Fig. 1

Unavoidable contamination on graphene surface during growth. a The competition between the formation of sp²-bonded graphene and defect-rich amorphous carbon (a–c) during the CVD growth of graphene. b AFM image of freshly prepared graphene on Cu foil after growth. c TEM image of a commonly as-grown graphene surface. Inset: HRTEM image of the clean and contaminated regions with atomic resolution. d TERS spectra of the unclean (blue line) and clean (red line) graphene regions in unclean graphene sample with Lorentzian line fit analysis, and in-situ far-field Raman spectrum of graphene in the same region (dark cyan line). Inset: TERS mapping of the D band intensity after smoothing. e Statistics of the D and G band positions from the contaminated regions of graphene grown by normal methane (blue) and ¹³C-labelled methane (red). Inset: representative TERS spectra of isotopically labelled graphene. f Photograph of 0.3 m × 1-m-sized unclean graphene after TiO₂ visualisation. An iPad was served as a size reference. Inset: dark-field optical microscopy (OM) image of the graphene surface decorated by TiO₂ particles. Note that, parameters such as the contact time and relative humidity of vaporised TiO₂ particles were kept identical, in order for a better compassion of graphene cleanness. Scale bar: 100 μm