Origin of the near 400 nm Absorption and Emission Band in the Synthesis of Cesium Lead Bromide Nanostructures: Metal Halide Molecular Clusters Rather Than Perovskite Magic-Sized Clusters

Abstract

In the synthesis of cesium lead bromide (CsPbBr 3 ) perovskite quantum dots, with an electronic absorption and emission band around 510 nm, and perovskite magic-sized clusters (PMSCs), with an electronic absorption and emission band around 430 nm, another distinct absorption and emission around 400 nm is often observed. While many would attribute this band to small perovskite particles, here we show strong evidence that this band is a result of the formation of lead bromide molecular clusters (PbBr 2 MCs) passivated with ligands, which do not contain the A component of the ABX 3 perovskite structure. This evidence comes from a systematic comparative study of the reaction products with and without the A component under otherwise identical experimental conditions. The results support that the near 400 nm band originates from ligand-passivated PbBr 2 MCs. This observation seems to be quite general and is significant in understanding the nature of the reaction products in the synthesis of metal halide perovskite nanostructures.

Distinguishing metal halide molecular clusters from perovskite magic sized clusters in the synthesis of metal halide perovskite nanostructures

“…There, LARP was utilized as is typical for the synthesis of perovskites, and showed that MHMCs can be formed in the absence of an A component both with and without an acidic passivating ligand, demonstrating an increased dependence on the amine ligand. However, like PMSCs, proportionally large quantities of passivating ligands are necessary as MHMCs have a large surface to volume ratio as a product of their small size and high degree of confinement [12,14,15]. Overall, the introduction of facile synthesis schemes such as LARP to produce stable, monodispersed nanocrystal populations has made the study of the varied size and morphological possibilities of PNCs more attainable.…”

“…Further investigation of this near 400 nm MHMC peak was undertaken to reconcile its observation in syntheses that contain the perovskite A component. A recent work by Win et al showed that in samples where there is an observed near 400 nm peak alongside PMSC and/or PQD peaks, this 400 nm peak should be attributed to MHMCs despite the presence of other, definitively perovskite, structures in the same solution [15]. Spectra from the 2023 Win et al study are shown in Figure 4, and show that in a synthesis which includes the perovskite A component ( a and c ), several peaks appear in the resultant absorbance and emission spectra.…”

“…UV–vis absorption and PL spectra of (a) MAA + BTYA CsPbBr 3 , (b) MAA + BTYA PbBr 2 MCs, (c) MAA + BZA CsPbBr 3 , and (d) MAA + BZA PbBr 2 MCs. Reproduced with permission [15]. …”

“…Recently, perovskite magic sized clusters (PMSCs), which are PNCs with single or uniform size smaller than typical PQDs, have been discovered and found to exhibit stronger QCE and significantly bluer optical absorption and emission than PQDs [11][12][13]. In the meantime, ligand-passivated metal halide molecular clusters (MHMCs) without the A component of the MHPs have been found, and they show slightly smaller size and blue optical absorption and emission compared to PMSCs [12,14,15]. These two types of clusters have smaller size distributions and larger S/V ratios compared to PQDs, so PMSCs and MHMCs are generally less stable and thereby require better passivation with stronger or higher concentration of ligands [16][17][18].…”

Ultrafast Study of Excited State Dynamics of Amino Metal Halide Molecular Clusters

“…Ligand-passivated PbBr 2 MCs exhibit first excitonic absorption and emission band peaked around 402 and 410 nm, respectively, which are bluer than both corresponding PQDs and perovskite magic-sized clusters (PMSCs). ,, Unlike their larger perovskite counterparts, the key difference among them is that metal halide MCs lack the A component that is present in PQDs and PMSCs with ABX 3 composition, as demonstrated by past work from our lab . Also, further study showed that metal halide MCs can be generated alongside PQDs and PMSCs even when the A component is present . More recent structural studies incorporating Raman, XRD, and TEM suggest that PbBr 2 -BTYA MCs consist of a possible layered structure where the layers are composed of tilted, corner-sharing [PbBr 6 ] 4– octahedra with a particle size of 2.0 ± 0.4 nm .…”