How to grow electronics that are one atom thick

Two atomically thin materials can form functional circuits given the right pattern.

John Timmer

Enlarge / Once a channel is cut into the graphene, a molybdenum disulfide crystal can grow within it.

Berkeley Lab

The features we're making in current semiconductor materials are shrinking to the point where soon, they will be just a handful of atoms thin. Unfortunately, the behavior you get from bulk materials is often different from what you see when there are just a few atoms present, and quantum effects begin to dominate. There is an alternative, however: start with a material that is already incredibly small and has well-defined properties. Graphene, for example, is a sheet of carbon just one atom thick, and it's an excellent conductor; a variety of similar materials have been also developed.

It's a big challenge to manipulate these things that are just one atom thick, so it's really hard to put together any sort of circuitry based on these materials. Now, however, researchers have figured out how create a template where single-atom-thick materials will grow to create functional circuitry.

As we noted above, graphene is an excellent conductor of electrons, so the authors of the new paper decided to use it to create wiring. But getting sheets of graphene lined up to consistently create the wiring of even simple circuitry has been nearly impossible. The authors didn't even try. Instead, they took a larger sheet of graphene, dropped it onto silicon dioxide, and then etched away any material they didn't want. The etching involved a plasma of oxygen ions, which burned channels in the graphene that were about 15µm wide.

Once the wiring was in place, the authors added semiconducting features. These were based on another atomically thin substance, molybdenum disulfide (MoS₂). This was put in place by a process called chemical vapor deposition, which is more or less exactly what it sounds like: the graphene/silicon substrate was placed in a chamber with vaporized MoS₂, which then formed crystals on the surface.

The key to this working is the fact that graphene doesn't provide a surface that MoS₂ is able to grow on; the silicon dioxide does. But the MoS₂ vapor also preferentially starts growing crystals along the edges of a graphene sheet, moving out from there to fill in the gaps between sheets. As a result, everywhere the graphene had been etched away ended up filled with a single-atom-thick sheet of MoS₂. And, since it started forming right at the edges of the graphene, the two materials integrated into a single electronic device.

Amazingly, all of this worked. After testing out the basic properties of the circuitry, the researchers constructed an inverter, which can also be viewed as a logical NOT gate.

The big limitation of the technique is our ability to control the oxygen plasma that etched away the graphene. Right now, the features are about 15µm wide, which is quite a bit larger than the latest silicon technology. But they are only a single atom thick, which means that you could potentially fit several layers of circuitry in the volume currently taken up by existing chips. And it should be possible to put the material between the layers to use for things like removing heat from the circuitry.

The real challenge will be showing that this approach can build something more complicated than the device demonstrated here. It's clear that the authors intend to try, as they've filed patent applications on this work.

Nature Nanotechnology, 2016. DOI: 10.1038/nnano.2016.115  (About DOIs).

Listing image by Berkeley Lab