"We wanted to explore the possibility of using nanotubes themselves as pattern masks for growing organized carbon nanotube array patterns of different length scales," Anyuan Cao of Rensselaer Polytechnic Institute told nanotechweb.org. "We found that while nanotubes selectively grow on silica as expected, they do not grow on the previously deposited nanotube films."

Firstly, Cao and colleagues deposited a film of vertically aligned carbon nanotubes onto a silica substrate from a mixture of ferrocene and xylene. The ferrocene acted as a catalyst precursor while the xylene provided a source of carbon. The scientists grew the film for 10 minutes, producing vertically aligned carbon nanotubes of roughly 40 µm in length. Then they scratched off selected parts of the film using tweezers with a 100 µm tip to form a pattern of parallel and crossing tracks that were roughly 1-2 mm apart.

A second chemical-vapour deposition sequence, this time for about 20 minutes, grew carbon nanotubes on the areas of exposed silica where the first layer of carbon nanotubes had been stripped away. This produced "stripes" of vertically aligned carbon nanotubes of roughly 100 µm long. Finally, the researchers used tweezers to peel off the original layer of carbon nanotubes, which had become brittle during the second deposition step.

"Building micron-to-millimetre scale architectures of nanotubes with well-defined orientations and configurations is essential to harness the properties of nanotubes for applications such as microprocessors with molecular switches, microelectromechanical systems, reinforcements in composites, field emitters and porous membranes for molecular separation," said Cao. "This could also open up a new approach for constructing complex mesoscale nanotube structures, such as multilayers with different predetermined nanotube orientations and lengths in each layer."

Now the scientists are looking to obtain more control over the selection of nucleation site and nanotube orientation and they want to measure properties of the arrays, such as electrical response and mechanical integrity. They are also investigating the growth of organized hybrid architectures consisting of one-dimensional nanostructures of different materials.