Doped Fluoride Phosphors Research Articles

High-efficiency RbASiF6:Mn4+ (A = K, Cs) phosphors obtained by one-step green synthesis method

Red-emitting Mn4+-doped fluoride phosphors are recognized as promising materials for high-performance warm white light-emitting diodes (WLEDs). However, the extensive use of toxic hydrofluoric acid in traditional synthesis methods hinders their development and commercialization. Furthermore, the traditional solid-phase method often has cumbersome sintering steps, high temperature, long holding time, and certain safety hazards, so it is very necessary to develop a green synthesis method with simple steps and low energy consumption. In this study, a low-temperature solid-state method is proposed to synthesize RbASiF6:Mn4+ (A = K, Cs) phosphors by one step. Optimal emission intensities were observed at Mn4+ concentrations of 1.2 % in RbKSiF6 and 2.1 % in RbCsSiF6. These phosphors demonstrated high thermal stability. At 423 K, the photoluminescence intensities of RbKSiF6:1.2 %Mn4+ and RbCsSiF6:2.1 %Mn4+ reached 114 % and 87 % of their respective values at room temperature. Meanwhile, the chromaticity shifts were almost negligible, and high color purity was achieved for both phosphors. The electro-optical performance of WLED devices utilizing these phosphors, in combination with YAG:Ce3+ and GaN chips, showed high color rendering indices (CRI) and low correlated color temperatures (CCT) across varying currents. Specifically, under a driving current of 20 mA, the device incorporating RbKSiF6:1.2 %Mn4+ exhibited a CRI of 94 and a CCT of 3732 K, with a luminous efficacy of 61 lm/W. These findings highlight the potential of green-synthesized RASFM phosphors in advancing the application of warm WLEDs.

Crystal field optimization, luminescence enhancement and thermal stability of novel K2NaAl0.4Ga0.6F6:Mn4+ solid solution red phosphor for high-performance warm WLED

Mn4+-doped fluoride phosphors have important application value in the background light and WLED lighting fields. Therefore, the synthesis of efficient and stable Mn4+-doped red phosphor is of great application value. In this study, a series of red-emitting K2NaAl1-yGayF6:xMn4+ (y = 0, 0.2, 0.4, 0.6, 0.8 and 1) (x = 0.01, 0.03, 0.05, 0.07 and 0.09) solid solution phosphors are successfully synthesized by simple coprecipitation method. The synthesis strategy of simple crystal structure optimization is proposed to realize simultaneously the crystal field optimization, luminescence enhancement and thermal stability of red phosphor. The calculation and theoretical analysis of crystal field strength are used to prove that solid solution matrix K2NaAl0.4Ga0.6F6 is the most ideal matrix material. X-ray powder diffraction, energy-dispersive X-ray spectrometer and scanning electron microscope are employed to determine the crystal structure, composition and morphology of all samples. Optical properties are characterized using excitation spectra, emission spectra and fluorescence lifetime curves. Moreover, the CIE coordinates of red-light show that the phosphor has low correlated color temperature and excellent color purity. Intriguingly, the as-prepared K2NaAl0.4Ga0.6F6:Mn4+ phosphor possesses admirable thermal quenching behavior and color stability at high temperature. At a temperature of 423 K, the photoluminescence intensity can be maintained at 52 % of the original intensity (298 K). The activation energy, the chromaticity shift and chromaticity coordinate variation are also systematically calculated. More importantly, A high-performance warm WLED with low correlated color temperature (CCT = 3982 K) and high color rendering index (Ra = 93.7) is achieved by using K2NaAl0.4Ga0.6F6:Mn4+ as a red-emitting material. These results demonstrate that K2NaAl0.4Ga0.6F6:Mn4+ red phosphor provides a novel choice for the warm WLED required for indoor lighting.

