Berlin 2012 – scientific programme
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O: Fachverband Oberflächenphysik
O 88: Electronic structure II
O 88.8: Talk
Friday, March 30, 2012, 12:15–12:30, MA 043
Charge transfer and dipole-dipole repulsion at ultra-thin organic heterojunction on metal — •Patrick Amsalem1, Jens Nierderhausen1, Andreas Wilke1, Raphael Schlesinger1, Stefanie Winkler1, Antje Vollmer2, Jurgen P. Rabe1, and Norbert Koch1 — 1Humboldt-Universität zu Berlin, Institut für Physik, Newtonstrasse 15, Berlin 12489, Germany — 2Helmholtz-Zentrum Berlin für Materialien und Energie GmbH - BESSY II, Albert-Einstein-Strasse 15, Berlin 12489, Germany
We investigate the electronic properties of a two monolayer thick organic heterojunction grown on a metal surface by photoelectron spectroscopy. The first molecular α-sexithiophene layer is physisorbed on the Ag(111) substrate and the sample work function decreases by 0.7 eV. Upon incremental deposition of an organic C60 overlayer, a charge transfer state, attributed to the partial filling of the C60 LUMO-derived states, is observed. This is accompanied by a work function increase which does not increase linearly as a function of coverage within the sub-monolayer regime. Analysis of the valence band and core level spectra reveals that the overlayer is actually composed of a mixture of charged and neutral species. Noteworthy, the work function behavior is correlated with the ratio of charged and neutral molecules, which depends on coverage. Dipole-dipole repulsion is proposed as the main driving force to explain these charge/neutral molecule ratio variations. The energy level alignment and structural properties are discussed by invoking simple electrostatic considerations.