Substrate Reflectance Controlled Optical Emission in a Chemical Vapor Deposition-Grown WS₂ Monolayer

Two-dimensional transition metal dichalcogenides (2D-TMDs) are the subject of enormous interest due to their exceptional optical and electronic properties, such as direct band gap, strong luminescence, high on–off current ratio ∼108, and device performance. Local environments and substrate effects are vital in determining the optical and electronic properties of 2D-TMDs, such as carrier mobility, charge carriers, absorption, and photoluminescence (PL). To evaluate the optical response of 2D-TMDs, previous reports suggest that strain, dielectric screening, and charge transfer are crucial parameters to consider. Previous reports mainly focused on mechanically exfoliated or transferred 2D-TMDs to investigate the optical response. However, there are challenges in pinpointing the mechanism behind the observed variation in the optical properties. Crucial factors such as substrate reflectivity are often missed when comparing optical responses on different substrates. This investigation presents the influence of substrate reflectance and transmittance on the PL of a chemical vapor deposition-grown WS2 monolayer. Here, we use SiO2/Si and Si as reflective substrates while sapphire and quartz are transparent substrates. WS2/SiO2/Si exhibits ∼2.7-fold and ∼2.3-fold enhancements in PL compared to WS2/sapphire and WS2/quartz, respectively. We demonstrate an ∼25% PL enhancement on the transparent sapphire substrate after Ag coating due to the reflected signal, proving the hypothesis.