Intercalation of Few-Layer Graphite Flakes with FeCl3: Raman Determination of Fermi Level, Layer Decoupling and Stability

We use anhydrous ferric chloride (FeCl3) to intercalate graphite flakes consisting of 2 to 4 graphene layers and to dope graphene monolayers. The intercalant, staging, stability and doping of the resulting intercalation compounds (ICs) are characterized by Raman scattering. The G peak of monolayer graphene heavily-doped by FeCl3 upshifts to~1627cm-1. 2-4 layer ICs have similar upshifts, and a Lorentzian lineshape for the 2D band, indicating that each layer behaves as a decoupled heavily doped monolayer. By performing Raman measurement at different excitation energies we show that, for a given doping level, the 2D peak can be suppressed by Pauli blocking for laser energy below the doping level. Thus, multi-wavelength Raman spectroscopy allows a direct evaluation of the Fermi level, complementary to that derived by Raman measurements at excitation energies higher than the doping level. We estimate a Fermi level shift of~0.9eV. These ICs are ideal test-beds for the physical and chemical properties of heavily-doped graphenes.