A scanning force microscope operated in the dynamic force mode has been used to analyze the arrangement of 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) molecules on a KBr(001) surface. The molecules do not form a close-packed layer but crystallites with the shape of truncated pyramids. Even after depositing only 0.3 molecular layers of PTCDA, these crystallites have a height of 20--30 molecular layers. We have been able to resolve individual molecules of the top layer and on some of the crystallites facets. The results agree with the known ``herringbone'' structure of the bulk. A comparison of the experimentally observed contrast to a simple force field calculation based on semiempirical interaction potentials is given.
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Kunstmann et al. (2005) studied this question.