  Nanotubes (NTs) of transition metal dichalcogenides (TMDs), such as MoS2 and
WS2, were first synthesized more than a quarter of a century ago; nevertheless,
many of their properties have so far remained basically unknown. This review
presents the state of the art in the knowledge of the optical properties of TMD
NTs. We first evaluate general properties of multilayered TMD crystals, and
analyze available data on electronic band structure and optical properties of
related NTs. Then, the technology for the formation and the structural
characteristics of TMD NTs are represented, focusing on the structures
synthesized by chemical transport reaction. The core of this work is the
presentation of the ability of TMD NTs to emit bright photoluminescence (PL),
which has been discovered recently. By means of micro-PL spectroscopy of
individual tubes we show that excitonic transitions relevant to both direct and
indirect band gaps contribute to the emission spectra of the NTs despite the
presence of dozens of monolayers in their walls. We highlight the performance
of the tubes as efficient optical resonators, whose confined optical modes
strongly affect the emission bands. Finally, a brief conclusion is presented,
along with an outlook of the future studies of this novel member of the family
of radiative NTs, which have unique potential for different nanophotonics
applications.
