Significant decrease in the reflectance of thin CNT forest films tuned by the Taguchi method

The Taguchi method of design of experiments was utilized to tune the optical properties of a carbon nanotube (CNT) forest by decreasing the total ultraviolet-visible (UV-Vis) reflectance. The CNT growth parameters included the acetylene flow rate, the thickness of the Fe catalyst film, substrate polarization potential (bias) during sputtering of the AlOx buffer layer, and acetylene-to-hydrogen gas flow ratio. A plot of the signal-to-noise (SN) ratio showed that the buffer-layer bias and catalyst thickness had the highest impact on the final results of the total optical reflectance. A verification experiment was conducted based on the optimized values of the growth parameters, and it yielded a 45% decrease in the UV-Vis reflectance to the lowest value (0.077% at λ = 750 nm) ever reported for a thin CNT forest film with relatively low height (∼20 μm). In addition, the structural parameters of the CNT forest were studied. The effects of the CNT structure (single-walled CNTs (SWCNTs), double-walled CNTs (DWCNTs), and multi-walled CNTs (MWCNTs)), filling factor (density), and alignment of CNTs on the total optical reflectance were investigated. The density and alignment of CNTs showed the highest impact, while the individual CNT structure exhibited a negligibly small effect on the low total optical reflectance.