Phase Transitions, Melting Dynamics, and Solid-State Diffusion in a Nano Test Tube
Vincent C. Holmberg,
Matthew G. Panthani,
Brian A. Korgel*
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.
* To whom correspondence should be addressed. E-mail: }{korgel{at}che.utexas.edu}{
