Enhanced Photoluminescence and Crystallinity in FAPbBr₃@Ni Core-Shell Nanoparticles Synthesized via Reverse Micelle Deposition

Organo-halide perovskites are known for being excellent candidates for optoelectronic devices due to their intense and narrow emission and absorption spectra, ease of fabrication, and low costs [1]. These properties can be utilized in optoelectronic devices like solar cells, LEDs, lasers, and sensors. However, they are extremely sensitive to air and moisture. Core-shell structures encapsulate the perovskite nanoparticle, offering a material that eliminates these short comings. Compared to nanocrystals, core-shell systems have also been shown to improve luminescence and charge carrier transport, reduce exciton recombination, and increase stability against environmental factors [2]. Recent studies have found that introducing transition metals as dopants also leads to an increase in these properties [3]. Reverse micelle deposition (RMD) is a bottom-up solution-based synthesis method for the fabrication of such nanoparticles [4][5]. This method enables the perovskites to form in an enclosed nanoreactor based on a diblock co-polymer, suspended in non-polar solvent. When the perovskite salts are loaded into the solution, they infiltrate the reverse micelle and form the nanoparticle core [6][7]. Subsequently, the shell salts are similarly loaded, and infiltration into the micelle allows final core-shell nanoparticle to form away from ambient conditions that may otherwise deteriorate the material [8][9]. Photoluminescence spectroscopy was used to study the emission of FAPbBr3 and FAPbBr3@Ni core-shell nanoparticles encapsulated by seven different polymer micelles. Using different molecular weights and diblock polymer ratios, FAPbBr3 nanoparticles with sizes ranging from 5 and 70 nm in radius can be produced. The most intense emission was observed from smaller particles, and with some polymers, emission was not observed though nanoparticles were formed. In the presence of the Ni shell, emission was observed in all cases, even with the polymer micelles that failed to form emissive perovskite crystals without the shell. Additionally, the FAPbBr3 emission was intensified with the presence of the Ni shell for many of the polymers used, as shown in Figure 1. The increase in emission varies from 1.5x to 20x. This indicates, along with supporting evidence from x-ray diffraction measured at the Canadian Light Source, that the Ni shell is enhancing the formation of FAPbBr3 crystals. These findings highlight that reverse micelle deposition enables the straightforward and reproducible synthesis of FAPbBr3@Ni core-shell nanoparticles, which show consistent increases in emission intensity across all polymer micelles. Synchrotron XRD data further suggest that the Ni shell enhances crystallinity, providing evidence of its facilitation of crystal formation. The increased emission suggests enhanced exciton recombination efficiency, possibly by reducing defect states at the core-shell interface [10][11]. These results underline the importance of understanding dynamic exciton processes and charge transfer at the core-shell interface, a topic central to advancing energy conversion materials. Future investigations will evaluate the stability of these nanoparticles over extended periods and under environmental exposure to moisture and oxygen, shedding light on the protective capabilities of Ni shells. Additionally, expansion of this study to include other transition metal shells will reveal new insights into optimizing the synthesis and performance of perovskite-based materials. References [1] R, S., Nayak, et al. Journal of Alloys and Compounds vol. 834 at https://doi.org/10.1016/j.jallcom.2020.155246 (2020). [2] Ahmed, G. H., et al. ACS Energy Letters vol. 6 1340–1357 at https://doi.org/10.1021/acsenergylett.1c00076 (2021). [3] Xu, L., et al. Materials Today Nano vol. 6 at https://doi.org/10.1016/j.mtnano.2019.100036 (2019). [4] Munir, M. et al. Enhanced Stokes Shift and Phase Stability by Cosynthesizing Perovskite Nanoparticles (MAPbI 3 /MAPbBr 3 ) in a Single Solution . Advanced Photonics Research 3, (2022). [5] Turak, A. Reverse Micelles as a Universal Route to Solution Processed Nanoparticles for Optical, Optoelectronic and Photonic Applications: A Story of Salt Complexation, Micellar Stability, and Nanoparticle Spatial Distribution. Vid. Proc. Adv. Mater. 2021, 2, 2103166. https://doi.org/10.5185/vpoam.2021.03166. [6] Hui, L. S. et al. Reverse Micelle Templating Route to Ordered Monodispersed Spherical Organo-Lead Halide Perovskite Nanoparticles for Light Emission. ACS Applied Nano Materials 2, 4121–4132 (2019). [7] Munir, M., et al. Unusual Phase Behaviour for Organo-Halide Perovskite Nanoparticles Synthesized via Reverse Micelle Templating. Chemistry (Switzerland) 5, 2490–2512 (2023). [8] Munir, M., et al. Core-Shell Perovskite-TiO2Nanoparticles for High Stability. in 2023 Photonics North, PN 2023 (Institute of Electrical and Electronics Engineers Inc., 2023). doi:10.1109/PN58661.2023.10223000. [9] Turak, A. Perovskite nanoparticles on demand: highly stable nanoparticles using reverse micelle templating. in 2023 Photonics North, PN 2023 (Institute of Electrical and Electronics Engineers Inc., 2023). doi:10.1109/PN58661.2023.10222995. [10] Zhang, C. et al. Core/Shell Perovskite Nanocrystals: Synthesis of Highly Efficient and Environmentally Stable FAPbBr3/CsPbBr3 for LED Applications. Advanced Functional Materials 30, (2020). [11] das Adhikari, S. et al. Chemical Science vol. 14 8984–8999 at https://doi.org/10.1039/d3sc02955g (2023). Figure 1

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