How atoms and molecules self-organize into crystals is important in a wide variety of applications from protein formation and biomineralization to ice and snow formation. Crystallization is a two-step process starting with nucleation and followed by growth. Because the nucleation process is understood to be stochastic, experimental studies to visualize it are very challenging. Researchers led by Eiichi Nakamura have now visualized the in situ formation of a NaCl nucleus in a nanocrystal with atomic resolution, confirming a multi-event stochastic process (DOI: 10.1021/jacs.0c12100).A NaCl nanocrystal was formed inside of a vibrating conical carbon nanotube. The crystallization process was studied at 298 K using a transmission electron microscope to create videos to capture the process. The researchers observed a two-step mechanism in which the final cluster formation that produces a crystalline nucleus occurred only after the formation of multiple non-productive semi-ordered clusters, confirming a stochastic process. They also studied the NaCl crystal at a higher 473 K temperature, where the NaCl crystal shrank very quickly. These exquisite videos of nucleation can be used to further understand the crystallization mechanisms prevalent in other processes and provide a dramatic demonstration of a key chemical process that will be valuable for educating students at all levels.
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