Commensurism at electronically weakly interacting phthalocyanine/PTCDA heterointerfaces

Interfaces in multilayered electronic devices are of paramount importance, especially for layer thicknesses in the nanometer range. Among the interfacial processes are charge injection or extraction and excitonic dissociation, the latter being particularly relevant if molecular components are involved. Highly ordered superstructures are preferable to prevent undesired losses of charge carriers and/or excitons. Epitaxial organic-inorganic systems have already received eminent attention, but only few studies have dealt with organic-organic heterointerfaces so far. Here, we focus on the adsorption of metal-phthalocyanines (MePc, Me = Sn or Cu) on 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA) in the form of stacked monolayers (ML) on Ag(111). Using scanning tunneling microscopy and low-energy electron diffraction we reveal an initial nonordered growth for dilute SnPc submonolayers and consecutively three condensed phases at coverages ranging up to $1\phantom{\rule{0.16em}{0ex}}\mathrm{ML}$---each possessing a distinct commensurate registry with the underlying PTCDA. By applying in situ optical differential reflectance spectroscopy and photoelectron spectroscopy we find that neither the SnPc nor the CuPc phases exhibit significant electronic or optical coupling with the PTCDA interlayer. Therefore, our results demonstrate that commensurism does not necessarily imply chemisorption, as stated previously in the literature, but that physisorption may be accompanied by commensurate superstructures.