[Editors] MIT tames tricky carbon nanotubes

[Elizabeth Thomson]

MIT News Office
Massachusetts Institute of Technology
Room 11-400
77 Massachusetts Avenue
Cambridge, MA  02139-4307
Phone: 617-253-2700
http://web.mit.edu/newsoffice/www

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MIT tames tricky carbon nanotubes
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For Immediate Release
MONDAY, SEP. 18, 2006
Contact: Elizabeth A. Thomson, MIT News Office
Phone: 617-258-5402
Email: thomson at mit.edu

IMAGE AVAILABLE


CAMBRIDGE, Mass.--Based on a new theory, MIT scientists may be able 
to manipulate carbon nanotubes -- one of the strongest known 
materials and one of the trickiest to work with -- without destroying 
their extraordinary electrical properties.

The work is reported in the Sept. 15 issue of Physical Review 
Letters, the journal of the American Physical Society.

Carbon nanotubes -- cylindrical carbon molecules 50,000 times thinner 
than a human hair -- have properties that make them potentially 
useful in nanotechnology, electronics, optics and reinforcing 
composite materials. With an internal bonding structure rivaling that 
of another well-known form of carbon, diamonds, carbon nanotubes are 
extraordinarily strong and can be highly efficient electrical 
conductors.

The problem is working with them. There is no reliable way to arrange 
the tubes into a circuit, partly because growing them can result in a 
randomly oriented mess resembling a bowl of spaghetti.

Researchers have attached to the side walls of the tiny tubes 
chemical molecules that work as "handles" that allow the tubes to be 
assembled and manipulated. But these molecular bonds also change the 
tubes' structure and destroy their conductivity.

Now Young-Su Lee, an MIT graduate student in materials science and 
engineering, and Nicola Marzari, an associate professor in the same 
department, have identified a class of chemical molecules that 
preserve the metallic properties of carbon nanotubes and their 
near-perfect ability to conduct electricity with little resistance.

Using these molecules as handles, Marzari and Lee said, could 
overcome fabrication problems and lend the nanotubes new properties 
for a host of potential applications as detectors, sensors or 
components in novel optoelectronics.

Marzari and Lee use the fundamental laws of quantum mechanics to 
simulate material properties that are difficult or impossible to 
measure, such as molten lava in the Earth's core or atomic motion in 
a fast chemical reaction. Then they run these simulations on 
interconnected PCs and use the results to optimize and engineer novel 
materials such as electrodes for fuel cells and polymers that 
contract and expand like human muscles.

With the help of a powerful algorithm created by Lee and published 
last year in Physical Review Letters, the theorists focused on 
solving some of the problems of working with carbon nanotubes.

Like fuzzy balls and Velcro, the hexagon of carbon that makes up a 
nanotube has a predilection for clinging to other hexagons. One of 
the many challenges of working with the infinitesimally small tubes 
is that they tend to stick to each other.

Attaching a molecule to the sidewall of the tube serves a double 
purpose: It stops nanotubes from sticking so they can be processed 
and manipulated more easily, and it allows researchers to control and 
change the tubes' electronic properties. Still, most such molecules 
also destroy the tubes' conductance because they make the tube 
structurally more similar to a diamond, which is an insulator, rather 
than to graphite, a semi-metal.

Lee and Marzari used Lee's algorithm to identify a class of 
"molecular handles" (carbenes and nitrenes) that stop this from 
happening and preserve the tubes' original conductivity. "We now have 
a way to attach molecules that allows us to manipulate the nanotubes 
without losing their conductance," Marzari said.

Carbenes and nitrenes work by breaking a molecular bond on the 
nanotube's wall while creating their own new bond to the tube. This 
process -- one bond formed, one bond lost -- restores the perfect 
number of bonds each carbon atom had in the original tube and 
"conductance is recovered," Marzari said.

Some molecular handles can even transform between a bond-broken and a 
bond-intact state, allowing the nanotubes to act like switches that 
can be turned on or off in the presence of certain substances or with 
a laser beam. "This direct control of conductance may lead to novel 
strategies for the manipulation and assembly of nanotubes in metallic 
interconnects, or to sensing or imaging devices that respond in 
real-time to optical or chemical stimuli," Marzari said.

The next step is for experiments to confirm that the approach works.

This work is supported by the MIT Institute for Soldier 
Nanotechnologies and the National Science Foundation.

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