Tuning the Photoluminescence and Raman Response of Single-Layer WS₂ Crystals Using Biaxial Strain

Chemical vapor deposited WS2 monolayers are subjected for the first time to controlled pure biaxial tensile strain up to 0.7%. From photoluminescence (PL) spectroscopy, the trion and neutral exciton deformation potentials are found to be similar, approximately −130 meV/%. It is shown that the excess carrier concentration as well as residual strain in WS2 samples can be determined from the PL spectra. The experimental Grüneisen parameter of the in-plane E′ Raman mode for 1L-WS2 is found to be equal to the corresponding mode (E2g) mode in bulk WS2. The impact of mechanical strain on the electronic and phonon band structures is also calculated in the framework of density functional theory. The theoretically obtained deformation potential for the direct transition is in very good agreement with the experiment. The reduced dimensionality of the monolayer enables the visualization over the entire Brillouin zone of both the calculated phonon dispersions and the Grüneisen parameters, which are compared with the experimentally accessible ones. This work contributes to the experimental implementation of mechanical strain engineering applications in semiconducting two-dimensional transition metal dichalcogenides.