Substrate-interaction, long-range order, and epitaxy of large organic adsorbates

Large and symmetric organic molecules (>200 amu) can form highly-ordered adsorbate layers and thin films when they are deposited by vacuum sublimation on clean reactive surfaces. In such cases covalent bonding often occurs via the molecular pi-system leading to a parallel orientation of the adsorbate as shown for oligothiophenes and PTCDA on Ag(111). A proper choice of the substrate and/or a preadsorbate may also cause an upright orientation with bonding via a reactive group of the molecule (example: NDCA/Ni(111)). Most of the used molecules yield long-range ordered monolayers with large, almost defect-free domains. The stronger the bonding and the smaller the molecule the more likely is the formation of commensurate superstructures which indicate site-specific adsorption even for such large molecules as PTCDA or EC4T. Organic epitaxy is discussed and shown for a particular system, PTCDA on Ag(111), for which the structure of the monolayer is nearly identical to that of the beta-modification of PTCDA crystals, whereas on other substrates (e.g. Si(111), Ge(100)) a disordered interface and hence no true epitaxy is found.