Adsorption behavior of metal-phthalocyanines on metal surfaces
Benjamin Stadtmüller, Ingo Kröger, Christoph Kleimann, Christian Kumpf
Metal-Phthalocyanine molecules are dyes which are frequently used for applications, also in the field of organic electronics, e.g. for solar or photovoltaic cells. They also became subject of intensive fundamental research due to some unexpected interaction mechanisms with metal surfaces. This has at first been found for tin-phthalocyanine (SnPc) and the (111) oriented silver surface (See our research project “Order/Disorder phenomena of sub-monolayer organic films“). In this context we are investigating the organic-metal contact interface for similar systems, in particular for CuPc, SnPc, TiOPc and H2Pc on Au, Ag and Cu surfaces. Also fluorinated phthalocyanine molecules like F16CuPc are under study.
One fundamental result of our ongoing work is the comprehensive understanding of different adsorption behaviors of sub-monolayer thin CuPc films on Au, Ag and Cu(111) surfaces. On first sight the phase diagrams of all three systems show many similarities. They all contain a disordered phase at low coverages and several long-range ordered phases at higher coverages. However, the systems differ strongly in interaction strengths between adsorbate molecules and the surface, and consequently the structure formation in the adlayer is fundamentally different. This is most obvious in the high-coverage regime: On Au(111) only two different point-on-line structures are found, whereas on Ag(111) a continuous series of structures occurs which reflects the repulsive intermolecular interaction found for this system. On Cu(111) the molecules grow in commensurate islands.
The different structure formation can be correlated directly to the differences in the interaction strength between molecules and substrate. In the case of weakest binding between molecules and surface (CuPc on Au(111): physisorption, no charge transfer between molecules and metal, i.e., bare van der Waals interaction) we find an adsorbate film which is hardly affected by the substrate. Its structure is dominated by the inter-molecular interaction. In the case of the stronger binding of the molecules on Ag(111) (weak chemisorption) a repulsive interaction dominates the structure formation caused by the charge exchange between the molecules and the surface and the consequent charge redistribution within the molecule. One might expect that this effect becomes even stronger for the adsorption of CuPc on Cu(111), due to the increased charge transfer with this surface. However, the interaction with the Cu(111) surface is so strong that the molecule becomes significantly polarized. This induces a quadrupole moment giving rise for an additional attractive interaction overcompensating the intermolecular repulsion and leading to the formation of a very site-specific, commensurate structure.
Hence, the adsorption of MePcs on noble metal surfaces represents an exceptional model system for studying all variations of molecule-molecule and molecule-substrate interactions. By selecting molecules and substrate materials properly the system can be arbitrarily tuned from weak physisorption to strong chemisorptions and from attractive to repulsive intermolecular interaction.
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M. Sokolowski, Univ. Bonn,
F. Reinert, A. Schöll, Univ. Würzburg,
J. Zegenhagen, ESRF, Grenoble, France,
T.-L. Lee, Diamond Light Source, Oxfordshire, UK
DFG Ku 1531 2-1