Strong Charge-Transfer Doping of 1 to 10 Layer Graphene by NO2

We use resonance Raman and optical reflection contrast methods to study charge transfer in 1-10 layer (1L-10L) thick graphene samples on which NO2 has adsorbed. Electrons transfer from the graphene to NO2, leaving the graphene layers doped with mobile delocalized holes. Doping follows a Langmuir-type isotherm as a function of NO2 pressure. Raman and optical contrast spectra provide independent, self-consistent measures of the hole density and distribution as a function of the number of layers (N). At high doping, as the Fermi level shift E-F reaches half the laser photon energy, a resonance in the graphene G mode Raman intensity is observed. We observe a decrease of graphene optical absorption in the near-IR that is due to hole-doping. Highly doped graphene Is more optically transparent and much more electrically conductive than intrinsic graphene: In thicker samples, holes are effectively confined near the surface, and in these samples, a small band gap opens near the surface. We discuss the properties and versatility of these highly charge-transfer-doped, few-layer-thick graphene samples as a new class of electronic materials.