AVS 51st International Symposium
    Surface Science Wednesday Sessions
       Session SS+OF-WeA

Paper SS+OF-WeA3
The Influence of Chemical Coupling Groups on the Electronic Structure of Conjugated Self-Assembled Molecular Monolayers

Wednesday, November 17, 2004, 2:40 pm, Room 213B

Session: Contacts to Molecules and Molecular Films
Presenter: S.W. Robey, NIST
Authors: S.W. Robey, NIST
C.D. Zangmeister, NIST
R.D. Van Zee, NIST
Correspondent: Click to Email
The performance of conjugated molecular systems in electronic applications, either for organic light emitting diodes (OLED's) and field effect transistors, or in more speculative applications proposed for molecular electronics, depends critically on coupling at the molecule-electrode interface. Interactions at this interface determine the alignment of the contact Fermi level with the transport levels in the molecular system, which in turn controls charge injection into the molecular @pi@ levels. Using one-and two-photon photoemission to access occupied and unoccupied levels, we have examined the influence of the metal-molecule coupling chemistry on the Fermi level alignment and electronic structure in the prototypical "molecular wire", 4,4'-(ethynylphenyl)-1-benzenethiol system on Au. Photoelectron spectroscopies reveal an increase in the binding energies (relative to the Fermi level of Au) for the C @sigma@ and @pi@ derived valence levels and the C (1s) core level upon substitution of the isocyanide coupling for thiol. No spectral changes are observed, however, in the region of the molecular @pi@ levels important for transport. Optical absorption measurements also reveal no change in optical band gap. These results indicate that substitution of the isocyanide linking chemistry for thiol based chemistry leads to a large shift, by ~ .5 eV, of E@sub f@ away from the highest occupied level in the molecule, but little modification of the extended @pi@ molecular electronic structure. The interaction at the Au-thiol-OPE interface will be compared and contrasted with the Au-isocyanide-OPE interface in terms of bonding and charge transfer effects and contact made to RAIRS and transport data for related systems.