One-Step Discrete Symmetric Arrangement of Magnetic Microspheres with Nanoscale Spacing Immobilized by Ultraviolet Irradiation toward Plasmonic Resonators

Micron-scale paramagnetic spheres are organized into discrete symmetric assemblies in a single step by overcoming the repulsive forces between them and balancing the magnetic attractive forces between the spheres and diamagnetic photoresist molds. Various symmetric arrangements of the spheres are obtained by modifying the shape and depth of the photoresist mold, the size of the spheres, and the magnetic susceptibilities of the paramagnetic spheres and the background medium. Permanent preservation of the assembled structure has been an issue for this type of assembly for years, because the magnetically assembled structures are maintained only under the external magnetic field. In this study, we overcome this issue by simple UV light irradiation during the magnetic assembly so that the surfaces of the sphere and substrate melt and fuse with each other, thus locking the position of the spheres on the substrate surface. This technique allowed permanent fixation of the symmetrically arranged paramagnetic spheres within 60 s, with the nanoscale spacings between adjacent spheres being maintained as narrow as 110 nm. This technique provides access to a wider variety of micro- and nanostructures and is thus useful in many research fields and industries. Particularly, such symmetric metal structures with nanogaps can serve as good media for plasmonic resonators.