Phys. Rev. B 107, 075420 (2023)
We present low-temperature Raman measurements on gate-tunable graphene encapsulated in hexagonal boron nitride, which allows us to study in detail the Raman G and 2D mode frequencies and linewidths as a function of the charge carrier density. We observe a clear softening of the Raman G mode (of up to 2.5 cm⁻¹) at low carrier density due to the phonon anomaly and a residual G mode linewidth of ≈3.5 cm⁻¹ at high doping. By analyzing the G mode dependence on doping and laser power we extract an electron-phonon-coupling constant of ≈4.4×10⁻³ (for the G mode phonon). The ultraflat nature of encapsulated graphene results in a minimum Raman 2D peak linewidth of 14.5 cm⁻¹ and allows us to observe intrinsic electron-electron scattering-induced broadening of the 2D peak of up to 18 cm⁻¹ for an electron density of 5×10¹²cm⁻² (laser excitation energy of 2.33 eV). Our findings not only provide insights into electron-phonon coupling and the role of electron-electron scattering in the broadening of the 2D peak but also crucially show the limitations when it comes to the use of Raman spectroscopy (i.e., the use of the frequencies and the linewidths of the G and 2D modes) to benchmark graphene in terms of charge carrier density, strain, and strain inhomogeneities. This is particularly relevant when utilizing spatially resolved 2D Raman linewidth maps to assess substrate-induced nanometer-scale strain variations.