Effect of Dielectric Environment on Excitonic Dynamics in Monolayer WS2

An experimental study on the effect of hexagonal boron nitride (h-BN) underlay and cap layers on excitonic dynamics in monolayer WS2 is reported. A monolayer WS2 flake is fabricated by mechanical exfoliation. By using a dry transfer technique, three regions of the sample are obtained: WS2 directly on SiO2, WS2 on h-BN, and WS2 sandwiched by two h-BN flakes. Photoluminescence measurements show higher yield and narrower linewidth of the h-BN/WS2/h-BN region. Transient absorption measurements reveal that the top h-BN layer enhances the exciton formation, prolongs the exciton lifetime, and slightly affects the exciton-exciton annihilation. By performing spatially resolved transient absorption measurements, exciton diffusion coefficients of about 100, 40, and 26 cm(2) s(-)(1) for the regions of WS2, h-BN/WS2, and h-BN/WS2/h-BN, respectively, are obtained. The suppression of exciton diffusion by h-BN is attributed to the additional phonon scattering mechanisms introduced by h-BN, which decreases the exciton mean free path and thus the diffusion coefficient. The findings provide useful information for designing and understanding the effect of h-BN layers interfacing with 2D semiconductors.