A bottom-up engineered broadband optical nanoabsorber for radiometry and energy and harnessing applications

Optical absorbers based on vertically aligned multi-walled carbon nanotubes (MWCNTs), synthesized using electric-field assisted growth, are described here that show an ultra-low reflectance, 100X lower compared to Au-black from wavelength λ ~ 350 nm - 2.5 μm. A bi-metallic Co/Ti layer was shown to catalyze a high site density of MWCNTs on metallic substrates and the optical properties of the absorbers were engineered by controlling the bottom-up synthesis conditions using dc plasma-enhanced chemical vapor deposition (PECVD). Reflectance measurements on the MWCNT absorbers after heating them in air to 400°C showed negligible changes in reflectance which was still low, ~0.022 % at λ ~ μm. In contrast, the percolated structure of the reference Aublack samples collapsed completely after heating, causing the optical response to degrade at temperatures as low as 200°C. The high optical absorption efficiency of the MWCNT absorbers, synthesized on metallic substrates, over a broad spectral range, coupled with their thermal ruggedness, suggests they have promise in solar energy harnessing applications, as well as thermal detectors for radiometry.