Single-Crystal Bilayer Graphene with Controlled Stacking from Ni-Cu Gradient Alloy

Previously reported approaches for bilayer graphene (BLG) growth include flat growth substrates of Cu or Ni-Cu uniform alloys and “copper pocket” structures. Use of flat substrates has the advantage of being scalable, but the growth mechanism is either “surface limited” (for Cu) or carbon precipitation (for uniform Ni-Cu), which results in multi-crystalline BLG grains. For copper pockets, growth proceeds through a carbon back-diffusion mechanism, which leads to the formation of highly crystalline BLG but scaling of the copper pocket structure is expected to be difficult. Here we demonstrate a Ni-Cu gradient alloy that combines the advantages of these earlier methods: the substrate is flat, so easy to scale, while growth proceeds by a carbon back-diffusion mechanism leading to high-yield growth of BLG with high crystallinity. The BLG layer stacking was almost exclusively Bernal or twisted with an angle of 30 °, consistent with first principles calculations we conducted. Furthermore, we demonstrated scalable production of transistor arrays based single-crystal Bernal-stacked BLG with a bandgap that was tunable at room temperature.