Original paper: Biomimetic Isotropic Nanostructures for Structural Coloration
There’s a reason why the word “peacock” has become a verb synonymous with commanding attention. Of course, the size of the peacock tail is enough to turn heads, but it wouldn’t be nearly as beautiful without its signature iridescent, or angle-dependent, color. The brilliant colors of the peacock come from the interaction of light with the nanoscale structure of the feathers, which is much different from the origin of color in regular dyes and pigments. In today’s paper, Jason Forster and his colleagues in the Dufresne group developed a simple way to make colors that is inspired by the structures in certain bird feathers.
Colors come from the way our brain interprets different wavelengths of light. Most colors we encounter in dyes and paints are a result of absorption. Certain chemicals absorb specific wavelengths of light, and the other wavelengths are reflected; the colors we see are due to those reflected wavelengths. However, not all colors come from absorption. The color of the sky is perhaps the most widely seen example of this. The molecules that make up air scatter much more light at small wavelengths, which corresponds to blue light.
The iridescence of the colors in the peacock feather is caused by constructive interference due to the nanoscale structure of the feather. To explain this, let’s look at a simplified picture. If you have a layered stack of materials, some light will be reflected from each layer in the stack (Figure 2). Since the light reflected from the layers at the bottom stack will have traveled farther, the different sets of reflected waves will be shifted out of phase. When the waves are shifted by exactly one wavelength, they add constructively and give a stronger reflection. This constructive interference happens at a wavelength which depends on the thickness of the layers, their index of refraction, and the angle at which the light is sent and detected. Structural color is a result of the stronger reflectance at a particular wavelength due to this constructive interference of light.
Structural color can arise in many different types of structures, from bird feathers and butterfly wings to soap bubbles and opals, but today’s paper is about a type of structural color made from plastic spherical particles. These spheres are only a few hundred nanometers in diameter, on the order of the wavelength of visible light, and they are so small that they can remain suspended in water for long periods of time, forming a colloidal suspension. Jason Forster and his colleagues in the Dufresne group made structurally colored films by starting with a small volume of a colloidal suspension of these particles and allowing it to dry, causing the particles to pack together and self-assemble into structures with color.
The way the particles packed greatly impacted the color of the film. When the researchers used spheres that were all the same size, the particles formed a crystal (an ordered arrangement made of a repeating unit cell) as the suspension dried. In a crystalline structure such as the peacock feather, the structural color is iridescent, or angle-dependent. This angle-dependence of color arises because the angle that light is sent into the sample will affect the distance it travels through the material, therefore changing the wavelength at which the light will constructively interfere. However, the researchers found that when they mixed spheres of two different sizes, the spheres could no longer form a crystal, and instead formed a disordered structure (Figure 3, top). This structure was isotropic, meaning that it looked the same from any angle. The structural color of a crystalline sample is iridescent because light travels different path lengths through it at different angles. Because the isotropic structure is essentially the same at all angles, the color is the same at all angles.
By making a more disordered structure, Forster and his colleagues were able to make a more uniform color! These disordered assemblies of spheres bear a striking resemblance to the nanoscale structures found in bird feathers such as Lipodothrix Coronata (Figure 3, bottom), which are made up air spheres embedded in a disordered array inside a matrix of beta-keratin. These bird feathers have a color similar to the particle films made by the researchers: a blue color that doesn’t change with angle.
Our eyes are a useful tool for observing colors, but they are not the most precise way to measure light. If we want to compare colors precisely and quantitatively, the best way to do that is by looking at a reflectance spectrum. A reflectance spectrum tells you the amount of light reflected from an object at a range of wavelengths. You can measure a reflectance spectrum by shining light at a colored sample and using a spectrometer to detect the reflected light. Combined with a computer, a spectrometer allows you to record an intensity value for a range of wavelengths, giving you a full intensity spectrum. The reflectance spectrum is found by normalizing this data against a perfect reflector such as a mirror or a white material, giving you the percent of light reflected at each wavelength. So if you were to measure the reflectance of a blue material, you would have a spectrum with a peak in the wavelengths that correspond to blue light (~450-495 nm).
One way to infer the reflectance spectrum of a material that has no absorption is to measure transmittance. To measure the transmittance spectrum, you can move the detector to the side opposite to the incident light, so it detects the light that goes through the sample. If you were to measure the transmittance spectrum of this same blue material, you would expect to see a dip corresponding to the blue wavelengths. The blue light would not make it through to the other side because it was reflected.
The researchers measured the transmittance spectra for their structurally colored samples and found that the blue isotropic structural color and the blue crystalline structures both showed a dip in the blue wavelengths (Figure 4). However, the dip in the isotropic structure data was much broader and more shallow, meaning that less light was reflected at that wavelength, making the color less bright and saturated.
But the quality of the color wasn’t the only thing that changed in the spectra of the isotropic structures. In these samples, the transmittance dip stayed at the same range of wavelengths even when the measurement angle changed, while the dip in the spectrum of the crystalline structure shifted as the measurement angle was changed. By eye, the researchers also saw that the disordered structures made angle-independent color, and the ordered structures made iridescent color. The measurements of the crystalline and isotropic structures show that there is a tradeoff between saturation and angle-independence in structural color.
The thickness of these isotropic structurally colored films also greatly affected the saturation of their color. Films that were just a few micrometers thick had a bright blue color, while much thicker films looked nearly white. The researchers found that adding some carbon black– black nanoparticles that absorb light at all visible wavelengths– made the colors of the thick films more vibrant (Figure 5). The carbon black works by reducing the effective thickness of the samples, absorbing light before it can travel through the entire layer of the sample and causing it to look like a thinner sample.
This work showed that structural color, both iridescent and angle-independent, can be made using simple methods that could potentially make the colors in large volumes for real-world applications. Because these colors come from structure and not absorption, they will not fade over time as current dyes do. In addition, one material can be used to make a range of different colors by tuning the structure, so these assemblies could be used as colorimetric sensors that change color in response to environmental changes such as strain or temperature.