Current-voltage measurements of metal-molecule-metal junctions formed from pi-conjugated thiols exhibit an inflection point on a plot of ln(I/V(2)) vs 1/V, consistent with a change in transport mechanism from direct tunneling to field emission. The transition voltage was found to scale linearly with the offset in energy between the Au Fermi level and the highest occupied molecular orbital as determined by ultraviolet photoelectron spectroscopy. Asymmetric voltage drops at the two metal-molecule interfaces cause the transition voltage to be dependent on bias polarity.

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Here Γ is the width at half maximum of the resonance produced by the hybridization with the conduction electrons in the right and left lead and ǫ 0 is the energy of the local state. The value of Γ depends on the height and width of the barrier potential between the central region and the leads.

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