Iridescence of Patterned Carbon Nanotube Forests on Flexible Substrates: From Darkest Materials to Colorful Films

In this study, we prepared carbon nanotube (CNT) forests exhibiting two types of optical properties: so-called "darkest materials" and films displaying iridescence phenomena. The darkest materials, comprising vertically aligned CNT (VA-CNT) forests, displayed extremely low reflections as a result of the rough surface and the trapping of light in the CNT forests. The lengths of the CNTs in the CNT forests had a strong influence on whether the light transmitted through the CNT forest or reflected from the substrate. From an investigation of the limitations of the darkest materials of CNT forests, we prepared CNT surfaces running the gamut from dark materials to iridescent. To study iridescence phenomena, we prepared two kinds of patterns of CNT forests on flexible polycarbonate substrates: CNTs arrayed in hexagonal-hole patterns and broccoli-like CNTs, the latter formed through a combination of inverse nanosphere lithography (INSL) and a poisoned-catalyst mechanism. In the patterned CNT forests, even though the refractive index difference between the CNT film and air was extremely low and even though the CNTs could trap the incident light, the iridescence phenomenon remained, inducing colorful images from the CNT films.