Fig. 3
Super-clean graphene surface after transfer onto target substrates. a–b AFM images of transferred super-clean graphene (a) and unclean graphene (b) onto SiO2/Si substrates. c Height histograms for super-clean graphene (grey, red line) and exfoliated graphene (black, blue line) on SiO2 substrate. The data of super-clean graphene is obtained from AFM image (b). d ToF-SIMS spectra of transferred super-clean (red line) and contaminated graphene (blue line) on SiO2/Si substrate after removal of 2H-PMMA. Note that the transfer process is kept the same for super-clean and unclean graphene. Inset (left): structural formula of 2H-PMMA. Inset (right): statistics of 2H- peak intensity of super-clean (red) and contaminated graphene (blue), as obtained from ToF-SIMS results
