11-06/07 :: June/July 2011
Nanocrystal Transformers // Structural Transformations in Single Nanocrystals © Text: Berkeley Lab
hile a movie about giant robots that undergo structural transformations is breaking box
office records this summer, a scientific study about structural transformations within single nanocrystals is breaking new ground for the design of novel mate- rials that will serve next-generation energy storage batteries and solar energy harvesting devices. Resear- chers at the U.S. Department of Energy (DOE)’s Lawrence Berkeley National Laboratory (Berkeley Lab) have reported the first direct observation of structural transformations within a single nanocry- stal of copper sulfide, a semiconductor expected to play an important role in future energy technologies.
Using TEAM 0.5, one of the world’s most powerful transmission electron microscopes, a research group led by Berkeley Lab director Paul Alivisatos, obser- ved structural fluctuations in a copper sulfide nano- crystal as it transitioned between the low- and high- chalcocite solid-state phases. These fluctuations are highly relevant to understanding such phenomena as how ion transport occurs within electrodes duri- ng the charging and discharging of batteries, or how the structures of a solid material might change at the interface between an electrode and an electrolyte.
“TEAM 0.5, with its advanced electron optics and re- cording systems, enables rapid sample imaging with single atom sensitivity across the periodic table and
greater collection efficiency. This provides extraordi- nary opportunities to study structural transformation dynamics in situ with atomic resolution,” Alivisatos says.
“In this study,” he adds, “we observed structural transformation dynamics in a copper sulfide nano- rod from a low- to a high-chalcocite structure with unprecedented detail, and found these dynamics to be strongly influenced by defects in the nanorod cry- stal. Our findings suggest strategies for suppressing or assisting such transformations that should aid in the future design of materials with new and controlled phases.”
The popular concept of phase transitions is that of a material, in response to temperature changes, under- going a transformation from a solid to a liquid or gas, i.e., ice to water to steam. But some solid materials, especially at the nanoscale, when subjected to tem- perature changes can transition between two more different phases in their crystal structure. Copper sulfide, for example, can be transformed from a complex hexagonal structure known as the low-chal- cocite phase, to a more simple hexagonal structure known as the high-chalcocite phase.
Because such “first-order structural transformations” can alter the properties of a nanocrystal, they are of