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Nanoparticles can self-assemble into quasicrystalline structures, according to researchers in the USA. The newly discovery structures could provide useful insights into how such non-periodic, and yet ordered, materials lying half way between amorphous solids and regular crystals can arise. It may also hint at a new approach to making photonics crystals through self-assembly. Quasicrystals combine indefinite numbers of regularly repeating elements with packing symmetries, like five- and twelve-fold rotations, that are forbidden in classical crystallography. However, scientists have previously only demonstrated their existence in particular systems such as intermetallic compounds based on tantalum telluride, for instance, or tri-block copolymers and branching polymers known as organic dendrimers. Now, chemist Dmitri Talapin and Maryna Bodnarchuk of The University of Chicago, Illinois and colleagues Elena Shevchenko of the Center for Nanoscale Materials, Argonne National Laboratory, Argonne, Illinois, Xingchen Ye, Jun Chen, and Christopher Murray of the University of Pennsylvania, Philadelphia have self-assembled nanoparticles into dodecagonal twelvefold symmetric quasicrystalline superlattices. They were able to do this by fine tuning the sizes of the particles to exploit a previously untested packing motif. "The discovery of quasicrystals in 1984 changed our view of ordered solids as periodic structures and introduced new long-range-ordered phases lacking any translational symmetry," the team says. The researchers add that, "Quasicrystals permit symmetry operations forbidden in classical crystallography, for example five-, eight-, ten- and 12-fold rotations, yet have sharp diffraction peaks." The quasicrystals can be formed from nanoparticles made from several different combinations of materials, the researchers explain. For instance, they have used different binary nanoparticles systems based on 13.4-nm iron(III) oxide (Fe2O3) and 5-nm gold nanocrystals. They produced the quasicrystals through slow evaporation of a colloidal solution containing monodisperse Fe2O3 and Au nanocrystals, capped with oleic acid and dodecanethiol molecules, respectively, in tetrachloroethylene as solvent. It was also possible to make related quasicrystals from 12.6-nm iron(II,III) oxide (Fe3O4) and 4.7-nm gold nanocrystals, and 9-nm lead sulfide and 3-nm palladium nanocrystals. These findings suggest that the formation of such species does not require a unique combination of interparticle interactions. Instead, the team suggests, it is a general phenomenon associated with the packing of the spheres, which is governed by simple interparticle potentials. The team also explains that their structures can also be connected to ordinary crystalline binary superlattices using a thin 'wetting layer'. Such layers were discussed in a distinct context by German mathematician Johannes Kepler in the early 17th century. The arrangement of equilateral squares and triangles is known in topology as Archimedean tiling, the team explains. These were first described by Kepler in 1619 and are defined as regular patterns of polygonal tessellation of a plane by regular polygons where only one type of vertex is permitted in each tiling. At first glance, the Archimedian tilings assembled from nanocrystals may appear to be irregular, however, they show sharp small-angle electron diffraction spots outlining fourfold rotational symmetry, the team explains. "A closer look reveals the packing principle: each iron(III) oxide nanocrystal is surrounded by five equidistant iron(III) oxide nanocrystals. Moreover, identical arrangements of Au nanocrystals surround each iron(III) oxide nanocrystal: the clockwise sequence is always (Au)6, Au, (Au)6, Au, Au," the team says. The researchers add that there are several ways to build square and triangular tiles from spherical particles. Their demonstration is the first of its kind having neither been observed experimentally nor anticipated in quasicrystal models and simulations previously. In terms of applications, the team points out that spherical nanoparticles can be used to model both geometric and electronic properties of individual atoms. Studies of the self-assembly of quasicrystal nanoparticles might therefore provides new insights into the formation of such crystal phases in atomic systems. Alfons van Blaaderen of the Debye Institute for Nanomaterials Science, at Utrecht University, affirms that the discovery of this new approach to quasicrystals might open the way to novel, related materials, such as photonic crystals. The fact that it is the size and shape of the nanoparticles that apparently controls the formation of the specific phases through spherical packing suggests that it might be relatively easy to control the physical properties of such photonic quasicrystals without too much concern for the chemistry. Related links:
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