Orientation Of Graphene Layers Research Articles

Probing symmetry-breaking defects in polished graphitizable sp2 carbons using angle-resolved polarized Raman scattering

Raman scattering is commonly used to quantify defects in $s{p}^{2}$ carbons, mostly based on the values of the double-resonant D-band intensity. It is well known that this peculiar band undergoes changes when the $s{p}^{2}$ carbon is polished. However, the physical process behind this phenomenon is unclear so far. In this paper, we provide a deep understanding of the fundamental optical properties of the polishing-induced defects by combining angle-resolved polarized Raman scattering and theoretical calculations. We address the question regarding the types of these additional defects that participate in the intervalley double-resonance Raman process and contribute to the spatially inhomogeneous increase of the relative D-band intensity. The study was performed on anthracene-based coke pyrolyzed at $2000\phantom{\rule{0.16em}{0ex}}{}^{\ensuremath{\circ}}\mathrm{C}$ and uses highly oriented pyrolytic graphite with known crystallographic orientations as a model material. The results shed light on the types of defects as well as the D-band peculiar behavior as a function of the spatial orientation of graphene layers dictated by polishing with respect to the fixed direction of the incident and scattered light. Moreover, the insignificant change in ${I}_{2\mathrm{D}}/{I}_{\mathrm{G}}$, the width and the position of the G band after polishing, and the insensitivity of less ordered carbons to this process are also discussed.

X‐Ray Spectromicroscopy Investigation of Heterogeneous Sodiation in Hard Carbon Nanosheets with Vertically Oriented (002) Planes

Hard carbon (HC) is a promising anode material for sodium-ion batteries, but the performance remains unsatisfactory and the sodiation mechanism in HC is one of the most debated topics. Here, from self-assembled cellulose nanocrystal sheets with crystallographic texture, unique HC nanosheets with vertically oriented (002) planes are fabricated and used as a model HC to investigate the sodiation mechanisms using synchrotron scanning transmission X-ray microscopy (STXM) coupled with analytical transmission electron microscopy (TEM). The model HC simplifies the 3D sodiation in a typical HC particle into a 2D sodiation, which facilitates the visualization of phase transformation at different states of charge. The results for the first time unveil that the sodiation in HC initiates heterogeneously, with multiple propagation fronts proceeding simultaneously, eventually merging into larger aggregates. The spatial correlation between the preferential adsorption and nucleation sites suggests that the heterogeneous nucleation is driven by the local Na-ion concentration, which is determined by defects or heteroatoms that have strong binding to Na ions. By identifying intercalation as the dominant sodium storage mechanism in the model HC, the findings highlight the importance of engineering the graphene layer orientation and the structural heterogeneity of edge sites to enhance the performances. Figure 1

Influence of layers orientation of graphene stacks in shungite disordered carbon to its integral electrical conductivity

The map of carbon distribution in the sample of graphene-containing shungite is investigated by the electron microscopy. It is found the large quantity of stacks which are formatted by parallel disposed layers of graphene. The model of electrical conductivity of graphene stack is proposed. This model takes into consideration the arbitrary orientation of layers relatively to the direction of current. It is made the numerical calculation of dependence of conductivity from the orientation of graphene layers. It is found the sharp abatement of conductivity by the orientation of graphene layers relatively to the current direction along the corner near the 45 degree. The obtained peculiarities are explained by the influence of geometrical factor.

Nanoscale imaging of oxidized copper foil covered with chemical vapor deposition‐grown graphene layers

Characterization of the geometrical and structural characteristics of oxidized Cu area in high resolution is crucial for tracking the change in morphology, exploring interactions between graphene layers and Cu substrates and revealing the mechanism for the orientation‐dependent oxidation of Cu. Here, we reported experimental results on nanoscale imaging of natural oxidation of the polycrystalline Cu substrate coated by partial‐coverage chemical vapor deposition (CVD)‐grown graphene stored in dryer under ambient conditions for up to 10 months. Scanning electron microscope (SEM), together with atomic force microscope (AFM), Raman, and X‐ray photoelectron spectroscopy (XPS), was used for systematically studying the morphological and compositional changes at nanoscale during oxidation. The appearance of oxidized Cu substrates could be unambiguously distinguished from the unoxidized regions based on their distinctly different morphologies in SEM images, and the underlying mechanism was discussed in detail. By analyzing a millimeter‐seized polycrystalline Cu substrate, we found that the oxidation of polycrystalline Cu substrate depends sensitively on both orientation of graphene layers and Cu substrates. Furthermore, the time‐dependent oxidation evolution of Cu substrate was also established, and the oxidation rate was readily determined. The findings reported here will have important implications for developing protection coatings for Cu.

