Sweep-Character-Dependent Switching of the Conductance State in Ferrocene-Substituted Thiofluorene Self-Assembled Monolayers

Self-assembled monolayers (SAMs) of ferrocene-substituted thiofluorene on Au(111) exhibit two distinct conductance states (CSs) in two-terminal junctions featuring a sharp tip of eutectic GaIn as the top electrode. The occurrence of these states and the resulting effective rectification by the SAM depend on the way the bias voltage is swept; when the junction is only negatively biased, the original, high CS is preserved, whereas the junction is switched to a low CS when applying only positive biases. This results in an exceptionally high effective rectification ratio (RR) of ∼2100 already at voltages as low as 0.1 V. In contrast, when sweeping the junction alternatingly to the maximum positive and negative bias voltages (as usually performed in the literature), fully symmetric J-V curves are observed. That is, for the present SAM, rectification disappears, and the effective RR is ≈1. It is noteworthy that whether the junction in these symmetric sweeps is in the high or low CS depends on the polarity of the first sweep. We attribute the occurrence of the two CSs to a (quasi) non-reversible oxidation of the ferrocenes in combination with structural changes in the monolayer geometry. The observed sweeping dependence of the conductivity switching is an additional parameter that needs to be considered when interpreting experimental J-V curves, especially when dealing with redox-active systems.