M.I. Heggie

Autocatalysis during fullerene growth

Total energy calculations with a local spin density functional have been applied to the Stone-Wales transformation in fullerene (C60). In the formation of the almost exclusively observed Ih isomer of C60 with isolated pentagons, the final transformation must be from a C2v isomer with two pentagon pairs. It was found that the energy barrier for this rearrangement was substantially reduced in the presence of an extra carbon atom. Such atoms were found to bind loosely, preferentially to regions in which there were paired pentagons. Pentagon rearrangements, which are necessary steps in the growth of fullerenes, may therefore result from autocatalysis by carbon.

Adatoms and nanoengineering of carbon

We present a new and general mechanism for inter-conversion of carbon structures via a catalytic exchange process, which operates under conditions of Frenkel pair generation. The mechanism typically lowers reaction barriers by a factor of four compared to equivalent uncatalysed reactions. We examine the relevance of this mechanism for fullerene growth, carbon onions and nanotubes, and dislocations in irradiated graphite

Radiation defects in graphite

This article discusses the nature of radiation defects in graphite, reviewing past and recent developments in understanding their structure, interactions and effect on physical properties. The principal focus is on behaviour at the atomic and microstructural level, with an interest both in understanding graphite moderator damage in nuclear reactors and building a foundation for the range of emerging technological applications of defect-engineered graphitic materials. It is hoped that this article will both clarify the picture that has emerged over the last 50 years and provide a useful background to ongoing efforts.

Effect of oxygen on the growth of (101̄0) GaN surfaces: The formation of nanopipes

Local density–functional methods are used to examine the behavior of O and O-related defect complexes on the walls of nanopipes in GaN. We find that O has a tendency to segregate to the (1010) surface and identify the gallium vacancy surrounded by three oxygen impurities [VGa–(ON)3] to be a particularly stable and electrically inert complex. We suggest that during Stranski–Krastanow growth, when interisland spaces shrink, these defects reach a critical concentration beyond which further growth is prevented and nanopipes are formed.

Low-Energy Termination of Graphene Edges via the Formation of Narrow Nanotubes

Viktoria V. Ivanovskaya, ∗ Alberto Zobelli, Philipp Wagner, Malcolm I. Heggie, Patrick R. Briddon, Mark J. Rayson, and Chris P. Ewels † Institut des Matériaux Jean Rouxel (IMN), UMR 6502 CNRS, University of Nantes, 44322 Nantes, France Institute of Solid State Chemistry, Ural division of Russian Academy of Science, 620041, Ekaterinburg, Russia Laboratoire de Physique des Solides, Univ. Paris-Sud, CNRS UMR 8502, F-91405, Orsay, France Department of Chemistry, University of Sussex, Falmer, Brighton BN1 9QJ, United Kingdom School of Electrical, Electronic and Computer Engineering, University of Newcastle upon Tyne, Newcastle NE1 7RU, United Kingdom Dept. Eng. Sciences and Mathematics, Lule̊a University of Technology, S-97187 Lule̊a, Sweden

Stacking fault and dislocation glide on the basal plane of graphite

Generalized stacking-fault energies for the basal plane of graphite are calculated from first principles for slip along two high-symmetry directions. The rhombohedral fault energy compares well with experiment and the anisotropy in behaviour is consistent with observed dislocation network geometry. Utilizing these calculated fault energies within a modified Peierls-Nabarro model, we estimate the barrier for basal dislocation motion based on lattice friction. This is found to be extremely small, from which we conclude that dislocation network interaction and pinning, rather than the Peierls barrier, must determine the practical shear strength of graphite. However, at low dislocation densities or over small crystallite regions, the shear strength should tend to this lower limit. We discuss the relevance of this to the mechanism of lubrication.

Dislocations in hexagonal and cubic GaN

The structure and electronic activity of several types of dislocations in both hexagonal and cubic GaN are calculated using first-principles methods. Most of the stoichiometric dislocations investigated in hexagonal GaN do not induce deep acceptor states and thus cannot be responsible for the yellow luminescence. However, it is shown that electrically active point defects, in particular gallium vacancies and oxygen-related defect complexes, can be trapped at the stress field of the dislocations and may be responsible for this luminescence. For cubic GaN, we find the ideal stoichiometric 60° dislocation to be electrically active and the glide set to be more stable than the shuffle. The dissociation of the latter is considered.

Interaction of Oxygen with Threading Dislocations in GaN

A review is given of the results of first principles calculations used to investigate the structures and electronic properties of screw and edge dislocations in GaN. The atoms at the core of the full core screw dislocation possess heavily strained bonds leading to deep gap states. Removing the first shell of Ga and N atoms gives a screw dislocation with a small open core consisting of {1010} type surfaces. Therefore open-core screw dislocations induce only shallow gap states. In the same way we found the core of the threading edge dislocation to be reconstructed without any deep states. The interaction of oxygen with the cores of open-core screw and edge dislocations is considered and it is found that the impurity has a strong tendency to be bound by Ga vacancies leading to three types of defect trapped in the strain field. We suggest that the most stable defect leads to a poisoning of growth centres on the walls of nanopipes.

Nanometric Resolved Luminescence in h-BN Flakes: Excitons and Stacking Order

Romain Bourrellier, Michele Amato, 2 Luiz Henrique Galvão Tizei, Christine Giorgetti, Alexandre Gloter, Malcolm I. Heggie, Katia March, Odile Stéphan, Lucia Reining, Mathieu Kociak, and Alberto Zobelli ∗ Laboratoire de Physique des Solides, Univ. Paris-Sud, CNRS UMR 8502, F-91405, Orsay, France Laboratoire des Solides Irradiés, Ecole Polytechnique, Route de Saclay, F-91128 Palaiseau and European Theoretical Spectroscopy Facility (ETSF), France Department of Chemistry, University of Surrey, Guildford GU2 7XH, United Kingdom

Straight and kinked 90° partial dislocations in diamond and 3C-SiC

Density-functional based calculations are used to investigate low energy core structures of 90° partial dislocations in diamond and 3C-SiC. In both materials dislocation glide is analysed in terms of kink formation and migration and the fundamental steps to kink migration are investigated. We find the C terminated core structure in SiC to be more mobile than the Si core. However, the Si partial is electrically active and this opens the possibility of recombination-enhanced glide under ionizing conditions or an enhanced mobility in doped material.