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

Abstract

Chemical vapor deposited WS 2 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 WS 2 samples can be determined from the PL spectra. The experimental Gruneisen parameter of the in-plane E′ Raman mode for 1L-WS 2 is found to be equal to the corresponding mode (E 2g ) mode in bulk WS 2 . 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 Gruneisen 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.

Large Biaxial Compressive Strain Tuning of Neutral and Charged Excitons in Single-Layer Transition Metal Dichalcogenides

“…On the other hand, we obtain a significantly more efficient transfer of strain than in previous thermal compression experiments, likely due to performing the experiments under vacuum and maintaining precise control of temperature change rates (see the Materials and Methods section). In fact, our gauge factor values are nearly twice as high as those reported in previous experiments involving thermally induced strain. , Furthermore, our gauge factor values are comparable to the best values achieved for biaxial tensile strain using bending techniques, ,, or pressurized membranes , (see Supporting Information Section S8 for a detailed comparison). Moreover, these values closely align with those predicted by ab initio calculations , (see Table ).…”

“…22,35,58,60 However, the effect of strain on the binding energies of trions has been less studied and only in the case of tensile strain experiments. 26,32 In a first approximation, the binding energies of excitons for an intrinsic 2D semiconductor depend solely on the excitonreduced mass μ and the screening length ρ for the Coulomb interactions within the material, 4,61−63 which are expected to change under applied strain. 58,64 From first-principles calculations (see the Materials and Methods section), we estimated that a 1.5% biaxial compressive strain can result in an ∼10% increase in the reduced mass for both 1L-MoS 2 and 1L-WS 2 (Supporting Information Section S11).…”

“…However, for 1L-WS 2 on PC, E b T is significantly larger and seems to increase with compressive strain, reaching ∼80 meV (see the inset of Figure c). The effect of strain on the binding energies of neutral excitons has been both experimentally and theoretically explored, demonstrating changes of the order of 10 meV/%. ,,, However, the effect of strain on the binding energies of trions has been less studied and only in the case of tensile strain experiments. , …”

“…Large amounts of uniform tensile strain, both uniaxial and biaxial, have been successfully induced in 2D semiconductors by bending them on flexible substrates. ,,,,− For instance, uniaxial strain up to ∼2.8% has been achieved in 1L-MoS 2 , and gauge factors for excitons up to 100 meV/% have been demonstrated for biaxial strain . The effects of larger, nonuniform, biaxial tensile strain on the optical properties of TMDs have also been explored by nanoindentation methods and by pressurizing single-layer membranes. , Some of these methods have also been applied to explore the impact of tensile strain on the binding energies and valley selectivity of excitonic species in single-layer TMDs. − However, experimental methods to apply compressive strain and to study its effects on the properties of 2D materials are limited.…”

Fine-Tuning of the Excitonic Response in Monolayer WS2 Domes via Coupled Pressure and Strain Variation

“…The different pressure trends exhibited by the in-plane and out-of-plane phonon frequencies can be ascribed to different responses of the strain components at work in the 1L-dome. According to ref the in-plane E 2 g 1 mode of monolayer WS 2 exhibits a redshift rate of ∼−6 cm –1 /% with increasing tensile biaxial strain, meaning that even a tiny strain variation causes a well visible shift in the peak center. Therefore, the unresponsiveness we observed for the 1L-dome E 2 g 1 frequency indicates the absence of pressure-induced modifications of the in-plane strain in the dome system.…”