Fabrication of height-modulated carbon nanotube forests: Morphologies and prospects for broadband absorption

Carbon nanotube (CNT) forests can be the darkest manmade surfaces in the visible to near-infrared range of the electromagnetic spectrum, if the dimensions and alignment of the individual tubes and the densities and heights of the forests are in an appropriate range. However, it has been recognized that to push the absorption range beyond mid-infrared, hierarchical structures are needed. Site-selective CNT forest synthesis has been well-developed but not height-modulated CNT forest. Four different approaches and their resulting forest morphologies are analyzed here. Patterning substrates by either reactive ion etching or wet-chemical etching could create height- modulated forests but with regions of imperfect alignment for light absorption. Patterning diffusion barriers with the same catalyst do not always create forest height modulation because lower CNTs were uprooted by the van der Waals forces. Differential growth created by catalyst patterns results in height-modulated forests with vertically aligned CNTs, which is promising for broadband absorption. Novel CNT forest morphologies are revealed by the non-planar substrates and uneven distribution of catalyst. From the forest height ratios, the lifetime of Fe catalyst on silicon dioxide can be estimated to be ~60% of that on aluminum oxide.