A variety of synthetic methods are used to approach the preparation of materials with nanoscale features.  In several of the programs our teams in which are teams are involved, nanoparticles (metal, alloy, oxide, core-shell, etc.) are sought; these are used in bulk, as precursors to other materials, or in assembled arrays.   Other efforts are examining the production of structured materials for incorporation into technologically significant architectures.  Here building blocks such as various wire structures (simple wires, hollow, core-shell, superlattice, etc.), spheres, rings and disks are being sought.  Some of these components can be used as prepared, while others will be dispersed and then re-assembled into new organized structures.  The latter will be directed such that particular properties will be sought.

­­­­­­Summary of this page’s contents

Simple wires

Hierarchical structures

Core-shell wires

Coaxial wires

Superlattice wires

Topologically varied wires

Tubules (hollow wires)

Porous wires

Corrugated wires

Wires with structured tips

Wires with furrows

Other building blocks

Spheres

Rings

Disks

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  Simple wires

Template methods for growing wires in porous membranes were developed by Possin, Williams, and Martin several years ago [1].  (For more details on templates, go to porous membranes.)  Both chemical and electrochemical methods can be used to form these components.  Wires as small as 10 nm and as large as 1-micron diameter are easily accessible by this approach. The length of the wires can be varied from nanometers to 10’s of microns.

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Hierarchical structures

 We are working to fabricate hierarchical structures including core-shell, coaxial (core-shell-shell), and superlattice wires.   Such materials will prove important to a variety of applications (magnetics, magnetotransport, high frequency devices, etc.) as well as provide insight into fundamental properties relating to magnetism, transport and microwave materials.

Core-shell and coaxial (core-shell-shell) wires

Some researchers have reported core-shell wire containing an oxide layer and a metal core [2,3].  We have recently prepared similar wires containing metals and metal oxide components (gold with silica/titania, see below) and two metal components, e.g. gold-cobalt and gold-nickel (not shown).   

 We are currently working to expand these methods to make coaxial wires.  These wires will have electro- and/or magneto-active (EMA) layers, separated by an insulating layer, either a polymer or metal oxide.  The EMA layers will be composed of simple ferromagnetic metals (Fe, Co, Ni) or magnetic oxides such as CoO, NiO, CoFe2O4, etc.  Combinations of hard and soft magnetic materials will be especially intriguing.  The oxide layers are prepared by reported sol-gel techniques and the metal layers by electrodeposition.   

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 Superlattice wires

Methods are well established for the preparation of superlattice wires [4,5].  These structured materials can be easily grown electrochemically by simply alternating between two plating solutions or from a solution with two components where the one that is more readily reduced than the other is in relatively low concentration.  The image to the right shows our initial efforts in preparing superlattice wires. This wire is a superlattice of nickel and gold; the brigher regions are gold.

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 Topologically varied wires

 Here we are preparing hollow wires (tubules), porous wires, corrugated wires, wires with structured tips and wires with furrows along the circumference of the wire. 

Tubules

 A variety of methods are available for the preparation of tubules.  Such materials have been prepared as metals, oxides, polymers, etc [6-13].  The preparation of these structured materials relies mostly on chemical methodologies. We have developed a slightly different approach.  Here, we partially coat the surface of an alumina membrane with Pd/Au by sputtering.  After attaching a wire lead with silver paste to this surface, the metal side of the template is sealed with insulating glue (Scotch 3M Super strength).  In this step, the glue diffuses into the pores to produce “glue wires”.  Subsequently on deposition of metal into the modified porous membrane, the metal is directed along the pore wall to produce tubules.  The fabrication scheme (Fig. 4) outlined below highlights the steps in this process and the SEM’s  (Fig. 5) show the glue wires and nickel tubules, as well as glue wires protruding from tubules.  (A manuscript including this approach has been submitted for review [14].)

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Porous wires

 

Porous metal wires have been fabricated by electrodeposition of metal into porous polymer or alumina membranes that have been infiltrated with spheres.  The fabrication scheme for this process is presented below (Fig. 6).  After deposition, both spheres and the template are removed.  This work has been recently accepted for publication [15]. These SEM’s illustrate some of the wires formed by this approach.  Both 1-micron wires (Fig. 7) with 500 and 300 nm spheres and 300 nm wires with 140 nm wires were prepared.

Corrugated wires

Mechanical processing of metal-opal compositions can be used to produce corrugated wires.  After electrodeposition of metal into opal pieces, controlled polishing of the composite samples can lead to the formation of corrugated wires.  This zigzag motif consists of alternating polyhedra formed from the opal template.  The corrugated wires are a relatively minor component in the mechanical processing – thin mesh layers are more readily obtained [16]. 

Wires with structured tips

Wires with structured tips can be made from modified templates [14].  Initially, nickel tubules are formed as described above.  Then after oxidation of the tubules, this creates modified templates where metal can be deposited to produce wires with smaller diameter tips.

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Wires with furrows around their circumference

It is also possible to fabricate wires with furrows around the circumference of the wire.   Here tubules are prepared as described above except with alternating gold and nickel sections.  Subsequent growth of gold into the tubules followed by removal of the nickel component, produces wires with furrows around the wire.  The wires are about 300 nm in diameter and the furrows are about 100 nm deep [17].

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Other Building blocks

 

We have also developed methods for the fabrication of other building blocks including spheres, rings and disks.

Spheres

Opal templates can be used to create mesh arrays.  If this mesh is PMMA, then we can use this as a template to form sphere arrays [18].  Subsequent sonication of the sphere arrays results in clusters of spheres of individual isolated spheres.

Disks and Rings

Initial efforts have yielded some successes in production of disks and rings [20].  Figure 18 shows images of approximately 300 nm diameter rings. 

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For more information please contact: Professor John Wiley (e-mail: jwiley@uno.edu)