandgap in MoS2-graphene heterostructures

Using density functional theory calculations with van der Waals corrections, we investigated how the interlayer orientation affects the structure and electronic properties of MoS2-graphene bilayer heterostructures. Changing the orientation of graphene with respect to MoS2 strongly influences the type and the value of the electronic bandgap in MoS2, while not significantly altering the binding energy between the layers or the interlayer spacing. We show that the physical origin of this tunable bandgap arises from variations in the S–S interplanar distance (MoS2 thickness) with the interlayer orientation, variations which are caused by electron transfer away from the Mo–S bonds.

The crystal orientation relation and macroscopic surface roughness in hetero-epitaxial graphene grown on Cu/mica

Clean, flat and orientation-identified graphene on a substrate is in high demand for graphene electronics. In this study, the hetero-epitaxial graphene growth on Cu(111)/mica(001) by chemical vapor deposition is investigated to check the applicability for top-gate insulator research on graphene, as well as graphene channel research, by transferring graphene on to SiO2/Si substrates. After adjusting the graphene growth conditions, the surface roughness of the graphene/Cu/mica substrate and the average smoothed areas are ∼0.34 nm and ∼100 μm2, respectively. The orientation of graphene in the graphene/Cu/mica substrate can be identified by the hexagonal void morphology of Cu. Moreover, we demonstrate a relatively high mobility of ∼4500 cm2 V−1 s−1 in graphene transferred on the SiO2/Si substrate. These results suggest that the present graphene/Cu/mica substrate can be used for top-gate insulator research on graphene.

Friction anisotropy dependence on lattice orientation of graphene

The observation of friction anisotropy on graphene by friction measurement at atomic scale has been reported in this paper. Atomic-scale friction measurement revealed friction anisotropy with a periodicity of 60°, which is consistent with the hexagonal periodicity of the graphene. Both experiments and theory show that the value of the friction force is related to the graphene lattice orientation, and the friction force along armchair orientation is also larger than the one along zigzag orientation. These results will play a critical role in the use of graphene to manufacture nanoscale devices.

Chemically Engineered Graphene-Based 2D Organic Molecular Magnet

Carbon-based magnetic materials and structures of mesoscopic dimensions may offer unique opportunities for future nanomagnetoelectronic/spintronic devices. To achieve their potential, carbon nanosystems must have controllable magnetic properties. We demonstrate that nitrophenyl functionalized graphene can act as a room-temperature 2D magnet. We report a comprehensive study of low-temperature magnetotransport, vibrating sample magnetometry (VSM), and superconducting quantum interference (SQUID) measurements before and after radical functionalization. Following nitrophenyl (NP) functionalization, epitaxially grown graphene systems can become organic molecular magnets with ferromagnetic and antiferromagnetic ordering that persists at temperatures above 400 K. The field-dependent, surface magnetoelectric properties were studied using scanning probe microscopy (SPM) techniques. The results indicate that the NP-functionalization orientation and degree of coverage directly affect the magnetic properties of the graphene surface. In addition, graphene-based organic magnetic nanostructures were found to demonstrate a pronounced magneto-optical Kerr effect (MOKE). The results were consistent across different characterization techniques and indicate room-temperature magnetic ordering along preferred graphene orientations in the NP-functionalized samples. Chemically isolated graphene nanoribbons (CINs) were observed along the preferred functionality directions. These results pave the way for future magnetoelectronic/spintronic applications based on promising concepts such as current-induced magnetization switching, magnetoelectricity, half-metallicity, and quantum tunneling of magnetization.

A comparison of electronic transport properties of graphene with hexagonal boron nitride substrate and graphane, a first principle study

In this paper, a first-principle investigation of the electronic properties of graphene on hexagonal boron nitride substrate, and of one-sided and two-sided fully saturated hydrocarbons with C-H formula derived from a single sheet of graphene, tablelike and chairlike graphane, are presented within density functional theory (DFT). We obtain the most stable orientation of graphene on the substrate, the adsorption energy, the charge transfer and density of states (DOS) for these systems. We discuss the changes in the density of states as well as the extent of charge transfer, band gap and finally quantum conductivity and current for graphene due to the presence of the substrate and H atoms. We show that the band gap of 64 meV induced by the BN substrate can greatly improve the electrical characteristics of graphene-based field effect transistors (FETs) and its on/off ratio and decreases the minimum conductance by a factor three. We identify that the substrate is acting as a donor for graphene layer and graphene is acting as an acceptor with respect to H atoms after saturation with hydrogen. We show that graphene on h-BN substrate has higher on/off ratio respect to pristine graphene and higher conductance respect to tablelike graphane.

