Alkyl-Functionalized ZnS Nanoparticles for Optical Management in Quantum Dot Color Conversion Layers with Inkjet Printing Compatibility

Alkyl-Functionalized ZnS Nanoparticles for Optical Management in Quantum Dot Color Conversion Layers with Inkjet Printing Compatibility | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Article Alkyl-Functionalized ZnS Nanoparticles for Optical Management in Quantum Dot Color Conversion Layers with Inkjet Printing Compatibility Yoon-Jeong Choi, Mir Jeong, Min Su Lee, Jeong-Yeol Yoo, Byung Doo Chin This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-9229334/v1 This work is licensed under a CC BY 4.0 License Status: Under Revision Version 1 posted 9 You are reading this latest preprint version Abstract Achieving high color conversion efficiency by optimizing the optical properties of quantum dots (QDs) color conversion layers (CCLs) remains a challenge for next-generation display fabrication. CCLs, which convert blue light into red or green emission via photoluminescence, are key components in QD-based displays. However, their optical efficiency and inkjet printability remain limited due to poor compatibility between QDs and conventional light-scattering materials such as titanium oxide (TiO 2 ). In this study, alkyl-functionalized ZnS nanoparticles were synthesized and incorporated into a QD ink formulation to enhance conversion efficiency and reduce blue leakage. The functionalized ZnS exhibited improved dispersibility in polymeric resins, ensuring uniform mixing. To enable precise inkjet-printed patterning, ink viscosity was optimized using a viscosity modifier, and substrate wettability was controlled via CF 4 plasma treatment to confine ink droplets within defined areas. The results confirm that ZnS-enhanced QD CCLs improve color conversion efficiency (up to 78.1%) and reduce blue leakage below 1%, leading to superior optical performance. This study highlights the synergistic role of light-scattering nanoparticles and inkjet printability in enabling scalable, high-resolution inkjet-printed QD-CCLs, offering a promising approach for next-generation display technologies. Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 1. Introduction Colloidal QDs have garnered significant attention because of their size-dependent tunable emission, high color purity, high quantum efficiency, and photoluminescence quantum yield (PLQY) [1–3]. These remarkable optical and electrical properties of QDs stem from the quantum confinement effect, in which the energy band gap increases as the particle sizes decrease [4]. In particular, QDs exhibit narrow full width at half-maximum (FWHM), wide color gamut coverage, and excellent stability under continuous photoexcitation, making them highly suitable as emissive or color-converting materials in advanced display applications [5–9]. Recently, the application of QDs as color conversion materials has transitioned from research to commercial success. In 2022, Samsung introduced its first commercial QD-organic light-emitting diode (QD-OLED) displays at CES 2022, integrating QD as color converters to achieve a wide color gamut and viewing experience, showing the practical viability of QDs in display technology. In the architecture of QD-OLEDs, a blue OLED serves as the light source, while a layer of red or green QDs is deposited on top layers as a PL color conversion layer (CCL), selectively converting portions of the blue emission into red/green light [10–14]. This structure eliminates the need for conventional color filters and enables a wide color gamut and higher efficiency by using the narrow emission bandwidth of QDs. Consequently, the QD-OLED structure not only simplifies the optical stack but also maximizes energy efficiency and color performance, which are key metrics in modern display engineering. However, a persistent challenge in QD-based CCLs is the incomplete absorption of incident blue light, which results in blue light leak leakage and limits the overall color conversion efficiency (CCE). Effective management of photon propagation within the CCL is therefore essential to maximize interaction between the excitation light and QDs. To further improve the performance of such QD-based architecture, significant research efforts have been focused on enhancing the optical performance of QDs and optimizing fabrication strategies for high-performance CCLs [15–19]. One widely explored strategy to address this challenge is the incorporation of light-scattering materials into the CCLs. Instead of relying on direct absorption along a single optical path, dispersed light scattering particles within the CCLs induce multiple scattering of incident blue light. This produced redirection of light increases the effective optical path length within the film, thereby enhancing the likelihood of improving excitation efficiency. To achieve this effect, materials with high-refractive index with wide band gaps, such as titanium oxide (TiO 2 ), zinc oxide (ZnO), and barium titanium oxide (BaTiO 3 ), have been employed as scattering materials [18, 20–24]. Nevertheless, these high-refractive index scatterers are difficult to apply stably in solution-processed CCLs due to their surface characteristics. Generally possessing hydrophilic surfaces, they exhibit low compatibility with hydrophobic QDs. Consequently, phase separation and precipitation readily occur in organic solvent systems, and in more severe cases, agglomeration occurs. This not only degrades the optical quality of the resulting film but also causes practical issues such as nozzle clogging during inkjet printing, significantly limiting their suitability for high resolution solution-processed display fabrication [25]. Importantly, dispersion stability is not only a performance issue but also a manufacturing constraint in inkjet-based CCL fabrication. Scattering additives that precipitate or aggregate typically require additional formulation complexity or hardware-level mitigation such as circulation to maintain jetting reliability, thereby increasing process costs and operational burdens. Therefore, the selection of a scattering material in QD-based CCL cannot be determined solely by scattering performance. It must simultaneously satisfy the requirements of minimizing visible light absorption loss, compatibility and stability with QDs, resins, and organic solvents, and inkjet printability. Considering these requirements, zinc sulfide (ZnS) can be considered an attractive substitute. While its scattering intensity may be somewhat lower than TiO 2 , which is widely used in research, ZnS overcomes a critical optical limitation of TiO 2 . Unlike TiO 2 , which has a relatively narrow bandgap that can inadvertently absorb blue excitation light, ZnS possesses a significantly wider bandgap of approximately 3.5–3.7 eV [26, 27]. This exceptionally wide bandgap results in negligible absorption and maintains high optical transparency across the entire visible spectrum, including blue excitation wavelengths. In this study, we investigated alkyl- functionalized ZnS nanoparticles as a process-compatible scattering material for QD-based CCLs. The alkyl-functionalized ZnS nanoparticles developed in this study provide intrinsic compatibility with solvent-free UV-curable resin/QD systems, enabling stable storage and dispensing without relying on complex dispersion strategies. This simplicity presents a practical pathway for cost-effective and scalable inkjet printing of QD-CCLs, where particle stability maintenance under actual printing conditions is often a limiting factor. Furthermore, by systematically varying the ZnS content, we investigated its impact on CCE and blue light suppression, while maintaining stable dispersion and printing suitability. Through a combined theoretical and experimental approach, including single-particle scattering considerations and hazed-based film-level scattering parameters, we analyzed scattering behaviors beyond traditional performance metrics. 2. Experiment 2.1 Chemicals Zinc acetate (Zn(ac) 2 , 99.99%), sulfur (S, 99.998%), 1-octadecene (ODE, 90%), oleic acid (OA, 90%), isobornyl acrylate (technical grade, contains 200 ppm monomethyl ether hydroquinone as inhibitor, 92.7%), and 4-(1,1,3,3-Tetramethylbutyl)phenyl-polyethylene glycol (Triton X-45), and 1-hydroxycyclohexyl phenyl ketone (HPK, 99%) were purchased from Sigma-Aldrich. Bisphenol A epoxy diacrylate was purchased from BIOSYNTH. N-hexane (above 95.0%), ethyl alcohol (99.5%), and methyl alcohol (99.8%) were purchased from DAEJUNG. Diiodomethane (99%, stab.) was purchased from Alfa Aesar. All chemicals were used without further purification. 2.2 Synthesis of ZnS nanoparticles 2 mmol of Zn(ac) 2 , 20 mmol of S, 36 mL of ODE, and 12 mL of OA were mixed in the three-neck round-bottom flask. The reaction mixture was degassed at 120 ℃ for 90 min. After that, the reaction flask was heated up to 300 or 320 ℃ under the nitrogen flow and kept for 3 h. The resulting reaction is then cooled to room temperature, and 10 mL of hexane is injected quickly into the mixture. Then, ethanol/butanol 4:3 solutions were added to the mixture and centrifuged at 3800 rpm for 10 min. The obtained product was dried in a vacuum oven for 12 h and then washed with hexane/methanol. 2.3 Preparation of UV-curable QDs/ZnS nanoparticles composite films UV-curable QDs/ZnS nanoparticles composite films were prepared by mixing 20 mg/mL QDs with IBOA, Triton X-45 (1.6 v/v%), HPK (33 mg/mL), and ZnS nanoparticles as light scattering materials (10–40 mg/mL). IBOA was used as a monomer, while Triton X-45 and HPK acted as dispersant and a photo-initiator respectively. The mixture was stirred for 1 h to make a uniform dispersion. 3 mL of the mixture was cast into a mold and subjected to a UV-curing process (λ em = 365 nm, 1185 mW/cm 2 ) for less than 10 min to form the polymerized film. 2.4 Characterization For the characterization of synthesized ZnS scattering nanoparticles, the structural properties were evaluated by X-ray diffraction (XRD)(Ultima Ⅳ, Rigaku). We utilized a Fourier-transform infrared (FT-IR) (Nicolet 380, Thermo) to analyze ligand compositions of ZnS. The average size with a standard deviation of the ZnS nanoparticles was determined by field-emission transmission electron microscopy (FE-TEM) (JEM-F200, TFEG). Absorbance spectrum observations were evaluated through a UV-Vis spectrometer (1601PC, Shimadzu), and photoluminescence properties were analyzed using a spectrofluorophotometer (Fluoromax-4, Horiba). The Dispersion of QDs and ZnS nanoparticles in an ink was studied by a dynamic light scattering (DLS) instrument (Zeta-Potential & Particle Size Analyzer ELSZ-200 series, Otsuka Electronics). To investigate the effect of the viscosity modifier content, ink viscosity was measured using a viscometer (DV2TLVTJ10, Brookfield). The refractive index of synthesized ZnS nanoparticles was analyzed using an ellipsometer (Alpha-SE, J.A. Woollam). To evaluate the blue light leakage ratio, a mini blue LED panel was chosen to demonstrate QDs color-converted optoelectronics device. Device performance was characterized using a Keithley 2400 source meters and a CS-2000 spectroradiometer (Konica-Minolta). The Commission International de 1’Ecalirage (CIE 1931) parameters and EL spectra were also obtained from the coupled CS-2000 spectroradiometer. A custom-built inkjet printing system (OmniJet 300, Unijet) with a commercially available print cartridge (Sapphire QS-256/30 AAA, Fujifilm) served as the patterning unit. Unless stated otherwise, the jetting voltage was set to 100V, the frequency to 1000 Hz, and the print bed temperature to 30 ℃. To prepare the ink, CdSe/ZnCdS QDs were dispersed in formulated viscous polymer-based inks at a concentration of 20 mg/mL. A fiducial recognition camera mounted on a printer was used to visualize the droplet/pixel formation photographs. 3-dimensional contour images of the dried samples were obtained on a 3D laser confocal microscope (OLS4100, Olympus). 