Extended Data Figure 1: Characterization of representative graphene.

a, Scanning electron microscope image of chemical vapour deposition graphene on copper foil. All the graphene used in these experiments was predominantly single-layer, with some small bilayer patches. The larger-scale contrast variations show the copper grains. Scale bar is 10 µm. b, Raman spectrum of chemical vapour deposition graphene transferred to SiO₂/Si (285-nm oxide layer) substrate. The spectrum shows graphene’s characteristic G peak at 1,580 cm⁻¹ and two-dimensional peak at 2,700 cm⁻¹; the ratio between the two indicates that the graphene is primarily monolayer. A small D peak at 1,350 cm⁻¹ indicates low disorder. The small peak at 2,450 cm⁻¹ is background. c, High-contrast bright-field TEM image of graphene transferred over 10-nm-thick Si₃N₄ windows shows continuous monolayer graphene. Scale bar is 1 µm. d, False-colour composite image of dark-field TEM images showing grain size and shape. The graphene is polycrystalline, with grain sizes of the order of micrometres. Scale bar is 1 µm. e, TEM diffraction pattern for region shown in c and d.