3 research outputs found
The Stone-Wales transformation: from fullerenes to graphite, from radiation damage to heat capacity
The Stone-Wales (SW) transformation, or carbon-bond rotation, has been fundamental to understanding fullerene growth and stability, and ab initio calculations show it to be a high-energy process. The nature and topology of the fullerene energy landscape shows how the Ih-C60 must be the final product, if SW transformations are fast enough, and various mechanisms for their catalysis have been proposed. We review SW transformations in fullerenes and then discuss the analogous transformation in graphite, where they form the Dienes defect, originally posited to be a transition state in the direct exchange of a bonded atom pair. On the basis of density functional theory calculations in the local density approximation, we propose that non-equilibrium concentrations of the Dienes defect arising from displacing radiation are rapidly healed by point defects and that equilibrium concentrations of Dienes defects are responsible for the divergent ultra-high-temperature heat capacity of graphite. This article is part of the themed issue Fullerenes: past, present and future, celebrating the 30th anniversary of Buckminster Fullerene
A ReaXFF carbon potential for radiation damage studies
Although molecular dynamics simulations of energetic impacts and collision cascades in graphite have been investigated for over 25 years, recent investigations have shown a difference between the types of defects predicted by the commonly used empirical potentials compared to ab-initio calculations. As a result a new ReaXFF potential has been fitted which reproduces the formation energies of many of the defects predicted by the ab-initio calculations and the energy pathways
between different defect states, important for investigating long term defect evolution. The data sets in the fitting have been have been added to the existing data sets used for modelling hydrocarbons and fullerenes. The elastic properties of the potential are less well modelled than the point defect structures with the elastic constants c33 being too high
and c44 too low compared to experiment. Preliminary results of low energy collision cascades show many point defect structures develop that are in agreement with those predicted from the ab-initio results
Interlayer vacancy defects in AA-stacked bilayer graphene: Density functional theory predictions
© 2017 IOP Publishing Ltd.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