Exploring two-phonon Raman scattering in bilayer graphene by carbon isotope labeling

The scattering processes of Raman modes in monolayer graphene have been extensively discussed, whereas they become more complex in bilayer graphene due to interlayer interaction. Here, we present a Raman study of bilayer graphene utilizing isotope labeling to investigate significant two-phonon scattering processes. Our findings reveal that the G*, 2D, and 2D′ modes can be explicitly decoupled into three components originating from the top layer, the bottom layer, and both layers combined. This indicates that the scattering processes of all three modes inherently involve interlayer electron–phonon coupling. Furthermore, as the twist angle of bilayer graphene increases, the proportion of these interlayer scattering components exhibits a non-monotonic evolution. The local enhancement of the interlayer components closely tracks the line shape broadening of the two-phonon mode, which is fundamentally driven by the resonance between the emitted photon energy and the energy gap of the Van Hove singularities. Importantly, we clarify that this optically derived interlayer electron–phonon coupling fraction should be distinguished from the static structural van der Waals interaction strength. This work establishes a more comprehensive framework for analyzing the Raman spectra of graphene under diverse conditions, advancing the utility of Raman spectroscopy in probing the intricate physical properties of low-dimensional 2D materials.