No abstract

The understanding of the fundamental physical properties of metal-organic and organic-organic interfaces is crucial for improving the performance of organic electronic devices. This is particularly true for (multilayer) systems containing several molecular species due to their relevance for donor-acceptor systems. A prototypical heteromolecular bilayer system is copper-II-phthalocyanine (CuPc) on 3,4,9,10-perylene-tetra-carboxylicdianhydride (PTCDA) on Ag(111). In an earlier work we have reported a commensurate registry between both organic layers and an enhanced charge transfer from the Ag substrate into the organic bilayer film [Phys. Rev. Lett. 108, 106103 (2012)], which both indicate an unexpectedly strong intermolecular interaction across the organic-organic interface. Here we present new details regarding electronic and geometric structure for the same system. In particular, we provide evidence that the enhanced charge transfer from the substrate into the organic bilayer does not involve CuPc electronic states, hence, there is no significant charge transfer into the second organic layer. Furthermore, we report vertical bonding distances revealing a shortening of the PTCDA-Ag(111) distance upon CuPc adsorption. Thus, electronic and geometric properties (charge transfer and bonding distance, respectively) both indicate a strengthening of the PTCDA-Ag(111) bond upon CuPc adsorption. We explain these findings-in particular the correlation between CuPc adsorption and increased charge transfer into PTCDA-in a model involving an intermolecular screening mechanism.

Tailoring the properties of molecular thin films and interfaces will have decisive influence on the success of future organic electronic devices. This is equally true for metal–organic and hetero–organic contacts as they occur, for example, in donor–acceptor systems. Here, we report on the structure formation and interaction across such a heteromolecular interface. It is formed by monolayers of F16CuPc and CuPc stacked on a Ag(111) surface. We investigated the lateral and vertical structure using spot-profile analysis low energy electron diffraction and normal incidence X-ray standing waves, and performed pair potential calculations to understand the driving forces for the structure formation. Most surprisingly, for one phase we found a commensurate registry between the two organic layers, usually a sign for a strong (chemisorptive) interaction often involving metallic states of the surface. However, because the organic bilayer is not commensurate with the underlying Ag substrate in our case, the dominating factor must be the intermolecular interaction. Pair potential calculations suggest a site-specific adsorption that leads to a commensurate registry at the heteromolecular interface. The adsorbate system was further characterized by measuring adsorption heights, indicating flat-lying molecules and a CuPc–F16CuPc layer spacing of 3.06 Å.