Nanoparticles Get Wired

Tight spot. A single palladium nanoparticle, wedged between two electrodes, can have its electrical properties tested.

Scientists have succeeded in the delicate feat of trapping a single metal particle, just 17 nanometers (billionths of meter) wide, and measuring its electrical properties. The handy technique, to be reported next week in Applied Physics Letters, might be extended to measure the electrical behavior of single conducting molecules--testing that is needed before fantastically small electronic devices can be built.

It's not easy to strap electrodes onto a nanoparticle or molecule. Instead, researchers have resorted to indirect approaches, such as dropping tubular nanoparticles onto grids of fine wires and hoping some make contact. Dreaming of an easier way, physicists Cees Dekker and Alexey Bezryadin of Delft Technical University in the Netherlands etched a channel 20 nanometers wide into a substrate of silica nitrate. They left two "fingers" that protruded into the channel to catch a nanoparticle, then deposited platinum electrodes with traditional ion-beam sputtering. The electrodes grew out over the fingers, until the gap between the electrodes narrowed to an unprecedented 4 nm.

To trap a single nanoparticle, the team bathed the electrodes in water containing suspended nanoparticles of palladium. A small voltage across the electrodes, about 4.5 volts, polarized the particles and attracted them to the gap. The first particle to lodge itself in the gap allowed a current to flow across the electrodes. A resistor then cut the voltage. Without the electric field to attract them to the electrodes, the other particles froze in their tracks.

The lodged nanoparticle remained in place, even when the solvent was evaporated away. "We found this quite amazing," says Dekker. He and his colleagues proceeded to measure how much current could flow through the particle and can study quantum effects like electron tunneling. Dekker believes that the technique could also measure the electrical properties of biological molecules, such as DNA.

Experts are impressed by the team's work. "My group has done similar things with other techniques, but their technique is better," says Paul McEuen of the University of California, Berkeley. McEuen says that it could also open the way to making new devices as well as measuring them: "One may be able to make reliably very small objects trapped between two electrodes for a variety of applications."

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