In Situ Formation of Ultrathin Composite Shell with Strong Water Resistance on the Surface of K2SiF6:Mn4+ via NaClO Treatment

AbstractSolving the problem of poor weather resistance of Mn4+‐doped fluoride phosphors is pivotal for their application in white‐light‐emitting diodes (WLEDs). This study introduces a one‐step synthesis strategy for the phosphor T‐KSF:Mn (KSF:M = K2SiF6:Mn4+ and T = surface‐treated with NaClO) that involves the use of a NaClO solution as a treatment agent to augment the water resistance of this phosphor. The results of transmission electron microscopy, electron diffraction tomography, and X‐ray photoelectron spectroscopy, analyses reveal the formation of an ultrathin composite shell (<5 nm) comprising K2SiF6 and KNaSiF6 on the surface of T‐KSF:Mn. The fluorescence intensity of the treated phosphor is 1.09 times that of the untreated phosphor, and its internal quantum efficiency reaches 82.72%. Even after 60 days of testing in a harsh hydrolysis environment, the fluorescence intensity of T‐KSF:Mn remains at 86.24%. At 150 and 200 °C, the integrated fluorescence intensity of T‐KSF:Mn surge to 256.84% and 292.29% of its initial value at 30 °C, respectively, demonstrating a substantial negative thermal quenching (NTQ) effect. Furthermore, the results of aging tests conducted at 85 °C and 85% relative humidity substantiate the effectiveness of the aforementioned strategy in considerably enhancing WLED stability under practical conditions.

Highly effective red phosphor for warm white LEDs without rare earth elements

The use of red fluorescent powder derived from nitrides and oxides in warm white LEDs is a pressing concern due to its lower quantum efficiency. Fluoride red phosphors activated by Mn4+ ions show promising potential for application in white light-emitting diodes (LEDs) due to their distinctive narrowband red emission and broad blue excitation, along with the relatively mild synthesis conditions required. However, the quantum efficiency of early Mn4+-doped fluoride phosphors was not optimal, highlighting the importance of identifying a suitable preparation method and matrix. Here, we introduce an efficient and thermally stable red fluoride phosphor, Cs2SiF6:Mn4+, which demonstrates a remarkable quantum efficiency of 98.32 % achieved through ion exchange techniques. However, even at a temperature of 423 K, the luminous intensity still maintained at 95 % of that under room temperature conditions. By incorporating Cs2SiF6:Mn4+ as a red emitter, a stable 10 W high-power warm white LEDs with a luminous efficiency of approximately 126.1 lm/W can be realized. These findings underscore the significant potential of the red phosphor Cs2SiF6:Mn4+ in advancing high-performance, high-power warm white LEDs.

Striking antithermal quenching and waterproofness of Mn4+-doped fluoride phosphors induced by synergy of double coating and single passivation

Manganese (IV) (Mn4+)-doped fluoride phosphors generally suffer two pressing problems, thermal quenching (TQ) and poor water resistance. This study reports striking luminous intensity enhancement, anti-TQ property, and waterproofness of Mn4+-doped potassium fluosilicate phosphors induced by synergy of double coating and single passivation (coating with hydroxylated graphene quantum dots (GQDs), passivation with H2O2, and coating with K2SiF6). The luminous intensity of the phosphor is approximately 163 % of that of K2SiF6:Mn4+. The integrated emission intensity at 150 °C is 129.3 % higher than that at 30 °C. The emission intensity remains 93.2 % of the initial value after immersion in water for 480 min. These results are attributed to energy transfer from GQDs to Mn4+ and the formation of a stable Mn-free shell layer. In particular, the white light-emitting diodes produced using the phosphors maintained 99.6 % luminescence efficiency after 240 h of operation in high-temperature, high-humidity environments. We believe that this study provides a feasible approach to simultaneously enhance the thermal stability and waterproofness of Mn4+-doped fluoride phosphors.

Harnessing solid-state ion exchange for the environmentally benign synthesis of high-efficiency Mn4+-doped phosphors.

