A. De Vita

Theory of composite BxCyNz nanotube heterojunctions

Examines the stability and electronic properties of composite B[sub x]C[sub y]N[sub z] nanotube heterojunctions. Use of ab initio density functional calculations and semi-empirical approaches; Advantage of nanotubes; Independence of junction characteristics from nanotube factors.

Low-speed fracture instabilities in a brittle crystal

When a brittle material is loaded to the limit of its strength, it fails by the nucleation and propagation of a crack. The conditions for crack propagation are created by stress concentration in the region of the crack tip and depend on macroscopic parameters such as the geometry and dimensions of the specimen. The way the crack propagates, however, is entirely determined by atomic-scale phenomena, because brittle crack tips are atomically sharp and propagate by breaking the variously oriented interatomic bonds, one at a time, at each point of the moving crack front. The physical interplay of multiple length scales makes brittle fracture a complex ‘multi-scale’ phenomenon. Several intermediate scales may arise in more complex situations, for example in the presence of microdefects or grain boundaries. The occurrence of various instabilities in crack propagation at very high speeds is well known, and significant advances have been made recently in understanding their origin. Here we investigate low-speed propagation instabilities in silicon using quantum-mechanical hybrid, multi-scale modelling and single-crystal fracture experiments. Our simulations predict a crack-tip reconstruction that makes low-speed crack propagation unstable on the (111) cleavage plane, which is conventionally thought of as the most stable cleavage plane. We perform experiments in which this instability is observed at a range of low speeds, using an experimental technique designed for the investigation of fracture under low tensile loads. Further simulations explain why, conversely, at moderately high speeds crack propagation on the (110) cleavage plane becomes unstable and deflects onto (111) planes, as previously observed experimentally.

Deposition of calcium ions on rutile (110) : A first-principles investigation

Abstract The deposition of calcium ions is the first and most crucial step of apatite nucleation on ceramic supports from ionic solution. This process is believed to initiate the growth of bone-like material on the surface of biocompatible implants. We have investigated the adsorption of Ca2+ from water solution on the rutile TiO2 (110) surface by means of first principles techniques. The preferential binding site of the calcium ion on the hydrated oxide surface was determined through a series of static calculations. Molecular dynamics simulations were then performed to elucidate the deposition pathway. The driving force for adsorption is identified in the electrostatic interaction between the Ca2+ complexes and negatively charged deprotonated sites present on the hydrated TiO2 (110) surface.

Multiscale hybrid simulation methods for material systems

We review recent progress in the field of multiscale hybrid computer simulations of materials, and present an overview of a novel scheme that links arbitrary atomistic simulation techniques together in a truly seamless manner. Rather than constructing a new hybrid Hamiltonian that combines different models, we use a unique short range classical potential and continuously tune its parameters to reproduce the atomic trajectories at the prescribed level of accuracy throughout the system.

Macroscopic scattering of cracks initiated at single impurity atoms

Brittle crystals, such as coloured gems, have long been known to cleave with atomically smooth fracture surfaces, despite being impurity laden, suggesting that isolated atomic impurities do not generally cause cracks to deflect. Whether cracks can ever deviate when hitting an atomic defect, and if so how they can go straight in real brittle crystals, which always contain many such defects, is still an open question. Here we carry out multiscale molecular dynamics simulations and high-resolution experiments on boron-doped silicon, revealing that cracks can be deflected by individual boron atoms. The process, however, requires a characteristic minimum time, which must be less than the time spent by the crack front at the impurity site. Deflection therefore occurs at low crack speeds, leading to surface ridges which intensify when the boron-dopage level is increased, whereas fast-moving cracks are dynamically steered away from being deflected, yielding smooth cleavage surfaces.

