Charge transfer and optical phonon mixing in few-layer graphene chemically doped with sulfuric acid

Chemical doping is expected to substantially increase the density of free charge carriers by charge transfer and modify the Fermi level and screening effect of doped materials. Here, along with Raman identification of 3 and 4 graphene layers by a 633-nm laser excitation, we investigated charge transfer and optical phonon mixing in few layer graphenes in detail by utilizing sulfuric acid as an electron-acceptor dopant. Sulfuric acid molecules are found to be only physically adsorbed on the surface layers of graphenes without intercalations. The top and bottom layers of bilayer graphene can be intentionally doped differently by concentrated sulfuric acid. The difference of the hole doping between the top and bottom layers results in phonon mixing of symmetric and antisymmetric modes in bilayer graphene. The Raman frequency evolution with the doping level is in agreement with recent ab initio density-functional theory calculations [P. Gava, M. Lazzeri, A. M. Saitta, and F. Mauri, Phys. Rev. B 80, 155422 (2009)]. Chemical doping by adsorption-induced charge transfer offers a way to study the electronic and vibrational behaviors of few layer graphenes at high-carrier concentration.