Electrochemically tunable photonic diffraction colours

What is it about?

This work reports a metal ion-responsive photonic colloidal crystalline (PCC) microspheric material and the convenient, reversible switching of its photonic diffraction properties by the redox reaction of copper in a simple electrochemical cell. The PCC micro-beads were fabricated from the orderly three-dimensional packing of core-shell nanoparticles with hydrogel coatings laden with anionic phosphate functionalities. Electrostatic binding of metal cations by the hydrogel coating lowered its internal osmotic pressure and caused shrinkage of the core-shell nanoparticles. This brought about a blue-shift of the photonic diffraction max of the micro-beads. The maximum shift, from 650 to 590 nm (colour change from magenta to green) was found to be produced by Cu2+ at a concentration of 1 mM. A simple electrochemical photonic device was constructed by sandwiching a suspension of the photonic micro-beads in 1 mM Cu2+ solution between two transparent ITO-glass electrodes. Photonic diffraction colour of the microbeads was reversibly switched via the electrochemical reduction/oxidation of Cu2+ inside the device. The colour change can be observed by naked eyes under ambient light.

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Why is it important?

Having said that, it is still a considerable challenge to produce PCC thin-films massively and accurately into small pixels in which the modulation of photonic band-gap can be readily controlled individually or collectively with the application of electric fields. Furthermore, since most of the systems have to be operated in organic solvent-based electrolytes, the indium tin oxide (ITO) electrodes have to be sealed with epoxy. This increases the production costs and introduces toxicity.

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