Reconfigurable Photonic Crystal Reversibly Exhibiting Single and Double Structural Colors.

Unlike absorption‐based colors of dyes and pigments, reflection‐based colors of photonic crystals, so called "structural colors", are responsive to external stimuli, but can remain unfaded for over ten million years, and therefore regarded as a next‐generation coloring mechanism. However, it is a challenge to rationally design the spectra of structural colors, where one structure gives only one reflection peak defined by Bragg's law, unlike those of absorption‐based colors. Here, we report a reconfigurable photonic crystal that exhibits single‐peak and double‐peak structural colors. This photonic crystal is composed of a colloidal nanosheet in water, which spontaneously adopts a layered structure with single periodicity (407 nm). After a temperature‐gradient treatment, the photonic crystal segregates into two regions with shrunken (385 nm) and expanded (448 nm) periodicities, and thus exhibits double reflection peaks that are blue‐ and red‐shifted from the original one, respectively. Notably, the transition between the single‐peak and double‐peak states is reversible. [ABSTRACT FROM AUTHOR]