Berlin 2008 – scientific programme

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DS: Fachverband Dünne Schichten

DS 19: Organic Interfaces (SYSA 6)

DS 19.7: Talk

Wednesday, February 27, 2008, 16:15–16:30, H 2013

Direct Optical Observation of Charge Transfer between Metals and Epitaxial Organic Layers — •Roman Forker, Giovanni Pizzi, Christian Golnik, Thomas Dienel, and Torsten Fritz — Institut für Angewandte Photophysik, Technische Universität Dresden, 01062 Dresden, Germany

We investigate epitaxial mono- and multilayers of 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) in situ on various metal surfaces by a variant of optical absorption spectroscopy. During film growth we observe pronounced changes in the spectra.

Unlike on insulating substrates, such as mica [1], the 1 ML spectra on Au(100), Au(111), and Ag(111) are broad and essentially featureless. Detailed theoretical work for PTCDA/Au(111) suggested significant molecular level broadening and interface electron density rearrangement induced by the metal proximity [2], which might be applicable to Ag as well despite reports of gap states in that case [3]. In the second ML on these metals, however, our spectra exhibit monomeric character similar to the first ML on mica, which indicates weak interaction with the underlying PTCDA layer. Besides the monomeric part of the 2 ML spectra on Au, a new feature at around 2.0 eV appears. By comparison to K-doped PTCDA on mica (where the anion shows up at 1.85 eV) and to semi-empirical calculations we reason that we optically probe cationic PTCDA in the partly charged second ML on Au.

[1] H. Proehl et al., Phys. Rev. Lett. 93 (2004), 097403.

[2] H. Vázquez et al., Europhys. Lett. 65 (2004), 802.

[3] Y. Zou et al., Surf. Sci. 600 (2006), 1240.