Role of Surface Bands in the Photogeneration, Cooling, and Recombination of Charge Carriers in Two-Dimensional Bi₂Se₃

Bi₂Se₃, a layered three-dimensional topological insulator, exhibits intriguing changes in its band structure when its thickness is reduced below 7 quintuple layers. The reduction in thickness leads to hybridization between the surface states and the opening of a gap between these states. We combine density functional theory calculations with pump-probe spectroscopy to explore how these hybridized states affect the photogeneration, cooling, and recombination of charge carriers in two-dimensional Bi₂Se₃ nanoplatelets. Our calculations reveal that the hybridized surface states are crucial for understanding the optical transitions. By comparing the experimental absorption spectrum with the calculated absorptance in the near-infrared-visible region, we identify key transitions within the 2D Brillouin zone. We distinguish transitions involving the hybridized surface states from those involving the interior layers. We observe a significant delay of several picoseconds in carrier recombination when surface state transitions are excited, which we attribute to carrier accumulation in the valleys of the Rashba-shaped surface-state valence band and in higher-lying surface states of the conduction band. These findings emphasize the important role of surface state bands in the optical behavior of Bi₂Se₃ and their potential for manipulating carrier dynamics in two-dimensional materials.