@article {837596,
	title = {Color from hierarchy: Diverse optical properties of micron-sized spherical colloidal assemblies},
	journal = {Proc. Nat. Acad. Sci.},
	volume = {112},
	year = {2015},
	note = {This work was supported by the National Science Foundation Materials Research Science and Engineering Center at Harvard University under Award DMR-1420570 and by the Badische Anilin und Sodafabrik{\textquoteright}s North American Center for Research on Advanced Materials. N.V. acknowledges funding of the Deutsche Forschungsgemeinschaft (DFG) through the Cluster of Excellence Engineering of Advanced Materials. S.U. acknowledges funding from DFG. K.R.P. acknowledges support from a graduate research fellowship from the Department of Defense. I.B.B. acknowledges support from a Banting Postdoctoral Fellowship funded by the Natural Sciences and Engineering Research Council of Canada. M.K. acknowledges financial support from the Massachusetts Institute of Technology Mechanical Engineering Department.},
	pages = {10845-10850},
	abstract = {Materials in nature are characterized by structural order over multiple length scales have evolved for maximum performance and multifunctionality, and are often produced by self-assembly processes. A striking example of this design principle is structural coloration, where interference, diffraction, and absorption effects result in vivid colors. Mimicking this emergence of complex effects from simple building blocks is a key challenge for man-made materials. Here, we show that a simple confined self-assembly process leads to a complex hierarchical geometry that displays a variety of optical effects. Colloidal crystallization in an emulsion droplet creates micron-sized superstructures, termed photonic balls. The curvature imposed by the emulsion droplet leads to frustrated crystallization. We observe spherical colloidal crystals with ordered, crystalline layers and a disordered core. This geometry produces multiple optical effects. The ordered layers give rise to structural color from Bragg diffraction with limited angular dependence and unusual transmission due to the curved nature of the individual crystals. The disordered core contributes nonresonant scattering that induces a macroscopically whitish appearance, which we mitigate by incorporating absorbing gold nanoparticles that suppress scattering and macroscopically purify the color. With increasing size of the constituent colloidal particles, grating diffraction effects dominate, which result from order along the crystal{\textquoteright}s curved surface and induce a vivid polychromatic appearance. The control of multiple optical effects induced by the hierarchical morphology in photonic balls paves the way to use them as building blocks for complex optical assemblies{\textemdash}potentially as more efficient mimics of structural color as it occurs in nature.},
	doi = {10.1073/pnas.1506272112},
	url = {http://www.pnas.org/content/112/35/10845},
	author = {Vogel, N. and S. Utech and England, G.T. and Shirman, T. and Phillips, K.R. and N. Koay and Burgess, I.B. and Kolle, M. and D. A. Weitz and Aizenberg, J.}
}
