# Bound Exciton Complexes in Near-Infrared Emitting Quantum Shells

## TL;DR

Researchers developed quantum shells that emit near-infrared light efficiently by suppressing harmful recombination processes and observing new exciton behaviors.

## Contribution

The study introduces quantum shells that suppress Auger recombination and exhibit bound exciton complexes at room temperature.

## Key findings

- CdS/HgS/CdS quantum shells show tunable NIR emission with high photoluminescence quantum yields.
- CdS/HgCdSe/ZnS quantum shells display photoinduced absorption due to bound multiexciton complexes.
- Bound exciton complexes enable long-lived sub-bandgap states and potential nonlinear photonic applications.

## Abstract

Near-infrared (NIR)
light sources based on colloidal
semiconductor
nanocrystals (NCs) represent a scalable, low-cost alternative to epitaxial
semiconductor platforms. However, their performance remains hindered
by rapid Auger recombination, a problem that is particularly pronounced
in narrow-bandgap materials. Here, we report on CdS/HgS/CdS and CdS/HgCdSe/ZnS
quantum shells (QSs), a class of spherical quantum wells specifically
engineered for a suppression of nonradiative Auger processes. Fabricated
QSs exhibit tunable NIR emission with photoluminescence quantum yields
reaching ∼60% below 1000 nm and up to 30% near 1300 nm. Optical
gain and stimulated emission were observed in CdS/HgS/CdS QSs. In
contrast, CdS/HgCdSe/ZnS QSs displayed a photoinduced absorption in
lieu of optical gain despite demonstrating a comparatively stronger
Auger suppression. Transient absorption spectroscopy revealed that
this phenomenon arises from the formation of bound multiexciton complexes
that induce long-lived sub-bandgap multiexciton states. The observation
of such bound excitonic clusters at room temperature offers a pathway
toward nonlinear NIR photonic phenomena, including biexciton–exciton
cascade emission, optical modulation, and single-exciton gain.