Ordering in weakly bound molecular layers: organic-inorganic and organic-organic heteroepitaxy
It is an aim of this work to provide insight into the energetic influence on the ordering of molecular thin films on crystalline substrates. Here, the term substrate either refers to inorganic crystal surfaces or highly ordered layers of another organic molecular species. In order to calculate the total interface potential of extended molecular domains, a new calculation technique (GRID technique) is developed in the first part of this work. Compared to the standard approach, this method accelerates the potential calculation drastically (times 10000). The other parts of the thesis are dedicated to the comparison of experimental results (obtained by scanning tunneling microscopy and electron diffraction) to the optimal layer structure as predicted by optimization calculations. Potential calculations which are performed for the system perylenetetracarboxylicdianhydride (PTCDA) on graphite demonstrate that point-on-line coincident structures correspond to energetically favorable alignments of the molecular lattice with respect to the substrate lattice. The capability of the GRID technique to predict the optimal layer structure is demonstrated for the system peri-hexabenzocoronene (HBC) on graphite. The organic-organic heteroepitaxy system PTCDA on HBC on graphite is investigated in order to clarify to which extent the ordering mechanism there differs from that of the organic-inorganic heteroepitaxy system PTCDA on graphite. As a result of this investigation, a new type of epitaxy, i.e., substrate induced ordering is found. This new epitaxy type is governed by the inner structure of the substrate lattice unit cell. Here, the substrate surface is a layer of organic molecules itself, hence the substrate surface unit cell does indeed exhibit a complex inner structure. A generalized classification scheme for epitaxial growth incorporating this new type of epitaxy is proposed. In the last chapter, the structure of the first layers of titanylphthalocyanine (TiOPc) on Au(111) is investigated and compared to potential optimization calculations. The correspondence of experimental and theoretical results provides evidence that the GRID technique can, in principle, also be applied to molecular layers on metal surfaces.
Epitaxie, Niederenergetische Elektronenbeugung, Organische Dünnschichten, Potentialberechnung, Rastertunnelmikroskopie, epitaxy, low energy electron diffraction, organic thin films, potential energy calculation, scanning tunneling microscopy
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