Environment-induced effects on the temperature dependence of Raman spectra of single-layer graphene

Environment-induced effects on the E _2G G-band and A ₁′ 2D-band Raman spectral features of single-layer graphene provide insights on the intrinsic and extrinsic dependences of the phonon energy and line width on temperature. Graphene prepared via mechanical exfoliation in air exhibits a G-band line width that increases with temperature between 298 and 573 K but shows an opposite trend after annealing under Ar. The opposing temperature dependences are considered within the context of Kohn anomaly induced phonon softening and broadening. The primary cause of the changes in the E _2g phonon energy and the electron-phonon coupling is attributed to ambient O ₂ shifting the Fermi level away from the Dirac point. Our results emphasize the need to carefully consider the sample environment when investigating electronic and vibrational properties of graphene.