Researchers at the National Institute of Standards and Technology (NIST) have demonstrated a prototype nanoscale electronic switch that works like lightning - except for the speed. Their proof-of-concept experiments reported last week 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.
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 NIST researchers have demonstrated that this can be a feature, not a bug, in an elegant experiment that uses this growth to make a nanoscale binary switch. 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 kilohertz in the prototype; and a tendency of the switch to freeze permanently closed after a large number of cycles.