Structural color

To make a material that is colored, one normally uses a dye or pigment. But another way to make color is to make a nanostructure that reflects or scatters light so that waves of certain frequencies can constructively interfere. These nanostructured materials are said to have structural color. Unlike traditional color, which comes from dyes or pigments that absorb light, structural color can be made resistant to fading. We use colloidal self-assembly to make nanostructures that have a variety of different colors. At the same time, we aim to understand the physics of the scattering process so that we can optimize these nanostructures for applications. 

Picture of blue, green, and red photonic capsules

We are particularly interested in making structural color that is angle independent. That means that the color is the same, regardless of how you rotate the material, and regardless of the angle between the light source and your eyes. There are many structurally colored materials that, like an opal stone, are iridescent, meaning that the color changes with the viewing angle and orientation. The reason for this change in color is that the nanostructure of these materials is well-ordered (or crystalline). Sometimes these kinds of ordered structures are called photonic crystals.

To make materials that have angle-independent color, we need to make nanostructures that are disordered. We call these materials photonic glasses (a term that we believe was first coined by John Ballato in 2000). Examples of naturally occurring photonic glasses are the feathers of blue birds and the wings of blue butterflies. Eric Dufresne and collaborators were the first to demonstrate synthetic structures that mimic the blue color of the bird feathers (Forster, Noh, Liew, Saranathan, Schreck, Yang, Park, Prum, Mochrie, O'Hern, Cao, Dufresne Advanced Materials 2010). They made these structures from spherical colloidal particles, and they taught us their techniques.

Our goal is to study, both theoretically and experimentally, how the optical properties of these glasses relate to their structure and constituent particles. Based on our observations from photonic glasses of conventional particles, we constructed a theoretical model that explains why it is difficult to make yellow, orange, and red photonic glasses. Guided by this model, we have developed new colloidal systems that give a higher degree of control over structural color. These systems might be used to make reflective displays or paints and coatings. 

Understanding why blue structural colors are easier to make than red

Early in our work on angle-independent structural color, we found that it was easy to make blue colors but hard to make yellow, orange, or red. It also turns out that nearly all examples of angle-independent color in nature are blue. Sofia Magkiriadou and Jin-Gyu Park set out to understand why. Sofia made photonic glasses of different particle sizes and showed that the wavelength of the structural color scaled with the size of the particle, as expected.  She was able to make blue and green samples, but the sample that should have been red turned out to be magenta:

At left, measured reflectivity spectra for three similarly prepared colloidal glasses of PMMA particles in air. Photographs of the samples are shown at right. The magenta sample would appear red if it weren't for the high reflectivity in the blue, indicated by the arrow in the spectra. 

Sofia developed a theoretical model to explain the blue peak in the reflectivity spectrum that, when mixed with the red structural color, made these samples magenta. She showed that this blue peak arises because the individual colloidal particles tend to scatter light more strongly in the blue than in the red (the same argument can be used to explain why the sky is blue). More technically, she and her colleagues showed how the backscattering resonances of the individual particles arise from cavity modes and how these resonances interact with the constructive interference of the nanostructures to suppress the structural red color. From the model that Sofia developed, we were able to establish some design rules for making red structural color. Our findings were highlighted in PhysicsNew Scientistand Chemistry World

Making angle-independent red structural colors

Based on the design rules established in our theoretical study, our group is trying to make photonic pigments that are non-fading and non-toxic: these pigments can potentially be used in cosmetics, reflective displays, and inks. 

Jin-Gyu Park, working together with Sofia and Shin-Hyun Kim, were able to demonstrate a colloidal assembly method to make microcapsules that showed non-iridescent structural colors spanning the entire visible range. The microcapsules are made using a microfluidic device that produces a droplet surrounded by a thin shell of monomer. The droplet contains colloidal particles with a core-shell structure, the core consisting of polystyrene, which has a high-refractive index, and the shell consisting of a hydrogel, which has a low refractive index. The core-shell structures of the particles allows us to decouple the distance between the particles, which sets the structural color, from the scattering strength of the individual particles, which sets the opacity (to understand this, have a look at one of our earlier papers).

After making the droplets, Jin-Gyu and colleagues were able to concentrate the core-shell particles into a dense, disordered nanostructure by pulling out the water. Then they polymerized the shell to make photonic capsules:

Top photographs show a droplet containing core-shell scatterers. By removing the water, as shown in the diagram at bottom,  we concentrate the particles. The color of the droplet becomes more yellow as the particles get more concentrated. We can then lock in the structure by polymerizing the shell, creating a photonic capsule. The electron microscope image at  bottom right shows the random and isotropic nanostructure formed by the core-shell particles inside the capsule.

