CeNS Colloquium

Prof. Leonhard Grill
Department of Physical Chemistry, Fritz Haber Institut, Berlin

The scanning tunneling microscope (STM), invented in the 1980-ies by Binnig and Rohrer, is not only a powerful instrument for imaging surfaces with atomic resolution, but can also be used for the controlled manipulation of single atoms and molecules by taking advantage of chemical or electrostatic forces or electronic processes. The investigation of functional molecules on surfaces is of high interest in this regard as they can act as model systems for future applications in molecular nanotechnology where they should be used as single-molecule "devices".

In this presentation, various examples of manipulations of single molecules and inorganic nanostructures on metal surfaces by low temperature STM will be given. On the one hand molecular switches, i.e. molecules that exhibit at least two stable states, will be discussed. Trans-cis isomerizations of azobenzene derivatives are obtained on a surface by light and STM manipulation and different mechanisms are identified. By slightly modifying the chemical structure of the molecular switches, it is found that the atomic-scale environment of each individual molecule, i.e. the surrounding molecules and the surface underneath, strongly influence the isomerization process and can either allow or suppress it. Furthermore, the controlled assembly of functional molecules will be presented, whereas covalent bonding is the desired intermolecular interaction, because it provides high stability and the possibility of efficient charge transfer. With this technique, fluorene polymers as model systems for molecular wires were grown on a gold surface. The conductance of such a molecular wire was measured as a continuous function of the electrode-electrode distance by pulling a single polymer from a metallic surface. The method of molecular assembly has been extended to functional molecules that contain wheel-like side groups. Finally, the combination of inorganic, crystalline growth and the organic growth of such nanostructures, thus creating a hybrid system, will be discussed.