Magnetic Route To Photonic Crystals

From magnetic drawing toys to grade-school science demonstrations, tiny particles of iron oxide can be delightfully entertaining when subjected to magnetic forces. Modify the particles a bit via chemical means and the stuff of child's play can turn into building blocks of future high-tech devices.

That strategy is being pursued by a team of California scientists who recently developed a synthesis for preparing self-assembling polymer-capped magnetite (Fe3O4) nanoclusters of uniform size. As a result of the clusters' highly charged polymer coating and the core particles' strong magnetic character, exposing an aqueous suspension of the clusters to a magnetic field causes the nanoparticles to come together in the form of colloidal photonic crystals—well-ordered assemblies of colloidal particles that selectively diffract light of specific wavelengths (Angew. Chem. Int. Ed. 2007, 46, 7428).

The crystals may come in handy as photonics researchers aim to develop new types of circuits and high-tech instruments that relay signals among circuit components by light beams—as opposed to electric currents, which flow through all electronic devices. Optoelectronic devices, which are driven by a combination of light and electronic signals, are already commonplace in telecommunications and other areas.

But additional applications—for example, new types of flexible color displays, sensors, and devices controlled entirely by light—might be made possible by photonic crystals. These materials restrict the flow of photons of selected wavelengths (those that fall within an optical bandgap), much as semiconductors restrict the the flow of electrons with energies that lie in an electronic bandgap. Depending on the intended application, bandgaps in semiconductors can be tailored by modifying the composition of the material via doping or in other ways. Until now, researchers have had little success in controlling bandgaps in colloidal crystals.

The new synthesis, which was developed at the University of California, Riverside, by assistant chemistry professor Yadong Yin, postdoc Jianping Ge, and graduate student Yongxing Hu, takes a step in that direction by providing a simple and inexpensive way to make photonic crystals that can be tuned to diffract light across the entire visible spectrum.

By hydrolyzing FeCl3 at high temperature in the presence of polyacrylic acid, the team forms crystallites of Fe3O4—roughly 10 nm in diameter—which in turn assemble into uniformly sized polyacrylate-capped clusters ranging in size from 30 to 180 nm in diameter. The cluster size is controlled synthetically. Then by subjecting the colloidal clusters to a magnetic field—from a bar magnet in this study—the researchers cause the particles to order into a photonic crystal.

Evidence of the ordering is easily observed with the naked eye. Bringing a magnet close to a vial containing the colloidal suspension induces an immediate and reversible color change. In addition, by carefully adjusting the distance between the magnet and the vial—which alters the internal crystal spacings and thus selects the wavelengths that are diffracted efficiently and those that aren't (the ones that lie in the bandgap)—the Riverside group can tune the material to exhibit every color of the rainbow in a controlled manner.

At the heart of the crystals' three-dimensional ordering is a delicate balance between interparticle forces, Yin explains. On one side of the equation, polymer carboxylate groups that assemble on the clusters' surfaces render the particles highly charged and repulsive to one another. On the other side, the clusters are drawn together by strong magnetic forces, but only under the influence of an external magnetic field. In the absence of the field—when the magnet is moved away—the clusters remain well-dispersed, a quality required for reversibly forming tunable colloidal crystals.

That field dependence is a manifestation of the clusters' paramagnetic nature, which in turn is a consequence of the synthesis technique. As Yin explains, the clusters are paramagnetic because they are composed of discrete 10-nm magnetite crystallites. If the method yielded individual crystals with diameters larger than a critical size of roughly 30 nm, then the basic building blocks and the clusters they would form would exhibit ferromagnetism, the kind that makes certain magnets stick to refrigerators. In that case, Yin says, the particles would attract one another, agglomerate into large chunks, and fall out of solution.

Yin adds that it took a while for the group to strike the necessary balance between forces. "At first, we didn't see the optical response we were looking for," he acknowledges. Eventually the team learned that they needed to wash the product repeatedly to remove excess surfactant and ions. That procedure reduced the ionic strength of the solutions and increased electrostatic repulsion between the clusters.

Orlin D. Velev, an associate professor of chemical and biomolecular engineering at North Carolina State University, Raleigh, comments that tuning the crystal spacing with a magnetic field is "elegant and innovative." The work is also important, he says, because understanding how to control the bandgap in colloidal photonic crystals has the potential to bring about truly usable precursors for a variety of photonic devices.