Nanoscale Imaging of Oxidized Copper Foil Covered with CVD-Grown Graphene Layers

Orientation-dependent oxidation of Cu substrates covered by CVD-grown graphene under ambient conditions has been intensively studied. However, the effect of orientation of graphene layers has been largely overlooked. Here, we reported experimental results on natural oxidation of the polycrystalline Cu substrate coated by partial-coverage CVD-grown graphene stored in dryer under ambient conditions for up to 10 months. SEM, together with AFM, Raman and XPS, was used for systematically studying the morphological and compositional changes at nanoscale during oxidation. Oxidation of the polycrystalline Cu substrate depends sensitively on both orientation of graphene layers and Cu substrates as revealed by the distinctly different morophologies between oxidized and unoxidized regions in SEM images. The time dependence of oxidation for Cu was established. The findings reported here will have important implications for developing protection coatings for Cu.

X-Ray Spectromicroscopy Investigation of Heterogeneous Sodiation in Hard Carbon Nanosheets with Vertically Oriented (002) Planes.

Hard carbon (HC) is a promising anode material for sodium-ion batteries, but the performance remains unsatisfactory and the sodiation mechanism in HC is one of the most debated topics. Here, from self-assembled cellulose nanocrystal sheets with crystallographic texture, unique HC nanosheets with vertically oriented (002) planes are fabricated and used as a model HC to investigate the sodiation mechanisms using synchrotron scanning transmission X-ray microscopy (STXM) coupled with analytical transmission electron microscopy (TEM). The model HC simplifies the 3D sodiation in a typical HC particle into a 2D sodiation, which facilitates the visualization of phase transformation at different states of charge. The results for the first time unveil that the sodiation in HC initiates heterogeneously, with multiple propagation fronts proceeding simultaneously, eventually merging into larger aggregates. The spatial correlation between the preferential adsorption and nucleation sites suggests that the heterogeneous nucleation is driven by the local Na-ion concentration, which is determined by defects or heteroatoms that have strong binding to Na ions. By identifying intercalation as the dominant sodium storage mechanism in the model HC, the findings highlight the importance of engineering the graphene layer orientation and the structural heterogeneity of edge sites to enhance the performances.

Graphene-based polymer composite films with enhanced mechanical properties and ultra-high in-plane thermal conductivity

Graphene has very high electrical and thermal conductivities and thus is a promising candidate for use as a filler to enhance the conductivity of polymer composites. The main challenge is properly dispersing and aligning graphene nanoflakes (GNFs) within a polymer matrix. We report here a simple and scalable solution mixing and molding process to make such a composite film. These films were analyzed using SEM, ATR-FTIR, XRD, DSC, and TGA. An optical tensiometer and a laser flash analyzer were used to measure the water contact angle and in-plane thermal diffusivity of the films, respectively. The poly(vinylidene fluoride-co-hexafluoropropylene) composite films had an in-plane thermal conductivity (κ) that reached a new record of ~25 W m−1 K−1 at a GNF concentration of 20 wt%. The presence of GNFs had a noticeable effect on the surface morphology, crystal structure, and hydrophobicity of the polymer matrix. The tensile strength and Young's modulus of the composite films increased by the addition of GNFs up to 20 wt%. The composite films showed very high electrical conductivity due to the presence of highly conductive graphene layers. This manufacturing process ensured the in-plane orientation of graphene layers, which allowed the transport of phonons and electrons through the composite films.

Effect of Liquid Crystalline Texture of Mesophase Pitches on the Structure and Property of Large-Diameter Carbon Fibers.

