Nanotubes are ubiquitous in the world of science. Although several methods for making them exist, little is known about how these techniques physically produce the hollow fibers of carbon molecules known as nanotubes. That is, until now. A multinational team of scientists has discovered that multi-walled carbon nanotubes made by the pure carbon arc method are, in fact, carbon crystals that form inside drops of glass-coated liquid carbon.

"We were doing research on the electrical transport properties of carbon nanotubes when we noticed that the nanotubes had these little beads that looked like liquid drops on them,” said lead author Walt A. de Heer, physics professor at the Georgia Institute of Technology.

The research team saw that the beads had the disordered grouping characteristic of glass, while the nanotubes they surrounded had an orderly crystalline pattern. This lead them to conclude that the carbon arc must have melted the graphite into drops of liquid carbon, which had cooled at a much faster rate on the outside, giving it a glassy appearance.

Since the nanotubes in the interior had a crystalline structure, the team reasoned that the liquid carbon on the inside of the drops had cooled so slowly it became a super-cooled liquid, which is a liquid below the temperature that normally turns it into a solid. As the temperature of any super-cooled liquid drops to a certain critical temperature, it begins to crystallize. In this case, researchers reasoned, it resulted in the orderly molecular structure of the nanotubes.

As the nanotubes continue to crystallize, they lengthen, poking through the glass layer and causing the glass to bead on the tubes much like water beads on pine needles. This final portrait of the beads on nanotube fibers is the photo that began the research team's initial questions.

Find out more at: www.physics.gatech.edu/people/faculty/wdeheer.html

Glassy drops of carbon coat the fibers that house nanotubes after their synthesis with a carbon arc.

Nanotubes coated with glassy drops of carbon poke through the surface of a column housing nanotubes.