Fluorescent organic nanoparticles (NPs) are gaining significant attention due to their unique optical and chemical properties, facile synthesis, easy processability, and excellent biocompatibility, making them suitable for various applications where inorganic counterparts fall short. Existing methods for organic NP synthesis, such as emulsification, nanoprecipitation, and solvent evaporation, often suffer from poor uniformity and low yields. Surfactants have been widely used in inorganic NP synthesis to improve stability and yield, but their application in organic NP synthesis remains limited. This research aims to address the lack of systematic analysis of surfactant effects on organic NP properties (yield, morphology, photophysical properties) and explore the potential of these surfactant-stabilized organic nano-dots as high-performance color conversion layers (CCLs) in optoelectronic devices. Inorganic QDs, though promising, have drawbacks such as sensitivity to moisture and temperature, and involve toxic or hazardous materials. Organic NPs offer a potential solution, providing enhanced photochemical and moisture stability, high uniformity, and facile, environmentally friendly synthesis. This study focuses on a comprehensive investigation of surfactant effects on organic nano-dot properties and their application as CCLs, offering a potential improvement in the stability and performance of light-emitting devices.

The literature highlights the advantages of fluorescent organic NPs over their inorganic counterparts, particularly in bio-applications. Several synthesis methods exist, but often lack uniformity and high yields of smaller particles. While surfactants are extensively used in inorganic NP synthesis, their systematic application in organic NP synthesis is scarce. Existing studies demonstrate sporadic use of surfactants in producing specific organic NPs, such as cholesterol NPs or NPs of fluorescent organic materials. However, a comprehensive, quantitative analysis of surfactant effects on organic NP properties and the influence of surfactant properties (e.g., CMC) is lacking. The use of fluorescent and phosphorescent organic dyes in light emission applications, including OLEDs, OLETs, and CCLs, is well-established. However, NPs of fluorescent organic materials are less explored despite their potential advantages in optoelectronic devices. While various materials have been used in CCL fabrication, such as phosphors, inorganic QDs, and perovskites, organic dyes suffer from aggregation-induced emission quenching and poor stability. Inorganic QDs, while promising, exhibit limitations in moisture and temperature sensitivity and involve toxic synthetic procedures. Carbon dots and carbonized polymer dots have shown some potential but a comprehensive analysis of the use of surfactants to improve organic nano-dot synthesis and their use as CCLs is needed.

The study systematically investigates the impact of both ionic (tetra-butyl ammonium oleate, TBA oleate) and non-ionic (Triton X-100) surfactants on the synthesis and properties of fluorescent organic nano-dots. A thermally activated delayed fluorescent (TADF) emitter, Ttrz-DI, was used as a model fluorophore, along with other fluorophores (TNAP, 4CzIPN, CzDABNA, 4tBuMB) to demonstrate broader applicability. Nano-dots were synthesized using an anti-solvent precipitation method, where a solution of the fluorophore in THF was rapidly mixed with water (anti-solvent) in the presence of varying concentrations of surfactant. The resulting dispersions were filtered and dialyzed to remove excess surfactant. The influence of surfactant concentration on particle size and morphology was examined using optical microscopy, atomic force microscopy (AFM), scanning electron microscopy (SEM), and fluorescence microscopy. Photoluminescence (PL) spectroscopy was employed to assess the optical properties of the nano-dots, and dynamic light scattering (DLS) was used to measure particle size distribution. The yield of nano-dots was determined spectrophotometrically. The effect of different solvents (THF, 1,3-dioxalane, 1,4-dioxane, NMP, N-methyl imidazole) and anti-solvents (water, methanol, ethanol, iso-propanol, 2-methoxyethanol) was investigated. The influence of anti-solvent temperature on nano-dot synthesis was also explored. Color conversion applications were studied by fabricating films of organic nano-dots dispersed in polyvinyl alcohol (PVA). A blue LED (400 nm) was used as the excitation source, and color conversion efficiency (CCE) was calculated using an integrating sphere. A comparison was made with inorganic green fluorescent QDs (InP/ZnSe/ZnS) dispersed in PMMA. The long-term stability of the nano-dot films was assessed under accelerated aging conditions (constant UV radiation) and by monitoring their CCE over time. Additionally, the CCE performance was compared with that of bulk fluorophore films.