3. Results and discussion One of the technical goals of this work is to synthesize ZnS nanoparticles with organic alkyl chain ligands that can be well dispersed with QDs without aggregation. ZnS nanoparticles were synthesized via a heating-up method using zinc acetate and sulfur as precursors in an organic solvent [28,29]. The reaction was carried out at 300–320 ℃ under an inert atmosphere for a controlled duration, allowing organic ligand coverage. These ligands consist of a carboxylate headgroup anchored on the particles and the alkyl chains at the end of ligand tails. As a result, these particles can be well dispersed in solvent-free polymer ink for further work. First, as shown in Figure S1 (Supporting information, Figure S1 ), XRD and FT-IR were conducted to verify the properties of ZnS samples synthesized at different temperatures, at 300℃ (black line) and 320 ℃ (red line). As shown in Figure S1 a, the XRD pattern shows that they have a cubic (zinc blende) structure. The XRD pattern of ZnS exhibits distinguishable peaks but broader than those of in a bulk form, implying that the nano-sized particles are well synthesized. The surface chemistry of ZnS was characterized by using FT-IR spectroscopy. Figure S1 b shows the FT-IR spectra of ZnS both show characteristic bands at ~ 2900 cm − 1 and ~ 2850 cm − 1 , corresponding to the asymmetric and symmetric CH 2 stretching modes of oleic acid. These confirm the successful surface functionalization of ZnS with oleic acid, hence enabling well-dispersibility with QDs synthesized with organic ligands. In addition, ZnS synthesized at 300 ℃ exhibited stronger asymmetric and symmetric CH 2 peaks of oleic acid compared to ZnS synthesized at 320 ℃, indicating improved dispersibility in organic solvents. To further explore the dispersion stability, we conducted a comparative analysis between our synthesized ZnS nanoparticles and commercially available TiO 2 nanoparticles. Both materials were dispersed at equal concentrations (10, 20, and 40 mg/mL) in a solvent-free UV-curable resin matrix and stored at room temperature. The dispersion was monitored over 10 days to assess sedimentation behavior. As shown in Fig. 1 , TiO 2 dispersion exhibited sedimentation even at a low concentration of 10 mg/mL after just 1 day. In contrast, ZnS dispersions remained stable and optically uniform throughout the 10-day test period, even at a high concentration of 40 mg/mL. These results indicate that the alkyl-chain-functionalized surfaces of ZnS nanoparticles effectively suppress aggregation, ensuring excellent colloidal stability over time. Such dispersion stability is a foundational requirement in inkjet printing, as particle aggregation or sedimentation can lead to solution degradation and clogging of inkjet nozzles [30,31]. Therefore, the high stability of ZnS even under concentrated conditions highlights its practical advantages as a printable light-scattering material over conventional inorganic particles like TiO 2 . As observed in transmission electron microscopy (TEM) images (Figure S2 (a)-(b) for TEM images and Figure S2 (c)-(d) for histograms), the average size of ZnS synthesized at 300 ℃ was estimated to be 7.6 nm (standard deviation: 1.05 nm). In contrast, ZnS nanoparticles prepared at 320 ℃ possessed a larger average size of 11.0 nm with a higher standard deviation of 2.6 nm. This size difference is attributed to the temperature-dependent nucleation followed by suppressed growth yields smaller and more uniform particles. In contrast, at 320 ℃, elevated thermal energy facilitates particle growth and promotes Ostwald ripening, leading to larger average size and broader size distribution [32]. Although both nanoparticles synthesized at different temperatures exhibited similar crystallinity and surface chemistry, we confirmed that the qualitative difference in the number of ligands attached to the surface and the difference in the size distribution affected dispersibility. Based on these results, ZnS synthesized at 300 ℃ was chosen as the light scattering nanoparticles for use in CCLs. The refractive index of ZnS synthesized at 300 ℃ was measured to be 1.37 at a wavelength of 632 nm (Supporting information, Figure S3), which is smaller than those of conventional scattering materials (TiO 2 : 2.49 (anatase type), BaTiO 3 : 1.59) [18,33]. Selecting an appropriate monomer is essential for ensuring the stable distribution of QDs without aggregation in resin formulations. IBOA, a commercial acrylate-based resin monomer, was used for this purpose [34]. Subsequently, to prevent aggregation upon mixing with the synthesized ZnS scattering particles, Triton X-45 as a dispersant was used. The films exhibit high transparency in the visible region after the UV curing process, implying that the CCL film does not absorb the blue light of the blue LED, as shown in Figure S4. Figure 2 (a) shows a schematic image of the structure of the QD CCL with the scattering particles ZnS. Figure 2 (b) shows a schematic representation of the overall process flow for the fabrication and evaluation of QD CCL films. FT-IR analysis was conducted to confirm whether UV curing was complete by monitoring changes in the C = C stretching vibration of IBOA, as shown in Figure S5. Before the UV curing step, a symmetric C = C stretching peak was observed at 1650 and 1407 cm − 1 . However, after the curing step, a decrease in the intensity of the C = C stretching peak was detected, confirming that the curing process occurred through the cleavage of C = C double bonds in IBOA [35]. The optical properties of QDs and the blue LED are shown in Fig. 2 (c). The PL spectra of the QD solution and CCL film exhibit consistent profiles with a peak wavelength at 646 nm and FWHM of 30.7 nm, indicating that the QDs remain well-dispersed within the resin without aggregation, even after the incorporation of ZnS nanoparticles. The blue LED used in this study emits at a peak wavelength of 469 nm with a FWHM of 21.8 nm, which substantially overlaps with the absorption spectrum of the QDs, thereby facilitating efficient energy transfer. Table 1 Summary of EL peak wavelength (λmax), CCE, and blue leakage of QD CCLs at different concentrations. Samples Blue LED QD-10 mg/mL QD-15 mg/mL QD-20 mg/mL QD- 30 mg/mL EL λ max (nm) 469 640 644 644 648 CCE (%) N/A 24.7 55.2 66.0 98.4 Blue leakage (%) N/A 38.26 24.97 18.52 1.88 To investigate the effect of QD concentration on the color conversion behavior of QD-based films, since the color conversion efficiency changes depending on the QD concentration, luminescence intensity at different concentrations (10 to 30 mg/mL) was measured by assembling them with blue LEDs to select an appropriate QD concentration. As shown in Fig. 3 (b), the intensity of blue light decreases as QD concentration increases up to 30 mg/mL. At a relatively low concentration of QDs (10–15 mg/mL), color conversion is not saturated yet, and the difference in conversion efficiency and the presence of scattering material are not distinguishable. To clearly evaluate the effect of ZnS nanoparticles on light scattering, QDs were used at a fixed concentration of 20 mg/mL. This concentration was experimentally determined to provide partial color conversion while retaining a measurable blue emission peak, thereby enabling distinguishable analysis of the blue leakage reduction. As shown in Fig. 3 (a) and Table 1 , the color conversion efficiency was calculated for each sample using Eq. (1) [36]: $$\:{\eta\:}_{CCE}\:\left(\%\right)=\frac{{\eta\:}_{QD}}{{\eta\:}_{Blue}-\:{\eta\:}_{Blue,\:QD}}\times\:100\%=\:\frac{Area\:D}{Area\:B}\:\times\:100\%$$ Where η CCE is color conversion efficiency (CCE), η QD is the total intensity of converted red light by QDs CCL, η Blue is the total intensity of the blue LED used as a light source, and η Blue, QD is the intensity of the blue LED that remains through the CCL. Even though the blue light passes through the CCL, some portion of blue light still exists. This residual blue leakage interferes with achieving a high color gamut. Blue light leakage must be minimized to improve the color conversion efficiency. The blue leakage ratio is calculated as described in Eq. (2) [37]: $$\:Blue\:leakage\:\left(\%\right)=\:\frac{{\eta\:}_{Blue,QD}}{{\eta\:}_{Blue}}\:\times\:100\:\left(\%\right)=\frac{Area\:C}{Area\:A}\:\times\:100\%$$ As shown in Fig. 3 (c), the CCE increases with QD concentration, while blue leakage decreases significantly. The calculated results indicate that as the QD concentration increases, CCE increases while blue leakage decreases. Table 2 Summary of EL peak wavelength (λ max ), color conversion efficiency (CCE), and blue leakage of QD CCLs with fixed QD concentration (20mg/mL) varying ZnS concentrations Samples REF Sample A Sample B Sample C EL λ max (nm) 644 644 644 648 CCE (%) 66.0 75.6 76.9 78.1 Blue leakage (%) 18.52 2.43 0.26 0.32 Subsequently, four different film samples were prepared to evaluate the influence of ZnS nanoparticles on the CCE and blue light leakage in CCLs. All samples contained a fixed QD concentration of 20 mg/mL, while the ZnS content was varied as 0 mg/mL (REF), 10 mg/mL (Sample A), 20 mg/mL (Sample B), and 40 mg/mL (Sample C). As summarized in Table 2 and shown in Fig. 4 , the incorporation of ZnS did not significantly alter the emission peak position for Samples A and B (λ max = 644 nm), while a slight redshift to 648 nm was observed for Sample C. Furthermore, in terms of CCE, a clear enhancement was observed with the increase of ZnS nanoparticles. CCE increased from 66.0% (REF) to 78.1% (Sample C), while blue leakage drastically reduced from 18.52% to as low as 0.26% (Sample B) and 0.32% (Sample C). However, explaining this performance improvement solely based on the scattering characteristics of individual ZnS nanoparticles has limitations. While ZnS is classified as a high-refractive-index inorganic material in its bulk form, the refractive index extracted from the ZnS-polymer composite film in this study (n \(\:\approx\:\) 1.37 at 632 nm) makes it difficult to expect strong single-particle scattering based on refractive index alone. Furthermore, TEM analysis confirmed the average diameter of ZnS nanoparticles to be approximately 7.6 nm. This implies a sufficiently small size parameter ( \(\:x\ll\:1\) ) in the visible light region, corresponding to the Rayleigh scattering regime. Strong angular redistribution associated with classical Mie resonances is not anticipated under these circumstances, and the scattering cross-section of isolated particles scales with the sixth power of the particle radius [38,39]. As a result, it is expected that single-particle scattering will be weak and almost isotropic. Nevertheless, the experimental results showing that blue leakage decreases and CCE increases as ZnS content rises suggest that additional optical mechanisms beyond isolated primary nanoparticle scattering alone must be considered. In this regard, Park group reported that introducing scattering particles into the QD CCL increases the optical path length of blue excitation light due to light scattering. Consequently, this enhances blue light absorption by the QDs, leading to increased conversion luminance while simultaneously reducing blue light leakage. Specifically, they interpreted that this optical path length increase effect could be amplified when using a mixture of spherical and rod-shaped scattering particles, suggesting that changes in the film-level light transport within the CCL could contribute to performance enhancement [22]. Therefore, in this study, to complement single-particle-level scattering analysis, the mean free path (MFP) was introduced as an indicator to quantify how light transport varies at short length scales at the film level. The MFP was calculated as follows form the film thickness t and total transmittance (T t ), based on the transmittance-based definition proposed by Shin et al. [40]. $$\:Mean\:free\:path=-\frac{t}{\text{ln}\left({T}_{t}\right)}$$ Table 3 Film-level scattering parameters derived from standard haze measurements for ZnS-incorporated composite films Sample A Total transmittance (T t ) Diffuse transmittance (T d ) Diffuse fraction (f d ) Mean free path (MFP) (mm) 0.889 0.119 0.134 3.42 Sample B 0.801 0.258 0.321 1.80 Sample C 0.753 0.367 0.488 1.41 In this study, the thickness of all samples was fixed at 0.4 mm for comparison. Furthermore, to determine the relative proportion of the diffusion component, the diffusion fraction (f d ) was calculated from the T t and the diffuse transmittance (T d ) (Table 3 ) [41]. As a result, the diffusion characteristics were significantly