Fabricating high-quality opal films with uniform structure over a large area.

Films of opal, a colloidal crystalline lattice with closely packed structure, are anticipated to become a fundamental material in photonic crystal engineering. One of the technological issues is forming the opal film with a flat and uniform surface over a large area. This article describes a new and simple method for forming an opal film without special equipment. The opal film is formed by drying a colloidal suspension covered on a hydrophilic solid substrate. In the conventional method, a ring-shaped opal usually forms at the edge (contact line) of the suspension on the substrate. The new method improved the process of drying the colloidal suspension free from the ring formation. The driving force of this ring formation is based on capillary flow in the suspension from inside to outside because of the high evaporation rate at the contact line. To prevent capillary flow, the contact line of the suspension was covered with hydrophobic silicone liquid. As a result, ring formation was depressed and flat opal films with uniform structure were formed. The structure comprised cubic closely packed (111) planes, and the opal films were grown to grain sizes larger than 200 microm. In addition, the coating area of the opal film was greater than 75 cm2 using a 4-in. silicone wafer. This new method should be useful for coating high-quality opal film over large areas on solid substrates.