a, Typical Raman spectra of wrinkled graphene film after ICP treatment at 650 °C. b, Inhomogeneous distribution of ω2D collected from 2 mm × 2 mm region, showing that the strains are inhomogeneous on a large scale. c, Typical AFM image of Cu foil fully covered by graphene films. Scale bar, 1 μm. d, Typical Raman spectra of graphene films grown on different Cu substrates or suspended on a micro-sized hole. e, Raman spectra of wrinkled graphene films on Cu(111) after different treatments and of ultra-flat graphene films grown by ICP–CVD. All the processes here use D2. f, Distribution of Raman frequency collected from multiple points after different treatments.