By changing the sizes of the cores and the shells, Jin-Gyu and colleagues were able to make photonic capsules with blue, green, and red colors:

Picture of blue, green, and red photonic capsules

Our results were highlighted in Phys.org, Chemical & Engineering NewsHarvard Magazineand in a press release from Harvard SEAS.

Materials with responsive structural color

Jin-Gyu Park and Ben Rogers realized that the hydrogel core-shell particles we were using to make the photonic capsules could also be used to make responsive structural color, that is, color that can change with solution conditions or temperature. They made the photonic crystals shown below, which can shift their color rapidly in response to temperature. These materials might be used as sensors or as resonators for organic lasers.  

Colloidal photonic crystals made of hydrogel building blocks can rapidly change colors without melting

Publications

Choi, T. M. ; Park, J. - G. ; Kim, Y. - S. ; Manoharan, V. N. ; Kim, S. - H. Osmotic-Pressure-Mediated Control of Structural Colors of Photonic Capsules. Chemistry of Materials 2015, 27, 1014–1020. Publisher's VersionAbstract

Crystalline or glassy materials made of colloidal nanoparticles show distinctive photonic effects; the crystals exhibit sparkling colors with strong iridescence, while the glasses show noniridescent colors. Both colors are the results of constructive interference of the reflected light by the nonadsorbing nanostructures. Such colored materials have potential applications as nonfading colorants in reflective color displays, optical sensors, coatings, and cosmetics. All of these applications require granular format of the nanostructures; however, precise control of the nanostructures from amorphous to crystalline over the submillimeter length scale remains challenging. Here, we present micrometer-level control of photonic nanostructures confined in microcapsules through osmotic-pressure-mediated concentration. We encapsulate aqueous suspensions of colloidal particles using double-emulsion drops with ultrathin layers of photocurable resin. The microcapsules are then isotropically compressed by imposing a positive osmotic pressure difference that forces the water out through the thin resin membrane. We find that the internal nanostructure of our photonic microcapsules can be kinetically controlled from crystalline to amorphous; slow concentration in small pressure gradients yields colloidal crystals with sparkling color patterns, whereas fast concentration in large pressure gradients yields glassy packing with only short-range order, which show uniform color with little iridescence. By polymerizing the thin monomeric shell, we permanently fix these nanostructures. Our findings provide new insights into the design and synthesis of optical materials with controlled structural colors.

Magkiriadou, S. Structural Color From Colloidal Glasses, 2015. Publisher's VersionAbstract

When a material has inhomogeneities at a lengthscale comparable to the wavelength of light, interference can give rise to structural colors: colors that originate from the interaction of the material's microstructure with light and do not require absorbing dyes. In this thesis we study a class of these materials, called photonic glasses, where the inhomogeneities form a dense and random arrangement. Photonic glasses have angle-independent structural colors that look like those of conventional dyes. However, when this work started, there was only a handful of colors accessible with photonic glasses, mostly hues of blue.

We use various types of colloidal particles to make photonic glasses, and we study, both theoretically and experimentally, how the optical properties of these glasses relate to their structure and constituent particles. Based on our observations from glasses of conventional particles, we construct a theoretical model that explains the scarcity of yellow, orange, and red photonic glasses. Guided by this model, we develop novel colloidal systems that allow a higher degree of control over structural color. We assemble glasses of soft, core-shell particles with scattering cores and transparent shells, where the resonant wavelength can be tuned independently of the reflectivity. We then encapsulate glasses of these core-shell particles into emulsion droplets of tunable size; in this system, we observe, for the first time, angle-independent structural colors that cover the entire visible spectrum. To enhance color saturation, we begin experimenting with inverse glasses, where the refractive index of the particles is lower than the refractive index of the medium, with promising results. Finally, based on our theoretical model for scattering from colloidal glasses, we begin an exploration of the color gamut that could be achieved with this technique, and we find that photonic glasses are a promising approach to a new type of long-lasting, non-toxic, and tunable pigment.

