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"Interface properties of metal-molecule-metal junctions for molecular electronics applications."

The permanent miniaturization in the field of microelectronics requires new concepts beside the  established Si based technology. The use of single molecules or organic thin layers as active components in nano-scaled devices represents one promising option. In this thesis, metal-molecule-metal junctions with Self Assembled Monolayers (SAMs) as molecular building blocks were intensively investigated. Especially the metal-molecule interface properties are of high interest due to their fundamental importance concerning the junctions’ transport properties. In this respect,the phenomenon of the contact resistance plays a major role. Various SAMs on Au(111) and their metallization based on an electrochemical approach to build up metal-molecule-metal junctions were characterized concerning the structural and chemical properties. The interfaces of the different systems were investigated by photoelectron spectroscopic methods to unravel the impact of the interfacial chemical interactions onto the electronic structure of the metal electrodes.

SAMs of 4-Mercaptopyridine and 4-(4-Mercaptophenyl)Pyridine and their metallization with closed Pd overlayers were in the focus within the concept of molecular families. It could be demonstrated, that the interactions at the junction interfaces, resulting in local density of states (LDOS) effects in the electronic structure of the metal electrodes, do not solely depend on the head and terminal group of the molecules, but also on the molecular backbone. 4-Mercaptophenol SAMs as an example of non-pyridine terminated SAMs could not be metallized, most probably due to the hydroxyl type terminal group. Thiazole based SAMs could successfully be metallized with Pd, resulting in a stable junction. Again, at the interfaces of the junction LDOS effects were detected, however weaker pronounced compared to similar systems. In addition, thiol-free SAMs of Dicyanobenzene (DCB) were in the focus of the project. The DCB molecules arrange as flat lying arrangement on the Au surface. The metallization with Pd islands on top was carried out resulting in a junction with ultrathin molecular spacing. Finally the concept was extended towards building up multilayer systems. The molecular “double-decker” could successfully be prepared. The Pd interlayer, spacing the molecular SAMs, as well as the Pt toplayer exhibit metallic properties, opening the door towards Molecular Electronics in the 3rd dimension.