Structural and electronic investigations on homo- and hetero-organic layers involving CuPc on silver single crystal surfaces

In this work we investigate variations of a homo-molecular layer of CuPc adsorbed on the Ag(111) surface, which is a well known example in research on organic electronics where the structural and electronic properties at the metal-organic interface are of interest. Three modifications of the mentioned system are realized by addition of a second layer, exchange of the substrate, and addition of a second type of organic molecules. Measurements on the lateral structure are performed by STM and LEED. For experiments on the electronic structure, STM-based differential conductance spectroscopy and angle-resolved PES are applied. For a second layer of CuPc on top of the first layer of CuPc on Ag(111) we observe a weaker interaction between the two molecular layers than between the substrate and the first molecular layer. This allows molecules in the second layer to adsorb in an inclined configuration in contrast to the flat lying geometry of molecules in the first layer. The HOMO of CuPc shifts towards larger binding energies with increasing coverage. The (former) LUMO, which in the first layer is weakly occupied by charge donation from the silver substrate, is unoccupied in the second layer because of a significantly weaker interaction with the underlying material. Experiments on a dense, closed layer of CuPc molecules on the Ag(110) surface reveal a stronger effect of this substrate on the layer formation than the Ag(111) surface. The stronger interacting substrate of lower symmetry dominates the formation of the lateral molecular arrangement interspersed by dislocation lines where the intermolecular interaction breaks through. The initially 4-fold symmetry of the molecules is reduced to2-fold due to a combination of geometric and electronic effects. The part of the molecule that is is aligned with a more acute angle to the Ag[001] direction is slightly bent down, interacting stronger with the substrate and receiving charge donated by the silver. By this asymmetry the original degeneracy of the two parts of the LUMO is lifted. Laterally mixed hetero-organic layers of CuPc and PTCDA on Ag(110) show the stronger influence of the substrate on the formation of ordered structures compared to mixed ordered layers on Ag(111). A tendency to form complex packing motifs is observed and we investigate two different structures that are described by large unit cells comprising 5 and 9 molecules, respectively. Measurements on the local electronic structure are dominated by signals from PTCDA molecules and we observe that the PTCDA LUMO is occupied to at least the same degree as it is in a homo-molecular PTCDA layer. Th CuPc LUMO is unoccupied indicating a molecule-molecule interaction with an unequal charge distribution for the two types of molecules