Spontaneous Formation of Single-Crystalline Spherulites in a Chiral 2D Hybrid Perovskite

In two-dimensional (2D) chiral metal-halide perovskites (MHPs), chiral organic spacers not only induce chiroptical properties, but also impart structural chirality to the metal halide sublattice. For instance, this structural chirality enables the reversible crystalline-glass transition and formation of spherulites in (S-NEA)₂PbBr₄, a prototype chiral MHP where NEA⁺ stands for 1-(1-naphthyl)ethylammonium cation. Here, we investigate the formation of two spherulite phases of (S-NEA)₂PbBr₄, which form either radial-like or ring-banded patterns depending on the annealing conditions of thin film in amorphous phase. We carefully characterized the optical and structural properties of the two spherulite phases using a multimodal approach. Despite similarities in absorption and photoluminescence, the ring-banded spherulite exhibits higher crystallinity and consequently, better transparency than radial-like spherulites. Remarkably, X-ray nanoprobe measurements on ring-banded spherulites reveal a tilting-angle modulation in the octahedral plane, consistent with the geometric tilting of the film surface. The transfer matrix method demonstrates that the optical contrast of the ring-banded patterns, observed under a brightfield optical microscope, arises from optical interference between the film’s upper and lower surface, which is distinct from the origin of contrast of polymer spherulites observed under a polarized optical microscope. Additionally, ultrafast carrier dynamics indicate that ring-banded spherulites resemble single crystals more closely than radial-like spherulites. Our results provide key insights into differences between spherulites with the same composition but distinct morphologies, paving the way for their potential applications in optoelectronic devices.