Aligned nanowires: carbon nanotubes grow along atomic steps on sapphire surfaces

17-Nov-2004

Defined structures of nanowires on a wafer consisting of a whisper-thin disc of semiconducting material are excellent candidates for the production of nanoscopic electrical circuits. A team at the Weizmann Institute of Science in Israel has now developed a new technique for forcing wires of carbon nanotubes into a parallel orientation: the nanotubes grow along atomic "stair steps" on a sapphire surface.

The team headed by Ernesto Joselevich was initially researching in a different direction: they were trying to give carbon nanotubes a preferred orientation on a wafer by applying an electrical field as the tubes were being formed. This works very well with silicon dioxide wafers. On a sapphire support (sapphire is a form of aluminum oxide), however, it didn't work: the nanotubes were beautifully arranged in parallel, but with an orientation that was completely independent of the electrical field-even when no field was applied at all!

Closer examination of the sapphire surface solved the mystery: commercial sapphire wafers are generally not cut exactly along the plane of the crystal. Their surface is thus not completely smooth; instead, it has parallel steps-of atomic dimensions-between the different planes of the crystal. The nanotubes end up lying along these steps. The researchers explain it like this: the nanotubes form from a catalyst of nanoscopic iron particles. It is clear that these iron particles don't like "climbing stairs"; instead, they "glide" along the inner edge of the step, like on a track. They thus remain continuously in contact with two surfaces, rather than just one, which seems to stabilize the catalyst. Just as an airplane leaves behind a condensation trail, the iron particles leave the newly formed nanotubes lying along their "tracks". The nanotubes even follow bends in the steps which are caused by defects in the crystal. This results in both straight and zigzag-shaped tubes, which are rumored to have particularly interesting electronic properties.

"The orientation and form of the atomic steps on a crystal surface can be controlled by the cutting process, and defects can be created artificially," says Joselevich. "It should thus be possible to produce different nanowire arrangements in a controlled fashion."

https://www.chemeurope.com/en/news/40894/aligned-nanowires-carbon-nanotubes-grow-along-atomic-steps-on-sapphire-surfaces.html