Electrochemical and theoretical investigation on the behavior of the Co2+ ion in three eutectic solvents.

Deep eutectic solvents (DESs) have many advantages, making them a promising alternative in replacing ionic liquids and organic solvents. Besides, DESs have received much prominence due to their diverse applications: Electrodeposition of metals, organic synthesis, gas adsorption, and biodiesel production. Therefore, this work analyzed the effect of the temperature increase (298 K-353 K) on the behavior of the Co2+ ions in three eutectic solvents through electrochemical techniques and computational simulations. From the electrochemical analysis realized, the increase in temperature caused a reduction in specific mass and an increase in the diffusion coefficient. Besides, the activation energy values were of 15.3, 29.9, and 55.2 kJ mol-1 for 1ChCl:2 EG, 1ChCl:2U, and 1ChCl:2G, respectively. The computational simulations indicate that the increased temperature effect caused the replacement of HBD molecules by anions chloride around Co2+ ions for the SDW1 and SDW3 systems between the temperatures of 298 K-353 K, except for the SDW2 system that the replaced occurred in the interval of 313 K-353 K. Besides, the increase of temperature occasioned the increase of strength for Co-Cl interaction and weakened the interactions between the Co2+ ions with the oxygen of HBD molecules.

Electrochemical Scanning Tunneling Microscopy Study of Bismuth Chalcogenide Single Crystals.

We use electrochemical scanning tunneling microscopy (EC-STM) to image single-crystal surfaces of the layered bismuth chalcogenide Sn0.01Bi1.99Te2Se in situ under electrochemical control for the first time. The Bi chalcogenides are of interest for their thermoelectric properties and as model topological insulators (TIs). We show that oxidative dissolution takes place via the progressive nucleation of pits in the initially smooth surface terraces rather than at their edges. Nanometer-resolution EC-STM images show that the pit depth is generally equal to the thickness of a complete chalcogenide quintuple layer. The preferential redeposition of dissolved components at step and defect edges on application of a more negative potential after oxidation is observed. Our work demonstrates the ability to control and characterize the surface morphology of single-crystal TIs in an electrochemical environment.

Electrochemical Modification and Characterization of Topological Insulator Single Crystals.

We compare electrochemically modified or thiol-functionalized single-crystal samples of the topological insulator (TI) Bi2Te0.9Se2.1 to freshly cleaved/air-exposed control samples and use X-ray photoelectron spectroscopy (XPS) to investigate the extent of any surface oxidation. XPS spectra for a TI sample maintained at an appropriate potential for 2 h demonstrate the feasibility of protecting the TI surface from oxidation while working in an electrochemical environment. Deliberate electrochemical oxidation, in contrast, generates prominent Bi, Te, and Se peaks associated with oxidation. However, this change is reversible, as further XPS spectra following electrochemical reduction are similar to those measured for an in situ cleaved sample. XPS also shows that adsorption of pentanedithiol (PDT) protects the TI surface from oxidation. Cyclic voltammetry shows that PDT adsorption suppresses electrochemical oxidation and reduction, while electrochemical impedance spectroscopy shows that it increases the charge transfer resistance significantly. Our work demonstrates the ability to control and characterize the surface chemistry of single-crystal TIs in an electrochemical environment for the first time.

Charge transport at a molecular GaAs nanoscale junction.

In recent years, the use of non-metallic electrodes for the fabrication of single-molecule junctions has developed into an elegant way to impart new properties to nanodevices. Integration of molecular junctions in a semiconducting platform would also speed technological deployment, as it would take advantage of established industrial infrastructures. In a previous proof-of-concept paper, we used simple α,ω-dithiol self-assembled monolayers on a gallium arsenide (GaAs) substrate to fabricate molecular Schottky diodes with a STM. In the devices, we were also able to detect the contribution of a single-molecule to the overall charge transport. The prepared devices can also be used as photodiodes, as GaAs is a III-V direct bandgap (1.42 eV at room temperature) semiconductor, and it efficiently absorbs visible light to generate a photocurrent. In this contribution, we demonstrate that fine control can be exerted on the electrical behaviour of a metal-molecule-GaAs junction by systematically altering the nature of the molecular bridge, the type and doping density of the semiconductor and the light intensity and wavelength. Molecular orbital energy alignment dominates the charge transport properties, resulting in strongly rectifying junctions prepared with saturated bridges (e.g. alkanedithiols), with increasingly ohmic characteristics as the degree of saturation is reduced through the introduction of conjugated moieties. The effects we observed are local, and may be observed with electrodes of only a few tens of nanometres in size, hence paving the way to the use of semiconducting nanoelectrodes to probe molecular properties. Perspectives of these new developments for single molecule semiconductor electrochemistry are also discussed.

Dual Control of Molecular Conductance through pH and Potential in Single-Molecule Devices.