Two types of carbon fibers with a large diameter of ∼22 μm, derived from unstirred and vigorously stirred mesophase pitch melts with different liquid crystalline mesophase textures, were prepared by melt-spinning, stabilization, carbonization, and graphitization treatments. The morphology, microstructure, and physical properties of the carbon fibers derived from the two kinds of mesophase precursors after various processes were characterized in detail. The results show that the optical texture (i.e., size and orientation) of the liquid crystalline mesophase in the molten pitch is obviously modified by thermomechanical stirring treatment, which has a significant effect on the texture of as-spun pitch fibers, and finally dominates the microstructure and physical properties of the resulting carbon and graphite fibers. These large-diameter fibers expectedly maintain their morphological and structural integrity and effectively avoid shrinkage cracking during subsequent high-temperature heat treatment processes, in contrast to those derived from the unstirred pitch. This is due to the smaller crystallite sizes and lower orientation of graphene layers in the former. The tensile strength and axial electrical resistivity of the 3000 °C-graphitized large fibers derived from the unstirred pitch are about 1.8 GPa and 1.18 μΩ m, respectively. In contrast, upon melt stirring treatment of the pitch before spinning, the resulting large-diameter graphite fibers possess the corresponding values of 1.3 GPa and 1.86 μΩ m. Despite the acceptable decrease of mechanical properties and axial electrical and thermal conduction performance, the latter possesses relatively high mechanical stability (i.e., low strength deviation) and ideal morphological and structural integrity, which is beneficial for the wide applications in composites.

ReaxFF Molecular Dynamics Simulation for the Graphitization of Amorphous Carbon: A Parametric Study.

A parametric study of ReaxFF for molecular dynamics simulation of graphitization of amorphous carbon was conducted. The responses to different initial amorphous carbon configurations, simulation time steps, simulated temperatures, and ReaxFF parameter sets were investigated. The results showed that a time step shorter than 0.2 fs is sufficient for the ReaxFF simulation of carbon using both Chenoweth 2008 and Srinivasan 2015 parameter sets. The amorphous carbon networks produced using both parameter sets at 300 K are similar to each other, with the first peak positions of pair distribution function curves located between the graphite sp2 bond peak position and the diamond sp3 bond peak position. In the graphitization process, the graphene fragment size increases and the orientation of graphene layers transforms to be parallel with each other with the increase of temperature and annealing time. This parallel graphene structure is close to the crystalline graphite. Associated with this graphitization is the presence of small voids and pores which arise because of the more efficient atomic packing relative to a disordered structure. For all initial densities, both potential parameter sets exhibit the expected behavior in which the sp2 fraction increases significantly over time. The sp2 fraction increases with increasing temperature. The differences of sp2 fraction at different temperatures are more obvious in lower density at 1.4 g/cm3. When density is increased, the gap caused by different temperatures becomes small. This study indicates that both Chenoweth 2008 and Srinivasan 2015 potential sets are appropriate for molecular dynamics simulations in which the growth of graphitic structures is investigated.

Anisotropy of functional properties of SiC composites with GNPs, GO and in-situ formed graphene

This paper reports on anisotropy of functional properties of different silicon carbide-graphene composites due to preferential orientation of graphene layers during sintering. Dense silicon carbide/graphene nanoplatelets (SiC/GNPs) and silicon carbide/graphene oxide (SiC/GO) composites were sintered in the presence of yttria (Y2O3) and alumina (Al2O3) sintering additives at 1800°C in vacuum by the rapid hot pressing (RHP) technique. It is found that electrical conductivity of SiC/GNPs and SiC/GO composites increases significantly in the perpendicular direction to the RHP pressing axis, reached up to 1775S/m in the case of SiC/GO (for 3.15wt.% of rGO). Also, thermal diffusivity was found to increase slightly by the addition of GNPs in the SiC/GNPs composites in the perpendicular direction to the RHP pressing axis. But, in the parallel direction, the addition of GNPs showed a negative effect. The formation of graphene domains was observed in reference sample SiC-Y2O3-Al2O3 sintered by RHP, without any addition of graphene. Their presence was confirmed indirectly by increasing electrical conductivity about three orders of magnitude in comparison to the reference sample sintered by conventional hot press (HP). Raman, SEM and TEM analysis were used for direct evidence of presence of graphene domains in RHP reference sample.

