Atomic Graphene Research Articles

The computational design of junctions by carbon nanotube insertion into a graphene matrix

Using first-principles density functional theory calculations, two types of junction models constructed from armchair and zigzag carbon nanotube (CNT) insertion into a graphene matrix have been envisioned. It has been found that the insertion of the CNT into the graphene matrix leads to the formation of C–C covalent bonds between graphene and the CNT that distort the CNT geometry. However, the hydrogenation of the suspended carbon bonds on the graphene resumes the graphene-like structure of the pristine tube. The calculated band structure of armchair CNT insertion into graphene or hydrogenation graphene opens up a band gap and converts the metallic CNT into a semiconductor. For the zigzag CNT, the sp3 hybridization between the graphene and nanotube alters the band structure of the tube significantly, whereas saturating the dangling bonds of terminal carbon atoms of graphene makes the CNT almost keep the same character of the bands as that in the pristine tube. The synthesis of our designed hybrid structures must be increasingly driven by an interest in molecules that not only have intriguing structures but also have special functions such as hydrogen storage.

2D ISING MODEL WITH COMPETING INTERACTIONS AND ITS APPLICATION TO CLUSTERS AND ARRAYS OF π-RINGS, GRAPHENE AND ADIABATIC QUANTUM COMPUTING

We study the two-dimensional Ising model with competing nearest-neighbour and diagonal interactions and investigate the phase diagram of this model. We show that the ground state at low temperatures is ordered either as stripes or as the Néel antiferromagnet. However, we also demonstrate that the energy of defects and dislocations in the lattice is close to the ground state of the system. Therefore, many locally stable (or metastable) states associated with local energy minima separated by energy barriers may appear forming a glass-like state. We discuss the results in connection with two physically different systems. First, we deal with planar clusters of loops including a Josephson π-junction (a π-rings). Each π-ring carries a persistent current and behaves as a classical orbital moment. The type of particular state associated with the orientation of orbital moments in the cluster depends on the interaction between these orbital moments and can be easily controlled, i.e. by a bias current or by other means. Second, we apply the model to the analysis of the structure of the newly discovered two-dimensional form of carbon, graphene. Carbon atoms in graphene form a planar honeycomb lattice. Actually, the graphene plane is not ideal but corrugated. The displacement of carbon atoms up and down from the plane can be also described in terms of Ising spins, the interaction of which determines the complicated shape of the corrugated graphene plane. The obtained results may be verified in experiments and are also applicable to adiabatic quantum computing where the states are switched adiabatically with the slow change of coupling constant.

Embedding Transition-Metal Atoms in Graphene: Structure, Bonding, and Magnetism

We present a density-functional-theory study of transition-metal atoms (Sc-Zn, Pt, and Au) embedded in single and double vacancies (SV and DV) in a graphene sheet. We show that for most metals, the bonding is strong and the metal-vacancy complexes exhibit interesting magnetic behavior. In particular, an Fe atom on a SV is not magnetic, while the Fe@DV complex has a high magnetic moment. Surprisingly, Au and Cu atoms at SV are magnetic. Both bond strengths and magnetic moments can be understood within a simple local-orbital picture, involving carbon sp(2) hybrids and the metal spd orbitals. We further calculate the barriers for impurity-atom migration, and they agree well with available experimental data. We discuss the experimental realization of such systems in the context of spintronics and nanocatalysis.

Topology and electron scattering properties of the electronic interfaces in epitaxial graphene probed by resonant tunneling spectroscopy

$Z\text{\ensuremath{-}}V$ scanning tunneling spectroscopy is used to probe the topology and electron scattering properties of the electronic interfaces of monolayer and bilayer graphenes, expitaxially grown on SiC(0001). The $dZ/dV$ spectra validate existing calculations of the interface topology and provide evidence for new electron scattering properties due to changes in the electronic character of the bonding. Two sharp boundaries are observed: between the vacuum and the graphene $\ensuremath{\pi}$ state lying above the graphene atom plane and a subsurface barrier between the carbon-rich layer and the bulk SiC.

