Intrinsic lineshape of the Raman 2D-mode in freestanding graphene monolayers

TL;DR

This study reveals that the intrinsic 2D Raman mode in freestanding graphene exhibits a bimodal lineshape that is sensitive to doping levels and photon energy, requiring a full two-dimensional model for accurate description.

Contribution

It demonstrates the intrinsic bimodal lineshape of the 2D Raman mode in freestanding graphene and how doping and photon energy influence this feature, challenging simpler models.

Findings

Bimodal 2D-mode lineshape in freestanding graphene

Doping obscures the bimodal lineshape, mimicking supported samples

Full two-dimensional phonon wavevector contributions are necessary for accurate modeling

Abstract

We report a comprehensive study of the two-phonon inter-valley (2D) Raman mode in graphene monolayers, motivated by recent reports of asymmetric 2D-mode lineshapes in freestanding graphene. For photon energies in the range $1.53 \rm eV - 2.71 \rm eV$, the 2D-mode Raman response of freestanding samples appears as bimodal, in stark contrast with the Lorentzian approximation that is commonly used for supported monolayers. The transition between the freestanding and supported cases is mimicked by electrostatically doping freestanding graphene at carrier densities above $2\times 10^{11} \rm cm^{-2}$. This result quantitatively demonstrates that low levels of charging can obscure the intrinsically bimodal 2D-mode lineshape of monolayer graphene, which can be utilized as a signature of a quasi-neutral sample. In pristine freestanding graphene, we observe a broadening of the 2D-mode feature with decreasing photon energy that cannot be rationalized using a simple one-dimensional model based on resonant \textit{inner} and \textit{outer} processes. This indicates that phonon wavevectors away from the high-symmetry lines of the Brillouin zone must contribute to the 2D-mode, so that a full two-dimensional calculation is required to properly describe multiphonon-resonant Raman processes.