New nano-sized particles set to revolutionise data storage

Researchers have discovered an entire new class of phase-change materials that could be applied to phase change random access memory (PCM) technologies and possibly optical data storage as well. 

Phase-change materials’ ability to readily and swiftly transition between different phases has made them valuable as a low-power source of non-volatile or “flash” memory and data storage.

The new phase-change materials, nanocrystal alloys of a metal and semiconductor, found by researchers with the Lawrence Berkeley National Laboratory (Berkeley Lab) and the University of California (UC) Berkeley, are called “BEANs,” for binary eutectic-alloy nanostructures.

“Phase changes in BEANs, switching them from crystalline to  amorphous and back to crystalline states, can be induced in a matter of nanoseconds by electrical current, laser light or a combination of both,” said Daryl Chrzan, a physicist.

“Working with germanium tin nanoparticles embedded in silica as our initial BEANs, we were able to stabilise both the solid and amorphous phases and could tune the kinetics of switching between the two simply by altering the composition,” he added.

“What we have shown is that binary eutectic alloy nanostructures, such as quantum dots and nanowires, can serve as phase change materials. The key to the behaviour we observed is the embedding of nanostructures within a matrix of nanoscale volumes. The presence of this nanostructure/matrix interface makes  possible a rapid cooling that stabilises the amorphous phase, and also enables us to tune the phase-change material’s transformation kinetics,” said Chrzan.

A eutectic alloy is a metallic material that melts at the lowest possible temperature for its mix of constituents.

The germanium tin compound is a eutectic alloy that has been considered by the investigators as a prototypical phase-change material because it can exist at room temperature in either a stable crystalline state or a metastable amorphous state.

Chrzan and his colleagues found that when germanium tin nanocrystals were embedded within amorphous silica the nanocrystals formed a bilobed nanostructure that was half crystalline metallic and half crystalline semiconductor.

“Rapid cooling following pulsed laser melting stabilizes a metastable, amorphous, compositionally mixed phase state at room temperature, while moderate heating followed by slower cooling returns the nanocrystals to their initial bilobed crystalline state,” said Chrzan.

“The silica acts as a small and very clean test tube that confines the nanostructures so that the properties of the BEAN/silica interface are able to dictate the unique phase-change properties,” he added.

The study has been published in the journal NanoLetters.