PORTLAND, Ore. -- When Harold Kroto, Robert Curl and the late Richard Smalley won the 1996 Nobel Prize for their 1985 discovery of carbon-60, they speculated on how they believed it was created. The trio called these hollow spheres of 60 carbon atoms arranged in interlocking pentagons "fullerenes," (and sometimes "buckyballs") to celebrate their resemblance to the geodesic dome invented by Buckminster Fuller. They called fullerenes' construction method "shrink wrapping," because they believed that fullerenes started out as sheets of graphene that were wrapped into giant spheres of a thousand or more atoms, then shed atoms by "evaporation" until they reached the smallest possible formation, carbon-60. Unfortunately, when Smalley died, in 2005, the shrink-wrap hypothesis had yet to be confirmed. Now, however, Sandia National Laboratories claims to have experimental confirmation.
"Since they are so small--just a few nanometers--no one had been able to observe a buckyball forming. Consequently, the shrink-wrap hypothesis remained speculative--a fact that prompted others to propose alternative theories," said Sandia National Laboratories researcher Jianyu Huang. "But seeing is believing, and now we have the first video [see below] showing the shrink-wrapping of giant fullerenes by evaporation."
The problem with observing the formation of fullerenes had been an inability to get them to hold still for a photo at such high temperatures. Huang solved this problem by containing them inside a multi-walled nanotube, which held them still enough to be captured on video. Huang made the observations using a high-resolution transmission electron microscope (TEM).
Huang, however, was not out to prove the shrink-wrap theory when he made his observations, but rather was characterizing the durability of multi-walled nanotubes when he saw fullerenes forming inside them.
"I discovered these buckyballs forming by accident when I was studying high temperature stability of multi-walled nanotubes," said Huang.
At 2000 degrees Celsius, the inside walls of the multi-walled nanotubes, measuring about 10 nanometers in diameter, begin to shed layers. As the layers shed--each an atomically thin sheet of graphene--they peeled off the inside of the nanotube, whereupon some of them began rolling up into spheres. These rolled-up spheres began as giant fullerenes of more than a thousand carbon atoms each. But the high-temperature evaporation caused them to quickly shed atoms, while maintaining their pentagon structure, until they reached the smallest possible size of carbon-60, after which they disintegrated under the high heat.
In addition to confirming the shrink-wrap construction method for fullerenes, Huang and his colleagues at Rice University (materials science professor Boris Yakobson, research associate Feng Ding and doctoral candidate Kun Jiao) claim that their results also indicate that various sizes of fullerenes could someday be constructed.
"Our work suggests possible routes to making fullerenes of different sizes for different applications, from fabricating electronic devices to providing a medium for energy storage," said Huang.
Since large fullerenes can contain other substances inside their hollow core while completely isolating them from their environment, giant fullerenes could someday be filled with hydrogen to serve as energy storage reservoirs for hydrogen fuel cells.
Yakobson's team at Rice had previously predicted Huang's observations using a detailed computer simulation, but Huang's observations are the first direct experimental confirmation that their model was accurate. Now the team is expanding its computer model to incorporate Huang's new data and to search for control mechanisms by which fullerenes of different sizes can be constructed for various applications.