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Soldering iron added to nano-toolkit

New Scientist

An atomic-scale conveyor belt may also be the smallest soldering iron ever created. The new device, which ferries molten metal, is made from carbon nanotubes just 20 millionths of a millimetre in diameter.

The discovery could pave the way for nano-machines that are pieced together from smaller components, rather than emerging from chemical reactions.

"There has been a dream for many years to build nano-structures piece by piece, like building a large-scale machine," explains Alex Zettl, who built the nano-soldering iron at the University of California in Berkeley. Then the structure is no longer constrained by the chemistry of its components, he says.

Currently, nano-probes can nudge atoms one at a time from one place to another. But to generate the flow of molten material necessary to solder parts together, hundreds of thousands of atoms must be moved.

Zettl begins by spraying pure carbon nanotubes with gaseous indium. The metal then condenses into solid droplets between one and 10 nanometres wide. Using a nano-manipulator built in his lab he connects the tubes to a circuit and applies a small voltage.

Heat from the resulting current melts the droplets, which scurry along the nanotube's surface and collect as a bubbling liquid at the negative end. Reversing the voltage shunts indium to the other end, meaning the movement is driven by electricity rather than a thermal effect. And varying the voltage changes the flow of liquid indium from a "drip-drip" to a "surge", Zettl says.

The liquid bubbles could be wicked off the ends of the tubes and used to solder tiny parts together, he says, though this has yet to be demonstrated.

Zettl is unsure why neutral indium atoms move in response to a voltage. "It looks like the indium is being positively charged," he says. He suggests that electrons may be migrating from the indium to the nanotubes.

Droplets of gold, tin and platinum can also be made to move from one end of the tube to the other. "It's the first time we have controllable, atomic scale motion of mass," Zettl told New Scientist.

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