Herein, we report ultrasonic generation of thiyl radicals as a general method for functionalizing a range of surfaces with organic molecules. The method is simple, rapid, can be utilized at ambient conditions and involves sonicating a solution of disulfide molecules, homolytically cleaving S-S bonds and generating thiyl radicals that react with the surfaces by forming covalently bound monolayers. Full molecular coverages on conducting oxides (ITO), semiconductors (Si-H), and carbon (GC) electrode surfaces can be achieved within a time scale of 15-90 min. The suitability of this method to connect the same molecule to different electrodes enabled comparing the conductivity of single molecules and the electrochemical electron transfer kinetics of redox active monolayers as a function of the molecule-electrode contact. We demonstrate, using STM break-junction technique, single-molecule heterojunction comprising Au-molecule-ITO and Au-molecule-carbon circuits. We found that despite using the same molecule, the single-molecule conductivity of Au-molecule-carbon circuits is about an order of magnitude higher than that of Au-molecule-ITO circuits. The same trend was observed for electron transfer kinetics, measured using electrochemical impedance spectroscopy for ferrocene-terminated monolayers on carbon and ITO. This suggests that the interfacial bond between different electrodes and the same molecule can be used to tune the conductivity of single-molecule devices and to control the rate of charge transport in redox active monolayers, opening prospects for relating various types of interfacial charge-transfer rate processes.
Keywords: electron transfer kinetics; molecular electronics; molecule−electrode contact; self-assembled monolayers; single-molecule conductance; thiyl radicals.