Carbon nanotubes get into the groove

Ultrathin carbon nanotubes could have wonderous uses, like converting waste heat into electricity with near-perfect efficiency, and engineers at Rice University have now taken a big step toward that goal. This latest step continues a story that began in 2013, when Rice University's Junichiro Kono and his students discovered a breakthrough method for making carbon nanotubes line up in thin films on a filter membrane.

Nanotubes are long, hollow and notoriously tangle-prone. Imagine a garden hose that's dozens of miles long, then shrink the diameter of the hose to the width of a few atoms. Anyone who's ever fought with a knotted hose can appreciate Kono's feat: he and his students had turned a mob of unruly nanotubes into a well-ordered collective. Of their own accord, and by the billions, nanotubes would willingly lie down side-by-side, like dry spaghetti in a box. The problem was that Kono and his students had no idea why it was happening.

"It was magical. I mean, really mysterious," said Kono, an electrical engineer, applied physicist and materials scientist who has studied carbon nanotubes for more than two decades. "We had no idea what was really happening on a microscopic scale. And most importantly, we did not even know in which direction that nanotubes would align."

He and his team published their findings in 2016, and the field weighed in with possible explanations. The answer, as described in a new paper in Nano Letters by Kono's team and collaborators in Japan, is both unexpected and simple: tiny parallel grooves in the filter paper – an artifact of the paper's production process – cause the nanotube alignment.

A graduate student in Kono’s lab, study lead author Natsumi Komatsu, was the first to notice the grooves and associate them with nanotube alignment. "I found that any commercially purchased filter membrane paper used for this technique has these grooves," said Komatsu. "The density of grooves varies from batch to batch. But there are always grooves."

To form the two-dimensional (2D) crystalline films, researchers first suspend a mixture of nanotubes in a water-surfactant solution. The soap-like surfactant coats the nanotubes and acts as a detangler. In 2013, Kono's students were using vacuum filtration to draw these mixtures through membrane filter paper. The liquid passed through the paper membrane, leaving a film of aligned nanotubes on top.

In an exhaustive set of experiments, Komatsu and her colleagues, including Kono group postdoctoral researcher Saunab Ghosh, have now shown that the alignment of nanotubes in these films correspond to parallel, sub-microscopic grooves on the paper. These grooves likely form when the filter paper is pulled onto rolls at the factory, Kono said.

Komatsu examined dozens of samples of filter paper, and used scanning electron microscopes and atomic force microscopes to characterize the grooves and patterns of grooves. She cut filters into pieces, reassembled the pieces with grooves facing different directions and showed they produced films with matching alignments.

Komatsu and her colleagues also used heat and pressure to remove the grooves from the filter paper, similar to the way ironing removes wrinkles from clothing. They showed that films made with groove-free paper had nanotubes aligned in several directions.

Finally, starting with groove-free paper, they used a very fine reflective grating with periodic grooves to create their own patterns of grooves, finding that corresponding nanotube films would follow these new patterns.

Kono said the method is exciting because it brings a needed level of predictability to the production of 2D crystalline nanotube films. "If the nanotubes are randomly oriented, you lose all of the one-dimensional properties," Kono said. "Being one-dimensional is key. It leads to all of the unusual but important properties."

While Kono group's films are essentially 2D – as much as 1 inch in diameter but just a few billionths of a meter thick – the individual nanotubes behave like one-dimensional (1D) materials, especially in terms of their optical and electronic properties.

The extraordinary optical and electronic properties of carbon nanotubes depend on their diameter and structure, or chirality, which determines whether a carbon nanotube acts like a metal or a semiconductor. Researchers have struggled for decades to find a way to make large, macroscopic objects like a wire or one of Kono's 1-inch diameter films purely from nanotubes with a single diameter and chirality.

"That's obviously the next step," Ghosh said. "In this study, we still used a mixture of metallic and semiconducting carbon nanotubes with a diameter distribution. The next step is to apply this new method based on intentional groove-making using a grating to achieve total control of the alignment direction."

Kono said his team has made highly aligned 2D crystals from solutions with a diverse mixture of nanotubes. "But when we go to a single-chirality solution, we were never satisfied with the alignment," he said. "Now, with this knowledge of grooves, we are confident we can improve the degree of alignment in the case of single-chirality carbon nanotube films."

Single-chirality films could open the door to applications with mind-boggling potential – for example, sheets of pure carbon that can convert heat into light with almost perfect efficiency. Marrying such a sheet to a photovoltaic material could provide a way to turn heat into electric power very efficiently, creating the possibility of radiators that both cool motors and electronics while also powering them.

According to Kono, single-chirality crystalline films could also be used to study new states of matter, such as exciton polaritons and Bose-Einstein condensates, and for applications that haven't yet been envisioned.

"At this moment, only a small number of groups in the world can make these aligned, highly dense, heavily packed carbon nanotube films," he said. "And the work we just finished, the groove-assisted work, offers more control. This will lead to better films, new applications and new science. We are very excited."

This story is adapted from material from Rice University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.