Physicists Find Key To Long-lived Nanowires

Researcherspredict that such nanotechnology will be the next Big Thing torevolutionize the computing, medical, power and other industries incoming decades.

Although researchers in Japan, the Netherlands,Spain, Brazil and the United States have had some success at makingnanowires -- extremely small filaments that transport electrons -- thewires don't last long except at low temperatures.

What researchers need are robust nanowires that will take repeated use without failing at room temperature and higher.

UApost-doctoral associate Jerome Buerki and physics Professors CharlesStafford and Daniel Stein developed a theory that explains whynanowires thin away to nothing at non-zero temperatures. Energyfluctuations in a nanowire at higher temperatures create a collectivemotion, or "soliton," among atoms in the wire. As each of thesekink-like structures propagates from one end of the wire to the other,the wire thins.

Stafford has posted movies that show thisphenomenon on his Web page,http://www.physics.arizona.edu/~stafford/necking.htmlThe movie was madeby the Takayanagi group at the Tokyo Institute of Technology.

"Ourtheory makes one very simple prediction, which is that the energybarrier that creates these kinks depends, very simply, on the squareroot of the surface tension of the wire," Stafford said. "That's quitecounterintuitive, because naively you'd think that surface tensionshould actually make the filament unstable. But the larger the surfacetension, the more stable the wire, regardless of the radius of thewire."

Creation of solitons, or kinks, in the wire depends on twocompeting forces - the surface tension and a quantum force that holdsthe wire together, Stafford explained. "It just so happens that thecompetition between those two forces leads to a kind of universalenergy barrier which goes as the square root of the surface tension."

Thediscovery explains why experimentalists have had more luck at makinglonger-lived nanowires using such noble metals as gold and silverrather than sodium or other alkali metals. According to the UAphysicists' theory, copper is the best metal for making nanowiresbecause it has the largest natural surface tension of the nanowiremetals.

"The hardest thing with developing nanowires, I think, ishow to fabricate them in a controlled way," Stafford said. "The moviesshow how researchers can fabricate one tiny wire, but that's notconnecting many such wires, or connecting them to make a circuit.

"Butat least, our work says that these wires are very stable, and that weunderstand exactly how stable they are. I think that can give peopleconfidence to move ahead with trying to do something practical withthem."

The research, funded by the National Science Foundation,will be published this week in Physical Review Letters. The article,"Theory of Metastability in Simple Metal Nanowires,"appears online at http://link.aps.org/abstract/PRL/v95/390601.