Biomedical subjects

John H Harding

Publications and source records attributed to John H Harding.

7 recordsLinked to original sources

PubMed2006

Graphitic nanofilms as precursors to wurtzite films: theory.

Periodic ab initio density functional calculations on ultrathin films of AlN, BeO, GaN, SiC, ZnO, and ZnS demonstrate the stabilization of thicker films terminating with the polar {0001} surface via charge transfer and metallization of the surface layers. In contrast thinner films remove the dipole by adopting a graphiticlike structure in which the atoms are threefold coordinate. This structure is thermodynamically the most favorable for these thinner films. Implications for the crystal growth of wurtzite materials are discussed.

Journal Article↗

PubMed2005

Modeling the properties of self-assembled monolayers terminated by carboxylic acids.

Self-assembled monolayers of long-chain carboxylic acids are often used as substrates to promote the growth of oriented crystals. Recent work has shown that the length of the chain (odd or even number of carbon atoms) determines whether oriented growth is observed. We use molecular dynamics simulations to investigate whether the configuration of the headgroups is significantly different in the two cases. We conclude that there are differences between odd- and even-length chains, even at 300 K and in the presence of water for some packings of the monolayer. We discuss whether these differences are large enough to account for the different behavior.

Journal Article↗

PubMed2005

Effect of bicarbonate ions on the crystallization of calcite on self-assembled monolayers.

We use molecular dynamics simulations to investigate the nucleation of calcite crystals on self-assembled monolayers. We show how the presence of bicarbonate ions adsorbed on the monolayer surface can both aid nucleation and control the orientation of the growth of the crystal. Using a simple model of the nucleation process and calculated interfacial energies, we calculate the enhancement (with respect to the homogeneous nucleation rate) of the nucleation of calcite on the (012) and (0001) faces. The calculations show clearly that the (012) face is favored over the (0001) face and that the nucleation rate is enhanced for self-assembled monolayers made from molecules containing an even number of carbon atoms in the alkyl chain over those containing an odd number.

Journal Article↗

PubMed2004

Growth of polar crystal surfaces on ionized organic substrates.

Calcite crystals nucleate on the (01.2) face on a diverse range of organic substrates, including self-assembled monolayers, hydrogen-bonded ribbons, and polymer rafts. The (01.2) face of calcite is a polar surface. Therefore macroscopic crystal growth can only occur if the dipole moment is quenched. We demonstrate that the dipole moment can be quenched for a given polar direction by adsorption onto an organic substrate with arbitrary charge density. The density of ions in the outer calcium plane must be modified, by introducing rows of vacancies, to fulfill the condition of zero net dipole moment. Interfacial energies are calculated for interfaces between the polar (01.2) and (00.1) faces of calcite and stearic acid monolayers with a range of densities. It was found that, contrary to the experimental evidence, the (00.1) face has lower interfacial energy than the (01.2) face with monolayers with equivalent densities. We give an explanation for this discrepancy based on kinetic models.

Journal Article↗

PubMed2004

Simulation of organic monolayers as templates for the nucleation of calcite crystals.

Living organisms can control the size, shape, and structure of minerals. Attempts to reproduce this biological control in the laboratory often use Langmuir monolayers of long-chain carboxylic acids. We use large-scale molecular dynamics simulations to calculate the interfacial energies of calcite crystals grown on stearic (octadecanoic) acid monolayers. In light of these simulations we discuss the argument that the orientation of the growing mineral is controlled by the organic substrate acting as a template which the mineral must fit in order to grow.

Adsorption↗

PubMed2004

Simulating surface diffusion and surface growth in ceramics.

We examine the movement of ion pairs on the surfaces of simple oxides. Using temperature-accelerated dynamics the elementary processes involved are identified and the activation energies of these used as input to kinetic Monte Carlo simulations. Results are presented for the motion of BaO and SrO ion pairs on the (100) surfaces of BaO and SrO, respectively, and the formation of island pairs on these surfaces is studied. The simulations reveal the importance of exchange mechanisms in surface diffusion and growth of oxides. The importance of such reactions has been recognised previously for metallic surfaces but not for ionic systems, where it has been assumed that ionic surface diffusion is surface diffusion via the hopping motion of ion pairs from one surface site to another. Exchange mechanisms can dominate transport processes both on terraces and steps for both homoepitaxial and heteroepitaxial growth. We suggest the unavoidable mixing when an exchange mechanism operates must be considered when attempting to grow sharp interfaces in oxide nanostructures.

Journal Article↗

PubMed2003

Not too big, not too small: the appropriate scale.

The tools we use at the human scale, whether mechanical, medical or microelectronic, depend on materials for which some other scale of length or time is critical. Often this is the mesoscale, between the scales of engineering and of atomic science. Linking underlying processes to what we handle is sometimes called 'spanning' (or 'bridging') length scales, giving the impression that the mesoscale is a swamp to be crossed without getting mud on our boots. This is misleading: we do not wish to span the mesoscale, but to work at the appropriate scale, and to connect that to our human needs. The appropriate scale need not rule out multiscale computer modelling, in which some supercode integrates relevant scales in one pass, hoping to combine the best of methods for two or more levels. But the reality for such attempts, too often, is that the worst of both regimes are found. Happily, simpler strategies at a judicious scale will often suffice.

Bone and Bones↗