Lithographically-featured colloidal photonic crystals and applications

Abstract

Colloidal crystals, periodic structures composed of colloidal particles, have been re-ceiving increasing attention in materials chemistry owing to their unique optical properties as 3D photonic crystals. However, conventional approaches for creation of such colloidal photonic crystals, including vertical deposition and spin-casting, lack in physical stability of the materials and have a difficulty in micro-patterning, thereby severely limiting practical usages. In this thesis, I propose a novel platform to produce robust colloidal photonic crys-tals which can be patterned by simple photolithography. To achieve this, monodisperse silica particles are dispersed in photocurable resin and the suspension is used for crystal formation and structural fixation. The silica particles dispersed in the resin are spontaneously crystallized under assistance of shear flow owing to repulsive interparticle potential and the crystal structures are easily solidified through UV-induced polymerization of the resin. The resultant colloidal photonic crystals are highly transparent due to low contrast in refractive index between the particles and the medium. This enables the production of multiple layers of photonic crystals and therefore the generation of multiple distinct reflectance peaks. In addition, the photonic crystals can be patterned by photolithography. Selective irradiation of UV light through photomask results in photonic crystal micro-pattern after development of unpolymerized suspension. These multi-colored and micro-patterned colloidal photonic crystals are used for making security materials for anti-counterfeiting

this is potentially useful for reflection-mode color display.

structural colors from colloidal photonic crystals are difficult to duplicate and strong and narrow bandwidth of multiple peaks provide high optical selectivity. Photolithographically-designed micropattern can be released from substrate to pro-vide freely-suspended microparticles with disk shape. Moreover, multiple steps of photoli-thography may provide photonic microparicles composed of multi-layers. Exploiting these features, multi-layered microparticles with circular-disk shape are prepared. The photonic microparticles can be further functionalized by making magnetic layer which makes overall microparticles magneto-responsive. Such field-responsive photonic disks can be used as active color pigments for microdisplay operated at reflection mode. For example, tri-layered photonic microdisks can be designed to have two distinct photonic crystal layers at the top and bottom and magnetic layer at the middle. The microdisks align their magnetic moment along the direction of external magnetic field, thereby enabling the manipulation of the flipping motion. Therefore, reflection colors from the photonic disks can be switched from one to the other