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


Other title
Light vs. molecular tunnel junctions
Probing internal energy levels
Photoresponse of molecular tunnel junctions
Type of item
Degree grantor
University of Alberta
Author or creator
Fereiro, Jerry Alfred
Supervisor and department
McCreery, Richard(Chemistry)
Examining committee member and department
Freeman, Mark (Physics)
Frank, Natia (Chemistry)
Veinot, Jonathan (Chemistry)
Petersen, Nils (Chemistry)
Department of Chemistry

Date accepted
Graduation date
Doctor of Philosophy
Degree level
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.
Permission is hereby granted to the University of Alberta Libraries to reproduce single copies of this thesis and to lend or sell such copies for private, scholarly or scientific research purposes only. The author reserves all other publication and other rights in association with the copyright in the thesis and, except as herein before provided, neither the thesis nor any substantial portion thereof may be printed or otherwise reproduced in any material form whatsoever without the author's prior written permission.
Citation for previous publication
Sayed Y. Sayed*, Jerry A. Fereiro*, Haijun Yan, Richard L. McCreery, Adam J. Bergren. Proceedings to National Academy of Science (PNAS-USA). 2012, vol. 109, No.29, 11498-11503. (*equally contributed, Invited article).Jerry A. Fereiro, Adam J. Bergren, Richard L. McCreery. Journal of the American Chemical Society (JACS). 2013, vol. 135, NO.26, 9584-9587.Jerry A. Fereiro, Mykola Kondratenko, Adam J. Bergren, Richard L. McCreery. Journal of the American Chemical Society (JACS). 2015, vol. 137, NO.3, 1296-1304.

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