Nano parfait a treat for scientists
Rice researchers spin pure batches of nanotubes species

BY MIKE WILLIAMS
Rice News staff

In two new papers, Rice University researchers report using ultracentrifugation (UCF) to create highly purified samples of carbon nanotube species.

	JEFF FITLOW
	Pictured clockwise from left are Ben Lu, Erik Haroz, Sergei Bachilo, William Rice, Robert Hauge, Bruce Weisman, Junichiro Kono and Saunab Ghosh.

One team, led by Rice Professor Junichiro Kono, temporary research associate Erik Haroz and graduate student William Rice, has made a small but significant step toward the dream of an efficient nationwide electrical grid that depends on highly conductive quantum nanowire.

The other, led by Rice Professor Bruce Weisman and graduate student Saunab Ghosh, employed UCF to prepare structurally sorted batches of semiconducting nanotubes that could find critical uses in medicine and electronics.

UCF is what it sounds like: a superfast version of the centrifuge process medical lab technicians use to separate blood cells from plasma.

The process involves suspending mixtures of single-walled carbon nanotubes (SWNTs) in combinations of liquids of different densities. When spun by a centrifuge at up to 250,000 times the force of gravity, the nanotubes migrate to the liquids that match their own particular densities. After several hours in the centrifuge, the test tube becomes a colorful parfait with layers of purified nanotubes. Each species has its own electronic and optical characteristics, all of which are useful in various ways.

Weisman’s lab reported its results in this week’s online edition of Nature Nanotechnology. Weisman is a professor of chemistry.

Kono’s lab reported its results in the online edition of ACS Nano. Kono is a professor in electrical and computer engineering and professor of physics and astronomy.

The lack of pure batches of nanotubes species “has been a real hindrance in the field for nearly 20 years,” Weisman said. While the UCF technique is not new, Ghosh found careful fine-tuning of the gradient structure let him sort at least 10 of the numerous species of nanotubes contained in a single sample produced by the Rice-created HiPco process.

WEISMAN LAB/RICE UNIVERSITY
Layers of nanotubes separated from an ultracentrifuge tube, left, show sharp and distinct spectral signatures in the Rice lab of Bruce Weisman.

“Now the process can be used to prepare a greater variety of purified samples,” Weisman said.

Basic research is a big early winner, “because when you can get pure samples of nanotubes, you can learn so much more about them,” Weisman said. “Secondly, some electronic applications become much simpler because the tube type determines the nanotube’s band gap, a crucial electronic property.” Biomedical applications may benefit by exploiting the optical properties of specific types of nanotubes.

In the Kono lab, metallic nanotubes rose to the top of the spinning vial while nearly all of the semiconducting nanotubes sank to the bottom. What surprised lead researchers Haroz and Rice was that nearly all of the metallic tubes were armchair SWNTs, the most desirable species for the manufacture of quantum nanowire. Zigzag and near-zigzag species, also considered metallic, would also sink.

There are many kinds of nanotubes among the single-walled variety. (Multi-walled nanotubes are like Russian nested dolls, with smaller tubes inside larger ones.) Imagine a sheet of carbon atoms laid out like chicken wire. Rolling it produces a tube of a particular diameter, and the angle at which the sides meet determines how conductive it will be.

Armchair nanotubes are so-called because of their “U”-shaped end segments. Theoretically, armchairs are the most conductive nanotubes, letting electrons charge down the middle with nothing to slow them.

As so often happens, the project started as something else. “I was trying to produce more metallic nanotubes for my research into electron spin resonance,” Rice said. Eventually, the process of purifying metallics took on a life of its own, and though it ultimately proved unsuitable for his research, the results were worth the considerable effort Rice and Haroz put in.

Fluorescence and absorption spectroscopies can identify semiconducting nanotubes clearly, but not metallic nanotubes. Resonant Raman spectroscopy worked well, though.

Haroz traveled to the lab of Stephen Doorn, a Los Alamos National Laboratory researcher and scientist with the Center for Integrated Nanotechnologies, and used Doorn’s unique, high-resolution Raman system and its vast array of widely tunable laser sources. For five weeks last year, Haroz used hundreds of laser wavelengths to systematically interrogate his purified metallic nanotubes samples. ”It’s really through the unique combination of lasers and detectors in Dr. Doorn’s Raman system that we were able to discover that armchair nanotubes dominate our samples,” Haroz said. ”His system is probably one of the best in the world for studying nanotubes.”

The composition of the gradient solution made a difference in the quality of the samples, Haroz said. “One of the surfactants we’re using, sodium cholate, has a molecular structure that’s similar to a nanotube — basically hexagons put together,” he said. “We think there’s a match between the sodium cholate and the structure of nanotubes, and it binds just a little bit better to an armchair than it does to zigzags.”

Hurdles remain in the path to quantum armchair nanowires that nanotechnology pioneer and Nobel laureate Richard Smalley, Haroz’s first mentor at Rice who died in 2005, felt would be a panacea for many of the world’s problems. Fix the distribution of energy and solutions to other challenges — clean water, food, environmental woes — will fall into place, he believed.

“Step 1 of the armchair quantum nanowire project is, ‘Can we get armchairs?’ We’ve done that,” Haroz said. “Now let’s make macroscopic structures — not necessarily long cables, but small structures — to test their conductivity.”

Rice research scientist Sergei Bachilo is co-author of the Nature Nanotechnology paper with Weisman and Ghosh. Grants from the National Science Foundation and the Welch Foundation supported the research.

Co-authors of the ACS Nano paper with Kono, Haroz, Rice, Weisman and Ghosh are Robert Hauge, Distinguished Faculty Fellow in Chemistry at Rice, Rice junior Ben Lu and Los Alamos National Laboratory researcher Stephen Doorn.

The Department of Energy Office of Basic Energy Sciences, the Welch Foundation, the Air Force Research Laboratories, the National Science Foundation and the Laboratory Directed Research and Development program at Los Alamos National Laboratory supported the research.