In this study, a solid-state ion exchange (SSIE) method is proposed to synthesize a series of Mn4+-doped fluoride phosphors, which avoids the use of HF solution during the Mn4+-doping process. The obtained Mn4+-doped fluoride phosphors exhibit strong red emission with a quantum yield of 46%.

Application of chelation effect in improving the triple properties of K2SiF6Mn4+ phosphors through composite modification

Mn4+-doped fluoride phosphors are limited by their poor moisture resistance in practical applications of white light-emitting diodes (WLEDs). Herein, a new red light-emitting phosphor KSF:MnK2SiF6:Mn4+ was synthesized based on a novel strategy combining surface treatment with hexamethylene tetramine (HMTA) and coating with a matrix, hereafter referred to as P-KSF:Mn. Given the formation of the coordination bond between N and Mn4+ and the strong hydrogen bond between N and F in the shell after the protonation of HMTA, a dense and firm shell of rare Mn4+ formed on the sample surface. P-KSF:Mn showed strong emission, waterproof property and luminescent thermal stability, and its fluorescence intensity was 1.81 times that of KSF:Mn. After soaking in water for 90 d, the fluorescence intensity of P-KSF:Mn was 90.7 % of the initial value. The integrated fluorescence intensity at 200 °C was approximately 416 % of the initial value at 25 °C. The strong luminous thermal stability was supported by strong negative thermal quenching (NTQ), which is explained by the phonon induction mechanism. The WLED prototype assembled with P-KSF:Mn emitted warm white light with correlated color temperature (CCT) = 3112 K, color rendering index (CRI) = 90.7, and luminous efficiency (LE) = 119 lm/W. The results show that P-KSF:Mn is a potential candidate for applications in the field of warm WLEDs.

Enhancing quantum efficiency of Mn4+-doped fluoride phosphors through reverse doping strategy toward potential biological detection

Mn4+-doped fluoride red-emitting materials are renowned for their distinct characteristics, including fixed emission peaks and narrow spectral emission, rendering them highly favored in photoluminescent light-emitting diodes (LEDs). However, achieving ultra-high-concentration Mn4+ doping in fluoride phosphors is imperative to enhance quantum efficiency. Nevertheless, attaining such doping levels in metastable compound systems poses considerable challenges. In this study, we introduce an innovative reverse doping strategy, leading to the synthesis of Mn4+-doped cubic K2SiF6 phosphor (11.38 mol %) from hexagonal K2MnF6. Remarkably, this approach significantly improves the external quantum efficiency (EQE) to 66.7 % upon 450 nm excitation. Through systematic comparisons with conventional co-precipitation and ion exchange methods, our findings unequivocally demonstrate the superior efficacy of this approach in achieving high-concentration Mn4+ ion doping within relatively insoluble matrices. The successful application of our optimized phosphor enabled the fabrication of a red light-emitting diode (LED) that exhibits an exceptional photoelectric efficiency of 31.67 % at 350 mA, surpassing the performance of a commercial red LED chip. Furthermore, the stable red-light emission from the phosphor-converted LED (pc-LED) proves instrumental in rapid-response vascular imaging, showcasing the potential of our findings in advancing light sources for biological detection applications. This research provides valuable insights into enhancing red emission efficiency in slightly-soluble fluoride phosphors via novel synthetic strategy, unlocking possibilities for the development of highly efficient light sources in biological detection and imaging applications.

Novel Mn4+-activated fluoride red phosphors induced by alkaline metal ions replacement: Green synthesis, luminescence enhancement, high stability and application in warm WLED

