# Halide‐Exchange Arrest Enables Reabsorption‐Free CsPbCl3/CsPbI3 Perovskite Core/Shell Nanocrystals

## TL;DR

This paper introduces a method to create perovskite nanocrystals with a large Stokes shift by using CdCl2 passivation to stabilize core/shell structures, enabling efficient and reabsorption-free emitters for photonic and quantum technologies.

## Contribution

The novel use of CdCl2 passivation to arrest halide exchange and enable stable, reabsorption-free perovskite core/shell nanocrystals.

## Key findings

- CdCl2 passivation blocks inward I− diffusion, enabling a ≈1.2 eV Stokes shift in CsPbCl3/CsPbI3 core/shell nanocrystals.
- The heterostructures exhibit ≈70% photoluminescence quantum yield and fast emission lifetime (≈10 ns).
- Ultrafast core-to-shell exciton transfer (≈60 ps) is confirmed via transient absorption and DFT modeling.

## Abstract

Expanding the Stokes shift of lead‐halide perovskite nanocrystals (NCs) without compromising their sharp, fast excitonic emission has remained elusive, as high halide mobility erases the compositional gradients required for stable core/shell architectures. Here, it is shown that introducing a CdCl2 passivation step prior to halide exchange provides a simple solution. Treating CsPbCl3 NCs with CdCl2 eliminates halide‐vacancy traps, enhances emission yield, and crucially blocks inward diffusion of I−, arresting the Cl− → I− exchange after just a few monolayers. This produces CsPbCl3/CsPbI3 core/shell NCs that absorb at 3.14 eV from the core and emit at 1.91 eV from the shell, achieving an apparent Stokes shift of ≈1.2 eV. The heterostructures exhibit ≈70% photoluminescence quantum yield, fast emission lifetime (≈10 ns) and complete suppression of reabsorption losses, as confirmed by liquid‐waveguiding experiments. Transient absorption spectroscopy and DFT modeling reveal an inverted type‐I band alignment with ultrafast (≈60 ps) core‐to‐shell exciton transfer. This fully solution‐processed chemistry enables heterostructuring‐based wavefunction engineering – long employed to expand the capabilities of conventional quantum dots – now realized in perovskite NCs, which provides a practical route to reabsorption‐free perovskite emitters for advanced photonic and quantum technologies.

Achieving large Stokes shifts in perovskite nanocrystals is challenging due to halide mobility that disrupts stable core–shell structures. Using CdCl2 passivation, this work stabilizes CsPbCl3/CsPbI3 heterostructures, yielding a ≈1.2 eV Stokes shift, high quantum yield, and fast exciton transfer, enabling reabsorption‐free emitters for advanced photonic and quantum technologies.