enhanced as the ZnS content increased. For Sample A (ZnS 10 mg/mL), f d = 0.134 and MFP = 3.42 mm, whereas for Sample B (ZnS 20 mg/mL), f d = 0.321 and MFP = 1.80 mm, and for Sample C (ZnS 40 mg/mL), f d = 0.488 and MFP = 1.41 mm. That is, as the ZnS concentration increased from 10 to 40 mg/mL, the proportion of the diffuse component increased significantly, while the MFP decreased from approximately 3.42 mm to 1.41 mm. This indicates that, under identical film thickness conditions, the tendency for light to interact with particles (or microstructures) at shorter length scales and have its propagation direction redistributed became stronger, rather than maintaining the characteristic of light propagating forward. In other words, the increase in f d and decrease in MFP shown in Table 3 support the possibility that the observed reduction in blue leakage and increase in CCE are associated with film-level light transport changes, such as the formation of aggregate/microstructure-based effective scattering units within in film or increased optical path length due to multiple scattering, rather than being explained solely by single scattering from isolated primary ZnS nanoparticles. In the present study, the ZnS content was systematically varied up to 40 mg/mL to investigate the effect of scattering enhancement while maintaining dispersion stability within the QD matrix. Up to this concentration, the composite films exhibited uniform optical appearance without visible phase separation, indicating that the ZnS nanoparticles remained well dispersed within the cured films. In contrast, when the ZnS content exceeded 40 mg/mL, clear aggregation between QDs and ZnS nanoparticles was observed in the films, despite the alkyl-chain functionalized employed to improve compatibility. Such aggregation led to the formation of non-uniform regions within the composite layer, indicating a dispersion stability threshold beyond which further increases in ZnS loading detrimental to film uniformity and optical reliability. Accordingly, 40 mg/mL was identified as the practical upper limit for ZnS in the QD matrix, as summarized in Fig. 5 . Since this nonlinear behavior between light scattering materials concentration and blue leakage highlights the need for precise engineering to leverage scattering benefits while avoiding saturation effects. To evaluate the practical suitability of scattering materials in QD CCL fabrication, films were also prepared using commercially available TiO 2 nanoparticles under the same formulation and processing conditions as the ZnS samples. Despite the immediate use after mixing, visible sedimentation and particle aggregation were observed during the short period required for film casting and UV curing, especially at higher TiO 2 concentrations. As shown in Fig. 6 , the resulting films exhibited a complete separation bilayer of the top and bottom surfaces due to the rapid layer separation of TiO 2 particles. This behavior is consistent with the poor dispersion stability observed in the long-term storage test (Fig. 6 ), confirming that TiO 2 suffers from both temporal and process-related issues. In contrast, the synthesized ZnS maintained excellent colloidal stability even at high concentrations, ensuring uniform film morphology during actual fabrication. These findings reinforce the practical advantages of the alkyl-functionalized ZnS system over conventional light scattering materials. Table 4 Summary of ink formulations and physical properties HPK (mg/mL) Triton X-45 (v/v %) BED (mg/mL) Viscosity (cP) Ink #1 33 1.6 0 7.46 Ink #2 100 15.0 Ink #3 200 20.0 Ink #4 300 66.8 While the earlier experiments investigated the effects of the ZnS-induced light scattering properties in enhancing the optical performance of QD CCLs, the subsequent studies focused on evaluating the ink printability of the formulated ink. Ensuring successful inkjet printing requires careful control of the ink’s rheological properties, particularly viscosity and surface tension, which directly affect droplet formation, deposition accuracy, and film quality. Low-viscosity inks tend to spread excessively upon substrate contact, often overflowing the pixel boundary and resulting in non-uniform film formation [42]. To control ink viscosity, we added bisphenol A epoxy diacrylate (BED) as a viscosity modifier. As shown in Table 4 , the initial viscosity of Ink #1 was 7.46 cP, which increased to 15.0 cP with 100 mg/mL addition of BED. Further increases in BED concentration enabled the formulation of high-viscosity inks, reaching 66.8 cP at 300 mg/mL (Ink #4). The droplet formation of Ink #1 and #2 is shown in Fig. 7 . While Ink #1 exhibited an elongated tail extending from the main droplet, Ink #2 achieved stable single-droplet ejection without any tailing or satellite formation. This accomplished by increasing the viscosity of the Ink #2 through the addition of a viscosity modifier and optimizing the jetting waveform to match the adjusted fluid properties. At even higher viscosity (Ink #3 and #4), it was difficult to make droplet ejections under normal conditions. However, as shown in Figure S6, droplet ejection from high-viscosity inks could be conducted by controlling the printing head temperature. Although further optimization is needed to improve surface tension and ejection properties for high viscosity Ink #3 and #4, Ink #2 was successfully printed without tail or satellite, demonstrating adequate pattern formation for formulated inks. These results highlight the feasibility of using UV-curable inks in inkjet printing processes for display applications. Future efforts will focus on suppressing the formation of satellite droplets, which are critical for achieving high-resolution, defect-free pattering. Table 5 Calculated surface energy of the substrate from the measured contact angles W/O CPT CA of DI water (º) CA of Diiodomethane (º) Surface energy (mN/m) 61.2 32.0 54.4 W/CPT 30 sec 85.5 74.6 27.0 W/CPT 1 min 88.3 73.7 26.2 W/CPT 2 min 88.9 73.0 26.2 W/CPT 5 min 89.0 67.8 28.2 As shown in the inset image of Fig. 8 (b), when a 3 \(\:\times\:\) 3 array of dots was jetted using Ink #2, the diameter of the dot was 150 µm, and when a pixel-confined substrate with a width of 160 µm was used as a test bed to attempt to form a film inside the pixel, overflow occurred regardless of changes in dot pitch. Wettability is not only a crucial factor in determining the desired position and shape of printed droplets but also plays a key role in forming micro-thickness optimized QD films for CCLs using inkjet printing technology [43]. To control the surface energy of the substrate, CF 4 plasma treatment (CPT) was used to optimize the surface wettability. Figure 8 (a) shows the contact angle of diluted (DI) water, diiodomethane, and Ink #2, respectively. The contact angle (CA) of DI water and diiodomethane increased from 61.2º to 88.9º and from 32.0º to 73.0º after 2 min surface treatment. However, as shown in the results after 5 min of CPT, the CA of diiodomethane showed a slight decrease. Nevertheless, for Ink #2, the CA increased as the CPT time increased, even though the non-treated substrate was too spread out to even measure. As a result, it formed a CA of 52.2º and 54.0º after 2 and 5 min of CPT, respectively. Based on the measured CA, the calculated surface energy value of the substrate is shown in Table 5 . As shown in Table 5 , the surface energy of the substrate decreased as the CPT time increased and showed a slight increase after 5 min of treatment. For some reason, surface energy increased again after 2 min of treatment. Based on this, subsequent inkjet printing tests were conducted after 2 min of CPT. As shown in the inset image in Fig. 8 (c), when the same Ink #2 was used to form dots on the substrate after 2 mins CPT, the dot size was 65 µm, indicating a significant reduction in diameter after surface treatment. The surface treatment was applied to enhance ink confinement within the pixels, enabling the formation of CCLs. To verify higher-resolution patterning capability, we fabricated using 80 \(\:\times\:\) 240 µm pattern (168 ppi, Fig. 9 (a) and (b)) and 44 \(\:\times\:\) 212 µm pattern (231 ppi, Fig. 9 (c) and (d)), as shown in Fig. 9 . By using the same composition of ink and adjusting the number of dots, a pattern with a thickness of approximately 9 µm was formed. Additionally, we attempted to form a higher-resolution pattern with a 453 ppi pixel-confined cell, corresponding to a cell width of 33 µm and a height of 119 µm, as shown in Fig. 10 . While the ink successfully filled in the patterned regions, minor overflow between adjacent pixels was observed due to the narrow pixel pitch. Although the current formulation and printing conditions were not fully optimized for 453 ppi patterning, this experiment demonstrates the potential for high-resolution inkjet-printed CCLs. Further optimization of ink viscosity, pattern size tuning, and droplet spacing control will be required to achieve precise patterning at this resolution. These experiments demonstrate the feasibility of ZnS-incorporated QD CCLs and their compatibility with inkjet printing technology. In the future, we will focus on refining the ink formulation and jetting parameters to enable ultra-high-resolution patterning, along with droplet behavior control through surface energy engineering. These efforts will contribute to the reliable fabrication of inkjet-printed color conversion layers, facilitating their integration into next-generation display architectures. 4. Conclusion We investigated the optical properties, inkjet printability, and high-resolution patterning capabilities of ZnS-incorporated QD CCLs. A systemic evaluation revealed that ZnS nanoparticles enhance light scattering while effectively reducing blue light leakage. Notably, the ZnS nanoparticles synthesized in this work exhibited high dispersion compatibility with the UV-curable QD ink system, enabling stable inkjet processing without requiring complex formulations or additional circulation hardware system. This processing simplicity can reduce operational burden and supports a more cost-effective manufacturing route for inkjet-printed CCLs. To further study the feasibility of inkjet printing for CCL fabrication, we optimized the ink formulation by adjusting viscosity using BED as a viscosity modifier. This modification enabled droplet ejection and improved film uniformity. However, pristine substrates exhibited ink spreading across the pixels. To address this, CPT was considered to modify the surface energy, which enhanced wettability and confirmed ink droplets within defined pixel boundaries. Following CPT, we successfully fabricated high-resolution patterns at 168 ppi (80 \(\:\times\:\) 240 µm) and 231 (44 \(\:\times\:\) 212 µm) with a uniform thickness of approximately 9 µm. Additionally, a higher resolution 453 ppi pattern was attempted, demonstrating the potential of inkjet printing for ultra-high-resolution patterning. While minor overflow was observed at 453 ppi, further optimization of ink rheology, surface energy control, and print parameters will be necessary to suppress lateral spreading between adjacent pixels. Compared with conventional scattering additive approaches that often improve conversion efficiency at the expense of residual blue leakage and/or increased formulation complexity, the ZnS-integrated CCLs demonstrated a favorable balance between enhanced scattering and pronounced suppression of blue leakage. Overall, these findings highlight the potential of QD CCLs as a practical material for high-resolution inkjet-printed displays. The result provides valuable insights into material selection, ink formulation, and surface engineering to enhance printing resolution and uniformity. Declarations Author Contribution Y.-J. Choi conceived the study, conducted the main experiments, analyzed the data and prepared the figures, and wrote the manuscript. M. Jeong and M.S. Lee assisted with nanoparticle synthesis, sample preparation, and related experiments. J.-Y. Yoo and B.D. Chin supervised the study, contributed to interpretation of the results and revised the manuscript. 