Here we describe a doping approach that enables selective and variable doping on graphene. The doping level reflected in the successive shift of the Raman G mode can be progressively changed by varying the coverage of molecular adsorption on graphene. We make use of lattice defects which serve as anchor groups for the non-covalent functionalization on graphene to enhance molecule adsorption on defective sites at the elevated processing temperatures and also orbital overlap between graphene and adsorbates (melamine). Low density of defects, which can be monitored by seeing the intensity ratio of the D to G mode in the Raman spectra, was generated by exposing graphene to short Ar plasma pulses, followed by dopant adsorption. The controllable creation of defects makes the precise doping on graphene feasible. Systematic characterizations by Raman scattering show that holes are transferred to graphene, with the doping level depending on the surface coverage of melamine. The charge transfer is also identified by the downshift of the charge neutrality point in the transfer characteristics.
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