Facile Preparation of Lightweight Microcellular Polyetherimide/Graphene Composite Foams for Electromagnetic Interference Shielding

We report a facile approach to produce lightweight microcellular polyetherimide (PEI)/graphene nanocomposite foams with a density of about 0.3 g/cm3 by a phase separation process. It was observed that the strong extensional flow generated during cell growth induced the enrichment and orientation of graphene on cell walls. This action decreased the electrical conductivity percolation from 0.21 vol % for PEI/graphene nanocomposite to 0.18 vol % for PEI/graphene foam. Furthermore, the foaming process significantly increased the specific electromagnetic interference (EMI) shielding effectiveness from 17 to 44 dB/(g/cm3). In addition, PEI/graphene nanocomposite foams possessed low thermal conductivity of 0.065-0.037 W/m·K even at 200 °C and high Young's modulus of 180-290 MPa.

Orientation-dependent binding energy of graphene on palladium

Using density functional theory calculations, we show that the binding strength of a graphene monolayer on Pd(111) can vary between physisorption and chemisorption depending on its orientation. By studying the interfacial charge transfer, we have identified a specific four-atom carbon cluster that is responsible for the local bonding of graphene to Pd(111). The areal density of such clusters varies with the in-plane orientation of graphene, causing the binding energy to change accordingly. Similar investigations can also apply to other metal substrates and suggests that physical, chemical, and mechanical properties of graphene may be controlled by changing its orientation.

Weak mismatch epitaxy and structural Feedback in graphene growth on copper foil

Abstract Graphene growth by low-pressure chemical vapor deposition on low cost copper foils shows great promise for large scale applications. It is known that the local crystallography of the foil influences the graphene growth rate. Here we find an epitaxial relationship between graphene and copper foil. Interfacial restructuring between graphene and copper drives the formation of (n10) facets on what is otherwise a mostly Cu(100) surface, and the facets in turn influence the graphene orientations from the onset of growth. Angle resolved photoemission shows that the electronic structure of the graphene is decoupled from the copper indicating a weak interaction between them. Despite this, two preferred orientations of graphene are found, ±8° from the Cu[010] direction, creating a non-uniform distribution of graphene grain boundary misorientation angles. Comparison with the model system of graphene growth on single crystal Cu(110) indicates that this orientational alignment is due to mismatch epitaxy. Despite the differences in symmetry the orientation of the graphene is defined by that of the copper. We expect these observations to not only have importance for controlling and understanding the growth process for graphene on copper, but also to have wider implications for the growth of two-dimensional materials on low cost metal substrates.

Grain boundaries orientation effects on tensile mechanics of polycrystalline graphene

Molecular dynamics simulations were performed to investigate how the orientation of grain boundary (GB) affects the tensile mechanics of polycrystalline graphene, where two opposite GB groups, i.e., armchair (AC) and zigzag (ZZ)-oriented tilted GBs were considered for the anisotropic study. We found very close mechanical similarities between the two groups in misorientation angle effect and critical bond length effect to determine the tensile strength. Mono-atomic carbon chains (MACCs) were commonly generated at tensile failure in both groups, as bridged between fractured sections, yielding the considerably higher population density and achievable length (4.51 nm−2 and 1.47 nm, maximally) compared to pristine graphene. Notably, we found that polycrystalline graphene exhibited distinctly different behaviors in this MACC production depending on GB orientation, being 1.2–3.0 times denser and 1.6–5.0 times longer for ZZ-oriented GBs. Atomic stress analyses indicated that all key reactions emerging before tensile failure would not be affected by the GB orientation of polycrystalline graphene since the reactions only occurred along GBs, explaining why anisotropic mechanical GB response has not been observed so far, in contrast to the MACC dynamics occurring after tensile failure.

