A Neural Theory of Visual Search: Recursive Attention to Segmentations and Surfaces

A neural theory is proposed in which visual search is accomplished by perceptual grouping and segregation, which occurs simultaneous across the visual field, and object recognition, which is restricted to a selected region of the field. The theory offers an alternative hypothesis to recently developed variations on Feature Integration Theory (Treisman, and Sato, 1991) and Guided Search Model (Wolfe, Cave, and Franzel, 1989). A neural architecture and search algorithm is specified that quantitatively explains a wide range of psychophysical search data (Wolfe, Cave, and Franzel, 1989; Cohen, and lvry, 1991; Mordkoff, Yantis, and Egeth, 1990; Treisman, and Sato, 1991).

Involvement of incipient hydrous oxide species in electrocatalysis at some metal electrode surfaces

In this thesis a novel theory of electrocatalysis at metal (especially noble metal)/solution interfaces was developed based on the assumption of metal adatom/incipient hydrous oxide cyclic redox transitions. Adatoms are considered as metastable, low coverage species that oxidise in-situ at potentials of often significantly cathodic to the regular metal/metal oxide transition. Because the adatom coverage is so low the electrochemical or spectroscopic response for oxidation is frequently overlooked; however, the product of such oxidation, referred to here as incipient hydrous oxide seems to be the important mediator in a wide variety of electrocatalytically demanding oxidation processes. Conversely, electrocatalytically demanding reductions apparently occur only at adatom sites at the metal/solution interface - such reactions generally occur only at potentials below, i.e. more cathodic than, the adatom/hydrous oxide transition. It was established that while silver in base oxidises in a regular manner (forming initially OHads species) at potentials above 1.0 V (RHE), there is a minor redox transition at much lower potentials, ca. o.35 v (RHE). The latter process is assumed to an adatom/hydrous oxide transition and the low coverage Ag(l) hydrous oxide (or hydroxide) species was shown to trigger or mediate the oxidation of aldehydes, e. g. HCHO. The results of a study of this system were shown to be in good agreement with a kinetic model based on the above assumptions; the similarity between this type of behaviour and enzyme-catalysed processes - both systems involve interfacial active sites - was pointed out. Similar behaviour was established for gold where both Au(l) and Au(lll) hydrous oxide mediators were shown to be the effective oxidants for different organic species. One of the most active electrocatalytic materials known at the present time is platinum. While the classical view of this high activity is based on the concept of activated chemisorption (and the important role of the latter is not discounted here) a vital role is attributed to the adatom/hydrous oxide transition. It was suggested that the well known intermediate (or anomalous) peak in the hydrogen region of the cyclic voltanmogram for platinum region is in fact due to an adatom/hydrous oxide transition. Using potential stepping procedures to minimise the effect of deactivating (COads) species, it was shown that the onset (anodic sweep) and termination (cathodic sweep) potential for the oxidation of a wide variety of organics coincided with the potential for the intermediate peak. The converse was also shown to apply; sluggish reduction reactions, that involve interaction with metal adatoms, occur at significant rates only in the region below the hydrous oxide/adatom transition.

Flat surfaces of finite type in the 3-sphere

We introduce the notion of flat surfaces of finite type in the 3- sphere, give the algebro-geometric description in terms of spectral curves and polynomial Killing fields, and show that finite type flat surfaces generated by curves on S2 with periodic curvature functions are dense in the space of all flat surfaces generated by curves on S2 with periodic curvature functions.

A microscopic study of structural and electronic properties of functionalized silicon surfaces based on first-principles

