Process intensification by CO₂ for high quality carbon nanotube forest growth: Double-walled carbon nanotube convexity or single-walled carbon nanotube bowls?

Introduction of CO₂ is a facile way to tune the growth of vertically aligned double- or single-walled carbon nanotube (CNT) forests on wafers. In the absence of CO₂, a double-walled CNT convexity was obtained. With increasing concentration of CO₂, the morphologies of the forests transformed first into radial blocks, and finally into bowl-shaped forests. Furthermore, the wall number and diameter distribution of the CNTs were also modulated by varying the amount of CO₂. With increasing CO₂ concentration, CNTs with fewer wall number and smaller diameter were obtained. The addition of CO₂ is speculated to generate water and serve as a weak oxidant for high quality CNT growth. It can tune the growth rate and the morphologies of the forests, prevent the formation of amorphous carbon, and reduce the wall number of the CNTs.