Size effect of graphene nanoparticle modified epoxy matrix

The size effect of unmodified and graphene nanoparticle modified matrix fibres is experimentally investigated. Neat matrix fibres show a clear size effect of increasing tensile strength with decreasing volume due to a statistical defect distribution. The nanoparticle modified matrix shows no significant size effect. Nanoparticles act as crack initiators and consume fracture energy. The size of the particles is independent of specimen volume, so that the failure initiating as well as energy absorbing mechanisms are available, independently of the volume. Fractography analysis of SEM images shows different energy dissipation mechanisms such as micro-damage at the graphene particles. Graphene pull-out, layer separation, layer shearing, formation of micro voids as well as crack separation and crack bifurcation are observed that depend on the orientation of the graphite layers to the fracture plane. These mechanisms dissipate energy and so that a graphene nanoparticle modification result in an increased fracture toughness and thus increased strength of an epoxy matrix system if the volume is large enough. The maximum stress in specimen of small volume depends on graphene layer orientation, so that ideally, the covalent bonds of the nanoparticles should be orientated in loading direction.

Graphene nanocluster decorated niobium oxide nanofibers for visible light photocatalytic applications

In this paper, we report a novel nanocomposite of graphene nanocluster decorated Nb2O5 nanofibers. With the structural modification, the photocatalytically active region of Nb2O5 has been significantly extended from the UV region to the UV-Vis region (from ∼380 nm to ∼800 nm in the adsorption edge); and hence, the visible light photocatalytic performance has been greatly improved. Detailed structural analysis revealed that the enhanced photocatalytic activity of the graphene nanocluster decorated Nb2O5 nanofibers could be mainly attributed to the modification of the bandgaps by the clusters and the unique orientation of graphene layers (nearly perpendicular to the Nb2O5/C interface, which is quite different from classical “core–shell” composites). We believe that the findings suggest a potential candidate for visible-light photocatalytic applications and would inspire further studies.

The structure and properties of ribbon-shaped carbon fibers with high orientation

Using a naphthalene-derived mesophase pitch as a starting material, highly oriented ribbon-shaped carbon fibers with a smooth and flat surface were prepared by melt-spinning, oxidative stabilization, carbonization, and graphitization. The preferred orientation, morphology, and microstructure, as well as physical properties, of the ribbon-shaped carbon fibers were characterized. The results show that, the ribbon-shaped fibers possessed uniform shrinkage upon heat treatment, thereby avoiding shrinkage cracking commonly observed in round-shaped fibers. As heat treatment progressed, the ribbon-shaped graphite fibers displayed larger crystallite sizes and higher orientation of graphene layers along the main surface of the ribbon-shaped fiber in comparison with corresponding round-shaped fibers. The stability of the ribbon-shaped graphite fibers towards thermal oxidation was significantly higher than that of K-1100 graphite fibers. The longitudinal thermal conductivity of the ribbon fibers increased, and electrical resistivity decreased, with increasing the heat treatment temperatures. The longitudinal electrical resistivity and the calculated thermal conductivity of the ribbon-shaped fibers graphitized at 3000°C are about 1.1μΩm and above 1100W/mK at room temperature, respectively. The tensile strength and Young’s modulus of these fibers approach 2.53 and 842GPa, respectively.

Structure, Chemistry and Energy of Carbon Surfaces

In this paper, the graphene layer is considered as the basic structural unit in a range of carbon materials. The effects of 2D and 3D order in defining the microstructures and macrostructures of carbons are reviewed along with the related concept of graphene layer orientation within the structure and the resulting ratio of basal plane to edge site as well as its effects on surface chemical reactivity and energy. Both non-specific and specific surface forces and the resulting potentials are considered. Theoretical physical models based on the Lennard-Jones potential applied to the basic structural units, either as an ideal, i.e. perfect, single graphene layer or as multiple layers of varied inter-layer spacing and size, are presented. Mention is then made of non-ideal surfaces (stepped or containing vacancies) and their resulting enhanced potential. The effects of incorporating chemical heterogeneity into the surfaces of these structures are also considered: carbon oxidation mechanisms are discussed in ter...

X-Ray Diffraction and Raman Spectroscopy Study of Strain in Graphene Films Grown on 6H-SiC(0001) Using Propane-Hydrogen-Argon CVD

We have grown graphene films on 6H-SiC(0001) using propane CVD and evidenced the strong impact of the hydrogen/argon mixture used as the carrier gas on the graphene/SiC interface and on the orientation of graphene layers. By studying a set of samples grown with different hydrogen/argon mixture using Raman spectroscopy and grazing incidence X-ray diffraction, we evidence the links between graphene/SiC interface and strain in graphene.

