Visible iridescence from self-assembled periodic rippling in vertically aligned carbon nanotube forests

Abstract

We observe iridescence in the form of spectrally dispersed white light reflected from the structured sidewalls of vertically aligned carbon nanotube forests. The iridescence is a result of diffraction from a self-assembled periodic rippling pattern on the forest sidewalls that acts as a reflection grating. We measure the grating spacing via white light and laser diffraction experiments and see good agreement with the spacing of the rippling pattern as measured via scanning electron microscopy. The periodic rippling pattern self-assembles during chemical vapor deposition growth of the forests.

Origin of periodic rippling during chemical vapor deposition growth of carbon nanotube forests

“…The observed colour changes from red to blue (a-j) as the angle of incidence is swept across the sample in 4°i ntervals. Based on the diffraction data the grating spacing is 330 ± 140 nm, which agrees with the 470 ± 10 nm measured by SEM [20].…”

“…Previously, we showed that rippled forests, such as those in the present report, can be observed optically via diffraction because they behave as gratings [20]. This effect allows their periodicity to be directly calculated from the simple grating equation, and shows that the ripples are very consistent over significant areas, for if they were not, clear diffraction patterns, such as those observed here, would not be seen.…”

“…Remarkably, the ripples we obtain are sufficiently regular over 0008-6223/$ -see front matter Crown Copyright Ó 2011 Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2011.02.017 a large enough area that they produce an easily observed optical effect: iridescence caused by the diffraction of visible light via a surface grating effect [20]. This iridescence provides a means to observe the ripples optically since they are too small for direct observation with visible light.…”

Morphological Instabilities and the Dynamics of Carbon Nanotube Forest Growth

“…At lower temperatures, morphologies become even more complicated, with self-delamination common in the range 730°C and 760°C (figure 2c) and even cracking and "exploding" at lower temperatures (figure 2d). In the intermediate range of temperatures (~730°C to ~830°C), the forests reproducibly show periodic ripples on their outer surface of wavelength ~1 µm, extending over a large range (19,20). The ripples are so regular, over much of the forest height, that one can even obtain optical diffraction patterns from them.…”

Transforming carbon nanotube forest from darkest absorber to reflective mirror

“…Kempa et al 9 fabricated honeycomb-structure arrays of vertically aligned CNTs and observed strong colorful diffraction from the nanotubes because of their high metallicity (i.e., low dielectric loss). Vinten et al 10 reported visible iridescence from the sidewalls of CNT forests due to the ripples present on the sidewalls. Shoji et al 11 developed an optical polarizer made of aligned single-walled CNTs.…”