Germanium Nanocrystals Embedded in Glass Exhibit Large Melting-Point Hysteresis
- PDF / 87,825 Bytes
- 1 Pages / 576 x 783 pts Page_size
- 100 Downloads / 186 Views
conjugating nanoparticles with biomolecules opens up new possibilities for making functional electronic devices using biomaterial systems.” STEVEN TROHALAKI
Germanium Nanocrystals Embedded in Glass Exhibit Large Melting-Point Hysteresis J.W. Ager III, Q. Xu, D.C. Chrzan, and E.E. Haller of Lawrence Berkeley National Laboratory (LBNL) and the University of California, Berkeley, P. Kluth of the Australian National University, and their colleagues have discovered that nanocrystals of germanium embedded in silica glass do not melt until the temperature rises almost 200 K above the melting temperature of germanium in bulk. These melted nanocrystals have to be cooled more than 200 K below the bulk melting point before they resolidify. 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 mixed with silicon dioxide offers particular advantages for forming optical fibers for long-distance communication.” To exploit these properties means understanding the melting/freezing transition of Ge under a variety of conditions. The researchers embedded nanoparticles averaging 2.5 nm in diameter in silica. As they reported in the October 13, 2006, issue of Physical Review Letters (155701; DOI: 10.1103/PhysRevLett.97.155701), the researchers made silica glass samples 500 nm thick by oxidizing pure silicon wafers in steam. They implanted germanium ions in the amorphous silica and then annealed the sample at 900ºC to form nanocrystals. The transparent glass allowed characterization of the embedded nanocrystals by Raman spectroscopy. The glass was also readily etched away for examination of the nanocrystals with an atomic force microscope. Heating and cooling of the samples were performed in situ in a transmission electron microscope. By thinning the sam-
ples to less than 300 nm, the researchers could observe the electron diffraction rings produced by the crystal lattices of the embedded particles. When the particles began to melt, the diffraction rings weakened and vanished, allowing precise measurement of the temperature at which the embedded particles melted. As the temperature was lowered again, the appearance of the diffraction rings signaled resolidification. 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 nan
Data Loading...
