Scanning electron micrograph of a porous silicon photonic crystal. The sample is prepared from an aqueous suspension of small silicon particles about 10 nanometers in diameter and polystyrene spheres about one micrometer in diameter. When the water evaporates from the suspension, the polystyrene spheres self-assemble into a face-centered cubic (FCC) colloidal crystal, and the silicon particles are trapped in the voids. After all the water evaporates, the sample is heated to burn off the polystyrene, leaving a silicon matrix with an ordered FCC arrangement of pores. The final material is a photonic crystal—it diffracts light across a range of angles and wavelengths.

This image shows a grain boundary in the crystal, as well as some other defects. The small holes at the bottom of the pores correspond to contact points between polystyrene spheres in the template.

See G. Subramanian, V.N. Manoharan, J.D. Thorne, and D.J. Pine, “Ordered Macroporous Materials by Colloidal Assembly: A Possible Route to Photonic Bandgap Materials”, Advanced Materials 11 (15): 1261–1265 (1999)

Image credit: G. Subramanian