DOI: 10.1002/adma.200800782 Patterned Colloidal Photonic Domes and Balls Derived from Viscous Photocurable Suspensions** By Shin-Hyun Kim, Jong-Min Lim, Woong Chan Jeong, Dae-Geun Choi, and Seung- Man Yang* Many-body colloidal systems have been studied intensively on account of their phase behavior and photonic character- istics. Direct visualization of colloids using optical microscopy makes it possible to analyze colloidal crystal structures with ease; this allows such systems to be used as ‘‘visible’’ models of atomic or molecular assemblies. [1,2] The periodic modulation of the refractive index within a colloidal crystal induces photonic bandgaps at wavelengths comparable to the periodi- city or lattice constant. [3–6] Generally, colloidal crystals are prepared by the self-organization of colloidal particles, and therefore the photonic bandgap properties can readily be controlled by adjusting the size and concentration of the particles. More importantly, the fabrication of colloidal photonic crystals via self-organization is simple and cheap compared with the conventional lithography-based approach. However, evaporation-induced self-assembly, the most pop- ular method for fabricating close-packed colloidal crystals, requires long times and subtle fabrication conditions, and inevitably produces crystals with cracks due to non-uniformities in the capillary force. Although dispersion of charged colloidal particles in an aqueous medium and subsequent infiltration of a non-ionic monomer can produce crack-free non-close packed crystals, this approach still requires complicated processes to achieve large scale production. [7–10] Recently, we developed a novel and simple method for spontaneous crystallization of charged colloidal particles in a photocurable resin. [11] In this method, the viscous suspension is brought into contact with a template and crystallization occurs, starting at the interface between the colloidal suspension and the template. More importantly, the high viscosity of the colloidal suspension and strong interparticle repulsions serve to make the crystal structure resistant to external disturbances. Here, we report a simple and versatile method for patterning photonic domes and balls by dispensing colloidal suspension drops through a capillary nozzle over a substrate. Hemi- spherical domes and spherical colloidal crystal balls have the unique optical property that the stop band for normal incident light is independent of the direction of light propagation. [3,11–15] This property arises because colloidal crystallization occurs from the spherical interface, resulting in concentric particle arrangements. In fact, the interface is the (111) plane of a face-centered cubic (fcc) array and the stop band is the so called L-gap. [16] Therefore, in reflection-mode display applications in which colloidal crystals are used as color pigments, hemi- spherical domes can provide a wider viewing angle than conventional film-type colloidal crystals. Although many research groups have fabricated hemisphe- rical colloidal crystals by evaporation-induced self-assembly to utilize their isotropic optical properties, cracks form in these crystals during evaporation [17] or the hemisphere size is limited to a few tens of micrometers. [18] In addition, the red, green, and blue (RGB) color patterns required for display devices cannot be achieved due to poor processibility and the low surface coverage of hemispheres, the latter arising from the large volume shrinkage that occurs during evaporation. [18,19] From a practical standpoint, coffee-ring effects are a major obstacle for obtaining a hemispherical dome shape and the highly concentrated suspensions tend to undergo evaporation- induced agglomeration, thereby blocking the dispenser nozzle. [20–22] To solve these intrinsic problems, we employed an alternative strategy in the present work in which arrays of hemispherical domes of colloidal crystals are fabricated using colloidal particles dispersed in a refractive-index matching solvent with high polarity. Specifically, we used silica particles (refractive index n p ¼ 1.45) dispersed in the photocurable resin of ethoxylated trimethylolpropane triacrylate (ETPTA) (n m ¼ 1.4689). In our system, electrostatic interactions dom- inate because of diminishing Van der Waals interaction and the silica particles can form various crystalline phases depending on the strength of interactions for a given particle concentra- tion. [2,23,24] Indeed, the electrostatic repulsions between the silica particles cause them to organize into an fcc structure at concentrations as low as f ¼ 0.1, which gives rise to iridescent colors. Moreover, the crystal structures can be captured readily by photopolymerization of ETPTA under UV exposure for less than 1 s. COMMUNICATION [*] Prof. S.-M. Yang, S.-H. Kim, J.-M. Lim, W. C. Jeong National Creative Research Initiative Center for Integrated Optofluidic Systems and Department of Chemical and Biomolecular Engineering KAIST, Daejeon 305-701 (Korea) E-mail: smyang@kaist.ac.kr Dr. D.-G. Choi Nano-Mechanical System Research Center Korea Institute of Machinery and Materials Daejeon 305-343 (Korea) [**] This work was supported by a grant from the Creative Research Initiative Program of the Ministry of Education, Science and Technology for ‘‘Complementary Hybridization of Optical and Fluidic Devices for Integrated Optofluidic Systems.’’ The authors also appreciate partial support from the Brain Korea 21 Program. Supporting Information is available online from Wiley InterScience or from the author. Adv. Mater. 2008, 20, 3211–3217 ß 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 3211