Illinois researchers successful in measuring the “seeds” of crystallization
A novel microscopy technique—fluctuation transmission electron microscopy—allowed researchers at Illinois to detect subcritical nuclei in a glassy material, the first such measurements of the earliest stages of crystallization.
According to Bong-Sub Lee, who is the lead author of an article published this week in Science, this technique could help scientists gain a more complete understanding of how materials crystallize, and that may be the basis for building high-speed computer memories based on crystallization.
“The principal scope of this research is exploring basic science, but the insights from this work will also be valuable for technological development,” said Lee, who is a postdoctoral research associate in the Department of Materials Science and Engineering (MatSE) and the Coordinated Science Laboratory. Although the presence of these "subcritical" nuclei had been predicted decades ago, it has been extremely difficult to observe how they evolve and affect crystallization.
“Phase transformation, such as water turning into ice or graphite into diamond, generally starts with the formation of nanoscopic regions—called nuclei—of the new phase,” Lee explained. “Although the theory of phase transformation is well developed, observing nanoscale nuclei has not been possible except on exposed surfaces.”
The Illinois research team, which included MatSE professor John Abelson and Stephen Bishop, a professor of electrical and computer engineering, combined pulsed laser and electron microscope techniques to reveal the evolution of nanometer-scale nuclei embedded in glassy materials and the role of these nuclei in phase transformation. As a model system for this investigation, the researchers employed phase-change materials that are applied for data storage technology. AIST (an alloy of antimony and tellurium with small amounts of silver and indium) is commercially used for rewriteable CDs and DVDs that can repeatedly record and erase data.
“This and other materials including GST (a germanium-antimony-tellurium alloy) are also vigorously investigated by major companies for developing phase-change random access memory (PCRAM or PRAM)—a promising next-generation non-volatile memory for cell phones, computers, and many other applications,”Abelson explained. “In such a device, recording a bit of data corresponds to the transformation between the glassy and crystalline phases; this is one of the many practical examples where the fundamental understanding of nuclei and phase transformation is important.”