Although the experimental study of spherical colloids has been extensive, similar studies on rodlike particles are rare because suitable model systems are scarcely available. To fulfill this need, we present the synthesis of monodisperse rodlike silica colloids with tunable dimensions. Rods were produced with diameters of 200 nm and greater and lengths up to 10 μm, resulting in aspect ratios from 1 to ∼25. The growth mechanism of these rods involves emulsion droplets inside which silica condensation takes place. Due to an anisotropic supply of reactants, the nucleus grows to one side only, resulting in rod formation. In concentrated dispersions, these rods self-assemble in liquid crystal phases, which can be studied quantitatively on the single particle level in three-dimensional real-space using confocal microscopy. Isotropic, paranematic, and smectic phases were observed for this system.

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In this paper, the characterization and fluorescent labeling of silica rods are reported. These rods are synthesized following a recently reported method. Material properties of the silica rods measured with NMR, elemental analysis, TGA, and porosimetry are compared with those of well‐established Stöber silica spheres. Additionally, silica rods are made suitable for quantitative real‐space studies by confocal microscopy. Several methods of fluorescent labeling to prepare rods with different fluorescent patterning, ranging from uniform fluorescence levels to gradients from one rod‐end to the other, and even patterns of several colors are presented and discussed.

Binary colloidal crystals offer great potential for tuning material properties for applications in, for example, photonics, semiconductors and spintronics, because they allow the positioning of particles with quite different characteristics on one lattice. For micrometer-sized colloids, it is believed that gravity and slow crystallization rates hinder the formation of high-quality binary crystals. Here, we present methods for growing binary colloidal crystals with a NaCl structure from relatively heavy, hard-spherelike, micrometer-sized silica particles by exploring the following external fields: electric, gravitational, and dielectrophoretic fields and a structured surface (colloidal epitaxy). Our simulations show that the free-energy difference between the NaCl and NiAs structures, which differ in their stacking of the hexagonal planes of the larger spheres, is very small (Ϸ0.002 kBT). However, we demonstrate that the fcc stacking of the large spheres, which is crucial for obtaining the pure NaCl structure, can be favored by using a combination of the above-mentioned external fields. In this way, we have successfully fabricated large, 3D, oriented single crystals having a NaCl structure without stacking disorder.colloidal materials ͉ photonic crystals ͉ self-assembly ͉ surface patternings (epitaxy) R ecently, examples have been shown of the tunability of binary colloidal crystals in several binary systems of nanoparticles (1-5). This tunability of the material properties offers great potential for applications in materials science. However, in many cases, the absolute size of the building blocks is of crucial importance for the functionality of the material. For example, to obtain a photonic band gap in the visible or near-infrared range, which is important for applications in telecommunications, the lattice spacings have to be of the order of 0.1 to 1 m. Unfortunately, for larger colloids, gravity and slow crystallization rates hinder the formation of binary crystals (6-8). We recently found in calculations that a so-called inverse photonic crystal based on the binary sodium chloride has a larger band gap than that of an inverse fcc crystal which until now has been the structure almost exclusively focused on in methods using selfassembly (see SI 1 in the SI Text and Fig. S1). The finding of this property of the NaCl binary lattice motivated us to focus on this binary crystal, but the methods we present for the manipulation of the growth of high-quality binary crystals from dispersions of micrometer-sized spheres are quite general (see also SI 2 in the SI Text and Fig. S2). For instance, the results presented in this paper also directly indicate how to grow other binary crystals as discussed in the conclusion section. We explore here several combinations of external fields to optimize the number of small particles in the NaCl structure and to avoid stacking faults. The binary NaCl crystal structure consists of spheres of two different radii ( L and S ), both ordered in a fcc structure, where the small particle...

Confocal microscopy in combination with real-space particle tracking has proven to be a powerful tool in scientific fields such as soft matter physics, materials science and cell biology. However, 3D tracking of anisotropic particles in concentrated phases remains not as optimized compared to algorithms for spherical particles. To address this problem, we developed a new particle-fitting algorithm that can extract the positions and orientations of fluorescent rod-like particles from three dimensional confocal microscopy data stacks. The algorithm is tailored to work even when the fluorescent signals of the particles overlap considerably and a threshold method and subsequent clusters analysis alone do not suffice. We demonstrate that our algorithm correctly identifies all five coordinates of uniaxial particles in both a concentrated disordered phase and a liquid-crystalline smectic-B phase. Apart from confocal microscopy images, we also demonstrate that the algorithm can be used to identify nanorods in 3D electron tomography reconstructions. Lastly, we determined the accuracy of the algorithm using both simulated and experimental confocal microscopy data-stacks of diffusing silica rods in a dilute suspension. This novel particle-fitting algorithm allows for the study of structure and dynamics in both dilute and dense liquid-crystalline phases (such as nematic, smectic and crystalline phases) as well as the study of the glass transition of rod-like particles in three dimensions on the single particle level.