Surface modification of silicon with organic monolayers tethered to the surface by different linkers is an important process in realizing future (opto-)electronic devices. Understanding the role played by the nature of the linking group and the chain length on the adsorption structures and electronic properties of these assemblies is vital to advance this technology. This Thesis is a study of such properties and contributes in particular to a microscopic understanding of induced changes in the work function of experimentally studied functionalized silicon surfaces. Using first-principles density functional theory (DFT), at the first step, we provide predictions for chemical trends in the work function of hydrogenated silicon (111) surfaces modified with various terminations. For nonpolar terminating atomic species such as F, Cl, Br, and I, the change in the work function is directly proportional to the amount of charge transferred from the surface, thus relating to the difference in electronegativity of the adsorbate and silicon atoms. The change is a monotonic function of coverage in this case, and the work function increases with increasing electronegativity. Polar species such as −TeH, −SeH, −SH, −OH, −NH2, −CH3, and −BH2 do not follow this trend due to the interaction of their dipole with the induced electric field at the surface. In this case, the magnitude and sign of the surface dipole moment need to be considered in addition to the bond dipole to generally describe the change in work function. Compared to hydrogenated surfaces, there is slight increase in the work function of H:Si(111)-XH, where X = Te, Se, and S, whereas reduction is observed for surfaces covered with −OH, −CH3, and −NH2. Next, we study the hydrogen passivated Si(111) surface modified with alkyl chains of the general formula H:Si–(CH2)n–CH2 and H:Si–X–(CH2)n–CH3, where X = NH, O, S and n = (0, 1, 3, 5, 7, 9, 11), at half coverage. For (X)–Hexyl and (X)–Dodecyl functionalization, we also examined various coverages up to full monolayer grafting in order to validate the result of half covered surface and the linker effect on the coverage. We find that it is necessary to take into account the van der Waals interaction between the alkyl chains. The strongest binding is for the oxygen linker, followed by S, N, and C, irrespective of chain length. The result revealed that the sequence of the stability is independent of coverage; however, linkers other than carbon can shift the optimum coverage considerably and allow further packing density. For all linkers apart from sulfur, structural properties, in particular, surface-linker-chain angles, saturate to a single value once n > 3. For sulfur, we identify three regimes, namely, n = 0–3, n = 5–7, and n = 9–11, each with its own characteristic adsorption structures. Where possible, our computational results are shown to be consistent with the available experimental data and show how the fundamental structural properties of modified Si surfaces can be controlled by the choice of linking group and chain length. Later we continue by examining the work function tuning of H:Si(111) over a range of 1.73 eV through adsorption of alkyl monolayers with general formula -[Xhead-group]-(CnH2n)-[Xtail-group], X = O(H), S(H), NH(2). The work function is practically converged at 4 carbons (8 for oxygen), for head-group functionalization. For tail-group functionalization and with both head- and tail-groups, there is an odd-even effect in the behavior of the work function, with peak-to-peak amplitudes of up to 1.7 eV in the oscillations. This behavior is explained through the orientation of the terminal-group's dipole. The shift in the work function is largest for NH2-linked and smallest for SH-linked chains and is rationalized in terms of interface dipoles. Our study reveals that the choice of the head- and/or tail-groups effectively changes the impact of the alkyl chain length on the work function tuning using self-assembled monolayers and this is an important advance in utilizing hybrid functionalized Si surfaces. Bringing together the understanding gained from studying single type functionalization of H:Si(111) with different alkyl chains and bearing in mind how to utilize head-group, tail-group or both as well as monolayer coverage, in the final part of this Thesis we study functionalized H:Si(111) with binary SAMs. Aiming at enhancing work function adjustment together with SAM stability and coverage we choose a range of terminations and linker-chains denoted as –X–(Alkyl) with X = CH3, O(H), S(H), NH(2) and investigate the stability and work function of various binary components grafted onto H:Si(111) surface. Using binary functionalization with -[NH(2)/O(H)/S(H)]-[Hexyl/Dodecyl] we show that work function can be tuned within the interval of 3.65-4.94 eV and furthermore, enhance the SAM’s stability. Although direct Si-C grafted SAMs are less favourable compared to their counterparts with O, N or S linkage, regardless of the ratio, binary functionalized alkyl monolayers with X-alkyl (X = NH, O) is always more stable than single type alkyl functionalization with the same coverage. Our results indicate that it is possible to go beyond the optimum coverage of pure alkyl functionalized SAMs (50%) by adding a linker with the correct choice of the linker. This is very important since dense packed monolayers have fewer defects and deliver higher efficiency. Our results indicate that binary anchoring can modify the charge injection and therefore bond stability while preserving the interface electronic structure.

Gold nanoparticles on polarizable surfaces as Raman scattering antennas.

