Application and Practicality of Structural Coloration Brian Alewine
Abstract: Structural colors are created by the interference and absorption of light by nanostructures on a surface. The benefits of using structural color comes from the differences in its physical properties compared to dyes or pigments. Examples of structural color are commonly found in nature, yet some aspects of structural color synthesis have proved difficult. This review will cover the application and practicality of two types of structural color generation. Introduction Color in pigments and dyes comes from the reflection and absorption of light by molecules. Structural color is produced by light interfering with nanostructures on the scale of the wavelength of visible light.1 Because the molecules in pigments and dyes absorb light energy to produce color, over time they break down, leading to color fading. Since dyes break down over time, they have to constantly be produced, causing sustainability and environmental concerns. Structural color stays colorful as long as the structure is intact, leading to long lasting color even over millions of years. Structural colors can be made from a wider range of materials, such as metals which do not originally have bright colors, as it is the nanostructures that provide the color, not the types of molecules. Nanostructures that create structural color have been found in many animals, from butterfly wings to peacock feathers; however, the complicated structures involved are challenging to replicate artificially, so most coloration methods simplify the structure.5 Simpler synthesis methods would be less expensive, better suited for mass production, 10
and able to be applied in more areas. Structural color in nature is usually iridescent or non-iridescent which is caused by the structures being anisotropic or isotropic.23 For most applications, noniridescent color is desired because the color stays the same from different viewing angles. While iridescent structures have richer colors at certain viewing angles, noniridescent structures provide consistent color across any angle. Two of the primary methods for creating structural color are discussed in detail: using photonic pigments with microspheres, and the creation of color via subwavelength hole array. Structural Color from Photonic Pigments The most positive results in synthesis of artificial structural color have come from the formation of self-assembling crystalline structures. The distance between particles, determined by the diameter of each particle, changes the amount of light absorbed and scattered and thus shifts the resulting color.4 Because natural color structures can be extremely complex, the easiest way to replicate structural color artificially is to use self-assembling particles.5 A crystalline structure has the benefit of being easy to manufacture, not energy intensive, and highly repeatable. Crystals, also having amorphous properties, allow for bright colors without iridescence which can be used in a larger range of applications. The photonic nanoparticles that form the crystal are contained in a semi-permeable membrane which allows for changes in nanoparticle density with osmotic pressure. After the desired color is reached, microspheres are then cured with UV light, preventing further color change.4
