A research team headed by Yadong Yin at the University of California, Riverside has created a liquid that changes its color “on demand” and can take on any color of the rainbow.
Nanoscopic particles made of tiny magnetic crystals coated with a plastic shell self-assemble in solution to form photonic crystals—semiconductors for light. When a magnetic field is applied, the optical properties of the crystals change, allowing their color to be very precisely adjusted through variation of the strength of the field.
The crystals involved are not “conventional” lattices of ions or molecules like the ones for salt. They are colloidal crystals, periodic structures that form from uniform solid particles that are finely dispersed in a liquid. Colloidal crystals can be produced at little cost and on a large scale—and can be used as photonic crystals.
Photonic crystals are the optical analogues of electronic semiconductor materials. Like their electronic counterparts, they have photonic band gaps, forbidden energy levels, or wavelengths, at which the photonic crystal does not transmit light. These optical properties depend on the spatial relationships within the crystal.
Current research is concerned with photonic crystals whose forbidden bands are variable and can be adjusted quickly and precisely in response to an external stimulus. These requirements have been impossible to meet until now.
One stimulus they used is a magnetic field, using crystals made of magnetic materials, in this case iron oxide. Previously the problem with that solution is that magnetization is maintained when the particles grow into larger domains (ferromagnetism).
Yin and his team have found a solution: They coated nanoscopic iron oxide particles with a plastic called polyacrylate. This results in separate clusters of nanocrystals, which self-assemble in solution to form colloidal photonic crystals. The forces of the magnetic field affect every individual cluster, changing the cluster-to-cluster distances within the crystal lattice.
Depending on the distance from the magnet and thus the field strength, the color of the colloidal crystal changes right across the whole visible spectrum. This response is rapid and fully reversible because the nanocrystals in clusters are so small that they lose their magnetism when the magnetic field is shut off (superparamagnetism). Potential applications for these switchable “optical semiconductors” include novel optoelectronic components for telecommunications, displays, and sensors.
"Highly Tunable Superparamagnetic Colloidal Photonic Crystals",Yadong Yin, University of California, Riverside, Angewandte Chemie International Edition, doi: 10.1002/anie.200701992