The study found that surfactant concentration significantly influences the size, uniformity, and yield of organic nano-dots. Above the critical micelle concentration (CMC) of each surfactant, both ionic and non-ionic surfactants dramatically improved particle diameter and yield, with 6 mM concentration of either surfactant yielding optimal results (nano-dots ranging from 120 to 165 nm). Nearly 100% yield was achieved for several fluorophores. Optical and microscopic analyses consistently demonstrated the smallest average particle size at optimal surfactant concentrations. The PL spectra of nano-dot dispersions showed peak positions intermediate between solution and film states, consistent with the fluorophore existing in an intermediate state. The organic nano-dot films exhibited outstanding color conversion characteristics, with excellent air and photo-stability and high color purity (FWHM ranging from 21.1 to 87.4 nm). Compared to state-of-the-art InP-based QDs, organic nano-dot CCLs displayed superior CCE and stability. The highest CCE (31%) was obtained with TNAP nano-dots, exhibiting a narrow FWHM (35.4 nm) and high color purity, demonstrating potential for display applications. Nano-dots based on TADF materials showed broader emission spectra, suitable for white LEDs. The study also demonstrated the superior stability of nano-dot CCLs compared to both inorganic QD films and bulk fluorophore films, showing only marginal decline in CCE after one month of storage, in contrast to a significant decrease in CCE for QDs (52%). Boron-containing nano-dots (TNAP and CzDABNA) showed narrow FWHMs and high color purity, covering 105.96% of the NTSC color gamut. Nano-dots based on TADF materials (Ttrz-DI and 4CzIPN) with broader FWHMs (71.05% and 76.91% of NTSC gamut) demonstrated applicability as CCLs in white LEDs. The 4tBuMB nano-dot CCL showed the greatest stability under UV radiation, with less than 20% decay after 120 hours of exposure.

The findings demonstrate the significant impact of surfactants in achieving high-yield synthesis of uniform fluorescent organic nano-dots. The superior performance of these nano-dots as CCLs, exceeding that of inorganic QDs in terms of both efficiency and long-term stability, addresses the limitations of current CCL technologies. The wide color gamut coverage and high color purity of nano-dots, particularly boron-containing ones, highlight their potential in advanced display applications. The improved stability, stemming from the nano-dot structure and surfactant stabilization, is crucial for practical applications in LEDs and lighting. The environmentally benign, water-based synthesis method adds to the advantages of this technology, offering a cost-effective and sustainable alternative to conventional CCL materials. The results of this study contribute to the advancement of both display and lighting technologies by providing a superior and more environmentally friendly approach to CCL fabrication.

This work demonstrates a highly efficient and environmentally friendly method for synthesizing fluorescent organic nano-dots using surfactants, achieving near-100% yield and superior performance as CCLs. The resulting nano-dots exhibit excellent color purity, high conversion efficiency, and substantially improved long-term stability compared to inorganic QDs. The broad range of emission colors achievable and the high color gamut coverage suggest significant applications in both display and white LED technologies. Future research could focus on exploring other surfactant types, optimizing the synthesis parameters for specific fluorophores, and integrating these nano-dots into various device architectures for practical applications.

While this study demonstrates excellent results, several limitations should be noted. The study focused on a limited set of fluorophores and surfactants, and further investigation is needed to explore the generality of the findings. The accelerated aging tests, while informative, do not fully represent real-world operating conditions. Long-term field studies would provide further insights into the stability and longevity of these nano-dot CCLs. Finally, a comprehensive cost-benefit analysis compared to existing technologies would provide a complete picture of the economic viability of this method.