Surface plasmons supported by metal nanoparticles are perturbed by coupling to a surface that is polarizable. Coupling results in enhancement of near fields and may increase the scattering efficiency of radiative modes. In this study, we investigate the Rayleigh and Raman scattering properties of gold nanoparticles functionalized with cyanine deposited on silicon and quartz wafers and on gold thin films. Dark-field scattering images display red shifting of the gold nanoparticle plasmon resonance and doughnut-shaped scattering patterns when particles are deposited on silicon or on a gold film. The imaged radiation patterns and individual particle spectra reveal that the polarizable substrates control both the orientation and brightness of the radiative modes. Comparison with simulation indicates that, in a particle-surface system with a fixed junction width, plasmon band shifts are controlled quantitatively by the permittivity of the wafer or the film. Surface-enhanced resonance Raman scattering (SERRS) spectra and images are collected from cyanine on particles on gold films. SERRS images of the particles on gold films are doughnut-shaped as are their Rayleigh images, indicating that the SERRS is controlled by the polarization of plasmons in the antenna nanostructures. Near-field enhancement and radiative efficiency of the antenna are sufficient to enable Raman scattering cyanines to function as gap field probes. Through collective interpretation of individual particle Rayleigh spectra and spectral simulations, the geometric basis for small observed variations in the wavelength and intensity of plasmon resonant scattering from individual antenna on the three surfaces is explained.

The F4 fimbrial chaperone FaeE is stable as a monomer that does not require self-capping of its pilin-interactive surfaces.

Many Gram-negative bacteria use the chaperone-usher pathway to express adhesive surface structures, such as fimbriae, in order to mediate attachment to host cells. Periplasmic chaperones are required to shuttle fimbrial subunits or pilins through the periplasmic space in an assembly-competent form. The chaperones cap the hydrophobic surface of the pilins through a donor-strand complementation mechanism. FaeE is the periplasmic chaperone required for the assembly of the F4 fimbriae of enterotoxigenic Escherichia coli. The FaeE crystal structure shows a dimer formed by interaction between the pilin-binding interfaces of the two monomers. Dimerization and tetramerization have been observed previously in crystal structures of fimbrial chaperones and have been suggested to serve as a self-capping mechanism that protects the pilin-interactive surfaces in solution in the absence of the pilins. However, thermodynamic and biochemical data show that FaeE occurs as a stable monomer in solution. Other lines of evidence indicate that self-capping of the pilin-interactive interfaces is not a mechanism that is conservedly applied by all periplasmic chaperones, but is rather a case-specific solution to cap aggregation-prone surfaces.

Accurate ground-state potential energy surfaces of the C2H2–Kr and C2H2–Xe van der Waals complexes

Accurate ab initio intermolecular potential energy surfaces (IPES) have been obtained for the first time for the ground electronic state of the C 2H2-Kr and C2H2-Xe van der Waals complexes. Extensive tests, including complete basis set and all-electron scalar relativistic results, support their calculation at the CCSD(T) level of theory, using small-core relativistic pseudopotentials for the rare-gas atoms and aug-cc-pVQZ basis sets extended with a set of 3s3p2d1f1g mid-bond functions. All results are corrected for the basis set superposition error. The importance of the scalar relativistic and rare-gas outer-core (n.1)d correlation effects is investigated. The calculated IPES, adjusted to analytical functions, are characterized by global minima corresponding to skew T-shaped geometries, in which the Jacobi vector positioning the rare-gas atom with respect to the center of mass of the C2H2 moiety corresponds to distances of 4.064 and 4.229Å, and angles of 65.22° and 68.67° for C 2H2-Kr and C2H2-Xe, respectively. The interaction energy of both complexes is estimated to be -151.88 (1.817 kJ mol-1) and -182.76 cm-1 (2.186 kJ mol-1), respectively. The evolution of the topology of the IPES as a function of the rare-gas atom, from He to Xe, is also discussed. © 2012 Taylor and Francis.

Characterisation of freeze-dried wafers and solvent evaporated films as potential drug delivery systems to mucosal surfaces

Freeze-dried (lyophilised) wafers and solvent cast films from sodium alginate (ALG) and sodium carboxymethylcellulose (CMC) have been developed as potential drug delivery systems for mucosal surfaces including wounds. The wafers (ALG, CMC) and films (CMC) were prepared by freeze-drying and drying in air (solvent evaporation) respectively, aqueous gels of the polymers containing paracetamol as a model drug. Microscopic architecture was examined using scanning electron microscopy, hydration characteristics with confocal laser scanning microscopy and dynamic vapour sorption. Texture analysis was employed to investigate mechanical characteristics of the wafers during compression. Differential scanning calorimetry was used to investigate polymorphic changes of paracetamol occurring during formulation of the wafers and films. The porous freeze-dried wafers exhibited higher drug loading and water absorption capacity than the corresponding solvent evaporated films. Moisture absorption, ease of hydration and mechanical behaviour were affected by the polymer and drug concentration. Two polymorphs of paracetamol were observed in the wafers and films, due to partial conversion of the original monoclinic to the orthorhombic polymorph during the formulation process. The results showed the potential of employing the freeze-dried wafers and solvent evaporated films in diverse mucosal applications due to their ease of hydration and based on different physical mechanical properties exhibited by both type of formulations.

