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

Bismuth oxyselenide (Bi 2 O 2 Se), 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 Bi 2 O 2 Se. Our findings reveal that monolayer Bi 2 O 2 Se exhibits parity-forbidden transitions between the band edges at the Γ point, whereas bilayer Bi 2 O 2 Se 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 Bi 2 O 2 Se 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 Bi 2 O 2 Se layered materials, providing crucial insights for the strategic design of photonic devices utilizing 2D materials. There is a two-stage hot-electron relaxation process-initially fast followed by slow-in both monolayer and bilayer Bi 2 O 2 Se within the conduction band, and the monolayer displays a shorter relaxation time.