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.
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.
