"Carbon nanotubes in aligned forms are interesting for various scientific and practical applications ranging from electronics to biological devices," Hongjie Dai of Stanford told nanotechweb.org. "While several CVD methods have been developed to grow vertically aligned multi-walled carbon nanotubes, growth of vertical single-walled carbon nanotubes is still in an early stage and has been difficult to reproduce."

Dai and colleagues added roughly 1% molecular oxygen to methane, hydrogen and argon during PECVD using an inductively coupled radio-frequency plasma source. The process grew densely-packed single-walled carbon nanotubes over 4 inch wafers of SiO2/Si coated with iron clusters about 1.3 nm in diameter. The iron clusters acted as a catalyst for the growth of the tubes.

"We uncovered that the key role played by oxygen in the high-yield single-walled nanotube growth is to remove reactive hydrogen radicals, and to provide a carbon-rich and hydrogen-deficient condition to favour the formation of sp2-like graphitic single-walled nanotube structures," said Dai. "Reactive hydrogen species are unfavourable to single-walled nanotube formation and growth and can etch pre-formed single-walled nanotubes, especially for small-diameter tubes."

The team also developed a lift-off technique so that they could create a coating of vertically aligned nanotubes on "any desirable substrate", including metals, plastics and glasses. After deposition, the researchers lifted off the array of vertical nanotubes by etching away the SiO2 layer with hydrofluoric acid. Free-floating the array on a water surface enabled its transfer to another surface coated with a 50 nm layer of PMMA for adhesion. Finally, a heat treatment at 190 °C melted the polymer layer and "glued" the new substrate to the ends of the nanotubes.

Now Dai and colleagues say they plan to optimize the packing density of vertical single-walled nanotubes and pursue their applications.

The researchers reported their work in PNAS.