“If graphene can be grown on an insulating substrate with a clean interface, certain devices might function better,” said project scientist Bo Tian of KAUST – the King Abdullah University of Science and Technology. “This also opens the door to new types of graphene-based nanodevices.”
The technique involves depositing copper – a common catalyst for graphene growth – on top of a sapphire substrate. Initially polycrystalline, careful heat treatment turns the cooper into a single thin film-like Cu(111) crystal.
Inserted into a methane atmosphere, the copper decomposes the gas into carbon atoms, which diffuse into the copper.
Further heat treatment causes graphene to form on the upper surface of the copper – this is a traditional way to form graphene.
However, this graphene is not what the team is interested in.
Instead, they are interested in carbon atoms that diffuse through the copper and migrate towards the copper-sapphire interface.
This interface acts as a template where well-oriented graphene islands form.
Repeated rounds of diffusion, heat treatment and etching away the upper graphene layer – a form of plasma etch-assisted chemical vapour deposition – allow the graphene islands at the copper-sapphire interface to merge into a single graphene crystal the same size as the copper layer, with the similar crystal lattice symmetry of the copper and sapphire guiding the formation of a high crystallinity graphene monolayer.
To remove the copper without damaging the graphene, the assembly is repeatedly thermally shocked by cycling it between liquid nitrogen and a furnace. This weakens the copper-graphene attachment and buckles the copper film, without damaging the graphene-sapphire bond. Eventually the copper film can be peeled away, leaving a single graphene crystal on the sapphire wafer.
“Field-effect transistors manufactured on the sapphire-grown single-crystal graphene monolayer exhibited excellent performance with higher carrier mobilities,” according to KAUST. “The superior electronic performance of the graphene grown on sapphire results from its higher crystallinity and fewer folds on the surface.”
“Our team is now trying to grow other two-dimensional materials on the insulator-supported graphene to build functionalised large-scale heterostructures,” added Tian “These heterostructures held together by van der Waals interactions are expected to be useful in future nanodevices.”
The work is published as ‘Wafer-scale single-crystal monolayer graphene grown on sapphire substrate‘ in Nature Materials.