Waterproof perovskites: High fluorescence quantum yield and stability from a methylammonium lead bromide/formate mixture in water

We’ve observed intense fluorescence from the surface of lead formate crystals when they are precipitated from a CH₃NH₃PbBr₃(MAPbBr₃) perovskite precursor solution. The crystals exhibit emission in the range of 500-550 nm with a photoluminescence quantum yield (PLQY) of up to 70%. The fluorescence is stable in water and at elevated temperature without significant decrease in PLQY for months, conditions which instantly decompose MAPbBr₃. Fluorescence occurred with the highest quantum efficiency of 70% at an optimal 1?:?0.3 molar ratio of methylammonium formate and lead bromide and decreased rapidly for other ratios. Fluorescence was quenched using other halides (Cl or I) or other cations (Cs, ethylammonium, propylammonium,etc.). Single crystal analysis indicates that the material possesses the lead formate structure with lattice parameters which are identical to pristine lead formate, however, analysis of the particles by EDX, XPS and fluorescence microscopy confirms the presence of Br and fluorescence emission on the particle surfaces. The observed material characteristics indicate that the emissive species consists of a mixed-valence surface layer of Pb coordinated to both Br and formate ligands.