SELF ASSEMBLY · Structure and electronic properties of low-dimensional systems and molecular assemblies

Final Activity Report Summary - SELF ASSEMBLY (Structure and electronic properties of low-dimensional systems and molecular assemblies)

The goal of the project was to study the self-assembly of nanostructures on surfaces, including the self-organisation of surfaces and the self-assembly of molecular structures on surfaces. According to the work plan, all objectives of the project were archived. The geometric structure of the self-organisation of steps on stepped surfaces was studied using a Variable temperature scanning tunnelling microscope (VT-STM). It was found that there existed differences between the self-organisation of steps on gold and copper(111) surfaces. While vicinal copper (Cu) formed equally sized monoatomic steps, we found facetts of different step sizes on surfaces with the same miscut angles on gold (Au). By measuring not only the geometric but also the electronic structure we could show that the reason for these differences was the distinct behaviour of the electronic surface state of both materials. A similar influence of the surface electronic structure on the geometry of the self-organised triangular network of one monolayer of silver on a Cu(111) surface was found. In this case an incommensurate superstructure was formed, which was stabilised by the opening of an electronic gap at the Fermi energy. We showed that, in such a case, the energy that was gained by the system by opening the electronic gap was of the same order as the energy necessary to go from the usually energetically preferred commensurate structure to an incommensurate one. This was one of the first examples where it was shown that the energy of a surface state could influence the structure of a material. We also tested the self-assembly of molecules on top of stepped surfaces. For this purpose, we used an Au(887) surface that presented equally sized monoatomic steps of 4 nm in one direction and a further periodicity of 7 nm because of the so-called herringbone reconstruction in the other direction. Half a monolayer of fullerene-C60 molecules self-assembled on such a surface in rectangular arrays of three rows containing four to five molecules. This highly ordered nanostructure presented a new electronic state that resided inside the electronic band gap of monolayer thick or solid C60. Therefore, we could show that a monolayer thick film was not metallic as it was believed until the project realisation. Additionally, during the second year of the project, the applicant designed and built up a new photoemission machine that would be connected to the existing scanning tunnelling microscope. This new machine was characterised by the time of the project completion and first measurements were carried out. After finishing this phase, the combination of the VT-STM and the photoemission technique would be one among only a few machines in the world able to measure both the electronic and geometric structure of samples simultaneously.

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