Spontaneous Grafting of OH-Terminated Molecules on Si−H Surfaces via Si–O–C Covalent Bonding

Advances in single-molecule junctions as tools for chemical and biochemical analysis

The development of miniaturized electronics has led to the design and construction of powerful experimental platforms capable of measuring electronic properties to the level of single molecules, along with new theoretical concepts to aid in the interpretation of the data. A new area of activity is now emerging concerned with repurposing the tools of molecular electronics for applications in chemical and biological analysis. Single-molecule junction techniques, such as the scanning tunnelling microscope break junction and related single-molecule circuit approaches have a remarkable capacity to transduce chemical information from individual molecules, sampled in real time, to electrical signals. In this Review, we discuss single-molecule junction approaches as emerging analytical tools for the chemical and biological sciences. We demonstrate how these analytical techniques are being extended to systems capable of probing chemical reaction mechanisms. We also examine how molecular junctions enable the detection of RNA, DNA, and traces of proteins in solution with limits of detection at the zeptomole level.

Voltammetric Measurement of Rates and Energetics for Surface Methoxylation of Si(100) in Methanol with Dissolved Electron Acceptors Using Si Ultramicroelectrodes

The steady-state voltammetric responses of n-type Si(100) semiconductor ultramicroelectrodes (SUMEs) immersed in air- and water-free methanolic electrolytes have been measured. The response characteristics of these SUMEs in the absence of illumination were modeled and understood through a framework that describes the distribution of the applied potential across the semiconductor/electrolyte contact using four discrete regions: the semiconductor space charge, surface, Helmholtz, and diffuse layers. The latter region was described by the full Gouy-Chapman model. This framework afforded insight on how relevant parameters such as the semiconductor band edge potentials, the reorganization energies for charge transfer, the standard potential of redox species in solution, the density and energy of surface state populations, and the presence of an insulating (tunneling) layer individually and collectively dictate the observable current-potential responses. With this information, the methoxylation of Si surfaces was evaluated by analysis of the change in voltammetric responses during the course of prolonged immersion in methanol. The electrochemical data were consistent with a surface methoxylation mechanism that depended on the standard potential of redox species dissolved in solution. Estimates of the enthalpies of adsorption as well as the potential-dependent rate constant for surface methoxylation were obtained. Collectively, these measurements supported the contention that the rates of Si surface reactions can be systematically tuned by exposure to dissolved outer-sphere electron acceptors. Moreover, the data represent the quantitative utility of voltammetry with SUMEs for the measurement of semiconductor/liquid contacts.

High Shear Thin Film Synthesis of Partially Oxidized Gallium and Indium Composite 2D Sheets

Reducing resistance in silicon-based devices is important as they get miniaturized further. 2D materials offer an opportunity to increase conductivity whilst reducing size. A scalable, environmentally benign method is developed for preparing partially oxidized gallium/indium sheets down to 10 nm thick from a eutectic melt of the two metals. Exfoliation of the planar/corrugated oxide skin of the melt is achieved using the vortex fluidic device with a variation in composition across the sheets determined using Auger spectroscopy. From an application perspective, the oxidized gallium indium sheets reduce the contact resistance between metals such as platinum and silicon (Si) as a semiconductor. Current-voltage measurements between a platinum atomic force microscopy tip and a Si-H substrate show that the current switches from being a rectifier to a highly conducting ohmic contact. These characteristics offer new opportunities for controlling Si surface properties at the nanoscale and enable the integration of new materials with Si platforms.

Electro-polymerization rates of diazonium salts are dependent on the crystal orientation of the surface

Electro-polymerization of diazonium salts is widely used for modifying surfaces with thin organic films. Initially this method was primarily applied to carbon, then to metals, and more recently to semiconducting Si. Unlike on other surfaces, electrochemical reduction of diazonium salts on Si, which is one of the most industrially dominant material, is not well understood. Here, we report the electrochemical reduction of diazonium salts on a range of silicon electrodes of different crystal orientations (111, 211, 311, 411, and 100). We show that the kinetics of surface reaction and the reduction potential is Si crystal-facet dependent and is more favorable in the hierarchical order (111) > (211) > (311) > (411) > (100), a finding that offers control over the surface chemistry of diazonium salts on Si. The dependence of the surface reaction kinetics on the crystal orientation was found to be directly related to differences in the potential of zero charge (PZC) of each crystal orientation, which in turn controls the adsorption of the diazonium cations prior to reduction. Another consequence of the effect of PZC on the adsorption of diazonium cations, is that molecules terminated by distal diazonium moieties form a compact film in less time and requires less reduction potentials compared to that formed from diazonium molecules terminated by only one diazo moiety. In addition, at higher concentrations of diazonium cations, the mechanism of electrochemical polymerization on the surface becomes PZC-controlled adsorption-dominated inner-sphere electron transfer while at lower concentrations, diffusion-based outer-sphere electron transfer dominates. These findings help understanding the electro-polymerization reaction of diazonium salts on Si en route towards an integrated molecular and Si electronics technology.

On-Surface Azide-Alkyne Cycloaddition Reaction: Does It Click with Ruthenium Catalysts?

