Essential role of momentum-forbidden dark excitons in the energy transfer responses of monolayer transition-metal dichalcogenides

We present a theoretical investigation of exciton-mediated Förster resonant energy transfers (FRET’s) from photoexcited quantum dots (QD’s) to transition-metal dichalcogenide monolayers (TMD-ML’s), implemented by the quantum theory of FRET on the base of first-principles-calculated exciton fine structures. With the enhanced electron-hole Coulomb interactions, atomically thin TMD-MLs are shown to serve as an exceptional platform for FRET that are mediated purely by excitons and take full advantage of the superior excitonic properties. Remarkably, the energy-transfer responses of atomically thin TMD-ML’s are shown to be dictated by the momentum-forbidden dark excitons rather than the commonly recognized bright ones. Specifically, the longitudinal dark exciton states following the exchange-driven light-like linear band dispersion play a key role in grading up the efficiency and robustness of FRET of TMD-ML against the inhomogeneity of QD-donor ensembles. With the essential involvement of dark excitons, the FRET responses of TMD-ML’s no longer follow the distance power law as classically predicted and, notably, cannot manifest the dimensionality of the donor-acceptor system.