“This is an exciting new method for the manipulation of nanowires because it is so versatile,” Wendy Crone of the University of Wisconsin-Madison told nanotechweb.org. “The options are quite broad for the material deposited between the nickel.”
To make the nanowires, Crone and colleagues electrodeposited a 3 micron thick layer of nickel into an alumina membrane containing pores 200 nm in diameter. Then they electrodeposited a 7 micron layer of bronze alloy from a mixture of copper and tin plating solutions, before laying down a final 3 micron coating of nickel. Once the alumina template had been dissolved away, the result was 200 nm diameter bronze nanowires capped with 3 micron long nickel endcaps. In theory, the technique should be suitable for any template-synthesized nanowire system that contains conductive material.
Applying a rotating magnetic field to a suspension of the Ni-CuSn-Ni nanowires in ethylene glycol caused the nanowires to spin around. In contrast, CuSn nanowires without nickel caps did not respond to a magnetic field.
The researchers also used a magnetic field to position Ni-CuSn-Ni nanowires onto nickel stripes on a silicon substrate. The 100 nm thick nickel stripes were 20 microns wide and spaced between 10 and 40 microns apart. The scientists placed the substrate in a magnetic field oriented perpendicular to the long axis of the stripes. They added a suspension of nanowires in an acid surfactant solution, and as the water evaporated the nanowires oriented themselves horizontally across the stripes. Around 66% of the nanowires aligned within 10 degrees of the horizontal: almost all of these were in contact with a nickel stripe. For comparison, only 8% of CuSn nanowires without endcaps aligned horizontally in a similar experiment.
“We developed this method for manipulating and positioning nanowires because we were interested in performing mechanical testing on individual nanowires and needed to position our samples accurately,” said Crone. “Now that we can position nanowires over a trench, we can use a scanning probe technique to image the wires. What’s more, we can see how a wire bends downward when we push on its centre using the scanning probe tip. This will provide information about the mechanical response of the nanowires.”
The technique could also have applications in the assembly of nanoscale devices.
The scientists reported their work in Nano Letters.