Chemists decorate nanotubes for usefulness

In a step that could lead to harder materials and tinier electronic devices, researchers have found a promising new way to attach molecules to carbon nanotubes.

A transmission electron microscope image shows a simple carbon nanotube’s smooth edges (above). A similar nanotube has bumpy edges after groups of atoms bind to it (below) (c) 2001 Am. Chem. Soc.

In its simplest form, a carbon nanotube is a one-atom-thick sheet of carbon curved into a cylinder. Such tubes exhibit extraordinary strength and electrical conductivity. For many potential uses of carbon nanotubes, chemists need to attach clusters of atoms, called functional groups, to the outsides of the tubes. The new report, which will appear in an upcoming issue of the Journal of the American Chemical Society, demonstrates a novel way to do just that.

Researchers have had some success with adding functional groups to carbon nanotubes. But the new method is simpler and can attach a greater variety and number of groups, says research team member James M. Tour of Rice University in Houston. The process can attach a functional group to as many as 1 out of every 20 carbons on a nanotube, which can contain millions of carbon atoms.

Tour and his colleagues used a technique similar to one by which chemists link functional groups to graphite, which forms from flat sheets of carbon. The Rice researchers attached an electrode to apply a voltage to a mesh of carbon nanotubes known as bucky paper. Then, to link each type of chemical group to the nanotubes, they bathed the bucky paper in a solution containing a different aryl diazonium salt.

Each molecule of an aryl diazonium salt contains a six-carbon ring, to which the researchers had attached one of a variety of functional groups. Joined to one of the ring’s five other carbon atoms was a different chemical group that the scientists expected would readily get knocked off as the molecule approached the charged bucky paper. If that happened, the ring’s suddenly available carbon atom would bond to a carbon of the nearby nanotube.

A variety of tests by the Rice researchers revealed that the functional groups indeed attach to the carbon nanotubes. Using a scanning tunneling microscope, Paul Weiss of Pennsylvania State University in University Park has also confirmed that. He now is further characterizing the nanotubes.

“I think that [such] functionalization of the nanotubes is very important, because there is a whole host of applications,” comments Robert Haddon of the University of California, Riverside.

For example, nanotubes carrying certain functional groups could mix more readily with other materials. Scientists then might be able to create new conductive plastics or even plastics that are as hard as steel. The Rice group now is working to make carbon nanotubes compatible with the epoxy resins used by NASA on spacecraft, Tour says.

Another exciting vision would use carbon nanotubes for making electronic circuits that are far tinier than today’s silicon-based circuitry. Doing so will require chemically hooking carbon nanotubes to other microscopic electronic components, comments Weiss.

In fact, one of the functional groups that the Rice researchers successfully attached to carbon nanotubes has exhibited both memory and switching behaviors necessary for electronic devices, says Tour. The researchers are investigating whether a nanotube and its functional groups retain their desirable strength, conductivity, and chemical traits after they’re combined.

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