UC Riverside

Smart Optical Materials by Nanoscale Magnetic Assembly

Zhiwei Li

Doctor of Philosophy, Graduate Program in Chemistry, University of California, Riverside, Dr. Yadong Yin, Chairperson

The nanoscale magnetic assembly has been demonstrated as a simple yet reliable strategy for preparing responsive optical materials, including photonic and plasmonic nanostructures. Compared with other assembly approaches, the magnetic assembly provides many smart optical materials without the need for complicated surface or property engineering. It can remotely, reversibly, and rapidly control the orientational and/or positional order of nanoparticles, whose secondary structures represent open platforms for creating integrated devices and functional materials.

This dissertation covers a set of concepts and working principles of smart optical materials through the nanoscale magnetic assembly. In the first part, extending magnetic assembly to nanostructures with reduced symmetry represents a simple and effective strategy to create responsive photonic crystals. This is achieved by controlling the easy axis of magnetization in different nanostructures. Our systematical studies of the force dynamics by using finite element analysis appreciate the critical role of shape anisotropy: it decouples the favorable magnetic bonding between interacting colloids from any of their geometric axes, thus creating preferential 1D photonic chains, 2D centered rectangular photonic sheets, and 3D body-centered tetragonal photonic crystals from nanocubes, nanoplates, and nanorods, respectively.

In the second part, a space-confined, seed-mediated growth is used for preparing hybrid magnetic-plasmonic nanostructures. It demonstrates that combining the conventional seeded growth with new nanostructure engineering produces a robust and versatile synthetic strategy for preparing smart materials. The physical properties of hybrid Fe3O4/Au nanorods can be reversibly, rapidly tuned by changing the direction of the applied magnetic field. Fixing the orientation of the hybrid nanorods in the polymer film can be easily achieved by applying a magnetic field during lithography. It enables the programmable design of mechanochromic films with colorimetric responses to linear and nonlinear mechanical perturbations.

In the last part, we propose a background-free bioimaging strategy on the basis of magnetic modulation and fast Fourier transform. The interface-confined growth produces Fe3O4@Au nanorods, which serve as integrated contrast agents. Switching the directions of applied magnetic fields creates bright and deactivated imaging modes. The pixel subtraction or fast Fourier transform is applied to remove noises from any biological and synthetic backgrounds. It has the potentials to remarkably enhance the imaging contrast and specificity in the two near-infrared windows.