Magnetic-Field-Guided Assembly of Anisotropic Nanostructures for Optical Applications

Magnetic-field-guided assembly of colloidal matter has long been regarded as one of the most unique methods for bottom-up fabrication of functional materials, owing to the instantaneous and anisotropic nature of magnetic interactions. The magnetic assembly process is driven by magnetic dipole–dipole interactions which magnitude and direction can be conveniently controlled by the field strength and direction. The collective property of the resultant superstructures can often response to the external magnetic stimuli, allowing the facile design of smart and responsive devices. Tremendous efforts have been made in the development of magnetic-field-guided assembly strategies; however, current magnetic assembly processes were limited to spherical, isotropic building block; while little progress has been achieved to the assembly of anisotropic building blocks, which, however, hold great promise as they often possess shape-dependent physical and chemical properties and add more degrees of freedom to the materials design. In this dissertation, I summarized my explorations on magnetic anisotropic nanostructures, from their controlled synthesis, guided assembly, to the tuning of the resultant superstructures, with a special emphasis on the tuning of their collective optical. To start with, I developed a sophisticated method for the controlled synthesis of one-dimensional magnetic nanostructures, with tunable size, shape, aspect ratio and magnetic property. These magnetic nanostructures have great uniformity and serve as excellent building blocks for magnetic assembly. I studied the assembly behavior of these building blocks, investigated the effects of their morphology, volumetric fraction, and the direction and strength of external magnetic fields, and optimized the assembly process with respect to these factors. After successfully assembling the anisotropic magnetic building blocks into organized structures with positional periodicities and/or orientational orders, I investigated the optical tuning of the resultant superstructures. Owing to the anisotropy of building blocks, these superstructures exhibit unique angular-dependent optical property, which can be controlled by changing the direction of external magnetic fields, as anisotropic magnetic particles spontaneously align themselves parallel to the field direction. I demonstrated this concept by using the magnetic field direction to tune the photonic property, polarity and plasmonic property of as-assembled superstructures. Such method is instant, reversible, contact-less, and only requires a weak magnetic field. It is expected to provide a general and very effective mean to assemble a wide range of nanoscale objects, and paves the way towards fabrication of novel superstructures and design of function devices in many fields