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Existing carbon nanotube manufacturing technology—usually wrapped graphene sheets—cannot fully control their length, diameter, and type.

Recently, this problem has been solved for two of the three different types of nanotubes. However, a third type called "sawtooth" nanotubes is still out of reach. Now, scientists at the National Institute of Natural Sciences (NINS) in Japan have discovered how to synthesize a zigzag variant.

This new technology has been explained in a paper published in the journal Nature Chemistry on January 25, 2021.

Carbon exhibits the special ability to combine with other atoms to form molecules, so when it is combined with itself, it can be combined in a variety of different structural ways (such as graphite and diamond) and has different characteristics.

In the past few decades, shapes like graphene have been produced-a layer of carbon with a thickness of one atom formed by a hexagonal honeycomb lattice. Hollow graphene cylinders called nanotubes are another different form, or "allotropes."

In this honeycomb lattice, when the carbon hexagons in the nanotubes come together, they form a chiral, zigzag, or armchair structure. The term "sawtooth" is used for configurations where the "path" of each molecular bond between carbon atoms initially leads 60° to the left, then 60° to the right, then 60° to the left, and then to the right again 60°-zigzag pattern.

The term "armchair" means a path that moves to the left twice and then to the right twice before repeating the pattern. This path is obviously similar to an armchair, hence the name. The third substance called chirality and its mirror image are somewhere between these two forms.

If you use a knife to cut this tube horizontally with respect to the longitudinal axis twice, you can produce a nanotube "ribbon" formed by 12 carbon hexagonal rings. Such belts are called "nanobelts".

The measures taken to produce such nanoribbons have become the subject of many academic studies. This is due to the shortcomings of the traditional nanotube manufacturing method, which uses the so-called "top-down" form. Top-down manufacturing involves crushing large amounts of carbon into powder, and then randomly developing nanotubes into one or more of the three types.

The problem here is that you cannot control which configuration type, or diameter, or even length is formed. But if you can build nanotubes from the "seeds" of nanoribbons from the bottom up, then you can control all three aspects.

Yasutomo Segawa, Research Correspondence Author, Institute of Molecular Science, National Academy of Natural Sciences, USA

An early study conducted in 2019 can produce chiral nanoribbons and armchair nanoribbons, but not the third type-zigzag nanoribbons.

For the first time, NINS researchers can develop zigzag nanoribbons. The main aspect of the nanoribbon synthesis method is the bridging of hexagonal rings through oxygen atoms (addition of oxygen norbornadiene units). They can then use X-ray crystallography to verify that the structure predicted by theoretical calculations is indeed developing in the real world.

Due to the synthesis of the third type of nanoribbons, all three types of nanotubes-zigzag, chiral and armchair can be used in principle at present. This is a big step towards bottom-up production of customized carbon nanotubes.

The next step is to synthesize carbon nanotubes by using carbon nanotube nanoribbons as seeds, and extend this proof of principle to the actual structure-selective bottom-up carbon nanotube production.

Cheung, KY, etc. (2021) Synthesis of zigzag carbon nanobelt. Natural chemistry. doi.org/10.1038/s41557-020-00627-5.

Source: https://www.nins.jp/en/

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