Assemblies of organic molecules on insulating surfaces investigated by nc-AFM

The formation of ordered one- and two-dimensional molecular structures on insulating surfaces has been investigated by means of a room temperature scanning force microscope (SFM). In particular, one dimensional row structures along step edges of alkali halide crystals were obtained as well as two dimensional chain structures on the terraces. Various molecules have been deposited on different substrates. While most of the combinations didn’t show any ordered structures at all, some combinations led to ordered growth. Porphyrin based molecules have proven to be promising candidates to form self-assembled structures on insulating surfaces. Their close affinity to hemoglobine and chlorophyll makes them also interesting as functional building parts. Porphyrin molecules tend to π − π stack in crystals, allowing them to work as optical or electrical antennae. While the adsorption on metallic surfaces showed that this particular stacking was prevented by the flat alignment on the substrate, molecules were adsorbed standing upright on alkali-halide(100) surfaces maintaining the intermolecular π − π-stacking. Alkali-halide surfaces have been structured in different ways to study the molecular arrangements under various conditions. Micrometer long, one dimensional wires along step edges have been observed as well as large two-dimensional assemblies of parallel running wire structures across flat terraces. The orientation of those chains is guided by the underlying substrate and the equilibrium distance between the molecules. In order to check the feasibilty to contact the wires, experiments on gold cluster decorated insulating surfaces have been performed. Under certain premises of the cluster arrangement it proved to be successful to grow molecular wires between two gold clusters. At last, molecular assemblies on ultrathin salt films on Cu(111) were studied by means of Kelvin Probe Force Microscopy (KPFM) to obtain details about possible exciton or electron transport in the π − π stacks. The KPFM did not yet show conclusive results, but the dynamic compensation of the contact potential difference allowed ultrahigh resolution images, which revealed differences in the adsorption structures on layers of different heights. In summary, the adsorption of cyanoporphyrin molecules on various insulation surfaces has been studied and carefully analyzed.