Biomedical subjects

A S Foster

Publications and source records attributed to A S Foster.

10 recordsLinked to original sources

PubMed2006

Finite-element implementation for electron transport in nanostructures.

We have modeled transport properties of nanostructures using Green's-function method within the framework of the density-functional theory. The scheme is computationally demanding, so numerical methods have to be chosen carefully. A typical solution to the numerical burden is to use a special basis-function set, which is tailored to the problem in question, for example, the atomic-orbital basis. In this paper we present our solution to the problem. We have used the finite-element method with a hierarchical high-order polynomial basis, the so-called p elements. This method allows the discretation error to be controlled in a systematic way. The p elements work so efficiently that they can be used to solve interesting nanosystems described by nonlocal pseudopotentials. We demonstrate the potential of the implementation with two different systems. As a test system a simple Na-atom chain between two leads is modeled and the results are compared with several previous calculations. Secondly, we consider a thin hafnium dioxide (HfO2) layer on a silicon surface as a model for a gate structure of the next generation of microelectronics.

Journal Article↗

PubMed2005

Reactions and clustering of water with silica surface.

The interaction between silica surface and water is an important topic in geophysics and materials science, yet little is known about the reaction process. In this study we use first-principles molecular dynamics to simulate the hydrolysis process of silica surface using large cluster models. We find that a single water molecule is stable near the surface but can easily dissociate at three-coordinated silicon atom defect sites in the presence of other water molecules. These extra molecules provide a mechanism for hydrogen transfer from the original water molecule, hence catalyzing the reaction. The two-coordinated silicon atom is inert to the water molecule, and water clusters up to pentamer could be stably adsorbed at this site at room temperature.

Catalysis↗

PubMed2005

Probing organic layers on the TiO2(110) surface.

In this work, we use first principles simulations to provide features of the dynamic scanning force microscopy imaging of adsorbed organic layers on insulating surfaces. We consider monolayers of formic (HCOOH) and acetic (CH(3)COOH) acid and a mixed layer of acetic and trifluoroacetic acids (CF(3)COOH) on the TiO(2)(110) surface and study their interaction with a silicon dangling bond tip. The results demonstrate that the silicon tip interacts more strongly with the substrate and the COO(-) group than the adsorbed acid headgroups, and, therefore, molecules would appear dark in images. The pattern of contrast and apparent height of molecules is determined by the repulsion between the tip and the molecular headgroups and by significant deformation of the monolayer and individual molecules. The height of the molecule on the surface and the size of the headgroup play a large role in determining access of the tip to the substrate and, hence, the contrast in images. Direct imaging of the molecules themselves could be obtained by providing a functionalized tip with attraction to the molecular headgroups, for example, a positive potential tip.

Journal Article↗

PubMed2004

Adsorption of acetic and trifluoroacetic acid on the TiO2(110) surface.

We use the first-principles static and dynamic simulations to study the adsorption of acetic (CH(3)COOH) and trifluoroacetic (CF(3)COOH) acid on the TiO(2)(110) surface. The most favorable adsorption for both molecules is a dissociative process, which results in the two oxygens of the carboxylate ion bonding to in-plane titanium atoms in the surface. The remaining proton then bonds to a bridging oxygen site, forming a hydroxyl group. We further show that, by comparing the calculated dipoles of the molecules on the surface, it is possible to understand the difference in contrast over the acetate and trifluoroacetate molecules in the atomically resolved noncontact atomic force microscopy images.

Journal Article↗

PubMed2004

Irradiation-induced magnetism in graphite: a density functional study.

Recent experiments indicate that proton irradiation triggers ferromagnetism in originally nonmagnetic graphite samples while He ion bombardment has a much smaller effect. To understand the origin of irradiation-induced magnetism, we have performed spin-polarized density functional theory calculations of the magnetic properties of the defects which are most likely to appear under irradiation vacancies and vacancy-hydrogen complexes. Both defects are magnetic, but as for the latter we find that H adsorption on one of the vacancy dangling bonds gives rise to a magnetic moment double that of the naked vacancy. We show that for small irradiation doses vacancy-hydrogen complexes result in a macroscopic magnetic signal which agrees well with the experimental values.

Journal Article↗

PubMed2004

Interaction of silicon dangling bonds with insulating surfaces.

We use first principles density functional theory calculations to study the interaction of a model dangling bond silicon tip with the surfaces of CaF2, Al2O3, TiO2, and MgO. In each case the strongest interaction is with the highest anions in the surface. We show that this is due to the onset of chemical bonding with the surface anions, which can be controlled by an electric field across the system. Combining our results and previous studies on semiconductor surfaces suggests that using dangling bond Si tips can provide immediate identification of surface species in atomically resolved noncontact atomic force microscopy and facilitate selective measurements of short-range interactions with surface sites.

Journal Article↗

PubMed2003

Magnetic properties and diffusion of adatoms on a graphene sheet.

We use ab initio methods to calculate the properties of adatom defects on a graphite surface. By applying a full spin-polarized description to the system we demonstrate that these defects have a magnetic moment of about 0.5micro(B) and also calculate its role in diffusion over the surface. The magnetic nature of these intrinsic carbon defects suggests that it is important to understand their role in the recently observed magnetism in pure carbon systems.

Journal Article↗

PubMed2002

Mechanism of interstitial oxygen diffusion in hafnia.

We have performed density functional calculations of oxygen incorporation and diffusion in monoclinic hafnia (HfO2) for a range of oxygen charge states. The calculations demonstrate that oxygen favors atomic incorporation and that O2- is the most stable species. We find that oxygen interstitials diffuse via exchange with lattice oxygen sites in hafnia, and that O- species have the smallest diffusion barrier.

Journal Article↗

PubMed2001

Unambiguous interpretation of atomically resolved force microscopy images of an insulator.

The (111) surface of CaF2 was imaged with dynamic mode scanning force microscopy and modeled using atomistic simulation. Both experiment and theory showed a clear triangular contrast pattern in images, and theory demonstrated that the contrast pattern is due to the interaction of a positive electrostatic potential tip with fluorine ions in the two topmost surface layers. We find a good agreement of position and relative height of scan line features between theory and experiment and thus establish for the first time an unambiguous identification of sublattices of an insulator imaged by force microscopy.

Journal Article↗

PubMed1998

Reading achievement by learning disabled students in resource and regular classes.

K-TEA Comprehensive Reading scores of 34 elementary boys in either resource rooms or regular settings were compared. The boys were identified as learning disabled in reading. They were pretested at the beginning of the school year and posttested at the end. Treatment was one year of daily instruction in reading provided by six teachers in resource setting and six teachers in regular settings. K-TEA Reading Decoding and Reading Comprehension scores, separately compared in 2 x 2 repeated-measures analysis of variance, were not significantly different.

Child↗