Tunable Colloidal Crystalline Patterns on Flat and Periodically Micropatterned Surfaces as Antireflective Layers and Printable–Erasable Substrates

2D nonclose-packed colloidal crystal patterns have received considerable attention in various fields, but it remains a challenge to fabricate patterns and manipulate their geometries regardless of substrate types and structures. Herein, a simple approach is developed for producing nonclose-packed hydrogel colloidal crystalline patterns on flat and periodically micropatterned substrates by exposing close-packed colloidal crystal monolayers to salt aqueous solutions. The patterns are achievable on flat surfaces like silicon, glass, graphene, poly(ethylene terephthalate), and poly(dimethyl siloxane) surfaces. Hydrogel colloidal spheres can deform into disk-like or hemispherical particles on different material substrates. The colloid geometries and dimensions in the patterns are tunable by varying the salt type or concentration. The tunable colloidal crystalline patterns can form on periodically micropatterned substrates, resulting in novel hierarchical crystalline structures. The modification of the method is effective to easily fabricate nonclose-packed hybrid colloidal crystal patterns where plasmonic nanoparticles are selectively assembled on the hydrogel particle surfaces. These patterned substrates modulate colors, reduce reflectivity of visible to near-infrared light, and enhance transmittance. The nonclose-packed colloidal patterns are useful as antireflection coating layers to enhance the solar thermoelectricity efficiencies of flat and micropatterned substrates. The optical tunability of colloidal crystal patterns enables developing colloid crystal-based printable and erasable substrates.