Publisher's copyright statement: This is the accepted version of the following article: Alsnani, Hind, Goss, J. P., Briddon, P. R., Rayson, M. J. Horsfall, A. B. (2019). First Principles Study of the Stability and Di usion Mechanism of a Carbon Vacancy in the Vicinity of a SiO2/4HSiC Interface. physica status solidi (a) 216(17): 1900328 which has been published in nal form at https://doi.org/10.1002/pssa.201900328. This article may be used for non-commercial purposes in accordance With Wiley Terms and Conditions for self-archiving.

B G Breeze,1 C J Meara,2, 3 X X Wu,1 C P Michaels,1 R Gupta,1 P L Diggle,4 M W Dale,4 B L Cann,4 T Ardon,5 U F S D’Haenens-Johansson,5 I Friel,6 M J Rayson,2 P R Briddon,2 J P Goss,2 M E Newton,1, 3 and B L Green1, ∗ Department of Physics, University of Warwick, Coventry, CV4 7AL, UK School of Engineering, Newcastle University, Newcastle upon Tyne NE1 7RU, UK EPSRC Centre for Doctoral Training in Diamond Science and Technology, UK De Beers Group Technology, Maidenhead, Berkshire, SL6 6JW, UK Gemological Institute of America, 50 W 47th Street, New York, New York 10036, USA Element Six, Global Innovation Centre, Fermi Avenue, Didcot OX11 0QR, UK We report the first experimental observation of a doubly-charged defect in diamond, SiV2−, in silicon and nitrogen co-doped samples. We measure spectroscopic signatures we attribute to substitutional silicon in diamond, and identify a silicon-vacancy complex decorated with a nearestneighbor nitrogen, SiVN, supported by theoretical calculations. Samples containing silicon and nitrogen are shown to be heavily photochromic, with the dominant visible changes due to the loss of SiV0/− and gain in the optically-inactive SiV2−.

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 fluoroparaphenyl polymers, and functionalised armchair edges in graphene nanoribbons. Notably we consider both structural and electronic evolution. Finally we examine C-60@[10]CPP, i.e. C-60 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.

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.

We use first-principles models to demonstrate how an organic oxidizing agent, F4 -TCNQ (7,7,8,8- tetracyano-2,3,5,6-tetrafluoroquinodimethane), modifies the electronic structure of silicon nanocrys- tals, suggesting it may enhance p-type carrier density and mobility. The proximity of the lowest unoccupied level of F4 -TCNQ to the highest occupied level of the Si-nanocrystals promotes the formation of an empty hybrid state overlapping both nanocrystal and molecule, reducing the exci- tation energy to about 0.8-1 eV in vacuum. Hence, it is suggested that F4 -TCNQ can serve both as a surface oxidant and a mediator for hole hopping between adjacent nanocrystals.

First-principles methods are used to investigate the self-interstitial and its aggregates in diamond. The experimental assignment of the spin-1 $R2$ EPR center to the single interstitial has been questioned because of the small fine-structure term observed. We calculate the spin-spin interaction tensor for the three interstitial defects ${I}_{1}^{〈001〉},$ ${I}_{2}^{\mathrm{NN}},$ and ${I}_{3}$ and compare with the experimental D tensors. The results give support for the assignments of the single and di-interstitials to microscopic models and allow us to conclusively identify a recently observed EPR center, $O3,$ with ${I}_{3}.$ This identification, in turn, suggests a low-energy structure for ${I}_{4}$ and a generic model for an extended defect called the platelet. We also determine the optical properties of ${I}_{1}^{〈001〉}$ as well as its piezospectroscopic or stress tensor and find these to be in agreement with experiment. Several multi-interstitial defects are found to possess different structural forms which may coexist. We propose that a different form of the charged ${I}_{2}$ defect gives rise to the $3H$ optical peak. Several structures of the platelet are considered, and we find that the lowest-energy model is consistent with microscopic and infrared studies.