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Grenzflächen-Analyse durch Mie-Plasmon-Spektroskopie an Edelmetallclustern

  • Hilger, Almuth
Publication Date
Jan 01, 2003
Publikationsserver der RWTH Aachen University
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The topological and electronic structure of condensed matter in the nanometer range between molecule and solid state is an important topic of present fundamental research. Both size and interface phenomena and in particular the rise of interband transitions were determined in clusters on the nanometer scale with optical spectroscopy. Clusters made of silver are especially sensitive to these effects since they expose a very distinct Mie-resonance-absorption. Silver- and gold-cluster with the size of 2R = 2nm...4 nm were produced inside a thermal heater in a special high-vacuum-chamber by supersonic expansion. The cluster-density is high enough for an optical measurement in the free beam. Silver-clusters in the free beam can be compared with cluster of the same run deposited on different substrates or compared with clusters codeposited with various matrix-materials. For substrate coating and embedding matrix different materials were used: Oxides (Al2O3, Sb2O5, TiO2, SrTiO2, Cr2O3, SnO2, In2O3, ITO, SiO2), Fluorides (MgF2, CaF2, LiF), C60 and Si. The detected resonance-absorption cannot be described quantitatively within the classical Mie-Theory. The extended Mie-Theory allowed the identification of electron transfer between cluster and matrix and the interface-induced damping of the free electrons within the cluster. The width of the Mie-resonance absorption is caused mainly by the damping of the 5s-electrons in the silver-cluster. The damping depends on the cluster-size and on the chemical-interface damping at the cluster-matrix-interface. In this thesis the changes of interband-transitions in clusters compared to bulk silver were discovered by means of a Kramers-Kronig-Analysis. Electron-spill-out into the surrounding matrix, static charge transfer through chemical cluster-matrix-bonding and/or changed interband-transitions caused a shift between calculated and measured Mie-resonance maximum. The two-stage ellipsoidal model after the theory of Gans and Yamagushi takes coupling between clusters into account. The behavior at the cluster-matrix-interface is documented by an contact area of different size. Interband-transitions in the silver-cluster proved to be size-dependent and additionally matrix-dependent. This was shown for clusters in the free beam and for deposited and embedded clusters. The production of goldclusters and their measurement in the free beam should confirm the results obtained from silver-clusters. A new built thermal heater and first measurements were shown in this thesis. The last chapter is devoted to the deposition of silver-clusters on an epitactic silver film. The contact area between cluster and film has a huge influence on the specific resistance of the film. The effect was theoretically described by Kaser et al.. In the experiment performed with stepwise rise and drop of resistance at room-temperature and low temperatures. Yielded conclusions about the contact area and the electron-scatter-efficiency increased by the clusters.

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