Anomalous 2D and D + D'' Raman signatures in few-layer graphene

Since the early 1980s, Raman spectroscopy has been a key tool for characterizing carbon-based materials. The discovery of graphene has significantly boosted the use of Raman spectroscopy, leading to exponential growth in its applications. Despite this, anomalies in the Raman signatures of many graphene-based systems remain prevalent. In this study, we revisit the Raman spectroscopy of mono- and few-layer graphene using high-resolution experimental data, focusing on the Raman 2D and D + D'' peaks and their characterization using Lorentzian fitting. Our observations reveal that the Raman 2D peak of bilayer graphene consists of four Lorentzian peaks with uniform widths. In contrast, the Raman 2D peak of four-layer graphene displays an anomalous signature with eight Lorentzian peaks of varying widths. We interpret this anomaly through group theory analysis of electron-phonon interactions and compare our findings with existing literature. Additionally, we highlight that the Raman D + D'' peaks exhibit asymmetric profiles across monolayer, few-layer, and bulk graphite. For monolayer graphene and bulk graphite, the signature is characterized by a dominant peak at 2445 cm^-1, efficiently captured by two Lorentzian peaks at par with existing literature. However, for bilayer, trilayer, and four-layer graphene, the D + D'' Raman signatures show a broader peak profile with a dominant peak at 2460 cm^-1and multiple neighboring peaks. Instead of two Lorentzian sub-peaks, these signatures for few-layer graphene are better characterized using three or four Lorentzian sub-peaks, indicating more electron-phonon transitions.