Origin of the 2450cm−1 Raman bands in HOPG, single-wall and double-wall carbon nanotubes

The second order Raman signals around the G′-band region of graphite and carbon nanotubes have been investigated at more than 15 excitation laser lines. Two distinct Raman bands have been observed around 2700 cm−1; a prominent one is due to the so-called G′-band and the other is a weak band around 2450 cm−1. Both two bands can be from the double resonance process involving two phonons around the K-point in the phonon dispersion of a two-dimensional graphite. The 2450 cm−1-band has exhibited little power dependence, whereas the intensity of G′-band has shown large photon energy dependence as already reported. The 2450 cm−1-band and the G′-band correspond to non-dispersive q = 0 and fully-dispersive q = 2k, respectively. From the phonon dispersion and the corresponding phonon frequency, the 2450 cm−1-band can be assigned as an overtone mode of LO phonon (i.e. 2LO). This is revealed by calculated Raman spectra of graphite with proper electron–phonon matrix elements. The present study is the first report on the origin and assignment of the 2450 cm−1-band, which is based on the double resonance Raman scattering.