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Supplementary Files SupportingInformation.docx Cite Share Download PDF Status: Under Revision Version 1 posted Editorial decision: Revision requested 28 Apr, 2026 Reviews received at journal 28 Apr, 2026 Reviewers agreed at journal 25 Apr, 2026 Reviews received at journal 23 Apr, 2026 Reviewers agreed at journal 21 Apr, 2026 Reviewers invited by journal 03 Apr, 2026 Editor assigned by journal 28 Mar, 2026 Submission checks completed at journal 28 Mar, 2026 First submitted to journal 26 Mar, 2026 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-9229334","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":617048770,"identity":"b122d9aa-4814-4afc-a08b-ada2a0966cf2","order_by":0,"name":"Yoon-Jeong Choi","email":"","orcid":"","institution":"Dankook University","correspondingAuthor":false,"prefix":"","firstName":"Yoon-Jeong","middleName":"","lastName":"Choi","suffix":""},{"id":617048773,"identity":"157219ab-5797-409b-8107-886f96cc5add","order_by":1,"name":"Mir Jeong","email":"","orcid":"","institution":"Dankook University","correspondingAuthor":false,"prefix":"","firstName":"Mir","middleName":"","lastName":"Jeong","suffix":""},{"id":617048774,"identity":"db45b6b6-265d-41fa-beab-5966fc2866b9","order_by":2,"name":"Min Su Lee","email":"","orcid":"","institution":"Dankook University","correspondingAuthor":false,"prefix":"","firstName":"Min","middleName":"Su","lastName":"Lee","suffix":""},{"id":617048776,"identity":"9409e934-7701-4420-ba4e-efdf3316d517","order_by":3,"name":"Jeong-Yeol Yoo","email":"","orcid":"","institution":"Dankook University","correspondingAuthor":false,"prefix":"","firstName":"Jeong-Yeol","middleName":"","lastName":"Yoo","suffix":""},{"id":617048777,"identity":"0d8c4e0e-d27a-42c7-a4a0-806e6d21eb01","order_by":4,"name":"Byung Doo Chin","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA1klEQVRIie3QvQrCMBDA8SvCdSm4VqT4Ci0FRfDjVRoCcfEhCkJcBNeAoK9QN8dCoFPRNaBDXZy7utmik0hoN4f8txz8IHcAJtMf5kOEEMHEigGrZ1rPsBFhbQmAbEFGNn0U99PF2+44KcocBt0YWakj481j5JP8GopbJgOhIBApSqH9mFqiS/iVxGrB+04JVgL2Sr/Lm5zJ4UPmTUlKEsWyvqOAJIBSS6pdhhWh4VExGojcpUIi1ZL6Yr0nn3l7xYKizCbT7ZqHWvKdC9BpBUwmk8n0qxfThU1y5PRxJgAAAABJRU5ErkJggg==","orcid":"","institution":"Dankook University","correspondingAuthor":true,"prefix":"","firstName":"Byung","middleName":"Doo","lastName":"Chin","suffix":""}],"badges":[],"createdAt":"2026-03-26 05:08:41","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-9229334/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-9229334/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":106516187,"identity":"04094929-3222-4ec7-b02e-6cf1f3389a2e","added_by":"auto","created_at":"2026-04-09 11:57:29","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":686257,"visible":true,"origin":"","legend":"\u003cp\u003ePhotographic comparison of dispersion stability for synthesized ZnS and commercial TiO\u003csub\u003e2\u003c/sub\u003e nanoparticles at concentrations of 10, 20, and 40 mg/mL after 10 days storage of storage at room temperature.\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-9229334/v1/277e6dd444390c1ebe2a0199.png"},{"id":106516166,"identity":"6aa4c037-bfb7-47e6-a7c2-6b63e5dcd360","added_by":"auto","created_at":"2026-04-09 11:57:28","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":109706,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Schematic illustration of QD CCL film with ZnS nanoparticles, (b) overall process schematic for CCL fabrication, and (c) absorption and PL spectra of QD solution and QD-CCL film, and EL spectra of blue LED\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-9229334/v1/e5d70b1ecb3bc392304d8758.png"},{"id":106516186,"identity":"9ebf7704-56c4-46e7-b78a-5311c81b18b6","added_by":"auto","created_at":"2026-04-09 11:57:29","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":82950,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Illustration of CCE and blue leakage in QD CCLs. (b) Luminescence spectra and (C) CIE chromaticity diagram of QD CCLs with different QD concentrations: 10 mg/mL (red), 15 mg/mL (blue), 20 mg/mL (green), and 30 mg/mL (magenta). (d) The plot of calculated CCE and blue leakage from the spectra using equations (1) and (2).\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-9229334/v1/a2e0afe4693759d79700ecac.png"},{"id":106516111,"identity":"fef70d97-d623-4418-bc0e-11dded682561","added_by":"auto","created_at":"2026-04-09 11:57:11","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":86146,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Luminescence spectra and (b) CIE chromaticity diagram of QD/ZnS composite films with varying ZnS concentrations: REF (QD only), Sample A (ZnS 10 mg/mL), Sample B (ZnS 20 mg/mL), and Sample C (ZnS 40 mg/mL). (c) Calculated color conversion efficiency (blue circles) and blue leakage (black circles) based on equations [1] and [2].\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-9229334/v1/9d5f802b8a91e8f1a16e0e13.png"},{"id":106516116,"identity":"25bb2adb-414b-445e-b52b-57624452abbf","added_by":"auto","created_at":"2026-04-09 11:57:12","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":405696,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eOptical images of QD CCL films containing ZnS nanoparticles at concentration above 40 mg/mL, showing visible aggregation and non-uniform film morphology.\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-9229334/v1/7caaafe484c30cfe71237c61.png"},{"id":106516164,"identity":"4b570746-5bb0-4c36-ba2b-0dee232bd7b9","added_by":"auto","created_at":"2026-04-09 11:57:28","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":292473,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eOptical images of QD CCL films containing TiO\u003c/strong\u003e\u003csub\u003e\u003cstrong\u003e2\u003c/strong\u003e\u003c/sub\u003e\u003cstrong\u003e nanoparticles at concentrations of 10, 20 and 40 mg, showing sedimentation and inhomogeneity during film formation.\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"6.png","url":"https://assets-eu.researchsquare.com/files/rs-9229334/v1/ebe1bd325e0080e7c54c5194.png"},{"id":106516162,"identity":"148f3546-b5eb-4a6c-8676-23d664085dc2","added_by":"auto","created_at":"2026-04-09 11:57:26","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":120078,"visible":true,"origin":"","legend":"\u003cp\u003eImages of droplet formation and jetting behavior of Ink #1 (a) and Ink #2 (b).\u003c/p\u003e","description":"","filename":"7.png","url":"https://assets-eu.researchsquare.com/files/rs-9229334/v1/d117725843f43c002197b9f5.png"},{"id":106516190,"identity":"21821682-e47e-44d1-9d45-a35ff1093b92","added_by":"auto","created_at":"2026-04-09 11:57:30","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":499715,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Contact angle images of DI water, diiodomethane, and Ink #2. (b,c) Inkjet-printed patterns and inset dot-printed images using Ink #2 on substrates with 160 μm width and 480 μm height banks: (b) without CF\u003csub\u003e4\u003c/sub\u003e plasma treatment (CPT), and (c) with 2 minutes CPT.\u003c/p\u003e","description":"","filename":"8.png","url":"https://assets-eu.researchsquare.com/files/rs-9229334/v1/53f41a71476de0c4ce9ffd99.png"},{"id":106516192,"identity":"c3e870c2-92e1-41cc-a34d-074477eea3b7","added_by":"auto","created_at":"2026-04-09 11:57:31","extension":"png","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":117752,"visible":true,"origin":"","legend":"\u003cp\u003eInkjet-printed patterns on pixel-confined substrate with two different pixel sizes: 80 μm x 240 μm-sized pixel (a,b) and 44 μm x 212 μm-sized pixel (c,d). (a,c) CCD images of printed patterns and (b,d) corresponding 3D laser confocal images after UV curing.\u003c/p\u003e","description":"","filename":"9.png","url":"https://assets-eu.researchsquare.com/files/rs-9229334/v1/79d8148f858f6856618a0e58.png"},{"id":106516165,"identity":"04659470-3539-4586-953e-beba3bb09217","added_by":"auto","created_at":"2026-04-09 11:57:28","extension":"png","order_by":10,"title":"Figure 10","display":"","copyAsset":false,"role":"figure","size":193398,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003e(a) Optical image of pixel arrays, (b) fluorescence image, and (c-d) 3D-confocal laser microscopy images of inkjet-printed patterns on the 33 μm × 119 μm pixel-confined substrates\u003c/strong\u003e.\u003c/p\u003e","description":"","filename":"10.png","url":"https://assets-eu.researchsquare.com/files/rs-9229334/v1/1c65f3e125946f2ed6472fcb.png"},{"id":106993976,"identity":"4e120fd8-c6bf-43af-82af-554104fc18a3","added_by":"auto","created_at":"2026-04-15 15:01:26","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":4035200,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-9229334/v1/6d2f903f-fa54-454c-a2c0-04bd689b4c16.pdf"},{"id":106516113,"identity":"33d124b5-8466-42f7-89e1-4dd40626b200","added_by":"auto","created_at":"2026-04-09 11:57:12","extension":"docx","order_by":0,"title":"","display":"","copyAsset":false,"role":"supplement","size":1896771,"visible":true,"origin":"","legend":"","description":"","filename":"SupportingInformation.docx","url":"https://assets-eu.researchsquare.com/files/rs-9229334/v1/03016d9d1c3fcddeb40e9d51.docx"}],"financialInterests":"No competing interests reported.","formattedTitle":"Alkyl-Functionalized ZnS Nanoparticles for Optical Management in Quantum Dot Color Conversion Layers with Inkjet Printing Compatibility","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eColloidal QDs have garnered significant attention because of their size-dependent tunable emission, high color purity, high quantum efficiency, and photoluminescence quantum yield (PLQY) [1\u0026ndash;3]. These remarkable optical and electrical properties of QDs stem from the quantum confinement effect, in which the energy band gap increases as the particle sizes decrease [4]. In particular, QDs exhibit narrow full width at half-maximum (FWHM), wide color gamut coverage, and excellent stability under continuous photoexcitation, making them highly suitable as emissive or color-converting materials in advanced display applications [5\u0026ndash;9].\u003c/p\u003e \u003cp\u003eRecently, the application of QDs as color conversion materials has transitioned from research to commercial success. In 2022, Samsung introduced its first commercial QD-organic light-emitting diode (QD-OLED) displays at CES 2022, integrating QD as color converters to achieve a wide color gamut and viewing experience, showing the practical viability of QDs in display technology. In the architecture of QD-OLEDs, a blue OLED serves as the light source, while a layer of red or green QDs is deposited on top layers as a PL color conversion layer (CCL), selectively converting portions of the blue emission into red/green light [10\u0026ndash;14]. This structure eliminates the need for conventional color filters and enables a wide color gamut and higher efficiency by using the narrow emission bandwidth of QDs. Consequently, the QD-OLED structure not only simplifies the optical stack but also maximizes energy efficiency and color performance, which are key metrics in modern display engineering. However, a persistent challenge in QD-based CCLs is the incomplete absorption of incident blue light, which results in blue light leak leakage and limits the overall color conversion efficiency (CCE). Effective management of photon propagation within the CCL is therefore essential to maximize interaction between the excitation light and QDs.\u003c/p\u003e \u003cp\u003eTo further improve the performance of such QD-based architecture, significant research efforts have been focused on enhancing the optical performance of QDs and optimizing fabrication strategies for high-performance CCLs [15\u0026ndash;19]. One widely explored strategy to address this challenge is the incorporation of light-scattering materials into the CCLs. Instead of relying on direct absorption along a single optical path, dispersed light scattering particles within the CCLs induce multiple scattering of incident blue light. This produced redirection of light increases the effective optical path length within the film, thereby enhancing the likelihood of improving excitation efficiency. To achieve this effect, materials with high-refractive index with wide band gaps, such as titanium oxide (TiO\u003csub\u003e2\u003c/sub\u003e), zinc oxide (ZnO), and barium titanium oxide (BaTiO\u003csub\u003e3\u003c/sub\u003e), have been employed as scattering materials [18, 20\u0026ndash;24]. Nevertheless, these high-refractive index scatterers are difficult to apply stably in solution-processed CCLs due to their surface characteristics. Generally possessing hydrophilic surfaces, they exhibit low compatibility with hydrophobic QDs. Consequently, phase separation and precipitation readily occur in organic solvent systems, and in more severe cases, agglomeration occurs. This not only degrades the optical quality of the resulting film but also causes practical issues such as nozzle clogging during inkjet printing, significantly limiting their suitability for high resolution solution-processed display fabrication [25].