Engineering of Graphene Layer Orientation to Attain High Rate Capability and Anisotropic Properties in Li‐Ion Battery Electrodes

AbstractNovel carbon films with different graphene layer orientations are investigated as electrode materials for Li‐ion batteries. It is demonstrated that engineering the crystallographic orientation with graphene layers oriented perpendicular to the surface substantially alters stress evolution during Li insertion. With this crystallographic orientation the intercalating/de‐intercalating Li‐ions also have direct access to the graphene interlayer spaces, resulting in higher capacity at faster electrochemical cycling, compared to carbon films with graphene layers parallel to the film surface. Electrodes with perpendicular alignment are prepared by supramolecular synthesis using either spin coating or bar coating of chromonic liquid crystal precursors into precursor organic films followed by in situ carbonization. These materials are compared with in situ stress measurements during lithiation/delithiation cycles, and the bar‐coated films exhibit a highly anisotropic stress which is consistent with long‐range alignment of the graphene layers. In contrast, the in‐plane stresses in the spin‐coated films are isotropic, which is consistent with the presence of randomly oriented domains (still with graphene layers oriented perpendicular to the surface). Overall, the use of thin film graphitic materials with controlled crystallographic orientations provides a valuable platform for investigating the impact of graphene structure on the properties of Li‐ion battery electrode materials.

Homogenization of the elastic properties of pyrolytic carbon based on an image processing technique

AbstractIn this work, the linear elastic material properties of differently textured variants of pyrolytic carbon are homogenized from the submicro‐ to the micro‐scale. In high resolution transmission electron microscope (HRTEM) lattice fringe images, the microstructure of pyrolytic carbon manifests itself in terms of projections of graphene layers. According to their orientation distribution, different textures of pyrolytic carbon have been classified. Assuming a von Mises‐Fisher distribution for the spatial orientation of single graphene layers, the orientation distribution function of the projected layers in the image plane is analytically found to be a modified Struve function. For each pyrolytic carbon texture, Maximum‐likelihood estimates for the mean orientation and the concentration parameter of the von Mises‐Fisher distribution are obtained numerically. Hereby, Fourier transformation and appropriate filters are used to determine the probabilities for discrete orientations of the graphene layers directly from HRTEM images. First‐ and second‐order bounds of the linear elastic properties of pyrolytic carbon of the different textures are computed. Elastic constants of graphite and pyrolytic graphite have been used for modeling the elastic behavior of the graphene layers within a continuum mechanical setting. Due to the high anisotropy of all analyzed textures of pyrolytic carbon, the differences even between the second‐order bounds are quite large.

Grazing incidence X-ray diffraction study on carbon nanowalls

The growth and structure of carbon nanowalls (CNWs) on the substrate were investigated by grazing incidence X-ray diffraction. Fully-grown CNWs have a single phase of hexagonal graphite structure with relatively high degree of graphitization as the graphite powder. The preferred orientation of graphene layers is perpendicular to the substrate surface. At the initial growth state, the graphene layers are horizontally oriented. At the critical deposition time for changing the orientation, some diffraction peaks which cannot be assigned to the hexagonal graphite structure are observed. These peaks suggest the formation of a carbon allotrope which should contribute to the turning of the orientation of graphene layers.

Tunable amorphous carbon nanotubes prepared by a simple template

An easy versatile method is reported to synthesize tunable amorphous carbon nanotubes (a-CNTs) with different pore diameter, length and orientation of graphene layers at a low temperature of 450 °C in the absence of metal catalysts by using porous anodic aluminum oxide (AAO) as a template and citric acid as a precursor. Since citric acid was adsorbed by hydrogen bonds with the –OH groups at the walls of the AAO templates, which can be influenced by the concentration of H + ions in the solution, the pH leads to the change of the orientation of graphene layers and the graphitization degree.

Water-dispersible “carbon nanopods” with controllable graphene layer orientation

A pod-like form of a hollow nanocarbon "carbon nanopod" with controllable graphene layer orientation and good water-dispersibility has been synthesized with extremely high selectivity and without any impurities.