Ultrasonic Generation of Thiyl Radicals: A General Method of Rapidly Connecting Molecules to a Range of Electrodes for Electrochemical and Molecular Electronics Applications

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.