A first principles based polarizable O(N) interatomic force field for bulk silica

We present a reformulation of the Tangney-Scandolo interatomic force field for silica [J. Chem. Phys. 117, 8898 (2002)], which removes the requirement to perform an Ewald summation. We use a Yukawa factor to screen electrostatic interactions and a cutoff distance to limit the interatomic potential range to around 10 Å. A reparametrization of the potential is carried out, fitting to data from density functional theory calculations. These calculations were performed within the local density approximation since we find that this choice of functional leads to a better match to the experimental structural and elastic properties of quartz and amorphous silica than the generalized gradient approximation approach used to parametrize the original Tangney-Scandolo force field. The resulting O(N) scheme makes it possible to model hundreds of thousands of atoms with modest computational resources, without compromising the force field accuracy. The new potential is validated by calculating structural, elastic, vibrational, and thermodynamic properties of α-quartz and amorphous silica.

Ab initio study of tritium defects in lithium oxide

Lithium oxide has been suggested as a suitable breeder blanket material for fusion reactors. Tritium ions and lithium vacancies are created by neutron irradiation, forming bulk defect complexes whose exact character is experimentally unclear. We have used ab initio total energy pseudopotential methods to study the structure and relative energies of tritium as a substitutional defect, and of the separate tritium interstitial and lithium vacancy. For all stable defect geometries, the formation of an OT- complex with an O-T bond length of about 1 AA is found to be energetically favoured. In the case of the substitutional defect this bond is found to point towards the vacant Li site, but the direction is fairly free for the interstitial case. The binding energy of tritium to a lithium vacancy is found to be 1.3 eV. Structural relaxation effects are included throughout, and are found to significantly affect the relative energies of different defect geometries. The effects of zero-point fluctuations are estimated and found not to be very significant. The most probable migration path of interstitial tritium is identified as a jump between nearest-neighbour oxygen ions, with an activation energy of 0.45 eV, in agreement with experimental evidence. The results suggest a picture of thermally assisted diffusion of tritium interstitials and lithium vacancies along the anion and cation sublattices respectively, with the preferential trapping of the two defects into substitutional complexes.

Ab initio based multiscale modelling for materials science

Materials modelling of extended defects in semiconductors (and many other systems) requires both detailed electronic models of matter to account for bond breaking and formation at the atomic scale and the representation of material systems at large scales, in the micrometre–microsecond range. These twin demands, if implemented directly by ab initio calculations, are unachievable with potentially available computational resources for the foreseeable future. An alternative approach is to develop multiscale simulations, where the level of simulation detail can vary in time and space, thus saving on computational cost without sacrificing the necessary detailed modelling. This paper introduces the basic concepts and reviews some progress in this field, and the related challenges along two main strands: (i) sequential multiscale modelling to construct larger-scale material models from first principles and (ii) hybrid multiscale modelling for the description of unitary systems which are too large for monoscale modelling at the desired accuracy.

Chemically driven molecular decomposition at semiconductor surfaces

We present an analysis of the chemistry of dissociative adsorption based on an ab initio molecular dynamics simulation of Cl2 incident on Si(111)-2×1 at an energy of 1 eV. It is shown that the break-up of the molecule occurs because of charge transfer into a more antibonding molecular orbital. The process is strongly orientationally dependent and is absent if the molecule is oriented perpendicular to the surface. In this case the molecule enters a precursor state and subsequently dissociates when it escapes from the local energy minimum.

Two-dimensional core–shell donor–acceptor assemblies at metal–organic interfaces promoted by surface-mediated charge transfer

Organic charge transfer (CT) complexes obtained by combining molecular electron donors and acceptors have attracted much interest due to their potential applications in organic opto-electronic devices. In order to work, these systems must have an electronic matching - the highest occupied molecular orbital (HOMO) of the donor must couple with the lowest unoccupied molecular orbital (LUMO) of the acceptor - and a structural matching, so as to allow direct intermolecular CT. Here it is shown that, when molecules are adsorbed on a metal surface, novel molecular organizations driven by surface-mediated CT can appear that have no counterpart in condensed phase non-covalent assemblies of donor and acceptor molecules. By means of scanning tunneling microscopy and spectroscopy it is demonstrated that the electronic and self-assembly properties of an electron acceptor molecule can change dramatically in the presence of an additional molecular species with marked electron donor character, leading to the formation of unprecedented core-shell assemblies. DFT and classical force-field simulations reveal that this is a consequence of charge transfer from the donor to the acceptor molecules mediated by the metallic substrate.