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Photoresponse of Molecular Tunnel Junctions: Converting Photons to Charge Carriers and Probing Internal Energy Levels

  • Author / Creator
    Fereiro, Jerry Alfred
  • Molecular electronics seeks to build circuitry using organic components with atleast one dimension in the nanoscale domain. Progress in the field has been inhibited by the difficulty in determining the energy levels of molecules after being perturbed by interactions with the conducting contacts. We show the conductance of seven different aromatic molecules covalently bonded to carbon implies a modest range (<0.5 eV) in the observed transport barrier despite widely different free molecule HOMO energies (>2.0 eV range). Upon bonding, electronic inductive effects modulate the energy levels of the system, resulting in compression of the tunneling barrier. Modification of the molecule with electron donating or withdrawing groups modulate both the molecular orbital energies and the contact energy level, resulting in a levelling effect which compresses the tunneling barrier into a range much smaller than expected. While the value of the tunneling barrier can be varied by using a different class of molecules (alkanes), using only aromatic structures results in a similar equilibrium value for the tunnel barrier for different structures resulting from partial charge transfer between the molecular layer and the substrate. Thus, the system does not obey the Schottky-Mott limit, and the interaction between the molecular layer and the substrate acts to influence the energy level alignment. These results indicate that the entire system must be considered to understand the impact of a variety of electronic factors that act to determine the tunnel barrier. The photocurrent spectra for large-area aliphatic and aromatic molecular tunnel junctions with partially transparent copper top contacts have been measured. The effect of variation of the molecular structure and thickness are discussed. Internal photoemission (IPE), a process involving optical excitation of hot carriers in the contacts followed by transport across internal system barriers, is dominant when the molecular component does not absorb light. The IPE spectrum contains information regarding energy level alignment within a complete, working molecular junction, with the photocurrent sign indicating transport through either the occupied or unoccupied molecular orbitals. At photon energies where the molecular layer absorbs, a secondary phenomenon is operative in addition to IPE. In order to distinguish IPE from this secondary mechanism, the effect of the source intensity as well as the thickness of the molecular layer on the observed photocurrent has been shown. Our results clearly show that the IPE mechanism can be differentiated from the secondary mechanism by the effects of variation of experimental parameters. IPE may provide valuable information regarding interfacial energetics in intact, working molecular junctions, as long as the secondary mechanism is avoided.

  • Subjects / Keywords
  • Graduation date
    2015-11
  • Type of Item
    Thesis
  • Degree
    Doctor of Philosophy
  • DOI
    https://doi.org/10.7939/R30863G9R
  • License
    This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.
  • Language
    English
  • Institution
    University of Alberta
  • Degree level
    Doctoral
  • Department
    • Department of Chemistry
  • Supervisor / co-supervisor and their department(s)
    • McCreery, Richard(Chemistry)
  • Examining committee members and their departments
    • Veinot, Jonathan (Chemistry)
    • Freeman, Mark (Physics)
    • Frank, Natia (Chemistry)
    • Petersen, Nils (Chemistry)