Unraveling Piezoelectric Heterogeneity in All-Inorganic Metal Halide Perovskite Nanoparticles and Thin-Films

Perovskites have a unique crystal structure that allows tunability of physical properties through compositional modifications. Halide perovskites, particularly cesium lead halides (CsPbX₃; X = Cl, Br, I), exhibit outstanding optoelectronic properties, with recently demonstrated piezoelectric behavior arising from noncentrosymmetric lattice distortions under mechanical stress or electric fields. While iodine alloying has previously been shown to enhance piezoelectricity in perovskite bulk thin films, its effect in nanoparticles (NPs) remains unexplored. Here, we investigate the intrinsic piezoelectric properties of CsPbBr₃ and iodine-alloyed CsPb(Br_0.8I_0.2)₃ in both NP and thin-film morphologies using piezoresponse force microscopy (PFM). To quantify local piezoelectricity, we developed a high-resolution d₃₃ mapping strategy that resolves nanoscale heterogeneity and captures distinct piezoelectric subpopulations. The highest of three observed subpopulations in CsPbBr₃ NPs reached 15 ± 4 pm V^–1, while CsPb(Br_0.8I_0.2)₃ NPs reached up to 24 ± 6 pm V^–1. The alloyed thin-film perovskite exhibited remarkable local d₃₃ coefficients up to 80 ± 17 pm V^–1, the highest reported for this material. This work establishes a comparison between iodide alloying effects in CsPbBr₃ NPs and thin-film, as well as newfound nanoscale piezoelectric heterogeneity with potential impact on the design of tunable piezoelectric and nanoscale devices.