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June 19, 2012

Simulations show slower growth will enable defect free Meter Long Carbon Nanotubes

Through simulations, researchers at Rice, Hong Kong Polytechnic and Tsinghua universities find it theoretically possible to grow perfect nanotubes up to a meter long. A nanotube growing about 1 micrometer a second at 700 kelvins could potentially reach the meter milestone. The length would be reached in 11.5 days.

At the right temperature, with the right catalyst, there’s no reason a perfect single-walled carbon nanotube 50,000 times thinner than a human hair can’t be grown a meter long.

That calculation is one result of a study by collaborators at Rice, Hong Kong Polytechnic and Tsinghua universities who explored the self-healing mechanism that could make such extraordinary growth possible. That’s important to scientists who see high-quality carbon nanotubes as critical to advanced materials and, if they can be woven into long cables, power distribution over the grid of the future.

They determined that iron is the best and quickest among common catalysts at healing topological defects – rings with too many or too few atoms – that inevitably bubble up during the formation of nanotubes and affect their valuable electronic and physical properties. The right combination of factors, primarily temperature, leads to kinetic healing in which carbon atoms gone astray are redirected to form the energetically favorable hexagons that make up nanotubes and their flat cousin, graphene. The team employed density functional theory to analyze the energies necessary for the transformation.

Physical Review Letters - Efficient Defect Healing in Catalytic Carbon Nanotube Growth




The energetics of topological defects (TDs) in carbon nanotubes (CNTs) and their kinetic healing during the catalytic growth are explored theoretically. Our study indicates that, with the assistance of a metal catalyst, TDs formed during the addition of C atoms can be efficiently healed at the CNT-catalyst interface. Theoretically, a TD-free CNT wall with 10^8–10^11 carbon atoms is achievable, and, as a consequence, the growth of perfect CNTs up to 0.1–100 cm long is possible since the linear density of a CNT is about 100 carbon atoms per nanometer. In addition, the calculation shows that, among catalysts most often used, Fe has the highest efficiency for defect healing

“It is surprising that the healing of all potential defects — pentagons, heptagons and their pairs — during carbon nanotube growth is quite easy,” said Ding, who was a research scientist in Yakobson’s Rice lab from 2005 to 2009. “Only less than one-10 billionth may survive an optimum condition of growth. The rate of defect healing is amazing. If we take hexagons as good guys and others as bad guys, there would be only one bad guy on Earth.”

The researchers found that very transition happens best when carbon nanotubes are grown at temperatures around 930 kelvins (1,214 degrees Fahrenheit). That is the optimum for healing with an iron catalyst, which the researchers found has the lowest energy barrier and reaction energy among the three common catalysts considered, including nickel and cobalt.

Once a 5/7 forms at the interface between the catalyst and the growing nanotube, healing must happen very quickly. The further new atoms push the defect into the nanotube wall, the less likely it is to be healed, they determined; more than four atoms away from the catalyst, the defect is locked in.

Tight control of the conditions under which nanotubes grow can help them self-correct on the fly. Errors in atom placement are caught and fixed in a fraction of a millisecond, before they become part of the nanotube wall.

The researchers also determined through simulations that the slower the growth, the longer a perfect nanotube could be. A nanotube growing about 1 micrometer a second at 700 kelvins could potentially reach the meter milestone, they found.

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