Molecular Exchange in a Heteromolecular PTCDA/CuPc Bilayer Film on Ag(111)

The future success of organic semiconductors in electronic or spintronic devices depends crucially on the ability to control the properties of molecular thin films. Metal contacts as well as interfaces formed by different organic materials are of equal importance in this context. A model system contributing to the improvement of the fundamental understanding of such interfaces is the heteromolecular bilayer film formed by 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA) grown on a well ordered CuPc monolayer on Ag(111). Using complementary experimental techniques, we are able to reveal a molecular exchange across this heteromolecular interface. At the initial stage of the PTCDA deposition, some of these molecules diffuse into the CuPc layer and displace CuPc molecules to the second layer. This inhibits the formation of a smooth interface between both species and results in a structurally disordered heteromolecular CuPc-PTCDA film in the first and randomly arranged CuPc molecules as well as ordered PTCDA islands in the second layer. While the second organic layer is electronically decoupled from the underlying layer, the first layer, although disordered, shows a charge reorganization and an adsorption height alignment of CuPc and PTCDA as it is known for highly ordered heteromolecular monolayer structures on Ag(111). The molecular exchange, which we consistently find in all our experimental data, is the result of a lower adsorption energy gain of PTCDA on Ag(111) compared to CuPc on Ag(111).