Reconstructing the Surface of K2SiF6:Mn4+ Phosphors toward Enhanced Moisture Resistance for White LED Applications.

The K2SiF6:Mn4+ (KSFM) phosphor featuring efficient ultranarrow red emissions is an outstanding candidate for white light-emitting diode (WLED) applications. However, poor moisture resistance seriously affects its application performance. In this study, a two-step surface reconstruction strategy is proposed to dramatically enhance the moisture resistance of commercially available KSFM phosphors, involving treatment with H2NbF7 and subsequent hydrothermal treatment. The modified KSFM phosphor exhibits a high internal quantum efficiency (IQE) of 98.9% after the two-step surface treatment. Meanwhile, nearly 100% of the initial emission intensity is retained for the modified KSFM phosphor even after aging in high temperature (85 °C) and high relative humidity (85% RH) environments for 6 days, in sharp contrast to only 18.6% retention for the original KSFM phosphor. The relative emission intensity of the modified KSFM remains at 98.9% even after being immersed in water for 6 h. Additionally, the phosphor-converted LED fabricated with the modified KSFM phosphor demonstrated excellent long-term stability, retaining up to 97.9% of initial luminous efficacy after aging under 85 °C and 85% RH conditions for 500 h. The moisture-resistance mechanism is elucidated on the basis of spectroscopic analysis as well as structural and compositional characterization of the phosphor surface layer, which can be attributed to the formation of a robust Mn4+-rare shell with high crystalline quality following this two-step surface treatment. The findings contribute to the performance improvements of KSFM phosphors for industrial applications.

All-Inorganic Green Synthesis of Small-Sized and Efficient K2SiF6:Mn4+ Phosphor for Mini-LED Displays.

High-resolution liquid crystal display (LCD) backlight requires a color conversion layer featuring micrometer light-emitting particles and a uniform morphology. The widely used commercial red-emitting K2SiF6:Mn4+ phosphor, showing promise as a light-conversion candidate, faces limitations due to its toxic synthesis process, large particle size, and poor moisture resistance. We successfully demonstrated an efficient substitution of the highly toxic HF/TEOS/KHF2 solvent system with a commonly used HCl/SiO2/KF solvent system to synthesize the small-sized K2SiF6:Mn4+ phosphor. Additionally, surface passivation was performed to enhance the luminescence intensity and resistance to moisture, denoted as K2SiF6:Mn4+@CaF2. Accordingly, the K2SiF6:Mn4+@CaF2 phosphor presents a high luminescence efficiency (99.87%/32.84% IQE/EQE) with an average particle size of ∼2.67 µm. Notably, after exposure to 85% humidity and 85 °C temperature for 3 h, the luminescence intensity remains at 47.4% for K2SiF6:Mn4+@CaF2, while 21.2% for pristine K2SiF6:Mn4+, and only 3.5% for K2SiF6:Mn4+ synthesized by TEOS. These advancements hold great potential for improving high-resolution LCD backlighting, particularly for displays with micron-level pixels, opening up new possibilities for enhanced display technology.

HF-Free Microwave-Assisted Synthesis of Nano-Micro-Sized K2 SiF6 :Mn4+ for MicroLED Color Conversion.

Micro-light-emitting diodes (MicroLED) are considered to be the next generation of ideal display devices, with chip size requirements of less than 50 µm. To meet its micron-scale pixel size, submicron luminescent materials are needed. Mn4+ doped fluoride phosphor, K2 SiF6 :Mn4+ (KSFM) as a red luminescent material with excellent narrow-band emission sensitivity to human eyes, has great potential as a color conversion material for full-color MicroLED. However, it is difficult to obtain small-size KSFM efficiently by conventional synthesis methods. Here, a simple HF-free strategy for the rapid batch synthesis of nano-micro-sized KSFM based on a microwave-assisted method is reported. The synthesized KSFM shows uniform morphology, average particle size is less than 0.2 µm, and has 89.3% internal quantum efficiency under 455 nm excitation. It exhibits excellent thermal stability (97.4%@423 K of the integrated emission intensity at 298 K) and prominent moisture resistance (81.9% of its initial relative emission intensity after immersing in water for 30 min). By employing it as a red emitter, the authors fabricate high-performance white LEDs with high luminous efficacy of 116.1 lm W-1 and wide color gamut of 130.4% NTSC. In addition, self-luminous red-emitting arrays with a pixel size of 20 × 40 µm are constructed by nanoimprinting as-synthesized KSFM.

Influence of Isostatic Pressure on the Elastic and Electronic Properties of K2SiF6:Mn4.

Isostatic pressure effects on the elastic and electronic properties of non-doped and Mn4+-doped K2SiF6 (KSF) have been investigated by first-principles calculations within density functional theory (DFT). Bulk modulus was obtained by the Murnaghan's equation of states (EOS) using the relationship between volume and pressures at pressures between 0 and 40 GPa, and elastic constants were calculated by the stress-strain relationship giving small distortions at each pressure point. The other elastic parameters such as shear modulus, sound velocity and Debye temperature, which can be obtained from the elastic constants, were also estimated. The influence of external isostatic pressure on the electronic properties, such as crystal field strength 10Dq and emission energy of 2E → 4A2 transition (Eem), of KSF:Mn4+ was also studied. The results suggest that 10Dq and Eem linearly increase and decrease, respectively, with increasing pressure.

