Dresden 2011 – scientific programme

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O: Fachverband Oberflächenphysik

O 68: Polymeric biomolecular films

O 68.5: Talk

Thursday, March 17, 2011, 12:15–12:30, WIL B122

Polariton effect in α-Sexithiophene films revealed by reflectance difference spectroscopy — •Günther Weidlinger, Lidong Sun, and Peter Zeppenfeld — Institute of Experimental Physics, Johannes Kepler University Linz, Altenberger Straße 69, A-4040 Linz, Austria

α-Sexithiophene (α-6T) films consisting of either flat-lying or standing molecules were prepared on clean and carbon-terminated Ag(110) surfaces, respectively, using organic molecular beam epitaxy. The optical properties of the α-6T films were investigated by reflectance difference spectroscopy (RDS). For films composed of flat lying molecules the higher Davydov component derived from the intramolecular HOMO-LUMO transition is located at 2.6 eV, whereas for standing α-6T molecules the transition is shifted to 3.5 eV. This observation can be explained by the so-called polariton effect, i.e., the interaction between photons and excitons [1]. The energetic position of the higher Davydov component thus becomes a function of the angle between the α-6T molecular axis in the films and the propogation direction of the light. This result demonstrates that crystalline orientation and thus the excitonic optical properties of organic thin films can be controlled by the substrate and thin film preparation. Furthermore, RDS is shown to be a sensitive method not only for the determination of the in-plane alignment of organic molecules, but also for their out-of-plane orientation.

[1] G. Weiser and S. Möller, Organic Electronics 5 (2004) 91-97