Using gate-modulated Raman scattering and electron-phonon interactions to probe single-layer graphene: A different approach to assign phonon combination modes

Gate-modulated and laser-dependent Raman spectroscopy have been widely used to study q = 0 zone center phonon modes, their self-energy, and their coupling to electrons in graphene systems. In this work we use gate-modulated Raman of q not equal 0 phonons as a technique to understand the nature of five second-order Raman combination modes observed in the frequency range of 1700-2300 cm(-1) of single-layer graphene (SLG). Anomalous phonon self-energy renormalization phenomena are observed in all five combination modes within this intermediate frequency region, which can clearly be distinguished from one another. By combining the anomalous phonon renormalization effect with the double resonance Raman theory, which includes both phonon dispersion relations and angular dependence of the electron-phonon scattering matrix elements, and by comparing it to the experimentally obtained phonon dispersion, measured by using different laser excitation energies, we can assign each Raman peak to the proper phonon combination mode. This approach should also shed light on the understanding of more complex structures such as few-layer graphene (FLG) and its stacking orders as well as other two-dimensional (2D)-like materials.