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06/22/2020 - Mind-Blowing Sounds of the World's Thinnest Materials

by Alan S. Brown

a warm glowing background, and what looks like a rocky ridge in the foreground, and a small thin bar emerging from itImages, taken by a transmission electron microscope, show a single carbon nanotube protruding from an electrode. This nanotube is less than a micron long and only ten nanometers wide, or 10000 times thinner than the width of a single human hair. When a radio wave of a specific frequency impinges on the nanotube it begins to vibrate vigorously. An electric field applied to the nanotube forces electrons to be emitted from its tip. This electrical current may be used to detect the mechanical vibrations of the nanotube, and thus listen to the radio waves. Image courtesy of the Zettl Group.

Imagine a flat sheet of material so thin, your eyes cannot make it out. Yet place that material in a pair of ear buds and it will reproduce sound so accurately, you could distinguish the tone of an individual instrument in a symphony orchestra. 

interlocking hexagons on a black backgroundAn artist's conception of graphene's structure

That material is graphene, a form of carbon only a few atoms thick. After years of probing and manipulating graphene and other nanoscale objects, Alex Zettl, a physicist at University of California, Berkeley, and his former post-doctoral researcher Qin Zhou built their first graphene speaker five years ago. 

Now, Silicon Valley startup Graph Audio hopes to commercialize that technology within a year or two. At 64, Zettl, a member of UC Berkeley's Kavli Energy Nanoscience Institute, is suffused with wonder and enthusiasm when he talks about it.

"I listened to their prototype and it was unbelievable," he said. "The sound quality was so good, my eyes were as big as saucers."

He is not the only one. The ear buds drew a crowd of admirers at the Consumer Electronics Show last January. 

The physics behind their stellar performance lies in graphene's ability to reproduce sound accurately without any variations in volume. Every speaker tries to achieve this "flat" response. Few even get close.

To understand why, consider a conventional speaker. It starts with a diaphragm, usually a round cone of some lightweight but stiff material, attached to a magnet. Another magnet drives the speaker in and out, like a piston to create sound. The more precisely the magnet controls the cone's movement, the more accurate the sound.

Still, even the best speakers are never truly flat. Instead, their combination of mass, size, and magnet strength makes them more prone to emit sound in some frequencies more than others. By combining several speakers of different sizes into a single cabinet or headphone, however, engineers can achieve the illusion of a flat response. 

Graphene changes the equation by eliminating magnets and mass. Instead, the new speaker runs a current through a sheet of graphene sandwiched between a positive and a negative electrode. Alternating that current causes the sheet to vibrate and reproduce sounds with a very flat response.

"To get a wide frequency response, you want a material to have very low mass and enough strength to suspend over large area without tearing," Zettl said. "That's what graphene has. Its carbon atoms are very lightweight and they create very strong bonds. No other material can come close to that. It's so lightweight, air resistance does all the dampening for you, so you don't need a magnet."

Surprisingly, Zettl continued, graphene produces that flat response from subsonic through ultrasonic frequencies. This makes it useful for headphones, and also microphones, submarine communications, and even inexpensive ultrasonic medical imaging devices. 

But Zettl's speakers are only one of the interesting inventions that have grown out of Zettl's lab, which made its name by developing a way to manipulate atoms. 

Ambulatory Atoms

Zettl's parents both taught at San Francisco State, his father, media aesthetics and television production, and his mom, comparative literature. Zettl found it more fun to pull apart engines. That eventually led him to physics, which, he said, "best explained how nature works." 

After graduate school, he investigated low-dimensional materials, materials with at least one dimension so small that its properties fall between those of atoms and bulk materials. Zettl studied them using his lab's transmission electron microscope (TEM), a device that uses electrons instead of light to illuminate the ghostly outlines of atoms.

Then, in the early 1990s, researchers discovered nanotubes. They shared some properties with the materials Zettl was studying and he had a TEM, which turned out to be the instrument of choice for studying nanotubes.

"What was missing was the ability to manipulate those materials," he said. "It was like working with a Swiss watch with all those tiny gears. The TEM was our microscope and it let us see the gears. But we needed tools that matched the size of those gears move those things around and see how they worked, but all we had were hammers or chisels."

So Zettl built a nanomanipulator that fit inside the TEM. "It let us turn knobs and change voltages to manipulate things at the atomic scale," he said. The device, one of the first of its kind, now resides in the Smithsonian Institution. 

"Suddenly, we could try experiments we dismissed years before as impossible," Zettl said. "We could reach in and bend a molecule and see if it conducted electricity differently. For someone like me, who grew up when color television was a fantastic thing, the ability to manipulate atoms was amazing." 

One experiment involved nanotubes with multiple walls. Using the manipulator, he grabbed one of the tubes and pulled. The nanotube extended like an old-fashioned mariner's telescope. By measuring the force needed to pull out the tube, he calculated the friction between the walls, which was extremely low because the tubes' walls were atomically perfect. 

That experiment led to the world's first nanomotor. "We wired up a multiwalled nanotube and it spun around on concentric tubes, just like an electrical motor," Zettl said.

Using the nanomanipulator, the lab next turned its attention to complex nanoscale electric circuits. To see if they could do it, they tried to build a radio. 

"A radio is pretty sophisticated," Zettl explained. "It has an antenna, tuner, amplifier, and demodulator. We tried to build it out of discrete nanoscale components and wire them together on a chip. Sure, Intel does this all the time, but not at the size where we were working. And, basically, we failed. The components worked individually, but we could not integrate them in any meaningful way."

Then Zettl had an insight: He could leverage the quantum properties of a single nanotube to act like all those components at once. 

"The electrical engineers we showed this to said, 'No way that could work.' We had to show them how we could use quantum mechanics and tunneling--all kind of physics--to create a radio that worked by mechanical movement as well as electrons flowing."  

It was the ultimate integrated system. The nanotube worked like a mechanical antenna, vibrating as it picked up an FM signal. It captured the audio part of the vibrations like a demodulator. And it vibrated to play back voices and songs. 

Zettl's inventions--graphene speakers and nanotube motors and radios--sound suspiciously like engineering. Yet Zettl claims engineering is just a hobby, something his lab pursues because they are too cool to pass up. 

His real love is understanding how atoms, crystals, and defects behave at the smallest of scales. Sometimes, understanding those behaviors leads to something like a mind-blowing pair of ear buds. Other times, it brings Zettl closer to nature's most intimate mysteries. 

"It's a rich field," he said. "We will go wherever our research leads us.”

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