Red-emitting Mn4+-doped fluoride phosphors are promising fluorescent material for use in high-performance warm white light emitting diodes (WLEDs). However, low stability and non-green synthesis method limit the development and commercialization of such phosphor. Herein, a series of novel (Li/Na/Cs)0.4Ba0.8SiF6:Mn4+ phosphors with optimized fluorescence properties induced by alkaline metal ions replacement are successfully synthesized via a green and eco-friendly approach. Under the irradiation of 460 nm blue light, Cs0.4Ba0.8SiF6:Mn4+ red phosphor with narrow band emission peak has the best optical performance. The optimal doping concentration of Mn4+ in Cs0.4Ba0.8SiF6 host and the reason for concentration quenching are systematically analyzed. Furthermore, the moisture resistance and thermal stability of this phosphor are discussed. After being soaked in deionized water for 90 min, the PL intensity of Cs0.4Ba0.8SiF6:Mn4+ phosphor can maintain at 40% of that in air, which is 4 times higher than commercial K2SiF6:Mn4+. At the operating temperature of LED (423 K), Cs0.4Ba0.8SiF6:Mn4+ red phosphor can exhibit stable light output intensity and color. A warm WLED with low correlated color temperature of 4336 K, high color rendering index of 88.4 and high lumen efficiency of 105.96 lm/W is fabricated via using Cs0.4Ba0.8SiF6:Mn4+ phosphor as a component of red-light, and the warm WLED can maintain stable output even at high driving currents. Overall, the alkali metal ion replacement and green synthesis strategies have opened up a new perspective for the synthesis of efficient Mn4+-doped fluoride phosphors, which has dual benefits for optimizing the performance of WLED and environmental protection.

Synthesis of an efficient Mn4+-doped fluoride red phosphor with excellent fluorescence stability by green synthesis method for warm WLED

Currently, Mn4+-doped fluoride phosphors have become a research hotspot, which can help compensate for the missing red component in cold white light-emitting diodes (WLEDs). However, significant synthesis defects limit the development of such fluorescent powders due to the use of highly toxic solvent HF. To improve this situation, a simple green synthesis method should be explored to synthesize Mn4+-activated fluoride phosphor. In this study, a series of red-emitting Mn4+-activated BaTiF6 phosphors are successfully synthesized via the non-toxic reaction system (HNO3/HCl + NH4F). Its crystal structure, morphology, optical characteristics and thermal stability are systematically evaluated. Under blue light excitation, BaTiF6:Mn4+ exhibits a bright narrow-band red emission with exceptional correlated color temperature (3909 K) and color purity (90%). Additionally, the critical concentration of Mn4+ is determined to be 0.03 and the concentration quenching mechanism is also systematically discussed. By calculating the crystal field strength (Dq), Racah parameters (B and C) and nephelauxetic effect ratio parameter β1, it is confirmed that BaTiF6 host can provide a strong crystal field environment for Mn4+. Moreover, these results are compared with other matrix materials. Intriguingly, the optimal sample exists satisfactory thermal stability, whose fluorescence intensity at 423 K retains 49% of that at 298 K and the activation energy reaches up to 0.925 eV. In addition, the chromaticity shift and chromaticity coordinate variation are calculated as 17 × 10−3 and (−0.0063, 0.0063), respectively. Meanwhile, the feasibility of the as-fabricated WLEDs for interior illumination is examined. A high-performance warm WLED with low correlated color temperature (CCT = 4470 K) and high color rendering index (Ra = 88.3) is achieved by using BaTiF6:Mn4+ as a red-emitting material, moreover, WLED exhibits strong output stability under high driving current. The findings of this work provide a comprehensive understanding for rational design of high-performance Mn4+-activated fluoride red phosphors through a simple green synthesis method.

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 50µm. 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 0.2µm, and has 89.3% internal quantum efficiency under 455nm 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 30min). By employing it as a red emitter, the authorsfabricate high-performance white LEDs with high luminous efficacy of 116.1lmW-1 and wide color gamut of 130.4% NTSC. In addition, self-luminous red-emitting arrays with a pixel size of 20 ×40µm are constructed by nanoimprinting as-synthesized KSFM.

