Quantum Confinement Effects in Monolayer Black Phosphorus: Revision of the Nonradiative Recombination Dynamics

The paper reports a computational study of excited-state dynamics in pristine and divacancy-defect-containing monolayer black phosphorus (ML-BP), with the primary focus on the nonradiative recombination of the lowest energy exciton. The excitonic effects are treated using time-dependent density functional theory (TD-DFT) combined with several hybrid density functionals, while the excitonic dynamics is modeled using a variety of quantum-classical nonadiabatic molecular dynamics methods. Our calculations reveal that the nonradiative recombination rates obtained from the overcoherent quantum dynamics increase with the increase of the exact exchange fraction in hybrid functionals which increases the corresponding S₁–S₀energy gaps. The observed trend is reversed when electronic decoherence effects are accounted for due to the dominance of the energy-gap law effects. Using the decoherence-corrected trajectory surface hopping methods, the predicted recombination time in pristine ML-BP is found to vary from 12.7 ps to 13.7 ns, depending on the methodological details. In the divacancy-defect-containing ML-BP, it varies from 14.2 ps to 19.8 ns. While our computational results show certain consistency with prior studies, they also suggest that the proper treatment of excitonic effects undertaken in the present work leads to a qualitatively different view on the excitonic dynamics in ML-BP system: the nanosecond time scales reported in experiment may be characteristic to the excitonic dynamics in pristine multilayer BP, while the excitonic recombination in pristine and divacancy-containing ML-BP would be taking place on the progressively larger time scales.