PerspectiveMaterials Science

Viewing the Seeds of Crystallization

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Science  13 Nov 2009:
Vol. 326, Issue 5955, pp. 942-943
DOI: 10.1126/science.1182817

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All materials search for the lowest accessible energy state. As temperature is increased in disordered materials, atoms diffuse and explore different chemical and structural configurations. Crystalline phases may be favored, but a very small crystal is unstable, so there is a “nucleation barrier” to overcome; only after reaching a critical size can the nucleus grow. Although we understand the thermodynamics of the nucleation process [e.g., (1)], observation of the actual atomic-scale complexity during nucleation has remained elusive, despite its importance to the properties of materials. Taking nucleation out of the “black box” is one of the grand challenges to “materials by design” that is seen as the future solution to major societal problems such as sustainable energy (2). On page 980 of this issue, Lee et al. (3) use fluctuation electron microscopy to image subcritical nuclei in a solid material, observing metastable structural states that facilitate later nucleation in amorphous films. Their study is applied to a technologically important case of “phase-change memory” and therefore may facilitate efforts to design faster higher-density nonvolatile memory.