Resonance Raman study of polyynes inside single-walled carbon nanotubes.

We report a resonance Raman study of polyynes C$n$H2 molecules ($n $= 10 and 12) composed of linearly bonded sp-carbon atoms aligned inside the single-walled carbon nanotubes (SWNTs), using many laser lines in the range 1.9 to 2.7 eV. The C10H2@SWNT hybrid material exhibits a Raman peak at 2066 cm-1 related with the stretching vibration of the C10H2 molecules interacting with SWNTs, while the C12H2@SWNT exhibits a peak at a lower frequency, around 2020 cm-1. The intensities of these peaks are strongly dependent on the laser energy, and exhibit maxima around 2.15 and 2.10 eV for the C10H2@SWNT and C12H2@SWNT materials, respectively. However, the optical absorption spectrum of the purified C10H2 and C12H2 dispersed in n-hexane exhibits peaks in the UV, around 4.9 and 4.5 eV, respectively.This result can be associated with a two-photon Raman resonance enhancement or can reflect important changes in the electronic structure of the polyynes when they are encaged in a carbon nanotube. New experimental and theoretical works are needed to understand this result.