To create the arrays, Stephan Stranick and colleagues deposited gold nanoparticles onto an a-plane sapphire substrate. These acted as catalysts for the growth of zinc oxide nanowires from a mixture of zinc oxide and graphite in argon at 900°C. The wires grew on the plane of the substrate in just one direction - the result was arrays of aligned nanowires.

Zinc oxide is a wide-bandgap semiconductor with potential applications in blue-light sources, transparent displays, sensors and solar cells.

According to the researchers, other techniques for aligning nanowires involve growing the wires vertically on a substrate, cutting them off, mixing them in a solvent and forcing them to align by using systems such as fluid flow or an electric field. The NIST method grows aligned nanowires directly.

The team deposited the gold nanoparticles in two ways: from a colloidal solution of nanoparticles; and by patterning the substrate with thin gold lines that formed nanodroplets at the deposition temperature.

For the nanowires to grow horizontally, Stranick and colleagues found that the nanoparticles must have diameters smaller than about 20 nm and be separated by a minimum distance of 100-150 nm.

Nanowires formed from thin gold lines had an average width of 12 nm. For gold lines thicker than about 10 nm, the nanowires began to grow vertically because of the increase in nanodroplet size. The team found they could control the width of the horizontal nanowires by decreasing the line thickness below 10 nm, but that the range of control was limited because the nanodroplets had a minimum radius.

By using a colloidal solution of a nanoparticles, the scientists were able to achieve more control, growing nanowires with diameters between 1 and 20 nm.

The researchers, who reported their work in Applied Physics Letters, believe the technique may also suit the growth of other semiconductor nanowires.