Composition-dependent phase transitions and superlattice ordering in lead-iodide perovskite nanocrystals*

Confinement of lead-iodide perovskites to nanoscale dimensions via colloidal synthesis has enabled several emergent properties, notably including the stabilization of crystallographic phases known to be thermodynamically unstable in the bulk. Further, these halide perovskite quantum dots exhibit desirable optoelectronic properties for light emission and quantum optics applications, including efficient photoluminescence and superfluorescence arising from coherent coupling between neighboring nanocrystals.

            Using synchrotron grazing incidence wide-angle X-ray scattering (GIWAXS), we established three distinct perovskite phases exist at room temperature within 15-nm FA_xCs_1–xPbI₃ (x = 0, 0.1, 0.25, 0.5, 0.75, 1) NCs. These phase transitions demonstrate a unique ability to target perovskite crystal symmetry (i.e., cubic, tetragonal, or orthorhombic) with A-site composition. GIWAXS patterns collected on thin films of the FA_xCs_1–xPbI₃ NCs show a degree of inter-particle orientation (i.e., superlattice formation) that is dependent on the method of deposition. Thus, we further investigate the time-dependence of the inter-particle orientation via in situ GIWAXS measurements and introduce surface-ligand engineering approaches to achieve optimal superlattice formation.