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Manipulation Under the Microscope


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To perform these experiments, the researchers made silica glass samples 500 nanometers thick by oxidizing pure silicon in steam. They implanted germanium ions in the amorphous silicon and then annealed the sample at 900 degrees Celsius 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 at the Department of Energy’s National Center for Electron Microscopy, based at Berkeley Lab. By thinning the samples to less than 300 nanometers and looking straight through them with the microscope’s electron beam (with the beam itself masked off so as not to hit the camera), 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 finally 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.

“Melting and freezing points for materials in bulk have been well understood for a long time,” says Haller, “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.”