Pressure-Induced Phase Transformation and Band-Gap Engineering of Formamidinium Lead Iodide Perovskite Nanocrystals.

Formamidinium lead halide (FAPbX, X = Cl, Br, I) perovskite materials have recently drawn an increased amount of attention owing to their superior optoelectronic properties and enhanced material stability as compared with their methylammonium-based (MA-based) analogues. Herein, we report a study of the pressure-induced structural and optical evolutions of FAPbI hybrid organic-inorganic perovskite nanocrystals (NCs) using a synchrotron-based X-ray scattering technique coupled to in situ absorption and photoluminescence spectroscopies. As a result of their unique structural stability and soft nature, FAPbI NCs exhibit a wide range of band-gap tunability (1.44-2.17 eV) as a function of pressure (0-13.4 GPa). The study presented here not only provides an efficient and chemically orthogonal means to controllably engineer the band gap of FAPbI NCs using pressure but more importantly sheds light on how to strategically design the band gaps of FA-based hybrid organic-inorganic perovskites for various optoelectronic applications.