Effect of microstructure on the thermal conductivity of disordered carbon

Computational methods are used to control the degree of structural order in a variety of carbonmaterials containing primarily sp2 bonding. Room-temperature thermal conductivities arecomputed using non-equilibrium molecular dynamics. Our results reproduce experimental data foramorphous and glassy carbons and confirm previously proposed structural models for vitreouscarbons. An atomistic model is developed for highly oriented thin films seen experimentally, with amaximum computed thermal conductivity of 35 W m1 K1. This value is much higher than thatof the amorphous and glassy structures, demonstrating that the microstructure influences thethermal conductivity more strongly than the density

First Principles Simulations of Boron Diffusion in Graphite

Boron strongly modifies electronic and diffusion properties of graphite. We report the first ab initio study of boron interaction with the point defects in graphite, which includes structures, thermodynamics, and diffusion. A number of possible diffusion mechanisms of boron in graphite are suggested. We conclude that boron diffuses in graphite by a kick-out mechanism. This mechanism explains the common activation energy, but large magnitude difference, for the rate of boron diffusion parallel and perpendicular to the basal plane. © 2007 The American Physical Society

Nomenclature of sp2 carbon nanoforms

Carbon’s versatile bonding has resulted in the discovery of a bewildering variety of nanoforms which urgently need a systematic and standard nomenclature [1]. Besides fullerenes, nanotubes and graphene, research teams around the globe now produce a plethora of carbon-based nanoforms such as ‘bamboo’ tubes, ‘herringbone’ and ‘bell’ structures. Each discovery duly gains a new, sometimes whimsical, name, often with its discoverer unaware that the same nanoform has already been reported several times but with different names (for example the nanoform in Fig. 1h is in different publications referred to as ‘bamboo’ [2], ‘herringbone-bamboo’ [3], ‘stacked-cups’ [4] and ‘stacked-cones’ [5]). In addition, a single name is often used to refer to completely different carbon nanoforms (for example, the ‘bamboo’ structure in [2] is notably different from ‘bamboo’ in [6]). The result is a confusing overabundance of names which makes literature searches and an objective comparison of results extremely difficult, if not impossible

Molecular dynamics simulations of the transformation of carbon peapods into double-walled carbon nanotubes

The transformation of carbon peapods (encapsulated fullerenes in nanotubes) into doublewalled nanotubes was studied using molecular dynamics simulation. The simulations reproduce the two main trends known experimentally: the production of low-defect nanotubes and the templating effect of the outer tube. The process involves a low-temperature polymerization of the fullerenes followed by higher temperature self-assembly into a tube. Modelling of this second stage is made possible by the use of the Environment-Dependent Interaction Potential, a large number of atoms and long-time annealing. Analysis shows that the outer tube acts as a container for the self-assembly process, analogous to previous simulations and experiments in which free surfaces, either external or internal, template the formation of highly ordered sp2 phases

Graphitization of amorphous carbons: A comparative study of interatomic potentials

We perform a comparative study of six common carbon interatomic potentials: Tersoff, REBO-II, ReaxFF, EDIP, LCBOP-I and COMB3. To ensure fair comparison, all the potentials are used as implemented in the molecular dynamics package LAMMPS. Using the liquid quenching method we generate amorphous carbons at different densities, and subsequently anneal at high temperature. The amorphous carbon system provides a critical test of the transferability of the potential, while the annealing simulations illustrate the graphitization process and test bond-making and -breaking. A wide spread of behavior is seen across the six potentials, with quantities such as sp2 fraction, radial distribution function, morphology, ring statistics, and 002 reflection intensity differing considerably. While none of the potentials is perfect, some perform particularly poorly. The lack of transferability can be traced to the details of the functional form, suggesting future directions in the development of carbon potentials

A comparative study of density functional and density functional tight binding calculations of defects in graphene

The density functional tight binding approach (DFTB) is well adapted for the study of point and line defects in graphene based systems. After briefly reviewing the use of DFTB in this area, we present a comparative study of defect structures, energies and dynamics between DFTB results obtained using the dftb+ code, and density functional results using the localised Gaussian orbital code, AIMPRO. DFTB accurately reproduces structures and energies for a range of point defect structures such as vacancies and Stone-Wales defects in graphene, as well as various unfunctionalised and hydroxylated graphene sheet edges. Migration barriers for the vacancy and Stone-Wales defect formation barriers are accurately reproduced using a nudged elastic band approach. Finally we explore the potential for dynamic defect simulations using DFTB, taking as an example electron irradiation damage in graphene

Graphene edge structures: Folding, scrolling, tubing, rippling and twisting

Conventional three-dimensional crystal lattices are terminated by surfaces, which can demonstrate complex rebonding and rehybridisation, localised strain and dislocation formation. Two dimensional crystal lattices, of which graphene is the archetype, are terminated by lines. The additional available dimension at such interfaces opens up a range of new topological interface possibilities. We show that graphene sheet edges can adopt a range of topological distortions depending on their nature. Rehybridisation, local bond reordering, chemical functionalisation with bulky, charged, or multi-functional groups can lead to edge buckling to relieve strain, folding, rolling and even tube formation. We discuss the topological possibilities at a 2D graphene edge, and under what circumstances we expect different edge topologies to occur. Density functional calculations are used to explore in more depth different graphene edge types.Comment: Additional figure in published versio

The structure of junctions between carbon nanotubes and graphene shells

Junctions between carbon nanotubes and flat or curved graphene structures are fascinating for a number of reasons. It has been suggested that such junctions could be used in nanoelectronic devices, or as the basis of three-dimensional carbon materials, with many potential applications. However, there have been few detailed experimental analyses of nanotube-graphene connections. Here we describe junctions between nanotubes and graphene shells in a material produced by passing a current through graphite. Transmission electron micrographs show that the junction angles are not random but fall close to multiples of 30°. We show that connections with these angles are the only ones which are consistent with the symmetry of the hexagonal lattice, and molecular models show that a continuous lattice requires the presence of large carbon rings at the junction. Some of the configurations we propose have not been previously considered, and could be used to construct new kinds of three-dimensional carbon architecture. We also discuss the possible formation mechanism of the junctions

Comment on “Increase in specific heat and possible hindered rotation of interstitial C2 moleculesin neutron-irradiated graphite”

Iwata and Watanabe’s model for the observed low-temperature specific heat of neutron-irradiated graphite [T. Iwata and M. Watanabe, Phys. Rev. B 81, 014105 2010] assumes that self-interstitial atoms exist as clusters of nearly free C2 molecules. We suggest that their hypothesis is not supported by other experiments and theory, including our own calculations. Not only is it inconsistent with the long-known kinetics of interstitial prismatic dislocation loop formation, density-functional theory shows that the di-interstitial is covalently bonded to the host crystal. In such calculations no prior assumptions are made about the nature of the bonding, covalent or otherwise

Fullerene attachment to sharp-angle nanocones mediated by covalent oxygen bridging

Using a combination of transmission electron microscopy and density functional modeling we examine covalent bridging between carbon nanoforms, focusing on fullerene attachment to carbon nanocones (nanohorns). We show that oxygen mediates covalent cross-linking between carbon nanoforms, analogously to oxygen-mediated fullerene dimerisation (C120O). We confirm this theoretically and experimentally for fullerenes bonded to nanocone tips. Oxygen bridging only occurs in systems with relatively localized double bond character, i.e., in the case of nanocones, bridging only occurs between fullerenes and high angle nanocone tips