Silicon has been the material of choice for electronics for decades, but it’s beginning to bump up against efficiency limits. The next step could be carbon transistors and circuits, and now engineers at UC Berkeley have created metallic graphene nanoribbons, which can act as the wires in such all-carbon electronics.
Moore’s Law is a theory that describes the rate of technological progress, claiming that the number of transistors on a computer chip will double every two years or so. While it’s held true for decades, it’s beginning to slow down in recent years as we reach the physical limits of what's possible with silicon.
Abundant, cheap and available in a variety of forms, carbon is a great contender for keeping Moore’s Law going, particularly if all-carbon circuitry can be achieved. Graphite, diamond, and carbon nanotubes are all forms of carbon that have proven useful in various electronic components.
But perhaps most promising is graphene, a lattice of carbon just one atom thick. And even this can come in different shapes – as flat sheets, crumpled balls, tiny quantum dots, or long thin “nanoribbons.”
It’s that last shape that the UC Berkeley team has now made a breakthrough with. Graphene nanoribbons are usually semiconductors, but the team has managed to turn them into metals, which makes them conductive and able to act like wires to carry electrons around a circuit.
“We think that the metallic wires are really a breakthrough,” says Felix Fischer, an author of the study. “It is the first time that we can intentionally create an ultra-narrow metallic conductor – a good, intrinsic conductor – out of carbon-based materials, without the need for external doping.”
To create these metallic nanoribbons, the team stitched short segments together using heat to make the molecules chemically react and join together, forming a longer chain. In the end, the nanoribbon measured dozens of nanometers long and just 1.6 nanometers wide.
Once it was complete, the researchers found that the nanoribbons had the electronic properties of a metal, with each segment contributing just one conducting electron that can then flow freely along the ribbon. And finally, the team made one tiny change to the structure to boost its performance even further.
“Using chemistry, we created a tiny change, a change in just one chemical bond per about every 100 atoms, but which increased the metallicity of the nanoribbon by a factor of 20, and that is important, from a practical point of view, to make this a good metal,” says Michael Crommie, an author of the study.
While carbon nanotubes are excellent conductors and have shown promise in electronics, the team says that they’re harder to manufacture at scale. Nanoribbons are easier to make in bulk, making all-carbon electronics more viable.
“Nanoribbons allow us to chemically access a wide range of structures using bottom-up fabrication, something not yet possible with nanotubes,” says Crommie. “This has allowed us to basically stitch electrons together to create a metallic nanoribbon, something not done before. This is one of the grand challenges in the area of graphene nanoribbon technology and why we are so excited about it.”
The research was published in the journal Science.
Source: UC Berkeley