Abstract
A major fabrication challenge is producing disordered photonic materials with an angle-independent structural red color. Theoretical work has shown that such a color can be produced by fabricating inverse photonic glasses with monodisperse, nontouching voids in a silica matrix. Here, we demonstrate a route toward such materials and show that they have an angle-independent red color. We first synthesize monodisperse hollow silica particles with precisely controlled shell thickness and then make glassy colloidal structures by mixing two types of hollow particles with the same core size and different shell thicknesses. We then infiltrate the interstices with index-matched polymers, producing disordered porous materials with uniform, nontouching air voids. This procedure allows us to control the light-scattering form factor and structure factor of these porous materials independently, which is not possible to do in photonic glasses consisting of packed solid particles. The structure factor can be controlled by the shell thickness, which sets the distance between pores, whereas the pore size determines the peak wave vector of the form factor, which can be set below the visible range to keep the main structural color pure. By using a binary mixture of 246 and 268 nm hollow silica particles with 180 nm cores in an index-matched polymer matrix, we achieve angle-independent red color that can be tuned by controlling the shell thickness. Importantly, the width of the reflection peak can be kept constant, even for larger interparticle distances.
Highlights
Inspired by nanostructures in bird feathers or firefly lantern,[1,2,3] a new kind of structurally colored material, called a “photonic glass,” has recently been developed.[4,5,6] Unlike photonic crystals, which show iridescent colors—that is, colors that depend on both the viewing angle and the orientation of the sample—photonic glasses can have angle-independent colors that are indistinguishable from traditional dyes or pigments.One way to produce such materials is by quickly drying colloidal suspensions.[7,8,9] this approach yields only blue or green colors, and never red
Magkiriadou et al.[10] used a single-scattering model to explain why red is difficult to achieve: When colloidal glasses scatter light, two peaks arise in the reflection spectra: one comes from the form factor and is determined primarily by the size and refractive index of individual particles, while the other comes from the structure factor and is determined primarily by the inter-particle distance.[11]
When the sphere size is such that the peak of the structure factor corresponds to a reflected green or blue wavelength, the peak of the form factor is in the ultraviolet (UV) and cannot be seen by the human eye
Summary
Inspired by nanostructures in bird feathers or firefly lantern,[1,2,3] a new kind of structurally colored material, called a “photonic glass,” has recently been developed.[4,5,6] Unlike photonic crystals, which show iridescent colors—that is, colors that depend on both the viewing angle and the orientation of the sample—photonic glasses can have angle-independent colors that are indistinguishable from traditional dyes or pigments.One way to produce such materials is by quickly drying colloidal suspensions.[7,8,9] this approach yields only blue or green colors, and never red. When the sphere size is such that the peak of the structure factor corresponds to a reflected green or blue wavelength, the peak of the form factor is in the ultraviolet (UV) and cannot be seen by the human eye. If one increases the sphere size, so as to increase the average inter-particle distance and shift the peak of the structure factor wavelengths, the peak of the form factor enters the visible region, making the structural color a mix of blue and red that appears magenta.[12,13,14]
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