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A detailed high pressure X-ray diffraction and Raman spectroscopy study is carried out on monolayer WS2 and nanocrystalline WS2. The monolayer sample is obtained by liquid exfoliation. Photoluminescence and Raman measurements show it to consist of a monolayer. Careful analysis of ambient and high pressure data indicates the emergence of a triclinic phase at about 5.8 GPa in patches embedded in the parent hexagonal phase. This raises a question mark over the structural purity of the exfoliated monolayer materials beyond certain stress conditions. Raman mode values and their full width at half maximum of the monolayer sample show anomalous changes at about 27 GPa, the pressure where the sample completely gets converted to the triclinic structure indicating the importance of strain in structural as well as electronic properties of two dimensional materials.

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We report high pressure x-ray diffraction and systematic Raman measurements on a ReS 2 sample, which is mechanically exfoliated from a single crystal. A few new Bragg peaks are observed to emerge above 6 GPa indicating a structural transition from distorted 1T to distorted 1T 0 in a triclinic structure. The same is corroborated by the appearance of new Raman modes in the same pressure range. Softening of the Raman modes corresponding to Re atom vibrations is observed in the distorted 1T 0 phase in the pressure range of 15-25 GPa. In the same pressure range, the anomalous change in the volume is found to be induced by the lattice expansion. The volume expansion is related to the sliding of layers leading to octahedral distortion and an increase in octahedral volume. The sample is found to be very incompressible above 25 GPa with respect to below 15 GPa data. The same is also reflected in the Raman mode shifts with pressure.