Excitonic resonances in WS₂ nanotubes

Resonance Raman profiles of WS₂ nanotubes of different diameter are presented. We show that the A excitonic transition energy lies below the bulk value and is increasingly redshifted with decreasing diameter of the nanotubes. The findings are attributed to strain effects associated with the curvature of the nanotube walls. A silent Raman mode, the B_1u mode, is disorder enhanced in the Raman spectra of WS₂ nanomaterials.We discuss the development of the relative intensities of the B_1u mode and the nearby A_1g mode with nanotube diameter, excitation energy, and hydrostatic pressure in terms of a slight difference in resonance condition.