Regensburg 2007 – wissenschaftliches Programm
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O: Fachverband Oberflächenphysik
O 16: Organic, Polymeric, and Biomolecular Films I
O 16.13: Vortrag
Montag, 26. März 2007, 17:15–17:30, H42
Abnormal pinning of the Fermi and vacuum levels in monomolecular self-assembled films — Heejoon Ahn1, James Whitten1, and •Michael Zharnikov2 — 1Department of Chemistry, The University of Massachusetts Lowell, Lowell, MA 01854, USA — 2Angewandte Physikalische Chemie, Universität Heidelberg, D-69120 Heidelberg, Germany
Ultrathin organic monomolecular films, while electric insulators, are generally believed to be in electrical contact with a photoelectron spectrometer when adsorbed on metal substrates due to electron tunnelling that results in alignment of the Fermi levels of the film and the spectrometer. We show, however, that this generally applicable model is incorrect for these systems and that energy level alignment in photoemission experiments occurs in a complex fashion, involving both vacuum and Fermi levels of the monomolecular film, conductive substrate, and the spectrometer. While the Fermi levels of the substrate and spectrometer are aligned, vacuum level instead of Fermi level alignment occurs between the substrate and the SAM. As a result, the binding position of the photoemission peaks related to the monomolecular film is not only determined by its chemical identity, but also by changes in the work function of the substrate and film itself. This effect should certainly be taken into account in drawing conclusions from photoemission spectra of self-assembled organic films and adsorbate systems.