Nanocrystals (NCs) discussed in this Review are tiny crystals of metals, semiconductors, and magnetic material consisting of hundreds to a few thousand atoms each. Their size ranges from 2-3 to about 20 nm. What is special about this size regime that placed NCs among the hottest research topics of the last decades? The quantum mechanical coupling * To whom correspondence should be addressed. E-mail: dvtalapin@uchicago.edu. † The University of Chicago. ‡ Argonne National Lab. Chem. Rev. 2010, 110, 389–458 389

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Prospects of Colloidal Nanocrystals for Electronic and Optoelectronic Applications

The self-organization of CdSe nanocrystallites into three-dimensional semiconductor quantum dot superlattices (colloidal crystals) is demonstrated. The size and spacing of the dots within the superlattice are controlled with near atomic precision. This control is a result of synthetic advances that provide CdSe nanocrystallites that are monodisperse within the limit of atomic roughness. The methodology is not limited to semiconductor quantum dots but provides general procedures for the preparation and characterization of ordered structures of nanocrystallites from a variety of materials.

https://science.sciencemag.org/content/sci/270/5240/1335.full.pdf

Self-Organization of CdSe Nanocrystallites into Three-Dimensional Quantum Dot Superlattices

Chemical methods developed over the past two decades enable preparation of colloidal nanocrystals with uniform size and shape. These Brownian objects readily order into superlattices. Recently, the range of accessible inorganic cores and tunable surface chemistries dramatically increased, expanding the set of nanocrystal arrangements experimentally attainable. In this review, we discuss efforts to create next-generation materials via bottom-up organization of nanocrystals with preprogrammed functionality and self-assembly instructions. This process is often driven by both interparticle interactions and the influence of the assembly environment. The introduction provides the reader with a practical overview of nanocrystal synthesis, self-assembly, and superlattice characterization. We then summarize the theory of nanocrystal interactions and examine fundamental principles governing nanocrystal self-assembly from hard and soft particle perspectives borrowed from the comparatively established fields of micro...

Self-Assembly of Colloidal Nanocrystals: From Intricate Structures to Functional Materials

Ordered macroporous materials with pore diameters comparable to optical wavelengths are predicted to have unique and highly useful optical properties such as photonic bandgaps1,2,3 and optical stop-bands4. Tight control over the pore size distribution might also lead to improved macroporous materials (those with pores greater than approximately 50 nm) for application as catalytic surfaces and supports5, adsorbents, chromatographic materials, filters6, light-weight structural materials7, and thermal, acoustic8 and electrical insulators9. Although methods exist for producing ordered porous materials with pore diameters less than 10 nm (refs 10, 11), there is no general method for producing such materials with uniform pore sizes at larger length scales. Here we report a new method for producing highly monodisperse macroporous materials with pore sizes ranging from 50 nm to several micrometres. Starting with an emulsion of equally sized droplets (produced through a repeated fractionation procedure12), we form macroporous materials of titania, silica and zirconia by using the emulsion droplets as templates around which material is deposited through a sol–gel process13. Subsequent drying and heat treatment yields solid materials with spherical pores left behind by the emulsion droplets. These pores are highly ordered, reflecting the self-assembly of the original monodisperse emulsion droplets into a nearly crystalline array14. We show that the pore size can be accurately controlled, and that the technique should be applicable to a wide variety of metal oxides and even organic polymer gels.

Ordered macroporous materials by emulsion templating

Crystallization of concentrated colloidal suspensions was studied in real space with laser scanning confocal microscopy. Direct imaging in three dimensions allowed identification and observation of both nucleation and growth of crystalline regions, providing an experimental measure of properties of the nucleating crystallites. By following their evolution, we identified critical nuclei, determined nucleation rates, and measured the average surface tension of the crystal-liquid interface. The structure of the nuclei was the same as the bulk solid phase, random hexagonal close-packed, and their average shape was rather nonspherical, with rough rather than faceted surfaces.

https://science.sciencemag.org/content/sci/292/5515/258.full.pdf

Real-space imaging of nucleation and growth in colloidal crystallization.

Binary mixtures of suspended hard-sphere colloidal particles, radius ratio ${\mathit{R}}_{\mathit{B}}$/${\mathit{R}}_{\mathit{A}}$=0.58, were observed to undergo entropically driven freezing transitions into both the ${\mathit{AB}}_{2}$ and the ${\mathit{AB}}_{13}$ superlattice structures at different relative proportions of the two species. The structures were identified by powder light crystallography and by electron microscopy of the dried solid phases. An approximate (constant volume) phase diagram containing three eutectics was determined. The results are compared with earlier work at size ratio 0.62.

RH Ottewill

Papers

Superlattice formation in binary mixtures of hard-sphere colloids