Nanoelectronic Switch Works Like Frozen Lightning

Researchers have demonstrated a prototype nanoscale electronic switch that works like lightning -- except for the speed. Their proof-of-concept experiments reported last month demonstrate that nanoscale electrical switches can be built from self-assembled layers of organic molecules on silver wires. Potential applications range from a replacement technology for magnetic data storage to integrated circuit memory devices.

NIST chemist James Kushmerick adjusts test apparatus demonstrating a prototype silver nanoswitch.

Silver would be a natural choice for nanoscale and microscale electrical contacts because of its high conductivity, but it has one notorious drawback. In an electric field, silver ions readily form silver "whiskers," tree-like branching growths of crystals that can short-out microelectronic devices.

Two National Institute of Standards and Technology (NIST) researchers have demonstrated that this can be a feature, not a bug, in an experiment that uses this growth to make a nanoscale binary switch.

Silver nanoswitch: When the voltage between a gold conductor (top) and silver conductor (bottom) exceeds a critical point, silver ions rapidly assemble like a lightning strike to bridge the gap through a organic molecule monolayer. (Images: NIST )

In the experiment, an extremely fine silver wire is coated with a molecule that forms a self-assembled monolayer on the wire, typically some organic molecule with a sulfur group on one end to bond to the silver. An equally fine gold wire is laid crosswise to the silver wire and a small voltage is applied across the two wires. When the voltage is increased to a critical level, silver ions form and quickly branch through the organic monolayer to the gold wire just like a lightning bolt, except solid. When a silver filament reaches the gold, it forms a short circuit, causing a dramatic change in conductance, which is easily detectable. Reversing the voltage retracts the filament and "opens" the switch.

As a candidate logic switch for nanoscale memory circuits and similar devices, the silver whisker switch has several attractive features:

The chemistry of the organic monolayer is not critical; the switch works with many different molecules and so can be used with many different self-assembled molecular electronics systems.
The crossed-wire structure is very simple to engineer and lends itself to large arrays of switches.
The difference between "on" and "off" is huge -- electrical resistance ratios of a million or more. This makes it easier to reliably scale up the technology to very large arrays.
Problems to be overcome, according to the researchers, include volatility -- the voltage has to be kept on to retain the switch state; slow switching speeds -- about 10 kHtz in the prototype; and a tendency of the switch to freeze permanently closed after a large number of cycles.

NIST has applied for a patent on the switch. For more information, visit: www.nist.gov