Combinatorial catalyst approach for high-density growth of horizontally aligned single-walled carbon nanotubes on sapphire

We studied effects of metal catalyst and gas composition on the chemical vapor deposition (CVD) growth of horizontally aligned single-walled carbon nanotubes (SWCNTs) on r-plane sapphire substrates. The SWCNTs are sitting on the substrate and aligned along [11̄01̄] direction of the sapphire surface. A combinatorial metal deposition method was applied for single and binary metal catalysts to systematically investigate the thickness and the composition dependence. The horizontally-aligned SWCNTs grown from stripe-patterned catalysts enable the direct comparison of the catalytic activity based on nanotube density. We found that the SWCNT density strongly depends on the metal catalyst in the order Fe > Co ≫ Ni ≈ Cu, while no nanotubes were grown over Mo. In addition, the methane concentration during CVD strongly influenced the nanotube density, and the optimal concentration varied depending on the metal species and its thickness. The study on the binary metal catalysts revealed that Fe-Co combination increases the SWCNT density (7-9 tubes/μm) about twice of the original metal film. The Co-Cu binary catalyst also showed the high density (8-10 tubes/μm) under a limited methane concentration. Different catalytic activity of each metal is discussed.