Structural Design of Intelligent Reversible Two-Way Structural Color Films

Metal-organic framework photonic crystals with bidisperse particles-based brilliant structural colors and high optical transparency for elaborate anti-counterfeiting

Photonic crystals (PCs) have attracted great interest and wide applications in displays, printing, anti-counterfeiting, etc. However, two main challenges significantly hinder their applications: 1) the tradeoff between high optical transparency across the whole visible range and brilliant colors requiring a large refractive index contrast (Δn), and 2) the way of regulating structural colors by altering tens of different sizes. To address these issues, a new type of metal-organic framework (MOF)-based transparent photonic crystal (TPC) has been fabricated through self-assembling MOF particles into three-dimensional ordered structures which were then infiltrated by polydimethylsiloxane (PDMS). Compared to conventional PCs, these TPCs exhibit 1) both brilliant forward iridescent structural colors and high transmittance (>75 %) across the whole visible spectra range, and 2) conveniently adjustable colors based on bidisperse particles. The unique color-generating mechanism of the light diffraction by each plane lattice and the small Δn between MOF particles and PDMS are the keys to TPCs' characteristics. Moreover, the prepared invisible anti-counterfeit labels can reversibly hide-reveal patterns with elaborate and exchangeable color contrast in a non-destructive way, showing potential applications in anti-counterfeiting, information encryption, and optical devices.

New electric field responsive photonic crystals with remarkable yellow-to-green switch for adaptive camouflage

Electric field responsive photonic crystals take on critical significance in developing adaptive camouflage technology, which are promising materials for adaptive camouflage devices with better fabrication processes and color saturation. However, electric field responsive photonic crystals are primarily susceptible to poor fusion with typical background colors and necessitate the continuous implementation of electric fields to attain specific colors, thus limiting their practical applications. Monodisperse polyvinylpyrrolidone modified cadmium sulfide (PVP/CdS) microspheres with large refractive index are well prepared in this study. Liquid photonic crystals, exhibiting earth-yellow and light green under specific electric field, are obtained with PVP/CdS particles dispersed in propylene carbonate in accordance with the subtractive color mixing principle of structural color and initial color. The resulting electric field responsive photonic crystals are characterized by a simple preparation process, fast electrical response, long-time holding of the earth-yellow state, and reversible color changes between earth-yellow and light green in -3.5 V and + 3.5 V electric field switching. This study can contribute to the development of color-changing devices designed for adaptive camouflage applications.