Breaking the Boundaries of the Goldschmidt Tolerance Factor with Ethylammonium Lead Iodide Perovskite Nanocrystals.

We report the synthesis of ethylammonium lead iodide (EAPbI) colloidal nanocrystals as another member of the lead halide perovskites family. The insertion of an unusually large -cation (274 pm in diameter) in the perovskite structure, hitherto considered unlikely due to the unfavorable Goldschmidt tolerance factor, results in a significantly larger lattice parameter compared to the Cs-, methylammonium- and formamidinium-based lead halide perovskite homologues. As a consequence, EAPbI nanocrystals are highly unstable, evolving to a nonperovskite δ-EAPbI polymorph within 1 day. Also, EAPbI nanocrystals are very sensitive to electron irradiation and quickly degrade to PbI upon exposure to the electron beam, following a mechanism similar to that of other hybrid lead iodide perovskites (although degradation can be reduced by partially replacing the EA ions with Cs ions). Interestingly, in some cases during this degradation the formation of an epitaxial interface between (EACs)PbI and PbI is observed. The photoluminescence emission of the EAPbI perovskite nanocrystals, albeit being characterized by a low quantum yield (∼1%), can be tuned in the 664-690 nm range by regulating their size during the synthesis. The emission efficiency can be improved upon partial alloying at the A site with Cs or formamidinium cations. Furthermore, the morphology of the EAPbI nanocrystals can be chosen to be either nanocube or nanoplatelet, depending on the synthesis conditions.