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Phase Transitions, Melting Dynamics, and Solid-State Diffusion in a Nano Test Tube

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Science  16 Oct 2009:
Vol. 326, Issue 5951, pp. 405-407
DOI: 10.1126/science.1178179

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Confined Germanium Thermodynamics

When a material is confined to nanoscale volumes, the very high proportion of surface to bulk can alter its thermodynamic properties. This has been studied using in situ electron microscopy, but in most cases the volume of the material is not constrained. Holmberg et al. (p. 405) studied the thermodynamics of a germanium nanowire attached to a gold seed and coated with a carbon shell to restrict its volume, measuring the reaction temperature, as well as the liquid composition without changes in volume throughout the heating cycle. This enabled monitoring of phase behavior while the germanium was being heated, and tracking solid-state diffusion across the confined interface.

Abstract

Confined nanoscale geometry greatly influences physical transformations in materials. The electron microscope enables direct visualization of these changes. We examined the evolution of a germanium (Ge) nanowire attached to a gold (Au) nanocrystal as it was heated to 900°C. The application of a carbon shell prevented changes in volume and interfacial area during the heating cycle. Au/Ge eutectic formation was visualized, occurring 15°C below the bulk eutectic temperature. Capillary pressure pushed the melt into the cylindrical neck of the nanowire, and Ge crystallized in the spherical tip of the carbon shell. Solid-state diffusion down the length of the confined Ge nanowire was observed at temperatures above 700°C; Au diffusion was several orders of magnitude slower than in a bulk Ge crystal.

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