Supercrystals

Thus, we see that the digestive ripening process leads to highly monodispersed nanoparticles that can come together to form ordered superstructures similar to atoms or molecules that form crystals from a supersaturated solution. Then if the superstructure formation can indeed be related to atomic/molecular crystallization, it should also be possible to make these supercrystals more soluble in the solvent with a change of temperature. Indeed, the optical spectra of the three colloids prepared by the different thiols discussed above exhibit only the gold plasmon band at 80 °C suggesting the solubilization of these superlattices at the elevated temperatures [49]. 

Here, we mention self-assembly as one of the methods to synthesize the superlattices or supercrystals of various dimensionality. Self-assembly of colloidal crystals in two-dimensional (2D) and three-dimensional (3D) superlattice occurs spontaneously during solvent evaporation [341, 342]. Self-assembly is controlled by the isotropic van der Waals forces between the particles and ligands. 

FIGURE 13.9 Scanning electron micrograph of PtjNi nano-octahedra forming a supercrystal on a Si substrate, showing the uniform size and shape of the octahedra. Reprinted with permission from Ref. [87]. American Chemical Society. 

An alternative route to particle arrangements beyond dense packings are binary supercrystals. Such structures contain two particle populations, each of which is monodispersed with a different mean particle size. Complex crystals can be formed from such mixtures by concurrent sedimentation [63,64], butthis process is slow and unreliable. Convective particle assembly that exploits convective forces and capillary interactions has been shown to yield large, ordered domains [65] (Figure 9.9). The small particles fill the interstices between the larger ones, with the detailed structure depending both on the particle size ratio Y = s/ t and the ratio of the particle concentrations. Local fluctuations in the concentration ratio lead to domains of different structure [66]. Mixtures with y > 0.3 do not assemble into binary crystals in... 

Figure 9.9 Binary supercrystals assembled from polystyrene and silica particles having different diameters. Panels (a-c) are micrographs obtained with different diameter ratios. (Reproduced with permission from [65]. Copyright (C) (2003) Wiley-VCH Verlag GmbH Co. KCaA.)...

An ordered arrangement of particles, colloidal crystals, is found in a wide range of scales. Opal is a typical colloidal crystal with an ordered arrangement of silica particles." Photonic crystals have been developed for the control of optical properties." A variety of supercrystals and superlattices consisting of nanoparticles are fabricated through self-assembly." When the unit particles are an amorphous material and the crystal lattices of each unit particle are not oriented, the colloidal assembly is not regarded as a mesocrystal (Fig. Ig). In contrast, colloidal crystals... 

The crystallographic direction of the unit particles is oriented in these colloidal crystals. Therefore, these supercrystals are one of mesocrystals comprised of the isolated nanoscale units. These findings suggest that the self-assembled oriented architectures are easily formed by the faceted polyhedral units with the surface modification by the organic molecules. The shapes of the unit crystals are involved in the geometrical packing state. 

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