AFM study of charge transport mechanisms at electrical contacts in metal-molecule-metal systems

We investigated charge transport mechanisms at the interface between 1,8 octanedithiol (ODT) and gold and platinum electrodes. We used a voltage-biased, conductive-probe atomic force microscope tip (CP-AFM) to repeatedly break the bond between a molecule and an electrode and compiled statistics of the measured conductances for gold (Au) and platinum (Pt) electrodes. We examined the conductances of the Au-ODT-Au, Au-ODT-Pt, and Pt-ODT-Pt systems and observed at least two kinds of conductance mechanisms. The conductances were ordered, Pt-ODT-Pt > Pt-ODT-Au > Au-ODT-Au, which is consistent with a charge-transport mechanism through the junction that is proportional to the number of electron channels and depends on the molecular distance between the ligand group at the end of the ODT and the electrode. We investigated the electronic properties of the thiol-metal junction by measuring conductances as a function of bias voltage. We observed a nonlinear junction response that we attribute to a resonance among interface states in the junction. The junction stability was investigated by measuring the conductance step-length, which we correlated with the chemical binding energy.