Experimental study of Kohn anomalies and phonon dispersion in graphene using Raman spectroscopy

The phenomenon of Kohn anomalies (KA) in graphene is characterized by a linear phonon dispersion law at the Γ and K points of the first Brillouin zone. Until now, experimental confirmation of the KA has been based on the observation of phonon softening during inelastic x-ray scattering measurements, and quantitative extraction of the dispersion parameters using optical benchtop techniques remains a critical challenge. Here, we determine the numerical value of the linear dispersion coefficient for the first time through an analysis of Raman-line broadening in ion-irradiated graphene. We demonstrate that in the dilute defect regime, the Raman linewidth evolution is governed by rigorous phonon spatial confinement, wherein point defects act as absolute phase-breaking scatterers. Furthermore, the broadening slope remains independent of the mass and energy of the irradiating ions, confirming that this phenomenon is an intrinsic property of the graphene lattice. For the G-band, the broadening is primarily determined by the KA at the Γ-point of the Brillouin zone center. By deriving and applying a closed-form non-linear Voigt profile correlation, we extract a linear dispersion coefficient of 403.5 cm−1, demonstrating remarkable quantitative agreement with the Density Functional Theory predictions (397 cm−1). This methodology establishes Raman linewidth analysis as a highly accurate approach for probing electron–phonon interaction parameters in two-dimensional materials.