Role of dislocations in self-assembled 2D layered raspberry surface to 3D flower-like microstructural evolution in...

Molecular self-assembly is a unique process in nature leading to interesting growth of crystals and layered structures. However, understanding the nucleation pathways leading to the formation and evolution of self-assembled molecular structures is often more complex to decipher from experimental and theoretical investigations. Here, we report a unique growth of 2D layered microcrystals having 'raspberry surface' which further evolute to 3D 'flower-like' microcrystals observed for a hybrid bismuth based lead-free halide perovskite material. In contrast, in a mixed halide system obtained under bromine (Br) doping, the microcrystals grow as a 2D sheet which under evolution retains the shape of 'lotus leaf-like' microstructure. The growth mechanism proposed after comprehensive microscopic investigations reveal that the growth of 'raspberry surface' is induced by mixed dislocations following a 'terrace-ledge-kink' model while the growth of 2D sheets for mixed halide is driven by screw dislocations only. We further show from microscopic images how piling up of dislocations lead to the formation of cracks under compressive stress and finally led to the plastically deformed 3D flower-like structures. Molecular dynamics simulations are used to reconstruct the 3D flower structures from 2D sheets which further suggest a high bending stiffness (~ 200 to 25000 eV) of the flower petals. The results may be quite intriguing to the fundamental understanding of self-assembled molecular nucleation and growth of hybrid perovskites and their stability in device applications.