The second-order combination Raman modes of bilayer graphene in the range of 1800-2150 cm-1

In this paper, Raman modes of bi-layer graphene are investigated in detail in the frequency region between 1800 and 2150 cm-1. There are four dominant Raman modes in this frequency region. The frequencies of these modes dependent on excitation energy are revealed from the measurement with multi-wavelength lasers, which can be fully understood based on double resonance Raman scattering and the phonon dispersion relation of monolayer graphene. The results show that these Raman modes can be assigned to the combinational modes from the fundamental modes of iTA, LA and LO phonons, but not from iTO and oTO phonons as reported in the previous works. This study benefits us toward the full understanding of lattice dynamics of monolayer and multilayer graphenes.