The core structure and stability of the 90 ± partial dislocation in diamond is studied within iso... more The core structure and stability of the 90 ± partial dislocation in diamond is studied within isotropic elasticity theory and ab initio total energy calculations. The double-period reconstruction is found to be more stable than the single-period reconstruction for a broad range of stress states. The analysis of the ab initio results shows further that elasticity theory is valid for dislocation spacings as small as 10 -20 Å, thus allowing ab initio calculations to provide reliable parameters for continuum theory analysis.
We present a new and general mechanism for inter-conversion of carbon structures via a catalytic ... more 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 equivilent uncatalysed reactions. We examine the relevance of this mechanism for fullerene growth, carbon onions and nanotubes, and dislocations in irradiated
We demonstrate that free graphene sheet edges can curl back on themselves, reconstructing as nano... more We demonstrate that free graphene sheet edges can curl back on themselves, reconstructing as nanotubes. This results in lower formation energies than any other non-functionalised edge structure reported to date in the literature. We determine the critical tube size and formation barrier and compare with density functional simulations of other edge terminations including a new reconstructed Klein edge. Simulated high resolution electron microscopy images show why such rolled edges may be difficult to detect. Rolled zigzag edges serve as metallic conduction channels, separated from the neighbouring bulk graphene by a chain of insulating sp 3 -carbon atoms, and introduce Van Hove singularities into the graphene density of states.
Iwata and Watanabe's model for the observed low-temperature specific heat of neutron-irradiated g... more Iwata and Watanabe's model for the observed low-temperature specific heat of neutron-irradiated graphite ͓T. Iwata and M. Watanabe, Phys. Rev. B 81, 014105 ͑2010͔͒ assumes that self-interstitial atoms exist as clusters of nearly free C 2 molecules. We suggest that their hypothesis is not supported by other experiments and theory, including our own calculations. Not only is it inconsistent with the long-known kinetics of interstitial prismatic dislocation loop formation, density-functional theory shows that the di-interstitial is covalently bonded to the host crystal. In such calculations no prior assumptions are made about the nature of the bonding, covalent or otherwise.
Calculations of fluorine binding and migration on carbon nanotube surfaces show that fluorine for... more Calculations of fluorine binding and migration on carbon nanotube surfaces show that fluorine forms varying surface superlattices at increasing temperatures. The ordering transition is controlled by the surface migration barrier for fluorine atoms to pass through next neighbor sites on the nanotube, explaining the transition from semi-ionic low coverage to covalent high coverage fluorination observed experimentally for gas phase fluorination between 200 and 250 C. The effect of solvents on fluorine binding and surface diffusion is explored.
ABSTRACT An ab initio density functional technique (AIMPRO) has been employed to investigate the ... more ABSTRACT An ab initio density functional technique (AIMPRO) has been employed to investigate the structure, vibrational properties, and dissociation mechanisms of CO30, the important radical anion CO3− and the interaction of this species with the graphite basal plane. The results are discussed in the context of the radiolytic oxidation of graphite: a process of relevance to the British nuclear industry, which relies for the most part on graphite-cored, CO2-cooled reactors. The radiation field splits coolant molecules and produces, amongst other things, a very reactive radical anion CO3−, which has been suggested as the main agent for the accelerated oxidation of graphite. This paper shows that CO3− binds strongly to graphite after combining with an electronic hole and forming a long and strong ionic bond. It still remains mobile on the basal plane and can diffuse to a graphite edge and oxidize it.
We report the results of first-principles calculations on the early stages of hydrogen aggregatio... more We report the results of first-principles calculations on the early stages of hydrogen aggregation in silicon and diamond. We demonstrate that the hydrogenated glide dislocation dipole is the preferred structure for small numbers of H atoms in silicon and that it expands by dislocation glide, with hydrogen condensing in the shuffle plane between the dislocations. This structure is a good candidate for the initial stage in the development of hydrogeninduced platelets. We investigate the effect of shear and dilation on the energies of hydrogenated structures and compare the relative stabilities of these structures in silicon and diamond. We describe the method of determination of the Burgers vectors of dilation and shear for the dislocation dipoles by varying the lattice vectors of their supercells.
