Wires that are 10 or 20 nm in diameter turn out to be fragile and it has been difficult to get unbroken uniformity over any useful length. But in a recent issue of the journal Science, researchers Michael Zach, Kwok Ng and Reginald Penner reported success using the regular lattice structure of graphite as a template for growing molybdenum wires.

A block of graphite was cleaved at an angle to create a series of uniform steps defined by the regular arrangement of atoms in the crystal lattice. The edge of the steps becomes the site where molybdenum oxide crystals begin to grow from a solution via electrodeposition.

The oxide forms a connected wire along the graphite step and the diameter can be closely controlled by the deposition time. The oxide wires are not conducting, however, and they are also very fragile.

So the crucial step in the process was to transform the oxide into a pure metal using chemical reduction in a hydrogen atmosphere. Once that step was completed, the researchers were able to lift the wires off of the substrate by coating the graphite with polystyrene. The polystyrene film proved to be an ideal electrically insulating substrate for probing the conductivity of the wires, the team said.

One end of the molybdenum wires was coated with a silver paint to establish a contact. Individual wires were then contacted with a gold-coated glass probe. The researchers were able to test wires down to 300 nm in diameter with this apparatus. The data showed a straight-line I-V curve indicating typical ohmic conduction.

The process was easily controlled by timing the electrodeposition step. The smallest wires, 15-nm, were created by a 1-second deposition. By running the deposition longer, Zach, Ng and Penner found they could produce wires up to a micron in diameter. The control of the wire diameter seemed to be quite good over that large a range.

The researchers were also able to lift the wires off the polystyrene film to test their mechanical strength. Using the glass probe, they were able to bend 300-nm wires at 90 degrees without breaking them.

More typical metals such as silver, copper and platinum were also tried but failed to produce good wires, according to the team. Other conductors such as cadmium and a nickel molybdenum alloy produced good results, the team said. The difference seems to be the speed of electrodeposition. For the more common metals, the speed of this process seems to lead to nonuniform wire formation, while cadmium and molybdenum react more slowly.