Low-frequency modes in the Raman spectrum of sp-sp2 nanostructured carbon

Amorphous carbon films with sp-sp2 hybridization, produced by supersonic cluster beam deposition, shows the presence of both polyynic and cumulenic species [L.Ravagnan et al., Phys. Rev. Lett. 98, 216103 (2007)]. Here, we present an in situ Raman characterization of the low-frequency vibrational region 400–800 cm−1 of sp-sp2 films at different temperatures. We report the presence of two peaks at 450 and 720 cm−1. The lower frequency peak shows an evolution with the variation of the sp content and it can be attributed, with the support of density functional theory simulations, to bending modes of sp linear structures. The peak at 720 cm−1 does not vary with the sp content and it can be attributed to a feature in the vibrational density of states activated by the disorder of the sp2 phase.