Book of Abstracts Book of Abstracts - Universität Konstanz

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The Size-Evolution of the Optical Properties of Supported Gold
Clusters and Nanocrystals Studied by Cavity Ringdown Spectroscopy:
From the Atom to the Bulk
154
J. M. Antonietti, M. Michalski, H. Jones, and U. Heiz
Institute of Surface Chemistry and Catalysis, Universität Ulm, 89069 Ulm, Germany
Measuring the size-dependent electronic structure of supported clusters near the Fermi level
is of critical importance in order to understand the catalytic activities of supported clusters or to
tune the optical properties of cluster assembled materials with the number of atoms. Up to now,
extensive studies on the electronic structure of size-selected, supported clusters are sparse,
because of the experimental challenges which must be overcome: preparation of monodispersed
samples, high sensitivity required for recording the signal arising from a highly dilute sample,
detection of the signal due to the clusters from the intense background signal arising from the
substrate.
In order to overcome these problems, a new experimental setup has been designed and built to
perfom Cavity Ringdown Spectroscopy (CRDS) in the visible spectral range on size-selected
clusters deposited on silica in UHV conditions. The clusters are produced in a laser vaporization
source, size-selected in the gas phase, and soft-landed onto the substrate.
In this work, the ability to prepare monodispersed cluster samples is demonstrated. The ultimate
sensitivity of the setup is such that absorption cross-sections of 0.02 Å 2 with cluster coverage
down to 0.05% of a monolayer (~1 10 12 clusters/cm 2 ) can be detected. The absorption spectra of
gold clusters, Aun (n = 1, 4, 8, and 20) deposited on silica are presented. Additionally, the
optical properties of self-organized gold nanocrystals with well-defined diameters (1 to 20 nm),
prepared by chemical means and deposited on silica have been measured. The spectrum of the
atom exhibits sharp peaks which are attributed to atoms attached at various defect sites on the
surface. A dramatic size-sensitivity of the absorption properties of clusters is observed.
However, Au8 shows features which are associated with nanocrystals, rather than small clusters.