Observation of Low-Frequency Combination and Overtone Raman Modes in Misoriented Graphene

Abstract

Stacking disorder will significantly modify the optical properties and interlayer coupling stretch of few-layer graphene. Here, we report the observation of the Raman breathing modes in the low-frequency range of 100–200 cm–1 in misoriented few-layer graphene on a SiO2/Si substrate. Two dominant Raman modes are identified. The one at ∼120 cm–1 is assigned as the Eg + ZO′ combination mode of the in-plane shear and the out-of-plane interlayer optical phonon breathing modes. Another peak at ∼182 cm–1 is identified as the overtone mode 2ZO′. The appearance of these Raman modes for different twist angles indicates that stacking disorder in few-layer graphene significantly alters the Raman feature, especially for those combination modes containing the interlayer breathing mode. Further investigation shows that the two Raman vibrational modes (∼120 and ∼182 cm–1) are strongly coupled to the excitation laser energy, but their frequencies are independent of the number of graphene layers before folding. The present work provides a sensitive way to study the phonon dispersion, electron–phonon interaction, and electronic band structure of misoriented graphene layers.