Time-order Phonon Scattering Processes are Responsible for the Asymmetric G* Raman Band in Graphene

Background: While most of the prominent features in the Raman spectrum of graphene are well understood as mentioned in patents within the Double Resonance (DR) picture, the origin of the peak at 2450 cm-1 (also called the G* band) still remains unclear. Method and Objective: In this work, we performed detailed Raman studies of single- and few-layer graphene using multiple laser excitations to unravel the origin of G* band. Results: Based on our analyses, we conclude that the G* band arises from a combination of Transverse Optical (iTO) and Longitudinal Acoustic (LA) phonons, and exhibits an asymmetric peak structure due to the presence of two different time-order phonon processes. The lower (higher) frequency sub-peak is ascribed to an LA-first (iTO-first) process. We provide three strong experimental evidences for the time-ordered scattering processes: the dependence of the G* band sub-peaks with (i) increasing laser energy, (ii) increasing defects, and (iii) increasing temperature. Finally, we attribute the enhanced asymmetry of the G* band in multi-layer graphene to multiple processes between electronic sub-bands, similar to the G' band in multi-layer graphene. Conclusion: Our study uncovered the origin and nature of the G* peak in the Raman spectrum of graphene. We believe our results have important implications for processes such as graphene-enhanced Raman scattering, where the time-ordered scattering of optical and acoustic phonons can be very useful for sensing analytes.