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Synthesis and Photoluminescence Quantum Yield Studies of CsPbI₃ Perovskite Nanocrystals in Liquid Paraffin as a Novel and Low-Cost Reaction Medium

This study demonstrates that synthesizing CsPbI₃ perovskite nanocrystals in liquid paraffin yields uniform, size-controlled particles with significantly superior photoluminescence quantum yield compared to those made in conventional octadecene, establishing liquid paraffin as a novel, low-cost, and effective reaction medium for high-performance optoelectronic applications.

Original authors: Abbasali Ghorbani Kouhbanani, Vahid Saheb, S. Mohhamad Ali Hosseini

Published 2026-10-01

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CC BY 4.0

Original authors: Abbasali Ghorbani Kouhbanani, Vahid Saheb, S. Mohhamad Ali Hosseini

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

In the quest for cleaner energy and brighter lights, scientists are constantly searching for materials that can capture sunlight or emit it with incredible efficiency. For decades, silicon has been the king of solar panels, but making it requires intense heat and complex machinery, which drives up the cost. A newer class of materials, known as perovskites, has emerged as a promising alternative. These are crystals that can be grown in liquid solutions at much lower temperatures, offering a path to cheaper and more versatile devices. Among these, a specific type made from cesium, lead, and iodine stands out for its ability to glow brightly when hit with light. However, these tiny crystals are notoriously unstable; they can easily change their internal structure and lose their ability to function. The key to making them useful lies in finding the perfect liquid environment to grow them, one that keeps them stable and helps them form into perfect, uniform shapes without introducing flaws that would dim their light.

Researchers at Shahid Bahonar University of Kerman set out to solve this problem by testing a common, inexpensive liquid called liquid paraffin as a new home for growing these crystals. Instead of using the standard, expensive solvents typically found in laboratories, they mixed the chemical ingredients into liquid paraffin, a thick, clear oil often used in cosmetics and pharmaceuticals. They heated this mixture and injected a second chemical solution into it, a process that causes the crystals to form instantly. To see if this new method worked, they compared the crystals grown in the cheap paraffin against those grown in the traditional, more expensive solvent. The goal was simple: determine if the low-cost liquid could produce crystals that were just as good, or perhaps even better, than the high-end standard.

The results were striking. When the scientists looked at the crystals under a powerful electron microscope, they found that the ones grown in liquid paraffin were remarkably uniform. They were tiny cubes, each measuring about 11 nanometers across, with very little variation in size. This consistency is crucial because it means the crystals are well-behaved and predictable. Furthermore, the internal structure of these crystals was pristine. By analyzing how X-rays bounced off the material, the team confirmed that the crystals had formed in the correct, light-emitting shape, rather than a dull, inactive form that often plagues this type of material. The crystals were single, solid pieces of order, lacking the internal cracks or defects that usually trap energy and stop it from being released as light.

The most significant discovery, however, came when the researchers measured how brightly these crystals glowed. They used a precise method that compared how much light the crystals absorbed to how much they emitted, ensuring a fair test regardless of how many crystals were in the sample. The crystals grown in the liquid paraffin shone significantly brighter than those grown in the traditional solvent. In fact, the light output was roughly sixty percent stronger. This improvement suggests that the paraffin environment helped the crystals grow with fewer imperfections and a more perfect internal order. Because the crystals were so clean and uniform, they could convert energy into light much more efficiently.

This finding points toward a practical and affordable way to manufacture high-quality materials for future technology. By swapping a costly, specialized chemical for a common, low-cost oil, the researchers demonstrated that it is possible to create superior crystals without expensive equipment. The study suggests that this simple change in the liquid medium not only stabilizes the delicate crystals but actually enhances their performance. While further work is needed to see how these materials hold up over long periods of use, the research establishes that liquid paraffin is a powerful tool for producing the next generation of efficient, low-cost light-emitting and energy-harvesting devices.

