Breaking the Boundaries\nof the Goldschmidt Tolerance\nFactor with Ethylammonium Lead Iodide Perovskite Nanocrystals

We report the synthesis of ethylammonium lead iodide\n(EAPbI<sub>3</sub>) colloidal nanocrystals as another member of the\nlead halide\nperovskites family. The insertion of an unusually large <i>A</i>-cation (274 pm in diameter) in the perovskite structure, hitherto\nconsidered unlikely due to the unfavorable Goldschmidt tolerance factor,\nresults in a significantly larger lattice parameter compared to the\nCs-, methylammonium- and formamidinium-based lead halide perovskite\nhomologues. As a consequence, EAPbI<sub>3</sub> nanocrystals are highly\nunstable, evolving to a nonperovskite δ-EAPbI<sub>3</sub> polymorph\nwithin 1 day. Also, EAPbI<sub>3</sub> nanocrystals are very sensitive\nto electron irradiation and quickly degrade to PbI<sub>2</sub> upon\nexposure to the electron beam, following a mechanism similar to that\nof other hybrid lead iodide perovskites (although degradation can\nbe reduced by partially replacing the EA<sup>+</sup> ions with Cs<sup>+</sup> ions). Interestingly, in some cases during this degradation\nthe formation of an epitaxial interface between (EA<sub><i>x</i></sub>Cs<sub>1–<i>x</i></sub>)PbI<sub>3</sub> and\nPbI<sub>2</sub> is observed. The photoluminescence emission of the\nEAPbI<sub>3</sub> perovskite nanocrystals, albeit being characterized\nby a low quantum yield (∼1%), can be tuned in the 664–690\nnm range by regulating their size during the synthesis. The emission\nefficiency can be improved upon partial alloying at the A site with\nCs<sup>+</sup> or formamidinium cations. Furthermore, the morphology\nof the EAPbI<sub>3</sub> nanocrystals can be chosen to be either nanocube\nor nanoplatelet, depending on the synthesis conditions.