The technique could ultimately find a use in creating "vertically oriented" nanoelectronic devices.

"Verticality gives you the ability to fit more things into the same area, so you can add more and more layers while keeping the footprint the same size or smaller," said Timothy Fisher of Purdue University. "But before we can even think about using nanotubes in electronics, we have to learn how to put them where we want them."

To create the template, Fisher and colleagues prepared a film structure consisting of a silicon substrate coated with 50 nm of SiOx, 150 nm of titanium, 100–200 nm of aluminium, 0.5–20 nm of iron, and a final aluminium layer up to 1 µm thick. The titanium was present to act as an electrically conductive layer for attaching to electrical contacts at a later date.

The team used a double-anodization procedure to anodize the layers of aluminium and iron, creating vertical pores through the layers with a diameter of 20–50 nm. The resulting pores contained a layer of iron catalyst embedded in their walls.

Templates used to make multiwalled carbon nanotubes have generally either contained catalyst electrodeposited into the base of the pores or no catalyst at all.

The team grew nanotubes onto the template in a microwave plasma-enhanced chemical vapour deposition system. The result was vertically oriented arrays of a mixture of single-walled and double-walled carbon nanotubes.

Around 10% of the pores generated carbon nanotubes. The nanotubes were several microns long and had an average diameter of around 2 nm. The tubes generally kept a vertical orientation or formed vertically oriented loops on emergence from the pores, although in some cases they took up a horizontal configuration. Some of the nanotubes also bunched together.

"You get a single nanotube in each pore and that's important because we can start to think about controlling how and where to put nanotubes to vertically integrate them for future electronic devices and sensing technologies," said Sands.

Installing the catalyst layer further from the top of the template resulted in a lower density of carbon nanotubes. The thickness of the catalyst, however, did not appear to have a strong effect on nanotube density.

The researchers believe that further processing and optimization of the structure, as well as functionalization of the nanotubes within the vertical pores, should enable densely packed carbon nanotube-based electronic and sensing devices.

The researchers reported their work in Nanotechnology.