Elucidating the Mechanisms of Halide Perovskite Nanocrystal Syntheses

Halide perovskite nanoplatelets (NPLs) are a promising material for light-emitting applications, including LEDs and single-photon emitters. Two-dimensional Cs_{n – 1}Pb_nBr_3n+1 NPLs with monolayer-precise control over their thickness can be obtained through a ligand-assisted synthesis. Through quantum confinement, they exhibit narrow and bright photoluminescence (PL) with an emission wavelength tunable between 435 and 515 nm. While the optoelectronic properties of perovskite NPLs are widely understood, little is known about how the synthesis of these promising emitters proceeds, impeding further improvement.<br/><br/>To this end, we developed an in-situ reactor cell to simultaneously acquire small-angle and total X-ray scattering (SAXS and TS) and PL emission at the DESY synchrotron during the synthesis of halide perovskite NPLs with a time-resolution of &lt;50 ms. The NPL synthesis routine is based on combining a PbBr₂ ligand solution and Cs-oleate with a subsequent injection of the antisolvent acetone. The synthesis is carried out in a glass capillary within a copper block with stepper motor-controlled syringe pumps to inject precursor solutions.<br/>An analysis of the SAXS data reveals that the NPLs begin to form immediately upon the combination of the two precursor solutions and not upon injection of the antisolvent. Accordingly, contrary to the common understanding, the Cs_{n – 1}Pb_nBr_3n+1 NPL synthesis is not based on ligand-assisted reprecipitation but on well-defined precursor handling. However, the antisolvent still plays a crucial role, as it induces a superstructure assembly of the NPLs, leading to a greater monodispersity and enabling the sedimentation and purification of the NPLs. Furthermore, we investigated the structural and optical changes during the post-synthetic enhancement step and halide ion exchange within the same experimental setup. The presented combination of in-situ X-ray scattering and PL spectroscopy proves to be a powerful tool for understanding synthetic mechanisms.