Buckyball Formation Observed

A Sandia National Laboratories researcher looking for flaws in nanotube durability was unexpectedly able to experimentally confirm a hypothesis about how Buckyballs form.

“We have now the first direct, in situ, experimental proof of the hypothesis -- very significant to the scientific community -- that these structures are formed by the heated ‘shrink-wrapping’ of carbon sheets,” said Sandia researcher Jianyu Huang, who observed the Buckyball formation.

Buckyballs -- more formally known as buckminsterfullerene C60 -- are carbon-linked nanostructures named for their resemblance to the geodesic dome macrostructures favored for their strength by environmentalist Buckminster Fuller.

Atomic images of the inside of a nanotube show the formation of fullerenes, their reduction to C60 Buckyballs, and their dispersion when heated beyond that point. The images were taken by a transmission electron microscope (TEM). (Images courtesy Sandia National Laboratories)

The carbon-carbon bonds of Buckyballs are "the strongest chemical bonds in Mother Nature," said Huang. In addition to the strength generated by their bonds, 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.

Huang discovered that 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 C60 molecule. Continued application of heat reduces these fullerenes -- “shrink-wrapping” is the favored term -- to the size of stable C60 molecules. The Buckyball is 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.

Sandia researcher Jianyu Huang sits in front of a combination TEM-STM (scanning tunneling microscope) similiar to the one he used to image Buckyball formation. On the computer screen are images of flaws occurring in nanocylinders, a continuing area of research for Jianyu at the joint Sandia/Los Alamos CINT center.

Buckyball codiscoverer and Nobel laureate Richard Smalley had hypothesized that Buckyballs are formed in this fashion, but when he died in 2005 no experimental confirmation was yet available and other methods have since been proposed.

Huang’s discovery happened unexpectedly as he was looking for flaws in nanotube durability. Transmitting electric current through the atom-sized tip of a scanning tunneling microscope (STM) -- itself inside a transmission electron microscope (TEM) -- he had heated a 10-nm-diameter multiwalled carbon nanotube to approximately 2000 °C when he saw the exterior shells of giant fullerenes form from peelings within the nanotube. High-resolution 2-D images of the process taken by a CCD camera attached to the microscope showed the fullerenes reducing in diameter, linearly with time, until the structures became the size of C60, the smallest arrangement of carbon atoms that form the soccerball shape.

Then the Buckyballs vanished.

Simulations created at Huang’s request by Boris Yakobson’s team at Rice University show that heating could reduce fullerenes by emitting carbon dimers (pairs of atoms) until they reached the basic Buckyball shape. Further removal of carbon pairs collapsed the structure.

Buckyballs are formed by hexagonal and pentagonal arrangements of carbon atoms that seem stitched or welded together, in appearance much like a soccer ball. Their curvature, however, is caused by the pentagons alone, 12 to a Buckyball. Departing atoms leave the same number of pentagons until the fullerene shrinks below its smallest stable shape, below which the Buckyball disintegrates.

“I used to study metals,” said Huang, who grew up in a remote Chinese farming village and now uses the most complex instruments at Sandia’s Center for Integrated Nanotechnologies (CINT). “But carbon nanomaterials now are much more interesting to me.”

The Buckyball discovery was initially made by Huang on similar instruments at Boston College, and then interpreted at CINT, a joint effort of Sandia and Los Alamos national labs supported by the Department of Energy's Office of Science.

“The STM probe inside the TEM is a very powerful tool in nanotechnology,” Huang said. “The STM probe is like God’s finger: It can grab extremely small objects, as small as a single atomic chain, enabling me to do nanomechanics, nanoelectronics, and even thermal studies of carbon nanotubes and nanowires.”

The research was funded by CINT and Sandia’s Laboratory Directed Research and Development program.

A paper detailing the work coauthored by Huang and Yakobson's team was published in Physical Review Letters in October.

For more information, visit: www.sandia.gov