Spectroscopy of monolayer and multilayer tungsten disulfide under high pressure

Recently exfoliated monolayer and multilayered transition metal dichalcogenides have gathered significant interest based on their tunable bandgap and extremely high carrier mobility. We have investigated the Raman and photoluminescence spectra of monolayer and multilayer WS2 as a function of pressure. The Raman-inactive mode B-1u, which is activated by structural disorder, was revealed at 6.7 GPa in monolayers, at 8.0 GPa in bilayers, and at 13.7 GPa in multilayers, respectively. With the enhancement of pressure-induced interlayer interaction, the crystal phase transition due to layer sliding like 2H(c) to 2H(a) occurs at 14.8 and 18.7 GPa in bilayers and multilayers, as evidenced by the split of E-2g(1) and B-1u. The electronic phase transition of the monolayer is supposed to be a direct K-K bandgap changing to an indirect Lambda-K bandgap at 2.6 GPa. These observations contribute to a better understanding of the impact of interlayer interactions on the modulation of WS2 energy bands and structure, as well as fundamental studies of two-dimensional layered materials, which can inform the development of device applications.