Atom-scale covalent electrochemical modification of single-layer graphene on SiC substrates by diaryliodonium salts

•Controlled modification of epitaxial graphene is achieved by using diaryliodonium salts.•The grafting density can be reproducibly tuned from 4×1013 up to 3×1014moleculescm−2.•The grafting of nitrophenyl molecules occurs preferentially at basal planes.•Most of the grafts present a threefold-symmetry.