Researchers proposed a new theory to grow pure carbon nanotubes

Material theorists from Rice University have published a new theory (Nov .09) in Science Advances to make carbon nanotube batches with a single, desired chirality. Their method could simplify the purification of nanotubes that are all metallic or all semiconductors.

Carbon Nanotube (CNTs)

Carbon nanotubes (CNTs) are made from sheets of single-layer carbon atoms called graphene rolled into a cylinder. Since their discovery three decades ago, these nanostructures have attracted worldwide scientific attention due to their excellent properties.

Features such as lightweight, convenient structure, immense mechanical strength, superior thermal and electrical conductivities, and stability put CNTs a notch above other material alternatives.

Today, a multitude of industries, including optics, electronics, water purification, and biosciences, innovate at an unprecedented scale with these CNTs.

As impressive as it sounds, CNTs are not easy to grow. They usually grow randomly with single and multiple walls and various chiralities.

Chirality means that a molecule cannot be superimposed on its mirror image, or, in simpler terms, it refers to symmetry. Because chirality determines a nanotube's electrical properties, the ability to grow chiral-specific carbon nanotubes is a nanotechnology holy grail. It could lead to wires that, unlike copper or aluminum, transmit energy without loss.

Single Chirality Carbon Nanotube

Now Professor Boris Yakobson and researcher Ksenia Bets from Rice University's George R. Brown School of Engineering have shown a new strategy to grow a batch of carbon nanotubes that are all the same.

Carbon nanotubes with different chiralities grow at different speeds. This means that slower-growing tubes can be separated out and ultimately eliminated.

This can be done with nozzles that pipe feedstock – in the form of hot, carbon-based gas – into an area, along with catalysts to provoke the reaction. If these nozzles and catalysts are moving, then only the faster-growing tubes will form.

"The catalyst particles are moving as the nanotubes grow, and that's principally important. "If your feedstock keeps moving away, you get a moving window where you're feeding some tubes and not the others," said lead author Bets.

Once the speed is controlled, etching can be done to harvest nanotubes with specific chiralities.

Like a giraffe stretching for leaves on a tall tree, carbon nanotubes reach for food as they grow

The paper referenced Lamarck giraffes — a 19th-century theory of how they evolved such long necks. Like a giraffe stretching for leaves on a tall tree, carbon nanotubes reach for food as they grow.

“It works as a metaphor because you move your ‘leaves’ away and the tubes that can reach it continue growing fast, and those that cannot just die out. Eventually, all the nanotubes that are just a tiny bit slow will ‘die.’” said Bets.

The researchers, focusing on theory, haven't tested their idea experimentally. But other labs have turned past Rice theories into products like boron buckyballs.

Researchers are hopeful that other labs will test this theory with experiments. "In terms of science, it's usually more beneficial to give ideas to the crowd. That way, those who have interest can do it in 100 different variations and see which one works. One guy trying it might take 100 years," Bets said.

"We don't want to be that 'guy.' We don't have that much time," added Yakobson.

Their paper is published in Science Advances.