A Nano Conveyor

When an electrical current is applied to a multiwalled carbon nanotube (MWNT), the structure is transformed into a tiny conveyer belt that shuttles molten metal along the length of the tube, according to researchers at the University of California, Berkeley, and Lawrence Berkeley National Laboratory [Nature, 428, 924 (2004)]. Physics professor Alex Zettl, postdoc Chris Regan, and their coworkers liken the electrified tube to a nanosoldering iron that might someday be used to fabricate nanoscale devices.

Zettl and Regan's team demonstrate the mass transfer phenomenon by creating a circuit that runs through a MWNT dotted with indium nanocrystals. Increasing the applied voltage heats the tube and melts the indium crystals. With careful control of the voltage, the group can make the metal migrate along the tube from the anode to the cathode.

The metal does not actually move in discrete blobs like parts on an assembly line. Rather, the group observes that over time, metal particles near the anode shrink as particles closer to the cathode grow. "We believe the metal shuttles along the surface of the nanotube in atomic form," Zettl explains. "But the metal atoms are always stuck to the nanotube surface and never 'evaporate' off of it. So the transport process looks more like ants crawling along a thin, long branch, perhaps bunching up at nucleation sweet spots, and not like flies landing and taking off again."

By varying the voltage and polarity of the current, the team can precisely control the speed and direction of the particles' movement, even over distances greater than 2 mm. Furthermore, the group has used the technique to move other metals, such as gold, platinum, tin, and a tin-indium alloy.