Web Release Date: November 2,
Efficacy of Au-Au Contacts for Scanning Tunneling Microscopy Molecular Conductance Measurements
and
Chemistry Department, Boston University, 590 Commonwealth Avenue, Boston, Massachusetts 02215, Department of Chemistry, Barnard College, 3009 Broadway, New York, New York 10027, and Department of Applied Physics and Applied Mathematics and Center for Electron Transport in Molecular Nanostructures, Columbia University, New York, New York 10027
Received: July 17, 2007
In Final Form: September 1, 2007
Abstract:
We measured conductance traces while breaking gold point contacts in a solution of molecules containing
the 
-p-phenylenediethynyl X-C
C-C6H4-CC-X unit, with eight different capping X groups: Au-P(OMe)3 (1), H (2), SiMe3 (3), Au-P(cy)3 (4), Au-PMe2Ph (5), Au-PMePh2 (6), Au-PMe3 (7), and Au-PPh3 (8). Our goal with this work was to achieve a direct Au-C link with a conjugated organic group,
potentially forming a molecular junction without chemical link groups that typically decrease junction
conductances, such as thiols or amines. Conductance traces collected in the presence of molecules 1, 2, 3, 5,
and 7 reveal additional steps at conductances as high as 0.1 G0 (G0 = 2e2/h) down to the measurable limits
of the experimental setup. Conductance histograms generated from these traces therefore show a broad increase
of counts when compared to a control histogram collected in the solvent alone suggesting the binding of the
molecules to the broken Au contacts. The histograms for molecules 1, 5, 7, and 2 were not distinguishable,
although that of molecule 3 had considerably fewer counts over the entire conductance range, suggesting that
the steric bulk of the SiMe3 prevented frequent junction formation. The histograms collected in a solution of
molecules 4, 6, or 8 did not differ from that of the control histogram probably because of the steric bulk of
the Au-PR3 capping groups prevented the formation a molecular junction.
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