The effects of pre-annealing in either H2 or He on the formation of Fe nanoparticles for growing spin-capable carbon nanotube forests

Pre-annealing in either H₂ or He is shown to have a significant impact on the formation of either Fe nanoparticles or Fe contiguous films. This suggests various ways to form suitable-sized nanoparticles for the growth of well-aligned, spin-capable MWCNT forests. The reduction of the native iron oxide in as-deposited Fe films suppresses the formation of contiguous films, helping to form nanoparticles. In contrast, contiguous films are formed when the native iron oxide is crystallized into hematite, α-Fe₂O₃, by annealing the Fe film in He gas. The reduction of hematite in H₂ gas breaks up the contiguous films into nanoparticles. This break-up of Fe contiguous films into nanoparticles is suppressed by other effects such as Ostwald ripening processes at the forest growth temperature of 780 °C. Understanding the catalyst dynamics for forming either Fe nanoparticles or contiguous films provides ways to control the MWCNT alignment.