Interlayer vacancy defects in AA-stacked bilayer graphene: density functional theory predictions.

AA-stacked graphite and closely related structures, where carbon atoms are located in registry in adjacent graphene layers, are a feature of graphitic systems including twisted and folded bilayer graphene, and turbostratic graphite. We present the results of ab initio density functional theory calculations performed to investigate the complexes that are formed from the binding of vacancy defects across neighbouring layers in AA-stacked bilayers. As with AB stacking, the carbon atoms surrounding lattice vacancies can form interlayer structures with sp 2 bonding that are lower in energy than in-plane reconstructions. The sp 2 interlayer bonding of adjacent multivacancy defects in registry creates a type of stable sp 2 bonded 'wormhole' or tunnel defect between the layers. We also identify a new class of 'mezzanine' structure characterised by sp 3 interlayer bonding, resembling a prismatic vacancy loop. The V 6 hexavacancy variant, where six sp 3 carbon atoms sit midway between two carbon layers and bond to both, is substantially more stable than any other vacancy aggregate in AA-stacked layers. Our focus is on vacancy generation and aggregation in the absence of extreme temperatures or intense beams.

Cyclotetrahalo-p-phenylenes: simulations of halogen substituted cycloparaphenylenes and their interaction with C60.

Density functional calculations are used to study the role of edge-functionalization on the structure and electronic properties of cycloparaphenylene (CPPs) containing from six to twenty benzenoid rings. We substitute hydrogen by the halogens fluorine, chlorine and bromine. The resultant Cyclotetrahalo-p-phenylenes are compared with their hydrogenated equivalents, related linear paraphenyl and fluoro-paraphenyl polymers, and functionalised armchair edges in graphene nanoribbons. Notably we consider both structural and electronic evolution. Finally we examine C60@[10]CPP, i.e. C60 encapsulated within [10]CPP, with the various ring terminations. The effect of halogenation on electronic level position around the gap strongly affects their capacity to form donor-acceptor pairs with fullerenes.

Nitrogen segregation in nanocarbons.

We explore the behaviour of nitrogen doping in carbon nanomaterials, notably graphene, nanotubes, and carbon thin films. This is initially via a brief review of the literature, followed by a series of atomistic density functional calculations. We show that at low concentrations, substitutional nitrogen doping in the sp(2)-C graphenic basal plane is favoured, however once the nitrogen concentration reaches a critical threshold there is a transition towards the formation of the more thermodynamically-favoured nitrogen terminated 'zigzag' type edges. These can occur either via formation of finite patches (polycyclic aromatic azacarbons), strips of sp(2) carbon with zigzag nitrogen edges, or internal nitrogen-terminated hole edges within graphenic planes. This transition to edge formation is especially favoured when the nitrogen can be partially functionalised with, e.g. hydrogen. By comparison with available literature results, notably from electron energy loss spectroscopy and X-ray spectroscopy, the current results suggest that much of the nitrogen believed to be incorporated into carbon nanoobjects is instead likely to be present terminating the edges of carbonaceous impurities attached to nanoobject's surface. By comparison to nitrogen-doped tetrahedrally amorphous carbon, we suggest that this transition at around 10-20% nitrogen concentration and above towards sp(2) coordination via internal nitrogen-terminated edge formation may be a general property of nitrogen-doped carbon materials.