November 30, 2012 - Researchers have demonstrated a technique for growing virtually pure samples of single-wall carbon nanotubes (SWCNTs) with identical structures, a process they liken to "cloning" the nanotubes. If the process can be suitably scaled up, the approach could solve an important materials problem in nanoelectronics: producing carbon nanotubes of a specific structure to order.

Single-wall carbon nanotubes are hollow cylinders of carbon atoms bound together in a hexagonal pattern. There are many ways to wrap the hexagon sheet into a cylinder, one way being rows that wrap in spirals at various angles -- "chiralities," which are critical to the electronic properties of CNTs.

"Experts in the electronics industry believe that single-wall carbon nanotubes are a promising option for nanoelectronics -- semiconductor devices beyond today's CMOS technology," says Ming Zhengmaterials scientist at the National Institute of Standards and Technology (NIST).

CNTs are typically grown using chemical vapor deposition (CVD), using substrates with metal catalyst nanoparticles. Attempts to control the catalysts and thus achieve chirality-controlled nanotube growth have been unsuccessful, usually producing a mixture of many different chiralities or twists, along with a lot of junk.

The team, led by Zheng and Chongwu Zhou of the U. of Southern California (USC), took a different tack. NIST researchers had developed a technique for extracting nanotubes of a specific twist from a solution by using specially tailored DNA molecules that bind to one particular nanotube chirality. The DNA trick is very selective, but unfortunately only works well with fairly short pieces of nanotube.

"That approach laid the foundation for this work," says Zheng. "We are using the short purified nanotubes to grow bigger structures of the same kind. We call it 'cloning', like cloning an organism from its DNA and a single cell, but in this case, we use a purified nanotube as a seed." Instead of the metal catalyst seed, they use pieces of CNTs that have already been separated and preselected based on desired chirality, to generate much longer CNTs with the same chirality.

Cloning nanotubes: In this computer model, small, pre-selected nanotube "seeds" (yellow) are grown to long nanotubes of the same twist or "chirality" in a high-temperature gas of small carbon compounds. (Credit: Courtesy USC)

Next steps in the research will be to carefully study the mechanism of the nanotube growth in this system, to scale up the cloning process to get large quantities of chirality-controlled nanotubes and to use those nanotubes for electronic applications.

"I think the most important thing this work shows is that from a chemistry point of view, it's entirely possible to grow nanotubes without a catalyst, and even maintain control of the structure," says Zheng. Adds USC's Zhou: "Controlling the chirality of carbon nanotubes has been a dream for many researchers. Now the dream has come true."

The research was funded in part by the Semiconductor Research Corp.'s Focus Center Research Program, Functional Engineered Nano Architectonics, and the Office of Naval Research. The work was published Nov. 13 in Nature Communications.