  Organic color centers in single-walled carbon nanotubes have demonstrated
exceptional ability to generate single photons at room temperature in the
telecom range. Combining the color centers with pristine air-suspended tubes
would be desirable for improved performance, but all current synthetic methods
occur in solution which makes them incompatible. Here we demonstrate formation
of color centers in air-suspended nanotubes using vapor-phase reaction.
Functionalization is directly verified on the same nanotubes by
photoluminescence spectroscopy, with unambiguous statistics from more than a
few thousand individual nanotubes. The color centers show a strong
diameter-dependent emission intensity, which can be explained with a
theoretical model for chemical reactivity taking into account strain along the
tube curvature. We are also able to estimate the defect density by comparing
the experiments with simulations based on a one-dimensional diffusion equation,
whereas the analysis of diameter dependent peak energies gives insight to the
nature of the dopant states. Time-resolved measurements show a longer lifetime
for color center emission compared to E$_{11}$ exciton states. Our results
highlight the influence of the tube structure on vapor-phase reactivity and
emission properties, providing guidelines for development of high-performance
near-infrared quantum light sources.
