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

## TL;DR

Researchers developed a new method to study multiexcitons in quantum dots, revealing how exciton interactions change with size and enabling clearer insights into their behavior.

## Contribution

A novel ensemble time-gated heralded spectroscopy method is introduced to study biexciton and triexciton characteristics in quantum dot ensembles.

## Key findings

- Biexciton binding energy measurements in CdSe/CdS QDs show a transition between attractive and repulsive exciton interactions.
- Triexciton transitions and their spectral dynamics are resolved and isolated from higher-order multiexcitons.
- The method enables robust, low-noise characterization of multiexcitons in quantum dot systems.

## Abstract

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 1
s energy levels and one exciton in the 1
p 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.

The size‐dependent biexciton binding energy in CdSe/CdS quantum dots is measured via ensemble time‐gated heralded spectroscopy, showcasing a transition between repulsive and attractive exciton‐exciton interactions. The spectrum and dynamics of two triexciton transitions are resolved and isolated from emission of multiexcitons of higher order. This robust method provides rich emission signals and can be applied to diverse quantum dot systems.