HOUSTON, Nov. 17, 2006 -- Chemists today revealed what they said is the first method for cutting carbon nanotubes into "seeds" and using those seeds to sprout new nanotubes. The method may allow them to reproduce their very best nanotube samples for use in a variety of materials applications.
"Carbon nanotubes come in lots of diameters and types, and our goal is to take a pure sample of just one type and duplicate it in large quantities," said James Tour, director of Rice University's Carbon Nanotechnology Laboratory (CNL) and corresponding author of a study about the research. "We've shown that the concept can work."
The study's lead author, CNL founder and nanotube pioneer Richard Smalley, died in October 2005. Tour said Smalley devoted an enormous amount of time and energy to the seeded-growth nanotube amplification research in the final two years of his life.
A polymer-wrapped single-walled carbon nanotube.
"Rick was intent on using nanotechnology to solve the world's energy problems, and he knew we needed to find a way to make large quantities of pure nanotubes of a particular type in order to rewire power grids and make electrical energy widely available for future needs," Tour said. "Rick had a way of making things happen, and for six months during 2004, there were no fewer than 50 researchers in four Rice laboratories devoting their effort to this problem. It was unprecedented, and it paid off."
First discovered 15 years ago, single-walled carbon nanotubes (SWNTs) are molecules of pure carbon with many unique properties. Smaller in diameter than a virus, nanotubes are about 100 times stronger than steel, weigh about one-sixth as much and are among the world's best electrical conductors and semiconductors. Smalley, who devoted the last 10 years of his career to studying SWNTs, pioneered the first method for mass-producing them and many of the techniques scientists use to study them.
There are dozens of types of SWNTs, each with a characteristic atomic arrangement. These variations, though slight, can lead to drastically different properties: Some nanotubes are like metals, and others are semiconductors. While materials scientists are anxious to use SWNTs in everything from bacteria-sized computer chips to geostationary space elevators, most applications require pure compounds. Since all nanotube production methods, including the industrial-scale system Smalley invented in the 1990s, create a variety of 80-odd types, the challenge of making mass quantities of pure tubes -- which Smalley referred to as "SWNT amplification" -- is one of the major, unachieved goals of nanoscience.
"Rick envisioned a revolutionary system like PCR (polymerase chain reaction), where very small samples could be exponentially amplified," Tour said. "We're not there yet. Our demonstration involves single nanotubes, and our yields are still very low, but the amplified growth route is demonstrated."
The nanotube seeds are about 200 nanometers long and one nanometer wide -- length-to-diameter dimensions roughly equal to a 16-ft garden hose. After cutting, the seeds underwent a series of chemical modifications. Bits of iron were attached at each end, and a polymer wrapper was added that allowed the seeds to stick to a smooth piece of silicon oxide. After burning away the polymer and impurities, the seeds were placed inside a pressure-controlled furnace filled with ethylene gas. With the iron acting as a catalyst, the seeds grew spontaneously from both ends, growing to more than 30 times their initial length -- imagine that 16-ft water hose growing by more than 500 feet -- in just a few minutes.
Tour, Chao Professor of Chemistry, professor of mechanical engineering and materials science and professor of computer science, said CNL's team has yet to prove that the added growth has the same atomic architecture -- known as chirality -- of the seeds. However, he said the added growth had the same diameter as the original seed, which suggests that the method is successful.
Other researchers on the project include Yubao Li, postdoctoral researcher; Valerie C. Moore, former graduate student and assistant professor at the University of Texas Health Science Center at Houston; Katherine Price, graduate student; Ramon Colorado Jr., research scientist; Howard Schmidt, CNL executive director; Robert Hauge, distinguished faculty fellow and CNL technology director; and Andrew Barron, the Charles W. Duncan Jr.-Welch Professor of Chemistry, professor of materials science and associate dean for industry interactions and technology transfer.
The research, funded by DARPA, NASA and the Robert A. Welch Foundation, is slated to appear in an upcoming issue of the Journal of the American Chemical Society. For more information, visit: http://www.rice.edu
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