Researchers observe real-time homogeneous nucleation using cryo-TEM
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M tomogram showed that the MnPFOM molecules nucleated and grew in a two-dimensional (100) plane aligned with the x, y directions of the TEM foil. In the CsCl solution, the Cs+ cations induced a dense phase in which the MnPFOM molecules formed a nonordered aggregate in which they occupied positions close to, but not commensurate with, their ideal crystal lattice positions; when heated, this aggregate collapsed into an ordered lattice. Crystalline structure was determined by comparison with an x-ray diffraction structure. “The important thing about these compounds is that we can really control the formation of different types of phases, and this control allowed us to do this research,” Neumann says. “With Cs cations you get amorphous aggregates and with Na cations you get purely homogeneous nucleation.” The two-step aggregation-crystallization process of the Cs solution has a lower overall energy barrier than the one-step classical homogeneous spontaneous random aggregation of molecules in the Na solution, so the two-step process is faster, as predicted. Being able to observe this process with TEM could have implications down the road for a better understanding of protein crystallization and self-assembly of molecules, according to Neumann. “In recent years, evidence of nonclassicial nucleation pathways (often known as two-step nucleation models) has appeared in the literature of ionic systems, small molecule organic systems, and proteins and also has been supported by molecular modeling studies,” says Allan S. Myerson, professor of the practice of chemical engineering in the Department of Chemical Engineering at the Massachusetts Institute of Technology, who was not involved in this work. “In this new research, the authors experimentally observed nucleation using cryo-TEM. Their work demonstrates that both classical and nonclassical nucleation mechanisms can occur depending on the cation present. This observation demonstrates the complexity of nucleation and that nucleation pathways can depend on the nature of the substance involved.” Tim Palucka
• VOLUME 42 • FEBRUARYIP • www.mrs.org/bulletin 2017 Downloaded MRS fromBULLETIN https://www.cambridge.org/core. address: 80.82.77.83, on 24 Aug 2017 at 08:50:40, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1557/mrs.2017.10
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