Wolf-Dieter Schneider

Highly ordered structures and submolecular scanning tunnelling microscopy contrast of PTCDA and DM-PBDCI monolayers on Ag(111) and Ag(110)

Abstract Highly ordered monolayers of two perylene derivatives (perylene-3,4,9,10-tetracarboxylic-3,4,9,10-dianhydride (PTCDA) and N,N′-dimethylperylene-3,4,9,10-bis(dicarboximide) (DM-PBDCI)) were prepared by vacuum sublimation on Ag(111) and Ag(110) surfaces. Using scanning tunnelling microscopy (STM) and low energy electron diffraction, the nucleation, growth, step site interaction, and superstructures were investigated and compared. All adsorbate structures are commensurate, with the exception of DM-PBDCI on Ag(111). A similar, but not identical, submolecular STM resolution was obtained for both molecules and on both surfaces. The local distribution shows ten dominant intensity maxima per molecule and corresponds to that of the lowest unoccupied orbital. From these results we draw conclusions concerning the tunnelling mechanism, the adsorbate–substrate, and the adsorbate–adsorbate interactions, which are discussed in relation to the formerly published results on the electronic structure of PTCDA/Ag(111).

Photon Emission at Molecular Resolution Induced by a Scanning Tunneling Microscope

The tip-surface region of a scanning tunneling microscope (STM) emits light when the energy of the tunneling electrons is sufficient to excite luminescent processes. These processes provide access to dynamic aspects of the local electronic structure that are not directly amenable to conventional STM experiments. From monolayer films of carbon-60 fullerenes on gold(110) surfaces, intense emission is observed when the STM tip is placed above an individual molecule. The diameter of this emission spot associated with carbon-60 is approximately 4 angstroms. These results demonstrate the highest spatial resolution of light emission to date with a scanning probe technique.

Chemical reactivity of size-selected supported clusters: An experimental setup

We describe an experimental setup for the investigation of the chemical reactivity of size-selected, supported clusters. The clusters are generated by a high frequency laser vaporization source. After mass selection and energy control the clusters are deposited under ultrahigh-vacuum conditions and with low kinetic energy onto thin oxide films grown on metal single crystals. Such films are ideal in their function as supports for size-selected deposited metal clusters. Measured currents of mass-selected niobium cluster ions were of the order of 1 nA and are high enough for fast deposition rates. Adsorbed molecules used on the one hand as probes to characterize the supported clusters and on the other hand for studying chemical reactions on these clusters are investigated in situ by Fourier transform infrared spectroscopy, temperature desorption spectroscopy, and Auger electron spectroscopy. The performance of these analytical tools is illustrated with CO adsorbed on Ni atoms supported on thin MgO(100) films...

Nanoassembled model catalysts

The present review outlines, on the basis of specific examples taken from our laboratory, the most important steps in (i) preparing supported nanoassembled model catalysts and (ii) investigating their size-dependent catalytic properties. We describe the cluster generation and present evidence for softlanding of the clusters onto solid surfaces. Subsequently, the growth and the characterization of the cluster support material, thin magnesium oxide films, are discussed, including the role of surface defects. Then the thermal stability of Cu clusters and the individual electronic structure of small Ag and Cu clusters on MgO are addressed. Finally, two examples of cluster size-dependent heterogeneous catalytic reactions are presented, (i) the cyclotrimerization of acetylene on nanoassembled Pd catalysts, and (ii) the CO oxidation on nanoassembled Au catalysts. We show experimentally and together with first-principle calculations that the interaction of such small metal clusters with the oxide surface strongly changes their catalytic properties. In contrast to large particles an additional electron in localized valence states of small clusters represents a major change. It is this charge transfer which turns inactive gold and palladium clusters into active model catalysts opening new perspectives to tune catalytic processes on the nanoscale.

Fluorescence and Phosphorescence from Individual C60 Molecules Excited by Local Electron Tunneling

Using the highly localized current of electrons tunneling through a double barrier scanning tunneling microscope junction, we excite luminescence from a selected C60 molecule in the surface layer of fullerene nanocrystals grown on an ultrathin NaCl film on Au(111). In the observed fluorescence and phosphorescence spectra, pure electronic as well as vibronically induced transitions of an individual C60 molecule are identified, leading to unambiguous chemical recognition on the single-molecular scale.

Separation of a Racemic Mixture of Two‐Dimensional Molecular Clusters by Scanning Tunneling Microscopy

Adsorption of sub-monolayer amounts of 1-nitronaphthalene (NN) onto Au(111) leads to the aggregation of NN decamers, which exhibit two-dimensional chirality and represent a racemic mixture. In analogy to Pasteurs experiment of 1848 a scanning tunneling microscope can be used to discriminate and separate the enantiomers on a molecular scale.

Insulators at the ultrathin limit: electronic structure studied by scanning tunnelling microscopy and scanning tunnelling spectroscopy

Considerable progress has been made recently, using scanning tunnelling microscopy (STM), scanning tunnelling spectroscopy (STS) and local density functional theory (DFT), in examining the atomic structure and electronic properties of ultrathin insulating films. This article reviews pertinent results to date with special emphasis on ultrathin MgO films on Ag(001) surfaces. Using STS, the layer-by-layer resolved electronic structure up to 3 ML shows that the band gap of about 6 eV at the MgO surface develops within the first 3 ML confirmed by local density of states (LDOS) calculations. Using model calculations, the atomic species observed in STM on the MgO film are unambiguously identified. These results underline the importance of a combination of local spectroscopy, scanning probe techniques and local density of states calculations for the understanding of matter on the microscopic level.

Size-Selected Clusters on Solid Surfaces

The present critical review examines the status of the research on supported, size-selected clusters with specific emphasis on their size-dependent physical and chemical properties. We describe the most relevant experimental methods with their advantages and drawbacks for the production of size-selected clusters and look critically at the present evidence for soft-landing of the clusters onto solid surfaces. Growth and characterization of cluster support materials are surveyed and the important role of surface defects for the properties of the supported clusters is considered. Recent advances in the characterization of the supported, size-selected clusters with local and non-local surface science analytical tools are presented and a few illustrative examples for size effects are given. Of specific and possibly broad impact on heterogeneous catalysis, we consider the discovery of the size-dependence of chemical reactions on very small supported clusters where recent experimental and theoretical evidence indicates charge transfer from the support to the cluster. This charge transfer turns inactive clusters into active ones, opening new perspectives to tune catalytic processes on the nanoscale.

Tunnelling spectroscopy of surface state scattering and confinement

Using low-temperature scanning tunnelling microscopy we have investigated confinement of electronic surface states to hexagonal islands and other nanoscale structures on Ag(111). Local spectroscopy and spatial maps of the differential conductance are analysed using simple models of the electronic structure. We find that the concept of confinement of a two-dimensional surface state applies down to very small structures, and observe in real space how proximity to defects affects surface state lifetimes.

Two‐Dimensional Tiling by Rubrene Molecules Self‐Assembled in Supramolecular Pentagons, Hexagons, and Heptagons on a Au(111) Surface

Keywords: rubrene ; scanning probe microscopy ; self-assembly ; supramolecular chemistry ; Crystals ; Symmetry ; Graphite Reference EPFL-ARTICLE-161210doi:10.1002/anie.200704479View record in Web of Science Record created on 2010-11-30, modified on 2016-08-09