Carbon nanotubes are nearly atomically thin carbon structures — just 1-1.2 nanometers thin.

"Pure" carbon nanotubes are a powerful semiconductor, one that can compete with silicon for integration into microprocessors.

Today, scientists revealed a 16-bit microprocessor built from all carbon nanotubes, pushing us a step closer to integrating them into real world computers.

Carbon isn't just the stuff life is made of—it's also the stuff our future is being built on.

Carbon—a versatile element that frequently trades off its electrons to create various forms of itself—has been gaining an exciting reputation in tech thanks to the successful exfoliation of graphene, a sheet of carbon that's just one atom thick and has remarkable chemical properties.

But carbon nanotubes, a sort of cousin to graphene, has been quietly staking out its own place in the world of materials science.

A paper in Nature today announced one big step for carbon nanotubes: they've been successfully integrated into a 16-bit chip, one that sent out the message "Hello, World! I am RV16XNano, made from CNTs." Hello, World! is a computing trope going back to at least the 1970s.

"Carbon nanotubes have been a promising material for next generation electronics for almost two decades now," Max Shulaker, a professor of Electrical Engineering and Computer Sciences at MIT and lead author of the paper, says. "But there has always been a giant disconnect between the promise of carbon nanotubes and being able to build a real working system out of them."

What are Carbon Nanotubes?

A microscopy image of the carbon nanotube microprocessor. Shulaker et al

If graphene is a sheet of carbon just an atom thick, carbon nanotubes are sort of a rolled up version of graphene. They are lightweight and strong as steel. But most relevant to material scientists, they're a near perfect semi-conductor.

A semiconductor is a material that can conduct electricity, but which can also be shut off. This is built into the material itself and is temperature dependent. The most famous semi-conductor is silicon, whose ability to turn on and off holds the binary states that make up modern computing.

Carbon nanotubes are important because they're small size—again, in some cases, down to a couple atoms—makes them a compact semiconductor that could, as time goes on, make smaller and smaller transistors in microprocessors, leading to even more powerful computers.

How Are Carbon Nanotubes Made?

Carbon likes to assemble itself into various forms. For instance, under high pressure, carbon will arrange itself into a diamond. Similarly, carbon nanotubes are grown under pressure as well.

Essentially, graphite—a fairly common form of carbon—is placed in a furnace under high pressure and high temperature. Helium gas is pumped into the chamber, and certain metals are added as catalysts. As the carbon burns, it produces a soot. Within this soot are nanotubes.

But they're not necessarily pure nanotubes, at least not always. In order to build a chip, Shulaker and his team needed carbon nanotubes that were only semiconductors. But impure carbon nanotubes can take on the properties of metals, whose conductance can't be turned off.

"You can get 99.99% of CNTs to be semiconducting, but it’s darn near impossible to get that percentage to 100%," he says.

But thankfully, there's a way to sort this scientific wheat from the chafe.

"The best way to get the most pure group of carbon nanotubes is to get them in a solution form," Christian Lau, one of Shulaker's graduate research assistants, says. So the team took the furnaced nanotubes and placed them in a solution that separates "metallic" nanotubes from semiconducting ones. A polymer is then applied to help exfoliate off further imperfections.

From there, they're ready to be assembled into working electronics.

How Do Nanotubes Become a Chip?

A technician holds the completed RV16X-NANO microprocessor. Shulaker et al

Once the nanotubes are separated, they can be placed on a substrate—the surface that keeps them in place—and built into a fully functioning chip. Groupings of nanotubes on substrates can be integrated together to build the larger device. This isn't the first carbon nanotube microprocessor to be built, but it's the most complex. A previous device, for instance, had 178 transistors. This 16-bit chip has more than 14,000. Modern computer processors have billions of transistors inside, even in smartphone chips, so carbon nanotubes still have a long way to go.

But because of their compact size, nanotube computing components could be integrated with silicon components down the line, improving speeds and shrinking the sizes of chips. Carbon nanotubes can also be pushed into superconducting states at lower temperatures, potentially making them more energy efficient than silicon.

So Carbon Nanotubes Are Only Good for Computers?

Carbon nanotubes are so exciting to researchers because they're potential reaches beyond computing. In addition to building smaller scale computing components, metallic nanotubes can serve their own purpose, replacing copper in some applications where a powerful conductor is needed.

They can also be integrated into lithium ion batteries, which currently rely on graphite—and, increasingly, graphene—in electrodes. They've been shown in some cases to double storage capacity of batteries as a result.

They can also be used as components in solar cells, in biomedical devices, and in composite materials, where their strength and weight becomes an asset for building durable, light materials.

So When Does the Carbon Nanotubes Future Begin?

Shulaker's team is working with DARPA and companies like Analog Devices (who, contrary to their name, builds cutting edge semiconductors for digital computing), on scaling up the microprocessor, and making use of the SkyWater Technology Foundry to build a fuller-scale processor outside the lab.

In silicon microprocessors, Moore's Law—the speed and power of processors doubles nearly every two years—has always been in effect. Some have worried that we're reaching the end of what silicon can provide.

But for carbon nanotubes, the limit has yet to be discovered.

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