The reason for an upper limit to the height of spinnable carbon nanotube forests

The reason for the upper limit on the height of spinnable carbon nanotube (CNT) forests was studied. To analyze the differences between CNT forests with different heights, we synthesized CNT forests using different growth times (3, 6, 9, 12, 15, and 60 min). The height of the CNT forests increased from 260 μm at 3 min to 1.7 mm at 60 min, and the spinnability decreased sharply after 9 min of growth, where a wavy morphology first appeared. Raman analysis of the CNT forest grown for 9 min showed that the intensity ratio of G-band to D-band at the upper region was 1.50 and that near the bottom was 1.14. We also found that the reaction termination process affected the spinnability of the CNT forests. Depending on the termination process, both spinnable and non-spinnable CNT forests could be selectively synthesized, because of the different morphologies in their lower regions. The results suggested that any wavy morphology produced due to a disturbance in growth conditions causes a loss of spinnability.