Bimetallic Junctions Research Articles

Electrically Controlled Bimetallic Junctions for Atomic-Scale Electronics.

Forming atomic-scale contacts with attractive geometries and material compositions is a long-term goal of nanotechnology. Here, we show that a rich family of bimetallic atomic-contacts can be fabricated in break-junction setups. The structure and material composition of these contacts can be controlled by atomically precise electromigration, where the metal types of the electron-injecting and sink electrodes determine the type of atoms added to, or subtracted from, the contact structure. The formed bimetallic structures include, for example, platinum and aluminum electrodes bridged by an atomic chain composed of platinum and aluminum atoms as well as iron-nickel single-atom contacts that act as a spin-valve break junction without the need for sophisticated spin-valve geometries. The versatile nature of atomic contacts in bimetallic junctions and the ability to control their structure by electromigration can be used to expand the structural variety of atomic and molecular junctions and their span of properties.

Levelling the playing field: screening for synergistic effects in coalesced bimetallic nanoparticles.

Depending on the synthetic methods, bimetallic nanoparticles can have either core-shell, phase segregated, alloy, or partially coalesced structures, presenting different degrees of atomic mixing on their surface. Along with the variations of size and morphology, the structural differences make it difficult to compare the catalytic activity of bimetallic nanoparticles. In this article, we developed a facile screening method that can focus on the synergistic effects rather than structural differences. Prefabricated nanoparticles are mixed together to form linear aggregates and coalesced to form bimetallic junctions. Their hollow silica shells allow materials transport but prevent further aggregation. With a level playing field, this screening platform can identify the best bimetallic combination for a catalytic reaction, before optimizing the synthesis. This approach is more advantageous than the conventional approaches where structural difference may have dominant effects on the catalytic performance.

Fabrication of segmented Au/Co/Au nanowires: insights in the quality of Co/Au junctions.

Electrodeposition is a versatile method, which enables the fabrication of a variety of wire-like nanoarchitectures such as nanowires, nanorods, and nanotubes. By means of template-assisted electrodeposition, segmented Au/Co/Au nanowires are grown in anodic aluminum oxide templates from two different electrolytes. To tailor the properties of the cobalt segments, several electrochemical conditions are studied as a function of current density, pulse deposition, and pH. The morphology, crystal structure, and magnetic properties are accordingly investigated. Changes in the deposition conditions affect the cobalt electrocrystallization process directly. Cobalt tends to crystallize mainly in the hexagonal close-packed structure, which is the reason cobalt might not accommodate satisfactorily on the face-centered cubic Au surface or vice versa. We demonstrate that by modifying the electrolyte and the applied current densities, changes in the texture and the crystalline structure of cobalt lead to a good quality connection between dissimilar segments. In particular, lowering the bath pH, or using pulse plating at a high overpotential, produces polycrystalline fcc Co and thus well-connected Co/Au bimetallic junctions with smooth interface. These are crucial factors to be carefully considered taking into account that nanowires are potential building blocks in micro- and nanoelectromechanical systems.

Control of Catalytically Generated Electroosmotic Fluid Flow through Surface Zeta Potential Engineering

The catalytic decomposition of hydrogen peroxide at bimetallic junctions has been shown to produce a local electric field capable of driving both electroosmotic fluid flow and electrophoretic forces on charged particles. The direction of the electroosmotic fluid flow depends on the metal surface zeta potential. We demonstrate that the direction of fluid flow can be controlled by the use of self-assembled monolayers to engineer the surface charge. In particular, silver disks were patterned on a gold-coated silicon surface. In the presence of hydrogen peroxide, gold behaves as an anode generating protons, and silver behaves as a cathode consuming protons, thus forming a local electric field extending from the gold surface to the silver surface. This electric field distribution results in an outward (away from the silver disk) electrophoretic force on a negative tracer and an inward force on a positive tracer. By selectively functionalizing the gold surface with either negatively charged carboxylic-acid-functionalized monolayers or positively charged amine-functionalized monolayers, we demonstrate the ability to align the direction of the electroosmotic fluid flow with the direction of the electrophoretic force on a given charged particle. A method for photolithographically patterning the monolayers on the gold surface while protecting the silver surface with an inert polymer layer has also been developed. This ability to engineer the direction of local catalytically produced forces provides the possibility of creating numerous fluidic and sorting devices.

