Insights into electron dynamics in two-dimensional bismuth oxyselenide: a monolayer-bilayer perspective.

Bismuth oxyselenide (BiOSe), an emerging 2D semiconductor material, has garnered substantial attention owing to its remarkable properties, including air stability, elevated carrier mobility, and ultrafast optical response. In this study, we conduct a comparative analysis of electron excitation and relaxation processes in monolayer and bilayer BiOSe. Our findings reveal that monolayer BiOSe exhibits parity-forbidden transitions between the band edges at the point, whereas bilayer BiOSe demonstrates parity activity, providing the bilayer with an advantage in light absorption. Employing nonadiabatic molecular dynamics simulations, we uncover a two-stage hot-electron relaxation process-initially fast followed by slow-in both monolayer and bilayer BiOSe within the conduction band. Despite the presence of weak nonadiabatic coupling between the CBM + 1 and CBM, limiting hot electron relaxation, the monolayer displays a shorter relaxation time due to its higher phonon-coupled frequency and smaller energy difference. Our investigation sheds light on the layer-specific excitation properties of 2D BiOSe layered materials, providing crucial insights for the strategic design of photonic devices utilizing 2D materials.