Using the G′ Raman cross-section to understand the phonon dynamics in bilayer graphene systems

The G′ (or 2D) Raman band of AB stacked bilayer graphene comes from a double resonance Raman (DRR) process and is composed of four peaks (P ₁₁, P ₁₂, P ₂₁, and P ₂₂). In this work, the integrated areas (IA) of these four peaks are analyzed as a function of the laser power for different laser lines. We show that the dependence of the IA of each peak on temperature is different for each distinct laser excitation energy. This special dependence is explained in terms of the electron-phonon coupling and the relaxation of the photon-excited electron. In this DRR process, the electron is scattered by an iTO phonon from a K to an inequivalent K′ point of the Brillouin zone. Here, we show that this electron relaxes while in the conduction band before being scattered by an iTO phonon due to the short relaxation time of the excited electron, and the carrier relaxation occurs predominantly by emitting a low-energy acoustic phonon. The different combinations of relaxation processes determine the relative intensities of the four peaks that give rise to the G′ band. Some peaks show an increase of their IA at the expense of others, thereby making the IA of the peaks both different from each other and dependent on laser excitation energy and on power level. Also, we report that the IA of the G′ mode excited at 532 nm, shows a resonance regime involving ZO′ phonons (related to the interlayer breathing mode in bilayer graphene systems) in which a saturation of what we call the P ₁₂ process occurs. This effect gives important information about the electron and phonon dynamics and needs to be taken into account for certain applications of bilayer graphene in the field of nanotechnology.