Light emission from scanning tunnelling microscope on polycrystalline Au films - what is happening at the single-grain level?

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.

Investigation of dead-layer thickness in SrRuO3/Ba0.5Sr0.5TiO3/Au thin-film capacitors

Thin-film capacitors, with barium strontium titanate (BST) dielectric layers between 7.5 and 950 nm in thickness, were fabricated by pulsed-laser deposition. Both crystallography and cation chemistry were consistent with successful growth of the BST perovskite. At room temperature, all capacitors displayed frequency dispersion such that epsilon (100 kHz)/epsilon (100 Hz) was greater than 0.75. The dielectric constant as a function of thickness was fitted, using the series capacitor model, for BST thicknesses greater than 70 nm. This yielded a large interfacial d(i)/epsilon (i) ratio of 0.40 +/-0.05 nm, implying a highly visible parasitic dead layer within the capacitor structure. Modeled consideration of the dielectric behavior for BST films, whose total thickness was below that of the dead layer, predicted anomalies in the plots of d/epsilon against d at the dead-layer thickness. In the capacitors studied here, no anomaly was observed. Hence, either (i) 7.5 nm is an upper limit for the total dead-layer thickness in the SRO/BST/Au system, or (ii) dielectric collapse is not associated with a distinct interfacial dead layer, and is instead due to a through-film effect. (C) 2001 American Institute of Physics.

Dynamic, in situ optical, magnetic and magneto-optical monitoring of the growth of Co/Au and Pd/Co/Au multilayer systems

The growth of magnetron sputtered Co/Au and Pd/Co/Au superlattices on Au and Pd buffer layers, deposited onto glass substrates, has been monitored optically and magneto-optically in real time, using rotating analyser ellipsometry and Kerr polarimetry, at a wavelength of 633 nm. The magneto-optical traces, combined with ex situ and in situ hysteresis loops, provide a detailed and informative fingerprint of the optical and magnetic properties of the films as they evolve during growth. For Co/Au, oscillations in the polar magneto-optical effect developed during the deposition of An overlayers on Co and these may be attributed to quantum well states. However, the hysteresis measurements show that the magnetic field required to maintain saturation magnetization throughout the experiment was larger than available in situ, introducing a degree of confusion concerning the interpretation of the data. This problem was overcome by the incorporation of Pd layers into the Co/Au structure, thereby eliminating variation in magnetic orientation during growth of the Au layers as a contributory factor to the observations.

An isothermal-isobaric Langevin thermostat for simulating nanoparticles under pressure: Application to Au clusters

We present a method for simulating clusters or, molecules subjected to an external pressure, which is exerted by a pressure-transmitting medium. It is based on the canoninical Langevin thermostat, but extended in such a way that the Brownian forces are allowed to operate only from the region exterior to the cluster. We show that the frictional force of the Langevin thermostat is linked to the pressure of the reservoir in a unique way, and that this property manifests itself when the particle it acts upon is not pointlike but has finite dimensions. By choosing appropriately the strength of the random forces and the friction coefficient, both temperature and pressure can be controlled independently. We illustrate the capabilities of this new method by calculating the compressibility of small gold clusters under pressure.

Catalytic Role of Metal Oxides in Gold-Based Catalysts: A First Principles Study of CO Oxidation on TiO2 Supported Au

CO oxidation on TiO2 supported Au has been studied using density functional theory calculations. Important catalytic roles of the oxide have been identified: (i) CO oxidation occurs at the interface between Au and the oxide with a very small barrier; and (ii) O-2 adsorption at the interface is the key step in the reaction. The physical origin of the oxide promotion effect has been further investigated: The oxide enhances electron transfer from the Au to the antibonding states of O-2, giving rise to (i) strong ionic bonding between the adsorbed O-2, Au, and the Ti cation; and (ii) a significant activation of O-2 towards CO oxidation.

A combined experimental and theoretical study of the generation of palladium clusters on Au(111) with a scanning tunnelling microscope

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.

Generation of palladium clusters on Au(111) electrodes: Experiments and simulations

The properties of palladium clusters, generated with the electrochemical scanning tunneling microscope, have been investigated both by experiments and by computer simulations. The clusters are found to be larger and more stable if the tip is moved further towards the electrode surface in the generation process. The simulations suggest that the larger clusters consist of a palladium - gold mixture, which is more stable than pure palladium. Dissolution of the clusters occurs from the edges rather than layer by layer

Theoretical considerations of electrochemical phase formation for an ideal Frank-van der Merwe system - Ag on Au(111) and Au(100)

: Static calculation and preliminary kinetic Monte Carlo simulation studies are undertaken for the nucleation and growth on a model system which follows a Frank-van der Merwe mechanism. In the present case, we consider the deposition of Ag on Au(100) and Au(111) surfaces. The interactions were calculated using the embedded atom model. The kinetics of formation and growth of 2D Ag structures on Au(100) and Au(111) is investigated and the influence of surface steps on this phenomenon is studied. Very different time scales are predicted for Ag diffusion on Au(100) and Au(111), thus rendering very different regimes for the nucleation and growth of the related 2D phases. These observations are drawn from the application of a model free of any adjustable parameter.

