Effects of Interface Electronic Structures on Transition Voltage Spectroscopy of Alkanethiol Molecular Junctions

We investigated the charge transport characteristics of alkanemonothiol (CnH2n+1SH, n = number of carbons) molecular junctions by means of transition voltage spectroscopy (TVS) based on the observations of the interface electronic structures. The minimum in the Fowler–Nordheim plot was observed at the average positive and negative sample biases of +1.23 and −1.44 V. These voltages (Vmin) were insensitive to the molecular length. The low-energy ultraviolet photoelectron spectroscopy (LE-UPS) measurements revealed the presence of an Au–S bond at a binding energy of 1.4 eV with reference to the Fermi level of the Au substrates. The binding energy of the interface electronic state was independent of the molecular length. The TVS results were analyzed based on the LE-UPS results, including the differences in the measurement conditions. The results were consistently explained by the Au–S bond being responsible for Vmin at the negative bias. In addition, another interface state was suggested to be responsible for Vmin at the positive bias. The effects of the interface electronic structures besides the apparent barrier height should be considered to understand TVS of molecular junctions with wide energy gap molecules.