The atomic processes associated with energy storage and release in irradiated graphite have long ... more The atomic processes associated with energy storage and release in irradiated graphite have long been subject to untested speculation. We examine structures and recombination routes for interstitialvacancy (I-V) pairs in graphite. Interaction results in the formation of a new metastable defect (an intimate I-V pair) or a Stone-Wales defect. The intimate I-V pair, although 2.9 eV more stable than its isolated constituents, still has a formation energy of 10.8 eV. The barrier to recombination to perfect graphite is calculated to be 1.3 eV, consistent with the experimental first Wigner energy release peak at 1.38 eV. We expect similar defects to form in carbon nanostructures such as nanotubes, nested fullerenes, and onions under irradiation.
We demonstrate that free graphene sheet edges can curl back on themselves, reconstructing as nano... more We demonstrate that free graphene sheet edges can curl back on themselves, reconstructing as nanotubes. This results in lower formation energies than any other non-functionalised edge structure reported to date in the literature. We determine the critical tube size and formation barrier and compare with density functional simulations of other edge terminations including a new reconstructed Klein edge. Simulated high resolution electron microscopy images show why such rolled edges may be difficult to detect. Rolled zigzag edges serve as metallic conduction channels, separated from the neighbouring bulk graphene by a chain of insulating sp 3 -carbon atoms, and introduce Van Hove singularities into the graphene density of states.
ABSTRACT Ab initio methods are used to investigate hydrogen defects in diamond. For the isolated ... more ABSTRACT Ab initio methods are used to investigate hydrogen defects in diamond. For the isolated impurity, the bond-centered site is found lowest in energy and posses both donor and acceptor levels. The neutral defect possesses a single local mode with very small infrared effective charge, but the effective charge for the negative charge state is much larger. H+ is calculated to be very mobile with a low activation barrier. Hydrogen dimers are stable as H*2 defects which are also found to be almost IR-inactive. The complex between B and H is investigated and the activation energy for the reaction B–H → B— + H+ found to be in rasonable agreement with experiment. Hydrogen is strongly bound to dislocations which, together with H*2, may form part of the hydrogen accumulation layer detected in some plasma studies.
... S. Scarle a , CP Ewels , and MI Heggie ... If a silicon wafer after implantation is bonded ... more ... S. Scarle a , CP Ewels , and MI Heggie ... If a silicon wafer after implantation is bonded to another and then annealed, this blistering turns into delamination, producing a uniform layer of silicon on top of the newly bonded crystal [25,23,24]. ...
ABSTRACT We examine possible mechanisms for graphitization at dislocation cores in diamond, both ... more ABSTRACT We examine possible mechanisms for graphitization at dislocation cores in diamond, both as an explanation for brown coloration in natural brown diamond and as a model for annealing in amorphous carbon materials. We find a metastable structure for the 90° partial dislocation core where bonds are broken in both single- and double-period structures leading to a row of paired sp2 C atoms. We examine the effect of hydrogen and other impurities on this core structure both through direct interaction and potential charging effects. We also investigate a mechanism for progressive aromatization through the migration and aggregation of 30° partial dislocations.
Dislocations in semiconductors can be strongly affected by a hydrogen plasma; core states may be ... more Dislocations in semiconductors can be strongly affected by a hydrogen plasma; core states may be passivated and mobility changed. For example, in silicon the activation barrier for dislocation motion drops by 1.0 eV upon exposure to H plasma for one hour at 470-540 • C. If such an effect were to be found in diamond, a simple scaling argument would yield an activation energy of 1.9 eV. Here, density functional calculations have been applied to the 90 • partial dislocation in diamond which confirm this prediction. They also show that, energetically, the soliton model for motion of the 90 • partial is as viable as the strained-bond model.