Quantitative analysis of the reactivity of formate species seen by DRIFTS over a Au/Ce(La)O2 water–gas shift catalyst: First unambiguous evidence of the minority role of formates as reaction intermediates

The reactivity of the species formed at the surface of a Au/Ce(La)O2 catalyst during the water������¢���¯���¿���½���¯���¿���½gas shift (WGS) reaction were investigated by operando diffuse reflectance Fourier transform spectroscopy (DRIFTS) at the chemical steady state during isotopic transient kinetic analyses (SSITKA). The exchanges of the reaction product CO2 and of formate and carbonate surface species were followed during an isotopic exchange of the reactant CO using a DRIFTS cell as a single reactor. The DRIFTS cell was a modified commercial cell that yielded identical reaction rates to that measured over a quartz plug-flow reactor. The DRIFTS signal was used to quantify the relative oncentrations of the surface species and CO2. The analysis of the formate exchange curves between 428 and 493 K showed that at least two levels of reactivity were present. ������¢���¯���¿���½���¯���¿���½Slow formates������¢���¯���¿���½���¯���¿���½ displayed an exchange rate constant 10- to 20-fold slower than that of the reaction product CO2. ������¢���¯���¿���½���¯���¿���½Fast formates������¢���¯���¿���½���¯���¿���½ were exchanged on a time scale similar to that of CO2. Multiple nonreactive readsorption of CO2 took place, accounting for the kinetics of the exchange of CO2(g) and making it impossible to determine the number of active sites through the SSITKA technique. The concentration (in mol g������¢���¯���¿���½���¯���¿���½1) of formates on the catalyst was determined through a calibration curve and allowed calculation of the specific rate of formate decomposition. The rate of CO2 formation was more than an order of magnitude higher than the rate of decomposition of formates (slow + fast species), indicating that all of the formates detected by DRIFTS could not be the main reaction intermediates in the production of CO2. This work stresses the importance of full quantitative analyses (measuring both rate constants and adsorbate concentrations) when investigating the role of adsorbates as potential reaction intermediates, and illustrates how even reactive species seen by DRIFTS may be unimportant in the overall reaction scheme.

Molecular Plasmonics with Tunable Exciton-Plasmon Coupling Strength in J-Aggregate Hybridized Au Nanorod Assemblies

Controlling coherent electromagnetic interactions in molecular systems is a problem of both fundamental interest and important applicative potential in the development of photonic and opto-electronic devices. The strength of these interactions determines both the absorption and emission properties of molecules coupled to nanostructures, effectively governing the optical properties of such a composite metamaterial. Here we report on the observation of strong coupling between a plasmon supported by an assembly of oriented gold nanorods (ANR) and a molecular exciton. We show that the coupling is easily engineered and is deterministic as both spatial and spectral overlap between the plasmonic structure and molecular aggregates are controlled. We think that these results in conjunction with the flexible geometry of the ANR are of potential significance to the development of plasmonic molecular devices.

Progressive loss of ferroelectricity under bipolar pulsed fields and experimental determination of non-switchable polarization in Au/Ba0.5Sr0.5TiO3/SrRuO3 thin-film capacitors

175 nm-thick Ba0.5Sr0.5TiO3 (BST) thin film fabricated by pulsed laser deposition (PLD) technique is found to be a mixture of two distributions of material. We discuss whether these two components are nano-regions of paraelectric and ferroelectric phases, or a bimodal grain-size distribution, or an effect of oxygen vacancy gradient from the electrode interface. The fraction of switchable ferroelectric phase decreases under bipolar pulsed fields, but it recovers after removal of the external fields. The plot of capacitance in decreasing dc voltage (C(Vdown arrow) versus that in increasing dc 61 voltage C(Vup arrow) is a superposition of overlapping of two triangles, in contrast to one well-defined triangle for typical ferroelectric SrBi2Ta2O9 thin films.

Claves para el reconocimiento de la dehesa como "paisaje cultural" de Unesco

La dehesa española y su homónimo portugués (el montado) son sistemas agrosilvopastoriles mediterráneos que proyectan un paisaje excepcional y de elevados valores patrimoniales. Sobre ellos se ciernen nuevas o renovadas amenazas que comprometen su futuro, en un momento (el actual) de creciente revalorización socio-institucional de sus paisajes. Su declaración como paisaje cultural de Unesco puede constituir un potente instrumento para atajar dichas amenazas, pero requiere de la clarificación de ciertos interrogantes relacionados con las escalas. Este artículo se adentra en el valor universal excepcional de la dehesa y propone algunas claves para sustentar su posible incorporación a la Lista de Patrimonio Mundial de Unesco. El análisis se acomete a nivel de tipos de paisaje, pero se avanzan algunos criterios para la selección de ámbitos concretos y representativos susceptibles de integrar una candidatura como “bien en serie”.

Quantitative DRIFTS investigation of possible reaction mechanisms for the water-gas shift reaction on high-activity Pt- and Au-based catalysts

The present work emphasizes the importance of including a full quantitative analysis when in situ operando methods are used to investigate reaction mechanisms and reaction intermediates. The fact that some surface species exchange at a similar rate to the reaction product during isotopic transients is a necessary but not sufficient criterion for participation as a key reaction intermediate. This is exemplified here in the case of highly active low-temperature water-gas shift (WGS) catalysts based on gold and platinum. Operando DRIFTS data, isotopic exchanges, and DRIFTS calibration curves relating the concentration of formate species to the corresponding DRIFTS band intensity were combined to obtain a quantitative measure of the specific rate of formate decomposition. Despite displaying a rapid isotopic exchange rate (sometimes as fast as that of the reaction product CO2), the concentration of formates seen by DRIFTS was found to account for at most only 10% of the CO2 produced under the experimental conditions reported herein. These new results obtained on Au/CeZrO4 and Pt/CeO2 preparations (which are among the most active low-temperature WGS catalysts reported to date), led to the same conclusions regarding the minor role of IR-observable formates as those obtained in the case of less active Au/Ce(La)O-2 and Pt/ZrO2 catalysts. (c) 2007 Elsevier Inc. All rights reserved.