Researchers have been examining carbon nanotubes for use in solar cells (for example see: Carbon Nanotube Fibers Concentrate Solar Energy). The ability to make better nanotubes could make them even more attractive for this and other uses.
The process of making carbon nanotubes involves immersing single-atom thin graphene sheets in solution. The jumbled bundle of long nanotubes is then blasted by high-intensity sound waves that create cavities (or partial vacuums) in the solution. The bubbles that arise from these cavities expand and collapse so violently that the heat in each bubble's core can reach more than 5,000 degrees Kelvin, close to the temperature on the surface of the sun. Meanwhile, each bubble compresses at an acceleration 100 billion times greater than gravity.
Considering the energy involved, it isn't surprising that the tubes come out at random lengths. Sieves can be used to get tubes of the desired length, but no one was sure what caused the tubes to fracture. Materials scientists initially thought that the hot temperatures were responsible, and a group of German researchers proposed that it was the sonic boomlets caused by collapsing bubbles that pulled the tubes apart like a rope tugged at each end until it eventually rips.
Kyung-Suk Kim, professor of engineering in the School of Engineering at Brown, Brown postdoctoral researcher Huck Beng Chew, and engineers at the Korea Institute of Science and Technology investigated further by developing complex molecular dynamics simulations. They used an array of supercomputers to study what exactly made carbon nanotubes break. Rather than being pulled apart, as the German researchers had thought, the tubes were compressed from both ends. Buckling occurred in a roughly 5-nm section called the compression-concentration zone. In that zone, the tube is twisted into alternating 90-degree-angle folds and resembles a helix.
High-intensity atomic-level sonic boomlets cause nanotubes to buckle and twist at compression-concentration zones. Courtesy of Kyung-Suk Kim Laboratory, Brown University.
That discovery still did not explain fully how the tubes break. Through more computerized simulations, the group learned the force exerted by the bubbles' sonic booms caused atoms to shoot off the tube's lattice-like foundation.
"It's almost as if an orange is being squeezed, and the liquid is shooting out sideways," Kim said. "This kind of fracture by compressive atom ejection has never been observed before in any kind of material."
The team confirmed the computerized simulations through laboratory tests involving sonication and electron microscopy of single-walled carbon nanotubes.
The group also learned that cutting single-walled carbon nanotubes using sound waves in water creates multiple kinks, or bent areas, along the tubes' length. The kinks could be good spots for intramolecular junctions for building molecular-scale electronics.
Watch as compression causes nanotubes to buckle and twist and eventually to lose atoms from their lattice-like structure. Courtesy of Huck Beng Chew, Brown University.