Isotope-Labeled Graphene Reveals the Extrinsic Origin of Raman Signatures Assigned to Graphene Edge Modes.

Abstract

Sharp Raman bands near 1450 and 1530 cm^-1 observed under 633 nm excitation have previously been attributed to localized vibrational modes of zigzag and armchair graphene edges. Here, we employ an isotope-resolved Raman spectroscopy approach to investigate the origin of these features. Monolayer ¹³C graphene was synthesized and transferred alongside ¹²C graphene reference samples, enabling a direct comparison of isotope-dependent Raman signatures under identical processing conditions. Despite clear isotope-induced shifts of the intrinsic graphene modes, the peaks near 1450 and 1530 cm^-1 exhibit no isotope-dependent frequency shift, demonstrating that they do not originate from graphene lattice vibrations. Instead, their excitation-wavelength dependence and spectral characteristics are consistent with Raman enhancement of adsorbed molecular species under resonant conditions. These results establish isotope labeling as a robust experimental strategy for distinguishing intrinsic graphene vibrational modes from extrinsic Raman signals and provide a revised interpretation of Raman features previously attributed to graphene edge phonons.