C. D. Latham

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.

Stable and metastable states of C60H: buckminsterfullerene monohydride

Abstract Approximate ab initio Hartree—Fock and first-principles density functional calculations of potential energy surfaces and electronic structures of C 60 H show that the stable state has H attached to one C atom, outside the buckyball. This C atom is displaced radially outward and is close to being sp 3 hybridized. The unpaired electron of C 60 H is delocalized. The calculated Fermi contact density at the proton is in good agreement with recent low-temperature μSR data. A metastable configuration has atomic H at the center of the ball (H@C 60 ). Once trapped there, H must overcome a large barrier to go through the surface of C 60 . Other configurations we considered include H attached to one C atom but inside the buckyball, and H bridging one of the two inequivalent CC bonds. The barrier for diffusion of H from outside to the center of C 60 have also been calculated. The results are compared to recent muon spin rotation studies in solid C 60 and to the states of hydrogen in other forms of carbon.

Cooperative polarisation in ice Ih and the unusual strength of the hydrogen bond

Cooperative polarisation in ice Ih is illustrated in clusters of eight molecules using local density functional calculations where the exchange-correlation functional is a modified Ceperley-Alder expression. These parameter-free total energy calculations support the four-point charge/polarisable point dipole model of Kozack and Jordan and indicate that cooperative polarisation has a profound effect, not simply increasing the cohesive energy, but also making bond breaking disproportionately difficult.

Structures, Energetics and Electronic Properties of Complex III‐V Semiconductor Systems

A parallel implementation of the selfconsistent-charge density-functional based tight-binding (SCC-DFTB) method is used to examine large scale structures in III-V semiconductors. We firstly describe the parallel implementation of the method and its efficiency. We then turn to applications of the parallel code to complex GaAs systems. The geometries and energetics of different models for the √19 × √19 reconstruction at the (111) surface are investigated. A structure containing hexagonal rings of As at the surface consistent with STM experiments is found to be stable under Ga-rich growth conditions. We then examine voids in the bulk material which are mainly caused by the movement of dislocations. Void clusters of 12 missing atoms are found to be energetically favorable. This is in very good agreement with recent positron annihilation measurements. Additionally, we investigate the diffusion of C in p-type material and suggest a diffusion path with an activation energy of less than 1 eV which is consistent with experimental studies. Finally, focusing on GaN we provide atomistic insight into line defects in wurtzite GaN threading along the growing c-axis. We highlight the stability and electronic properties of screw and edge dislocations, discuss reasons for the formation of nanopipes and relate the yellow luminescence observed in highly defected materials to deep acceptors. V Ga and V Ga -(O N ) n , trapped at threading edge dislocations.

Molecular-diffusion of oxygen and water in crystalline and amorphous silica

Abstract The hydrolytic weakening of quartz requires diffusion of water to dislocation cores, while the oxidation of silicon requires diffusion of oxygen and/or water through amorphous silica (a-SiO2). Perfect α-quartz channels and larger dislocation channels are used to model the diffusion in α-quartz and a-SiO2 in conjunction with a classical interatomic potential for SiO2. Our results show that water and oxygen molecules are heavily compressed in perfect channels and much less so in the dislocation channel. The diffusion of these molecules through enlarged channels involves much lower activation energies and such channels will be preferred paths over the perfect channels.

The di-interstitial in graphite

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 has been assigned to them. The possibility that one of the structures may exhibit ferromagnetism is also investigated.

Ab initio energetics of CVD growth reactions on the three low-index surfaces of diamond

Two novel CVD diamond growth mechanisms are proposed which involve methyl radicals and acetylene molecules where carbon atoms are inserted into {100} 2 × 1 monohydride dimer surface reconstruction bonds. We have built models of the three principal surfaces of diamond and performed ab initio local density-based calculations to determine structures and energies of surface reactions including the two postulated for 2 × 1 reconstructed {100}. Both of these processes are found to be exothermic but with large barriers. However, they do demonstrate that reconstruction bonds can facilitate growth. The results are discussed with respect to the large body of recent experimental results and we conclude that the relative contributions of methyl and acetylene depend on the system used.

On the graphitization of diamond surfaces : the importance of twins

Ab initio total energy calculations reported recently (M.I. Heggie, C.D. Latham, R. Jones and P.R. Briddon, Phys. Rev. B, 50 (1994) 5937) revealed that the tetrahedrally bonded icosahedral C100 molecule decomposed spontaneously into two concentric fullerenes (C20 and C80). This C100 molecule belongs to a series of structures that may be viewed as the diamond analogues of fullerenes (L. Zeger and E. Kaxiras, Phys. Rev. Lett., 70 (1993) 2920). Since these molecules can be seen to be effectively a heavily twinned molecular diamond, their stability is important in the context of investigating the diamond “111” surface where a twin emerges. We present ab initio self-consistent calculations on a rather small C40H36 molecule representing the core of a twin intersecting two diamond “111” surfaces, and compare the results with those obtained with a non-self-consistent density-functional based tight-binding method. Since the latter is also capable of handling larger and periodic models in a molecular dynamics relaxation, we use it to study the graphitization effect as a function of temperature. We find nearly the same ground state for the small molecule which is clearly due to a graphitization, and find strong surface graphitization for a model of 128 carbon atoms at elevated temperatures. At 2700 K the top layer of this periodic model delaminates completely.

Ab initio calculations of the structure and dynamics of C60 and C60 3

A local-density-functional cluster method is used to calculate the structure and vibrational modes of C60. We find C–C lengths in good agreement with observed values. The effect of doping the molecule with three extra electrons is investigated and shown to result in a surprising shortening of the longer bonds. The second derivatives of the energy are evaluated and have enabled, for the first time, all the normal modes of the molecule to be found. We find these to be in fair agreement with the available experimental results.

The structures and properties of tetrafluoromethane, hexafluoroethane, and octafluoropropane using the AIMPRO density functional program

The AIMPRO density functional program was used to calculate the structures and properties of the three simplest perfluoroalkanes: tetrafluoromethane, hexafluoroethane, and octafluoropropane. The method reproduces both the molecular structures and the vibrational spectra well, and in addition, has identified a new C2-symmetry ortho-conformer of octafluoropropane that is approximately 0.525 kcal/mol (2.20 kJ/mol) higher in energy than the well-known C2v-symmetry global minimum staggered-conformer.