Ironing out Graphene’s Wrinkles

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Like a bullet train speeding down the track, electrons whizz through graphene at velocities approaching the speed of light. This makes the material a promising successor to silicon in electronic and photonic devices.

However, manufacturing graphene into a single perfectly flat ultrathin sheet is extremely difficult. Conventional manufacturing processes – like carbon vapor deposition (CVD) – can introduce wrinkles into the sheet, derailing the electron and significantly limiting graphene’s electrical performance.

During CVD, a single layer of carbon atoms are deposited onto a crystalline substrate such as copper foil before being submerged in acid to etch away the copper. This process produces large, macroscopic wrinkles in the graphene caused by the roughness of the underlying copper itself and the action of pulling the graphene from the acid. This uneven terrain prevents electrons from flowing at uniform rates.

Now, engineers at Massachusetts Institute of Technology (MIT) have found a way to manufacture graphene with fewer wrinkles, and to iron out any that do appear. Their work is important if we are to produce single-domain graphene – solitary layers of graphene which are uniform in both atomic arrangement and electronic performance – for use in industry.

“For graphene to play as a main semiconductor material for industry, it has to be single-domain, so that if you make millions of devices on it, the performance of the devices is the same in any location,” said Jeehwan Kim, the Class of 1947 Career Development Assistant Professor in the departments of Mechanical Engineering and Materials Science and Engineering at MIT. “Now we can really produce single-domain graphene at wafer scale.”

Previous work saw Kim and his team develop a method to produce single-crystalline graphene in which the orientation of carbon atoms is exactly the same throughout the wafer. They shunned CVD, and instead produced the single-crystalline graphene from a silicon carbide wafer with an anatomically smooth surface, albeit with nanometre step-like wrinkles. A thin sheet of nickel was used to peel off the topmost layer of graphene from the wafer in a process called layer-resolved graphene transfer.

In their latest paper- recently been published in Proceedings of the National Academy of Sciences - the scientists report that layer-resolved graphene transfer irons out the steps and tiny wrinkles in silicon carbide-fabricated graphene. Before transferring the graphene onto a silicon wafer, the team oxidized the silicon, thus creating a layer of silicon oxide that naturally exhibits electrostatic charges. When depositing the graphene, the scientists discovered that the silicon dioxide effectively pulled the carbon atoms in the graphene down onto the wafer, ironing out the steps and wrinkles.

To test whether the newly-produced graphene wafers were indeed single-domain, the team fabricated tiny transistors on multiple sites - including across previously wrinkled areas.

“We measured electron mobility throughout the wafers, and their performance was comparable,” Kim said. “What’s more, this mobility in ironed graphene is two times faster. So now we really have single-domain graphene, and its electrical quality is much higher [than graphene-attached silicon carbide].”

Each wafer exhibited uniform performance, meaning electrons were able to flow freely across each wafer, even at previously wrinkled regions.

Although there are still challenges to overcome before the graphene can be adapted for use in electronics, Kim says the work has provided a blueprint for how to reliably manufacture pristine, single-domain wrinkle-free graphene at the wafer scale.

“If you want to make any electronic device using graphene, you need to work with single-domain graphene,” said Kim. “There’s still a long way to go to make an operational transistor out of graphene. But we can now show the community guidelines for how you can make single-crystalline, single-domain graphene.”