Can finite armchair nanotubes host organic color centers?

Fluorescent sp³ defects made with organic molecules covalently bound to single-walled carbon nanotubes (nanotubes), known as organic color centers (OCCs), are promising room-temperature sources of single-photons. While creating an OCC in a metallic nanotube is an interesting prospect due to the ballistic transport properties of the charge carriers, to date all OCCs have been made in semiconducting nanotubes. Here we present time-dependent density functional theory calculations on (6,6) armchair nanotube segments that show that the introduction of sp³ defects to the surfaces of the nanotubes results in singlet excited states with non-zero oscillator strength, lower in energy than the previously dominant low-lying transition (within the set of the first 10 singlet excited states). This suggests the possibility of creating OCCs in metallic nanotubes, since OCCs fluoresce by providing a new radiative pathway for excitons to recombine via photoemission.