In conclusion, we assembled three-dimensional structures of colloidal particles on a water surface rather than on a planar substrate. The ordered structure was obtained by evaporating the solvent at elevated temperature, and the resulting crystal was transferred onto a planar substrate by simple sedimentation. Importantly, our method can be used to assemble colloidal particles three-dimensionally irrespective of the size of the colloidal particles. Therefore, when a generated colloidal opal structure is infiltrated by semi-conducting materials with large refractive indices and then templated, [6c,10] our method can be used for all-optical integrated circuits, optical filters, and sensors.
Experimental
Preparation of PS Colloidal Particles: a) 240 nm PS Colloid: PS sub-micrometer-sized particles as building blocks of a photonic crystal were synthesized by emulsifier-free emulsion polymerization. Deionized water (450 g) was poured into a reactor, kept at 80 C, and stirred at 350 rpm. Sodium styrene sulfonate (0.3 g) as emulsifier and sodium hydrogen carbonate (0.25 g) as buffer were added to the water. After 10 min, styrene monomer (50 g) was added to this solution. After 1 h, potassium persulfate (0.25 g) as initiator was introduced into the solution. Finally, polymerization was performed under nitrogen atmosphere for 18 h. b) 500 nm PS Colloid: Except for a different content of sodium styrene sulfonate (0.05 g), all the other conditions were kept the same as described under (a). c) 900 nm PS Colloid: Purchased from Aldrich.
PS Colloidal Particles Assembly: A 1 wt.-% PS colloidal solution was poured into a 50 ml Pyrex beaker, which was placed in a convection furnace at 90 C. After obtaining ordered microstructure on the air±water interface, a glass substrate was inserted under a tilted angle. The resulting ordered microstructure was transferred onto a glass substrate as a result of water evaporation.