LONDON A Duke University-led team of chemists has modified a method for growing exceptionally long, straight, numerous and well-aligned carbon cylinders only a few atoms thick that paves the way for manufacturing reliable electronic nanocircuits.
The team had already described a method last April for growing the crystals, but the modification is targeted at making a process specifically for producing semiconducting versions of the single-walled carbon nanotubes, sometimes called “buckytubes” because their ends, when closed, take the form of soccer ball-shaped carbon-60 molecules known as buckminsterfullerines, or "buckyballs".
The effort is being led by Jie Liu, Duke's Jerry G. and Patricia Crawford Hubbard Professor of Chemistry.
"I think it's the holy grail for the field," Liu said. "Every piece is now there, including the control of location, orientation and electronic properties all together. We are positioned to make large numbers of electronic devices such as high-current field-effect transistors and sensors."
A report on their achievement, co-authored by Liu and a team of collaborators from his Duke laboratory and Peking University in China, has just been published in the research journal Nano Letters.
The work was funded by the United States Naval Research Laboratory, the National Science Foundation of China, carbon nanotube manufacturer Unidym Inc., Duke University and the Ministry of Science and Technology of the People's Republic of China.
Liu has filed for a patent on the method. A post doctoral researcher in his laboratory, Lei Ding, was first author of the new report as well as the previous study published April 16, 2008, in the Journal of the American Chemical Society (JACS).
That earlier JACS report described how the researchers coaxed nanotubes to form in long, parallel paths that will not cross each other to impede potential electronic performance. Their method grows the nanotubes on a template made of a continuous and unbroken kind of single quartz crystal used in electronic applications. Copper is also used as a growth promoter.
But that method left one unresolved issue blocking the use of such nanotubes as electronic components. Only some of the resulting nanotubes acted electronically as semiconductors. Others were the electronic equivalent of metals. To work in transistors, the nanotubes must all be semiconducting, Liu said.
The researchers now say they have achieved virtually all-semiconductor growth conditions by making one modification.
In their earlier work they had used the alcohol ethanol in the feeder gas to provide carbon atoms as building blocks for the growing nanotubes. In the new work, they describe how they tried various ratios of two alcohols -- ethanol and methanol -- combined with two other gases they also used previously -- argon and hydrogen.
"We found that by using the right combination of the two alcohols with the argon and hydrogen we could grow exclusively semiconducting nanotubes," Liu said. "It was like operating a tuning knob." The inert argon gas was used to provide a steady feed of the ethanol and methanol, with hydrogen to keep the copper catalyst from oxidizing.
After making the nanotubes by chemical vapor deposition in a small furnace set to a temperature of 900 degrees Celsius, the researchers assembled some of them into field-effect transistors to test their electronic properties.
"We have estimated from these measurements that the samples consisted of 95 to 98 percent semiconducting nanotubes," the researchers reported.
The introduction of methanol to complement ethanol is also said to have shrunk the diameters of the resulting nanotubes and improved their atomic alignments with the underlying quartz crystal.
The group's next challenge is to understand at an atomic level how "just so" tuning of growth gas mixtures resulted in the right chirality to produce exclusively semiconducting nanotubes.
The researchers will also study whether another combination might produce all-metallic nanotubes.