lunes, 13 de abril de 2009

The nanoscale monorail

All aboard the nanotube freight train. It may be just a tiny gold flake moving half a millimeter, but new research from the Universitat Autònoma de Barcelona, Spain; the University of Vienna, Austria; and the Ecole Polytechnique Fédérale de Lausanne in Switzerland represents the first thermal nanomotor.
Adrian Bachtold and colleagues start with multi-walled nanotubes made by arc discharge evaporation and a gold ‘cargo’ tacked on by electron beam lithography. The remaining outer nanotubes are removed by electrical breakdown, leaving a nanotube sleeve loaded with cargo which can rotate around or translate along the inner nanotube track.
The assembly is suspended between two metal electrodes, and the sleeve can be seen to move when a current is applied. The team first surmised that the effect was due to electrons shuttling the cargo around, but when the motion showed no dependence on the current direction, the team was left perplexed.
Bachtold then enlisted the help of nanoscale theorist Eduardo R. Hernández and his team at Barcelona's Institute of Materials Science to help explain the results. Simulations show that heating in the nanotube leaves the center at some 1300 °C. In a perfect reversal of frictional heat dissipation, phonon waves along the temperature gradient shuffle the cargo along toward cooler regions [Barreiro et. al., Science (2008) 320, 775].
The authors note that the mutual chirality of the inner and outer nanotubes can define a track for the nanomotor, providing a custom mix of rotation or translation. And the cargo could be virtually anything that can go into a nanotube. “We have already obtained preliminary results indicating that fullerenes inside nanotubes should react in the same way to temperature gradients,” says Hernández. “It is certainly a new avenue of exploration, and with luck—and, no doubt, plenty of work—it may offer new ways of ‘taming the nanoscale’.”
D. Jason Palmer

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