Single-Step Synthesis of Dual Phase Bright Blue-Green Emitting Lead Halide Perovskite Nanocrystal Thin Films

Metal trihalide perovskites are rapidly redefining the landscape of solid-state semiconductors utilized as an active medium in photovoltaics and in light generation. Within this materials space, organic–inorganic hybrid formamidinium lead bromide (FAPbBr3) has arisen as a promising candidate for efferent light emitting devices, due to its capacity for sharp and bright green light emissions (530 nm). Herein we have applied a facile single-step ligand-mediated method for phase-controlled synthesis of FAPbBr3 cube- and rod-shaped nanocrystals (NCs), starting from different ratios of precursor agents. Examining their structural and optoelectronic properties—using a combination of synchrotron X-ray diffraction, X-ray spectroscopy, scanning electron microscopy, and steady-state and time-resolved photoluminescence (PL)—we reveal the two NC types to fundamentally differ. While the cube-shaped NCs exhibit properties aligning with those of bulk FAPbBr3, the nanorods exhibit a two-phase microstructure and the coexistence of both a typical cubic perovskite structure alongside the formation of a new low-symmetry monoclinic phase (P2/m). Further, the two-phase nanorods display a bright dual PL emission (peaks centered near 490 and 530 nm) and complex luminescence dynamics, properties characteristic of quasi-2D perovskites. The two phase nanorod generation can be assigned to the proton exchange in the presence of excess of FA+ during the synthesis.