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Carbon in drops

Theory sustains the idea that nanotubes form in a process that reminds one of dew production

Cylinders formed by a rolled up layer of graphite of only one atom in thickness, carbon nanotubes could be the raw material for a new generation of more efficient electronic components. Apparently versatile like no other physical structure, they could act as a conductor, semi-conductor or electrical insulator. In order to change their properties one only needs to alter the geometry of the atomic sheets. It looks simple but nobody knows how the nanotubes are formed and grow, and least of all how to control their production.

In the 11th of February issue of the American magazine Science, a research team from the United States, France and Brazil produced a theory to explain the formation of such structures when assembled while using the electric arc method, the same used to produce the first nanotubes of carbon back in 1991. The liquid theory – and for certain, they believe in it. Instead of building up starting from gaseous carbon, as until now had been thought, the nonocylinders of graphite, solid structures in spite of their infinitesimally small dimensions, are the children of drops of carbon generated at temperatures of some thousands of degrees Celsius. At least this is what the authors of the scientific paper are proposing.

The idea is controversial, as the very researchers themselves admit. “There’s no proof that carbon exists in its liquid state, although we believe this to be so”, explains the physicist Daniel Ugarte, from the National Synchrotron Light Laboratory (LNLS) and of the State University of Campinas (Unicamp), one of the authors of the work (his colleague Jefferson Bettini, from the LNLS, also participated in the study). As the melting point of the most abundant chemical element on the planet Earth is higher than 4,000°C, some scientists believe that carbon vaporizes before melting.

From a solid it turns directly into a gas without passing through the liquid phase, in a phenomenon known as sublimation, which happens at environmental temperatures with naphthalene placed in a clothes closet. So, did the researchers formulate a theory to explain the origin of nanotubes from a form of carbon whose existence is not yet totally accepted by the scientific community? Exactly. And they even published their text that presents the strange theory in one of the most important scientific magazines. “Some two years ago we drafted out our first version of the article, but is wasn’t all that good. We threw away the text and wrote another”, explains Ugarte. In spite of being questionable, the new explanation for the formation of nanotubes pleased them.

The theory applies only to the fibers of the carbon molecules produced by the old method of the electric arc, hardly ever used by researchers today, who prefer to turn to the less costly technique for generating their study material. By this method, the nanotubes mysteriously appear after the application of a high electrical discharge through graphite electrodes, graphite being the form of carbon that fills writing pencils, maintained within an atmosphere of helium, an inert gas. The most popular hypothesis attributed to the appearance of the nanotubes is a rearrangement of the solid, in the form of cylinders, made up of atoms of carbon that had evaporated on reaching temperatures in the order of 5,000° C. However, on examining the nanotubes generated by these experiments in detail, the American, French and Brazilian team of researchers saw something that nobody else had noticed or hadn’t given importance to: images through the electronic microscope revealed the occurrence of spheres upon some nanotubes. Bubbles that remind one of dew drops that are formed on the threads of a spider’s web.

This was the clue that they needed to formulate their theory. “Just by looking at the drops we were able to perceive that they had something to do with a liquid”, says the physicist Walt A. de Heer, from the Georgia Institute of Technology in the United States, the principal author of the article. “So then we asked ourselves the following question:  if the drops were one day liquid carbon, and the nanotubes to which they are linked are also carbon, why does the liquid carbon not dissolve the nanotube? The answer is that the liquid must have been a glassy carbon at a temperature less than that of the nanotube.”

According to the researchers, the sequence of events that leads to the formation of the nanotubes can be summarized as follows. First, drops of carbon are formed, a result of the liquidation of this element when submitted to very high temperatures. Next, due to the evaporation of the atoms, the most external portion of each drop cools very quickly. Such violent cooling creates on the surface of the drop a coating of a viscous liquid.

Of glassy carbon. In the interior of the glassy crust, nevertheless, there is still hot and liquid carbon. “The cooling of the external part of the drop comes about through the conduction of heat, in a process slower than that of the evaporation of the surface atoms”, explains Ugarte. As soon as the temperature falls within the drop, the nanotubes crystallize. In the end the drop falls off and the nanotubes pass along the viscous liquid which was covering them, resting against the nanocylinder portions of the glassy sphere. The final result is the image of the nanotube with small beads, a scenario similar to that of a spider’s web speckled with dew.