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.