Web Release Date: February 13,
Probing the Chemistry of Molecular Heterojunctions Using Thermoelectricity
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Department of Chemistry, Department of Mechanical Engineering, Applied Science and Technology Program, Department of Chemical Engineering, Department of Materials Science and Engineering, University of California Berkeley, California 94720, and Materials Science Division, Chemical Science Division, Lawrence Berkeley Laboratory, Berkeley, California 94720
Received October 23, 2007
Revised January 3, 2008
Abstract:
Thermopower measurements offer an alternative transport measurement that can characterize the dominant transport orbital and is independent of the number of molecules in the junction. This method is now used to explore the effect of chemical structure on the electronic structure and charge transport. We interrogate junctions, using a modified scanning tunneling microscope break junction technique, where: (i) the 1,4-benzenedithiol (BDT) molecule has been modified by the addition of electron-withdrawing or -donating groups such as fluorine, chlorine, and methyl on the benzene ring; and (ii) the thiol end groups on BDT have been replaced by the cyanide end groups. Cyanide end groups were found to radically change transport relative to BDT such that transport is dominated by the lowest unoccupied molecular orbital in 1,4-benzenedicyanide, while substituents on BDT generated small and predictable changes in transmission.
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