Preferred armchair edges of epitaxial graphene on 6H-SiC(0001) by thermal decomposition

Scanning tunneling microscopy is used to study the edge orientation of graphene fabricated by thermal decomposition of 6H-SiC. The exploration on the atomically resolved structures and the patterns in reciprocal space demonstrates that the armchair direction is always parallel to the basic vector of 6 × 6 reconstruction as well as the close-packed direction of 6H-SiC substrate. This can be used as the criterion to characterize the edge direction of graphene. With this method, it is found that armchair edges are preferred in both monolayer and bilayer regions. This special edge certainly will affect the electronic states and consequently the properties.

Highly coherent orientations of graphene on non-reconstructed silicon substrates

Silicon photovoltaics may benefit from the use of graphene by exploiting it both as a semitransparent electrode and an antireflection coating. It is well-known that various physical properties of interfaces are largely controlled by the interface crystallography. Highly coherent orientations of graphene on non-reconstructed Si (100) and Si (111) substrates are identified. The highly coherent junctions are likely to have distinguished physical properties that would give additional possibilities to optimize related devices in photovoltaic and other applications.

Cutting forces related with lattice orientations of graphene using an atomic force microscopy based nanorobot

The relationship between cutting forces and lattice orientations of monolayer graphene is investigated by using an atomic force microscopy (AFM) based nanorobot. In the beginning, the atomic resolution image of the graphene lattice is obtained by using an AFM. Then, graphene cutting experiments are performed with sample rotation method, which gets rid of the tip effect completely. The experimental results show that the cutting force along the armchair orientation is larger than the force along the zigzag orientation, and the cutting forces are almost identical every 60°, which corresponds well with the 60° symmetry in graphene honeycomb lattice structure. By using Poisson analysis method, the single cutting force along zigzag orientation is 3.9 nN, and the force along armchair is 20.5 nN. This work lays the experimental foundation to build a close-loop fabrication strategy with real-time force as a feedback sensor to control the cutting direction.

How the Orientation of Graphene Is Determined during Chemical Vapor Deposition Growth

We present a theoretical study on the determination of graphene orientation on the catalyst surface in chemical vapor deposition growth. Our study reveals that the interaction between the graphene wall and catalyst surface is weak and not sensitive to the orientation of graphene. The graphene edge–catalyst interaction is strong and sensitively depends on the graphene orientation. Therefore, the graphene edge–catalyst interaction is responsible for the orientation determination of a small graphene island in the early stage of graphene growth, and such an orientation can be inherited by the matured graphene due to the high barrier of graphene island rotation. On the basis of the mechanism of graphene orientation determination, various controversial-like experimental puzzles have been well-explained, and a potential of synthesizing large-area single-crystalline graphene on either single-crystalline or polycrystalline catalyst surfaces is revealed.

Mapping of the atomic lattice orientation of a graphite flake using macroscopic liquid crystal texture

The electronic properties of graphene depend critically on its lattice orientation and edge type. However, it is very difficult to identify them, and they are accessible only using sophisticated tools. In this paper, we show an easy and reliable way to reveal the lattice orientation and edge type of graphene and graphite flakes, i.e. multi-layered graphene. Nematic liquid crystals have the potential to align themselves into three symmetric and equivalent orientations on crystalline graphite. The director of macroscopic texture due to the elasticity indicates the lattice orientation of the top graphite layer. By analyzing the director orientation using a polarizing optical microscope, we were able to show the lattice orientation, chiral angle and edge type of graphene and graphite flakes on the macroscopic scale. As liquid crystals are soft and easily removable, our technique has little chance of influencing the following processes for graphene manipulation.

Graphene domain boundaries on Pt(111) as nucleation sites for Pt nanocluster formation

The growth of Pt nanoclusters on a graphene layer on Pt(111) was studied with ultra high vacuum scanning tunneling microscopy. Different periodicities in the moiré patterns of the graphene layer are observed corresponding to different orientations with respect to the Pt(111) lattice. Various graphene orientations are possible because of a relatively weak graphene–Pt interaction. Following Pt deposition onto the graphene-covered surface, small Pt nanoclusters are observed to preferentially form along the moiré domain boundaries. The weak interaction of graphene with Pt(111) leads to a weak corrugation in the superlattice compared to other transition metals, such as Ru, but it is found even this weak corrugation is sufficient to serve as a template for the formation of mono-dispersed one-dimensional Pt nanocluster chains. These Pt nanoclusters are relatively stable and only undergo agglomeration at annealing temperatures above 600K.