Ingo Barke

Combined temperature-programmed reaction and in situ x-ray scattering studies of size-selected silver clusters under realistic reaction conditions in the epoxidation of propene

The catalytic activity and dynamical shape changes in size-selected nanoclusters at work are studied under realistic reaction conditions by using a combination of simultaneous temperature-programmed reaction with in situ grazing-incidence small angle x-ray scattering. This approach allows drawing a direct correlation between nanocatalyst size, composition, shape, and its function under realistic reaction conditions for the first time. The approach is illustrated in a chemical industry highly relevant selective partial oxidation of propene on a monodisperse silver nanocatalyst. The shape of the catalyst undergoes rapid change already at room temperature upon the exposure to the reactants, followed by a complex evolution of shape with increasing temperature. Acrolein formation is observed around 50 degrees C while the formation of the propylene oxide exhibits a sharp onset at 80 degrees C and is leveling off at 150 degrees C. At lower temperatures acrolein is produced preferentially to propylene oxide; at temperatures above 100 degrees C propylene oxide is favored.

The 3D-architecture of individual free silver nanoparticles captured by X-ray scattering

The diversity of nanoparticle shapes generated by condensation from gaseous matter reflects the fundamental competition between thermodynamic equilibration and the persistence of metastable configurations during growth. In the kinetically limited regime, intermediate geometries that are favoured only in early formation stages can be imprinted in the finally observed ensemble of differently structured specimens. Here we demonstrate that single-shot wide-angle scattering of femtosecond soft X-ray free-electron laser pulses allows three-dimensional characterization of the resulting metastable nanoparticle structures. For individual free silver particles, which can be considered frozen in space for the duration of photon exposure, both shape and orientation are uncovered from measured scattering images. We identify regular shapes, including species with fivefold symmetry and surprisingly large aspect ratio up to particle radii of the order of 100 nm. Our approach includes scattering effects beyond Born’s approximation and is remarkably efficient—opening up new routes in ultrafast nanophysics and free-electron laser science.

Structure and energetics of Si(111)-(5×2)-Au

We propose a new structural model for the Si(111)-(5x2)-Au reconstruction. The model incorporates a new experimental value of 0.6 monolayer for the coverage of gold atoms, equivalent to six gold atoms per 5x2 cell. Five main theoretical results, obtained from first-principles total-energy calculations, support the model. (1) In the presence of silicon adatoms the periodicity of the gold rows spontaneously doubles, in agreement with experiment. (2) The dependence of the surface energy on the adatom coverage indicates that a uniformly covered phase is unstable and will phase-separate into empty and covered regions, as observed experimentally. (3) Theoretical scanning tunneling microscopy images are in excellent agreement with experiment. (4) The calculated band structure is consistent with angle-resolved photoemission spectra; analysis of their correspondence allows the straightforward assignment of observed surface states to specific atoms. (5) The calculated activation barrier for diffusion of silicon adatoms along the row direction is in excellent agreement with the experimentally measured barrier.

Design and capabilities of an experimental setup based on magnetron sputtering for formation and deposition of size-selected metal clusters on ultra-clean surfaces

The design and performance of an experimental setup utilizing a magnetron sputtering source for production of beams of ionized size-selected clusters for deposition in ultra-high vacuum is described. For the case of copper cluster formation the influence of different source parameters is studied and analyzed. Size-selected clusters are deposited on substrates and the efficiency of an electrostatic quadrupole mass selector is tested. Height analysis using atomic force microscopy (AFM) demonstrates relative standard size deviations of 7%-10% for the particles of various sizes between 6 nm and 19 nm. Combined analysis by AFM and transmission electron microscopy reveals that the clusters preserve almost spherical shape after the deposition on amorphous carbon substrates. Supported nanoparticles of a few nanometres in diameter have crystalline structure with a face-centered cubic (fcc) lattice.

Molecular nanostructures with strong dipole moments on the Si(111) 5 ? 2-Au surface

Switchable organic molecules adsorbed on a silicon surface combine the flexibility and the low cost of molecular electronic devices with the sophistication of modern silicon technology. The first step towards creating such hybrid devices is the formation of regular, ordered patterns of molecules on a silicon surface. A stepped Si surface passivated by a sub-monolayer of gold is found to provide a useful substrate for forming ordered molecular patterns. Molecules with strong dipole moments, such as fluorophenols, form a one-dimensional molecular array on such a substrate by adsorbing on top of the step edges. Local barrier height measurement by scanning tunneling spectroscopy demonstrates the possibilities to detect the direction of the dipole moment of an individual molecule. Polarization-dependent x-ray absorption spectroscopy reveals an oriented adsorption in both the azimuthal and polar directions.

