Characterization of Tunneling Junctions Comprising Self-Assembled Monolayers of Fully-Conjugated Molecular Wires using an eutectic Gallium-Indium Top Electrode

Understanding how molecular structure-function characteristics relate to the electrical properties of molecule-templated tunneling junctions is of primary importance to the realization of nanoscale electronic devices. In this study, we made use of a liquid top electrode made of eutectic Ga-In alloy (EGaIn)[1] for probing the transport properties across tunneling junctions obtained by contacting Self-Assembled Monolayers (SAMs) of fully-conjugated oligo(phenylene ethynylene) molecules (OPE) on template stripped Au. Compared to single molecule measurements, the use of SAMs enable the study of bottom-up large-area junctions, an environment more similar to what could be a final molecular electronic device. EGaIn-based techniques are well-established and have already been widely used to characterize the properties of molecular junctions comprising saturated and conjugated molecules investigating the effect of different lengths, anchoring groups, and functionalities on the tunneling transport properties. Anyway, few data are reported on fully conjugated systems. This may be due to the difficulties encountered when the measurements of these latter are performed under ambient conditions: large data dispersion, low current values and lower yield of working junctions make the data analysis complex and unreliable. Here we report how an accurate control of the environment conditions was successful in the measurements of a serie of thioacetate-terminated OPEs. In particular, preparation, handling and measurement of the samples were performed in a flowbox in which the O2 level was fixed between 1-3% and the humidity kept below 10%. Thanks to these favorable new conditions we were able to determine a beta value for the OPE wires and observe the effect on the current density of a different terminal group.