Hot injection synthesis has been established as the optimal method for the production of cesium lead halide perovskites (CsPbX3, X=Cl, Br, I). However, our fundamental understanding of the perovskite growth mechanism is very limited. Owing to the ionic nature of CsPbX3, the growing of nanocrystal occurs within just a few seconds, greatly hindering mechanistic studies. The challenge is further compounded by the fact that synthetic strategies for structural control established in other semiconductor nanomaterials do not work well for CsPbX3. Thus a better understanding of the growth mechanism may enable strategies for more sophisticated morphological targets which would further expand the range of applications that can benefit from the exceptional optical quality of the material. In this work, we demonstrate a method to slow down the growth of CsPbBr3 nanocrystals by limiting the chemical accessibility of bromine during a hot injection synthesis. Using 1-bromohexane as the halide source extends the growth time scale from a few seconds to tens of minutes. This slow growth allows us to monitor ex situ the evolution of products in the early stages of CsPbBr3 formation. Using a combination of powder X-ray diffraction and high-resolution transmission electron microscopy, we observe the initial formation of CsBr, followed by Cs4PbBr6, and eventually CsPbBr3. As the chemical environment remains substantially identical to that of a conventional preparation, these results suggest that the complexation of Br to Cs is, surprisingly, the critical step that initiates CsPbBr3 nanoparticle nucleation, with Cs4PbBr6 playing the role of intermediate. Furthermore, we use this fundamental insight to produce CsPbBr3 nanosheets through the simple control of Cs concentration under the slowed kinetics, demonstrating a manner of morphological control inaccessible under fast reaction kinetics.

Time evolution of CsPbBr3 nanocrystal synthesis: Cesium-bromine complexation dictates growth

2019 Spring National Meeting

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