The New Way to Create Color

A new approach to color creation uses scattering of light at specific wavelengths around small, nearly perfect spherical silicon crystals. Developed by Kobe University, this method enables structural colors that are independent of viewing angle, printable and fade-resistant. The material has low environmental and biological impact and can be applied extremely thinly, promising significant weight improvements over traditional paints. An object acquires color when light at a specific wavelength is reflected. In traditional pigments, this occurs as molecules absorb other colors from white light, but over time this interaction causes molecular degradation and color fading. Structural colors, by contrast, typically arise from light reflecting off parallel nanostructures positioned at the correct distance, so that only light at specific wavelengths survives while others cancel out, reflecting only the color we see. This phenomenon can be observed on butterfly wings or peacock feathers and has the advantage of color stability. From an industrial perspective, however, uniformly ordered nanostructures cannot easily be painted or printed. Since color depends on viewing angle, the material exhibits a color-shifting, iridescent appearance. Materials engineers Fujii Minoru and Sugimoto Hiroshi at Kobe University have developed an entirely new approach to color production. A single layer of silicon nanospheres produces bright structural colors independent of viewing angle. Color can be controlled by the sphere diameter, with smaller particles appearing more blue and larger ones more red. [caption id="attachment_161169" align="aligncenter"]
A single layer of silicon nanospheres produces bright structural colors independent of viewing angle[/caption] Regarding their work, the researchers stated: "Since 2020, in our previous studies, we were the first to achieve precise particle size control and develop colloidal suspensions of spherical and crystalline silicon nanoparticles. These individual silicon nanoparticles scatter light in bright colors through the 'Mie resonance' phenomenon, enabling us to develop structurally colored inks." In Mie resonance, spherical particles with dimensions comparable to the light wavelength reflect specific wavelengths particularly strongly. This essentially means that the color returning from the suspension can be controlled simply by changing the particle size. In their study published in ACS Applied Nano Materials, Fujii and Sugimoto demonstrate that the suspension can be applied to surfaces, thereby coating the underlying material with a structural color independent of viewing angle. This is because the color is produced not through interaction of light reflected from neighboring structures as in "conventional" structural colors, but through highly efficient scattering around individual nanospheres. Sugimoto explains another advantage: "A single layer of sparsely distributed silicon nanoparticles only 100-200 nanometers thick displays bright colors yet weighs less than half a gram per square meter. This makes our silicon nanospheres one of the world's lightest colored coatings." The Kobe University team used computational simulations to explore ink properties under different conditions, such as varying particle size and spacing, then experimentally validated the results. Counterintuitively, they found that reflectance is highest when particles are separated rather than tightly packed. The authors explain this as follows: "Despite minimal surface coverage of the nanospheres, this high reflectance stems from very high scattering efficiency. The requirement for only small amounts of silicon crystal is an advantage in applications as a color pigment." After further development and refinement, the researchers anticipate interesting applications for their technology. Sugimoto explained: "We could apply this to aircraft coatings, for example. The pigments and coatings on an aircraft weigh several hundred kilograms. Using our nanosphere-based ink, we could reduce that weight to less than 10 percent of the original." Source More information: Monolayer of Mie-Resonant Silicon Nanospheres for Structural Coloration, ACS Applied Nano Materials (2024). DOI: 10.1021/acsanm.3c04689 / Provided by Kobe University