Optimizing control of Fe catalysts for carbon nanotube growth

One must control the size distribution of catalyst Fe nano-particles (NPs) very carefully if one is to have any chance of growing "super-aligned" carbon nanotube (CNT) forests which can be spun directly into yarns and pulled directly into long sheets. Control of the Fe Nps size is important during all phases, including: the catalyst deposition, annealing and forest growth. As a result, it is important to understand how NPs are affected by various experimental factors as well as how those catalyst NPs then cause the growth of the forests. This paper focuses on two key experimental factors: The as-deposited thickness of the Fe catalyst ?lm and the use of hydrogen gas (H ₂) during anneal and growth. We found that the sheet resistance (R _s) of as-deposited Fe ?lms is directly related to the average ?lm thickness and can be used to estimate whether the ?lms can catalyze the growth of super-aligned forests. The height of the CNT forests decrease with decreasing R _s , but only slowly. More importantly, CNTs grown on the largest and the smallest R _s ?lms are less aligned. Instead, they are more curled and wavy due to the Fe NP dynamics. The use of Hydrogen (H ₂) affects the formation of Fe NPs from the as-deposited ?lm as well as their composition during the forest growth. We ?nd that the addition of H ₂ to a CNT forest growth process at 680 °C(C ₂H ₂/He [30/600 sccm]) increases the CNT alignment substantially. H ₂ can also reduce iron-oxides which otherwise would impede the formation of NPs. As a result, H ₂ has multiple roles: besides its chemical reactivity, H ₂ is important for catalyst reconstruction into NPs having a proper size distribution as well as surface density.