Dual-colour generation from layered colloidal photonic crystals harnessing "core hatching" in double emulsions

Colloidal photonic crystals (CPCs), which possess intrinsic colourization characteristics due to their internal periodic structure, have been extensively exploited for various optical applications. However, conventional droplet-based synthesis of CPCs has shown simple structure and mono-colour generating properties since they generally require a protective shell as the outermost confining layer, and the lattice spacing inside the CPCs is invariantly determined, which hinders further tunability for structural and optically responsive properties. To enhance the tunability for structural and optically responsive properties of CPCs, here, we present a means to induce a spontaneous core-hatching process of selectively removing the CPC-encapsulating oil layer in double emulsion droplets by harnessing interfacial instability. By virtue of the reinforced interconnectivity between colloidal polyvinylpyrrolidone (PVP)-decorated polystyrene (PS) nanoparticles, isolated CPCs can retain their structural integrity even without a protective outer shell. As a result, the hatched CPCs can be further re-ordered into a core-shell-like structure having dual internal spacings and dual-colour generation properties. PVP chains decorated on the PS nanoparticles dictate the lattice spacing not only through solvent swelling but also by imparting structural stability via formation of hydrogen bonds. Furthermore, the degree of hatching of CPCs can be modulated to either core-shell-like CPCs (complete hatching) or Janus-like CPCs (partial hatching). Therefore, the work presented here offers a new approach to create uniquely structured and multicoloured CPCs with stimulus responsiveness while fully utilizing the advantages of the double emulsion templating method.