A team of researchers from three institutions has carried out a chemical reaction in what may be the world's smallest set of test tubes: carbon nanotubes with inside diameters of less than ten nanometers and lengths of just one micron.
The work, reported in the December 13 issue of the journal Science, could ultimately have important applications in microelectronics and other fields in which extremely small conductors and other structures would allow production of new types of nanoscale devices.
"We have demonstrated that you can put materials into nanotubes and manipulate them to induce a chemical reaction," said Dr. Walter A. de Heer, a professor in the School of Physics at the Georgia Institute of Technology. "This work opens up new ways of thinking about structures that can act like extremely small test tubes."
In work conducted at the Ecole Polytechnique Federale de Lausanne in Switzerland, de Heer and his collaborators formed carbon nanotubes using well-established techniques. They opened the ends of the tubes and allowed capillary action to fill them with molten silver nitrate (AgNO3). In the final step of the process, they decomposed the silver nitrate into metallic silver by heating the tubes with a beam from an electron microscope.
The process resulted in chains of tiny silver beads within many of the nanotubes, each bead separated by a pocket of gas under pressure estimated to be as high as 1,300 atmospheres. Though not attached to the walls of the nanotubes, the beads remain wedged in place by high levels of friction.
Electron microscope study showed some thinning of the nanotube walls, indicating that the chemical reaction involved in metallizing the silver nitrate caused some damage. However, because the tubes are made up of multiple layers of carbon, the thinning of the walls should not diminish their ability to host chemical reactions.
The researchers, including D. Ugarte of the Laboratorio Nacional de Luz Sincotron in Brazil and A. Chatelain of the Ecole Polytechnique Federale de Lausanne in Switzerland, found that only a small percentage of the nanotubes -- those with diameters of at least four nanometers -- were filled with the silver nitrate.
This shows that the capillary action used to fill the tubes depends on the diameter of the nanotubes. Said de Heer: "If the diameter is relatively large, the liquid will go in easily, but if the tube is narrow, the liquid might not go in at all."
The relationship between size and capillary action is opposite of what would be expected in the "macro" world, where narrower tubes normally create a stronger attraction for liquids. But in nanoscale structures like the tiny tubes, de Heer believes the cylindrical shape alters electrical charges to cause reduced reactivity.
"The polarizability of the inner wall of the tube is reduced," de Heer explained. "The reactivity of the inner walls was lower when the tube diameter shrunk below a certain size."
Based on their studies, the researchers developed mathematical formulas that can be used to predict the capillary action associated with tubes of different diameters. Even small differences in the polarizations of the nanotubes had significant effects on the amount of capillary action that occurred.
Now that they have demonstrated that chemical reactions can be carried out within the nanotubes, the researchers would like to produce a continuous metallic wire. Such a structure would be "a new kind of conductor:" a metallic wire with a graphitic sheath around it.
"There are microelectronic applications for carbon nanotubes already indicated," de Heer added. "We see the potential for doping materials and for chemical reactions inside the nanotubes."
The technique could also be used in flat panel displays, to produce encapsulated compounds or to create other elongated nanostructures using a wide range of materials that will flow by capillary action. It provides an alternative to the electric arc techniques that have previously been used to fill nanotubes.
Carbon nanotubes have intrigued researchers because of their size. The research team that includes de Heer has previously investigated the electronic properties of closed tubes, particularly in applications where they could be electron emitters.
The work was supported by the Swiss National Science Foundation.
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