The ability of phase-change materials 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.
Now an entire new class of phase-change materials has been discovered by researchers
with the Lawrence Berkeley National
Laboratory (Berkeley Lab) and the University of California (UC) Berkeley
that could be applied to phase change random access memory (PCM) technologies
and possibly optical data storage as well. The new phase-change materials –
nanocrystal alloys of a metal and semiconductor – are called “BEANs,”
for binary eutectic-alloy nanostructures.
This schematic shows enthalpy curves sketched for the liquid, crystalline and amorphous phases of a new class of nanomaterials called “BEANs” for Binary Eutectic-Alloy Nanostructures. (Image courtesy of Daryl Chrzan)
“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,” says Daryl
Chrzan, a physicist who holds joint appointments with Berkeley Lab’s Materials
Sciences Division and UC Berkeley’s Department of Materials Science and
Engineering. “Working with germanium tin nanoparticles embedded in silica
as our initial BEANs, we were able to stabilize both the solid and amorphous
phases and could tune the kinetics of switching between the two simply by altering
the composition.”
Chrzan is the corresponding author on a paper reporting the results of this
research which has been published in the journal NanoLetters titled “Embedded
Binary Eutectic Alloy Nanostructures: A New Class of Phase Change Materials.”
Co-authoring the paper with Chrzan were Swanee Shin, Julian Guzman, Chun-Wei
Yuan, Christopher Liao, Cosima Boswell-Koller, Peter Stone, Oscar Dubon, Andrew
Minor, Masashi Watanabe, Jeffrey Beeman, Kin Yu, Joel Ager and Eugene Haller.
“What we have shown is that binary eutectic alloy nanostructures, such
as quantum dots and nanowires, can serve as phase change materials,” Chrzan
says. “The key to the behavior 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 stabilizes the amorphous phase,
and also enables us to tune the phase-change material’s transformation
kinetics.”
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,” Chrzan says. “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.”
While they have not yet directly characterized the electronic transport properties
of the bilobed and amorphous BEAN structures, from studies on related systems
Chrzan and his colleagues expect that the transport as well as the optical properties
of these two structures will be substantially different and that these difference
will be tunable through composition alterations.
“In the amorphous alloyed state, we expect the BEAN to display normal,
metallic conductivity,” Chrzan says. “In the bilobed state, the
BEAN will include one or more Schottky barriers that can be made to function
as a diode. For purposes of data storage, the metallic conduction could signify
a zero and a Schottky barrier could signify a one.”
Chrzan and his colleagues are now investigating whether BEANs can sustain repeated
phase-changes and whether the switching back and forth between the bilobed and
amorphous structures can be incorporated into a wire geometry. They also want
to model the flow of energy in the system and then use this modeling to tailor
the light/current pulses for optimum phase-change properties.
The in-situ Transmission electron microscopy characterizations of the BEAN
structures were carried out at Berkeley Lab’s National Center for Electron
Microscopy, one of the world’s premier centers for electron microscopy
and microcharacterization.