Molecular resolution in dynamic force microscopy: topography and dissipation for weakly interacting systems

摘要

Dynamic force microscopy can be used to study the growth of organic molecules on insulating substrates. The organic molecule 3,4,9,10- perylenetetracarboxylic- dianhydride ( PTCDA) exhibits Volmer - Weber growth on KBr( 001). While large areas are uncovered, some PTCDA crystallites of different heights, from two monolayers ( ML) up to 20 - 30 ML, can be seen, even at low coverages. On the topmost layer of the crystallites, single molecules can be resolved. Such a molecular contrast does not only show in topography but also in the dissipation signal. Here, the signal strongly depends on the frequency shift, i.e., the minimum distance between tip and sample. At large frequency shifts, the dissipation is enhanced between the molecules, if the frequency shift is lower, a shift of 0.2 nm between the peaks in topography and dissipation channel occurs. We propose a model that takes the crystalline growth of PTCDA into account, the shift between both signals being an effect of the two topmost layers of molecules.