Study on Sulfur Vacancies in CVD Grown Monolayer MoS2 & Strain Effects on Interlayer Coupling of MoS2/WSe2

Since the electrical and optical properties of TMD materials are directly related to their bandgaps, producing atomically thin TMDs with tunable band structures is important for practical device applications. Many techniques, including heterostructure engineering, doping, strain, surface functionalization, and the creation of semiconductor alloying structures, have been created to have the tunable band structures. The current study realizes band gap modulation through the use of heterostructure engineering and doping. Depending on the type of doping, the Raman A1g mode, which is linked to the out-of-plane mode, exhibits blue or red changes. In the current work, the out-of-mode A1g exhibits blue shifts for as-grown grains at 900°C caused by sulfur vacancies (n-type doping). The other methodology we employed is strain/heterostructure engineering to modulate the bandgap of individual layers of MoS2 and WSe2 and artificially stacked layer of MoS2/WSe2. We report fabrication of vertically stacked structures of monolayer MoS2 and multilayer WSe2 via wet transfer technique on both SiO2 and flexible PET (Polyethylene terephthalate). Fabricated heterostructures on SiO2 show shift, and reduction in intensity in E2g1 and A1g peak between the constituent layers of MoS2, WSe2. In addition to, there is a considerable shift (30 meV or more) in the MoS2 layer and the heterojunction. These heterostructures are then transferred to flexible substrate PET. On applying strain from a home-based equipment, the shifts in the Raman peak E2g1 and A1g, and the changes in the PL peak are recorded for the individual layers and heterostructures.