Magnetically Tunable Photonic Nanostructures: Making Colors with Magnets

Superparamagnetic iron oxide colloidal particles have been self-assembled into ordered photonic crystal structures in solution phase using external magnetic fields. The colloids form chain-like structures with regular interparticle spacings of a few hundred nanometers along the direction of the external field so that the system strongly diffracts visible light. The balance between attraction (in this case, magnetic dipole interaction) and repulsion (such as electrostatic and solvation force) dictates interparticle spacing and therefore optical properties. By changing the relative strength of these two forces, we can tune the peak diffraction wavelength over the entire visible spectrum. We have been able to optimize the diffraction intensity and the tuning range through studying their dependence on variables such as the size distribution and concentration of the iron oxide colloids or ionic strength of the solutions. By controlling the surface properties of the magnetic particles (so that the repulsive forces involved), we have been able to assemble the photonic structures in water, alcohols, and nonpolar solvents. The fast, reversible response and the feasibility for miniaturization impart these photonic materials great potential in applications such as optoelectronic devices, sensors, and color displays. In this presentation, we will discuss our recent success in direct imaging of the assembled structures using the electron microscopes. We will also demonstrate high resolution patterning of multiple structural colors by a single material, the color of which is magnetically tunable and lithographically fixable. The fabrication of color and shape coded micro-objects by using this unique photonic material will also be discussed.