Two members of our group, Adrián Francisco and Alejandro Goñi, published ‘On the impact of the stress situation on the optical properties of WSe2 monolayers under high pressure‘ in the open access journal Papers in physics. The insights of this study is also very relevant for high pressure measurements including 2D systems, such as 2D transition-metal dichalcogeneides, aand graphene using diamond cells.
We have studied the optical properties ofmonolayers (ML) by means of photoluminescence (PL), PL excitation (PLE) and Raman scattering spectroscopy at room temperature and as a function of hydrostatic pressure up to ca. 12 GPa. For comparison the study comprises two cases: A single ML directly transferred onto one of the diamonds of the diamond anvil cell and a ML encapsulated into hexagonal boron nitride (hBN) layers. The pressure dependence of the A and B exciton, as determined by PL and PLE, respectively, is very different for the case of the bare ML and the heterostructure. Whereas for the latter the A and B exciton energy increases linearly with increasing pressure at a rate of 3.5 to 3.8 meV/GPa, for the bare ML the A and B exciton energy decreases with a coefficient of -3.1 and -1.3 meV/GPa, respectively. We interpret that this behavior is due to a different stress situation. For a single ML the stress tensor is essentially uniaxial with the compressive stress component in the direction perpendicular to the plane of the ML. In contrast, for the substantially thicker heterostructure the compression is hydrostatic. The results from an analysis of the pressure dependence of the frequency of Raman active modes comply with the interpretation of having a different stress situation in each case.