Improved mechanical properties of magnesium–graphene composites with copper–graphene hybrids

New magnesium nanocomposites reinforced with copper–graphene nanoplatelet hybrid particles have been prepared through the semipowder metallurgy method. Compared with the monolithic Mg, the Mg–1Cu–xGNPs nanocomposites exhibited higher tensile and compressive strength. In tension, nanocomposites revealed substantial enhancement in elastic modulus, 0.2% yield strength, ultimate tensile strength and failure strain (up to +89, +117, +58 and +96% respectively) compared to monolithic Mg. In compression, the nanocomposites showed the greatest improvement in 0.2% yield strength, and the ultimate compressive strength and failure strain (%) (up to +34, +59 and +61% respectively), whilst the compressive elastic modulus first increases and then decreases with an increase in the graphene nanoplatelets (GNPs) contents. The enhanced strength of the composites is likely to result from strengthening mechanisms invoked by the addition of Cu–GNPs hybrids.     

Thermal conductivity of isotopically modified graphene

In addition to its exotic electronic properties graphene exhibits unusually high intrinsic thermal conductivity. The physics of phonons--the main heat carriers in graphene--has been shown to be substantially different in two-dimensional (2D) crystals, such as graphene, from in three-dimensional (3D) graphite. Here, we report our experimental study of the isotope effects on the thermal properties of graphene. Isotopically modified graphene containing various percentages of 13C were synthesized by chemical vapour deposition (CVD). The regions of different isotopic compositions were parts of the same graphene sheet to ensure uniformity in material parameters. The thermal conductivity, K, of isotopically pure 12C (0.01% 13C) graphene determined by the optothermal Raman technique, was higher than 4,000?W?mK(-1) at the measured temperature T(m)~320?K, and more than a factor of two higher than the value of K in graphene sheets composed of a 50:50 mixture of 12C and 13C. The experimental data agree well with our molecular dynamics (MD) simulations, corrected for the long-wavelength phonon contributions by means of the Klemens model. The experimental results are expected to stimulate further studies aimed at a better understanding of thermal phenomena in 2D crystals.     

Anomalous moire pattern of graphene investigated by scanning tunneling microscopy： Evidence of graphene growth on oxidized Cu（111）

The growth of graphene on oriented （111） copper films has been achieved by atmospheric pressure chemical vapor deposition. The structural properties of as-produced graphene have been investigated by scanning tunneling microscopy. Anomalous moir6 superstructures composed of well-defined linear periodic modulations have been observed. We report here on comprehensive and detailed studies of these particular moir6 patterns present in the graphene topography revealing that, in certain conditions, the growth can occur on the oxygen-induced reconstructed copper surface and not directly on the oriented （111） copper film as expected.     

Molecular dynamics simulations of the rheological properties of graphene–PAO nanofluids

Graphene is a promising additive for lubricants. The rheological properties of graphene nanofluids have a significant impact on the tribological performance of base oil. In this case, rheological properties including viscosity, density, mean square displacement and diffusion coefficient of graphene–PAO nanofluids were investigated by using the nonequilibrium molecular dynamics simulations in order to understand the effects of graphene on the rheological properties of base oil under extreme conditions. The molecular dynamics model was validated according to the experimental and numerical statistics reported by other researchers. The simulation results reflected that the viscosity of base oil was effectively improved by adding graphene nanoparticles. As the concentration of graphene increased, the viscosity of nanofluids becomes higher. However, the diffusion coefficient reached its highest value (3.73?×?10^?9 m²/s) with nanofluids containing two pieces of graphene in the system. Furthermore, we found that the graphene played a more significant role in enhancing the viscosity of base oil at high temperature and pressure. The viscosity was especially improved by 290.2% at 0.1 MPa, 500 K. The boiling point of the base oil became higher than 800 K after adding graphene. To our best knowledge, this work is the first study of the rheological properties of graphene–PAO nanofluids using molecular dynamic simulations.     

