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.