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Nanocrystals Are Hot

 

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.

How Materials Melt and Freeze

For almost a hundred years, theorists and experimenters have studied how the size of a crystal affects melting and freezing, the transition between the liquid and solid state of a material.  For most crystalline materials, the smaller the size, the lower the melting temperature. The melting temperature of a free-standing metal or semiconductor nanocrystal, typically comprised of a few hundred to a few thousand atoms, may be more than 300 degrees Kelvin below the melting temperature of the same material in bulk.

The reason for this, says Joel Ager of MSD, a coauthor of the Physical Review Letters report, is that “the smaller a solid object gets, the larger the percentage of its atoms residing at the surface. If it keeps shrinking, eventually it’s practically all surface.” Inside a crystalline solid the atoms are constrained by the crystal lattice, “but at the surface the atoms have more freedom to move. As the temperature increases, they begin to vibrate; when the vibration of the surface atoms reaches a certain percentage of the bond length between them, melting begins and then starts to propagate through the solid.”

Beginning in the 1950s, methods for accurately measuring the melting of crystalline solids were developed, and at the same time theories of melting and freezing became more sophisticated.

“Melting and freezing begin at the interface between the surface of the solid and its surroundings,” says theorist Daryl Chrzan of MSD, also a professor of materials science at UC Berkeley. “The solid phase has a certain free energy, the liquid another, vapor yet another, and interfaces between these phases have their own characteristic energies. The likelihood of a phase transition occurring in one direction or the other can be calculated based on the free energies of the material phases themselves and their interface energies, taking into account volume, geometry, density, and other factors.”

For most materials, interface energies between solid and vapor — for example, a bar of gold in air — favor the formation of a liquid surface layer as the temperature increases, which continues to grow until the entire object is melted; this liquid layer forms more readily at lower temperatures as the proportion of surface to volume increases. Haller notes that “if you make free-standing nanoparticles of gold small enough, they melt at room temperature.”

Embedded nanocrystals occasionally behave differently, however. Superheating has been observed in the case of nanocrystals embedded in a crystalline matrix, for example nanoparticles of lead embedded in an aluminum matrix. This is attributed to the lattice structures of the two crystals “locking up,” suppressing the vibration of the nanoparticles’ surface atoms that would lead to melting.