Origin of dispersive effects of the Raman<b><i>D</i></b>band in carbon materials

TLDR

The study investigates the origin and dispersion of the disorder‑induced Raman D band (~1350 cm⁻¹) in sp² carbon materials as a function of laser energy. The authors attribute the high dispersion and large Raman cross section to electron–phonon coupling at the K point, involving optic phonons of the D band and the transverse acoustic branch whose breathing motion enhances the signal. Their model demonstrates that the D peak results from a resonant Raman process, contradicting the notion that it stems from laser‑energy‑independent phonon density‑of‑states features.

Abstract

The origin and dispersion of the anomalous disorder-induced Raman band $(D$ band) observed in all ${\mathrm{sp}}^{2}$ hybridized disordered carbon materials near 1350 ${\mathrm{cm}}^{\ensuremath{-}1}$ is investigated as a function of incident laser energy. This effect is explained in terms of the coupling between electrons and phonons with the same wave vector near the K point of the Brillouin zone. The high dispersion is ascribed to the coupling between the optic phonons associated with the D band and the transverse acoustic branch. The large Raman cross section is due to the breathing motion of these particular phonons near the K point. Our model challenges the idea that the Raman D peak is due to laser-energy-independent features in the phonon density of states, but rather is due to a resonant Raman process.