Tuning the optoelectronic properties of a WS₂-monolayer in different dielectric environments

Monolayers of transition metal dichalcogenides (TMDs) are direct-gap semiconductors with their optoelectronic properties strongly affected by the dielectric environment. We investigate the optical properties of WS₂ monolayers mechanically transferred on a pre-patterned Si/SiO₂ substrate with cylindrical wells of 3 μm in diameter. A protocol has been developed in order to easily obtain areas fully conformed to the well (strained) or suspended due to air being trapped in some wells. As a result, exactly the same monolayer is investigated under different dielectric environments. Detailed Raman mapping was used to quantify the strain and a fully T-dependent spectroscopic characterization using PL and Reflectivity was performed. Comparison of suspended to strained areas reveals a 10-fold enhanced PL efficiency with strong neutral excitonic emission at 78K. A T-dependent polarization PL spectroscopy was also performed for the strained and suspended parts of the flake in order to study the spin-valley depolarization behavior. Our work offers a useful approach to better understand the fundamental intrinsic properties of TMDs and a simple experimental procedure towards device fabrication with spatially controlled valley optoelectronic properties.