One of the main reasons for the current interest in colloidal nanocrystals is their propensity to form superlattices, systems in which (different) nanocrystals are in close contact in a well-ordered three-dimensional (3D) geometry resulting in novel material properties. However, the principles underlying the formation of binary nanocrystal superlattices are not well understood. Here, we present a study of the driving forces for the formation of binary nanocrystal superlattices by comparing the formed structures with full free energy calculations. The nature (metallic or semiconducting) and the size-ratio of the two nanocrystals are varied systematically. With semiconductor nanocrystals, self-organization at high temperature leads to superlattices (AlB 2 , NaZn 13 , MgZn 2 ) in accordance with the phase diagrams for binary hard-sphere mixtures; hence entropy increase is the dominant driving force. A slight change of the conditions results in structures that are energetically stabilized. This study provides rules for the rational design of 3D nanostructured binary semiconductors, materials with promises in thermoelectrics and photovoltaics and which cannot be reached by any other technology.KEYWORDS Nanocrystals, self-assembly, superlattices, hard spheres, thermodynamics. C olloidal, monolayer-stabilized nanocrystals (NC) can be synthesized with a nearly spherical shape and with a well-defined diameter that does not vary by more than 5% in the sample. Because of their monodisperse size and shape, such NCs show a strong propensity to assemble into NC superlattices.1 More than a decade ago, the formation of single-component superlattices that consist of CdSe semiconductor NCs was reported.1 This was followed by the demonstration of binary superlattices, consisting of NCs of different diameters and different nature, that is, semiconductor, metallic, or magnetic. [1][2][3][4][5][6][7][8][9][10][11][12][13] In such systems, novel collective properties can arise from the (quantum mechanical) interactions between the different components that are in close contact in a three-dimensional (3D) ordered geometry. Some striking examples have already been reported. 3,4,14,15 For instance, a binary superlattice of two types of insulator nanoparticles is found to become conductive due to interparticle charge transfer.3 Colloidal crystallization is the only method known to date to obtain nanostructured systems with order in three dimensions. However, for further progress in the field of designed nanostructured materials, improved control of nanocolloid crystallization is a key factor.Apart from the viewpoint of emerging nanomaterials, the formation of (binary) NC superlattices is of strong interest in colloidal science. Nanocrystals have a mass that is about a million times smaller than that of the colloidal particles commonly used in crystallization studies; hence, their thermal velocity is about a factor of thousand higher. This agrees with the fact that nanocolloid crystallization is a relatively fast process, still oc...

The vertical deposition technique for creating crystalline microstructures is applied for the first time to nonspherical colloids in the form of hollow silica cubes. Controlled deposition of the cubes results in large crystalline films with variable symmetry. The microstructures are characterized in detail with scanning electron microscopy and small-angle X-ray scattering. In single layers of cubes, distorted square to hexagonal ordered arrays are formed. For multilayered crystals, the intralayer ordering is predominantly hexagonal with a hollow site stacking, similar to that of the face centered cubic lattice for spheres. Additionally, a distorted square arrangement in the layers is also found to form under certain conditions. These crystalline films are promising for various applications such as photonic materials.

We have investigated the effect of particle shape in Pickering emulsions by employing, for the first time, cubic and peanut-shaped particles. The interfacial packing and orientation of anisotropic microparticles are revealed at the single-particle level by direct microscopy observations. The uniform anisotropic hematite microparticles adsorb irreversibly at the oil-water interface in monolayers and form solid-stabilized o/w emulsions via the process of limited coalescence. Emulsions were stable against further coalescence for at least 1 year. We found that cubes assembled at the interface in monolayers with a packing intermediate between hexagonal and cubic and average packing densities of up to 90%. Local domains displayed densities even higher than theoretically achievable for spheres. Cubes exclusively orient parallel with one of their flat sides at the oil-water interface, whereas peanuts preferentially attach parallel with their long side. Those peanut-shaped microparticles assemble in locally ordered, interfacial particle stacks that may interlock. Indications for long-range capillary interactions were not found, and we hypothesize that this is related to the observed stable orientations of cubes and peanuts that marginalize deformations of the interface.

Particular types of solid-stabilized emulsions can be thermodynamically stable as evidenced by their spontaneous formation and monodisperse droplet size, which only depends on system parameters. Here, we investigate the generality of these equilibrium solid-stabilized emulsions with respect to the basic constituents: aqueous phase with ions, oil, and stabilizing particles. From systematic variations of these constituents, we identify general conditions for the spontaneous formation of monodisperse solid-stabilized emulsions droplets. We conclude that emulsion stability is achieved by a combination of solid particles as well as amphiphilic ions adsorbed at the droplet surface, and low interfacial tensions of the bare oil-water interface of order 10 mN/m or below. Furthermore, preferential wetting of the colloidal particles by the oil phase is necessary for thermodynamic stability. We demonstrate the sufficiency of these basic requirements by extending the observed thermodynamic stability to emulsions of different compositions. Our findings point to a new class of colloid-stabilized meso-emulsions with a potentially high impact on industrial emulsification processes due to the associated large energy savings.

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