Dynamic Switching of Ferrocene and Plasmonic Interactions in Au/Self-Assembled Monolayer/Single Ag Nanocube Molecular Junctions

We report the redox switching of ferrocene moieties embedded in a double tunnel barrier plasmonic cavity. A self-assembled monolayer of ferrocenyl-alkylthiol is formed by click chemistry on an ultraflat gold surface, and a poly(vinylpyrrolidone) capped silver single nanocube, AgNC, is deposited. The AgNC is contacted by the tip of a conductive-AFM to study the electron transport properties in the dark and under light irradiation at the plasmonic resonance wavelengths. We observe a dual behavior in the current–voltage (I–V) characteristics in the dark: a large hysteresis loop at positive voltages and a hysteretic negative differential conductance (NDC) at negative voltages, due to the redox switching of ferrocene between its oxidized (Fc⁺) and neutral (Fc⁰) states. The I–V curves are analyzed by a combined Marcus–Landauer model. We determine the highest occupied molecular orbital of the Fc⁺ and Fc⁰ states at 0.54 and 0.42 eV below the Fermi energy, respectively, with a weak reorganization energy <0.1 eV upon switching. Under plasmonic excitation, the hysteresis and NDC behaviors are no longer observed, and the I–V characteristics of the Au-ferrocenyl-alkylthiol/AgNC junctions become similar to Au-ferrocenyl-alkylthiol SAMs. A virtual molecular orbital due to the plasmon-induced coupling (fast electron transfer) between the two redox states of the Fc is determined at 0.46 eV. This dynamic behavior opens perspectives in artificial synaptic devices for neuromorphic computing with the additional function to turn on/off this synaptic behavior on-demand by light.