AVS 50th International Symposium
    Contacts to Organic Materials Topical Conference Monday Sessions
       Session OM-MoA

Paper OM-MoA9
Band Alignment on Patterned Surfaces

Monday, November 3, 2003, 4:40 pm, Room 318/319

Session: Contacts to Molecules and Molecular Films (II)
Presenter: G. Koller, Karl-Franzens University, Austria
Authors: G. Koller, Karl-Franzens University, Austria
F.P. Netzer, Karl-Franzens University, Austria
M.G. Ramsey, Karl-Franzens University, Austria
Correspondent: Click to Email
An important parameter for the function of organic electronic devices is the barrier to charge injection which is controlled by the interface dipole. This is determined by the details of the interaction of the first layer of molecules with the contact material. In reality the interface is not perfectly homogeneous and thus local differences in band alignment are to be expected although not necessarily observable with standard area averaging techniques. In this contribution we explore the local variation in band alignment on a nanoscopically patterned substrate surface by investigating the growth of bithiophene films, from sub-monolayer to multilayer coverages, with angle resolved UV-photoemission and workfunction measurements. The substrate used was the oxygen reconstructed Cu(110) surface. This surface reconstructs to form a mesoscopically patterned surface consisting of alternating stripes of clean Cu(110) and passive Cu(110)-(2x1)O, with stripe widths in the range of several nanometers depending on oxygen exposure. The bonding interaction of the probe molecule bithiophene is very different on the two surfaces resulting in band alignment differences of 1 eV for films grown on these surfaces. On the Cu(111)-(2x1)O striped surface for coverages of bithiophene up to two monolayers, the photoemission spectra is a result of the superposition of spectra offset by this difference in band alignment clearly indicating local differences in the charge injection barrier. However, beyond two monolayers the valence band spectra clearly indicate a single emitting species whose energy position is determined by the average interface dipole. The results thus show that inhomogenities of the substrate surfaces will lead to local differences in the charge injection barrier which are invisible to area averaging techniques if the molecular film thickness are greater than the lateral dimensions of the inhomogenities. Acknowledgments: Supported by the Austrian Science Foundation SFB Electroactive Materials