Structural Study of Paraffin-Stabilized Methylammonium Lead Bromide Magic-Sized Clusters

Abstract

Metal halide perovskites, such as methylammonium lead bromide, have recently attracted considerable attention due to their interesting and useful photoelectric properties. Here, two types of methylammonium lead bromide magic-sized clusters (MSCs), passivated with oleylamine and oleic acid, were synthesized using ligand-assisted reprecipitation (LARP) and heated LARP (HLARP) methods. The optical properties of these MSCs were characterized using UV−vis electronic absorption and photoluminescence (PL) spectroscopies. The HLARP synthesis resulted in a two-fold increase in the PL quantum yield of the MSCs to 76%. The stability of the MSCs was tested using timedependent PL spectroscopy. LARP MSCs in solution degraded completely after 14 days under ambient conditions, while HLARP MSCs lasted for 26 days. To stabilize them, the MSCs were added to a non-coordinating matrix, paraffin. Both MSCs showed significantly improved resistance to water with the addition of paraffin. Solid LARP MSCs lost all luminescence with and without the addition of paraffin by about 3 h. Solid HLARP MSCs without paraffin started to aggregate after 3 h, but paraffin stabilized HLARP MSC films were stable for 8 days. This improved stability in solid state form allowed for accurate, nonaggregated analysis using Raman spectroscopy, X-ray diffraction, and transmission electron microscopy. Raman spectroscopy revealed that the HLARP MSCs show an additional peak at 147 cm −1 compared to LARP MSCs, which is attributed to methylammonium. X-ray diffraction and transmission electron microscopy confirm that MSCs have a quasi-crystalline orthorhombic structure.

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

“…However, even with the addition of higher concentrations of passivating ligand, PMSCs aggregate to larger particles when dried. Recently, the introduction of a stabilizing matrix has allowed for this issue to be combatted and the PMSC structure to be examined using x-ray diffraction and transmission electron microscopy (TEM) [11]. Figure 6 shows TEM images of HLARP PMSCs with an average particle size of about 2.8 nm.…”

“…Similarly to PQDs, PMSCs need to be stabilized by surface passivation and/or encapsulation [16][17][18]. For example, transition from LARP to HLARP synthesis extends PMSC usable lifetime from 14 to 26 days in solution, and encapsulation with further stabilized the particle even when directly exposed to water [11].…”

“…Even more recently, a heated ligand‐assisted reprecipitation (HLARP) method was introduced, combining the benefits of both HI and LARP and producing PMSCs of higher quality than a standard LARP synthesis [25]. The effectiveness of HLARP was demonstrated by Guarino‐Hotz et al in a 2023 report which noted a two‐fold increase in the photoluminescence quantum yield (PLQY) of an HLARP PMSC compared to that of a standard LARP PMSC, as well as near doubled stability in the solution phase [11].…”

“…Images of HLARP MSCs at various resolutions using: (a–c) HRTEM and (d–f) HAADF STEM. Reproduced with permission [11]. …”

“…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].…”