The Raman spectra of graphene includes the G peak located at ∼ 1580 cm − 1 and 2D peak at ∼ 2700 cm − 1 , caused by the in-plane optical vibration (degenerate zone center E2g mode) and second-order zone boundary phonons, respectively. The D peak, located at ∼ 1350 cm − 1 due to first-order zone boundary phonons, is absent from defect-free graphene, but exists in defected graphene.