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 film and the use of hydrogen gas (H2) during anneal and growth. We found that the sheet resistance (Rs) of as-deposited Fe films is directly related to the average film thickness and can be used to estimate whether the films can catalyze the growth of super-aligned forests. The height of the CNT forests decrease with decreasing Rs, but only slowly. More importantly, CNTs grown on the largest and the smallest Rs films are less aligned. Instead, they are more curled and wavy due to the Fe NP dynamics. The use of Hydrogen (H2) affects the formation of Fe NPs from the as-deposited film as well as their composition during the forest growth. We find that the addition of H2 to a CNT forest growth process at 680 degrees C (C2H2/He [30/600 sccm]) increases the CNT alignment substantially. H2 can also reduce iron-oxides which otherwise would impede the formation of NPs. As a result, H2 has multiple roles: besides its chemical reactivity, H2 is important for catalyst reconstruction into NPs having a proper size distribution as well as surface density.