Development and mechanical characterization of solvent-cast polymeric films as potential drug delivery systems to mucosal surfaces

Solvent-cast films from three polymers, carboxymethylcellulose (CMC), sodium alginate (SA), and xanthan gum, were prepared by drying the polymeric gels in air. Three methods, (a) passive hydration, (b) vortex hydration with heating, and (c) cold hydration, were investigated to determine the most effective means of preparing gels for each of the three polymers. Different drying conditions [relative humidity - RH (6-52%) and temperature (3-45 degrees C)] were investigated to determine the effect of drying rate on the films prepared by drying the polymeric gels. The tensile properties of the CMC films were determined by stretching dumbbell-shaped films to breaking point, using a Texture Analyser. Glycerol was used as a plasticizer, and its effects on the drying rate, physical appearance, and tensile properties of the resulting films were investigated. Vortex hydration with heating was the method of choice for preparing gels of SA and CMC, and cold hydration for xanthan gels. Drying rates increased with low glycerol content, high temperature, and low relative humidity. The residual water content of the films increased with increasing glycerol content and high relative humidity and decreased at higher temperatures. Generally, temperature affected the drying rate to a greater extent than relative humidity. Glycerol significantly affected the toughness (increased) and rigidity (decreased) of CMC films. CMC films prepared at 45 degrees C and 6% RH produced suitable films at the fastest rate while films containing equal quantities of glycerol and CMC possessed an ideal balance between flexibility and rigidity.

General rules for predicting where a catalytic reaction should occur on metal surfaces: A density functional theory study of C-H and C-O bond breaking/making on flat, stepped, and kinked metal surfaces

To predict where a catalytic reaction should occur is a fundamental issue scientifically. Technologically, it is also important because it can facilitate the catalyst's design. However, to date, the understanding of this issue is rather limited. In this work, two types of reactions, CH4 CH3 + H and CO C + 0 on two transition metal surfaces, were chosen as model systems aiming to address in general where a catalytic reaction should occur. The dissociations of CH4 - CH3 + H and CO --> C + O and their reverse reactions on flat, stepped, and kinked Rh and Pd surfaces were studied in detail. We find the following: First, for the CH4 Ch(3) + H reaction, the dissociation barrier is reduced by similar to0.3 eV on steps and kinks as compared to that on flat surfaces. On the other hand, there is essentially no difference in barrier for the association reaction of CH3 + H on the flat surfaces and the defects. Second, for the CO C + 0 reaction, the dissociation barrier decreases dramatically (more than 0.8 eV on Rh and Pd) on steps and kinks as compared to that on flat surfaces. In contrast to the CH3 + H reaction, the C + 0 association reaction also preferentially occurs on steps and kinks. We also present a detailed analysis of the reaction barriers in which each barrier is decomposed quantitatively into a local electronic effect and a geometrical effect. Our DFT calculations show that surface defects such as steps and kinks can largely facilitate bond breaking, while whether the surface defects could promote bond formation depends on the individual reaction as well as the particular metal. The physical origin of these trends is identified and discussed. On the basis of our results, we arrive at some simple rules with respect to where a reaction should occur: (i) defects such as steps are always favored for dissociation reactions as compared to flat surfaces; and (ii) the reaction site of the association reactions is largely related to the magnitude of the bonding competition effect, which is determined by the reactant and metal valency. Reactions with high valency reactants are more likely to occur on defects (more structure-sensitive), as compared to reactions with low valency reactants. Moreover, the reactions on late transition metals are more likely to proceed on defects than those on the early transition metals.

Chiral discrimination within disordered adlayers on metal surfaces

Using a novel non-linear optical technique enantiomeric excess within a translationally disordered overlayer on a metal surface has been monitored for the first time.