Sub‐Micron Resolution, Large‐Scale Magnetic Printing of Orthorhombic Photonic Crystals

ABSTRACT We report direct printing of face‐centered orthorhombic photonic crystals with tunable optical properties and nanometer resolution by assembling magnetic nanorods at dynamic liquid‐solid interfaces. This strategy uses an aqueous dispersion of Fe 3 O 4 @SiO 2 nanorods as a magnetically responsive ink and assembles the nanorods into photonic crystals by applying a magnetic field at the solution's drying front. As water vaporizes, the drying front forms a dynamic liquid‐solid interface that moves in defined directions, while the concurrent application of a magnetic field assembles the nanorods into orthorhombic photonic crystals that rapidly solidify. The shape anisotropy of the nanorods further enables control over crystal orientation, unit‐cell parameters, and optical properties by varying the magnetic field direction. By carefully synchronizing the magnetic field duration and the solvent evaporation rate, we achieve free‐space, large‐scale printing of orthorhombic photonic structures with programmable crystal orientation and tunable structural colors. If microchannel templates are used, we further demonstrate direct printing of complex structural‐color patterns. While photonic structures with tunable crystal domains and orientation can be directly printed on substrates, the use of microchannel templates allows us to design photonic structures and patterns in extended dimensions, showcasing their promise for applications ranging from low‐cost displays and signage to advanced optoelectronic devices.