Carbon nanotubes behave as metals or semiconductors depending on their atomic arrangement, so structural uniformity is essential for their applications. In floating-catalyst chemical vapor deposition, the dominant route for industrial-scale production, catalyst particles spontaneously enlarge as they traverse continuously changing temperature and gas composition along the reactor. Whether an individual nanotube maintains its initial structure while growing under such conditions has remained unresolved, because of difficulty in monitoring a catalyst and its nanotube together over practical length scales.
The research team tracked the growth histories of individual nanotubes using digital isotope labeling, a technique developed by the team, in which the elongation history of each nanotube is encoded within its own structure as a sequence of carbon isotopes and read out afterward by Raman spectroscopy. Because a larger catalyst drives faster elongation, the recorded history also serves as an indicator of how the catalyst evolves. When the furnace temperature was raised and then lowered back, the nanotube growth rate was twice as high as before at the same temperature, indicating irreversible catalyst coarsening. The same nanotubes nevertheless retained atomically identical structures over lengths exceeding 100,000 times their diameter.
These findings show that nanotube structure is determined at nucleation and is robust to the subsequent growth environment, whereas the growth rate remains sensitive to it. This separation allows structure and yield to be optimized separately, in nucleation and elongation stages, offering a route toward resolving the long-standing trade-off between quality and quantity in nanotube synthesis.
Papers
Journal: Nature Communications
Title: Isotope-labeled growth histories reveal persistent chirality in individual carbon nanotubes despite catalyst evolution
Authors: Keigo Otsuka*, Ryuji Fujiwara, Shigeo Maruyama