(6,5) Carbon nanotubes: ultrahigh-purity synthesis achieved through trimetallic catalyst innovation

A central scientific question in carbon nanotube (CNT) research remains whether single-walled CNTs can be synthesized with direct control of chirality, particularly for small-diameter semiconducting species that are technologically useful. Achieving such control is essential because a nanotube’s chirality determines its electronic band gap, optical response, and device performance.

However, current growth methods produce broad mixtures of chiralities, and even syntheses termed “selective” rarely exceed moderate purity, typically in the 50–80 % range. While direct synthesis with over 90 % purity has been demonstrated for a small number of chiralities—most notably (14,4)- and (12,6)-CNTs—important targets such as (6,5)-CNTs have remained well out of reach.

In a 2024 article, an AIMR research team led by Toshiaki Kato reported a breakthrough approach that addresses this limitation by redesigning the catalyst itself¹. The team demonstrated that a carefully engineered trimetallic catalyst can precisely control the energetics of nanotube growth, enabling near-single-chirality synthesis that was previously inaccessible.

“The novelty of this work lies in the use of a NiSnFe trimetallic catalyst in which partial formation of Ni₃Sn intermetallic crystals occurs within nanoscale catalyst particles,” explains Kato. “This specific phase selectively lowers the activation energy for the growth of (6,5)-CNTs during plasma-enhanced chemical vapor deposition, while suppressing competing chiralities.”

Using this approach, the authors achieved an ultrahigh-purity synthesis of (6,5)-CNTs, reaching approximately 95.8% purity as confirmed by photoluminescence, UV–vis–NIR spectroscopy, and Raman analysis. They also observed the direct formation of chirality-pure (6,5)-CNT bundles and measured photoluminescence lifetimes more than 20 times longer than those of isolated tubes.

“Previous mono- and bimetallic catalysts can bias nanotube growth, but their ability to discriminate between closely competing chiralities is fundamentally limited,” says Kato. “We discovered that a trimetallic system can stabilize a specific intermetallic phase within the catalyst nanoparticles, which provides an additional level of thermodynamic control and—in this case—favors the nucleation of (6,5)-CNTs.”

The results from this work demonstrated that intermetallic catalyst phases can be used to engineer chirality selection at the atomic level, providing a general strategy for single-chirality nanotube growth. Beyond synthesis, the formation of chirality-pure bundles with enhanced excitonic properties opens new opportunities for optoelectronic and quantum photonic applications.

A future direction involves extending this multi-element catalyst approach to synthesize other single-chirality nanotubes on demand for semiconductor and quantum devices.

A personal insight from Dr. Toshiaki Kato 

Looking back on this project, what stands out most about the discovery process and its significance?

The most memorable aspect was the discovery itself—a master's student found the breakthrough catalyst somewhat by chance while systematically searching through catalyst combinations. What made this particularly satisfying was our ability to explain why it worked through detailed structural analysis, proposing a growth model that validated the experimental results. Catalysts with three or more elements had never been explored for CNT synthesis—it was outside conventional thinking. By challenging previous assumptions, we opened an entirely new research direction. Proving that our intuition was correct has been deeply encouraging and provides a design principle for future catalyst development.

This article was written by Patrick Han, Ph.D. (patrick@sayedit.com).

About the World Premier International Research Center Initiative (WPI)

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Advanced Institute for Materials Research (AIMR)

Tohoku University

Establishing a World-Leading Research Center for Materials Science

AIMR aims to contribute to society through its actions as a world-leading research center for materials science and push the boundaries of research frontiers. To this end, the institute gathers excellent researchers in the fields of physics, chemistry, materials science, engineering, and mathematics and provides a world-class research environment.