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.