Hybridization of Single Nanocrystals of Cs₄PbBr₆ and CsPbBr₃

Abstract

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