Charge transfer and screening effects in polyynes encapsulated inside single-wall carbon nanotubes

A resonance Raman study of ${\text{C}}_{10}{\text{H}}_{2}$ and ${\text{C}}_{12}{\text{H}}_{2}$ polyynes encapsulated in single-wall carbon nanotubes of different diameters is presented. We show that the energy of the optical transitions associated with excited dark states of the polyyne depends on the diameter of the nanotube, as a consequence of different dielectric screenings. Strong changes in the shape of the $G$ band of the metallic nanotubes are observed when they are encapsulating the carbon linear chains, reflecting charge transfers between these two systems. From ab initio calculations, we find that such electronic transfers are most likely occurring from the nanotubes to the polyyne molecules.