Partial dislocations under forward bias in SiC

First-principles calculations are used to investigate the partial dislocations in 4H-SiC. We have shown that the Peierls barriers are strongly dependent on the dislocation core structures. Our results have revealed that the asymmetric reconstruction does not possess midgap states while the symmetric reconstructions, characterized by dangling bond on like atoms along the dislocation line, are always electrically active. We suggested that under forward bias, the free energies of the symmetric reconstructions are dynamically lowered by continuous electron-hole transitions between the respective deep levels and valence/conduction bands

Structural and thermodynamic properties of water related defects in α-quartz

We have investigated the atomic geometries and thermodynamic properties of water related defects in α-quartz using first-principles calculation. We confirm that the (OH)4 group is thermodynamically most stable and aggregates to form platelets in the form of microcracks with hydrolysed surfaces. We also examine other forms of defects which can be accessible out of equilibrium at high temperature. Finally, we discuss the consequences of our results for the hydrolytic weakening of α-quartz. © Springer-Verlag 2005

Metastable Frenkel pair defect in graphite: source of Wigner energy?

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 interstitial-vacancy (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

Ab initio study of relative motion of walls in carbon nanotubes

We study the interwall interaction and relative motion of walls in carbon nanotubes using density functional theory. The interwall interaction energy surface as a function of relative rotation and sliding of walls is calculated for the (5,5)@(10,10) nanotube. The barriers to relative rotation and sliding are estimated ab initio for the chiral walls of the (8,2)@(16,4) nanotube. These results are used to extract information on experimentally measurable quantities, such as threshold forces, diffusion coefficients, and mobilities of walls. Possible applications of these nanotubes in mechanical nanodevices are discussed. Two distinct regimes of the wall movement exist: athermal, forced movement (accelerating mode) and movement controlled by thermal diffusion (Fokker-Planck mode). We calculate the limits of these regimes from first principles. ©2005 The American Physical Society

Structure and energetics of the vacancy in graphite

We determine properties of the vacancy in graphite from first principles calculations. The ground-state structure is associated with a formation energy of 7.4 eV and arises through a combination of symmetric relaxation and symmetry-breaking Jahn-Teller distortion to one of three degenerate, symmetry-related structures. The distortion results in a weak reconstructed bond and small out-of-plane atomic displacements. Dynamic switching between degenerate structures is activated by a barrier of 0.1 eV and we interpret scanning tunneling microscopy observations on the basis of thermal averaging between structures. The calculated migration energy of 1.7 eV is lower than that widely accepted from experiment, and we propose that the discrepancy is explained by a revised picture of trapping during vacancy transport, dependent on concentration. We discuss the significance of these findings in understanding defect behavior in irradiated graphite and related graphitic materials, in particular single-walled nanotubes

Electron spectroscopy of carbon materials: Experiment and theory

We present a comparative spectroscopic study of carbon as graphite, diamond and C60 using C1s K-edge electron energy-loss spectroscopy (EELS), X-ray emission spectroscopy, and theoretical modelling. The first principles calculations of these spectra are obtained in the local density approximation using a self-consistent Gaussian basis pseudo-potential method. Calculated spectra show excellent agreement with experiment and are able to discriminate not only between various carbon hybridisations but also local variation in environment. Core-hole effects on the calculated spectra are also investigated. For the first time, the EEL spectrum of carbyne is calculated. © 2006 IOP Publishing Ltd