\u003c/p\u003e \u003cp\u003eImportantly, dispersion stability is not only a performance issue but also a manufacturing constraint in inkjet-based CCL fabrication. Scattering additives that precipitate or aggregate typically require additional formulation complexity or hardware-level mitigation such as circulation to maintain jetting reliability, thereby increasing process costs and operational burdens. Therefore, the selection of a scattering material in QD-based CCL cannot be determined solely by scattering performance. It must simultaneously satisfy the requirements of minimizing visible light absorption loss, compatibility and stability with QDs, resins, and organic solvents, and inkjet printability. Considering these requirements, zinc sulfide (ZnS) can be considered an attractive substitute. While its scattering intensity may be somewhat lower than TiO\u003csub\u003e2\u003c/sub\u003e, which is widely used in research, ZnS overcomes a critical optical limitation of TiO\u003csub\u003e2\u003c/sub\u003e. Unlike TiO\u003csub\u003e2\u003c/sub\u003e, which has a relatively narrow bandgap that can inadvertently absorb blue excitation light, ZnS possesses a significantly wider bandgap of approximately 3.5\u0026ndash;3.7 eV [26, 27]. This exceptionally wide bandgap results in negligible absorption and maintains high optical transparency across the entire visible spectrum, including blue excitation wavelengths.\u003c/p\u003e \u003cp\u003eIn this study, we investigated alkyl- functionalized ZnS nanoparticles as a process-compatible scattering material for QD-based CCLs. The alkyl-functionalized ZnS nanoparticles developed in this study provide intrinsic compatibility with solvent-free UV-curable resin/QD systems, enabling stable storage and dispensing without relying on complex dispersion strategies. This simplicity presents a practical pathway for cost-effective and scalable inkjet printing of QD-CCLs, where particle stability maintenance under actual printing conditions is often a limiting factor. Furthermore, by systematically varying the ZnS content, we investigated its impact on CCE and blue light suppression, while maintaining stable dispersion and printing suitability. Through a combined theoretical and experimental approach, including single-particle scattering considerations and hazed-based film-level scattering parameters, we analyzed scattering behaviors beyond traditional performance metrics.\u003c/p\u003e"},{"header":"2. Experiment","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003e2.1 Chemicals\u003c/h2\u003e \u003cp\u003eZinc acetate (Zn(ac)\u003csub\u003e2\u003c/sub\u003e, 99.99%), sulfur (S, 99.998%), 1-octadecene (ODE, 90%), oleic acid (OA, 90%), isobornyl acrylate (technical grade, contains 200 ppm monomethyl ether hydroquinone as inhibitor, 92.7%), and 4-(1,1,3,3-Tetramethylbutyl)phenyl-polyethylene glycol (Triton X-45), and 1-hydroxycyclohexyl phenyl ketone (HPK, 99%) were purchased from Sigma-Aldrich. Bisphenol A epoxy diacrylate was purchased from BIOSYNTH. N-hexane (above 95.0%), ethyl alcohol (99.5%), and methyl alcohol (99.8%) were purchased from DAEJUNG. Diiodomethane (99%, stab.) was purchased from Alfa Aesar. All chemicals were used without further purification.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e2.2 Synthesis of ZnS nanoparticles\u003c/h2\u003e \u003cp\u003e2 mmol of Zn(ac)\u003csub\u003e2\u003c/sub\u003e, 20 mmol of S, 36 mL of ODE, and 12 mL of OA were mixed in the three-neck round-bottom flask. The reaction mixture was degassed at 120 ℃ for 90 min. After that, the reaction flask was heated up to 300 or 320 ℃ under the nitrogen flow and kept for 3 h. The resulting reaction is then cooled to room temperature, and 10 mL of hexane is injected quickly into the mixture. Then, ethanol/butanol 4:3 solutions were added to the mixture and centrifuged at 3800 rpm for 10 min. The obtained product was dried in a vacuum oven for 12 h and then washed with hexane/methanol.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec5\" class=\"Section2\"\u003e \u003ch2\u003e2.3 Preparation of UV-curable QDs/ZnS nanoparticles composite films\u003c/h2\u003e \u003cp\u003eUV-curable QDs/ZnS nanoparticles composite films were prepared by mixing 20 mg/mL QDs with IBOA, Triton X-45 (1.6 v/v%), HPK (33 mg/mL), and ZnS nanoparticles as light scattering materials (10\u0026ndash;40 mg/mL). IBOA was used as a monomer, while Triton X-45 and HPK acted as dispersant and a photo-initiator respectively. The mixture was stirred for 1 h to make a uniform dispersion. 3 mL of the mixture was cast into a mold and subjected to a UV-curing process (λ\u003csub\u003eem\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;365 nm, 1185 mW/cm\u003csup\u003e2\u003c/sup\u003e) for less than 10 min to form the polymerized film.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec6\" class=\"Section2\"\u003e \u003ch2\u003e2.4 Characterization\u003c/h2\u003e \u003cp\u003eFor the characterization of synthesized ZnS scattering nanoparticles, the structural properties were evaluated by X-ray diffraction (XRD)(Ultima Ⅳ, Rigaku). We utilized a Fourier-transform infrared (FT-IR) (Nicolet 380, Thermo) to analyze ligand compositions of ZnS. The average size with a standard deviation of the ZnS nanoparticles was determined by field-emission transmission electron microscopy (FE-TEM) (JEM-F200, TFEG). Absorbance spectrum observations were evaluated through a UV-Vis spectrometer (1601PC, Shimadzu), and photoluminescence properties were analyzed using a spectrofluorophotometer (Fluoromax-4, Horiba). The Dispersion of QDs and ZnS nanoparticles in an ink was studied by a dynamic light scattering (DLS) instrument (Zeta-Potential \u0026amp; Particle Size Analyzer ELSZ-200 series, Otsuka Electronics). To investigate the effect of the viscosity modifier content, ink viscosity was measured using a viscometer (DV2TLVTJ10, Brookfield). The refractive index of synthesized ZnS nanoparticles was analyzed using an ellipsometer (Alpha-SE, J.A. Woollam).\u003c/p\u003e \u003cp\u003eTo evaluate the blue light leakage ratio, a mini blue LED panel was chosen to demonstrate QDs color-converted optoelectronics device. Device performance was characterized using a Keithley 2400 source meters and a CS-2000 spectroradiometer (Konica-Minolta). The Commission International de 1\u0026rsquo;Ecalirage (CIE 1931) parameters and EL spectra were also obtained from the coupled CS-2000 spectroradiometer.\u003c/p\u003e \u003cp\u003eA custom-built inkjet printing system (OmniJet 300, Unijet) with a commercially available print cartridge (Sapphire QS-256/30 AAA, Fujifilm) served as the patterning unit. Unless stated otherwise, the jetting voltage was set to 100V, the frequency to 1000 Hz, and the print bed temperature to 30 ℃. To prepare the ink, CdSe/ZnCdS QDs were dispersed in formulated viscous polymer-based inks at a concentration of 20 mg/mL. A fiducial recognition camera mounted on a printer was used to visualize the droplet/pixel formation photographs. 3-dimensional contour images of the dried samples were obtained on a 3D laser confocal microscope (OLS4100, Olympus).\u003c/p\u003e \u003c/div\u003e"},{"header":"3. Results and discussion","content":"\u003cp\u003eOne of the technical goals of this work is to synthesize ZnS nanoparticles with organic alkyl chain ligands that can be well dispersed with QDs without aggregation. ZnS nanoparticles were synthesized via a heating-up method using zinc acetate and sulfur as precursors in an organic solvent [28,29]. The reaction was carried out at 300\u0026ndash;320 ℃ under an inert atmosphere for a controlled duration, allowing organic ligand coverage. These ligands consist of a carboxylate headgroup anchored on the particles and the alkyl chains at the end of ligand tails. As a result, these particles can be well dispersed in solvent-free polymer ink for further work. First, as shown in Figure \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e (Supporting information, Figure \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e), XRD and FT-IR were conducted to verify the properties of ZnS samples synthesized at different temperatures, at 300℃ (black line) and 320 ℃ (red line). As shown in Figure \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003ea, the XRD pattern shows that they have a cubic (zinc blende) structure. The XRD pattern of ZnS exhibits distinguishable peaks but broader than those of in a bulk form, implying that the nano-sized particles are well synthesized. The surface chemistry of ZnS was characterized by using FT-IR spectroscopy. Figure \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003eb shows the FT-IR spectra of ZnS both show characteristic bands at ~\u0026thinsp;2900 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and ~\u0026thinsp;2850 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, corresponding to the asymmetric and symmetric CH\u003csub\u003e2\u003c/sub\u003e stretching modes of oleic acid. These confirm the successful surface functionalization of ZnS with oleic acid, hence enabling well-dispersibility with QDs synthesized with organic ligands. In addition, ZnS synthesized at 300 ℃ exhibited stronger asymmetric and symmetric CH\u003csub\u003e2\u003c/sub\u003e peaks of oleic acid compared to ZnS synthesized at 320 ℃, indicating improved dispersibility in organic solvents.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eTo further explore the dispersion stability, we conducted a comparative analysis between our synthesized ZnS nanoparticles and commercially available TiO\u003csub\u003e2\u003c/sub\u003e nanoparticles. Both materials were dispersed at equal concentrations (10, 20, and 40 mg/mL) in a solvent-free UV-curable resin matrix and stored at room temperature. The dispersion was monitored over 10 days to assess sedimentation behavior. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e, TiO\u003csub\u003e2\u003c/sub\u003e dispersion exhibited sedimentation even at a low concentration of 10 mg/mL after just 1 day. In contrast, ZnS dispersions remained stable and optically uniform throughout the 10-day test period, even at a high concentration of 40 mg/mL. These results indicate that the alkyl-chain-functionalized surfaces of ZnS nanoparticles effectively suppress aggregation, ensuring excellent colloidal stability over time. Such dispersion stability is a foundational requirement in inkjet printing, as particle aggregation or sedimentation can lead to solution degradation and clogging of inkjet nozzles [30,31]. Therefore, the high stability of ZnS even under concentrated conditions highlights its practical advantages as a printable light-scattering material over conventional inorganic particles like TiO\u003csub\u003e2\u003c/sub\u003e.\u003c/p\u003e \u003cp\u003eAs observed in transmission electron microscopy (TEM) images (Figure S2 (a)-(b) for TEM images and Figure S2 (c)-(d) for histograms), the average size of ZnS synthesized at 300 ℃ was estimated to be 7.6 nm (standard deviation: 1.05 nm). In contrast, ZnS nanoparticles prepared at 320 ℃ possessed a larger average size of 11.0 nm with a higher standard deviation of 2.6 nm. This size difference is attributed to the temperature-dependent nucleation followed by suppressed growth yields smaller and more uniform particles. In contrast, at 320 ℃, elevated thermal energy facilitates particle growth and promotes Ostwald ripening, leading to larger average size and broader size distribution [32]. Although both nanoparticles synthesized at different temperatures exhibited similar crystallinity and surface chemistry, we confirmed that the qualitative difference in the number of ligands attached to the surface and the difference in the size distribution affected dispersibility. Based on these results, ZnS synthesized at 300 ℃ was chosen as the light scattering nanoparticles for use in CCLs. The refractive index of ZnS synthesized at 300 ℃ was measured to be 1.37 at a wavelength of 632 nm (Supporting information, Figure S3), which is smaller than those of conventional scattering materials (TiO\u003csub\u003e2\u003c/sub\u003e: 2.49 (anatase type), BaTiO\u003csub\u003e3\u003c/sub\u003e: 1.59) [18,33].