Probing the Chiral Domains and Excitonic States in Individual WS2 Tubes by Second-Harmonic Generation.

Distinct from carbon nanotubes, transition-metal dichalcogenide (TMD) nanotubes are noncentrosymmetric and polar and can exhibit some intriguing phenomena such as nonreciprocal superconductivity, chiral shift current, bulk photovoltaic effect, and exciton-polaritons. However, basic characterizations of individual TMD nanotubes are still quite limited, and much remains unclear about their structural chirality and electronic properties. Here we report an optical second-harmonic generation (SHG) study on multiwalled WS2 nanotubes on a single-tube level. As it is highly sensitive to the crystallographic symmetry, SHG microscopy unveiled multiple structural domains within a single WS2 nanotube, which are otherwise hidden under conventional white-light optical microscopy. Moreover, the polarization-resolved SHG anisotropy patterns revealed that different domains on the same tube can be of different chirality. In addition, we observed the excitonic states of individual WS2 nanotubes via SHG excitation spectroscopy, which were otherwise difficult to acquire due to the indirect band gap of the material.

Solving the "MoS2 Nanotubes" Synthetic Enigma and Elucidating the Route for Their Catalyst-Free and Scalable Production.

This study solves a more than two-decades-long "MoS2 Nanotubes" synthetic enigma: the futile attempts to synthesize inorganic nanotubes (INTs) of MoS2 via vapor-gas-solid (VGS) reaction. Among them was replication of the recently reported pure-phase synthesis of the analogous INT-WS2. During these years, successful syntheses of spherical nanoparticles of WS2 and MoS2 were demonstrated as well. All these nanostructures were obtained by VGS reaction of corresponding oxides with H2/H2S gases, at elevated temperatures (>800 °C), in a fluidized bed reactor (FBR) and a one-pot process. This success and apparent similarity between the two compounds "hid" from us the option of looking for the INT-MoS2 reaction parameters in entirely different regimes. The main challenge in the synthesis of INT-MoS2 via VGS was the instability of the in situ prepared suboxide nanowhiskers against over-reduction and recrystallization at high temperatures. The elucidated growth mechanism dictates separation of the reaction into five steps, as properties of the intermediate products are not consistent with a single process and require individual conditions for each step. A horizontal reactor with a porous-quartz reaction cell, which creates proper quasi-static (contrary to the FBR) conditions for the reaction involving sublimation, was imperative for the effective nanofabrication of INT-MoS2. These findings render a reproducible synthetic route for the production of highly crystalline pure-phase MoS2 nanotubes via a multistep VGS process, without the assistance of a catalyst and in a scalable fashion. Being a semiconductor, flexible, and strong, INT-MoS2 offers a platform for much research and numerous potential applications, particularly in the field of optoelectronics and reinforcement of polymer composites.