Formation and Near-Infrared Emission of CsPbI₃ Nanoparticles Embedded in Cs₄PbI₆ Crystals

Cs₄PbI₆, as a rarely investigated member of the Cs₄PbX₆ (X is a halogen element) family, has been successfully synthesized at low temperatures, and the synthetic conditions have been optimized. Metal iodides such as LiI, KI, NiI₂, CoI₂, and ZnI₂, as additives, play an important role in enhancing the formation of the Cs₄PbI₆ microcrystals. ZnI₂ with the lowest dissociation energy is the most efficient additive to supply iodide ions, and its amount of addition has also been optimized. Strong red to near-infrared (NIR) emission properties have been detected, and its optical emission centers have been identified to be numerous embedded perovskite-type α-CsPbI₃ nanocrystallites (∼5 nm in diameter) based on investigations of temperature- and pressure-dependent photoluminescent properties. High-resolution transmission electron microscopy was used to detect these hidden nanoparticles, although the material was highly beam-sensitive and confirmed a "raisin bread"-like structure of the Cs₄PbI₆ crystals. A NIR mini-LED for the biological application has been successfully fabricated using as-synthesized Cs₄PbI₆ crystals. This work provides information for the future development of infrared fluorescent nanoscale perovskite materials.