Two-Dimensional Square Lattice of Colloidal Particles Formed by Electrostatic Adsorption in Confined Space

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Abstract
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.

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