One of the principal aims of single-molecule electronics is to create practical devices out of individual molecules. Such devices are expected to play a particularly important role as novel sensors thanks to their response to wide ranging external stimuli. Here we show that the conductance of a molecular junction can depend on two independent stimuli simultaneously. Using a scanning tunnelling microscope break-junction technique (STM-BJ), we found that the conductance of 4,4'-vinylenedipyridine (44VDP) molecular junctions with Ni contacts depends on both the electrochemically applied gate voltage and the pH of the environment. Hence, not only can the Ni|44VDP|Ni junction function as a pH-sensitive switch, but the value of the pH at which switching takes place can be tuned electrically. Furthermore, through the simultaneous control of pH and potential the STM-BJ technique delivers unique insight into the acid-base reaction, including the observation of discrete proton transfers to and from a single molecule.

Single-Molecule Photocurrent at a Metal-Molecule-Semiconductor Junction.

We demonstrate here a new concept for a metal-molecule-semiconductor nanodevice employing Au and GaAs contacts that acts as a photodiode. Current-voltage traces for such junctions are recorded using a STM, and the "blinking" or "I(t)" method is used to record electrical behavior at the single-molecule level in the dark and under illumination, with both low and highly doped GaAs samples and with two different types of molecular bridge: nonconjugated pentanedithiol and the more conjugated 1,4-phenylene(dimethanethiol). Junctions with highly doped GaAs show poor rectification in the dark and a low photocurrent, while junctions with low doped GaAs show particularly high rectification ratios in the dark (>103 for a 1.5 V bias potential) and a high photocurrent in reverse bias. In low doped GaAs, the greater thickness of the depletion layer not only reduces the reverse bias leakage current, but also increases the volume that contributes to the photocurrent, an effect amplified by the point contact geometry of the junction. Furthermore, since photogenerated holes tunnel to the metal electrode assisted by the HOMO of the molecular bridge, the choice of the latter has a strong influence on both the steady state and transient metal-molecule-semiconductor photodiode response. The control of junction current via photogenerated charge carriers adds new functionality to single-molecule nanodevices.

Single-Molecule Transport at a Rectifying GaAs Contact.

In most single- or few-molecule devices, the contact electrodes are simple ohmic resistors. Here we describe a new type of single-molecule device in which metal and semiconductor contact electrodes impart a function, namely, current rectification, which is then modified by a molecule bridging the gap. We study junctions with the structure Au STM tip/X/n-GaAs substrate, where "X" is either a simple alkanedithiol or a conjugated unit bearing thiol/methylthiol contacts, and we detect current jumps corresponding to the attachment and detachment of single molecules. From the magnitudes of the current jumps we can deduce values for the conductance decay constant with molecule length that agree well with values determined from Au/molecule/Au junctions. The ability to impart functionality to a single-molecule device through the properties of the contacts as well as through the properties of the molecule represents a significant extension of the single-molecule electronics "tool-box".

The single-molecule electrical conductance of a rotaxane-hexayne supramolecular assembly.

Oligoynes are archetypical molecular wires due to their 1-D chain of conjugated carbon atoms and ability to transmit charge over long distances by coherent tunneling. However, the stability of the oligoyne can be an issue. Here we address this problem by two stabilization methods, namely sterically shielding endgroups, and rotaxination to produce an insulated molecular wire. We demonstrate the threading of a hexayne within a macrocycle to form a rotaxane and report measurements of the electrical conductance of this single supramolecular assembly within an STM break junction. The macrocycle is retained around the hexayne through the use of 3,5-diphenylpyridine stoppers at both ends of the molecular wire, which also serve as chemisorption contacts to the gold electrodes of the junction. Molecular conductance was measured for both the supramolecular assembly and also for the molecular wire in the absence of the macrocycle. The threaded macrocycle, which at room temperature is mobile along the length of the hexayne between the stoppers, has only a minimal impact on the conductance. However, the probability of molecular junction formation in a given break junction formation cycle is notably lower with the rotaxane. In seeking to understand the conductance behavior, the electronic properties of these molecular assemblies and the electrical behavior of the junctions have been investigated by using DFT-based computational methods.

Synthesis of Cationized Magnetoferritin for Ultra-fast Magnetization of Cells.

Many important biomedical applications, such as cell imaging and remote manipulation, can be achieved by labeling cells with superparamagnetic iron oxide nanoparticles (SPIONs). Achieving sufficient cellular uptake of SPIONs is a challenge that has traditionally been met by exposing cells to elevated concentrations of SPIONs or by prolonging exposure times (up to 72 hr). However, these strategies are likely to mediate toxicity. Here, we present the synthesis of the protein-based SPION magnetoferritin as well as a facile surface functionalization protocol that enables rapid cell magnetization using low exposure concentrations. The SPION core of magnetoferritin consists of cobalt-doped iron oxide with an average particle diameter of 8.2 nm mineralized inside the cavity of horse spleen apo-ferritin. Chemical cationization of magnetoferritin produced a novel, highly membrane-active SPION that magnetized human mesenchymal stem cells (hMSCs) using incubation times as short as one minute and iron concentrations as lows as 0.2 mM.

