In this thesis work, a combination of low-temperature scanning tunneling microscopy (STM) and atomic force microscopy (AFM) was used to study single atoms and molecules on thin insulating films. We show that noncontact-AFM can yield important additional information for these systems, which had previously been studied only with STM.
In particular, we demonstrate that the charge states of single gold adatoms can be detected with Kelvin probe force microscopy (KPFM). Furthermore, it is described how AFM can be used to image the chemical structure of a molecule with atomic resolution if the microscope tip is functionalized with suitable single atoms or molecules. This method was then applied to study the exact geometry of a molecular switch consisting of a single gold atom and a PTCDA molecule, and to help in the elucidation of the structure of an unknown molecule from the deep sea. Finally, we were able to combine the high resolution of our AFM molecular imaging technique with the charge sensitivity of KPFM to directly image for the first time the charge distribution within a single molecule.
These investigations show that by combining STM and AFM, the electronic and structural properties of single molecules can be revealed in unprecedented detail. The possibility of directly imaging the chemical structure and the intramolecular charge distribution could lead to new fundamental insights into single-molecule switching and bond formation – processes that are usually accompanied by a structural rearrangement and/or an intra- or intermolecular redistribution of charge
