Fig. 17 Magnetically responsive photonic nanocrystal arrays. (a) Photographs of Fe3O4@SiO2/PEGDA microspheres and their reflectance spectra showing diffraction peaks at the “on” stage and none at the “off” stage in response to heating–cooling and applied magnetic field. Reproduced with permission from ref. 54. Copyright 2009 American Chemical Society. (b) Photographs of a freestanding polydimethylsiloxane film displaying tunable iridescence in a nonuniform magnetic field. Reproduced with permission from ref. 15. Copyright 2012 American Chemical Society. (c) Reflective spectra of dualcolored photonic Janus balls of superparamagnetic nanoclusters/ polydimethylsiloxane composites; inset showing their corresponding reflective image. Reproduced with permission from ref. 162. Copyright 2012 The Royal Society of Chemistry. (d) Iridescent pattern formed by assembling polystyrene beads on dumbbell-shaped nonmagnetic templates. Reproduced with permission from ref. 163. Copyright 2013 American Chemical Society. (e) Photograph of iridescent letters written on the display manually by using a small magnet as a pen. Reproduced with permission from ref. 164. Copyright 2013 The Royal Society of Chemistry.