Manoharan, V. ; Magkiriadou, S. ; Park, J. - G. Photonic balls containing a microstructure of core-shell particles exhibiting angularly-independent structural color, 2014.Abstract

A photonic assembly for observing a preselected color includes an assembly of colloidal particles in a continuous liquid phase, the colloidal particles comprising a core scattering center and a shell layer surrounding the core, wherein the core scattering center is selected to scatter light having a predetermined wavelength, and wherein the shell has a thickness selected to provide an overall colloidal particle size that is about the same dimension as the wavelength of preselected color to be observed.

Magkiriadou, S. ; Park, J. - G. ; Kim, Y. - S. ; Manoharan, V. N. Absence of red structural color in photonic glasses, bird feathers, and certain beetles. Physical Review E 2014, 90, 062302. Publisher's VersionAbstract

Colloidal glasses, bird feathers, and beetle scales can all show structural colors arising from short-ranged spatial correlations between scattering centers. Unlike the structural colors arising from Bragg diffraction in ordered materials like opals, the colors of these photonic glasses are independent of orientation, owing to their disordered, isotropic microstructures. However, there are few examples of photonic glasses with angle-independent red colors in nature, and colloidal glasses with particle sizes chosen to yield structural colors in the red show weak color saturation. Using scattering theory, we show that the absence of angle-independent red color can be explained by the tendency of individual particles to backscatter light more strongly in the blue. We discuss how the backscattering resonances of individual particles arise from cavity-like modes and how they interact with the structural resonances to prevent red. Finally, we use the model to develop design rules for colloidal glasses with red, angle-independent structural colors.

Kim, S. - H. ; Park, J. - G. ; Choi, T. M. ; Manoharan, V. N. ; Weitz, D. A. Osmotic-pressure-controlled concentration of colloidal particles in thin-shelled capsules . Nature Communications 2014, 5 3068. Publisher's VersionAbstract

Colloidal crystals are promising structures for photonic applications requiring dynamic control over optical properties. However, for ease of processing and reconfigurability, the crystals should be encapsulated to form ‘ink’ capsules rather than confined in a thin film. Here we demonstrate a class of encapsulated colloidal photonic structures whose optical properties can be controlled through osmotic pressure. The ordering and separation of the particles within the microfluidically created capsules can be tuned by changing the colloidal concentration through osmotic pressure-induced control of the size of the individual capsules, modulating photonic stop band. The rubber capsules exhibit a reversible change in the diffracted colour, depending on osmotic pressure, a property we call osmochromaticity. The high encapsulation efficiency and capsule uniformity of this microfluidic approach, combined with the highly reconfigurable shapes and the broad control over photonic properties, make this class of structures particularly suitable for photonic applications such as electronic inks and reflective displays.

Park, J. - G. ; Kim, S. - H. ; Magkiriadou, S. ; Choi, T. M. ; Kim, Y. - S. ; Manoharan, V. N. Full-Spectrum Photonic Pigments with Non-iridescent Structural Colors through Colloidal Assembly. Angewandte Chemie International Edition 2014, 53, 2899-2903. Publisher's VersionAbstract

Structurally colored materials could potentially replace dyes and pigments in many applications, but it is challenging to fabricate structural colors that mimic the appearance of absorbing pigments. We demonstrate the microfluidic fabrication of “photonic pigments” consisting of microcapsules containing dense amorphous packings of core–shell colloidal particles. These microcapsules show non-iridescent structural colors that are independent of viewing angle, a critical requirement for applications such as displays or coatings. We show that the design of the microcapsules facilitates the suppression of incoherent and multiple scattering, enabling the fabrication of photonic pigments with colors spanning the visible spectrum. Our findings should provide new insights into the design and synthesis of materials with structural colors.

Magkiriadou, S. ; Park, J. - G. ; Kim, Y. - S. ; Manoharan, V. N. Disordered packings of core-shell particles with angle-independent structural colors . Optical Materials Express 2012, 2 1343-1352. Publisher's VersionAbstract

Making materials that display angle-independent structural color requires control over both scattering and short-range correlations in the refractive index. We demonstrate a simple way to make such materials by packing core-shell colloidal particles consisting of high-refractive-index cores and soft, transparent shells. The core-shell structure allows us to control the scattering cross-section of the particles independently of the interparticle distance, which sets the resonance condition. At the same time, the softness of the shells makes it easy to assemble disordered structures through centrifugation. We show that packings of these particles display angle-independent structural colors that can be tuned by changing the shell diameter, either by using different particles or simply by varying the concentration of the suspension. The transparency of the suspensions can be tuned independently of the color by changing the core diameter. These materials might be useful for electronic displays, cosmetics, or long-lasting dyes.