Transport properties of graphene nanoribbons with side-attached organic molecules

In this work we address the effects on the conductance of graphene nanoribbons (GNRs) of organic molecules adsorbed at the ribbon edge. We studied the case of armchair and zigzag GNRs with quasi-one-dimensional side-attached molecules, such as linear poly-aromatic hydrocarbons and poly(para-phenylene). These nanostructures are described using a single-band tight-binding Hamiltonian and their electronic conductance and density of states are calculated within the Green's function formalism based on real-space renormalization techniques. We found that the conductance exhibits an even-odd parity effect as a function of the length of the attached molecules. Furthermore, the corresponding energy spectrum of the molecules can be obtained as a series of Fano antiresonances in the conductance of the system. The latter result suggests that GNRs can be used as a spectrograph sensor device.     

Ion implantation assisted synthesis of graphene on various dielectric substrates

Direct synthesis of high-quality graphene on dielectric substrates is of great importance for the application of graphene-based electronics and optoelectronics. However, high-quality and uniform graphene film growth on dielectric substrates has proven challenging due to limited catalytic ability of dielectric substrates. Here, by employing a Cu ion implantation assisted method, high-quality and uniform graphene can be directly formed on various dielectric substrates including SiO2/Si, quartz glass, and sapphire substrates. The growth rate of graphene on the dielectric substrates was significantly improved due to the catalysis of Cu. Moreover, during the graphene growth process, the Cu atoms gradually evaporated away without involving any metal contamination. Furthermore, an interesting growth behavior of graphene on sapphire substrate was observed, and the results show the graphene domains growth tends to grow along the sapphire flat terraces. The ion implantation assisted approach could open up a new pathway for the direct synthesis of graphene and promote the potential application of graphene in electronics.     

Electrodeposition of graphene layers doped with Brϕnsted acids

A simple and fast method is demonstrated for the preparation of a thin film of graphene layers by the electrodeposition of positively doped graphene dispersion onto desired electrode substrates. A thin film of graphene layers was obtained by applying negative potentials according to the electrophoretic deposition mechanism. The doped graphene dispersion was prepared from expanded graphite treatment with various acids (HCl, HNO₃, and H₂SO₄) and an ultrasonication process. The doping and deposition processes are strongly dependent on the type of acid and the applied potential, which were monitored by Raman spectroscopy and quartz crystal microbalance, respectively. The morphology and electrochemical properties of the graphene film were characterized by scanning electron microscopy and cyclic voltammetry. The electrochemical performance of graphene film obtained using nitric acid or hydrochloric acid dopant is superior to that obtained with sulfuric acid doping. This technique could be a facile tool for the fabrication of a thin film of graphene layers on a desired substrate.     

Effect of strain rate on the fracture behaviour of epoxy–graphene nanocomposite

In this study, epoxy-based nanocomposite was fabricated by the addition of graphene nanosheet via a solution casting method. To investigate the effect of strain rate on tensile properties of epoxy, tensile tests were done on standard samples at different strain rates (0.05–1 min^?1). The role of strain rate and presence of graphene on fracture behaviour of epoxy were also studied by investigation of the fracture surfaces of some samples by scanning electron microscopy (SEM). Finally, Eyring’s model was performed to clarify the role of strain rate on activation volume and activation enthalpy of epoxy. The results of tensile tests showed a maximum strength of epoxy–graphene nanocomposite at the graphene wt% of 0.1%. Tensile strength of epoxy obviously improved with increasing strain rate, but tensile strength of epoxy/graphene nanocomposite sample was less sensitive. Fracture micrographs showed that the mirror zone of the fracture surface of epoxy diminished by increasing strain rate or addition of graphene; and final fracture zone also became rougher. Finally, by investigation of the activation enthalpies, it was showed that much higher enthalpy was needed to fracture the nanocomposite sample, as the activation enthalpy changed from 41.54 for neat epoxy to 67.34 kJ mol^?1 for EP–0.1% GNS sample.     

Hysteresis I–V nature of mechanically exfoliated graphene FET

Graphene is an attractive material for device applications due to its excellent electrical and mechanical properties. The mechanical exfoliation is an attractive method to fabricate graphene devices using mono and multilayer graphene flakes. As the graphene is very sensitive to atmosphere the occurrence of hysteresis and p-doping is common. This paper reports electrical characterization and hysteresis effect of graphene field effect transistor (FET) fabricated using mechanically exfoliated graphene flakes. Raman spectra and atomic force microscopy techniques have been used to examine the quality and thickness of the exfoliated graphene. This fabricated graphene FET has shown hysteresis nature with p-type doping. The possible reason for the observed hysteresis and p-doping has been explained.     

