CPM Seminar

Energy Dissipation by organic molecular crystals and by functional groups of organic molecules

Markus Fendrich

University of Duisburg-Essen

Frequency modulation atomic force microscopy (FM-AFM) has turned out a powerful tool in surface physics, allowing for real-space imaging of metals, semiconductors and insulators alike. The sample is scanned by an oscillating microfabricated cantilever with a tip, the image being a surface of constant demodulation of the cantilever's resonance frequency due to tip-sample forces. Beyond topographic information, the damping signal, i.e., the excitation that is needed to keep the oscillation amplitude of the cantilever constant, provides valuable information on energy dissipation processes on the sample.

This technique has been applied to analyze organic molecules on various surfaces. In the first part, results will be shown for the prototypical organic semiconductor molecule 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA) on KBr(001), where the damping signal reveals crystallographic information on the multilayered structures formed by the molecules. In the second part, I will show results of the same molecule on Ag(111). Here, an enhanced damping signal occurs which can be assigned to a switching of functional groups of PTCDA by the AFM tip. A comparison to a similar molecule, N-N'-dimethylperylene-3,4,9,10-dicarboximide (DiMe-PTCDI) will be given.