Interfacial B-Site Ion Diffusion in All-Inorganic Core/Shell Perovskite Nanocrystals.

All-inorganic metal halide perovskites (ABX3, X = Cl, Br, or I) show great potential for the fabrication of optoelectronic devices, but the toxicity and instability of lead-based perovskites limit their applications. Shell passivation with a more stable lead-free perovskite is a promising strategy to isolate unstable components from the environment as well as a feasible way to tune the optical properties. However, it is challenging to grow core/shell perovskite nanocrystals (NCs) due to the soft ionic nature of the perovskite lattice. In this work, we developed a facile method to grow a lead-free CsMnCl3 shell on the surface of CsPbCl3 NCs to form CsPbCl3/CsMnCl3 core/shell NCs with enhanced environmental stability and improved photoluminescence (PL) quantum yields (QYs). More importantly, the resulting core/shell perovskite NCs have color-tunable PL due to B-site ion diffusion at the interface of the core/shell NCs. Specifically, B-site Mn diffusion from the CsMnCl3 shell to the CsPbCl3 core leads to a Mn-doped CsPbCl3 core (i.e., Mn:CsPbCl3), which can turn on the Mn PL at around 600 nm. The ratio of Mn PL and host CsPbCl3 PL is highly tunable as a function of the thermal annealing time of the CsPbCl3/CsMnCl3 core/shell NCs. While the halide anion exchange for all-inorganic metal halide perovskites has been well-developed for band-gap-engineered materials, interfacial B-site diffusion in core/shell perovskite NCs is a promising approach for both tunable optical properties and enhanced environmental stability.

Inserting an "atomic trap" for directional dopant migration in core/multi-shell quantum dots.

Diffusion of atoms or ions in solid crystalline lattice is crucial in many areas of solid-state technology. However, controlling ion diffusion and migration is challenging in nanoscale lattices. In this work, we intentionally insert a CdZnS alloyed interface layer, with small cationic size mismatch with Mn(ii) dopant ions, as an "atomic trap" to facilitate directional (outward and inward) dopant migration inside core/multi-shell quantum dots (QDs) to reduce the strain from the larger cationic mismatch between dopants and host sites. Furthermore, it was found that the initial doping site/environment is critical for efficient dopant trapping and migration. Specifically, a larger Cd(ii) substitutional site (92 pm) for the Mn(ii) dopant (80 pm), with larger local lattice distortion, allows for efficient atomic trapping and dopant migration; while Mn(ii) dopant ions can be very stable with no significant migration when occupying a smaller Zn(ii) substitutional site (74 pm). Density functional theory calculations revealed a higher energy barrier for a Mn(ii) dopant hopping from the smaller Zn substitutional tetrahedral (Td) site as compared to a larger Cd substitutional Td site. The controlled dopant migration by "atomic trapping" inside QDs provides a new way to fine tune the properties of doped nanomaterials.