Effects of atomic layer deposition on the optical properties of two-dimensional transition metal dichalcogenide monolayers

Two-dimensional semiconducting transition metal dichalcogenides (TMDs) have attracted significant interest due to their unique optoelectronic properties. More often, these materials are enclosed inside a dielectric layer that can work as an insulator for field-effect transistors. The insulating layer is typically grown with atomic layer deposition (ALD). Here, we study the effects on bare and hBN-covered monolayer MoS2 and WSe2 flakes with ALD TiO2 films. Our results reveal a significant shift and decrease in intensity in photoluminescence and Raman signals of the monolayer TMDs. Further analysis suggests that these changes are caused by chemical doping, strain, and dielectric screening after the ALD. Our study not only sheds light on the impact of ALD on the properties of TMDs, but also indicates ALD can be an alternative method to engineer the doping, strain and dielectric environment for potential optoelectronics and photonics applications.