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 onthe Ag(111) surface, which is a well known example in research on organic electronicswhere 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 secondlayer, 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 experimentson the electronic structure, STM-based differential conductance spectroscopy andangle-resolved PES are applied.For a second layer of CuPc on top of the first layer of CuPc on Ag(111) we observe aweaker interaction between the two molecular layers than between the substrate and thefirst molecular layer. This allows molecules in the second layer to adsorb in an inclinedconfiguration in contrast to the flat lying geometry of molecules in the first layer. TheHOMO of CuPc shifts towards larger binding energies with increasing coverage. The(former) LUMO, which in the first layer is weakly occupied by charge donation fromthe silver substrate, is unoccupied in the second layer because of a significantly weakerinteraction with the underlying material.Experiments on a dense, closed layer of CuPc molecules on the Ag(110) surface reveala 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 thelateral molecular arrangement interspersed by dislocation lines where the intermolecularinteraction 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 moleculethat 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. Bythis 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 thestronger influence of the substrate on the formation of ordered structures compared tomixed ordered layers on Ag(111). A tendency to form complex packing motifs is observedand we investigate two different structures that are described by large unit cells comprising5 and 9 molecules, respectively. Measurements on the local electronic structure aredominated by signals from PTCDA molecules and we observe that the PTCDA LUMOis occupied to at least the same degree as it is in a homo-molecular PTCDA layer. TheCuPc LUMO is unoccupied indicating a molecule-molecule interaction with an unequalcharge distribution for the two types of molecules