|A key reason carbon nanotubes are not in computers right now is that they are difficult to manufacture in a predictable way. That may be soon to change as a team from NIST and the University of Southern California has developed a chemical vapor deposition technique that effectively allows carbon nanotubes with desired configurations to be 'cloned'.|
Single-wall carbon nanotubes are hollow cylinders of carbon atoms bound together in a hexagonal pattern, usually about a nanometer in diameter. A fascinating feature of nanotubes is that there are many ways to wrap the hexagon sheet into a cylinder, from perfectly even rows of hexagons that wrap around in a ring, to rows that wrap in spirals at various angles—"chiralities." Even more interesting, chirality is critical to the electronic properties of carbon nanotubes. Some structures are electrical conductors—nanowires.
"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 NIST materials scientist Ming Zheng, "But for that particular application, the structure is critically important. A fundamental issue in that field is how to make single-wall nanotubes with a defined structure."
A key problem is that methods for manufacturing carbon nanotubes, which often use a metal catalyst to start growth, usually produce a mixture of many different chiralities or twists—along with a lot of junk that's just carbon soot. Diverse research has concentrated on schemes for "purifying" the batch to extract one particular kind of nanotube and separate out the metallic catalyst.
“Controlling the chirality of carbon nanotubes has been a dream for many researchers. Now the dream has come true,” Zhou said. The team has already patented its innovation, and its research was published in Nature Communications.
The USC team’s innovation was to not use the metal catalyst and instead plant pieces of carbon nanotubes that have been separated and pre-selected based on chirality, using a nanotube-separation technique developed and perfected by Zheng and his co-workers. Using those pieces as seeds, the team used a chemical vapor deposition (CVD) to extend the seeds to get much longer nanotubes, which were found to have the same chirality as the seeds.
"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."
Small segments of nanotubes of identical chirality, extracted using the DNA technique, were put in a high-temperature reaction chamber at USC with methane gas, which breaks down in the heat to smaller carbon molecules that attach themselves to the ends of the nanotubes, gradually building them up—and preserving their structural chirality. "A bit like building a skyscraper," Zheng observes, though in these early experiments, the tubes are laying on a substrate.
"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, "It's a different approach, to do the separation first to obtain the 'seeds' and then do the synthesis to grow the desired nanotubes."
SOURCE NIST, USC
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