Direct wall number control of carbon nanotube forests from engineered iron catalysts

We present the direct wall number control of carbon nanotube (CNT) forests grown on engineered iron (Fe) catalysts in a catalytic chemical vapor deposition (CVD). Engineered Fe catalysts were fabricated by annealing thickness-tuned (0.8-3 nm) Fe films with small thickness variations prepared by a low-deposition-rate sputter deposition. Extensive scanning electron microcopy (SEM) characterization confirmed that vertically-aligned CNT forests were grown on Fe catalyst films with thickness larger than 1.5 nm. Detailed high-resolution transmission electron microscopy (HRTEM) and micro Raman spectroscopy analysis indicated that 75% of the CNTs grown on an Fe film with 1.5 nm mean thickness were single-walled CNTs while about 67% and 59% of CNTs grown on Fe films with 2.0 and 3.0 nm mean thickness were double-walled and triple-walled CNTs, respectively. The average wall number and outer diameter of CNT forests were found to linearly depend on the mean thickness of Fe catalyst films.