Unraveling Size Dependent Bi- and Tri-Exciton Characteristics in CdSe/CdS Core/Shell Quantum Dots via Ensemble Time Gated Heralded Spectroscopy

Multiexcitons in quantum dots (QDs) manifest many-body interactions under quantum confinement and are significant in numerous optoelectronic and quantum applications. Yet, the strong interactions between multiexcitons leading to rapid non-radiative Auger decay introduce challenges for their characterization. While so far, the measurement techniques rely either on indirect methods or on single particle studies, herein a new method is introduced to study multiexcitons in QD ensembles utilizing spectrally resolved time-gated heralded spectroscopy. With this approach, the biexciton binding energies is extracted in CdSe/CdS QD ensembles of several core/shell sizes, manifesting a transition between attractive to repulsive exciton-exciton interactions. Additionally, for triexcitons, involving occupation of two excitons in the 1s energy levels and one exciton in the 1p energy levels, the open issues of extracting the lifetime, the spectra of the two triexciton pathways and their branching ratio are resolved. The ensemble measurements provide high photon counts and low noise levels, and alongside the time-gated heralded approach, thus enable the observation of multiexciton characteristics that are often obscured in single particle studies. The approach can be further implemented in the characterization of the energies and lifetimes of multiexcitons in other QD systems to enable rapid characterization and understanding.