MoS₂ and WS₂ Nanotubes: Synthesis, Structural Elucidation, and Optical Characterization

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.