C. Daniel Frisbie Group
Chemical Engineering & Materials Science
University of Minnesota - Twin Cities 		Films & Interfaces,
Molecular Crystals,
Molecular Electronics,
Organic Semiconductors,
Scanning Probe Microscopy

Jeremy_Beebe Jeremy Beebe
NIST
Ph.D. Chemistry, 2005

Examination of the Resistance Profile of Nanoscopic Molecular Junctions

Junctions composed of a small number of organic molecules contacted on each side by metal pads have been proposed as important components of molecular circuitry. This proposed use remains a long way off, since the groundwork has not yet been laid for understanding how electron transport occurs in these systems. In order to address this issue, we make metal-molecule-metal junctions using the conducting probe atomic force microscopy (CP-AFM) technique. Specifically, we coat a silicon substrate and an AFM tip with a metal layer, allow a monolayer to self-assemble on the metal surface, and bring the tip into contact. We then source a voltage at the tip, ground the substrate, and measure the resulting current through the organic molecules. By examining the length dependence of resistance in a homologous series, we are able to determine both the structure-dependent decay constant b and the residual resistance when the series is extrapolated to zero repeat units, the contact resistance.

My research has focused on determining what role the metal-molecule interface plays in the overall junction resistance. By changing the metal at both the tip and substrate, we have found the junction resistance to be work function dependent. Contacting alkanethiols or alkanedithiols with a high work function metal results in increased current flow. This trend is indicative of HOMO-assisted transport, or 'hole-tunneling.' We have also seen that changing from monothiols to dithiols (1 chemisorbed to 2 chemisorbed contacts) results in essentially a factor of 100 increase in current for all metal types. Finally, we have also examined the effects of changing from a thiol contact to an isocyanide contact, and seen that for at least the case of alkanes, there is essentially no difference in the two contact types.

Publications
"Contact Resistance in Metal-Molecule-Metal Junctions Based on Aliphatic SAMs: Effects of Surface Linker and Metal Work Function," Beebe, J. M.; Engelkes, V. B.; Miller, L. L.; Frisbie, C. D., J. Am. Chem. Soc. 2002, 124(38), 11268-11269.

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