How can graphene reduce the flammability of polymer nanocomposites?

Nanocomposites based on poly(vinyl alcohol) (PVA) and graphene nanosheets have been prepared by polymer solution blending and their flame retardant properties have been evaluated by a cone calorimetry test. It has been shown that there is a strong influence of graphene nanosheets on the fire behaviour of the composites with a significant reduction in peak heat release rate (PHRR) and a much longer time to ignition. Compared to pure PVA, the PHRR of PVA filled with 3 wt.% graphene is reduced by 49%. The flame retardancy of graphene for PVA matrix surpasses that of both Na-MMT and MWNTs with the same addition content. Such a remarkable behaviour might be explained by the forming of a compact, dense and uniform char during combustion.     

Hybrid molecular dynamics–finite element simulations of the elastic behavior of polycrystalline graphene

In this paper, we develop an efficient multiscale molecular dynamics (MD)–finite element (FE) modeling scheme capable of determining the elastic and fracture properties of polycrystalline graphene. The local elastic properties of a grain boundary (GB) connecting two adjacent graphene grains, with different lattice orientations, were first determined using MD simulations. In a two-dimensional medium, randomly distributed grains connected with GBs were then created using the Voronoi tessellation method. The constructed Voronoi diagrams were used to create FE models of the polycrystalline graphene, where the GBs were represented by interphase regions with their local properties determined using MD. The grains were modeled as pristine graphene and the accuracy of the polycrystalline FE model was validated with MD simulations of a geometrically identical polycrystalline graphene. The results reveal good agreement between MD and FE simulations. They further show that the elastic and fracture properties of polycrystalline graphene are greatly influenced by the grain size and the misorientation angle. They also indicate that the predicted elastic properties are in agreement with earlier reported experimental and MD results. We believe that this newly proposed multiscale scheme could be easily integrated into current design software to model graphene based nano- and micro-devices.     

Strength of Mg–3%Al alloy in presence of graphene nano-platelets as reinforcement

Bulk Mg–3%Al alloy-based nanocomposites containing graphene nano-platelets (GNPs) were synthesised using the powder metallurgy technique incorporating energy efficient hybrid microwave sintering and hot extrusion. GNPs in amounts of 0.1, 0.3 and 0.5?wt-% were investigated as reinforcements. Microstructural characterisation accomplished using optical, scanning and transmission electron microscopy revealed the presence of minimal porosity (<1%), a uniform distribution of GNPs in the matrix and a reduced grain size as a result of the presence of GNPs. The results of mechanical property characterisation revealed an overall improvement in micro-hardness and compressive response, due to the presence of GNPs, with best results exhibited by the Mg–3Al/0.3% GNPs composite. The ductility under compressive loading for composite samples remained higher than 20%.     

Effect of processing routes on mechanical and thermal properties of copper–graphene composites

In this paper, copper–graphene composites were fabricated by using two different processing routes (ball milling (BM) and ultrasonication) followed by spark plasma sintering. Vickers hardness and anisotropic thermal conductivity of the composites were measured and observed that ultrasonicated fabricated composites gave better result compared with BM composite and even from pure copper. The hardness values obtained for ultrasonicated copper–graphene composite were 69?HV (57% higher) and thermal conductivity 387?W/m?K (13% higher) by using only 0.5?wt-% of graphene, while for pure copper the values were 44?HV and 341?W/m?K. The value of anisotropic thermal conductivity ultrasonicated composites was also 1.97 which is much higher than pure copper 0.94.     

Is graphene aromatic?

We analyze the chemical bonding in graphene using a fragmental approach, the adaptive natural density partitioning method, electron sharing indices, and nucleus-independent chemical shift indices. We prove that graphene is aromatic, but its aromaticity is different from the aromaticity in benzene, coronene, or circumcoronene. Aromaticity in graphene is local with two π-electrons delocalized over every hexagon ring. We believe that the chemical bonding picture developed for graphene will be helpful for understanding chemical bonding in defects such as point defects, single-, double-, and multiple vacancies, carbon adatoms, foreign adatoms, substitutional impurities, and new materials that are derivatives of graphene.      

