Technique offers support to single-crystal graphene monolayers

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Single-crystal graphene sheets produced on large-scale electrically insulating supports could help the development of next-generation nanomaterial-based devices, such wearables and solar cells.

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Rooftop solar installation (Image by Jyotiska Barua from Pixabay)

This is the claim of a team from KAUST, Saudi Arabia who said most graphene-based electronic devices require insulating supports. They added that high-quality graphene films destined for industrial use are typically grown on a metal substrate, such as copper foil, before being transferred to an insulating support for device fabrication. This transfer step can introduce impurities that affect how well the device performs. Consequently, efforts to grow graphene on insulating supports have not been able to produce the required high-quality single crystals.

“If graphene can be grown on an insulating substrate with a clean interface, certain devices might function better,” said Ph.D. student Bo Tian, who co-led the study under the supervision of Professor Xixiang Zhang from KAUST’s Physical Science and Engineering Division. “This also opens the door to new types of graphene-based nanodevices.”

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Zhang, Tian and co-workers from Asia and Europe adjusted the chemical vapour deposition method, which relies on the copper-catalysed decomposition of methane into carbon precursors, to generate smooth single-crystal graphene monolayers on wafer-scale single-crystal substrates called c-plane sapphire. Their findings are detailed in Nature Materials.

The researchers converted polycrystalline copper foil into its single-crystal counterpart Cu(111) on the sapphire surface and introduced active carbon atoms from the metal substrate-catalysed decomposition of methane on the resulting film. The carbon atoms diffused through the metallic film toward the copper–sapphire interface, which acted as a template, and formed well-oriented graphene islands that, after several growth cycles, merged to yield a sheet.

In addition to weak surface interactions, the copper film and sapphire displayed similar crystal lattice symmetry to that of graphene, Tian said in a statement, which explains the high crystallinity of the graphene monolayer.

The researchers etched away any graphene that had accumulated on top of the copper film using a hydrogen–argon plasma to facilitate carbon diffusion. They immersed the samples in liquid nitrogen before heating them rapidly to 500oC, making the copper film easy to peel off while keeping the graphene monolayer intact.

Field-effect transistors manufactured on the sapphire-grown single-crystal graphene monolayer are said to have exhibited excellent performance with higher carrier mobilities. The superior electronic performance of the graphene grown on sapphire results from its higher crystallinity and fewer folds on the surface, Tian said.

“Our team is now trying to grow other two-dimensional materials on the insulator-supported graphene to build functionalised large-scale heterostructures,” Tian said.