Magnetic assembly and manipulation of Janus photonic crystal supraparticles from a colloidal mixture of spheres and ellipsoids

Abstract

Photonic crystal supraparticles (PCSs) obtained by self-assembly of colloidal building blocks hold promise for applications in various areas. The majority of PCSs are based on isotropic colloidal spheres, fundamentally limiting their structural and functional diversity. Herein we demonstrate the magnetic assembly of Janus PCSs with increased complexity from a colloidal mixture of nonmagnetic spheres and magnetic ellipsoids. The key to this strategy is the use of non-uniform magnetic fields to simultaneously induce the phase separation of binary particles and assist the subsequent crystallization of anisotropic magnetic ellipsoids. Due to their different crystalline structures, the nonmagnetic and magnetic regions of Janus PCSs exhibit isotropic and anisotropic optical properties, respectively. Moreover, the structural colour can be tuned by magnetically controlling the orientation of supraparticles. Our study not only produces multi-functional PCSs with novel structures and properties but also provides a new model system for understanding the colloidal assembly of binary mixtures.