The birth secret of buckyballs -- hollow spheres of carbon no wider than a strand of DNA -- has been caught on tape by researchers at Sandia National Laboratory and Rice University. An electron microscope video and computer simulations show that "shrink-wrapping" is the key; buckyballs start life as distorted, unstable sheets of graphite, shedding loosely connected threads and chains until only the perfectly spherical buckyballs remain.
Buckyballs were discovered at Rice in 1985, but understanding the intimate details their formation has vexed scientists. Buckyballs form at high temperatures, and one long-standing theory of their genesis is the "hot giant" hypothesis, which suggests that the carbon atoms first assemble by the thousands in flat graphite sheets. Heat distorts the sheets, "shrink wrapping" them into ever-smaller shapes, and buckyballs survive thanks to their perfect symmetry.
"This 'hot evolution' is so rapid that it was nearly impossible to prove or disprove it by experimental observation," said study co-author Boris Yakobson, professor of mechanical engineering and materials science at Rice. "Sandia's Jianyu Huang solved this problem by creating an ingenious, controllable heat bath inside a 10-nanometer-wide nanotube. That allowed him to capture video of giant fullerenes gradually shrinking."
Huang, who performed the experiments while at Boston College and analyzed the data at Sandia, said the results constitute the first experimental evidence for the 'shrink-wrapping' and 'hot-giant' fullerene birth mechanisms.
That is, heating bends single-atomic-layer carbon sheets into nano bowls, and then adds more carbon atoms to the edge of the bowls until the formation of giant fullerenes — larger, less stable versions of the C-60 molecule. Continued application of heat reduces these fullerenes — “shrink-wrapping” is the favored term — to the size of stable C-60 molecules, the buckyball: the smallest stable arrangement of carbon atoms in that shape.
In further heating, the buckyball vanishes, providing more proof that the buckyball stage had been reached.
Huang captured the high-resolution images using a transmission electron microscope (TEM). The video shows a large fullerene, with an estimated 2,000 atoms of carbon gradually shrinking. It confirmed predictions about the atomic mechanisms that Yakobson's team at Rice had made based on detailed computer simulations.
"If heat is sustained, as it was when we took these images, the fullerenes undergo a further shrinking and vanish," Huang said. "This confirms an aspect of 'shrink wrapping' theory that was predicted by Rice's Rick Smalley and Bob Curl made shortly after they discovered fullerenes."
Huang and Yakobson said it may be possible to exploit the findings to control the fullerene formation process and tailor fullerenes for a variety of applications.
Buckyballs — more formally known as buckminsterfullerene C-60 — are carbon-linked nanostructures named for their resemblance to the geodesic dome macrostructures favored for their strength by environmentalist Buckminster Fuller.
In addition to the strength generated by their carbon-carbon bonds — “the strongest chemical bonds in Mother Nature,” says Huang, who still seems awed by the properties of the nanomaterial — the structure forms a relatively impermeable cage that conceivably could safely transport molecules of hydrogen for fuel, or tiny doses of medicine to targeted sites within the human body.
But before their widespread use is possible, buckyballs have to be available in large numbers. To achieve that, better understanding of how they form is crucial.
Video available at: http://www.youtube.com/watch?v=NSNlE8AreeM
The research is available online and slated to appear in the Oct. 26 issue of Physical Review Letters (PRL).
Co-authors of the research include research scientist Feng Ding and graduate student Kun Jiao, both of Rice. The research was funded by the Office of Naval Research and the Department of Energy's Center for Integrated Nanotechnologies.
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