Transition from Direct Tunneling to Field Emission in Metal-Molecule-Metal Junctions

Current-voltage measurements of metal-molecule-metal junctions formed from $\ensuremath{\pi}$-conjugated thiols exhibit an inflection point on a plot of $\mathrm{ln}(I/{V}^{2})$ 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.