Electronic Structure Engineering of Ultrahigh-Purity Single-Photon Emission in Monolayer and Bilayer 2D WSe₂

High quality single-photon emission (SPE) is essential for technological applications, such as quantum information science, but remains difficult to achieve. In this study, we demonstrate two effective approaches to suppress classical background emission, thereby achieving ultrahigh-purity SPE monolayer and bilayer WSe₂ beyond typical strain engineering approaches. Electronic structure calculations on monolayer WSe₂ with Se vacancy reveals strongly spin-split valence bands combined with degenerate defect states, which enables the reduction of classical emission. This mechanism leads to 98.3% purity in monolayer WSe₂ by exploiting the valley-dependent excitation of circularly polarized light in a cross-polarization photoluminescence setup. Second, bilayer WSe₂ exhibit degenerate valence bands and thus negligible valley dichroism; however, its indirect bandgap and inversion symmetry allows polarization-independent high-purity of 97% in bilayer WSe₂, as supported by the comparable results between cross- and co-polarization photoluminescence measurements. Overall, these two approaches enable consistent high purity single-photon emission in monolayer and bilayer WSe₂ among the highest SPE purities for two-dimensional transitional metal dichalcogenides.