Probing the electron-phonon coupling in ozone-doped graphene by Raman spectroscopy

We have investigated the effects of ozone treatment on graphene by Raman scattering. Sequential ozone short-exposure cycles resulted in increasing the p-doping levels as inferred from the blueshift of the 2D and G peak frequencies, without introducing significant disorder. The two-phonon 2D and 2D' Raman peak intensities show a significant decrease while, on the contrary, the one-phonon G Raman peak intensity remains constant for the whole exposure process. The former reflects the dynamics of the photoexcited electrons (holes) and, specifically, the increase in the electron-electron-scattering rate with doping. From the ratio of 2D to 2D' intensities, which remains constant with doping, we could extract the ratio of electron-phonon coupling parameters. This ratio is found independent on the number of layers up to ten layers. Moreover, the rate of decrease in 2D and 2D' intensities with doping was found to slowdown inversely proportional to the number of graphene layers, revealing the increase in the electron-electron collision probability.