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.