Novel coating method for artificial opal films and its process analysis

Closely packed colloidal crystals, artificial opal, have been actively studied as materials that exhibit structural colors and photonic band gap. Opal films were self-assembled with mono-dispersed submicron particles. The author proposed a novel and simple method to grow a uniform opal film comprised cubic closely packed, ccp (1 1 1) planes on a 4 in. silicon wafer without using any special equipment. The coating method enables to form colloidal crystal films on a solid substrate in an area over 70 cm2 under silicone oil. The author observed the change of structural color of the colloidal crystal from red to green and Bragg's diffraction peaks of 603.5 nm and 565.7 nm, respectively. This result suggests that colloidal crystal was grown on the substrate under covered silicone oil layer from colloidal suspension. Non-close-packed colloidal crystal was firstly formed during crystallization. Then the distance among the colloidal particles decreased evolution of evaporating water from the non-close-packed colloidal crystal. Finally, closely packed colloidal crystal, opal, films were formed on the substrate after perfectly evaporating water among the colloidal particles. The paper proposes a mechanism of packing process of oil covering method for opal film coating.