We measure the conductance of amine-terminated molecules by breaking Au point contacts in a molecular solution at room temperature. We find that the variability of the observed conductance for the diamine molecule-Au junctions is much less than the variability for diisonitrile and dithiol-Au junctions. This narrow distribution enables unambiguous conductance measurements of single molecules. For an alkane diamine series with 2-8 carbon atoms in the hydrocarbon chain, our results show a systematic trend in the conductance from which we extract a tunneling decay constant of 0.91 +/- 0.03 per methylene group. We hypothesize that the diamine link binds preferentially to undercoordinated Au atoms in the junction. This is supported by density functional theory-based calculations that show the amine binding to a gold adatom with sufficient angular flexibility for easy junction formation but well-defined electronic coupling of the N lone pair to the Au. Therefore, the amine linkage leads to well-defined conductance measurements of a single molecule junction in a statistical study.
@article{arxiv.cond-mat/0603281,
title = {Single-Molecule Circuits with Well-Defined Molecular Conductance},
author = {Latha Venkataraman and Jennifer E. Klare and Iris W. Tam and Colin Nuckolls and Mark S. Hybertsen and Michael L. Steigerwald},
journal= {arXiv preprint arXiv:cond-mat/0603281},
year = {2007}
}