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

Paper OM-MoM7
Electronic Structure and Hole Injection Barriers for Pure and Substituted Phenylene Ethynylene Oligomer Self-Assembled Monolayers

Monday, November 3, 2003, 10:20 am, Room 318/319

Session: Contacts to Molecules and Molecular Films (I)
Presenter: S.W. Robey, National Institute of Standards and Technology
Authors: S.W. Robey, National Institute of Standards and Technology
C. Zangmeister, National Institute of Standards and Technology
R.D. van Zee, National Institute of Standards and Technology
Correspondent: Click to Email
Ultraviolet photoelectron spectroscopy was used to determine the electronic structure of self-assembled monolayers (SAMs) of phenylene ethynylene oligomers on Au. Monolayers comprised of the three ring oligomer (OPE) and molecules with -NO@sub2@ (NO@sub2@-OPE) substitutions on the central ring were investigated. This study provides the first glimpse of the electronic structure and band alignment for SAMs of these molecular systems linked to interesting transport characteristics in nanoscale junctions.@super1@ For the unsubstituted oligomer, the electronic structure is interpreted using simple molecular systems and related polymers as guidance. The states above ~ 5 eV binding energy are primarily C-C and C-H @sigma@ bonding states. Their polarization dependence is found to be consistent with a nearly upright orientation. Near the Fermi level (E@subf@), orbitals traceable to benzene e@sub1g@ @pi@ states are split by the C-C coupling along the backbone into density localized primarily on the ortho-carbon atoms of individual phenyl rings (at ~ 3.9 eV) and potentially delocalized @pi@ levels along the backbone that can provide the primary channels for transport. The hole injection barrier (energy separation between E@subf@ and the HOMO level) is about 0.8 eV. Substitution of electron withdrawing groups on the central ring induces negative differential resistance in transport measurements.@footnote 1@ The effects on electronic structure were investigated in monolayers of the NO@sub2@-OPE oligomers. Compared to the unsubstituted OPE, there is an overall shift of intensity for @pi@ states near E@subf@ to higher binding energy by about 0.5 eV. The hole injection barrier also appears to increase to approximately 1.0 eV. Connections with calculations of transport properties for related molecular systems and experimental and theoretical results for substituent effects in conjugated polymers will be discussed. @FootnoteText@@footnote 1@Chen et al.,Science 286,1550(1999).