Carbon nanotubes have a colorful side. Although films made from forests of vertically aligned carbon nanotubes have been described as the darkest of dark materials, researchers in Taiwan demonstrate that patterning these nanotube arrays can render them iridescent (ACS Nano, DOI: 10.1021/nn901910h). These iridescent nanotube arrays could find use in flexible solar cells or displays, according to a team led by Hsuen-Li Chen of National Taiwan University and Nyan-Hwa Tai of National Tsing Hua University. The researchers studied dark films of nanotube forests and iridescent films in which the nanotube arrays had a hexagonal-hole pattern or were patterned to resemble broccoli florets. The dark films, they found, had an extremely low index of refraction, and nanotube length was critical to their extreme blackness. Vertically aligned nanotubes in such films should be at least 10 µm long if they are to entirely absorb transmitted light, the researchers say. Interestingly, in the patterned forests, the iridescence phenomenon was observed even though the index of refraction was also extremely low and the nanotubes were capable of trapping incident light.