The performance of organic semiconducting devices like organic field effect transistors (OFET) or organic light emitting diodes (OLED) crucially depends on structural details of the contacting interface between the organic semiconductor and the substrate [1]. At the contacting interface, the electronic interactions of organic molecules with metal or semiconducting substrates play a crucial role. The highest occupied molecular orbitals (HOMO) and the lowest unoccupied molecular orbitals (LUMO) of the organic molecules interact with bulk electronic states as well as with surface states of the substrate. This can lead to changes of the molecular orbitals as well as of the electronic structure of the substrate.

We have measured the modification of the Cu(110) Shockley surface state by the adsorption of an organic semiconducting molecule, namely pentacene with high-resolution angular-resolved photoelectron spectroscopy (ARPES). Two different adsorption structures of pentacene on Cu(110) depending on the molecular coverage are investigated (cf. Fig. 1). 

The results of the ARPES measurements on the two different adsorption structures are shown in Fig. 2. The red, dotted curves denote the fitted position of the surface state for the uncovered clean Cu(110) surface, where as the solid lines marked in blue denote the fitted position of the surface state on the pentacene covered samples. The binding energy of the lowest lying electronic state increases with increasing coverage, which is contradictory to former results for another organic molecule (PTCDA) on Au(111) [2]. Another interesting result of our measurements is, that the occupation of the surface state increases for the adsorption of 1 ML of pentacene. This can be seen in Fig 2e, where the Fermi surface contour encloses a bigger area as for the clean Cu(110) sample. The detailed physical mechanisms which lead to this change of the Shockley surface state of Cu(110) are still under investigation. We propose a combination of two mechanisms: (1) the Pauli repulsion between the surface electrons and the HOMO of the molecule and (2) the charge donation from the molecule to the substrate. We also aim at theoretical calculations to confirm our results and for a better understanding of the mechanism behind this particular modification of the Cu(110) Shockley surface state by adsorption of 1 ML of pentacene.

Fig. 2: Dispersion of the  Shockley surface state of a Cu(110) surface, covered with a-c) 0.8 ML pentacene and d-f) 1.0 ML pentacene. The lines marked in blue mark the fit of a paraboloid to the experimental data. The dashed lines marked in red correspond to the paraboloid fitted to the experimental data of the clean Cu(110) surface.

References

[1] S. Y. Yang, K. Shin, C. E. Park, Adv. Funct. Mater. 15, 1806 (2005)
[2] N. Nicoara, E. Román, J. M. Gómez-Rodríguez, J. A. Martín-Gago, and J. Méndez; Org. Electron. 7, 287 (2006)

Related publications
A. Scheybal et al. Phys. Rev. B 79, 115406, 2009.
K. Müller et al. Phys. Rev. B 79, 245421 (2009).

Contact
Kathrin Müller:kathrin.muellerpsi.ch
Dr. T.A. Jung: thomas.jungpsi.ch

Academic Partners:
Prof. Ph. Aebi, University of Neuchâtel, CH (http://www.unine.ch/phys/spectro/welcome.html)
Prof. J. Osterwalder, University of Zürich (http://www.physik.unizh.ch/groups/grouposterwalder/)

Financial support:
PSI, NCCR Nanosciences (http://www.nccr-nano.org/nccr), SNF