1000 resultados para reaction microscope


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The formation and reactivity of surface intermediates in the reverse water-gas-shift reaction on a Pt/CeO2 catalyst are critically dependent on the reaction conditions so that conclusionsregarding the reaction mechanism cannot be inferred using ex operando conditions.

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When operated with a metallic tip and sample the scanning tunnelling microscope constitutes a nanoscale, plasmonic light source yielding broadband emission up to a photon energy determined by the applied bias. The emission is due to tunnelling electron excitation and subsequent radiative decay of localized plasmon modes, which can be on the lateral scale of a single metal grain (similar to 25 nm) or less. For a Au-tip/Au-polycrystalline sample under ambient conditions it is found that the intensity and spectral content of the emitted light are not dependent on the lateral grain dimension, but are predominantly determined by the tip geometry. However, the intensity increases strongly with increasing film thickness (grain depth) up to 20-25 nm or approximately the skin depth of the Au film. Photon maps can show less emissive grains and two classes of this occurrence are distinguished. The first is geometrical in origin - a double-tip structure in this case - while the second is due to a contamination-induced lowering of the local work function that causes the tunnel gap to increase. It is suggested that differences in work-function lowering between grains presenting different crystalline facets, combined with an exponential decay in emitted light intensity with tip - sample distance, leads to grain contrast. These results are relevant to tip-enhanced Raman scattering and the fabrication of micro/nano-scale planar, light-emitting tunnel devices.

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First steps are taken to model the electrochemical deposition of metals in nanometer-sized cavities. In the present work, the electrochemical deposition of Cu atoms in nanometer-sized holes dug on Au(111) is investigated through Monte Carlo simulations using the embedded atom method to represent particle interactions. By sweeping the chemical potential of Cu, a cluster is allowed to grow within the hole rising four atomic layers above the surface. Its lateral extension remains confined to the area defined by the borders of the original defect. (C) 2004 Elsevier B.V. All rights reserved.

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In the present work we consider two aspects of the deposition of metal clusters on an electrode surface. The formation of such clusters with the tip of a scanning tunneling microscope is simulated by atom dynamics. Subsequently the stability of these clusters is investigated by Monte Carlo simulations in a grand-canonical ensemble. In particular, the following systems were considered explicitly: Pd clusters on Au(111), Cu on Au(111), Ag on Au(111), Pb on Au(111) and Cu on Ag(111). The analysis of the results obtained for the different systems leads to the conclusion that optimal systems for nanostructuring are those where the metals participating have similar cohesive energies and negative heats of alloy formation. In this respect, the system Cu-Pd(111) is predicted as a good candidate for the formation of stable clusters. (c) 2005 Elsevier B.V. All rights reserved.

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Palladium clusters have been deposited on the surface of a Au(111) electrode with the tip of a scanning tunnelling microscope. The distance over which the tip was moved towards the surface has a decisive influence on the properties of the clusters: the larger this distance, the larger the generated clusters, and the more stable they are. These findings are supported by computer simulations, which further suggest that the larger clusters contain a sizable amount of gold, which enhances their stability. Dissolution of the clusters occurs from the edges rather than layer by layer.

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A semiclassical complex angular momentum theory, used to analyze atom-diatom reactive angular distributions, is applied to several well-known potential (one-particle) problems. Examples include resonance scattering, rainbow scattering, and the Eckart threshold model. Pade reconstruction of the corresponding matrix elements from the values at physical (integral) angular momenta and properties of the Pade approximants are discussed in detail.