Dr. Kathrin SchönauerPhD Student Forschungszentrum JülichQuantum Nanoscience (PGI-3)
K. Schönauer, S. Weiß, V. Feyer, D. Lüftner, B. Stadtmüller, D. Schwarz, T. Sueyoshi, C. Kumpf, P. Puschnig, M. G. Ramsey, F. S. Tautz, S. Soubatch
In: Phys. Rev. B, vol. 94, pp. 205144, 2016.
On the Ag(110) surface copper phthalocyanine (CuPc) orders in two structurally similar superstructures, as revealed by low-energy electron diffraction. Scanning tunneling microscopy (STM) shows that in both superstructures the molecular planes are oriented parallel to the surface and the long molecular axes, defined as diagonals of the square molecule, are rotated by ≃±32° away from the high-symmetry directions [1-10] and  of the silver surface. Similarly to many other adsorbed metal phthalocyanines, the CuPc molecules on Ag(110) appear in STM as crosslike features with twofold symmetry. Photoemission tomography based on angle-resolved photoemission spectroscopy reveals a charge transfer from the substrate into the molecule. A symmetry analysis of experimental and theoretical constant binding energy maps of the photoemission intensity in the kx,ky-plane points to a preferential occupation of one of the two initially degenerate lowest unoccupied molecular orbitals (LUMOs) of eg symmetry. The occupied eg orbital is rotated by 32° against the  direction of the substrate. The lifting of the degeneracy of the LUMOs and the related reduction of the symmetry of the adsorbed CuPc molecule are attributed to an anisotropy in the chemical reactivity of the Ag(110) surface.
2015, ISBN: 978-3-95806-112-5.
In this work we investigate variations of a homo-molecular layer of CuPc adsorbed on the Ag(111) surface, which is a well known example in research on organic electronics where 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 second layer, 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 experiments on the electronic structure, STM-based differential conductance spectroscopy and angle-resolved PES are applied. For a second layer of CuPc on top of the first layer of CuPc on Ag(111) we observe a weaker interaction between the two molecular layers than between the substrate and the first molecular layer. This allows molecules in the second layer to adsorb in an inclined configuration in contrast to the flat lying geometry of molecules in the first layer. The HOMO of CuPc shifts towards larger binding energies with increasing coverage. The (former) LUMO, which in the first layer is weakly occupied by charge donation from the silver substrate, is unoccupied in the second layer because of a significantly weaker interaction with the underlying material. Experiments on a dense, closed layer of CuPc molecules on the Ag(110) surface reveal a 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 the lateral molecular arrangement interspersed by dislocation lines where the intermolecular interaction 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 molecule that is is aligned with a more acute angle to the Ag direction is slightly bent down, interacting stronger with the substrate and receiving charge donated by the silver. By this 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 the stronger influence of the substrate on the formation of ordered structures compared to mixed ordered layers on Ag(111). A tendency to form complex packing motifs is observed and we investigate two different structures that are described by large unit cells comprising 5 and 9 molecules, respectively. Measurements on the local electronic structure are dominated by signals from PTCDA molecules and we observe that the PTCDA LUMO is occupied to at least the same degree as it is in a homo-molecular PTCDA layer. Th CuPc LUMO is unoccupied indicating a molecule-molecule interaction with an unequal charge distribution for the two types of molecules.
M. Willenbockel, B. Stadtmüller, K. Schönauer, F. C. Bocquet, D. Lüftner, E. M. Reinisch, T. Ules, G. Koller, C. Kumpf, S. Soubatch, P. Puschnig, M. G. Ramsey, F. S. Tautz
In: New J. Phys., vol. 15, pp. 033017, 2013.
The compressed 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) herringbone monolayer structure on Ag(110) is used as a model system to investigate the role of molecule–molecule interactions at metal–organic interfaces. By means of the orbital tomography technique, we can not only distinguish the two inequivalent molecules in the unit cell but also resolve their different energy positions for the highest occupied and the lowest unoccupied molecular orbitals. Density functional theory calculations of a freestanding PTCDA layer identify the electrostatic interaction between neighboring molecules, rather than the adsorption site, as the main reason for the molecular level splitting observed experimentally.