Lattice doping of lanthanide ions in Cs₂ZrCl₆ nanocrystals enabling phase transition and tunable photoluminescence

Dopants can endow lead-free perovskite nanocrystals with novel photoelectric properties. However, understanding the effect of dopants on the structure and energy transfer of lead-free perovskite nanocrystals remains limited. In this work, we synthesize zero-dimensional Cs₂ZrCl₆ nanocrystals with a blue light quantum yield of up to 75.6% by an improved hot-injection method. And we introduce trace amounts of lanthanide ions (Ln³⁺) (<∼8%) in the lattice of nanocrystals and establish an effective energy transfer channel from self-trapped excitons (STEs) to various Ln³⁺ ions (Tb³⁺, Eu³⁺, Dy³⁺, Sm³⁺, and Pr³⁺), which can achieve tunable photoluminescence between red, green and blue. Interestingly, with increasing Ln³⁺ concentrations (>∼10%), the phase transition from the cubic phase Cs₂ZrCl₆:Ln³⁺ to the monoclinic phase Cs₃LnCl₆:Zr⁴⁺ occurred, while Zr⁴⁺ ions began to act as dopants. And a new energy transfer channel from dopant [ZrCl₆]^2- to host Ln³⁺ ions was established in the Cs₃LnCl₆ host accompanied by enhanced broadband photoluminescence excitation (PLE) and photoluminescence (PL). In particular, the photoluminescence quantum yield (PLQY) of Tb³⁺ ions increases from 0.77% to 54% upon the phase transition (under 276 nm excitation). Our study provides new insights into the effects of dopants on the structure of perovskite nanocrystals and is beneficial to the design of a variety of light-emitting materials for optoelectronic applications.