Available online 22 August 2011.
First order phase transitions are those which involve latent heat, where energy is absorbed or radiated while the temperature remains stable. These transitions range from changes in the state of matter (such as boiling water) to more subtle structural transformations.
As materials undergo such transformations, different regions change before others. Understanding exactly how these transformations progress is critical from both fundamental and technological perspectives.
Using one of the most powerful electron microscopes in the world, a team from Berkeley and Stanford has managed to perform the first study on the dynamics of a single nanocrystal undergoing a structural phase transition [Zheng et al., Science (2011) 333, 206]. Thanks to the high sensitivity and efficiency of the TEAM 0.5 microscope located at the DOE's National Center for Electron Microscopy, they have managed to measure the phase nucleation and propagation, and the dynamic fluctuations.
TEAM 0.5 is an electron microscope developed by the Transmission Electron Aberration-Corrected Microscope research project, as part of an initiative started in 2004 to develop a higher resolution electron microscope. The microscope corrects for the electron lens aberations that normally limit electron microscopes, to provide a resolution of 50 pm.
The team studied a rod shaped nanocrystal of copper sulphide, Cu2S, measuring 5 nm in diameter and around 40 nm in length. At lower temperatures Cu2S possesses a monoclinic structure, but when heated the copper atoms rearrange themselves to produce a simpler hexagonal structure.
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The low temperature domain (green) nucleates and fluctuates within the high temperature matrix (red).
Image courtesy of Haimei Zheng.
First author Haimei Zheng explained the importance of such an observation to Materials Today, “Such [an] observation gives important insights on [the] atomic pathways of phase transitions. Neither a study on bulk materials nor on the ensemble of nanomaterials has the capability of revealing such specific features of the phase transition. In bulk materials, the fluctuations are mostly not observable due to the significantly higher energy barrier [of] the fluctuation. In ensemble studies, only the average characteristics of the fluctuations can be observed, and many important features may be completely obscured.”
The team was able to directly measure how the high temperature phase propagates through the material. They found that after heating (via the electron beam) they were able to observe fluctuations between the two phases before the material stabilized in the high temperature structure. They also found that defects in the structure acted to separate the crystal into different domains, in which the phase transition would propagate with different trajectories.
Zheng revealed that they now hope to image the structural transformations in real time, and “aim to develop [a] microscopic understanding of structural transformations of materials that are important for energy applications.”
Volume 14, Issue 9, September 2011, Page 380