The problems with dislocations in semiconductors are becoming tractable with modern computing by ... more The problems with dislocations in semiconductors are becoming tractable with modern computing by hybrid techniques. These apply static first principles calculations of energetics for important processes (e.g. kink formation and migration energies) and kinetic Monte Carlo techniques to follow the dynamic interaction of these processes over length and time scales inaccessible to, for example, molecular dynamic simulation. The simplest model system for covalent and ceramic solids is silicon, but there is debate over the structure and properties of dislocations there. The movement of the dislocation by the simple bond switching mechanism was studied from first principles, finding activation energies close to experiment, but lately the alternative mechanism invoking free radicals or solitons was found to give similar energies. We report results from an n-fold way kinetic Monte Carlo approach, applied to a simple system to verify the standard model for kink pair nucleation limited dislocation glide (the Hirth-Lothe model). We then apply an improved technique to the kinetics of the soliton model and to hydrogen enhanced dislocation glide. r
replied: We did look very carefully at the C(1s) spectra but we are working with a single crystal... more replied: We did look very carefully at the C(1s) spectra but we are working with a single crystal sample rather than a high surface area powder and the surface concentrations we are dealing with (~20-30% monolayer) are too small to make a signicant difference against the bulk C(1s) background. However, we are condent of our assignments for a number of reasons: rstly, we only curve t where there is very good evidence that a peak is present; secondly, the curve ts have been tested against a series of spectra from different samples and the agreement is excellent; nally, we have since repeated this study using nitric acid in place of HCl and have obtained very similar results. 1
We demonstrate that free graphene sheet edges can curl back on themselves,reconstructing as nanot... more We demonstrate that free graphene sheet edges can curl back on themselves,reconstructing as nanotubes. This results in lower formation energies than any other non-functionalised edge structure reported to date in the literature. We determine the critical tube size and formation barrier and compare with density functional simulations of other edge terminations including a new reconstructed Klein edge. Simulated high resolution
Di-interstitial defects appear to play a key role in the microscopic understanding of radiation i... more Di-interstitial defects appear to play a key role in the microscopic understanding of radiation induced damage in graphite. Their formation has been invoked as both one of the main causes of dimensional change and as an energy releasing step in annealing cryogenic radiation induced damage. In the present work, first principles calculations are employed to examine several models for these defects. Two of the structures possess nearly equal energy, yet take very different forms. The results suggest that di-interstitial defects cannot play the principal role in radiation damage that have been assigned to them. The possibility that one of the structures may exhibit ferromagnetism is also investigated.
The core structure and stability of the 90 ± partial dislocation in diamond is studied within iso... more The core structure and stability of the 90 ± partial dislocation in diamond is studied within isotropic elasticity theory and ab initio total energy calculations. The double-period reconstruction is found to be more stable than the single-period reconstruction for a broad range of stress states. The analysis of the ab initio results shows further that elasticity theory is valid for dislocation spacings as small as 10 -20 Å, thus allowing ab initio calculations to provide reliable parameters for continuum theory analysis.
We present a new and general mechanism for inter-conversion of carbon structures via a catalytic ... more 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 equivilent uncatalysed reactions. We examine the relevance of this mechanism for fullerene growth, carbon onions and nanotubes, and dislocations in irradiated
We demonstrate that free graphene sheet edges can curl back on themselves, reconstructing as nano... more We demonstrate that free graphene sheet edges can curl back on themselves, reconstructing as nanotubes. This results in lower formation energies than any other non-functionalised edge structure reported to date in the literature. We determine the critical tube size and formation barrier and compare with density functional simulations of other edge terminations including a new reconstructed Klein edge. Simulated high resolution electron microscopy images show why such rolled edges may be difficult to detect. Rolled zigzag edges serve as metallic conduction channels, separated from the neighbouring bulk graphene by a chain of insulating sp 3 -carbon atoms, and introduce Van Hove singularities into the graphene density of states.
Iwata and Watanabe's model for the observed low-temperature specific heat of neutron-irradiated g... more Iwata and Watanabe's model for the observed low-temperature specific heat of neutron-irradiated graphite ͓T. Iwata and M. Watanabe, Phys. Rev. B 81, 014105 ͑2010͔͒ assumes that self-interstitial atoms exist as clusters of nearly free C 2 molecules. We suggest that their hypothesis is not supported by other experiments and theory, including our own calculations. Not only is it inconsistent with the long-known kinetics of interstitial prismatic dislocation loop formation, density-functional theory shows that the di-interstitial is covalently bonded to the host crystal. In such calculations no prior assumptions are made about the nature of the bonding, covalent or otherwise.
