Growth and Self-Assembly of CsPbBr₃ Nanocrystals in the TOPO/PbBr₂ Synthesis as Seen with X-ray Scattering

Abstract

Despite broad interest in colloidal lead halide perovskite nanocrystals (LHP NCs), their intrinsic fast growth has prevented controlled synthesis of small, monodisperse crystals and insights into the reaction mechanism. Recently, a much slower synthesis of LHP NCs with extreme size control has been reported, based on diluted TOPO/PbBr2 precursors and a diisooctylphosphinate capping ligand. We report new insights into the nucleation, growth, and self-assembly in this reaction, obtained by in situ synchrotron-based small-angle X-ray scattering and optical absorption spectroscopy. We show that dispersed 3 nm Cs[PbBr3] agglomerates are the key intermediate species: first, they slowly nucleate into crystals, and then they release Cs[PbBr3] monomers for further growth of the crystals. We show the merits of a low Cs[PbBr3] monomer concentration for the reaction based on oleate ligands. We also examine the spontaneous superlattice formation mechanism occurring when the growing nanocrystals in the solvent reach a critical size of 11.6 nm.

An Amorphous Phase Precedes Crystallization: Unraveling the Colloidal Synthesis of Zirconium Oxide Nanocrystals

“…Small Angle X-ray Scattering Analysis. Given the success of small-angle X-ray scattering (SAXS) in delivering insights into nucleation and growth mechanisms, 10,11,14,15,21 we also take it here as a starting point in our investigation. We performed ex situ SAXS measurements on aliquots, taken from the reaction mixture at 300 °C, 320 °C, and 340 °C during the heating ramp, and at different time intervals at 340 °C.…”

“…For example, the surfactant-free aqueous synthesis of calcium phosphate and magnetite involves amorphous prenucleation clusters. , In the surfactant-assisted synthesis of Ni nanocrystals, liquid cell TEM revealed again an amorphous phase . SAXS revealed amorphous agglomerates in the synthesis of CsPbBr 3 nanocrystals . Furthermore, magic-sized clusters with a well-defined structure have been identified in the synthesis of InP nanocrystals as intermediate species between precursors and nanocrystals .…”

“… 20 SAXS revealed amorphous agglomerates in the synthesis of CsPbBr 3 nanocrystals. 21 Furthermore, magic-sized clusters with a well-defined structure have been identified in the synthesis of InP nanocrystals as intermediate species between precursors and nanocrystals. 22 Finally, the conversion of iron oleate in iron oxide nanocrystals happens without nucleation.…”

Promoting solution-phase superlattices of CsPbBr3 nanocrystals

“…47 Additionally, the steep decline in the slope at low q −1 values for Fraction 5 suggests that the NCs were no longer freely dispersed and had formed larger structures greater than 90 nm in size. 2,48,49 More precise estimate of the size of the structure is not possible based on the detection limit of our SAXS instrument. The large structure was highly crystalline, with Bragg reflection peaks identified at 0.50 nm −1 , 0.69 nm −1 , and 1.39 nm −1 q −1 values.…”

Benzoyl Versus Phenacyl Bromide: The Strategic Differences in Halide Perovskite Nanocrystals Design

“…Unlike covalent chalcogenide nanocrystals, the chemistry of crystal modulations and surface engineering also could not be largely understood. Enormous efforts have been put forward in tuning ligands, designing new precursors, changing the reaction medium, and also varying different reaction parameters to slow down the rate of their formation, which remained a key parameter for facet dependent secondary growth and the shape modulation. ,,− In some cases, successes have also been achieved in the formation of different polyhedral nanocrystals, connected nanocrystals, nanorods/nanowires, nanoplatelets, and other anisotropic nanostructures with different anisotropy directions. − However, to rationalize this facet or shape control, there is no logical pathway or fundamental principle yet established which would provide guidelines to follow their formation. From the chronological development of these nanocrystals, it is revealed that the halide source remained as one of the major key parameters for controlling the surface facets of these nanocrystals. ,− Among these, benzoyl halides and phenacyl halides, which have broadly similar structures, but differing in only one −CH 2 – group, made a significant impact in designing the nanocrystals. , One resulted in the cube shape, and other turned halide perovskite nanocrystals to a dodecahedron shape having different groups of surface facets and symmetry. This indeed created the benchmark and provided hopes for the revolutionary pathway in nanocrystals engineering in halide perovskite nanocrystals.…”