Colloidal particles may be considered as building blocks for materials, just like atoms are the bricks of molecules, macromolecules, and crystals. Periodic arrays of colloids (colloidal crystals) have attracted much interest over the last two decades, largely because of their unique photonic properties. The archetype opal structures are based on close-packed arrays of spheres of submicrometer diameter. Interest in structuring materials at this length scale, but with more complex features and ideally by self-assembly processes, has led to much progress in controlling features of both building blocks and assemblies. The necessary ingredients include colloids, colloidal clusters, and colloidal "molecules" which have special shapes and the ability to bind directionally, the control over short-range and long-range interactions, and the capability to place and orientate these bricks. This Review highlights recent experimental and theoretical progress in the assembly of colloids larger than 50 nm.

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Abstract. Porous silica, structured as three-dimensionally ordered macroporous (3DOM) photonic crystal particles, is investigated as a pigment material with structural color. Brightly and uniformly colored 3DOM SiO 2 powders were obtained by templating of a tetraethoxysilane-containing precursor in polymeric colloidal crystals, pyrolysis in an inert atmosphere to maintain a small amount (ca. 5-8 wt %) of residual carbon as a background absorber, and particle size reduction. The color depended on placing optical stop bands in the appropriate range of the visible spectrum. This was achieved through selection of the polymer sphere diameter in the colloidal crystal template and finetuning by the pyrolysis temperature, which controls the extent of condensation and shrinkage of the 3DOM SiO 2 structure. The stop bands

Three-dimensionally ordered macroporous (3DOM) materials prepared by colloidal crystal templating are examples of photonic crystals that can exhibit structural color. The color intensity can vary widely, from a pale, nearly white opalescence to vivid, brilliantly metallic colors. Such variations are observed even for 3DOM materials of a single nominal composition that exhibit virtually identical structural order in scanning electron micrographs and are prepared from the same colloidal crystal templates. In this study we investigate the cause of the variations in color intensity for 3DOM ZrO 2 systems, considering both the role of zirconia grains in the skeleton of the photonic crystal and the presence or absence of carbonaceous components in the material. Such components act as broad spectral light absorbers and are introduced either directly in the synthesis through the precursor and the polymeric template or by postsynthesis addition and carbonization of sucrose solutions. We conclude that grain-size effects do not play a significant role but that the carbon content in 3DOM ZrO 2 provides direct control over the intensity of structural color in these photonic pigment materials.

Kolloidale Partikel können genau wie Atome (die Bausteine der Moleküle, Makromoleküle und Kristalle) als Baueinheiten für Materialien betrachtet werden. Periodische Anordnungen von Kolloiden – kolloidale Kristalle – haben in den letzten 20 Jahren viel Aufmerksamkeit erregt, im Wesentlichen wegen ihrer einzigartigen photonischen Eigenschaften. Die archetypischen Opalstrukturen beruhen auf dicht gepackten Anordnungen von Kügelchen mit Durchmessern im Submikrometerbereich. Das Interesse an der Strukturierung von Materialien in diesem Größenbereich, aber mit komplexeren Eigenschaften und idealerweise durch Selbstorganisationsprozesse, hat zu großen Fortschritten bei der Kontrolle der Eigenschaften sowohl der Baueinheiten als auch ihrer Anordnungen geführt. Erforderlich dafür sind Kolloide, kolloidale Cluster und kolloidale “Moleküle” mit speziellen Formen und der Fähigkeit zur Bildung einer gerichteten Bindung, eine Kontrolle über kurz‐ und langreichweitige Wechselwirkungen sowie Möglichkeiten zur Positionierung und Orientierung dieser Bausteine. Dieser Aufsatz beleuchtet jüngste experimentelle und theoretische Fortschritte bei der Organisation von Kolloiden mit einer Größe von mehr als 50 nm.