Calculations of fluorine binding and migration on carbon nanotube surfaces show that fluorine for... more Calculations of fluorine binding and migration on carbon nanotube surfaces show that fluorine forms varying surface superlattices at increasing temperatures. The ordering transition is controlled by the surface migration barrier for fluorine atoms to pass through next neighbor sites on the nanotube, explaining the transition from semi-ionic low coverage to covalent high coverage fluorination observed experimentally for gas phase fluorination between 200 and 250 C. The effect of solvents on fluorine binding and surface diffusion is explored.
ABSTRACT An ab initio density functional technique (AIMPRO) has been employed to investigate the ... more ABSTRACT An ab initio density functional technique (AIMPRO) has been employed to investigate the structure, vibrational properties, and dissociation mechanisms of CO30, the important radical anion CO3− and the interaction of this species with the graphite basal plane. The results are discussed in the context of the radiolytic oxidation of graphite: a process of relevance to the British nuclear industry, which relies for the most part on graphite-cored, CO2-cooled reactors. The radiation field splits coolant molecules and produces, amongst other things, a very reactive radical anion CO3−, which has been suggested as the main agent for the accelerated oxidation of graphite. This paper shows that CO3− binds strongly to graphite after combining with an electronic hole and forming a long and strong ionic bond. It still remains mobile on the basal plane and can diffuse to a graphite edge and oxidize it.
We report the results of first-principles calculations on the early stages of hydrogen aggregatio... more We report the results of first-principles calculations on the early stages of hydrogen aggregation in silicon and diamond. We demonstrate that the hydrogenated glide dislocation dipole is the preferred structure for small numbers of H atoms in silicon and that it expands by dislocation glide, with hydrogen condensing in the shuffle plane between the dislocations. This structure is a good candidate for the initial stage in the development of hydrogeninduced platelets. We investigate the effect of shear and dilation on the energies of hydrogenated structures and compare the relative stabilities of these structures in silicon and diamond. We describe the method of determination of the Burgers vectors of dilation and shear for the dislocation dipoles by varying the lattice vectors of their supercells.
The atomic processes associated with energy storage and release in irradiated graphite have long ... more The atomic processes associated with energy storage and release in irradiated graphite have long been subject to untested speculation. We examine structures and recombination routes for interstitialvacancy (I-V) pairs in graphite. Interaction results in the formation of a new metastable defect (an intimate I-V pair) or a Stone-Wales defect. The intimate I-V pair, although 2.9 eV more stable than its isolated constituents, still has a formation energy of 10.8 eV. The barrier to recombination to perfect graphite is calculated to be 1.3 eV, consistent with the experimental first Wigner energy release peak at 1.38 eV. We expect similar defects to form in carbon nanostructures such as nanotubes, nested fullerenes, and onions under irradiation.
We demonstrate that free graphene sheet edges can curl back on themselves, reconstructing as nano... more We demonstrate that free graphene sheet edges can curl back on themselves, reconstructing as nanotubes. This results in lower formation energies than any other non-functionalised edge structure reported to date in the literature. We determine the critical tube size and formation barrier and compare with density functional simulations of other edge terminations including a new reconstructed Klein edge. Simulated high resolution electron microscopy images show why such rolled edges may be difficult to detect. Rolled zigzag edges serve as metallic conduction channels, separated from the neighbouring bulk graphene by a chain of insulating sp 3 -carbon atoms, and introduce Van Hove singularities into the graphene density of states.
ABSTRACT Ab initio methods are used to investigate hydrogen defects in diamond. For the isolated ... more ABSTRACT Ab initio methods are used to investigate hydrogen defects in diamond. For the isolated impurity, the bond-centered site is found lowest in energy and posses both donor and acceptor levels. The neutral defect possesses a single local mode with very small infrared effective charge, but the effective charge for the negative charge state is much larger. H+ is calculated to be very mobile with a low activation barrier. Hydrogen dimers are stable as H*2 defects which are also found to be almost IR-inactive. The complex between B and H is investigated and the activation energy for the reaction B–H → B— + H+ found to be in rasonable agreement with experiment. Hydrogen is strongly bound to dislocations which, together with H*2, may form part of the hydrogen accumulation layer detected in some plasma studies.
