Excitonic Emission in van der Waals Nanotubes of Transition Metal Dichalcogenides

Nanotubes of transition metal dichalcogenides (TMDs), were firstsynthesized more than a quarter of a century ago; nevertheless, many of theiroptical properties have so far remained basically unknown. Herein, the stateof the art in the knowledge of the optical properties of TMD NTs is presented.First, general properties of multilayered crystals are evaluated, and availabledata on related NTs are analyzed. Then, the technology for the formation andthe structural characteristics of NTs are represented, focusing on thestructures synthesized by chemical transport reaction. The core of this work isthe presentation of the ability of synthesized TMD NTs to emit brightphotoluminescence (PL), which has been discovered recently. By means ofmicro-PL spectroscopy of individual tubes, we show that excitonic transitionsrelevant to both direct and indirect band gaps contribute to the emissionspectra of the NTs despite having the dozens of monolayers in their walls. Theperformance of the tubes as efficient optical resonators is highlighted, whereconfined optical modes strongly affect the emission. Finally, a brief conclusionis presented, along with an outlook of the future studies of this novel radiativemember of the NTs family, which have unique potential for differentnanophotonics applications