A research group led by Yadong Yin of the University of California, Riverside published an article in Angewandte Chemie, announcing that they have made new progress: the group coated a plastic shell with small magnetic crystals to form nanoparticles, and then the particles self-aggregated into photonic crystals in solution . When an external magnetic field is applied, the optical properties of the crystals will change, so the color of these crystals can be accurately changed by adjusting the magnetic field strength.
The crystals mentioned here are not the same as traditional crystals, they are colloidal crystals. The manufacturing cost of colloidal crystal is low, and it can be produced on a large scale, so it is suitable for photonic crystals. Photonic crystals are similar to semiconductor materials in electronics, they also have band gaps, band gaps, and so on. These optical characteristics depend on the spatial relationship of the crystal.
The current research is mainly aimed at photonic crystals whose forbidden band can be quickly and precisely adjusted by external stimulation. But these requirements have not been fulfilled.
One of the possible stimuli is a magnetic field, if the crystal is made of magnetic material, such as iron oxide. But the problem is that the magnetization can only be maintained when the particles reach a larger scale, and Yin's team found a solution: to coat the nano-iron oxide particles with a polyethylene shell.
The result of this is that nanocrystals self-aggregate into colloidal photonic crystals in solution. The magnetic field force acts on every single cluster, changing the lattice distance between clusters. By changing the strength of the magnetic field and the distance of the magnet, the color of the colloidal crystal can be changed throughout the visible spectrum. The entire process is rapid and reversible, because the clustered nanocrystals are so small that they can lose their magnetism immediately when the magnetic field is turned off. This can be used in communications, displays, sensors and other fields.
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