Ferromagnetism revealed in suspensions of magnetic nanoplatelets in liquid crystal
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Ferromagnetism revealed in suspensions of magnetic nanoplatelets in liquid crystal
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nspired by their beautiful color patterns viewed in an optical microscope and their intriguing behavior, Alenka Mertelj from the J. Stefan Institute in Ljubljana, Slovenia, developed an interest in the dynamics of complex fluids. Now she and her colleagues at the Insti-
Transmission electron microscope image of magnetic nanoplatelets. Image credit: Alenka Mertelj.
Soluble 2D supramolecular organic frameworks created
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upramolecular chemistry, in which molecules and molecular complexes are held together by noncovalent bonds, is just beginning to come into its own with the emergence of nanotechnology. Metal– organic frameworks (MOFs) are commanding much of the attention because of their appetite for greenhouse gases, but a new player has joined the field—supramolecular organic frameworks (SOFs). Researchers with Lawrence Berkeley National Laboratory (Berkeley Lab) have unveiled the first known two-dimensional (2D) SOFs that self-assemble in solution, an important breakthrough that holds implications for sensing and separation technologies, energy sciences, and, perhaps most importantly, biomimetics. The researchers report their work in the September 30, 2013 online edition of the Journal of the American Chemical Society (DOI: 10.1021/ja4086935).
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MRS BULLETIN
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VOLUME 39 • FEBRUARY 2014
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tute—Darja Lisjak, Miha Drofenik, and Martin Čopič—have added a new dimension to that complexity: ferromagnetism. Previous to their work, ferromagnetic complex fluids had only been observed at either liquid helium temperatures or above 1000 K. By mixing nanoparticles in a nematic liquid crystal, the research team succeeded in making a roomtemperature fluid ferromagnetic phase, thereby solving a 40-year-old problem. The issue at hand: avoid aggregation of the nanoparticles while maintaining sufficient magnetic interaction between them. Their trick: the use of magnetic nanoplatelets. As reported in the December 12, 2013 issue of Nature (DOI:10.1038/nature12863; p. 237), the researchers found that the platelet shape of barium hexaferrite particles allows for a suitable interplay between the magnetic and nematic-elastic interactions, and combined with quench-cooling of the suspension from the isotropic into the nematic phase, stable, aggregate-free samples are formed.
Traditional molecular chemistry involves strong covalent bonds formed by the sharing or exchange of electrons between the atoms comprising a molecular system. Supramolecular chemistry involves systems that are held together by weaker, noncovalent connections, such as hydrogen bonds, electrostatic, and van der Waals forces. While nature uses supramolecular chemistry to form the double-helix of DNA or to fold proteins, this research team believes that these ideas could also be translated to nanotechnology, where single layers of 2D structurally ordered materials—such as graphene—could fulfill many requirements. The key is that they should be processed in solution. “Solution-b
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