Scientists at the Lawrence Berkeley National Laboratory in California, USA, 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.1 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.
“Melting and freezing points for materials in bulk have been well understood for a long time,” says Eugene Haller (one of the authors) , “but whenever an embedded nanoparticle’s melting point goes up instead of down, it requires an explanation. With our observations of germanium in amorphous silica and the application of a classical thermodynamic theory that successfully explains and predicts these observations, we’ve made a good start on a general explanation of what have until now been regarded as anomalous events.”
The research was conducted because 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.”
Abstract:1 The melting behavior of Ge nanocrystals embedded within SiO2is evaluated using in situ transmission electron microscopy. The observed melting-point hysteresis is large (±17%) and nearly symmetric about the bulk melting point. This hysteresis is modeled successfully using classical nucleation theory without the need to invoke epitaxy.
WebElements October 9th, 2006