Magnesia supported Au and Ag catalysts for the preparation of few-layer graphene–metal nanocomposites: relationship between catalyst structure and the properties of graphene composites

Magnesia supported Au, Ag, and Au–Ag nanostructured catalysts were prepared, characterized, and used to synthesize few-layer graphene–metal nanoparticle (Gr–MeNP) composites. The catalysts have a mezoporous structure and a mixture of MgO and MgO·H₂O as support. The gold nanoparticles (AuNPs) are uniformly dispersed on the surface of the Au/MgO catalysts, and have a uniform round shape with a medium size of ~8 nm. On the other hand, the silver nanoparticles (AgNPs) present on the Ag/MgO catalyst have an irregular shape, larger diameters, and less uniform dispersion. The Au–Ag/MgO catalyst contains large Au–Ag bimetallic particles of ~20–30 nm surrounded by small (5 nm) AuNPs. Following the RF-CCVD process and the dissolution of the magnesia support, relative large, few-layer, wrinkled graphene sheets decorated with metal nanoparticles (MeNPs) are observed. Graphene–gold (Gr–Au) and graphene–silver (Gr–Ag) composites had 4–7 graphitic layers with a relatively large area and similar crystallinity for samples prepared in similar experimental conditions. Graphene–gold–silver composites (Gr–Au–Ag) presented graphitic rectangles with round, bent edges, higher crystallinity, and a higher number of layers (8–14). The MeNPs are encased in the graphitic layers of all the different samples. Their size, shape, and distribution depend on the nature of the catalyst. The AuNPs were uniformly distributed, had a size of about 15 nm, and a round shape similar to those from Au/MgO catalyst. In Gr–Ag, the AgNPs have a round shape, very different from that of the Ag/MgO catalyst, large size distribution and are not uniformly distributed on the surface. Agglomerations of AgNPs together with large areas of pristine few-layer graphene were observed. In Gr–Au–Ag composites, almost exclusively large bimetallic particles of about 25–30 nm, situated at the edge of graphene rectangles have been found.     

Tribological performance of carbon nanotube–graphene oxide hybrid/epoxy composites

An investigation is conducted on the effect of the hybrid of multi-wall carbon nanotubes (MWCNTs) and graphene oxide (GO) nanosheets on the tribological performance of epoxy composites at low GO weight fractions of 0.05–0.5 phr. The MWCNT amount is kept constant at 0.5 phr, which is typical for CNT/epoxy composites with enhanced mechanical properties. Friction and wear tests against smooth steel show that the introduction of 0.5 phr MWCNTs into the epoxy matrix increases the friction coefficient and decreases the specific wear rate. When testing the tribological performance of MWCNT/GO hybrids, it is shown that at a high GO amount of 0.5 phr, the friction coefficient is decreased below that of the neat matrix whereas the wear rate is increased above that of the neat matrix. At an optimal hybrid formulation, i.e., 0.5 phr MWCNTs and 0.1 phr GO, a further increase in the friction coefficient and a further reduction in the specific wear rate are observed. The specific wear rate is reduced by about 40% down to a factor of 11 relative to the neat epoxy when the GO content is 0.1 phr.     

Structural and electronic decoupling of C₆₀ from epitaxial graphene on SiC

We have investigated the initial stages of growth and the electronic structure of C(60) molecules on graphene grown epitaxially on SiC(0001) at the single-molecule level using cryogenic ultrahigh vacuum scanning tunneling microscopy and spectroscopy. We observe that the first layer of C(60) molecules self-assembles into a well-ordered, close-packed arrangement on graphene upon molecular deposition at room temperature while exhibiting a subtle C(60) superlattice. We measure a highest occupied molecular orbital-lowest unoccupied molecular orbital gap of ～3.5 eV for the C(60) molecules on graphene in submonolayer regime, indicating a significantly smaller amount of charge transfer from the graphene to C(60) and substrate-induced screening as compared to C(60) adsorbed on metallic substrates. Our results have important implications for the use of graphene for future device applications that require electronic decoupling between functional molecular adsorbates and substrates.