Resolving localized phonon modes on graphene/Ir(111) by inelastic atom scattering

Low energy (few meV) acoustic phonon modes of graphene/Ir (111) have been measured using inelastic helium atom scattering (HAS). The data clearly show two vibrational modes with energies 4.6 and 7.2  meV at Γ¯ with dispersionless branches due to Umklapp scattering. The phonons observed are not the ones expected for a free-standing graphene monolayer, in spite of the weak interaction between graphene and Ir (111). The measured parabolic dispersion of the flexural mode along the ΓM¯ direction allows determining the bending rigidity κ=(0.80±0.07)eV. Inelastic HAS measured at diffraction conditions proves that the moiré superstructure results in localized phonon modes related to the upper and the lower regions of the graphene superstructure. Our study shows that inelastic HAS is suitable to resolve low-energy vibrations spatially localized on structures in the nanometer range.