Nanocrystals are hot

[nanoorg]

Nanocrystals are hot

Germanium Nanocrystals Embedded in Glass: They're Hotter Before They Melt and Colder Before They Freeze
Scientists at the Department of Energy's Lawrence Berkeley National Laboratory have discovered that nanocrystals of germanium embedded in silica glass don't melt until the temperature rises almost 200 degrees Kelvin above the melting temperature of germanium in bulk. What's even more surprising, these melted nanocrystals have to be cooled more than 200 K below the bulk melting point before they resolidify. Such a large and nearly symmetrical "hysteresis" - the divergence of melting and freezing temperatures above and below the bulk melting point - has never before been observed for embedded nanoparticles.

Phase transitions between solid and liquid or liquid and vapor are familiar phenomena in the everyday world, for example between solid water ice, liquid water, and water vapor, or steam. Eugene Haller of Berkeley Lab's Materials Sciences Division (MSD), who is also a professor of materials science at the University of California at Berkeley, uses an epicurean example: "When a solid piece of chocolate melts in the mouth, it releases a burst of flavors."

Haller explains that beyond broad scientific interest, the properties of germanium nanoparticles embedded in amorphous silicon dioxide matrices have promising applications. "Germanium nanocrystals in silica have the ability to accept charge and hold it stably for long periods, a property which can be used in improved computer memory systems. Moreover, germanium dioxide (germania) mixed with silicon dioxide (silica) offers particular advantages for forming optical fibers for long-distance communication."

To exploit these properties means understanding the melting/freezing transition of germanium under a variety of conditions. The researchers embedded nanoparticles averaging 2.5 nanometers in diameter (a nanometer is a billionth of a meter) in silica. What they encountered when they heated and cooled this system was completely unexpected. Their results are published in the October 13, 2006 issue of Physical Review Letters.


Source: Research News, Lawrence Berkeley National Laboratory

More information: http://www.lbl.gov/Science-Articles/Archive/MSD-hot-nanocrystals.html



This story was first posted on 10th October 2006.