The origin of the transition voltage of gold–alkanedithiol–gold molecular junctions

Abstract

We studied the origin of the transition voltage of octanedithiol (C8DT) molecules sandwiched between two gold electrodes. Au–C8DT–Au molecular junctions were fabricated using the feedback controlled electromigration technique. The conductance measurement and the inelastic electron tunneling spectrum validate that the observed current–voltage characteristics indeed originate from the C8DT molecule in the junction. The transition voltage of Au–C8DT–Au junctions is determined to be about 0.95V, which cannot be interpreted using the energy difference between the highest occupied molecular orbital or lowest unoccupied molecular orbital of the alkyl chain and the Fermi level of electrodes. Instead, we relate the measured transition voltage to the Au–S bonds, which are about 1.4eV below the gold Fermi level. Our interpretation is supported by the ultraviolet photoelectron spectroscopy measurements and first-principles quantum transport simulations. These findings provide convincing evidence that the Au–S occupied states play a significant role in the charge transport of Au–alkanedithiol–Au molecular devices.