... S. Scarle a , CP Ewels , and MI Heggie ... If a silicon wafer after implantation is bonded ... more ... S. Scarle a , CP Ewels , and MI Heggie ... If a silicon wafer after implantation is bonded to another and then annealed, this blistering turns into delamination, producing a uniform layer of silicon on top of the newly bonded crystal [25,23,24]. ...
ABSTRACT We examine possible mechanisms for graphitization at dislocation cores in diamond, both ... more ABSTRACT We examine possible mechanisms for graphitization at dislocation cores in diamond, both as an explanation for brown coloration in natural brown diamond and as a model for annealing in amorphous carbon materials. We find a metastable structure for the 90° partial dislocation core where bonds are broken in both single- and double-period structures leading to a row of paired sp2 C atoms. We examine the effect of hydrogen and other impurities on this core structure both through direct interaction and potential charging effects. We also investigate a mechanism for progressive aromatization through the migration and aggregation of 30° partial dislocations.
Dislocations in semiconductors can be strongly affected by a hydrogen plasma; core states may be ... more Dislocations in semiconductors can be strongly affected by a hydrogen plasma; core states may be passivated and mobility changed. For example, in silicon the activation barrier for dislocation motion drops by 1.0 eV upon exposure to H plasma for one hour at 470-540 • C. If such an effect were to be found in diamond, a simple scaling argument would yield an activation energy of 1.9 eV. Here, density functional calculations have been applied to the 90 • partial dislocation in diamond which confirm this prediction. They also show that, energetically, the soliton model for motion of the 90 • partial is as viable as the strained-bond model.
The problems with dislocations in semiconductors are becoming tractable with modern computing by ... more The problems with dislocations in semiconductors are becoming tractable with modern computing by hybrid techniques. These apply static first principles calculations of energetics for important processes (e.g. kink formation and migration energies) and kinetic Monte Carlo techniques to follow the dynamic interaction of these processes over length and time scales inaccessible to, for example, molecular dynamic simulation. The simplest model system for covalent and ceramic solids is silicon, but there is debate over the structure and properties of dislocations there. The movement of the dislocation by the simple bond switching mechanism was studied from first principles, finding activation energies close to experiment, but lately the alternative mechanism invoking free radicals or solitons was found to give similar energies. We report results from an n-fold way kinetic Monte Carlo approach, applied to a simple system to verify the standard model for kink pair nucleation limited dislocation glide (the Hirth-Lothe model). We then apply an improved technique to the kinetics of the soliton model and to hydrogen enhanced dislocation glide. r
replied: We did look very carefully at the C(1s) spectra but we are working with a single crystal... more replied: We did look very carefully at the C(1s) spectra but we are working with a single crystal sample rather than a high surface area powder and the surface concentrations we are dealing with (~20-30% monolayer) are too small to make a signicant difference against the bulk C(1s) background. However, we are condent of our assignments for a number of reasons: rstly, we only curve t where there is very good evidence that a peak is present; secondly, the curve ts have been tested against a series of spectra from different samples and the agreement is excellent; nally, we have since repeated this study using nitric acid in place of HCl and have obtained very similar results. 1
We demonstrate that free graphene sheet edges can curl back on themselves,reconstructing as nanot... more We demonstrate that free graphene sheet edges can curl back on themselves,reconstructing as nanotubes. This results in lower formation energies than any other non-functionalised edge structure reported to date in the literature. We determine the critical tube size and formation barrier and compare with density functional simulations of other edge terminations including a new reconstructed Klein edge. Simulated high resolution
Di-interstitial defects appear to play a key role in the microscopic understanding of radiation i... more Di-interstitial defects appear to play a key role in the microscopic understanding of radiation induced damage in graphite. Their formation has been invoked as both one of the main causes of dimensional change and as an energy releasing step in annealing cryogenic radiation induced damage. In the present work, first principles calculations are employed to examine several models for these defects. Two of the structures possess nearly equal energy, yet take very different forms. The results suggest that di-interstitial defects cannot play the principal role in radiation damage that have been assigned to them. The possibility that one of the structures may exhibit ferromagnetism is also investigated.
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Papers by M. Heggie