AVS 46th International Symposium
    Nanometer-scale Science and Technology Division Tuesday Sessions
       Session NS2-TuM

Paper NS2-TuM7
Self-assembled Molecular Electronics: Is the Interface Conducting?

Tuesday, October 26, 1999, 10:20 am, Room 6C

Session: Molecular Electronics
Presenter: X.-Y. Zhu, University of Minnesota
Authors: T. Vondrak, University of Minnesota
C.J. Cramer, University of Minnesota
X.-Y. Zhu, University of Minnesota
Correspondent: Click to Email
The use of a single molecule as a 'quantum dot' or 'quantum wire' in charge transport has attracted considerable attention due to its exciting potential in future electronic devices. A number of groups have studied the electron transport in single aromatic molecules using thiol self-assembled monolayers (SAMs) on gold surfaces. A critical issue in interpreting experimental current-voltage measurements and in designing self-assembled monolayer of molecular electronics is understanding the interfacial electronic structure. We present a systematic study to address the title question. We probe both occupied and unoccupied electronic states at the interface using laser two-photon photoemission spectroscopy, in conjunction with electronic structural calculations. We choose phenyl or fluorophenyl group tethered to the metal surface at various distance in self-assembled monolayers on Cu(111). We found that, for phenyl attached to Cu via the -S- linker, the molecular LUMO can be stabilized by as much as 3 eV. This large change cannot be accounted for exclusively by polarization effects. The majority of the stabilization energy must come from direct, strong electronic coupling between the substrate and the adsorbate. Ab initio calculations on model molecules confirms this conclusion. This kind of strong electronic coupling is absent when the molecule is located at a similar distance, but weakly coupled to the surface. Thus, we may view the -S-metal linker as a conducting contact in SAMs of molecular electronics. Our result also suggests the importance of sigma states for electron transport in short molecular wires.