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 I(2) 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.