Double-encapsulated red-emitting formamidinium lead halide perovskite nanocrystals for fluorescence sensing and lighting applications

In recent years, metal halide perovskite nanocrystals (NCs) have offered several optoelectronic and sensing applications due to their unique photophysical properties. Formamidinium (FA) based perovskite NCs exhibit better thermal and structural stability than the corresponding volatile methylammonium (MA)-based counterpart, which quickly decomposes to release gaseous methylamine. FA-based perovskite NCs also demonstrate fantastic responses towards different environmental stimuli, which is helpful for different sensing applications. However, FAPbI₃ NCs suffer from phase instability and degrade very fast under external stimuli. Here, we synthesized mixed halide FAPb(Br/I)₃ NCs by partially replacing I-ions with Br-ions to address the key challenge of phase instability of FAPbI₃ NCs. Furthermore, with a surface modification approach such as encapsulating the surface of NCs with silica and also a double-encapsulated silica-polymethyl methacrylate (PMMA) polymer, we enhanced the stability of the NCs against heat, ion migration, and UV irradiation. These double-coated red-emitting FAPb(Br/I)₃ NCs (emission peak ∼ 642 nm) were tested for temperature sensing, exhibiting a relative sensitivity (<i>S</i> _r) of ∼12.5% K^-1. We also prepared fluorescent humidity sensors that revealed the lowest detection limit of ∼5% relative humidity (RH). Finally, down-converted WLEDs were fabricated using double-coated green-emitting Cs-doped FAPbBr₃ NCs and red-emitting FAPb(B/I)₃ NCs in thin-film form and embedded on a blue LED chip. These results will boost the development of high-performance sensors and lighting technologies.