Dec 21 2008
The paradox of perfection – that flaws make things perfect – could be the key to designing nanoelectronic circuits from carbon nanotubes, according to US scientists. They have discovered that a circuit of nanotubes can only guide a current if some of the tubes carry structural defects. Individual carbon nanotubes are exceptionally good conductors because they are essentially a single carbon molecule.
They can even outdo silicon at transmitting charge, which means nanotube circuits could boost computing speeds while reducing chip size (see our feature What happens when silicon can shrink no more? ). But connecting nanotubes into such circuits is not easy, says Vincent Meunier at Oak Ridge National Laboratory in Tennessee. "The connections between individual nanotubes do not conduct well," he says. Instead of jumping easily into an adjacent nanotube, as they would between metal wires, electrons are more likely to bounce back when they reach the end of a tube, says Meunier. Electrons treat junctions between nanotubes as barriers – what scientists call "opaque".
Now Meunier's team has discovered that it could improve the transparency of the junctions by adding flaws to the connecting ends of nanotubes. The carbon atoms within a nanotube are normally arranged in a hexagonal lattice similar to chicken wire. But the researchers used detailed simulations to see what happens when a few pentagons and heptagons are added to the otherwise regular structure. The results show that such irregularities can make connections between nanotubes much better conductors. A Y-junction of three nanotubes with no defects usually behaves as an insulator. But a virtual version with added flaws switched into an excellent conductor. Adding just two defects creates a small "window" of conductance for certain strengths of current, but adding more faults lets almost any current make the jump.
Ordinarily, electrons arriving at a three-pronged Y junction will simply bounce off the far wall and reflect back the way they have come. The defects effectively change the angle of that far wall so that electrons do not simply bounce back. Careful positioning of the defects can redirect the bouncing electrons into one of the other nanotubes.
"It's an indirect effect," says Meunier, "no current passes through the defect itself." Meunier's team say that structural defects could be used to precisely guide an electric current on a particular path through a honeycomb network of nanotubes. "Then you can start to make all kinds of things," Meunier says.
"You could make really complicated networks." Defects can be added to nanotubes exactly where needed using a focused electron beam, he says. Chuanhong Jin at the National Institute of Advanced Industrial Science and Technology in Tsukuba, Japan, is impressed. "This is a very interesting theoretical proposal for designing new carbon nanotube-based 3D nano-circuits," he says. "The method should be of particular importance for the realisation of the long-expected all-carbon programmable nano-electronics circuits."