Colloidal particles with uniform particle sizes are widely used as building blocks for constructing functional optical materials. Colloidal particles dispersed in a solvent self-assemble during an evaporation-induced aggregation process, driven by electrostatic interactions, capillary forces, and van der Waals forces, to form colloidal crystals with long-range order. The resulting colloidal crystals are broadly classified into two types: non-contact colloidal crystals, in which particles are arranged while maintaining a certain distance between them, and close-packing colloidal crystals, in which particles form a close-packed structure. These exhibit different optical properties depending on particle size, interparticle distance, and refractive index contrast. Since colloidal crystals possess periodic structures on the order of light wavelengths, they exhibit unique optical functions such as selective reflection based on Bragg diffraction, stopband gaps (SBG), and even photonic bandgaps, enabling the control of light propagation and localization. Consequently, they are expected to find applications in advanced optical devices such as photonic crystals, optical filters, sensors, and laser resonators. Furthermore, by utilizing selective reflection in the visible light range, they are also being utilized as structural color materials that do not require dyes. On the other hand, to suppress iridescence—a challenge in the practical application of structural color materials—aggregates of colloidal particles with amorphous or glassy structures lacking long-range order have also been developed in recent years. By utilizing scattering phenomena derived from short-range order, these materials can achieve non-iridescent structural colors that are independent of the observation angle, and their application in inks, paints, and cosmetics is currently being pursued. This figure schematically illustrates the process in which colloidal particles serve as the starting point to form ordered structures through self-organization, and the optical functions derived from these structures are utilized to develop structural color materials and photonic crystal materials.

Figure: Concept of nanophotonic material formation via colloidal self-assembly