This study solves a more than two-decades-long "MoS 2 Nanotubes" synthetic enigma: the futile attempts to synthesize inorganic nanotubes (INTs) of MoS 2 via vapor−gas−solid (VGS) reaction. Among them was replication of the recently reported pure-phase synthesis of the analogous INT-WS 2 . During these years, successful syntheses of spherical nanoparticles of WS 2 and MoS 2 were demonstrated as well. All these nanostructures were obtained by VGS reaction of corresponding oxides with H 2 /H 2 S 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-MoS 2 reaction parameters in entirely different regimes. The main challenge in the synthesis of INT-MoS 2 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-MoS 2 . These findings render a reproducible synthetic route for the production of highly crystalline pure-phase MoS 2 nanotubes via a multistep VGS process, without the assistance of a catalyst and in a scalable fashion. Being a semiconductor, flexible, and strong, INT-MoS 2 offers a platform for much research and numerous potential applications, particularly in the field of optoelectronics and reinforcement of polymer composites.

The synthesis of high-quality WS2 and more so of MoS2 (multiwall) nanotubes in substantial amounts from oxide precursors is a very challenging and important undertaking. While progress has been offered by a recent report, the present work presents another step forward in the synthesis of MoS2 nanotubes with a narrow size distribution and better crystallinity than before. Williamson–Hall analysis of the X-ray diffraction data is used to analyze the crystallinity and strain in the nanotubes. This analysis shows that the crystallinity and average diameter of the WS2 and MoS2 nanotubes reported here (type II) are better than those obtained according to the previous methods (type I). Size selection by centrifugation reported by others is used here to prepare several fractions of WS2 and MoS2 nanotubes according to their average diameter. The high refractive index of WS2 and more so MoS2 enables the nanotubes to trap light by total internal reflection, turning them into nanocylindrical resonators and thereby supporting cavity-mode resonances. The extinction, net absorption, and transient absorption of suspensions of MoS2 and WS2 nanotubes of different (average) diameters were investigated. A strong coupling effect between optical cavity modes and the A and B excitons was observed for the WS2 and MoS2 nanotubes with diameters above 80 and 60 nm, respectively. These conclusions are also supported by transient absorption measurements. Finite-difference time-domain (FDTD) calculations support the experimental findings, confirming the strong coupling effect in the WS2 and MoS2 nanotubes. These results are important not only for their own sake but also because they may bear on the new photocatalytic applications of such nanotubes.

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 WS 2 nanotubes on a single-tube level. As it is highly sensitive to the crystallographic symmetry, SHG microscopy unveiled multiple structural domains within a single WS 2 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 WS 2 nanotubes via SHG excitation spectroscopy, which were otherwise difficult to acquire due to the indirect band gap of the material.

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