“One of the key problems for electronics is in getting high-quality (defect-free) carbon nanotubes that are also ‘clean’ (i.e. amorphous carbon/particle by-product free), as undesirable by-products from the growth process can cause short circuits,” said Rodrigo Lacerda of the University of Cambridge. “Traditional high-temperature methods like arc discharge or laser ablation do indeed produce high-quality nanotubes but have 30-60% carbon by-products or particulates, which need to be filtered off.”

Lacerda and colleagues used chemical vapour deposition to grow the nanotubes onto an oxidized silicon substrate coated with a triple-layer thin-film catalyst of aluminium, iron and molybdenum. They heated the samples to 1000 ° C in helium in a quartz-tube vacuum furnace, before evacuating the chamber and introducing a burst of acetylene for about 5 seconds. Then they refilled the chamber with helium and cooled it to room temperature.

The resulting nanotubes were around 5 to 10 microns long, with a diameter of approximately 1.3 nm. This corresponds to a deposition rate of about 1-2 microns per second. Auger electron spectroscopy showed that amorphous carbon was not present on the surface of the tubes, but nanotubes grown with acetylene present for 10 minutes had a surface layer of amorphous carbon roughly 30 nm thick.

“We needed to use high temperatures to achieve highly crystalline nanotubes, but this also meant significant amounts of amorphous carbon were deposited, which formed an undesirable conductive layer on our substrates together with the nanotubes,” said Lacerda. “So our strategy was to use rapid growth. This ensures clean deposition because catalytic growth of nanotubes occurs faster than the creation of unwanted by-products.”

The researchers reported their work in Applied Physics Letters.