Electronic and geometric structure of heterorganic interfaces
Benjamin Stadtmüller, Christoph Kleimann, Sonja Schröder, Caroline Henneke, Christian Kumpf
Contacts between organic molecules and metal surfaces have been intensively investigated within the last decades and a good understanding of the key properties of such interfaces has been obtained. The most fundamental findings were achieved by investigating model systems like 3,4,9,10-perylene-tetracarboxylicdianhydride (PTCDA), metal phthalocyanines (MePc), or other π-conjugated molecules. However, regarding organic-organic heterojunctions, only a very small number of investigations can be found in the literature so far. This is possibly due to the common believe that interfaces between different organic materials are generally dominated by a very weak van der Waals interaction. This assumption was recently proven to be incorrect.
We have investigated the geometric and electronic structure of the hetero-organic interface between copper-II-phthalocyanine (CuPc) and PTCDA. The heterojunction is formed by adsorbing CuPc molecules on top of a closed layer of PTCDA on Ag(111). Fig. 1 shows a layer of CuPc molecules (bright contrast in the upper part of the figure) lying on top of the PTCDA film (dark contrast in the lower part). Individual molecules of both kinds can be resolved and their orientation investigated. A commensurate registry between both layers, i.e., a common crystallographic lattice for CuPc and PTCDA films (as well as for the Ag(111) surface), is found and indicates that the growth of the upper layer (CuPc) is dominated by the structure of the lower (PTCDA). This indicates an unexpectedly strong interaction between the different molecules in the upper and lower film, i.e., across the heterojunction.
The interaction unmasks itself as charge-transfer effect in ultra-violet photoelectron spectroscopy (UPS). The electronic states of the PTCDA molecule change significantly when the CuPc molecules are adsorbed on top of the PTCDA layer. In particular, the lowest unoccupied molecular orbital (LUMO) of PTCDA, which became already partly filled due to the interaction with the Ag surface, is gradually filled up further upon adsorption of CuPc. This is evidenced by a large shift of the F-LUMO peak toward the high binding-energy side in Fig. 2. Against common belief, this result proves that rather strong, chemisorptive interaction can occur also at hetero-organic interfaces.
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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