Graphene crystals grown on insulating supports
An approach that produces single-crystal graphene sheets on large-scale electrically insulating supports, developed by an international research team, could help with the development of next-generation nanomaterial-based devices such as touchscreens, wearable electronics and solar cells. Their method has been published in the journal Nature Materials.
Most graphene-based electronic devices require insulating supports — yet high-quality graphene films destined for industrial use typically are 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. 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 Bo Tian, a PhD student at Saudi Arabia’s King Abdullah University of Science and Technology (KAUST). “This also opens the door to new types of graphene-based nanodevices.”
Together with colleagues from KAUST as well as from Asia and Europe, Tian tweaked 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.
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 towards 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, 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 500°C, 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 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 added. These heterostructures held together by van der Waals interactions are expected to be useful in future nanodevices.
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