Polymethyl methacrylate (PMMA) was spin-coated on top of the graphene of one side of the Cu foil (the other side was etched out by O2 plasma) and on the MoS2 monolayers to provide support during the transfer process. The Cu foil was etched by floating the PMMA-coated graphene/Cu foil on ammonium persulfate (APS) solution (20 g ℓ−1) for 5 h. After etching the Cu foil, the PMMA/graphene film was floated multiple times on deionized (DI) water to completely wash away the APS residue. Finally, the graphene was transferred to the desired wafer and the PMMA was removed by acetone. The SiO2 on the MoS2 was etched by floating the PMMA-coated MoS2 SiO2/Si wafer on diluted (1%) hydrogen fluoride (HF) solution. Afterward, the HF residues were washed away by floating the PMAA/MoS2 film on DI water in a manner similar to that for the PMMA/graphene film.