Electrical properties of Au 55 cluster systems
A very interesting topic of nanotechnology today is the investigation of clusters exhibiting electrical quantum size effects, with the future aim of fabricating single-electron devices. One of the most suitable model compound is Au55 cluster, protected with various ligand shells of phosphorous- and sulfur organic molecules. In this work two main themes will be addressed: the effect of the covalent cross-link on the charge transfer mechanism of these nanoparticle systems and the conductive properties of the dense-packed cluster-monolayers. The current literature opinion, whether the chemical nature of the ligands bridging two clusters influence the charge-transfer mechanism or not, is non-equivocate. Reifenberger et. al. assume that the chemical nature of the ligands, especially their end-groups have an impact on the conductance of a certain sample. Simon found that the activation energy of nanoparticle systems does not depend on the chemical nature of the ligands, but on the distance between the nanoparticles. To clarify this apparent contradiction, the electrical properties of two-, quasi-two and three-dimensional systems were investigated. The measurements indicate that Simon's model is valid for physically cross-linked systems only. For covalently (chemically) cross-linked nanoparticle systems the activation energy is, however, smaller than expected from this and depend strongly on the chemical nature of the cross-linker ligand. The activation energy exhibits no trends as the function of the inter particle distance in this case. In the last part of this work the electrical properties of cluster monolayers are presented. For the measurements Si substrates were used, equipped with tungsten contact structures by electron beam lithography. In order to get an electrically insulating layer under the electrical contacts, the top of the sample was oxidized to SiO2. It could be shown that a nanoparticle system on this substrate can be activated by an exposition to a low energy electron beam. We explain this phenomenon with trapping of excess electrons in the SiO2 layer, which induce image charges of opposite charge in the cluster layer. This effect causes changes in the electron-configuration and perhaps also in the structure of the arrangement. Asymmetrical spatial exposition of the layer leads to asymmetrical I-V characteristics. This phenomenon can be explained similar to the rectifying behavior of the semiconductor diode: the arising image charges acts as electron holes in the layer.
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