Biomedical subjects

Laurence D Marks

Publications and source records attributed to Laurence D Marks.

6 recordsLinked to original sources

PubMed2006

Charge defects glowing in the dark.

We investigate the effects of local charge defects in HREM imaging, using electron densities calculated by density functional methods. As a model of a planar interface with a local charge defect we use the polar MgO square root (111)-3 x 3R30 degrees surface, which has an additional hole per surface unit cell. A complimentary example, the non-polar MgO (100) surface that has no local charge defect is simulated for comparison. We show that the contrast due to local charge defects is rather high, and suggest that they should be directly observable.

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PubMed2006

Fitting valence charge densities at a crystal surface.

A procedure is reported for obtaining a much better initial parameterization of the charge density than that possible from a neutral atom model. This procedure involves the parameterization of a bulk charge density model in terms of simple variables such as bond lengths, which can then be transferred to the problem of interest, for instance a surface. Parameterization is accomplished through the fitting of density functional theory calculations for a variety of crystal distortions. The details of the parameterization are discussed for the specific case of silicon. This parameterized model can then be applied to surfaces or to other problems where an initial higher-order model is needed without the addition of any extra fitted parameters. The non-convexity of the charge density problem is also discussed.

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PubMed2005

Solution-phase, triangular ag nanotriangles fabricated by nanosphere lithography.

A novel method to produce solution-phase triangular silver nanoparticles is presented. Ag nanoparticles are prepared by nanosphere lithography and are subsequently released into solution. The resulting nanoparticles are asymmetrically functionalized to produce either single isolated nanoparticles or dimer pairs. The structural and optical properties of Ag nanoparticles have been characterized. Mie theory and the Discrete Dipole Approximation method (DDA) have been used to model and interpret the optical properties of the released Ag nanoparticles.

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PubMed2004

Quantitative analyses of precession diffraction data for a large cell oxide.

Kinematical and two-beam calculations have been conducted and are compared to experimental precession data for the large unit cell crystal La4Cu3MoO12. Precession electron diffraction intensities are found to exhibit approximate two-beam behavior and demonstrate clear advantages over conventional SADP intensities for use in structure solution.

Copper↗

PubMed2003

Surface structures of SrTiO3 (001): a TiO2-rich reconstruction with a c(4 x 2) unit cell.

We report the solution of the c(4 x 2) reconstruction of SrTiO(3) (001), obtained through a combination of high-resolution transmission electron microscopy, direct methods analysis, and density functional theory. The structure is characterized by a single overlayer of TiO(2) stoichiometry in which TiO(5) polyhedra are arranged into edge-shared structures, in contrast to the corner-shared TiO(6) polyhedra in bulk. This structural pattern is similar to that reported by us earlier for the (2 x 1) reconstruction of the same crystal face formed at higher temperature. We discuss probable mechanisms of surface stabilization as revealed by these two solutions which are likely to apply to other reconstructions of SrTiO(3) (001) and, possibly, other perovskites in general.

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PubMed2002

The structure and chemistry of the TiO(2)-rich surface of SrTiO(3) (001).

Oxide surfaces are important for applications in catalysis and thin film growth. An important frontier in solid-state inorganic chemistry is the prediction of the surface structure of an oxide. Comparatively little is known about atomic arrangements at oxide surfaces at present, and there has been considerable discussion concerning the forces that control such arrangements. For instance, one model suggests that the dominant factor is a reduction of Coulomb forces; another favours minimization of 'dangling bonds' by charge transfer to states below the Fermi energy. The surface structure and properties of SrTiO(3)--a standard model for oxides with a perovskite structure--have been studied extensively. Here we report a solution of the 2 x 1 SrTiO(3) (001) surface structure obtained through a combination of high-resolution electron microscopy and theoretical direct methods. Our results indicate that surface rearrangement of TiO(6-x) units into edge-sharing blocks determines the SrO-deficient surface structure of SrTiO(3). We suggest that this structural concept can be extended to perovskite surfaces in general.

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