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Pillars of C60 and nickel are used to grow unprecedented nanotube crystals
At first, they couldn't believe their results. But after checking and rechecking to make sure they hadn't been fooled by some sort of "magical dream," James K. Gimzewski and colleagues convinced themselves that they had indeed achieved the unexpected: They had prepared rod-shaped single crystals of single-walled carbon nanotubes.
Each of these crystals consists of a perfectly ordered array of nanotubes with identical diameters and chirality, although these characteristics vary from crystal to crystal, the researchers report in the online edition of Science [Science Express, published April 5, http://www.sciencemag.org/fea ture/express/expresstwise.shl].
"This is the first time that anyone has been able to produce perfectly ordered material made of single-walled carbon nanotubes," says Gimzewski, who joined the University of California, Los Angeles, as a professor of chemistry in February.
|A growing carbon nanotube crystal Image courtesy of Mark E. Welland
Nanotubes typically are prepared in the form of an entangled and poorly ordered mat, like a serving of cooked spaghetti. But unlike spaghetti, the nanotubes vary in their diameter and structure (chirality), and it has not been possible to separate them. Gimzewski's nanotubes, by contrast, are arranged in a nearly perfect crystal reminiscent of a bundle of uncooked spaghetti.
Nanotube expert Daniel T. Colbert, who heads business development at Houston-based Carbon Nanotechnologies Inc., confessed to being "almost speechless" when he read Gimzewski's paper. "I think this is the most stunning result in buckytube research in at least a year," he tells C&EN. Being able to make such large, defect-free crystals of a single type of nanotube represents "a sea change" in the field, he adds.
The discovery was made by Gimzewski and coworkers at the IBM Zurich Research Laboratory, Rüschlikon, Switzerland, in collaboration with engineering professor Mark E. Welland's group at the University of Cambridge. They were trying to use a previously reported technique to make multiwalled nanotubes filled with metal. After several attempts, they succeeded in producing nanotubes--but not the kind they expected. The product contained thousands to millions of nanotubes bundled together in crystalline arrays with no metal present, and "that was absolutely mind-blowing," Gimzewski recalls.
The synthesis involves alternately evaporating C60 and nickel through the 300-nm holes of a ceramic membrane in a vacuum. The vapor that diffuses through the holes deposits C60 or nickel at discrete sites on a nearby substrate. The result is an array of
pillars consisting of alternating nanometer-thick layers of C60 and nickel. These pillars are then heated to 950 C in the presence of a magnetic field. "And voilá!"--the crystals self-assemble as if by magic, Gimzewski says.
The mechanism by which the crystals grow is still a mystery. But these are the type of crystals one would expect if the nanotubes are packed in the most thermodynamically stable arrangement, Gimzewski explains.
One of the most common crystals produced in this synthesis is made of 1.6-nm-wide nanotubes having the "armchair" arrangement of hexagons. These tubes are electrically conductive. Gimzewski expects that by fine-tuning the process, it should be possible to controllably make crystals consisting of specific kinds of tubes.
| CRYSTAL GROWER Gimzewski calls results "absolutely mind-blowing."
So far, the longest crystals made at the IBM Zurich lab are only in the micrometer range. At his new lab at UCLA, Gimzewski plans to investigate whether the crystals can be grown as continuous threads.
Because nanotubes have exceptional properties, bulk crystalline forms of nanotubes would be of enormous value, Colbert says. Nanotubes have been proposed for many applications, including composite materials, hydrogen storage, nanoelectronic components, and gas sensors.
A key question about the synthesis is economic: "It's hard to imagine right now an economic production method for nanotubes that's based on using C60 as a feedstock," Colbert says. However, he thinks it might be possible to modify the process so that a cheaper form of carbon could be used as a feedstock.
Even if Gimzewski's work doesn't lead to large-scale buckytube production, Colbert remarks, the nanotube crystals will still be "fascinating objects of study."
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