Giant single-molecule anisotropic magnetoresistance at room temperature.

We report an electrochemically assisted jump-to-contact scanning tunneling microscopy (STM) break junction approach to create reproducible and well-defined single-molecule spintronic junctions. The STM break junction is equipped with an external magnetic field either parallel or perpendicular to the electron transport direction. The conductance of Fe-terephthalic acid (TPA)-Fe single-molecule junctions is measured and a giant single-molecule tunneling anisotropic magnetoresistance (T-AMR) up to 53% is observed at room temperature. Theoretical calculations based on first-principles quantum simulations show that the observed AMR of Fe-TPA-Fe junctions originates from electronic coupling at the TPA-Fe interfaces modified by the magnetic orientation of the Fe electrodes with respect to the direction of current flow. The present study highlights new opportunities for obtaining detailed understanding of mechanisms of charge and spin transport in molecular junctions and the role of interfaces in determining the MR of single-molecule junctions.

Single-molecule electrochemical transistor utilizing a nickel-pyridyl spinterface.

Using a scanning tunnelling microscope break-junction technique, we produce 4,4'-bipyridine (44BP) single-molecule junctions with Ni and Au contacts. Electrochemical control is used to prevent Ni oxidation and to modulate the conductance of the devices via nonredox gating--the first time this has been shown using non-Au contacts. Remarkably the conductance and gain of the resulting Ni-44BP-Ni electrochemical transistors is significantly higher than analogous Au-based devices. Ab-initio calculations reveal that this behavior arises because charge transport is mediated by spin-polarized Ni d-electrons, which hybridize strongly with molecular orbitals to form a "spinterface". Our results highlight the important role of the contact material for single-molecule devices and show that it can be varied to provide control of charge and spin transport.

Ionic liquid based approach for single-molecule electronics with cobalt contacts.

An electrochemical method is presented for fabricating cobalt thin films for single-molecule electrical transport measurements. These films are electroplated in an aqueous electrolyte, but the crucial stages of electrochemical reduction to remove surface oxide and adsorption of alkane(di)thiol target molecules under electrochemical control to form self-assembled monolayers which protect the oxide-free cobalt surface are carried out in an ionic liquid. This approach yields monolayers on Co that are of comparable quality to those formed on Au by standard self-assembly protocols, as assessed by electrochemical methods and surface infrared spectroscopy. Using an adapted scanning tunneling microscopy (STM) method, we have determined the single-molecule conductance of cobalt/1,8-octanedithiol/cobalt junctions by employing a monolayer on cobalt and a cobalt STM tip in an ionic liquid environment and have compared the results with those of experiments using gold electrodes as a control. These cobalt substrates could therefore have future application in organic spintronic devices such as magnetic tunnel junctions.

Fe3O4 nanoparticles: protein-mediated crystalline magnetic superstructures.

The synthesis of magnetic, monodisperse nanoparticles has attracted great interest in nanoelectronics and nanomedicine. Here we report the fabrication of pure magnetite nanoparticles, less than ten nanometers in size, using the cage-shaped protein apoferritin (Fe(3)O(4)-ferritin). Crystallizable proteins were obtained through careful successive separation methods, including a magnetic chromatography that enabled the effective separation of proteins, including a Fe(3)O(4) nanoparticle (7.9 ± 0.8 nm), from empty ones. Macroscopic protein crystals allowed the fabrication of three-dimensional arrays of Fe(3)O(4) nanoparticles with interparticle gaps controlled by dehydration, decreasing their magnetic susceptibilities and increasing their blocking temperatures through enhanced dipole-dipole interactions.

Surface functionalization of electro-deposited nickel.

A new in situ electrochemical method of functionalizing an oxide-free Ni surface is demonstrated using octanethiol. Initial adsorption results in a multilayer molecular film, which blocks both the hydrogen evolution reaction (HER) and re-oxidation of the Ni by ambient oxygen. However, excess octanethiol can be removed by rinsing with ethanol, leaving behind a monolayer that continues to protect against re-oxidation but gives rise to an unexpected enhancement in the HER, with a greater enhancement for longer film formation times. The presence of an octanethiol monolayer on the surface was confirmed by spectroscopic observation of the CH(2), CH(3) and thiolate groups using infra red spectroscopy, while X-ray photo-electron spectroscopy demonstrated the effectiveness of the thiol layer as a barrier to surface oxidation. The electrochemically prepared octanethiol film impedes oxidation of the Ni in air more effectively than a film formed by immersion in a solution of octanethiol in ethanol.