Pronounced Size Dependence in Structure and Morphology of Gas-Phase Produced, Partially Oxidized Cobalt Nanoparticles under Catalytic Reaction Conditions

It is generally accepted that optimal particle sizes are key for efficient nanocatalysis. Much less attention is paid to the role of morphology and atomic arrangement during catalytic reactions. Here, we unravel the structural, stoichiometric, and morphological evolution of gas-phase produced and partially oxidized cobalt nanoparticles in a broad size range. Particles with diameters between 1.4 and 22 nm generated in cluster sources are size selected and deposited on amorphous alumina (Al2O3) and ultrananocrystalline diamond (UNCD) films. A combination of different techniques is employed to monitor particle properties at the stages of production, exposure to ambient conditions, and catalytic reaction, in this case, the oxidative dehydrogenation of cyclohexane at elevated temperatures. A pronounced size dependence is found, naturally classifying the particles into three size regimes. While small and intermediate clusters essentially retain their compact morphology, large particles transform into hollow spheres due to the nanoscale Kirkendall effect. Depending on the substrate, an isotropic (Al2O3) or anisotropic (UNCD) Kirkendall effect is observed. The latter results in dramatic lateral size changes. Our results shed light on the interplay between chemical reactions and the catalysts structure and provide an approach to tailor the cobalt oxide phase composition required for specific catalytic schemes.

Morphological impact on the reaction kinetics of size-selected cobalt oxide nanoparticles

Apart from large surface areas, low activation energies are essential for efficient reactions, particularly in heterogeneous catalysis. Here, we show that not only the size of nanoparticles but also their detailed morphology can crucially affect reaction kinetics, as demonstrated for mass-selected, soft-landed, and oxidized cobalt clusters in a 6 nm to 18 nm size range. The method of reflection high-energy electron diffraction is extended to the quantitative determination of particle activation energies which is applied for repeated oxidation and reduction cycles at the same particles. We find unexpectedly small activation barriers for the reduction reaction of the largest particles studied, despite generally increasing barriers for growing sizes. We attribute these observations to the interplay of reaction-specific material transport with a size-dependent inner particle morphology.

Current-Dependent Periodicities of Si(553)-Au

We investigate quasi one-dimensional atomic chains on Si(553)-Au with a scanning tunneling microscope (STM). The observed periodicity at the Si step edge can be altered by the STM and depends on the magnitude of the tunneling current. In a recent report this reversible structural transition was attributed to transient doping with a characteristic time scale of a few milliseconds. Here we explore the evolution of the STM topography as a function of the magnitude of the tunneling current for a wide temperature range. Based on a decomposition of topographic line profiles and a detailed Fourier analysis we conclude that all observed currentdependent STM topographies can be explained by a time-averaged linear combination of two fluctuating step-edge structures. These data also reveal the precise relative alignment of the characteristic STM features for both phases along the step edges. A simple diagram is developed, presenting the relative contribution of these phases to the STM topography as a function of tunneling current and temperature. Time- and current-dependent measurements of fluctuations in the tunneling current reveal two different transition regimes that are related to two specific current injection locations within the surface unit cell. A method based on spatially resolved I(z)curves is presented that enables a quantitative analysis of contributing phases.

Correlative microscopy of morphology and luminescence of Cu porphyrin aggregates

Transfer of energy and information through molecule aggregates requires as one important building block anisotropic, cable-like structures. Knowledge on the spatial correlation of luminescence and morphology represents a prerequisite in the understanding of internal processes and will be important for architecting suitable landscapes. In this context we study the morphology, fluorescence and phosphorescence of molecule aggregate structures on surfaces in a spatially correlative way. We consider as two morphologies, lengthy strands and isotropic islands. It turns out that phosphorescence is quite strong compared to fluorescence and the spatial variation of the observed intensities is largely in line with the amount of dye. However in proportion, the strands exhibit more fluorescence than the isotropic islands suggesting weaker non-radiative channels. The ratio fluorescence to phosphorescence appears to be correlated with the degree of aggregation or internal order. The heights at which luminescence saturates is explained in the context of attenuation and emission multireflection, inside the dye. This is supported by correlative photoemission electron microscopy which is more sensitive to the surface region. The lengthy structures exhibit a pronounced polarization dependence of the luminescence with a relative dichroism up to about 60%, revealing substantial perpendicular orientation preference of the molecules with respect to the substrate and parallel with respect to the strands.

Ex situ investigations of MOCVD-grown gallium nitride nanowires using reflection high energy electron diffraction

Vertically oriented nanowires (NWs) of single-crystalline wurtzite GaN have been fabricated on sapphire substrates, via metal organic chemical vapor deposition (MOCVD). We present ex situ investigations on orientation and structure of grown GaN nanowires on GaN(0001) surface using reflection high energy electron diffraction (RHEED). Both ordered and randomly oriented GaN crystalline structures have been detected.