Multiphonon Raman scattering in graphene

We report on multiphonon Raman scattering in graphene samples. Higher-order combination modes involving three and four phonons are observed in single-layer, bilayer, and few-layer graphene samples prepared by mechanical exfoliation. The intensity of the higher-order phonon modes (relative to the G peak) is highest in single-layer graphene and decreases with increasing layers. In addition, all higher-order modes are observed to upshift in frequency almost linearly with increasing graphene layers, betraying the underlying interlayer van der Waals interactions.

Figure 1

Higher-order Raman spectra from graphene samples collected with $E_{\mathrm{laser}}$ = 2.33 eV. All spectra have been normalized by the G peak intensity. Some weak intensity combination modes appearing above 4500 cm${}^{-1}$ are magnified for clarity.

Figure 2

Higher-order combination modes between 4000 and 4400 cm${}^{-1}$ in graphene samples collected with $E_{\mathrm{laser}}$ = 2.33 eV. All spectra have been normalized with respect to the G + 2D peak intensity for clarity and have been fitted with Lorentzian peaks. The D + 2D and G + 2D peaks upshift in frequency with increasing layers.

Figure 3

(a) Dispersion of the G + 2D and D + 2D peaks in SLG versus laser energy. (b) Peak frequencies of the G + 2D and D + 2D peaks versus 1/n. The error bars for the FLG samples were obtained from atomic force microscopy (AFM) measurements, which confirmed the presence of three to five layers.

Figure 4

Dispersion of four components within the G + 2D and 2D peaks in BLG versus laser energy. The dispersion of the highest frequency component in the G + 2D peak (∼98 cm${}^{-1}$/eV) is indicated in the figure.