The structure of certain materials can refract light in a way that creates brilliant colors. This was previously thought to require a periodic structure in the materials, however, it has been found that short-range order can also create structural colors. For such amorphous objects, the presence of black materials can enhance the colors. Some bird feathers, for example, have black melanin particles covered by an amorphous spongy network, which together causes a brilliant blue color.

Inspired by these natural materials, Shinya Yoshioka, Tokyo University of Science, Noda, Japan, Yukikazu Takeoka, Nagoya University, Japan, and colleagues have created angle-independent structurally colored materials, which are composed of amorphous arrays of submicrometer-sized fine spherical silica colloidal particles. The team used a black quartz plate with added carbon black as a background and covered this material with alternating layers of the charged colloidal particles and an oppositely charged polyelectrolyte. This allowed them to control the thickness of the resulting colloidal amorphous array (pictured).

Comparatively thick arrays after ca. 50 cycles of layering particles onto the substrate appear almost white due to light scattering. Thinner arrays with ca. 5 to 15 cycles, on the other hand, display colors such as bright blue, green, or purple, depending on the diameter of the silica particles (ranging from 190 nm to 300 nm). The color of the prepared material does not change with the angle of observation.

• Bio-Inspired Bright Structurally Colored Colloidal Amorphous Array Enhanced by Controlling Thickness and Black Background,
  Masanori Iwata, Midori Teshima, Takahiro Seki, Shinya Yoshioka, Yukikazu Takeoka,
  Adv. Mater. 2017.
  DOI: 10.1002/adma.201605050