Charge transfer chemical doping of few layer graphenes: charge distribution and band gap formation.

The properties of few layer (one layer (1 L) to four layer (4 L)) graphenes doped by adsorption and intercalation of Br-2 and I-2 vapors are investigated. The Raman spectra of the graphene G vibrations are observed as a function of the number of layers. There is no evidence for chemical reaction disrupting the basal plane pi electron conjugation. Adsorption of bromine on 1 L graphene creates a high doped hole density, well beyond that achieved by electrical gating with an ionic polymer electrolyte. In addition, the 2D Raman band is completely quenched. The 2 L bilayer spectra indicate that the doping by adsorbed 12 and Br-2 is symmetrical on the top and bottom layers. Br-2 intercalates into 3 L and 4 L graphenes. The combination of both surface and interior doping with Br-2 in 3 L and 4 L creates a relatively constant doping level per layer. In contrast, the G spectra of 3 L and 4 L with surface adsorbed I-2 indicate that the hole doping density is larger on the surface layers than on the interior layers and that I-2 does not intercalate into 3 L and 4 L. This adsorption-induced potential difference between surface and interior layers implies that a band gap opens in the bilayer type bands of 3 L and 4 L.