Investigating the Transition from 2D Monolayers to 3D Assemblies of Metal Halide Perovskite Nanoparticles

Introduction Organo-metal halide perovskite nanoparticles have been a subject of major scientific interest in the last decade, owing to their excellent optical, electronic, chemical, and structural properties [1]. These materials are particularly appealing due to their ease of fabrication, as they can be synthesized using simple solution processing techniques. Unlike traditional synthesis methods for inorganic semiconductors, solution processing enables a broader variety of device architectures by eliminating the need for lattice matching or high-temperature processing. Additionally, the high defect tolerance of perovskites further enhances their performance in various applications [2]. When scaled down to the nanoscale, perovskite nanoparticles inherit these advantageous properties while also gaining unique characteristics intrinsic to their size. These include quantum confinement effects, high surface-area-to-volume ratios, and enhanced reactivity, which distinguish nanoparticles from their bulk counterparts and open new possibilities for advanced materials design. Diblock copolymer reverse micelle synthesis (RMD) presents an effective approach for controlling the size and spatial arrangement of metal halide perovskite nanoparticles by adjusting key fabrication parameters such as polymer molecular weight, solvent, loading ratio, and solution deposition approach [3]. In RMD, an amphiphilic diblock copolymer such as poly(styrene)-b-poly(vinlyl pyridine) is added to a non-polar solvent such as o-xylene, forming reverse micelles. When precursor salts are added to the micellular solutions, they migrate to micelle cores under entropic and chemical pressures, reacting within the micellular nanoreactors to form perovskite nanoparticles. RMD offers distinct advantages over other synthesis methods for producing perovskite nanoparticles, including ease of fabrication, high particle homogeneity, enhanced stability against moisture and oxygen due to polymer encapsulation, slower reaction kinetics enabling the formation of unique phases, and tunability of particle size [3,4]. Transitioning From 2D Monolayers to 3D Assemblies Typical RMD synthesis procedures yield well-ordered 2D arrangements of perovskite nanoparticles when spin coating, dip coating, and slot-die coating [5]. However, achieving uniform 3D assemblies remains a significant challenge. Many potential applications, including light-emitting diodes (LEDs), solar cells, photodetectors, and integrated photonic devices, require multilayered 3D structures of perovskite nanoparticles [6-8]. The primary goal of this investigation is to adjust the fabrication parameters—including the solution recipe (e.g., polymer molecular weight, concentration, precursor ratios, and solvent) and solution deposition techniques (e.g., spin coating, dip coating, slot-die coating, and electrospray deposition)—to create uniform 3D assemblies of perovskite nanoparticles and to characterize their internal structure. X-ray reflectivity (XRR), grazing incidence small-angle X-ray scattering (GISAXS), ellipsometry, and confocal microscopy are employed to characterize the internal structure of the nanoparticle assemblies. In parallel, photoluminescence, time-resolved photoluminescence, UV-Vis spectroscopy, and ellipsometry are used to investigate their optical properties. Prior studies have shown that the maximum thickness achievable by spin coating, dip coating, and slot-die coating is constrained by their respective coating windows, necessitating adjustments to the solution recipe for creating multilayered structures [5]. To address this, the polymer concentration was significantly increased, with ellipsometry and XRR measurements used to correlate the concentration with the resulting film thickness. To compensate for the increase in the polymer amount, the perovskite precursor amounts were also increased. While this adjustment had minimal impact on film thickness or internal morphology, it did significantly alter the optical properties, indicating substantial changes in the perovskite formation process. GISAXS and XRR analyses revealed that variations in polymer molecular weight profoundly affected the internal film structure, leading to distinct outcomes such as polymer phase transitions resulting in cylindrical structures, high disordered morphologies, or retention of the micellular architecture. These finding show the intricate relationship between fabrication parameters and film structure, highlighting opportunities for precise tuning of polymer-perovskite based films. References [1] Fu, Y., Zhu, H., et al. (2019). Nature Reviews Materials , 4 (3), 169-188. [2] Huang, H., Bodnarchuk, M. I., et al. (2017). ACS energy letters , 2 (9), 2071-2083. [3] Hui, L. S., Beswick, C., et al. (2019). ACS Applied Nano Materials , 2 (7), 4121-4132. [4] Munir, M., Salib, A., et al. (2023). Chemistry , 5 (4), 2490-2512. [5] Oliveira, P. Q., Arbi, R., et al. (2024). Flexible and Printed Electronics , 9 (2), 025019. [6] Li, X., Aftab, S., et al. (2025). Nano-Micro Letters , 17 (1), 28. [7] Liu, D. S., Wu, J., et al. (2021). Advanced Materials , 33 (4), 2003733. [8] Chandra, S., Mustafa, M. A., et al. (2024). Naunyn-Schmiedeberg's archives of pharmacology , 1-42. Figure 1