On Carbon Nanotubes in the Interstellar Medium
  Since their discovery in 1991, carbon nanotubes (CNTs) -- a novel
one-dimensional carbon allotrope -- have attracted considerable interest
worldwide because of their potential technological applications such as
electric and optical devices. In the astrophysical context, CNTs may be present
in the interstellar space since many of the other allotropes of carbon (e.g.,
amorphous carbon, fullerenes, nanodiamonds, graphite, polycyclic aromatic
hydrocarbons, and possibly graphene as well) are known to be widespread in the
Universe, as revealed by presolar grains in carbonaceous primitive meteorites
and/or by their fingerprint spectral features in astronomical spectra. In
addition, there are also experimental and theoretical pathways to the formation
of CNTs in the interstellar medium (ISM). In this work, we examine their
possible presence in the ISM by comparing the observed interstellar extinction
curve with the ultraviolet/optical absorption spectra experimentally obtained
for single-walled CNTs of a wide range of diameters and chiralities. Based on
the absence in the interstellar extinction curve of the ~4.5 and 5.25 eV
$\pi$-plasmon absorption bands which are pronounced in the experimental spectra
of CNTs, we place an upper limit of ~10 ppm of C/H (i.e., ~4% of the total
interstellar C) on the interstellar CNT abundance.
