Optical reflection and absorption of carbon nanotube forest films on substrates

The correlation between the height and density of multi-walled carbon nanotube forests and their optical properties in the mid-infrared region was investigated using nanotube forests grown on Al/Si, Al/Nb/Si, and fused silica substrates. Measurements of the hemispherical reflectance and transmittance of carbon nanotube forests are presented. Analyses by an effective medium approximation and a circular waveguide model are compared. It is found that circular waveguides with graphite walls of reduced conductivity can generate similar spectra of the absorption coefficients as carbon nanotube forests do. Parameters from the waveguide model can describe qualitatively the density and alignment of carbon nanotubes in the forest. With a proper density, a randomly modulated forest of less than 20 lm in height can generate a hemispherical reflectance of less than 0.002 in the mid-infrared region. (C) 2015 AIP Publishing LLC.