\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eSelecting an appropriate monomer is essential for ensuring the stable distribution of QDs without aggregation in resin formulations. IBOA, a commercial acrylate-based resin monomer, was used for this purpose [34]. Subsequently, to prevent aggregation upon mixing with the synthesized ZnS scattering particles, Triton X-45 as a dispersant was used. The films exhibit high transparency in the visible region after the UV curing process, implying that the CCL film does not absorb the blue light of the blue LED, as shown in Figure S4. Figure\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e(a) shows a schematic image of the structure of the QD CCL with the scattering particles ZnS. Figure\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e(b) shows a schematic representation of the overall process flow for the fabrication and evaluation of QD CCL films. FT-IR analysis was conducted to confirm whether UV curing was complete by monitoring changes in the C\u0026thinsp;=\u0026thinsp;C stretching vibration of IBOA, as shown in Figure S5. Before the UV curing step, a symmetric C\u0026thinsp;=\u0026thinsp;C stretching peak was observed at 1650 and 1407 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. However, after the curing step, a decrease in the intensity of the C\u0026thinsp;=\u0026thinsp;C stretching peak was detected, confirming that the curing process occurred through the cleavage of C\u0026thinsp;=\u0026thinsp;C double bonds in IBOA [35]. The optical properties of QDs and the blue LED are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e(c). The PL spectra of the QD solution and CCL film exhibit consistent profiles with a peak wavelength at 646 nm and FWHM of 30.7 nm, indicating that the QDs remain well-dispersed within the resin without aggregation, even after the incorporation of ZnS nanoparticles. The blue LED used in this study emits at a peak wavelength of 469 nm with a FWHM of 21.8 nm, which substantially overlaps with the absorption spectrum of the QDs, thereby facilitating efficient energy transfer.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eSummary of EL peak wavelength (λmax), CCE, and blue leakage of QD CCLs at different concentrations.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"6\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSamples\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eBlue LED\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eQD-10 mg/mL\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eQD-15 mg/mL\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eQD-20 mg/mL\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003eQD- 30 mg/mL\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eEL λ\u003csub\u003emax\u003c/sub\u003e (nm)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e469\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e640\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e644\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e644\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e648\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCCE (%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eN/A\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e24.7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e55.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e66.0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e98.4\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eBlue leakage (%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eN/A\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e38.26\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e24.97\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e18.52\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e1.88\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eTo investigate the effect of QD concentration on the color conversion behavior of QD-based films, since the color conversion efficiency changes depending on the QD concentration, luminescence intensity at different concentrations (10 to 30 mg/mL) was measured by assembling them with blue LEDs to select an appropriate QD concentration. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e(b), the intensity of blue light decreases as QD concentration increases up to 30 mg/mL. At a relatively low concentration of QDs (10\u0026ndash;15 mg/mL), color conversion is not saturated yet, and the difference in conversion efficiency and the presence of scattering material are not distinguishable. To clearly evaluate the effect of ZnS nanoparticles on light scattering, QDs were used at a fixed concentration of 20 mg/mL. This concentration was experimentally determined to provide partial color conversion while retaining a measurable blue emission peak, thereby enabling distinguishable analysis of the blue leakage reduction. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e(a) and Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e, the color conversion efficiency was calculated for each sample using Eq.\u0026nbsp;(1) [36]:\u003cdiv id=\"Equa\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equa\" name=\"EquationSource\"\u003e\n$$\\:{\\eta\\:}_{CCE}\\:\\left(\\%\\right)=\\frac{{\\eta\\:}_{QD}}{{\\eta\\:}_{Blue}-\\:{\\eta\\:}_{Blue,\\:QD}}\\times\\:100\\%=\\:\\frac{Area\\:D}{Area\\:B}\\:\\times\\:100\\%$$\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003eWhere η\u003csub\u003eCCE\u003c/sub\u003e is color conversion efficiency (CCE), η\u003csub\u003eQD\u003c/sub\u003e is the total intensity of converted red light by QDs CCL, η\u003csub\u003eBlue\u003c/sub\u003e is the total intensity of the blue LED used as a light source, and η\u003csub\u003eBlue, QD\u003c/sub\u003e is the intensity of the blue LED that remains through the CCL. Even though the blue light passes through the CCL, some portion of blue light still exists. This residual blue leakage interferes with achieving a high color gamut. Blue light leakage must be minimized to improve the color conversion efficiency. The blue leakage ratio is calculated as described in Eq.\u0026nbsp;(2) [37]:\u003cdiv id=\"Equb\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equb\" name=\"EquationSource\"\u003e\n$$\\:Blue\\:leakage\\:\\left(\\%\\right)=\\:\\frac{{\\eta\\:}_{Blue,QD}}{{\\eta\\:}_{Blue}}\\:\\times\\:100\\:\\left(\\%\\right)=\\frac{Area\\:C}{Area\\:A}\\:\\times\\:100\\%$$\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003eAs shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e(c), the CCE increases with QD concentration, while blue leakage decreases significantly. The calculated results indicate that as the QD concentration increases, CCE increases while blue leakage decreases.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab2\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eSummary of EL peak wavelength (λ\u003csub\u003emax\u003c/sub\u003e), color conversion efficiency (CCE), and blue leakage of QD CCLs with fixed QD concentration (20mg/mL) varying ZnS concentrations\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"5\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSamples\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eREF\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eSample A\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eSample B\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eSample C\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eEL λ\u003csub\u003emax\u003c/sub\u003e (nm)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e644\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e644\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e644\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e648\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCCE (%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e66.0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e75.6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e76.9\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e78.1\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eBlue leakage (%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e18.52\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e2.43\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.26\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.32\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eSubsequently, four different film samples were prepared to evaluate the influence of ZnS nanoparticles on the CCE and blue light leakage in CCLs. All samples contained a fixed QD concentration of 20 mg/mL, while the ZnS content was varied as 0 mg/mL (REF), 10 mg/mL (Sample A), 20 mg/mL (Sample B), and 40 mg/mL (Sample C). As summarized in Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e and shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e, the incorporation of ZnS did not significantly alter the emission peak position for Samples A and B (λ\u003csub\u003emax\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;644 nm), while a slight redshift to 648 nm was observed for Sample C. Furthermore, in terms of CCE, a clear enhancement was observed with the increase of ZnS nanoparticles. CCE increased from 66.0% (REF) to 78.1% (Sample C), while blue leakage drastically reduced from 18.52% to as low as 0.26% (Sample B) and 0.32% (Sample C).\u003c/p\u003e \u003cp\u003eHowever, explaining this performance improvement solely based on the scattering characteristics of individual ZnS nanoparticles has limitations. While ZnS is classified as a high-refractive-index inorganic material in its bulk form, the refractive index extracted from the ZnS-polymer composite film in this study (n\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\approx\\:\\)\u003c/span\u003e\u003c/span\u003e1.37 at 632 nm) makes it difficult to expect strong single-particle scattering based on refractive index alone. Furthermore, TEM analysis confirmed the average diameter of ZnS nanoparticles to be approximately 7.6 nm. This implies a sufficiently small size parameter (\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:x\\ll\\:1\\)\u003c/span\u003e\u003c/span\u003e) in the visible light region, corresponding to the Rayleigh scattering regime. Strong angular redistribution associated with classical Mie resonances is not anticipated under these circumstances, and the scattering cross-section of isolated particles scales with the sixth power of the particle radius [38,39]. As a result, it is expected that single-particle scattering will be weak and almost isotropic. Nevertheless, the experimental results showing that blue leakage decreases and CCE increases as ZnS content rises suggest that additional optical mechanisms beyond isolated primary nanoparticle scattering alone must be considered.\u003c/p\u003e \u003cp\u003eIn this regard, Park group reported that introducing scattering particles into the QD CCL increases the optical path length of blue excitation light due to light scattering. Consequently, this enhances blue light absorption by the QDs, leading to increased conversion luminance while simultaneously reducing blue light leakage. Specifically, they interpreted that this optical path length increase effect could be amplified when using a mixture of spherical and rod-shaped scattering particles, suggesting that changes in the film-level light transport within the CCL could contribute to performance enhancement [22].\u003c/p\u003e \u003cp\u003eTherefore, in this study, to complement single-particle-level scattering analysis, the mean free path (MFP) was introduced as an indicator to quantify how light transport varies at short length scales at the film level. The MFP was calculated as follows form the film thickness t and total transmittance (T\u003csub\u003et\u003c/sub\u003e), based on the transmittance-based definition proposed by Shin et al. [40].