Assembly of Nanometer-Sized Hollow Sphere Colloidal Crystals for Applications as Tunable Photonic Materials

Electrically responsive photonic crystals, capable of transforming crystal structures and changing intrinsic structural colors in response to external electrical energies, can serve as optically active components for promising technological applications. Unfortunately, the deformation of inverse opal photonic crystals generally weakens the structural stability and leads to poor color tuning repeatability, while most color-tunable colloidal photonic crystals suffer from low color saturation as a result of small refractive index difference between the colloids and matrices. Inspired by cephalopod skins, nanometer-sized hollow silica sphere/poly(3,4-ethylenedioxythiophene)–polystyrene sulfonate photonic crystals are self-assembled using a scalable coating technique. The as-engineered photonic crystals exhibit a conspicuous structural color that is tunable on demand by applying varied voltages. Importantly, their appearance and expanded crystalline lattice can be maintained without any electric field under ambient conditions and simultaneously recovered by applying an oxidation potential. The reversibility and the dependence of hollow sphere size and thickness on electrochromic behaviors are also investigated in this study.