Witnessing a birth

By Neal Singer

Jianyu Huang at a microscope similar to the one he used to image buckyball births

Buckyballs – more formally known as buckminsterfullerene C-60 – are nanostructures formed by arrangements of carbon atoms that seem stitched or welded together, in appearance much like a soccer ball.

The carbon-carbon bonds give the buckyball great strength, and the spherical 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.

“We have now the first direct, in situ, experimental proof of the hypothesis that these structures are formed by the heated ‘shrinkwrapping’ of carbon sheets,” says Jianyu Huang, who first watched the structures form under a specialized microscope.

The hypothesis: Heating bends single-atom-layer carbon sheets into nano bowls, and then adds more carbon atoms to the edges of the bowls until fullerenes – larger, less stable versions of the C-60 molecule – shrink to form stable and spherical C-60 molecules.

Atomic images of the inside of a nanotube show the formation of fullerenes, their reduction to C-60 buckyballs, and their dispersion.

Buckyball codiscoverer (1985) and Nobel laureate (1996) Richard Smalley had hypothesized that buckyballs are formed in this fashion, but at his death in 2005 no experimental confirmation was yet available, and other methods of C-60 formation have been proposed.

Huang’s discovery happened unexpectedly. 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-nanometer-diameter multiwalled carbon nanotube to approximately 2,000 degrees Celsius when he saw the exterior shells of giant fullerenes form from peelings within the nanotube.

High-resolution two-dimensional 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 C-60. Then the buckyballs vanished.

Simulations created at Huang’s request by Boris Yakobson’s team at Rice University showed 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.

A movie clip showing the fullerenes’ collapse and disappearance is available at www.sandia.gov/news/resources/releases/2007/buckyball.html.

Interpretation of the research was paid for by Sandia’s Center for Integrated Nanotechnologies (CINT) and the program. CINT is a joint effort of Sandia and Los Alamos national labs and is supported by the DOE’s Office of Science.

“I used to study metals,” says Huang, who grew up in a remote Chinese farming village and now utilizes the most complex instruments at 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.

One promising candidate, called a Comparative Vacuum Monitoring (CVM) sensor, is a self-adhesive rubber patch, ranging from dimeto credit-card-sized. The rubber’s underside is laser-etched with rows of tiny, interconnected channels or galleries to which an air pressure is applied. Any propagating crack in the material under the sensor breaches the galleries and the resulting change in pressure is monitored.

“The STM probe inside the TEM is a very powerful tool in nanotechnology,” Huang says. “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.”

A paper detailing the work, co-authored by Yakobson, was published in the Oct. 26, 2007, issue of Physical Review Letters.