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.

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.

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Simpler synthesis methods would be less expensive, better suited for mass production, 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.

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

Figure 1: The osmotic pressure of the microsphere condenses the nanoparticles together, shifting the resulting color, and the microsphere is cured with UV light to lock the nanoparticles in place.

A benefit of the microsphere method is that the colors can be tuned in production all the way up until curing, unlike with chemical coloring which needs to be mixed with other colors or derived from a new source for a change in color. See Figure 2 below.

Figure 2: As the diameter of the microspheres decreases and the nanoparticle concentration increases, the wavelength that the particles absorb shifts higher, resulting in a different color.

The color flexibility of microspheres is unique even in regard to other methods of structural coloration such as the hole-array method which is subtractive, meaning that once material is removed, it cannot be added back. This method of structural color generation lets the photonic pigments be applied as a liquid, allowing for a greater range of application than coloration that is synthesized on a 2D plane. This method is best optimized for use inks or dyes as it is already in liquid form. One issue that was found was that red pigments were not as saturated as blue or green pigments, but this could be addressed in future studies by reducing incoherent scattering of light. Incoherent light scattering increases the whiteness of the sample because the color of the sample is mixed with the incoherently scattered light, reducing saturation. The best use for this method of structural color would be for inks that remain as a fluid, but a way to have the microspheres exist as a powder would be to make the shell out of a more rigid material so it does not compress under pressure.4

Structural coloration can also be generated by an array of holes in a metal surface. This method differs from the microsphere method as it physically alters the structure of the material the color is on. By creating circular holes in the metal sheet with a diameter smaller than the wavelength of light using electron-beam lithography (EBL) or focused ion beam (FIB), the transmittance wavelength of the material can be changed.6 The amount of light that can pass through the hole determines what wavelengths get absorbed rather than scattered.

See Figure 2 below.

Figure 2: Holes in a dimple array generating the letters “h” and “v” in red and green. The lattice constant for red was 550 nm and for green was 450 nm.6

Changing the hole shape from square to triangle can increase the intensity of the colors by reducing backscattering from light diffusing on the opposite side of the material. A change in the hole diameter is what has the greatest effect on the color produced. Unlike other methods that use structures generated on top of a sheet of metal or nanoscopic polarizing filters, hole arrays are

less complex to produce while still showing a high brightness and range of colors. 6 The benefits of nanohole structural coloration are that it is extremely thin at 300 nm and can be used as an electrode because it is made of silver, as well as being resistant to chemicals and UV radiation.6 Hole arrays have the potential to be used in applications with intense UV light exposure or chemical bleaching agents. This method of coloration could be used to create ultrathin high resolution displays, security patterns, or high density information storage.

These two methods of structural color production are effective because they generate color while still having a simple synthesis process. Although the techniques of creating the color differ, both methods can produce a range of colors. Structural color

1Xu, Ting, et al. "Structural Colors: From Plasmonic to Carbon Nanostructures." Small, vol. 7, no. 22, 20 Sept. 2011, pp. 3128-36, https://doi.org/10.1002/smll.201101068. 2 Forster, Jason D., et al. "Biomimetic Isotropic Nanostructures for Structural Coloration." Advanced Materials, vol. 22, nos. 26-27, 22 Apr. 2010, pp. 2939-44, https://doi.org/10.1002/adma.200903693. 3 Kohri, Michinari, et al. "Biomimetic Non-iridescent Structural Color Materials from Polydopamine Black Particles That Mimic Melanin Granules." Journal of Materials Chemistry C, vol. 3, no. 4, 2015, pp. 720-24, https://doi.org/10.1039/C4TC02383H. 4 Park, Jin-gyu, et al. "Full-Spectrum Photonic Pigments with Non-iridescent offers more advantages than pigmented color in durability, longevity, thickness, and color specificity. The scale at which structural color can be produced as a whole allows for pixels below the resolution of the human eye. This would be useful in high resolution displays, however, it would be more difficult to manufacture for large displays. Due to the microspheres properties as a liquid and the production method that does not directly apply them to a surface compared to the hole array method, microspheres would be better suited for large displays and could even be adapted to work in existing printing technology. Both of these methods show promise in a wide range of applications due to their color permanence, flexibility, and nanoscopic size.

Structural Colors through Colloidal Assembly." Angewandte Chemie International Edition, vol. 53, no. 11, 12 Feb. 2014, pp. 2899-903, https://doi.org/10.1002/anie.201309306. 5 Saito, Akira. "Material Design and Structural Color Inspired by Biomimetic Approach." Science and Technology of Advanced Materials, vol. 12, no. 6, Dec. 2011, p. 064709, https://doi.org/10.1088/1468-6996/12/6/064709. 6 Gu, Yinghong, et al. "Color Generation via subwavelength Plasmonic Nanostructures." Nanoscale, vol. 7, no. 15, 2015, pp. 6409-19, https://doi.org/10.1039/C5NR00578G.