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

Abstract

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.

Absence of a Relationship between Surface Conductivity and Electrochemical Rates: Redox-Active Monolayers on Si(211), Si(111), and Si(110)

“…Current–potential characteristics acquired by cAFM reveals that surface conductivity decreases in the order Si(211) ≫ Si(110) > Si(111). Unlike systems where differences in electrical conductivity are imparted by changes to the chemical nature of the monolayer anchoring group and unlike the inverse relationship between electron-transfer rate constants and the length of alkyl spacers separating the electrode and redox unit, , we found that identical surface chemistry on Si(211), Si(110), and Si(111) leads to comparable kinetics of a redox reaction occurring at the monolayer distal end. While other higher-index substrates, such as Si(311) and Si(411), remain to be investigated, the current findings demonstrate a pronounced facet-dependent electrical conductivity, expand the range of silicon orientations viable as electrode materials, and suggest literature discrepancies in electrochemical rate constants not to be linked to substrate defects, such as ubiquitous surface miscuts.…”

“…First, it is now generally agreed that common mathematical models used in the kinetic analysis of electrochemical data fall short of capturing all the important descriptors of an electrified interface. − Second, minor changes to the surface coverage of the redox molecule, monolayer order, and intermolecular interactions , are known to have dramatic kinetic effects. ,, It remains less clear to what extent the chemical nature of the interfacial bondbetween the organic monolayer and electrodeaffects the rates of surface-confined electrochemical reactions. , An important step in the direction of clarifying this overlooked factor was recently published. Dief and Darwish reported that for monolayers grafted on either indium–tin oxide or silicon or carbon electrodes, there is a link between changes in molecular conductivitiesascribed unambiguously to different interfacial bondsand differences in electrochemical rates …”

“…Dief and Darwish reported that for monolayers grafted on either indium−tin oxide or silicon or carbon electrodes, there is a link between changes in molecular conductivitiesascribed unambiguously to different interfacial bondsand differences in electrochemical rates. 40 The origin of a putative link between electrical conductivity and electrochemical rates still lacks a conclusive explanation, 39 but its existence and manifestation would have immediate implications in silicon electrochemistry applied to sensing and catalysis. This is because an atomically flat silicon electrode is an idealization, and even well-prepared surfaces are a continuous structure of flat terraces separated by small vertical steps.…”

Impermeable Graphene Oxide Protects Silicon from Oxidation

“…This shadows the semiconducting properties of Si and becomes more critical for nanoscale devices whose output will be dominated by the SiO x layer . Conventionally, self-assembled monolayers comprising alkyl chains are used to protect Si from oxidation by blocking oxygen and water from reaching the bulk silicon. ,− This however comes with an expense of the need for complicated fabrication procedures that often require external effects, such as heat, light, or radical initiators, to enable the formation of covalent bonds between the monolayer-forming molecules and the surface. − In addition, the length of the alkyl chain on the molecules in the monolayer should be long enough to enable efficient packing . However, longer-chain molecules also come with the expense of blocking electron transfer. − Packing defects in monolayers, which allows for oxygen and water to reach the Si surface, lead to monolayer degradation over time. ,, Hence, there is a need to find a procedure to protect Si from oxidation while preserving the electron-transferring capability of the protecting layer.…”

Nanoscale Silicon Oxide Reduces Electron Transfer Kinetics of Surface-Bound Ferrocene Monolayers on Silicon

“…Fundamental and applied research on the self-assembly of organic molecules on electrode surfaces continues to be an area of intense research. To date, the focus has been on the formation of self-assembled monolayers (SAMs) on gold electrodes using thiol or disulfide-terminated molecules, − but there is increasing interest of forming monolayers on a range of other electrodes. − The study by Chidsey and co-workers on Si–C-bound monolayers has helped to expand this research from metals to semiconductors. − In particular, monolayers on oxide-free silicon (Si–H) have gained significant interest because they enable combining the traditional semiconducting properties of Si with those of functional organic molecules. ,− For example, Si-based SAMs have been exploited in solar cell research, − biosensors, − fundamental electron transfer studies, − and molecular electronics. − …”