Quantifying Image Charge Effects in Molecular Tunnel Junctions Based on Self-Assembled Monolayers of Substituted...

A number of factors contribute to orbital energy alignment with respect to the Fermi level in molecular tunnel junctions. Here, we report a combined experimental and theoretical effort to quantify the effect of metal image potentials on the highest occupied molecular orbital to Fermi level offset, ε_h, for molecular junctions based on self-assembled monolayers (SAMs) of oligophenylene ethynylene dithiols (OPX) on Au. Our experimental approach involves the use of both transport and photoelectron spectroscopy to extract the offsets, ε_h^trans and ε_h^UPS, respectively. We take the difference in these quantities to be the image potential energy eV_image. In the theoretical approach, we use density functional theory (DFT) to calculate directly eV_image between positive charge on an OPX molecule and the negative image charge in the Au. Both approaches yield eV_image ∼ -0.1 eV per metal contact, meaning that the total image potential energy is ∼-0.2 eV for an assembled junction with two Au contacts. Thus, we find that the total image potential energy is 25-30% of the total offset ε_h, which means that image charge effects are significant in OPX junctions. Our methods should be generally applicable to understanding image charge effects as a function of molecular size, for example, in a variety of SAM-based junctions.