Computation of elastic fields in bimetals and metals with coatings under heating conditions

We analyze the functioning of bimetallic junctions consisting of different materials in a temperature field. The computational formulas obtained make it possible to choose optimal combinations and thicknesses of materials, predict the failure properties of junctions, and give recommendations for suitable treatment of the surface.

A microscopic model of metal–semiconductor contacts

A one-dimensional local-density model of the charge density, electrostatic potential, and energetics of metal–semiconductor contacts is constructed. This model is an extension of analogous models of bimetallic junctions to include the gap in the semiconductor excitation spectrum, space charge effects in the semiconductor, and fabrication-induced charges near the interface. Self-consistent analysis of the valence-electron charge redistribution at the model metal–semiconductor interface relative to the corresponding vacuum surfaces reveals a cancellation which makes the height of the rectifying Schottky barrier directly proportional to the observed vacuum work function of the metal in the absence of additional charges induced by atomic relaxations or chemical reactions which occur during the fabrication of the interface. The experimentally observed stability of the Schottky barrier heights against changes is applied bias and semiconductor doping are predicted correctly. The occurrence of interfacial atomic rearrangements and/or chemical reactions is incorporated into the model via the inclusion of charge centers near the interface. Four types of centers are considered: donors, acceptors, and three-state centers with both positive (U>0) and negative (U<0) effective electron–electron interactions. Applications of the model to describe measured barrier heights on GaSe(0001) and GaAs(110) require approximately 1014 negative-U centers per cm2 except in the case of noble metals and Sn on GaSe(0001).

The nature of interface plasmon modes at bimetallic junctions

A simple theoretical analysis of the properties of plasmon modes at a bimetallic interface reveals that the observed excitation known as an 'interface plasmon' is not a pure mode but arises from excitations of plasmons in a narrow band of states. The interface is responsible for the presence of this band but the states within the band are not spatially localised in the interface region.

Phase transformation in bismuth under shock loading

The behavior of bismuth under shock loading was studied by means of a shock-detection technique based on the electrical response of bimetallic junctions and on the polarization effect of thin dielectric sheets under shock. The experiments were performed in the pressure range 50–180 kbar. A new shock-induced phase transformation was found to take place at about 70 kbar. A rough estimate of the temperature increase associated with shock loading, and comparison with data of static compression experiments suggest that this new shock-induced phase change in bismuth corresponds to the Bi III-V transformation, previously observed only under static pressure loading.

The Bioelectric Field Pattern in the Salamander and Its Simulation by an Electronic Analog

The dc field potential has been determined on the surface of the intact salamander, Trituris viridescens. A complex field was found which did not correspond to a simple dipole. The field was found to vary in a dynamic fashion with changes in the level of anesthesia. The spatial organization of the field correlated well with the anatomical organization of the central nervous system of the animal, in that areas of nerve cell aggregations within the central nervous system appeared to serve as both sources and sinks of the potential, with transmission of the steady potential along major nerve trunks. An analog model of the central nervous system was constructed utilizing several bimetallic junctions and a conducting network. This model produced a field in a modified electrolytic tank which corresponded well with the field found in the intact animal. It was concluded that the dc bioelectric potential in organisms may be generated within and conducted by portions of the central nervous system. The possibility that this is a means of data transmission and/or control (in addition to the usual action potential system) is being evaluated.

Protection of Bimetallic Junctions

Metals and other materials require a satisfactory life or service under practical conditions and to ensure this a knowledge of the developments in the field of anti‐corrosion is necessary. The assessment of the suitability of materials to be used in a particular structure presents corrosion as a serious and related problem.