AVS 58th Annual International Symposium and Exhibition
    Surface Science Division Tuesday Sessions
       Session SS2-TuM

Paper SS2-TuM11
Electronic Structure and Charge Injection Barriers of Self Assembled Peptide Nucleic Acid Monolayers on Au

Tuesday, November 1, 2011, 11:20 am, Room 110

Session: Self Assembled Monolayers and Networks
Presenter: Matt Wolak, University of South Florida
Authors: M.A. Wolak, University of South Florida
A. Balaeff, Duke University
S. Gutmann, University of South Florida
M.M. Beerbom, University of South Florida
E. Wierzbinski, University of Pittsburgh
D.H. Waldeck, University of Pittsburgh
S. Bezer, Carnegie Mellon University
C. Achim, Carnegie Mellon University
D.N. Beratan, Duke University
R. Schlaf, University of South Florida
Correspondent: Click to Email
Peptide nucleic acids (PNA) are a promising alternative to DNA for bio-sensing applications as well as for strategies for self assembly based on nucleic acid hybridization. This potential is a result of the PNA’s neutral pseudopeptide backbone, which eliminates inter-strand electrostatic repulsion. In recent years charge transfer through PNA molecules has been a focus of research due to potential applications in self-assembled molecular circuits. This makes it interesting to investigate the electronic structure of PNA interfaces to electrode materials. A widely used strategy to ‘connect’ PNA molecules to metallic electrodes is through thiol-Au bonds using a terminal cysteine appended to PNA oligomers. This motivated the here presented research where the electronic structure of self-assembled PNA monolayers on Au substrates was investigated. Cys-appended PNA 7-mers of thymine (Cys-T7) were incubated on Au substrates in a nitrogen glove box attached to a photoemission spectrometer. Ultraviolet and x-ray photoemission spectroscopy (UPS and XPS) measurements on the resulting SAMs revealed the hole injection barrier at the interface and the interface dipole. Electronic structure calculations based on molecular dynamics sampling of the PNA structure yielded the band gap and the electronic density of states for PNA. Combined with the UPS data, the theoretical calculation enabled the estimate of the electron injection barrier at the interface, as well as the assignment of individual UP spectral features to specific molecular orbitals. Control measurements on Cys-appended, abasic PNA backbone 7-mers allowed the identification of the emissions related to the PNA backbone in the UP spectra. The orbital line-up at the interface between the Au substrate and the Cys-PNA indicates a significant interface dipole resulting in the alignment of the Au Fermi level near the center of the PNA HOMO-LUMO gap. This alignment causes large charge injection barriers for both holes and electrons, and thus impedes charge transfer from Au into the Cys-PNA SAM.