Hybridization of Single Nanocrystals of Cs₄PbBr₆ and CsPbBr₃

Nanocrystals of all-inorganic cesium lead halide perovskites (CsPbX₃, X = Cl, Br, I) feature high absorption and efficient narrow-band emission which renders them promising for future generation of photovoltaic and optoelectronic devices. Colloidal ensembles of these nanocrystals can be conveniently prepared by chemical synthesis. However, in the case of CsPbBr₃, its synthesis can also yield nanocrystals of Cs₄PbBr₆ and the properties of the two are easily confused. Here, we investigate in detail the optical characteristics of simultaneously synthesized green-emitting CsPbBr₃ and insulating Cs₄PbBr₆ nanocrystals. We demonstrate that, in this case, the two materials inevitably hybridize, forming nanoparticles with a spherical shape. The actual amount of these Cs₄PbBr₆ nanocrystals and nanohybrids increases for synthesis at lower temperatures, i.e., the condition typically used for the development of perovskite CsPbBr₃ nanocrystals with smaller sizes. We use state-of-the-art electron energy loss spectroscopy to characterize nanoparticles at the single object level. This method allows distinguishing between optical characteristics of a pure Cs₄PbBr₆ and CsPbBr₃ nanocrystal and their nanohybrid. In this way, we resolve some of the recent misconceptions concerning possible visible absorption and emission of Cs₄PbBr₆. Our method provides detailed structural characterization, and combined with modeling, we conclusively identify the nanospheres as CsPbBr₃/Cs₄PbBr₆ hybrids. We show that the two phases are independent of each other's presence and merge symbiotically. Herein, the optical characteristics of the parent materials are preserved, allowing for an increased absorption in the UV due to Cs₄PbBr₆, accompanied by the distinctive efficient green emission resulting from CsPbBr₃.