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STRUCTURAL COLORATION

Dyes and pigments are chemical colorants.  They are commonly used to impart color to objects.  The major difference between dyes and pigments is that dyes have much finer particle sizes than pigments.  Dyes, in which the coloring matter is dissolved in liquid, are absorbed into the material to which they are applied.  Pigments, on the other hand, consist of extremely fine particles of ground colorant suspended in a liquid which forms a paint film, with this paint film bonding to the surface to which it is applied.

An intriguing alternative approach is the coloration of objects by the modulation of their surfaces.  This approach, known as structural coloration, is the production of color by microscopically structured surfaces with features in just the right fineness range to interfere with visible light, sometimes in combination with pigments.  The development of structurally colored synthetic structures is a major area of research activity.  Many structurally colored synthetic materials have already been developed for various applications, but a lot more work remains to be done, and structural coloration is still far from reaching its full potential.

Example:  Observation of structural colors in random metallic networks with subwavelength dielectric coatings.  (a) Schematic illustration of an Al2O3-coated PtYAl nanomaterial, based on a three-dimensional (3D) reconstruction of a completely dealloyed PtYAl thin film obtained via FIB-assisted thin film tomography.  (b) Photographs of deposited, dealloyed and Al2O3-coated PtYAl metamaterial networks, illustrating the formation of vibrant colors and the continuous color change with increasing coat thickness.  The photographs were taken under illumination from ceiling lights.  Each image is 2 × 2 mm2.  (c) Experimental and FDTD simulated structural color reported in a standard CIE 1931 (x, y) space, depicting the chromaticity visible to the average person.  The red-green-blue (RGB) color space is marked by the triangle area. The chromaticity is calculated directly from reflectance spectra obtained either experimentally (circles markers) or by FDTD simulations (dashed line).  The edges of the tongue-shaped plane correspond to color values of maximal saturation.  (d) Color palette calculated by FDTD simulations for increasing thickness of Al2O3.  This image is reproduced from H. Galinski, G. Favraud, H. Dong, J. S. T. Gongora, G. Favaro, M. Döbeli, R. Spolenak, A. Fratalocchi, and F. Capasso, Scalable, Ultra-Resistant Structural Colors Based On Network Metamaterials, Light: Science & Applications, 6, e16233 (2017).

Structural coloration often enables color to be imparted in a more environmentally friendly manner by avoiding the use of chemical colorants and coloration processes.

Another advantage of structural coloration is that it is a biomimetic approach.  Structural color is abundant in nature.  Structural coloration hence possesses a potential inherent advantage over chemical coloration in that it allows us to learn from the successful outcomes of billions of years of evolution and then to recreate these outcomes synthetically.

Example:  Electron microscopy reveals the nanoscales on a butterfly wing and the nanospheres inside an opal.  The images are from Wikimedia Commons, with the exception of the opal micrograph which is by George Rossman at Caltech.

Classical optical physics can be used as the foundation for modeling and simulations that can assist in the creative design of new and/or improved structurally colored materials.

Many approaches have been developed for achieving structural coloration and continue to be explored and improved.  Some of these approaches are summarized below.

As reviewed by Aguirre et al., colloidal photonic crystals and materials derived from colloidal crystals can exhibit distinct structural colors that result from incomplete photonic band gaps.  Color-producing photonic crystals are periodically structured materials with a periodicity whose length scale is proportional to visible wavelengths.  Through rational materials design, the colors of such photonic crystals can be tuned reversibly by external physical and chemical stimuli.  Such stimuli include solvent and dye infiltration, applied electric or magnetic fields, mechanical deformation, light irradiation, temperature changes, changes in pH, and specific molecular interactions.  Reversible color changes result from alterations in lattice spacings, filling fractions, and refractive index of system components.

Iridescence is undesirable in many applications where it is usually preferable to have omnidirectional (angle-independent) structural color.  Many approaches are under development to achieve omnidirectional structural color.  The key to success appears to be the development of a structure where the reflectance as a function of the wavelength shows a pronounced maximum over a narrow wavelength range.  For example, (a) PMMA photonic crystals infiltrated with carbon black nanoparticles and exhibiting structural color that shows little dependence on the viewing angle were developed, and (b) some amorphous colloidal arrays manifest omnidirectional structural color although it is difficult to produce such arrays because submicron-sized particles have a strong tendency to crystallize.

Another approach for obtaining structural color is to prepare multilayer fibers, films, or coatings where the refractive indices, numbers, sequences, and thicknesses of the layers are chosen so as to provide the desired structural color characteristics.  Such multilayer (stacked) structures, which are morphologically quite different from colloidal photonic crystals, are sometimes referred to as one-dimensional photonic crystals.  Examples include inorganic films comprising alternating nanoscale layers of titanium dioxide and silicon dioxide and nanoscale nylon-polyester laminate fibers.

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