[show abstract][hide abstract] ABSTRACT: Thin films of the organic molecule perylene-3,4,9,10-tetracarboxylic-dianhydride (PTCDA) on graphite (0001) have been investigated from the mono- to the multilayer regime with low energy electron diffraction (LEED), X-ray-diffraction in Bragg-Brentano geometry, and high resolution scanning tunneling microscopy (STM). These different methods proved epitaxial growth in a coincident superstructure and yielded congruent results concerning details of the crystallographic structure of the epilayer. In addition it was possible to resolve submolecular structures in high resolution STM images; a comparison of the 10 resolved maxima of the tunneling current with the molecular structure leads us to question the conventional model description of tunneling.
Zeitschrift für Physik B Condensed Matter 09/1992; 86(3):397-404.
[show abstract][hide abstract] ABSTRACT: The molecular order in thin films of the aromatic hydrocarbon coronene, C24H12, and its larger homologue, hexa-peri-benzocoronene (“HBC”), C42H18, obtained by vacuum-deposition onto the (0001) cleavage plane of graphite and molybdenite, MoS2, has been investigated by low-intensity low-energy electron diffraction (LEED). Under well-defined preparation conditions (low evaporation rates and moderate substrate temperatures), epitaxial films could be obtained; their epitaxial relations could be characterized by well-resolved LEED-patterns. For coverages in the monolayer region, commensurate graphite (0001)-p(√21 × √21 R ± 10.9°)-coronene and MoS2(0001)-p(√13 × √13 R ± 13.9°)-coronene superstructures are formed, while hexa-peri-benzoncoronen forms commensurate graphite (0001)-p(√31 × √31 R ± 9.0°)-HBC and MoS2(0001)-p(√21 × √21 R ± 10.9°)-HBC superstructures. From the dependence of the observed superstructure lattice constants on the size and shape of the adsorbed molecules relative to the respective substrate surface lattice constants a parallel orientation of the molecular planes with respect to the substrate surface can be inferred. It is shown that under the influence of the substrate the distances and azimuthal orientations of the molecules are determined by the closest packing permitted by their in-plane van der Waals radii. These structures are different from the respective bulk crystal structures. For the system coronene on MoS2 a model of film formation is proposed.