Technical Summary: Synthesis and Photoluminescence Quantum Yield Studies of CsPbI₃ Perovskite Nanocrystals in Liquid Paraffin

Problem Statement
Cesium lead iodide (CsPbI₃) perovskite nanocrystals (PNCs) are promising candidates for optoelectronic applications due to their tunable bandgaps and high photoluminescence efficiency. However, maintaining the phase stability of CsPbI₃—specifically preserving the photoactive black cubic ( $\alpha$ ) phase over the thermodynamically stable but photoinactive yellow orthorhombic ( $\delta$ ) phase—remains a significant challenge. This stability is heavily dependent on reaction conditions, solvent environments, and precursor composition. Furthermore, conventional synthesis often relies on high-cost solvents like 1-octadecene (ODE), limiting scalability. There is a need for low-cost, reproducible reaction media that can effectively control nucleation, morphology, and phase evolution to produce high-quality nanocrystals.

Methodology
The study employed a modified hot-injection synthesis method to produce CsPbI₃ nanocrystals using liquid paraffin (PaL) as the reaction medium, comparing the results against a standard synthesis using ODE.

Precursor Preparation: Cesium oleate was prepared by reacting cesium carbonate with oleic acid in ODE.
Synthesis Routes:
- ODE Route: PbI₂ was dissolved in ODE at 160°C with oleylamine (OLA) and oleic acid (OA), followed by the rapid injection of preheated cesium oleate. The reaction was quenched after 5 seconds.
- PaL Route: A modified procedure utilized liquid paraffin (PaL) as the solvent. Due to PaL's higher viscosity and slightly higher polarity, the reaction temperature was optimized to 140°C (lower than ODE's 160°C) to account for slower solute diffusion and earlier supersaturation. The injection and quenching protocols were otherwise identical.
Purification: Both samples underwent precipitation using methyl acetate and redispersion in n-hexane. For PaL samples, an initial dilution with n-hexane was required due to the solvent's high viscosity.
Characterization:
- Structural/Morphological: X-ray diffraction (XRD) was used for phase identification, and Transmission Electron Microscopy (TEM) was used to analyze morphology and size distribution (n=270 particles).
- Optical: To rigorously compare Photoluminescence (PL) performance, an "Integrated PL Intensity vs. Absorbance" method was employed. Five serially diluted solutions for each medium were prepared to ensure absorbance at the excitation wavelength (450 nm) remained below 0.25, mitigating Inner Filter Effects. The slope of the linear regression between integrated PL intensity and absorbance served as a metric for relative quantum yield.

Key Results

Phase and Structure: XRD analysis confirmed that PaL-synthesized nanocrystals formed the photoactive cubic $\alpha$ -phase (space group $Pm\bar{3}m$ ) with a dominant (200) peak at $2\theta \approx 28.94^\circ$ . No peaks corresponding to the $\delta$ -phase or unreacted precursors were detected.
Morphology and Size: TEM imaging revealed well-defined cubic nanocrystals with a mean edge length of $10.97 \pm 2.16$ nm and a polydispersity index (PDI) of approximately 19.7%. The particles exhibited spontaneous self-assembly into large-area, square-packed superlattices.
Crystallinity: The crystallite size calculated via the Debye-Scherrer equation from XRD data was 11.72 nm, showing excellent agreement (<7% deviation) with the TEM-measured mean size. This suggests the nanocrystals are single-crystalline with a low density of internal grain boundaries.
Photoluminescence Performance: The comparative PL analysis showed that the regression slope for PaL-synthesized PNCs was approximately 60% higher than that of ODE-synthesized PNCs. This indicates a substantially superior photoluminescent performance for the PaL medium.

Significance and Claims
The paper claims that liquid paraffin is an effective, reproducible, and low-cost alternative to conventional solvents like ODE for synthesizing CsPbI₃ perovskite nanocrystals. The authors attribute the enhanced optical quality (60% higher relative PL intensity) to the PaL medium's ability to promote improved crystallinity, reduce the density of non-radiative defect sites, and favor specific morphology and size distributions. The study establishes PaL as a viable medium for producing size-controlled, high-quality CsPbI₃ PNCs, offering potential implications for the scalable fabrication of next-generation optoelectronic and photovoltaic devices. The research concludes that this low-cost approach facilitates further process development without compromising optical quality.

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