Abnormal pinning of the Fermi and vacuum levels in monomolecular self-assembled films

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. (c) 2006 Elsevier B.V. All rights reserved.