\u003cdiv id=\"Equc\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equc\" name=\"EquationSource\"\u003e\n$$\\:Mean\\:free\\:path=-\\frac{t}{\\text{ln}\\left({T}_{t}\\right)}$$\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab3\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 3\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eFilm-level scattering parameters derived from standard haze measurements for ZnS-incorporated composite films\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"5\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003eSample A\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eTotal transmittance (T\u003csub\u003et\u003c/sub\u003e)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eDiffuse transmittance (T\u003csub\u003ed\u003c/sub\u003e)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eDiffuse fraction (f\u003csub\u003ed\u003c/sub\u003e)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eMean free path (MFP) (mm)\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.889\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.119\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.134\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003e3.42\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSample B\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0.801\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.258\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0.321\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e1.80\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSample C\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0.753\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.367\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0.488\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e1.41\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eIn this study, the thickness of all samples was fixed at 0.4 mm for comparison. Furthermore, to determine the relative proportion of the diffusion component, the diffusion fraction (f\u003csub\u003ed\u003c/sub\u003e) was calculated from the T\u003csub\u003et\u003c/sub\u003e and the diffuse transmittance (T\u003csub\u003ed\u003c/sub\u003e) (Table\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e) [41]. As a result, the diffusion characteristics were significantly enhanced as the ZnS content increased. For Sample A (ZnS 10 mg/mL), f\u003csub\u003ed\u003c/sub\u003e = 0.134 and MFP\u0026thinsp;=\u0026thinsp;3.42 mm, whereas for Sample B (ZnS 20 mg/mL), f\u003csub\u003ed\u003c/sub\u003e = 0.321 and MFP\u0026thinsp;=\u0026thinsp;1.80 mm, and for Sample C (ZnS 40 mg/mL), f\u003csub\u003ed\u003c/sub\u003e = 0.488 and MFP\u0026thinsp;=\u0026thinsp;1.41 mm. That is, as the ZnS concentration increased from 10 to 40 mg/mL, the proportion of the diffuse component increased significantly, while the MFP decreased from approximately 3.42 mm to 1.41 mm. This indicates that, under identical film thickness conditions, the tendency for light to interact with particles (or microstructures) at shorter length scales and have its propagation direction redistributed became stronger, rather than maintaining the characteristic of light propagating forward. In other words, the increase in f\u003csub\u003ed\u003c/sub\u003e and decrease in MFP shown in Table\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e support the possibility that the observed reduction in blue leakage and increase in CCE are associated with film-level light transport changes, such as the formation of aggregate/microstructure-based effective scattering units within in film or increased optical path length due to multiple scattering, rather than being explained solely by single scattering from isolated primary ZnS nanoparticles.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eIn the present study, the ZnS content was systematically varied up to 40 mg/mL to investigate the effect of scattering enhancement while maintaining dispersion stability within the QD matrix. Up to this concentration, the composite films exhibited uniform optical appearance without visible phase separation, indicating that the ZnS nanoparticles remained well dispersed within the cured films. In contrast, when the ZnS content exceeded 40 mg/mL, clear aggregation between QDs and ZnS nanoparticles was observed in the films, despite the alkyl-chain functionalized employed to improve compatibility. Such aggregation led to the formation of non-uniform regions within the composite layer, indicating a dispersion stability threshold beyond which further increases in ZnS loading detrimental to film uniformity and optical reliability. Accordingly, 40 mg/mL was identified as the practical upper limit for ZnS in the QD matrix, as summarized in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eSince this nonlinear behavior between light scattering materials concentration and blue leakage highlights the need for precise engineering to leverage scattering benefits while avoiding saturation effects. To evaluate the practical suitability of scattering materials in QD CCL fabrication, films were also prepared using commercially available TiO\u003csub\u003e2\u003c/sub\u003e nanoparticles under the same formulation and processing conditions as the ZnS samples. Despite the immediate use after mixing, visible sedimentation and particle aggregation were observed during the short period required for film casting and UV curing, especially at higher TiO\u003csub\u003e2\u003c/sub\u003e concentrations. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e, the resulting films exhibited a complete separation bilayer of the top and bottom surfaces due to the rapid layer separation of TiO\u003csub\u003e2\u003c/sub\u003e particles. This behavior is consistent with the poor dispersion stability observed in the long-term storage test (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e), confirming that TiO\u003csub\u003e2\u003c/sub\u003e suffers from both temporal and process-related issues. In contrast, the synthesized ZnS maintained excellent colloidal stability even at high concentrations, ensuring uniform film morphology during actual fabrication. These findings reinforce the practical advantages of the alkyl-functionalized ZnS system over conventional light scattering materials.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab4\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 4\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eSummary of ink formulations and physical properties\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"5\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eHPK\u003c/p\u003e \u003cp\u003e(mg/mL)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eTriton X-45\u003c/p\u003e \u003cp\u003e(v/v %)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eBED\u003c/p\u003e \u003cp\u003e(mg/mL)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003eViscosity\u003c/p\u003e \u003cp\u003e(cP)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eInk #1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\" morerows=\"3\" rowspan=\"4\"\u003e \u003cp\u003e33\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\" morerows=\"3\" rowspan=\"4\"\u003e \u003cp\u003e1.6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e7.46\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eInk #2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e100\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e15.0\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eInk #3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e200\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e20.0\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eInk #4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e300\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e66.8\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eWhile the earlier experiments investigated the effects of the ZnS-induced light scattering properties in enhancing the optical performance of QD CCLs, the subsequent studies focused on evaluating the ink printability of the formulated ink. Ensuring successful inkjet printing requires careful control of the ink\u0026rsquo;s rheological properties, particularly viscosity and surface tension, which directly affect droplet formation, deposition accuracy, and film quality. Low-viscosity inks tend to spread excessively upon substrate contact, often overflowing the pixel boundary and resulting in non-uniform film formation [42]. To control ink viscosity, we added bisphenol A epoxy diacrylate (BED) as a viscosity modifier. As shown in Table\u0026nbsp;\u003cspan refid=\"Tab4\" class=\"InternalRef\"\u003e4\u003c/span\u003e, the initial viscosity of Ink #1 was 7.46 cP, which increased to 15.0 cP with 100 mg/mL addition of BED. Further increases in BED concentration enabled the formulation of high-viscosity inks, reaching 66.8 cP at 300 mg/mL (Ink #4).\u003c/p\u003e \u003cp\u003eThe droplet formation of Ink #1 and #2 is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e. While Ink #1 exhibited an elongated tail extending from the main droplet, Ink #2 achieved stable single-droplet ejection without any tailing or satellite formation. This accomplished by increasing the viscosity of the Ink #2 through the addition of a viscosity modifier and optimizing the jetting waveform to match the adjusted fluid properties. At even higher viscosity (Ink #3 and #4), it was difficult to make droplet ejections under normal conditions. However, as shown in Figure S6, droplet ejection from high-viscosity inks could be conducted by controlling the printing head temperature. Although further optimization is needed to improve surface tension and ejection properties for high viscosity Ink #3 and #4, Ink #2 was successfully printed without tail or satellite, demonstrating adequate pattern formation for formulated inks. These results highlight the feasibility of using UV-curable inks in inkjet printing processes for display applications. Future efforts will focus on suppressing the formation of satellite droplets, which are critical for achieving high-resolution, defect-free pattering.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab5\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 5\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eCalculated surface energy of the substrate from the measured contact angles\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"4\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003eW/O CPT\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eCA of DI water\u003c/p\u003e \u003cp\u003e(\u0026ordm;)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eCA of Diiodomethane\u003c/p\u003e \u003cp\u003e(\u0026ordm;)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eSurface energy\u003c/p\u003e \u003cp\u003e(mN/m)\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003e61.2\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003e32.0\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003e54.4\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eW/CPT 30 sec\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e85.