Colloidal Synthesis and Tunable Multicolor Emission of Vacancy-Ordered Cs₂HfCl₆ Perovskite Nanocrystals

Vacancy-ordered perovskite nanocrystals (NCs) have recently attracted much interest owing to their unique structure and optical properties; however, regulating self-trapped excitons (STEs) in vacancy-ordered perovskite NCs to exhibit tunable multicolor emission is still challenging because of the highly localized charge distribution and strong carrier-phonon coupling. Herein, the first colloidal synthesis of lead-free vacancy-ordered Cs₂HfCl₆ perovskite NCs by a hot-injection method using hafnium acetylacetonate as metal precursors is reported. The passivation strategy of sub-bandgap trap states inside Cs₂HfCl₆ NCs is proposed by doping Sb³⁺. X-ray photoelectron spectroscopy (XPS) measurements verify that the passivation of sub-bandgap trap states may be attributed to the enhanced chemical interaction between the Hf⁴⁺ cations and Cl^– anions. Simultaneously, the introduction of Sb³⁺ ions yields a bright orange emission. Employing lanthanide acetylacetonate compounds, four lanthanide ions including Pr³⁺, Eu³⁺, Tb³⁺, and Ho³⁺ are doped into the Cs₂HfCl₆ NCs, which achieves tunable multicolor emissions (from blue to green to pink) through the energy transfer from STEs to lanthanide ions. This investigation not only develops a novel vacancy-ordered perovskite NCs, but also provides effective strategies for tuning the optical properties of lead-free vacancy-ordered perovskite NCs.