A Reverse Strategy to Restore the Moisture-deteriorated Luminescence Properties and Improve the Humidity Resistance of Mn4+ -doped Fluoride Phosphors.

Fluoride phosphors as red components for warm white LEDs have attracted a tremendous amount of research attention. But these phosphors are extremely sensitive to moisture, which seriously limits their practical industrial applications. To tackle this problem, unlike all the straightforward preventive strategies, a reverse strategy "Good comes from bad" was successfully developed to treat the degraded K2 SiF6 : Mn4+ (D-KSFM) phosphor in the present study, which not only completely restores the luminescence properties, but also significantly enhances the moisture resistance at the same time. After treatment with an oxalic acid solution as restoration modifier, the emission intensity of the D-KSFM phosphor can be restored to 103.7% of the original K2 SiF6 : Mn4+ red phosphor (O-KSFM), and the moisture resistance is remarkably improved. The restored K2 SiF6 : Mn4+ (R-KSFM) maintains approximately 62.3% of its initial relative emission intensity after immersing in deionized water for 300 min, while the reference commercial K2 SiF6 : Mn4+ with a protective coating (C-KSFM) is only 33.2%. As a proof of general applicability, this strategy was also conducted to K2 TiF6 : Mn4+ phosphor, which is less moisture-stable than K2 SiF6 : Mn4+ . The luminescence intensity of the degraded K2 TiF6 : Mn4+ (D-KTFM) phosphor can be restored to 162.6% of original level of the K2 TiF6 : Mn4+ synthesized through a cation exchange approach without any treatment (O-KTFM). The emission intensity of the restored K2 TiF6 : Mn4+ (R-KTFM) phosphor retains 62.8% of its initial emission intensity after soaking in deionized water for 300 min. Finally, the R-KSFM phosphors were packaged into white light-emitting diodes with blue InGaN chips and Y3 Al5 O12 : Ce3+ yellow phosphors. The WLEDs display excellent color rendition with higher color rendering index, lower color temperature (WLED-II: Ra =83.6, R9 =57.3, 3743 K, ηl =199.68 lm/W; WLED-III: Ra =90.4, R9 =94.2, 2892 K, ηl =183.3 1 m/W). The above results show that the reverse strategy can be applied in those phosphor materials with poor moisture resistance to restore luminescence properties and improve moisture resistance without excessively care about the deterioration during the production, storage and transportation.

Hydrophobic Organic Skin as a Protective Shield for Moisture-Sensitive Phosphor-Based Optoelectronic Devices.

A moisture-stable, red-emitting fluoride phosphor with an organic hydrophobic skin is reported. A simple strategy was employed to form a metal-free, organic, passivating skin using oleic acid (OA) as a hydrophobic encapsulant via solvothermal treatment. Unlike other phosphor coatings that suffer from initial efficiency loss, the OA-passivated K2SiF6:Mn4+ (KSF-OA) phosphor exhibited the unique property of stable emission efficiency. Control of thickness and a highly transparent passivating layer helped to retain the emission efficiency of the material after encapsulation. A moisture-stable KSF-OA phosphor could be synthesized because of the exceptionally hydrophobic nature of OA and the formation of hydrogen bonds (F···H) resulting from the strong interactions between the fluorine in KSF and hydrogen in OA. The KSF-OA phosphor exhibited excellent moisture stability and maintained 85% of its emission intensity even after 450 h at high temperature (85 °C) and humidity (85%). As a proof-of-concept, this strategy was used for another moisture-sensitive SrSi2O2N2:Eu2+ phosphor which showed enhanced moisture stability, retaining 85% of emission intensity after 500 h under the same conditions. White light-emitting devices were fabricated using surface-passivated KSF and Y3Al5O12:Ce3+ which exhibited excellent color rendering index of 86, under blue LED excitation.

Nanocrystallization in Oxyfluoride Glasses Controlled by Amorphous Phase Separation.

Transparent bulk glass-ceramics containing ZnF2, K2SiF6, and KZnF3 nanocrystals are successfully obtained from xKF-xZnF2-(100 - 2x)SiO2 oxyfluoride glasses for the first time to the best of our knowledge. The glass transition temperatures of heat-treated samples increase with time and approach values that resemble the temperatures chosen for thermal treatment. During nucleation and crystal growth, the residual glass around the crystals is depleted in fluoride which as glass component usually leads to a decrease in viscosity. The crystallization behavior notably depends on the glass composition and changes within a small range from x = 20 to 22.5 mol %. The occurrence of liquid/liquid phase separation in dependence of the composition is responsible for the physicochemical changes. Two different microstructures of droplet and interpenetrating phase separation and their compositional evolution are observed by replica transmission electron microscopy technique in the multicomponent glassy system. This study suggests that the size and crystal phase of precipitated crystallites can be controlled by the initial phase separation.