Charge transport in molecular electronic junctions: compression of the molecular tunnel barrier in the strong coupling regime

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DOIResolve DOI: http://doi.org/10.1073/pnas.1201557109
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TypeArticle
Journal titleProceedings of the National Academy of Sciences of the United States of America
ISSN0027-8424
1091-6490
Volume109
Issue29
Pages1149811503; # of pages: 6
Subjectenergy alignment; molecular electronics; electronic coupling; charge transport; Fermi-level pinning
AbstractMolecular junctions are essentially modified electrodes familiar to electrochemists where the electrolyte is replaced by a conducting “contact.” It is generally hypothesized that changing molecular structure will alter system energy levels leading to a change in the transport barrier. Here, 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 eV range). These results are explained by considering the effect of bonding the molecule to the substrate. Upon bonding, electronic inductive effects modulate the energy levels of the system resulting in compression of the tunneling barrier. Modification of the molecule with donating or withdrawing groups modulate the molecular orbital energies and the contact energy level resulting in a leveling effect that compresses the tunneling barrier into a range much smaller than expected. Whereas 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 determine the impact of a variety of electronic factors that act to determine the tunnel barrier.
Publication date
LanguageEnglish
AffiliationSecurity and Disruptive Technologies; National Research Council Canada
Peer reviewedYes
NPARC number21269078
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Record identifier2c24db58-f704-430e-95fe-7924c0171d0b
Record created2013-12-05
Record modified2016-05-09
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