Superheating and supercooling of Ge nanocrystals embedded inSiO2
Free-standing nanocrystals exhibit a size-dependant thermodynamic melting point reduction relative to the bulk melting point that is governed by the surface free energy. The presence of an encapsulating matrix, however, alters the interface free energy of nanocrystals and their thermodynamic melting point can either increase or decrease relative to bulk. Furthermore, kinetic contributions can significantly alter the melting behaviors of embedded nanoscale materials. To study the effect of an encapsulating matrix on the melting behavior of nanocrystals, we performed in situ electron diffraction measurements on Ge nanocrystals embedded in a silicon dioxide matrix. Ge nanocrystals were formed by multi-energy ion implantation into a 500 nm thick silica thin film on a silicon substrate followed by thermal annealing at 900 C for 1 h. We present results demonstrating that Ge nanocrystals embedded in SiO{sub 2} exhibit a 470 K melting/solidification hysteresis that is approximately symmetric about the bulk melting point. This unique behavior, which is thought to be impossible for bulk materials, is well described using a classical thermodynamic model that predicts both kinetic supercooling and kinetic superheating. The presence of the silica matrix suppresses surface pre-melting of nanocrystals. Therefore, heterogeneous nucleation of both the liquid phase and the solid phase are required during the heating and cooling cycle. The magnitude of melting hysteresis is governed primarily by the value of the liquid Ge/solid Ge interface free energy, whereas the relative values of the solid Ge/matrix and liquid Ge/matrix interface free energies govern the position of the hysteresis loop in absolute temperature.
- Research Organization:
- Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States)
- Sponsoring Organization:
- USDOE Director. Office of Science. Office of AdvancedScientific Computing Research. Office of Basic Energy Sciences. MaterialsSciences and Engineering Research; National Science Foundation GrantDMR-0405472
- DOE Contract Number:
- DE-AC02-05CH11231
- OSTI ID:
- 902451
- Report Number(s):
- LBNL-61866; R&D Project: 513310; BnR: KC0201030; TRN: US200717%%336
- Resource Relation:
- Conference: International Conference on Nanoscience andTechnology (ICN+T 2006), Basel, Switzerland,7/30-8/4/2006
- Country of Publication:
- United States
- Language:
- English
Similar Records
In Situ Characterization of Ge Nanocrystals Near the Growth Temperature
Germanium Nanocrystals Embedded in Sapphire