Researchers at Stanford University, US, have found that a plasma-enhanced chemical vapour deposition (PECVD) technique can produce high-quality single-walled carbon nanotubes at low growth temperatures - around 600°C. What's more, in an unexpected bonus, almost 90% of the resulting nanotubes were semiconducting.

"The preferential growth of semiconducting nanotubes will enable chemical and biological nanosensors with higher sensitivities and reliability, and a higher yield of nanotube-based field effect transistors for basic research or hybrid electronics with MOS technology," Hongjie Dai told nanotechweb.org.

The researchers used a radiofrequency 4 inch PECVD system, growing nanotubes onto SiO2/Si wafers or holey-SiO2 films. Iron, either in the form of ferritin particles containing an average of roughly 300 iron atoms, or in the form of 1 Å thick films, acted as a catalyst for the process. The scientists heated the substrates to 600°C in argon, before introducing methane and turning on the 75 W plasma source for three minutes. The resulting nanotubes had a mean diameter of 1.2 nm.

"The initial motivation for pursuing PECVD synthesis of single-walled carbon nanotubes was to lower the minimum growth temperature for high-quality nanotubes," said Dai. "Lower temperature translates into higher compatibility of nanotube growth with CMOS fabrication technology for potential hybrid electronics, and higher degrees of control over catalyst nanoparticles and nanotubes."

But, by using electrical characterization techniques, the researchers also found that the process created an unusually high proportion (around 89%) of semiconducting, as opposed to metallic, nanotubes. The team examined other nanotube production methods for comparison: HiPco made roughly 60% semiconducting nanotubes (which is within the margin of error for materials with no chirality preference), while laser ablation grew around 30% semiconducting nanotubes. Previous work has indicated that single-walled carbon nanotubes grown by regular CVD techniques do not show a chirality preference.

Now the scientists, who reported their work in Nano Letters, say they will work on understanding the synthesis technique, increasing the preferential production of semiconducting nanotubes, and combining preferential growth and chemical separation to make purely semiconducting or metallic nanotubes.

About the author
Liz Kalaugher is editor of nanotechweb.org.