Synthesis and utilization of monodisperse superparamagnetic colloidal particles for magnetically controllable photonic crystals

We demonstrate fabrication of novel magnetically controllable photonic crystals formed through the self-assembly of highly charged, monodisperse superparamagnetic colloidal spheres. These superparamagnetic monodisperse charged polystyrene particles containing nanoscale iron oxide nanoparticles were synthesized through emulsion polymerization. They self-assemble into crystalline colloidal arrays (CCAs) in deionized water and Bragg diffract visible light. The diffraction from these superparamagnetic CCAs can be controlled by imposition of magnetic fields, which readily alter the CCA lattice constant. We also observe magnetically induced self-assembly of these superparamagnetic particles into CCAs in media such as NaCl aqueous solutions and organic polar solvents, which normally do not permit spontaneous CCA self-assembly. We also find that magnetic fields can strain the face-centered cubic lattice of superparamagnetic CCAs polymerized within hydrogels. The lattice symmetry of this photonic crystal becomes tetragonal. The observed magnetically induced CCA selfassembly enables the development of novel photonic crystal materials and devices.