Modulating Non-Iridescent Structural Colors by Controlling Shell Thickness of Inverse Opal Photonic Glasses with Atomic Layer Deposition Inspired by Avian Feathers

In nature, birds and insects have evolved to exhibit a diverse array of vivid colors based on structural colors. Methods to manifest structural colors have also been optimized with their nanostructures. While many of our industrial colorants are produced with toxic materials and harsh processes, these structural colors are formed from the environmentally benign and natural processes in living creatures. They also provide significant advantages such as better durability from photobleaching and brighter colors under sunlight compared to dye- or pigment-based colors.<br/><br/>Recently, increasing environmental concerns require the development of environment-friendly products that can be produced and recycled with carbon-neutral or energy-efficient processing in industries. Since nature already provides solutions on environmentally friendly and brilliant coloration, numerous research has been conducted to mimic the structural colors and to utilize them, especially for outdoor applications such as color coatings, reflective displays, and digital signages. Thanks to those efforts, rapid progress has been made in understanding the mechanism of structural colors and applying bio-inspired nanostructures to develop technologies such as aesthetic design and anti-counterfeiting with the iridescence of artificial structural colors. While there are applications unique to iridescent structural colors, many naturally-looking color surfaces and reflective displays require non-iridescent structural colors and controlled angle dependency for vivid reflective colors, and challenges remain for this type of application.<br/><br/>In this study, the nanostructures of avian feathers and their color-producing mechanisms are investigated, especially for non-iridescent photonic glass structures. The color spectrums from various types of birds showing non-iridescent structural colors were acquired, and their nanostructures were analyzed with a scanning electron microscope (SEM) and a transmission electron microscope (TEM). From the results, black-headed stork with irregularly arranged hollow nano-cylinders was selected for the bio-inspired non-iridescent colors because their structures are less affected by scattering from each individual cylinder. Also, Eurasian jay showing the periodic change from blue to white due to the thickness of the amorphous photonic nanostructures was chosen, and the hybrid structures inspired by both Black-headed stork and Eurasian jay was designed. To fabricate such structures, we first optimized conditions for synthesizing polystyrene nanoparticles (PSNPs) to control the dimension and the size distribution for making photonic glasses with controlled angle dependency. With the size-controlled PSNPs, the photonic glasses were fabricated via self-assembly, and the thickness of the oxide shell layer on the PSNPs within the photonic glass structure was varied by the atomic layer deposition (ALD) to modulate colors. The final structure was obtained by removing the PSNPs, and the obtained structures exhibit more vivid non-iridescent colors and cover wider color spectrums than the ones fabricated with conventional methods. Our result of modulating colors with the controlled shell thickness of inverse opal photonic glass nanostructures provides a more efficient and scalable process compared to changing the overall photonic structures.<br/><br/>This study was funded by the National Institute of Ecology through the grant number (NIE-C-2021-18) and Human Frontier Science Program through the grant number (RFP0047/2019).