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e74.6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e27.0\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eW/CPT 1 min\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e88.3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e73.7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e26.2\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eW/CPT 2 min\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e88.9\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e73.0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e26.2\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eW/CPT 5 min\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e89.0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e67.8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e28.2\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eAs shown in the inset image of Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e(b), when a 3\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\times\\:\\)\u003c/span\u003e\u003c/span\u003e3 array of dots was jetted using Ink #2, the diameter of the dot was 150 \u0026micro;m, and when a pixel-confined substrate with a width of 160 \u0026micro;m was used as a test bed to attempt to form a film inside the pixel, overflow occurred regardless of changes in dot pitch. Wettability is not only a crucial factor in determining the desired position and shape of printed droplets but also plays a key role in forming micro-thickness optimized QD films for CCLs using inkjet printing technology [43]. To control the surface energy of the substrate, CF\u003csub\u003e4\u003c/sub\u003e plasma treatment (CPT) was used to optimize the surface wettability. Figure\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e(a) shows the contact angle of diluted (DI) water, diiodomethane, and Ink #2, respectively. The contact angle (CA) of DI water and diiodomethane increased from 61.2\u0026ordm; to 88.9\u0026ordm; and from 32.0\u0026ordm; to 73.0\u0026ordm; after 2 min surface treatment. However, as shown in the results after 5 min of CPT, the CA of diiodomethane showed a slight decrease. Nevertheless, for Ink #2, the CA increased as the CPT time increased, even though the non-treated substrate was too spread out to even measure. As a result, it formed a CA of 52.2\u0026ordm; and 54.0\u0026ordm; after 2 and 5 min of CPT, respectively. Based on the measured CA, the calculated surface energy value of the substrate is shown in Table\u0026nbsp;\u003cspan refid=\"Tab5\" class=\"InternalRef\"\u003e5\u003c/span\u003e. As shown in Table\u0026nbsp;\u003cspan refid=\"Tab5\" class=\"InternalRef\"\u003e5\u003c/span\u003e, the surface energy of the substrate decreased as the CPT time increased and showed a slight increase after 5 min of treatment. For some reason, surface energy increased again after 2 min of treatment. Based on this, subsequent inkjet printing tests were conducted after 2 min of CPT. As shown in the inset image in Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e(c), when the same Ink #2 was used to form dots on the substrate after 2 mins CPT, the dot size was 65 \u0026micro;m, indicating a significant reduction in diameter after surface treatment. The surface treatment was applied to enhance ink confinement within the pixels, enabling the formation of CCLs.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eTo verify higher-resolution patterning capability, we fabricated using 80\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\times\\:\\)\u003c/span\u003e\u003c/span\u003e240 \u0026micro;m pattern (168 ppi, Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e(a) and (b)) and 44\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\times\\:\\)\u003c/span\u003e\u003c/span\u003e212 \u0026micro;m pattern (231 ppi, Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e(c) and (d)), as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e. By using the same composition of ink and adjusting the number of dots, a pattern with a thickness of approximately 9 \u0026micro;m was formed. Additionally, we attempted to form a higher-resolution pattern with a 453 ppi pixel-confined cell, corresponding to a cell width of 33 \u0026micro;m and a height of 119 \u0026micro;m, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003e. While the ink successfully filled in the patterned regions, minor overflow between adjacent pixels was observed due to the narrow pixel pitch. Although the current formulation and printing conditions were not fully optimized for 453 ppi patterning, this experiment demonstrates the potential for high-resolution inkjet-printed CCLs. Further optimization of ink viscosity, pattern size tuning, and droplet spacing control will be required to achieve precise patterning at this resolution. These experiments demonstrate the feasibility of ZnS-incorporated QD CCLs and their compatibility with inkjet printing technology. In the future, we will focus on refining the ink formulation and jetting parameters to enable ultra-high-resolution patterning, along with droplet behavior control through surface energy engineering. These efforts will contribute to the reliable fabrication of inkjet-printed color conversion layers, facilitating their integration into next-generation display architectures.\u003c/p\u003e"},{"header":"4. Conclusion","content":"\u003cp\u003eWe investigated the optical properties, inkjet printability, and high-resolution patterning capabilities of ZnS-incorporated QD CCLs. A systemic evaluation revealed that ZnS nanoparticles enhance light scattering while effectively reducing blue light leakage. Notably, the ZnS nanoparticles synthesized in this work exhibited high dispersion compatibility with the UV-curable QD ink system, enabling stable inkjet processing without requiring complex formulations or additional circulation hardware system. This processing simplicity can reduce operational burden and supports a more cost-effective manufacturing route for inkjet-printed CCLs. To further study the feasibility of inkjet printing for CCL fabrication, we optimized the ink formulation by adjusting viscosity using BED as a viscosity modifier. This modification enabled droplet ejection and improved film uniformity. However, pristine substrates exhibited ink spreading across the pixels. To address this, CPT was considered to modify the surface energy, which enhanced wettability and confirmed ink droplets within defined pixel boundaries. Following CPT, we successfully fabricated high-resolution patterns at 168 ppi (80\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\times\\:\\)\u003c/span\u003e\u003c/span\u003e240 \u0026micro;m) and 231 (44\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\times\\:\\)\u003c/span\u003e\u003c/span\u003e212 \u0026micro;m) with a uniform thickness of approximately 9 \u0026micro;m. Additionally, a higher resolution 453 ppi pattern was attempted, demonstrating the potential of inkjet printing for ultra-high-resolution patterning. While minor overflow was observed at 453 ppi, further optimization of ink rheology, surface energy control, and print parameters will be necessary to suppress lateral spreading between adjacent pixels. Compared with conventional scattering additive approaches that often improve conversion efficiency at the expense of residual blue leakage and/or increased formulation complexity, the ZnS-integrated CCLs demonstrated a favorable balance between enhanced scattering and pronounced suppression of blue leakage. Overall, these findings highlight the potential of QD CCLs as a practical material for high-resolution inkjet-printed displays. The result provides valuable insights into material selection, ink formulation, and surface engineering to enhance printing resolution and uniformity.\u003c/p\u003e"},{"header":"Declarations","content":"\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eY.-J. Choi conceived the study, conducted the main experiments, analyzed the data and prepared the figures, and wrote the manuscript. M. Jeong and M.S. Lee assisted with nanoparticle synthesis, sample preparation, and related experiments. J.-Y. Yoo and B.D. Chin supervised the study, contributed to interpretation of the results and revised the manuscript. All authors reviewed and approved the final manuscript.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eY.-H. Won, O. Cho, T. Kim, D.-Y. Chung, T. Kim, H. Chung, H. Jang, J. Lee, D. Kim, and E. Jang, Highly efficient and stable InP/ZnSe/ZnS quantum dots light-emitting diodes, Nature \u003cstrong\u003e575\u003c/strong\u003e, 634-638 (2019). doi: 10.1038/s41586-019-1771-5\u003c/li\u003e\n\u003cli\u003eG. Ba, Q. Xu, X. Li, Q. Lin, H. Shen, and Z. Du, Quantum dot light-emitting diodes with high efficiency at high brightness via shell engineering, Opt. Express \u003cstrong\u003e29\u003c/strong\u003e, 12169-12178 (2021). doi: 10.1364/OE.421029\u003c/li\u003e\n\u003cli\u003eW. H. Jung, J.-Y. Yoo, H. J. Kim, J.-G. Kim, B.D. Chin, and J. S. 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Interfaces. \u003cstrong\u003e10\u003c/strong\u003e, 2201851-2201858 (2023). doi: 10.1002/admi.202201851\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"journal-of-information-display","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"","sideBox":"Learn more about [Journal of Information Display](https://link.springer.com/journal/44469)","snPcode":"44469","submissionUrl":"https://submission.springernature.com/new-submission/44469/3?","title":"Journal of Information Display","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Springer Open","inReviewEnabled":true,"inReviewRevisionsEnabled":true},"keywords":"","lastPublishedDoi":"10.21203/rs.3.rs-9229334/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-9229334/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eAchieving high color conversion efficiency by optimizing the optical properties of quantum dots (QDs) color conversion layers (CCLs) remains a challenge for next-generation display fabrication. CCLs, which convert blue light into red or green emission via photoluminescence, are key components in QD-based displays. However, their optical efficiency and inkjet printability remain limited due to poor compatibility between QDs and conventional light-scattering materials such as titanium oxide (TiO\u003csub\u003e2\u003c/sub\u003e).\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;In this study, alkyl-functionalized ZnS nanoparticles were synthesized and incorporated into a QD ink formulation to enhance conversion efficiency and reduce blue leakage. The functionalized ZnS exhibited improved dispersibility in polymeric resins, ensuring uniform mixing. To enable precise inkjet-printed patterning, ink viscosity was optimized using a viscosity modifier, and substrate wettability was controlled via CF\u003csub\u003e4\u003c/sub\u003e plasma treatment to confine ink droplets within defined areas. The results confirm that ZnS-enhanced QD CCLs improve color conversion efficiency (up to 78.1%) and reduce blue leakage below 1%, leading to superior optical performance. This study highlights the synergistic role of light-scattering nanoparticles and inkjet printability in enabling scalable, high-resolution inkjet-printed QD-CCLs, offering a promising approach for next-generation display technologies.\u003c/p\u003e","manuscriptTitle":"Alkyl-Functionalized ZnS Nanoparticles for Optical Management in Quantum Dot Color Conversion Layers with Inkjet Printing Compatibility","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2026-04-09 11:55:39","doi":"10.21203/rs.3.rs-9229334/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2026-04-28T06:17:29+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-04-28T05:37:14+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"103846588928135312028603195573530200413","date":"2026-04-25T06:07:48+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-04-23T23:34:49+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"176115340762818611295189621253851612944","date":"2026-04-21T05:06:11+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2026-04-03T08:56:49+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2026-03-28T10:12:45+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2026-03-28T10:11:58+00:00","index":"","fulltext":""},{"type":"submitted","content":"Journal of Information Display","date":"2026-03-26T05:04:45+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"journal-of-information-display","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"","sideBox":"Learn more about [Journal of Information Display](https://link.springer.com/journal/44469)","snPcode":"44469","submissionUrl":"https://submission.springernature.com/new-submission/44469/3?","title":"Journal of Information Display","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Springer Open","inReviewEnabled":true,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"6f79f73e-19f8-4f39-a649-b59e5f368472","owner":[],"postedDate":"April 9th, 2026","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"in-revision","subjectAreas":[],"tags":[],"updatedAt":"2026-04-28T06:25:41+00:00","versionOfRecord":[],"versionCreatedAt":"2026-04-09 11:55:39","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-9229334","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-9229334","identity":"rs-9229334","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}