Structural phase transition from 1H to 1T′ at low pressure in supported monolayer WS2 : Raman study

We present pressure-dependent Raman studies on monolayer 1H tungsten disulfide (WS2) supported on a polymer substrate up to 20 GPa. The structural deformation starts at similar to 2.9 GPa by observing a split in A(1)' mode. At similar to 8 GPa, first order Raman modes exhibit a change in pressure coefficients of phonon frequencies, and four new modes at 187, 248, 282, and 317 cm(-1) appear, which become increasingly dominant with pressure. As the new modes are identified with the 1T' phase, our Raman results show a mixed phase of 1T' and 1H above 8 GPa. Our results suggest that biaxial (planar isotropic) compressive strain plays in inducing the phase transition at a relatively lower pressure in polymer-supported monolayer as compared to bulk WS2.