Magnetic assembly approach to hierarchical photonic stripes

Abstract

Magnetic north and south poles attract, leading to the well-known end-on attachment between two magnets. In this work, we leverage distinct sizedependent dipole-dipole interactions between nanorod magnets-magnetized nanorods-to assemble hierarchical photonic stripes and provide an effective mechanism to prepare complex photonic materials with precisely tunable structural colours. Under a magnetic field, Fe3O4@SiO2 nanorods are magnetized with an induced parallel magnetic dipole inside, favour a sizedependent equilibrium bonding state, and then assemble along a critical angle into an off-set packing instead of the end-on attachments. Our experimental results and computational model indicate that magnetic nanorods have a different bonding mode than that of magnetic nanospheres, underpinning the important role of magnetic anisotropy in determining the nanoscale magnetic interactions, structural symmetry, and photonic properties. This scale-dependent interaction produces tetragonal photonic crystals, which further assemble into hierarchical photonic stripes driven by magnetic repulsion between crystals at a high concentration under the assistance of a template. This work provides an assembly approach to fabricating photonic materials and devices that have orders across different length scales based on unique interactions between nanostructured materials.