Nanotubes get organized to form films

tubes all going the same way

A simple filtration process helped researchers create flexible, wafer-scale films of highly aligned and closely packed carbon nanotubes.

Scientists have made inch-wide films of densely packed, chirality-enriched single-walled carbon nanotubes through a process revealed in Nature Nanotechnology.

In the right solution of nanotubes and under the right conditions, the tubes assemble themselves by the millions into long rows that are aligned better than once thought possible, the researchers report.

The thin films offer possibilities for making flexible electronic and photonic (light-manipulating) devices, says Rice University physicist Junichiro Kono, whose lab led the study. Think of a bendable computer chip, rather than a brittle silicon one, and the potential becomes clear, he says.

“Once we have centimeter-sized crystals consisting of single-chirality nanotubes, that’s it,” Kono says. “That’s the holy grail for this field. For the last 20 years, people have been looking for this.”

The lab is closing in, he says, but the films reported in the current paper are “chirality-enriched” rather than single-chirality. A carbon nanotube is a cylinder of graphene, with its atoms arranged in hexagons. How the hexagons are turned sets the tube’s chirality, and that determines its electronic properties. Some are semiconducting like silicon, and others are metallic conductors.

[Can graphene oust silicon from memory chips?]

A film of perfectly aligned, single-chirality nanotubes would have specific electronic properties. Controlling the chirality would allow for tunable films, Kono says, but nanotubes grow in batches of random types.

For now, the researchers use a simple process developed at the National Institute of Standards and Technology to separate nanotubes by chirality. While not perfect, it was good enough to let the researchers make enriched films with nanotubes of different types and diameters and then make terahertz polarizers and electronic transistors.

Nanotubes like pickup sticks

The lab discovered the filtration technique in late 2013 when graduate students and lead authors Xiaowei He and Weilu Gao inadvertently added a bit too much water to a nanotube-surfactant suspension before feeding it through a filter assisted by vacuum. (Surfactants keep nanotubes in a solution from clumping.)

The film that formed on the paper filter bore further investigation. “Weilu checked the film with a scanning electron microscope and saw something strange,” He says. Rather than drop randomly onto the paper like pickup sticks, the nanotubes—millions of them—had come together in tight, aligned rows.

“That first picture gave us a clue we might have something totally different,” He says. A year and more than 100 films later, the students and their colleagues had refined their technique to make nanotube wafers up to an inch wide (limited only by the size of their equipment) and of any thickness, from a few to hundreds of nanometers.

[Why nanotubes now have their own alphabet]

Further experiments revealed that each element mattered: the type of filter paper, the vacuum pressure, and the concentration of nanotubes and surfactant. Nanotubes of any chirality and diameter worked, but each required adjustments to the other elements to optimize the alignment.

The films can be separated from the paper and washed and dried for use, the researchers say.

They suspect multiwalled carbon nanotubes and non-carbon nanotubes like boron nitride would work as well.

How do the tubes line up?

Coauthor Wade Adams, a senior faculty fellow at Rice who specializes in polymer science, says the discovery is a step forward in a long quest for aligned structures.

“They formed what is called a monodomain in liquid crystal technology, in which all the rigid molecules line up in the same direction,” says Adams, senior faculty fellow in materials science and nanoengineering.

“It’s astonishing. (The late Rice Nobel laureate) Rick Smalley and I worked very hard for years to make a single crystal of nanotubes, but these students have actually done it in a way neither of us ever imagined.”

Why do the nanotubes line up? Kono says the team is still investigating the mechanics of nucleation—that is, how the first few nanotubes on the paper come together.

“We think the nanotubes fall randomly at first, but they can still slide around on the paper,” he says. “Van der Waals force brings them together, and they naturally seek their lowest-energy state, which is in alignment.” Because the nanotubes vary in length, the researchers suspect the overhangs force other tubes to line up as they join the array.

The researchers found their completed films could be patterned with standard lithography techniques. That’s yet another plus for manufacturers, says Kono, professor of electrical and computer engineering, of physics and astronomy, and of materials science and nanoengineering.

“I gave an invited talk about our work at a carbon nanotube conference, and many people are already trying to reproduce our results,” he says. “I got so much enthusiastic response right after my talk. Everybody asked for the recipe.”

Coauthors are from Rice, Zhejiang University, and Los Alamos National Laboratory. The Department of Energy and the Robert A. Welch Foundation supported the research.

Source: Rice University

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