Ultrafast Bright-to-Dark Exciton Relaxation in Bilayer Borophene Driven by Strong Excitonic Effects.

Exciton dynamics in the recently discovered bilayer borophene (BL-α, consisting of two stacked boron sheets) are of great interest due to this material's promising electronic and optical properties for nano-optoelectronic applications. Using a GW plus real-time Bethe-Salpeter equation (GW-rtBSE) approach and nonadiabatic molecular dynamics (NAMD), we identify a Frenkel-type lowest-energy bright exciton and a spatially delocalized dark exciton in BL-α, with large binding energies of ∼700 and ∼502 meV, respectively. The electron-hole (e-h) Coulomb interaction (exciton effect) dominates over electron-phonon (e-ph) scattering, playing a pivotal role in an ultrafast bright-to-dark exciton transition with a relaxation time of ∼150 fs. Furthermore, the dark excitons undergo nonradiative recombination on a picosecond time scale (∼14 ps at room temperature). These results provide a theoretical foundation for potential nano-optoelectronic and light-energy harvesting applications of bilayer borophene.