Reports: B547506-B5: Electrical Conductivity of Molecular Assemblies

Krzysztof Slowinski, PhD, California State University (Long Beach)

Our research efforts have focused on experiments involving the use of GaIn/Hg and Hg/Hg tunnel junctions to investigate electrical properties of molecular assemblies.  We focused on junctions containing monolayers and bilayers of  mercaptoundecanoic acid assembled in aqueous environment. We determined that mercaptoundecanoic acid forms well organized monolayers on Hg surfaces with carboxylic group facing towards solution side of the junction. This experiment, performed using bipotentiostatically controlled tunnel junction device, allows independent control of electrochemical potential of each Hg or GaIn electrode with respect to common reference (SSCI). Once monolayer(s) are formed on the elctrodes, the drops are brought together and tunnel junction containing either monoleyr of bilayer of molecules is formed. The fundamental unresolved question in this area of research is the nature if the non-covalent interface between the base electrode and the monolayer or between 2 monolayers. Our experiments demonstrate that the solution conditions are critically important. The rate of electron transfer across a junction containing bilayer of undecanoic acid in a non-buffered water/ethanol solution containing lithium perchlorate as supporting electrolyte, is 7 orders of magnitude lower than the rate measure for a bilayer if undecanethiol under identical experimental conditions. It was determined previously in numerous experiments that the rate of electron tunneling across the junction depends exponentially on distance with tunneling coefficient of about 1 per methylene group. The observed, seven orders of magnitude, difference in rate between two systems that differ only by the presence of a carboxylic group, indicates that accumulation of water and/or ions at the junction interface plays critical role in its conductivity.  We investigated this phenomenon in detail by looking at the junction behavior in solutions at different pH. We determined that protonation of the terminal carboxylic group plays central role in the composition and properties of interfacial layer of water and ions which determine the overall thickness of the tunnel junction. Specifically, while pH has no influence on the tunneling currents flowing across the methyl-terminated monolayer/Hg interface, it significantly affects currents flowing across the carboxy-terminated monolayer/Hg interface.  The presented experimental approach allows, for the first time, systematic investigation of the electrical properties of electrochemically controlled tunnel junctions, through examination of the electrical double-layer formed at the monolayer/electrode interface.