Nanotube chirality set early and preserved during growth — Structural memory in carbon nanotubes revealed

A research group including Assistant Professor Keigo Otsuka of the School of Engineering at the University of Tokyo, clarified that even if ambient temperatures and dimensions of catalyst particles change during growth of carbon nanotubes, winding ways (chirality) of nanotubes themselves are maintained as initial stages of generation. It indicates that processes where structures of nanotubes are determined and processes where they elongate can be designed separately. The findings were published in Nature Communications.

Carbon nanotubes can exhibit either metallic or semiconducting properties depending on their chirality. Nanotubes with well-defined structures are essential for realizing properties suited to specific applications. However, in floating catalyst chemical vapor deposition (FCCVD), a process widely used in industry, nanotubes are synthesized while catalyst nanoparticles flow through a reaction tube. As a result, the temperature and feedstock gas concentration experienced by the particles continually change, while the particles themselves gradually increase in size. It is generally accepted that larger catalyst particles produce wider nanotubes. Previous indirect observations and simulations have also suggested that nanotube diameters can change during growth in response to temperature, feedstock gas composition, and catalyst size.

The researchers asked whether the structure of a nanotube changes in response when its catalyst particle grows larger during growth. Because continuously monitoring catalyst particles during growth is extremely difficult, the team conceived a method that uses nanotube growth rates as a proxy for catalyst size. Since the growth rate is proportional to the surface area of the catalyst particle, tracking changes in growth rate over time makes it possible to estimate changes in catalyst dimensions.

To do this, the researchers applied a digital isotope labeling technique they had developed. The growth history of each nanotube was recorded directly within the nanotube itself using carbon isotope labels and later read out from Raman scattering spectra. Because the same spectra can also be used to determine chirality, the method provides a large dataset containing the growth histories of both catalyst particles and nanotubes.

Using iron catalyst particles supported on a substrate, the team synthesized carbon nanotubes while repeatedly varying the furnace temperature between 800℃ and 873℃. In one nanotube, the growth rate doubled as the catalyst particle coarsened, even at the same temperature, yet the nanotube maintained the same chirality over a length exceeding 150 µm.

Analysis of a total nanotube length of 9.4 mm showed that chirality changed only once every approximately 400 µm on average, a frequency comparable to that observed under steady-state growth conditions. Molecular dynamics simulations further confirmed that nanotubes continued to elongate while preserving their chirality, even as catalyst particles gradually increased in size.

A change in chirality requires the incorporation of defects, such as five- and seven-membered carbon rings, into the hexagonal carbon lattice. However, these unstable defects are removed on the catalyst surface before becoming incorporated into the nanotube wall, allowing the initially established chirality to be maintained.

The findings demonstrate a form of structural memory in which chirality is determined at an early stage and retained throughout growth, even as catalyst particle size and growth conditions change. Given that catalyst particles can evolve significantly during growth, post-growth observations alone are insufficient for understanding how nanotube diameter is governed by catalyst size.

From an industrial perspective, the findings suggest that nanotube structure control and yield optimization can be designed independently. The results therefore support strategies that temporally or spatially modulate growth conditions and may provide design guidelines for synthesis processes that achieve both high structural quality and high productivity.

Journal Information
Publication: Nature Communications
Title: Isotope-labeled growth histories reveal persistent chirality in individual carbon nanotubes despite catalyst evolution
DOI: 10.1038/s41467-026-77101-2