Phonon dispersion in graphene

Taking into account constraints imposed by the lattice symmetry, we calculate analytically [1] the phonon dispersion for graphene with interactions between first and second neighbors. We find that the out-of-plane (bending) modes are not coupled with in-plane modes and described only with two force constants, one of which is determined by the corresponding Raman frequency and another by the smallest elastic constant C44. In contrast to calculations by Saito et al, we find the linear dispersion of the bending (out-of-plane) mode around the Γ point with a small but finite sound velocity =1.57 km/s. The sound velocity of this mode is very sensitive to small variations of the force constants. The sound velocities of in-plane modes are = 20.3 km/s and =13.1 km/s. Because of the lack of information for graphene, we compare the present theory with experiments on graphite. The low phonon frequencies in the critical points turn out less than their values in graphite, since the atoms in graphene are more free to move in the out-of-plane direction in comparison with graphite. Accuracy of the comparison can be estimated using the value of the observed splitting of the ZA and ZO' modes in graphite which is around 130 1/cm. [1] L.A. Falkovsky, cond-mat/0702409.

One‐ and Two‐Dimensional Diffusion of Metal Atoms in Graphene

In the present work, individual Au or Pt atoms in layersconsisting of one or twographene planes have been monitoredin real time at high temperature by high-resolution TEM. Weobtain information about the location of metal atoms ingrapheneandthediffusionmechanisms.Activationenergiesfordiffusion are obtained in a temperature range close to thetemperature of the technically important metal-assisted CVDprocess.Thematerialwassynthesizedinanarcdischarge

Fullerene-like CP x: A first-principles study of the relative stability of precursors and defect energetics during synthetic growth

Inherently nanostructured CP x compounds were studied by first-principles calculations. Geometry optimizations and cohesive energy comparisons show stability for C 3P, C 2P, C 3P 2, CP, and P 4 (P 2) species in isolated form as well as incorporated in graphene layers. The energy cost for structural defects, arising from the substitution of C for P and intercalation of P atoms in graphene, was also evaluated. We find a larger curvature of the graphene sheets and a higher density of cross-linkage sites in comparison to fullerene-like (FL) CN x , which is explained by differences in the bonding between P and N. Thus, the computational results extend the scope of fullerene-like thin film materials with FL-CP x and provide insights for its structural properties.

Effective Pathway for Hydrogen Atom Adsorption on Graphene

We investigate and discuss the interaction of a hydrogen atom (H) with graphene based on the density functional theory (DFT). Our calculation results show that reconstructions of carbon atoms play an important role in the H adsorption on graphene. When constituent carbon atoms are held rigid, endothermic H adsorption is about 0.2 eV, and the activation barrier is 0.3 eV for H adsorption, due to the strong π-bonding network of the hexagonal carbon. On the other hand, when carbon atoms are allowed to relax, the carbon atom directly below the H atom moves 0.33 Å upward towards the gas phase, and an s p 3 -like geometry is formed between the H and carbon atoms of graphene. This relaxation stabilizes the hydrogen–carbon interaction, and the exothermic hydrogen adsorption on the graphene has a binding energy of 0.67 eV. We also show that the effective pathway for H adsorption on graphene, which gives an activation barrier for the H adsorption on graphene of 0.18 eV.

Hamworthy KSE divests non-core units

Graphene can be utilized as reinforcing interlayers in metal-matrix composites to improve their mechanical properties. This improvement is due to the ability of the graphene layers to transfer their exceptional strength to the metallic matrix, and their ability to hinder the propagation of dislocations inside the metallic systems. In these composites, the structure of the graphene layer (or nanoparticles) in the matrix and the interaction between graphene and metallic atoms play a vital role in the improvement of metal-graphene composites’ mechanical properties. While the interaction between metal and graphene interface, when the graphene is on the top of a metallic layer, has been extensively studied, details of the interaction when the graphene is embedded inside a metal matrix are still highly unexplored. In this paper, we considered the structure of graphene and the interaction between graphene nanoparticle edges and nickel atoms when the graphene is embedded in the nickel matrix. Using ab-initio calculations, we found that when the graphene is embedded inside a nickel matrix, the most stable graphene structure is topfcc (with graphene nano-sheets being parallel to Ni(111) surface), which is a different configuration compared to the case of having graphene on the surface of a nickel slab. Also, we found that armchair edge energy is 0.8022(eV/Å) and zigzag edge energy is 2.5048(eV/Å) which implies armchair edge is more stable and creates less distortion inside the nickel matrix. Less distortion by armchair edge implies that nucleation of dislocation in composites will start later at armchair edges in comparison to zigzag edges during mechanical loading.