Fig. 2: Flat-to-flat transfer of wafer-scale monolayer graphene films.
From: Stacking transfer of wafer-scale graphene-based van der Waals superlattices
a Typical photo of 4-inch graphene transferred onto SiO2/Si wafer, the film looks homogenous at the macroscopic scale and the graphene can be distinguished from the reserved blank region. b Typical optical image of graphene on SiO2/Si, there are no cracks, folds, tears in the film and graphene can be only distinguished from the scratch. c Typical AFM height image of graphene and the inset height profile is taken along the dashed line, the thickness is homogenous with ~1.1 nm. d Statistical chart of the residual nanoparticles (NPs) on the as-transferred graphene, after vacuum annealing and after PAT, the annealing process will remove some NPs and the PAT can make graphene films much cleaner. e Statistical distribution of Raman intensity ratio of 2D to G peak (I2D/IG) in 100 × 100 μm2 of the as-transferred graphene, after vacuum annealing and after PAT, inset is the corresponding Raman mapping of the graphene after PAT. The distribution of I2D/IG is homogeneous after PAT with the value barely unchanged, while annealing will cause the decrease of the values. f Statistical distribution of the extracted shift values of 2D peak (Δω2D) measured from room temperature (RT) to 200 °C for the as-transferred, after vacuum annealing and after PAT graphene, the grey lines are the fit curves of the normal distributions. The PAT results in the similar coupling state to the as-transferred film. g Longitudinal resistances (Rxx) of the as-transferred, after vacuum annealing and after PAT graphene films as a function of gate voltage (Vbg), all the samples are fabricated into the large-sized Hall bars and the inset is the typical optical image. The PAT process can well decouple graphene and neutralize the doping level. h Photograph of the centimetre-sized Hall bar fabricated by transferred graphene on SiO2/Si after PAT, which is fixed on a chip carrier. i, Measured quantum Hall effect (QHE) for the centimetre-sized graphene Hall bars in h. Rxx and Hall conductivity (σxy) as a function of Vbg at different temperatures under perpendicular magnetic field B┴ = 7 T, the horizontal dashed lines are the guide lines of the Hall plateaus. Robust Hall plateaus can be easily observed even at RT, indicating the homogenous electrical properties over the wafer scale.
