Bimetallic nanowires that are compositionally modulated along the axis of the nanowire can form the basis for nanowire-based devices including diodes, spin valves and optical labels.

In this paper, we describe a method for preparing long (> 100 µm) bimetallic nanowires that are compositionally modulated along the axis of the nanowire. Essentially, the new method described here involves " wiring " together particles of one metal using nanowires of a second. This is accomplished by combining two new electrodeposition methods that we have recently described: " Slow growth " for preparing metal nanoparticles that are narrowly dispersed in diameter, and nanowire growth. Using this approach, beaded bimetallic nanowires that are up to a millimeter in length can be prepared in parallel arrays on a graphite surface. These wires can then be transferred onto a second, insulating surface (e.g., glass) using a method we have previously described. The procedure for preparing beaded, bimetallic nanowires is summarized in Scheme 1.

Scheme 1. Schematic diagram illustrating the preparation of beaded, bimetallic nanowires using electrochemical step edge decoration. The steps involved 1) Nanoparticles of Metal 1 are electrodeposited from an aqueous plating solution using the " slow growth " method described previously. The nucleation of these particles occurs preferentially at step edges present on the graphite surface, as shown. 2) The surface is rinsed with water, and exposed to a solution containing an n-alkanethiol (heavy lines). A monolayer of the thiol self-assembles on the surface of the particles of Metal 1. 3) The surface is rinsed with ethanol, immersed in a second plating solution and nanowires of Metal 2 are electrodeposited as previously described. During this process, the presence of the n-alkanethiol monolayer prevents electrodeposition onto the particles of Metal 1. 4) Finally, if necessary the n-alkanethiol monolayer is thermally removed from Metal 1 by heating the surface in hydrogen for an hour at 500 ^oC.

Figure 1. SEMs of a bunch of different beaded nanowire systems prepared using the basic scheme described in this paper.

Figure 2. Scanning electron micrographs (a,c) and corresponding EDX elemental maps (b,d) for copper/silver bimetallic nanowires. a,b. Bimetallic nanowires with silver particles and copper connecting segments after synthesis on a graphite surface. c,d. The same nanowires after embedding in epoxy and transfer to a glass surface. The surface of this sample was sputter coated with several nanometers of gold-palladium to eliminate charging effects. Collectively, these two elemental maps which show the same nanowires from opposite sides proves that the silver nanoparticles are disposed in series with the copper nanowires; the copper nanowires do not " shunt " these silver particles.