A Feb. 18, 2014 news item on ScienceDaily features a story not about a breakthrough but about a discovery that* could lead to one,
A single-walled carbon nanotube grows from the round cap down, so it’s logical to think the cap’s formation determines what follows. But according to researchers at Rice University, that’s not entirely so.
Theoretical physicist Boris Yakobson and his Rice colleagues found through exhaustive analysis that those who wish to control the chirality of nanotubes — the characteristic that determines their electrical properties — would be wise to look at other aspects of their growth.
The scientists have provided this image to illustrate chirality (‘twisting’) in carbon nanotubes,
To get a clear picture of how caps are related to nanotube chirality, the Rice group embarked upon a detailed, two-year census of the 4,500 possible cap formations for nanotubes of just two diameters, 0.8 and 1 nanometer, across 21 chiralities.
The cap of every nanotube has six pentagons – none of which may touch each other — among an array of hexagons, Penev said. They pull the cap and force it to curve, but their positions are not always the same from cap to cap.
But because a given chirality can have hundreds of possible caps, the determining factor for chirality must lie elsewhere, the researchers found. “The contribution of the cap is the elastic curvature energy, and then you just forget it,” Penev said.
“There are different factors that may be in play,” Yakobson said. “One is the energy portion dictated by the catalyst; another one may be the energy of the caps per se. So to get the big picture, we address the energy of the caps and basically rule it out as a factor in determining chirality.”
A nanotube is an atom-thick sheet of carbon atoms arranged in hexagons and rolled into a tube. Chirality refers to the hexagons’ orientation, and that angle controls how well the nanotube will conduct electricity.
A perfect conducting metallic nanotube would have the atoms arranged in “armchairs,” so-called because cutting the nanotube in half would make the top look like a series of wells with atoms for armrests. Turn the hexagons 30 degrees, though, will make a semiconducting “zigzag” nanotube. Nanotubes can be one or the other, or the chiral angle can be anything in between, with a shifting range of electrical properties.
Getting control of these properties has been a struggle. Ideally, scientists could grow the specific kinds of nanotubes they need for an application, but in reality, they grow as a random assortment that must then be separated with a centrifuge or by other means.
Yakobson suspects the answer lies in tuning the interaction between the catalyst and the nanotube edge. “This study showed the energy involved in configuring the cap is reasonably flat,” he said. “That’s important to know because it allows us to continue to work on other factors.
Here’s a link to and a citation for the paper,
Extensive Energy Landscape Sampling of Nanotube End-Caps Reveals No Chiral-Angle Bias for Their Nucleation by Evgeni S. Penev, Vasilii I. Artyukhov, and Boris I. Yakobson. ACS Nano, Article ASAP DOI: 10.1021/nn406462e Publication Date (Web): January 23, 2014
Copyright © 2014 American Chemical Society
This article is behind a paywall.
One final comment, it took these scientists two years of painstaking work to establish that caps are not the determining factor for chirality. It’s this type of story I find as fascinating, if not more so, as the big breakthroughs because it illustrates the extraordinary drive it takes to extract even the smallest piece of information. I wish more attention was given to these incremental efforts.
* March 7, 2014 changed ‘while’ to ‘that’.