Phase Change Transformations Observed In Nanocrystals By Berkeley Labs

It is a well known fact that a transformation in structure of any material brings about property changes in it. The same is perhaps achieved by Scientists from Lawrence Berkeley National laboratory. The scientists have observed structural changes in a single nanocrystal and this seems to be a ground breaking discovery in the arena of material sciences. The team was led by Berkeley Lab’s director Paul Alivisatos.

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Fast Fourier transform pattern (left) and high resolution TEM images of the low-chalcocite (green) and high-chalcocite (red) domains in a copper sulfide nanorod.

Moreover the scientists also verified the much talked about Phase transition theory which suggest that whenever the transition point for such transformation is reached, the particles oscillates between the two states finally settling down with the most stable configuration. The researchers employed one of the world’s most powerful microscopes called Transmission Electron Aberration-corrected Microscope 0.5 (abbreviated as TEAM 0.5) which can give you a resolution of half Angstrom (0.5X10^-10 m) which even less than a Hydrogen atom. With the aid of TEAM researchers observed a phase change in the crystal of Copper Sulfide (Cu₂S) as the temperature was varied. The copper Sulfide structure transitioned from low-chalcocite which is a complex hexagonal structure to High Chalcocite, a simple hexagonal structure with variation in temperature. TEAM made the scientists observe this transformation with unprecedented details. Scientists believe that such transformations contribute majorly to the alteration of properties of materials.

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TEAM 0.5 micrographs showing the low-chalcocite (left) and high-chalcocite atomic structures of a copper sulfide nanorod.

Speaking about the phase change transformation, Alivisatos said that whenever the thermal energy in the crystal atoms is equal to the energy barrier in the nanorod, a phase change transformation takes place. It is similar to those in solid to liquid or liquid to gas, however Copper Sulfide shows such transformation in its solid state itself. Berkeley labs have two such microscopes named TEAM 0.5 and TEAM 1.0 installed at Berkeley Lab in DOE’s National Center for Electron Microscopy (NCEM).

The study by team suggests various ways either to suppress or promote these phase changes to control the physical properties of materials at structural level. The research is aimed at giving the theorists a base for simulating structural transitions in solids. The paper in this regard titled Observation of Transient Structural-Transformation Dynamics in a Cu₂S Nanorod is published in journal Science.

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