Two-Dimensional Nonclose-Packed Colloidal Crystals by the Electrostatic Adsorption of Three-Dimensional Charged...

We demonstrate that nonclose-packed two-dimensional (2D) colloidal crystals fixed on flat solid surfaces can be obtained by the electrostatic adsorption of three-dimensional (3D) charged colloidal crystals onto oppositely charged substrates. 3D colloidal crystals of negatively charged polystyrene (diameter d = 500 nm) and silica (d = 510 and 550 nm) particles were formed in their aqueous dispersions. Then, a single layer of the 3D crystals (the particle volume fraction = ∼0.07-0.3) was adsorbed onto a glass surface, which was earlier modified with 3-aminopropyltriethoxysilane (APTES), a cationic silane coupling reagent. Under salt-free conditions, the lowermost layer of the 3D crystals, which was oriented parallel to the substrate, was adsorbed onto the substrate surface, forming 2D crystals. Centimeter-sized, large 2D silica crystals were produced by combining a unidirectional 3D crystallization of the silica colloid under a base concentration gradient and a unidirectional adsorption under an acidic concentration gradient, which allowed tuning of the charge number on the APTES-modified substrate. The interparticle separations of the resulted 2D crystals did not vary greatly (within 5%) over a large area (length: 2 cm); however, the separations were smaller than the initial value because of gravitational sedimentation. We also produced 2D crystals of gold particles (d = 250 nm), which we expect to be applicable as plasmonic materials. The present study will provide a facile strategy to produce nonclose-packed 2D colloidal crystals of various types of particles, including large and high-density particles.