The nature of condensed single molecules : local electronic and mechanical characteristics

In order to advance the performance of molecule based electronic devices a detailed and fundamental knowledge about the underlying physical aspects is mandatory. It is well known that the performance of any organic electronic device is influenced by the physics at the interfaces between different molecules or molecule and substrate. Tracing down these phenomena towards the single molecular scale could highly improve the understanding and broaden the insight into the physics involving interfaces with organic compounds.
In this manner, the present thesis is concerned with the nature of condensed single molecules studied by means of tuning fork based scanning tunneling- and atomic force microscopy in ultra-high-vacuum and at low temperature. The appealing local character of scanning probe based investigation tools is very well suited for investigations at the sub-nanometer scale. Particularly, the various spectroscopic operation modes directly enable to extract present interaction forces, to visualize molecular frontier orbitals, or to study local work function - or electrostatic potential variations. In order to apply these techniques towards single molecules on a surface, in a first step different spectroscopy data acquisition modes were compared with respect to the various experimental challenges that need to be regarded during long term high data density measurements.
In a second step, the elasticity of a single molecule on a metal surface was analyzed via three dimensional force spectroscopy data. By observing a vertical elastic lifting process of certain functional side groups by the scanning tip, a controlled manipulation process based on the rotation of single molecules could be established.
Similarly, the electronic properties of s single molecule on two different substrates were addressed by scanning tunneling- as well as three dimensional local Kelvin probe bias spectroscopy. By comparing the adsorption on a strongly interacting metal surface with that on an intervening epitaxial NaCl bilayer, due to which the molecule electronic structure is only weakly perturbed, the influences of charge transfer became directly visible. Supported by first principles calculations, it was shown, that even adsorption asymmetries concerning the second substrate layer below the molecule affect the distribution of charge.