(659a) Phase Transitions, Melting Dynamics, and Diffusion in a Nano Test Tube

The geometry of a confined nanoscale material can have a large influence on its physical transformations, and the electron microscope enables the 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. In many cases, when a nanostructure undergoes a phase change, the nanostructure is either destroyed or it changes substantially in shape and size. In order to preserve the geometry of the nanostructure throughout the experiment, a rigid carbon shell was deposited to prevent changes in volume and interfacial area during the heating cycle. The formation of a Au/Ge eutectic mixture was visualized inside of the carbon ?test tube,? occurring 15°C below the bulk eutectic temperature. Capillary pressure pushed the melt into the cylindrical neck of the nanowire, causing the simultaneous crystallization of Ge in the spherical tip of the carbon shell. Diffusion down the length of the confined Ge nanowire was observed at temperatures above 700°C, with diffusion coefficients much lower than those found in bulk Ge crystals.