Study of intermolecular interactions in hetero-organic thin films

In this work we present a systematic study of the structure formation in hetero-organic systems consisting of the prototype molecules 3,4,9,10-perylene-tetra-carboxylic-dianhydride (PTCDA) and copper-II-phthalocyanine (CuPc) adsorbed on the Ag(111) surface. The geometric structure of these systems is investigated with established surface science techniques like low energy electron diffraction, scanning tunneling microscopy or the X-ray standing wave technique. The electronic structure of the individual molecules in the mixed films is revealed by angle resolved photoemission spectroscopy data which are analyzed in the orbital tomography approach introduced recently [PBF$^{+}$09, PRU$^{+}$11]. Laterally mixed films of CuPc and PTCDA were studied in order to reveal the influence of the substrate mediated intermolecular interaction on the geometric and electronic properties of the mixed film. The lateral order, i.e., the size and shape of the unit cell, can be tuned by changing the relative coverage of the molecules on the surface. A highly surprising finding is that the charge transfer between the individual molecules in the mixed film and the substrate is no longer reflected by their adsorption height on the surface. We explain this finding by a coupling of the electronic levels of the molecules via a hybrid state, which results in an additional population of the PTCDA LUMO and a complete depopulation of the CuPc LUMO level. Vertically stacked bilayer films allow to study both the intermolecular interaction strength along the vertical stacking direction and the influence of the second organic layer on the properties of the metal organic interface. For the adsorption of CuPc on a closed PTCDA layer on Ag(111), a smooth organic-organic interface was formed. CuPc adsorbs in the second layer on PTCDA and does not destroy the lateral order of the PTCDA layer. The vertical distance between the organic layers indicates a mainly electrostatic and van der Waals interaction across the hetero-organic interface. However, the chemical bonding between PTCDA and the silver surface is changed upon the adsorption of CuPc. This is reflected in an enhanced charge transfer into the PTCDA LUMO level coinciding with an altered vertical adsorption height of PTCDA which depends on the CuPc coverage. These findings can be explained by an additional screening effect, induced by the adsorption of CuPc