Michel Kazempoor, Gerhard Pirug, Stefan Tautz
Applying surface sensitive spectroscopies the electronic and geometric structure of self assembled monolayers of organic molecules are studied on metal single crystal surfaces. These experiments resemble fundamental aspects of surface ordering of organic molecules as a result of balanced driving forces such as molecule-substrate and intermolecular interactions. We will focus on the characterization of SAMs on surfaces with special emphasis on their local and long range order, phase transions, stress relaxation and intermolecular interactions, chemical composition, electronic and vibrational properties.
In a first systematic LEED study of biphenyls (BPn)* with different numbers (n=2-6) of the alkane spacers different phases could be clearly distinguished according to their well resolved sharp LEED pattern. A comparison of the corresponding intrinsically correct unit cells with the periodically repeating units imaged by STM showed not only that lateral measurement by STM suffer from various inaccuracies due to the lack of references from the substrate or distortions but also that the contrast mechanism for this two methods is characteristically different. While LEED is much more sensitive to the adsorbate substrate ordering STM probes mainly the outer electron density controlled by intermolecular interaction. However the combined view of the molecular packing seen by STM and the unit cell determined by LEED leads to reasonable surface structure models which may be important for possible future applications in molecular devices based on these SAMs.
In order to obtain a more reliable comparability of spectroscopic results from the surface science approach with respect to data obtained under more realistic conditions a new transfer system containing UHV based complementary methods such as AES, LEED, XPS, UPS, XPD or HREELS will be available soon which allow the preparation of self assembled monolayers under various conditions such as thermal or electron irradiated evaporation, from solution eventually under electrochemical control or other ambient conditions.
* prepared by M. Homberger and U. Simon, Institute of Inorganic Chemistry, RWTH Aachen