Dresden 2009 – scientific programme
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O: Fachverband Oberflächenphysik
O 4: Metal substrates: Adsorption of organic / bio molecules I
O 4.3: Talk
Monday, March 23, 2009, 11:45–12:00, SCH A118
Layer growth and desorption kinetics of organic acceptor molecules on Au(111) — Paul Frank1, Norbert Koch2, Ralph Riegler3, Klaus Müllen3, and •Adolf Winkler1 — 1Institute of Solid State Physics, Graz University of Technology, Graz, Austria — 2Institute of Physics, Humboldt University Berlin, Germany — 3Max Planck Institute of Polymer Research, Mainz, Germany
The layer growth and desorption kinetics of the organic acceptor molecules hexaazatriphenylene-hexacarbonitrile (HAT-CN) on Au(111) surfaces has been studied by TDS, XPS and AFM. Ultra-thin films of HAT-CN show a quite unexpected thermal stability. The first two layers are wetting the surface. They desorb at 460 K (2nd layer) and 610 K (1st layer), respectively. For higher initial coverage a layer like film forms at and below room temperature, which transforms into a Stranski-Krastanov film upon heating at ca. 400 K. Interestingly, during this transformation also the 2nd wetting layer is incorporated into the nascent islands which then desorb at a higher temperature of ca. 490 K. At even higher initial coverage eventually the 1st monolayer becomes incorporated into the islands during heating and all the molecules desorb already at ca. 500 K. This shows that the two wetting layers are meta-stable and depend on the initial coverage and the temperature. The reason for this unusual behavior can be identified by the evaluation of the desorption spectra for the individual desorption peaks. It turns out that the desorption energies for all layers are rather similar, but that the pre-exponential factors (in particular for the 1st layer) differ by many orders of magnitude.