Christopher D. Latham

The contribution made by lattice vacancies to the Wigner effect in radiation-damaged graphite

Models for radiation damage in graphite are reviewed and compared, leading to a re-examination of the contribution made by vacancies to annealing processes. A method based on density functional theory, using large supercells with orthorhombic and hexagonal symmetry, is employed to calculate the properties and behaviour of lattice vacancies and displacement defects. It is concluded that annihilation of intimate Frenkel defects marks the onset of recovery in electrical resistivity, which occurs when the temperature exceeds about 160 K. The migration of isolated monovacancies is estimated to have an activation energy of E(a) ≈ 1.1 eV. Coalescence into divacancy defects occurs in several stages, with different barriers at each stage, depending on the path. The formation of pairs ultimately yields up to 8.9 eV energy, which is nearly 1.0 eV more than the formation energy for an isolated monovacancy. Processes resulting in vacancy coalescence and annihilation appear to be responsible for the main Wigner energy release peak in radiation-damaged graphite, occurring at about 475 K.

On the validity of empirical potentials for simulating radiation damage in graphite: a benchmark

In this work, the ability of methods based on empirical potentials to simulate the effects of radiation damage in graphite is examined by comparing results for point defects, found using ab initio calculations based on density functional theory (DFT), with those given by two state of the art potentials: the Environment-Dependent Interatomic Potential (EDIP) and the Adaptive Intermolecular Reactive Empirical Bond Order potential (AIREBO). Formation energies for the interstitial, the vacancy and the Stone-Wales (5775) defect are all reasonably close to DFT values. Both EDIP and AIREBO can thus be suitable for the prompt defects in a cascade, for example. Both potentials suffer from arefacts. One is the pinch defect, where two α-atoms adopt a fourfold-coordinated sp(3) configuration, that forms a cross-link between neighbouring graphene sheets. Another, for AIREBO only, is that its ground state vacancy structure is close to the transition state found by DFT for migration. The EDIP fails to reproduce the ground state self-interstitial structure given by DFT, but has nearly the same formation energy. Also, for both potentials, the energy barriers that control diffusion and the evolution of a damage cascade, are not well reproduced. In particular the EDIP gives a barrier to removal of the Stone-Wales defect as 0.9 eV against DFTs 4.5 eV. The suite of defect structures used is provided as supplementary information as a benchmark set for future potentials.

Core properties and mobility of the basal screw dislocation in wurtzite GaN: a density functional theory study

We have performed first principles simulations, based on density functional theory (DFT), to investigate the core properties of the basal a -type screw dislocation in wurtzite gallium nitride. Our calculations demonstrate that the fully coordinated shuffle core configuration is the most energetically favourable. The calculated electronic structure of the a -type screw dislocation was found to exhibit exclusively shallow gap states which are not associated with any extended metallization. This may explain why a -type screw dislocations are less detrimental to the performance of GaN based electronic devices than c -type screw dislocations.

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’.

The influence of structural characteristics on the electronic and thermal properties of GaN/AlN core/shell nanowires

Interatomic potential based molecular dynamics and ab initio calculations are employed to investigate the structural, thermal, and electronic properties of polar GaN/AlN core/shell nanowires. Nanowire models for the molecular dynamics simulations contain hundreds of thousands of atoms with different shell-to-nanowire ratios. The energetic and structural properties are evaluated through a detailed examination of the strain, the stress, and the displacement fields. It is found that the relaxation of the AlN shell is initiated at the edges, with the shell becoming increasingly stress free when the shell-to-nanowire ratio is increased. The basal lattice parametera of the AlN shell is found to have a smaller value than the value predicted by the elasticity theory. The stresses on the GaN core are strongly influenced by the shell. The core retains the alattice parameter of bulk GaN only up to a shell-to-nanowire ratio equal to 0.10 and is significantly compressed beyond this point. Concerning the thermal properties, the molecular dynamics simulations conclude that there is a linear relationship between the thermal conductivity and the shell-to-core area ratio of the GaN/AlN core/shell nanowires. The bandgaps of the nanowires are calculated through ab initio calculations of 103 atoms and the influence of the structural characteristics on the electronic properties is investigated. A well-defined relationship that predicts the bandgap of the GaN/AlN nanowires, follows the 2nd order Vegards law and taking into account the shell-to-nanowire ratio, is established. Finally, the valence band maximum is found to be dominated by the surface N-2p levels, while the conduction band minimum is dominated by the core and interface Ga-3s, and the surface Al-2s levels.

Chapter 12:The Atomic-, Nano-, and Mesoscale Origins of Graphite's Response to Energetic Particles

A rich variety of phenomena are observed when graphite is exposed to high doses of radiation from energetic particles. Most notably, the crystals expand along their c-axes, and dimension changes of tens of percent or even more are easily achieved. There are significant changes to the thermal and electrical properties of the material as well. When irradiation occurs below about 400 K, energy accumulates in the material, and the amount can be large in proportion to the specific heat. Known as Wigner energy, this is released by annealing, and is accompanied by a partial reversal of the initial changes, including conservation of the crystal volume. Nevertheless, the original dimensions of the crystals are not restored. The origins of this behaviour are spread over the atomic, nanometre, and mesoscale. They lie in the generation of Frenkel pairs and, we argue, dislocations. Models based on density functional theory provide insight into the likely nature and evolution of the defect structure during and after radiation.