Analysis of Raman scattering spectra of single-layer graphene doped with an ionic liquid*

Fundamental understanding of the electronic states of electrochemically doped single-layer graphene is essential for achievement of electric and electronic graphene devices. We fabricated a planar-type electrochemical cell operated with an ionic liquid, which is suitable for spectroscopic measurements. In the cell, the electric double layer induced by application of voltage results in high doping density. We observed Raman active phonon modes of graphene at various doping levels. The spectral shape of the G phonon mode reflects the effect of Fano resonance between the phonon excitation and the electronic excitation and becomes asymmetric depending on the degree of the effect. We performed a fitting analysis using a Breit-Wigner-Fano function to estimate the strength of the Fano coupling. On basis of the analysis, we discuss the electronic states of the single-layer graphene doped with ionic liquid.