Adsorption geometry of a large ð conjugated molecule (PTCDA) on different metal surfaces analysed x-ray standing waves: The role of interfacial and intermolecular interactions

Hauschild, Annegret1; Henze, Stina1; Bauer, Oliver1; Temirov, Ruslan1; Lassise, Adam1; Ikonomov, Julian1; Soubatch, Serguei1; Cowie, Bruce2; Tin-Lin, Lee2; Tautz, Stefan1; Sokolowski, Moritz1 1Germany; 2France

Presently there exists a large interest to understand the details of the bonding geometry of large planar ð conjugated molecules on metal surfaces both by experiment and theory. In particular the vertical bonding distances and possible distortions of the molecules due to the adsorption are of interest in relation to the interfacial bonding strength and the intermolecular interactions. We have investigated the model molecule PTCDA on a series of metal surfaces (Ag(111), Au(111), Ag(110), and Ag(100)) by the normal incidence x-ray standing wave (NIXSW) technique. For the carbon skeleton we find that the bonding distance on Au(111) [1] is by 0.4 Ang. (or 15%) larger than on Ag(111) (d= 2.86±0.01 Å) [2], in agreement with the expectation on the basis of a weaker bonding on Au. However, for PTCDA on Ag(111), we also find a strong influence of the intermolecular interactions, because for the disordered low temperature phase the bonding distance is found to be smaller (by 0.05 Å) compared the order phase at room temperature. A differential analysis of the two chemically different O atoms in the anhydride groups furthermore reveals a significant distortion of the molecule and points to their role in the interfacial bonding in addition to the perylene core. This work was supported by the DFG and the ESRF.
[1] S.K.M. Henze et al., Surf. Sci., in press.; [2] Hauschild et al. Phys. Rev. Lett. 94 (2005) 036106.

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