Construction And Properties Of Multilayer Structural Color Film Based On SiO₂ Colloidal Microspheres

Photonic crystals?PCs?are artificial periodic structures with spatially periodic arrangement of the refractive index,which results in the existence of photonic bandgap in the structure.If the photonic bandgap locates in the visible region,the PCs will show brilliant colors,which are called structural colors.In recent years,colloidal photonic crystals have been well developed due to their simple preparation method,low cost,and easy modification and functionalization.Patterned colloidal photonic crystals?PCs?provide a new way to build high-performance PCs with unique structures and specific functions.Structural colors originating from the photonic bandgap have several characteristics such as angle dependence,crystal lattice dependence,antiphotobleaching,and metal luster.Due to their unique optical properties,many efforts have been made for the application of structural colors in the fields of anticounterfeiting,color displays,colorimetric chemical sensors,etc.SiO₂ colloidal microsphere is one of the ideal materials for photonic crystals due to their simple preparation and self-assembly methods.Patterned colloidal PCs contribute a novel approach to constructing high-performance PC devices with unique structures and specific functions.However,traditional photonic crystal composed of a single particle size of microsphere has a specific photonic band gap,which limits the range of color variations.In order to enrich the structural color of PCs,we have prepared multilayer photonic structure films with multiple band gaps.?1?Inspired by the theory of color superposition,we successfully prepared a bilayer SiO₂photonic crystal film with two band gaps by consecutively dip-coating method.The multilayer films can not only realize the structural colors mixing by superposition of two bandgaps,but also realize the"circulation"of structural color with increase the incident angle.The synthesis procedure of silica microspheres was a modified St?ber process.A bilayer photonic crystal film was successfully prepared by consecutively dip-coating SiO₂ colloidal microspheres with different particle sizes.In the paper,the structural colors mixing was successfully achieved by controlling the thickness of the top photonic crystal.For example,we have obtained a bilayer PC with cyan color,which is formed by superposition of PCs with blue and green band gaps.We summarize the generation mechanism of the structural color of the bilayer PCs under different angles according to the reflectance spectra and digital photos of the bilayer PC?210/250 nm?.As a benefit of the tunable optical properties of this bilayer,we designed a special bilayer PC?250/330 nm?structure and the structural color could achieve circulation of green to magenta,orange,yellow,and green again with the increase of the incident angle.This phenomenon is greatly different from the monodirectional blue shift of the structural color in single-layer PCs and simple supposition of the structural color in reported multilayer PCs.The enhanced tunability of the structural color in the bilayer structure provides them strong anticounterfeiting capability and will show promising potential in this field.?2?In order to further enrich the mode of structural color and pattern of photonic crystals,multilayer photonic structure with different structural colors or different patterns on the front and back sides were designed by consecutively dip-coating/spraying/dip-coating method.When the thickness of the amorphous layer is sufficient,light is prohibited from propagating in the multilayer photonic film for strong incoherent scattering and the photonic band gaps of the top and the bottom photonic crystals cannot be superimposed,so that the structural color is only provided by the band gap on the side directly viewed layer.A pattern with mixed color of both ordered layers can be realized by opening a window in the disordered layer using a patterned mask,which was obviously different from the background color.Furthermore,completely different patterns on each side were also realized by spraying amorphous SiO₂ on both sides using templates with different patterns as mask.The different colors or patterns on each side of the multilayer photonic crystals will enhance their anticounterfeiting ability and provide them with double-side color display properties.