Junpei Yamanaka

In this study, we demonstrate a novel and efficient fabrication methodology for nonclose-packed, two-dimensional (2D) colloidal crystals exhibiting square lattice structures. In our recent work, we detailed the formation of 2D colloidal crystals via the electrostatic adsorption of three-dimensional (3D) charged colloidal crystals onto oppositely charged substrates. These 3D colloidal crystals possessed a face-centered cubic (FCC) lattice structure with their (111) planes aligned parallel to the substrate, facilitating the formation of 2D crystals with triangular lattice arrangements upon adsorption. This work presents the synthesis of 2D crystals with square lattices─a configuration widely used in photonics. We prepared 3D colloidal crystals of silica particles with four-fold symmetry in a micrometer-scale gap between two coverslips. The bottom glass surface is modified with a cationic silane coupling reagent, aminopropyltriethoxysilane, generating pH-responsive charge characteristics with an isoelectric point (iep) near pH 8. When the pH is greater than iep, the surface is charged negatively. As pH decreases below iep, the sign of the surface charge reverses to positive. Controlled pH lowering below the iep induces adsorption of the lowermost lattice plane of 3D crystals onto the substrate, yielding 2D crystals with a distinct square lattice. We further synthesized three-layer body-centered cubic (BCC) structures by stacking alternating layers of the 2D square lattices of silica and polystyrene particles. By aligning the refractive index of the surrounding medium (aqueous solution of ethylene glycol) with that of silica particles, we successfully fabricated a structure that is optically identical to a simple cubic lattice. These findings advance the development of 2D crystalline materials for photonic and plasmonic applications.

We present a novel methodology for immobilizing 2D nonclose-packed colloidal crystals in polymer resins. In our previous work (Aoyama Y, Toyotama A, Okuzono T, Yamanaka J. Two-Dimensional Nonclose-Packed Colloidal Crystals by the Electrostatic Adsorption of Three-Dimensional Charged Colloidal Crystals. 2019. Langmuir 35, 9194.), we detailed a fabrication technique involving electrostatic adsorption of 3D charged colloidal crystals onto a glass substrate with an opposite charge. Drying is essential for producing durable materials as these structures are formed in an aqueous environment. However, merely drying the crystals can lead to considerable disruption of the particle arrangement because of the capillary forces that arise between the particles during water evaporation. In this study, we demonstrate the immobilization of 2D colloidal silica crystals in a polymer resin. First, we introduced a water-soluble polymer, poly(dimethyl acrylamide), which effectively adsorbs both the particles and the substrate to stabilize the crystals, followed by the evaporation of water. Various configurations of 2D colloidal silica crystals were successfully fixed onto the glass substrates. After immobilization with poly(dimethyl acrylamide), the dimethyl acrylamide monomer was added and underwent polymerization, resulting in a more robust entrapment of the colloidal crystals in the polymer resin. This method can be applied to other polymers and hydrophobic particles. The current findings are expected to be valuable for a broad range of photonic applications.

We report on light refraction by titania particles with a high refractive index (approximately 2.4). Clusters of titania particles modified using a fluorescent dye, rhodamine B isothiocyanate, and nonfluorescent titania particles were prepared. When the clusters were irradiated using light at the excitation wavelength of the fluorescent dye, the edges of the bound nonfluorescent particles glowed brightly. Geometric optics and finite difference time domain calculations revealed that this phenomenon was due to a lens effect caused by titania particles.

We report here that a two-dimensional diamond-like structure of micron-sized colloidal particles can be obtained by layer-by-layer self-assembly.