Nanometer-Thick Bilayers by Stepwise Electrochemical Reduction of Diazonium Compounds for Molecular Junctions

This work describes an electrochemical bottom-up approach for the modification of carbon and gold electrodes with two nanometer-thick molecular layers. The strategy adopted is based on two successive electroreductions of a diazonium salt and is used in the fabrication of molecular junctions with gold and titanium/gold contacts. The ultrathin layers deposited are an electron donor, oligo(bisthienylbenzene) (BTB), and an electron acceptor, oligo(phenyl methylviologen) (PMV). The bilayers generated are characterized by atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS) depth profile analysis, and electrochemical techniques. The study demonstrates the possibility of grafting one layer over an initial one to create strongly coupled donor-acceptor or acceptor-donor bilayer systems with minimal interpenetration and overall thicknesses between 5 and 10 nm. Electron transfer to several outer-sphere redox probes in solution and electron transport in solid-state molecular junctions are studied. The electrochemical response of redox probes on these modified electrodes is close to that for a diode, thanks to the easily p-dopable oligo(BTB) or easily reducible oligo(PMV) moieties. Moreover, electron transport in the molecular junctions exhibits strong rectification. Both electrochemical response and electron transport depend on the order of deposition of the two layers.