Precursor Chemistry of Lead Bromide Perovskite Nanocrystals.

We investigate the early stages of cesium lead bromide perovskite formation through absorption spectroscopy of stopped-flow reactions, high-throughput mapping, and direct synthesis and titration of potential precursor species. Calorimetric and spectroscopic measurements of lead bromide complex titrations combined with theoretical calculations suggest that bromide complexes with higher coordination numbers than previously considered for nonpolar systems can better explain observed behaviors. Synthesis mapping of binary lead halides reveals multiple lead bromide species with absorption peaks higher than 300 nm, including a previously observed species with a peak at 313 nm and two species with peaks at 345 and 370 nm that also appear as reaction intermediates during the formation of lead bromide perovskites. Based on theoretical calculations of excitonic energies that match within 50 meV, we give a preliminary assignment of these species as two-dimensional magic-sized clusters with side lengths of 2, 3, and 4 unit cells. Kinetic measurements of the conversion of benzoyl bromide precursor are connected to stopped-flow measurements of product formation and demonstrate that the formation of complexes and magic-sized clusters (i.e., nucleation) is controlled by precursor decomposition, whereas the growth rate of 2D and 3D perovskites is significantly slower.

Dynamic lattice distortions driven by surface trapping in semiconductor nanocrystals.

Nonradiative processes limit optoelectronic functionality of nanocrystals and curb their device performance. Nevertheless, the dynamic structural origins of nonradiative relaxations in such materials are not understood. Here, femtosecond electron diffraction measurements corroborated by atomistic simulations uncover transient lattice deformations accompanying radiationless electronic processes in colloidal semiconductor nanocrystals. Investigation of the excitation energy dependence in a core/shell system shows that hot carriers created by a photon energy considerably larger than the bandgap induce structural distortions at nanocrystal surfaces on few picosecond timescales associated with the localization of trapped holes. On the other hand, carriers created by a photon energy close to the bandgap of the core in the same system result in transient lattice heating that occurs on a much longer 200 picosecond timescale, dominated by an Auger heating mechanism. Elucidation of the structural deformations associated with the surface trapping of hot holes provides atomic-scale insights into the mechanisms deteriorating optoelectronic performance and a pathway towards minimizing these losses in nanocrystal devices.