Reflectance measurements for black absorbers made of vertically aligned carbon nanotubes

Black coatings have important applications in space-borne infrared systems, absolute radiometers, and radiometric temperature measurements. Recently, researchers have demonstrated close-to-unity absorptance, with diffuse reflection, by using vertically aligned carbon nanotube (VACNT) arrays. The present study deals with the optical properties of highly absorbing VACNT arrays, with surface features from diffuse to specular. Three CNT arrays were fabricated using a thermal chemical vapor deposition (CVD) technique with different growth conditions to produce highly aligned multiwalled CNT arrays. The bidirectional reflectance distribution functions (BRDFs) were measured with a laser scatterometer at a wavelength of 635 nm. Sharp specular peaks can be seen from the BRDF plots for the relatively smooth sample; while for the relatively diffuse samples, the specular peaks are significantly lower. The directional-hemispherical reflectance (DHR) at wavelengths from 400 to 1000 nm was measured with an integrating sphere and a monochromator. Based on Kirchhoff's law, the absorptance was obtained from the DHR to be between 99.5% and 99.9% for all samples in the measured spectral region.