Owing to its simplicity, selectivity, high yield, and the absence of byproducts, the "click" azide-alkyne reaction is widely used in many areas. The reaction is usually catalyzed by copper(I), which selectively produces the 1,4-disubstituted 1,2,3-triazole regioisomer. Ruthenium-based catalysts were later developed to selectively produce the opposite regioselectivity─the 1,5-disubstituted 1,2,3-triazole isomer. Ruthenium-based catalysis, however, remains only tested for click reactions in solution, and the suitability of ruthenium catalysts for surface-based click reactions remains unknown. Also unknown are the electrical properties of the 1,4- and 1,5-regioisomers, and to measure them, both isomers need to be assembled on the electrode surface. Here, we test whether ruthenium catalysts can be used to catalyze surface azide-alkyne reactions to produce 1,5-disubstituted 1,2,3-triazole, and compare their electrochemical properties, in terms of surface coverages and electron transfer kinetics, to those of the compound formed by copper catalysis, 1,4-disubstituted 1,2,3-triazole isomer. Results show that ruthenium(II) complexes catalyze the click reaction on surfaces yielding the 1,5-disubstituted isomer, but the rate of the reaction is remarkably slower than that of the copper-catalyzed reaction, and this is related to the size of the catalyst involved as an intermediate in the reaction. The electron transfer rate constant (ket) for the ruthenium-catalyzed reaction is 30% of that measured for the copper-catalyzed 1,4-isomer. The lower conductivity of the 1,5-isomer is confirmed by performing nonequilibrium Green's function computations on relevant model systems. These findings demonstrate the feasibility of ruthenium-based catalysis of surface click reactions and point toward an electrical method for detecting the isomers of click reactions.

Effect of Electric Fields on Silicon-Based Monolayers

Electric fields can induce bond breaking and bond forming, catalyze chemical reactions on surfaces, and change the structure of self-assembled monolayers on electrode surfaces. Here, we study the effect of electric fields supplied either by an electrochemical potential or by conducting atomic force microscopy (C-AFM) on Si-based monolayers. We report that typical monolayers on silicon undergo partial desorption followed by the oxidation of the underneath silicon at +1.5 V vs Ag/AgCl. The monolayer loses 28% of its surface coverage and 55% of its electron transfer rate constant (k_et) when +1.5 V electrochemical potential is applied on the Si surface for 10 min. Similarly, a bias voltage of +5 V applied by C-AFM induces complete desorption of the monolayer at specific sites accompanied by an average oxide growth of 2.6 nm when the duration of the bias applied is 8 min. Current-voltage plots progressively change from rectifying, typical of metal-semiconductor junctions, to insulating as the oxide grows. These results define the stability of Si-based organic monolayers toward electric fields and have implication in the design of silicon-based monolayers, molecular electronics devices, and on the interpretation of charge-transfer kinetics across them.

Electrochemical Detection of Dinitrobenzene on Silicon Electrodes: Toward Explosives Sensors

Detection of explosives is vital for protection and criminal investigations, and developing novel explosives’ sensors stands at the forefront of the analytical and forensic chemistry endeavors. Due to the presence of terminal nitro groups that can be electrochemically reduced, nitroaromatic compounds (NACs) have been an analytical target for explosives’ electrochemical sensors. Various electrode materials have been used to detect NACs in solution, including glassy carbon electrodes (GCE), platinum (Pt), and gold (Au) electrodes, by tracking the reversible oxidation/reduction properties of the NACs on these electrodes. Here, we show that the reduction of dinitrobenzene (DNB) on oxide-free silicon (Si–H) electrodes is irreversible with two reduction peaks that disappear within the successive voltammetric scanning. AFM imaging showed the formation of a polymeric film whose thickness scales up with the DNB concentration. This suggest that Si–H surfaces can serve as DNB sensors and possibly other explosive substances. Cyclic voltammetry (CV) measurements showed that the limit of detection (LoD) on Si–H is one order of magnitude lower than that obtained on GCE. In addition, EIS measurements showed that the LoD of DNB on Si–H is two orders of magnitude lower than the CV method. The fact that a Si–H surface can be used to track the presence of DNB makes it a suitable surface to be implemented as a sensing platform. To translate this concept into a sensor, however, it would require engineering and fabrication prospect to be compatible with the current semiconductor technologies.

Optimized Technologies for Cointegration of MOS Transistor and Glucose Oxidase Enzyme on a Si-Wafer

The biosensors that work with field effect transistors as transducers and enzymes as bio-receptors are called ENFET devices. In the actual paper, a traditional MOS-FET transistor is cointegrated with a glucose oxidase enzyme, offering a glucose biosensor. The manufacturing process of the proposed ENFET is optimized in the second iteration. Above the MOS gate oxide, the enzymatic bioreceptor as the glucose oxidase is entrapped onto the nano-structured TiO2 compound. This paper proposes multiple details for cointegration between MOS devices with enzymatic biosensors. The Ti conversion into a nanostructured layer occurs by anodization. Two cross-linkers are experimentally studied for a better enzyme immobilization. The final part of the paper combines experimental data with analytical models and extracts the calibration curve of this ENFET transistor, prescribing at the same time a design methodology.

Impermeable Graphene Oxide Protects Silicon from Oxidation

The presence of a natural silicon oxide (SiO_x) layer over the surface of silicon (Si) has been a roadblock for hybrid semiconductor and organic electronics technology. The presence of an insulating oxide layer is a limiting operational factor, which blocks charge transfer and therefore electrical signals for a range of applications. Etching the SiO_x layer by fluoride solutions leaves a reactive Si-H surface that is only stable for few hours before it starts reoxidizing under ambient conditions. Controlled passivation of silicon is also of key importance for improving Si photovoltaic efficiency. Here, we show that a thin layer of graphene oxide (GO_x) prevents Si surfaces from oxidation under ambient conditions for more than 30 days. In addition, we show that the protective GO_x layer can be modified with molecules enabling a functional surface that allows for further chemical conjugation or connections with upper electrodes, while preserving the underneath Si in a nonoxidized form. The GO_x layer can be switched electrochemically to reduced graphene oxide, allowing the development of a dynamic material for molecular electronics technologies. These findings demonstrate that 2D materials are alternatives to organic self-assembled monolayers that are typically used to protect and tune the properties of Si and open a realm of possibilities that combine Si and 2D materials technologies.