A novel thiourea loading strategy for improving the moisture resistance of K2SiF6: Mn4+ red phosphors

Recently, Mn4+ doped fluoride phosphors have been considered as ideal candidate materials employed in white light-emitting diodes (WLEDs). However, the extremely serious performance deterioration in a wet environment has affected their practical commercial applications. Herein, we developed a novel thiourea loading strategy for greatly improving the moisture resistance of typical K2SiF6: Mn4+ (KSF) red phosphors through treated by thiourea ethanol solution evaporation. The luminescent properties and moisture resistance have been discussed in detail, and the underlying moisture resistance mechanism was further proposed. The results show that the moisture resistance of KSF phosphor treated by thiourea significantly increased approximately sevenfold than that of untreated counterpart, exhibiting a potential application in the WLEDs backlight with high gamut display. These works brings up a promising alternative strategy for enhancing moisture resistance and provides a deeper understanding on the performance degradation mechanism for a variety of Mn4+ doped fluoride phosphors system.

Recent Research Progress of Mn4+-Doped A2MF6 (A = Li, Na, K, Cs, or Rb; M = Si, Ti, Ge, or Sn) Red Phosphors Based on a Core-Shell Structure.

White light emitting diodes (WLEDs) are widely used due to their advantages of high efficiency, low electricity consumption, long service life, quick response time, environmental protection, and so on. The addition of red phosphor is beneficial to further improve the quality of WLEDs. The search for novel red phosphors has focused mainly on Eu2+ ion- and Mn4+ ion-doped compounds. Both of them have emissions in the red region, absorption in blue region, and similar quantum yields. Eu2+-doped phosphors possess a rather broad-band emission with a tail in the deep red spectral range, where the sensitivity of the human eye is significantly reduced, resulting in a decrease in luminous efficacy of WLEDs. Mn4+ ions provide a narrow emission band ~670 nm in oxide hosts, which is still almost unrecognizable to the human eye. Mn4+-doped fluoride phosphors have become one of the research hotspots in recent years due to their excellent fluorescent properties, thermal stability, and low cost. They possess broad absorption in the blue region, and a series of narrow red emission bands at around 630 nm, which are suitable to serve as red emitting components of WLEDs. However, the problem of easy hydrolysis in humid environments limits their application. Recent studies have shown that constructing a core-shell structure can effectively improve the water resistance of Mn4+-doped fluorides. This paper outlines the research progress of Mn4+-doped fluoride A2MF6 (A = Li, Na, K, Cs, or Rb; M = Si, Ti, Ge or Sn), which has been based on the core-shell structure in recent years. From the viewpoint of the core-shell structure, this paper mainly emphasizes the shell layer classification, synthesis methods, luminescent mechanism, the effect on luminescent properties, and water resistance, and it also gives some applications in terms of WLEDs. Moreover, it proposes challenges and developments in the future.

Morphology controllable BaNbF5.5(OH)1.5:Mn4+ red phosphor with strong zero phonon line for WLED application

A hydroxy fluoride red phosphor BaNbF5.5(OH)1.5:Mn4+ with dual morphology was designed and manufactured by co-precipitation strategy. The crystal phase structure, controlled morphology and photoluminescence performances were systematically investigated. The Mn4+ non-equivalent doped hydroxy fluoride phosphor has similar luminescence performance to fluoride phosphors, and it is worth noting that it presents a very strong zero phonon line (ZPL) at ∼629 nm at blue light excitation and has high color purity (98.5%). Crystal field intensity (Dq), Racah parameters (B and C) and nephelauxetic effect ratio parameter β1 of the sample were calculated and analyzed, the results indicate that the BaNbF5.5(OH)1.5 matrix provides a strong crystal field environment for Mn4+. The effects of the experimental conditions on the morphology of the phosphors were analyzed, and optimal synthetic conditions were determined. Because of the short fluorescence decay lifetime (τ < 4 ms), this red phosphor may be a prospective red emitting candidate for backlight displays. Moreover, the as-prepared BaNbF5.5(OH)1.5:Mn4+ red phosphor possesses distinguished UV light-induced stability. Thermal stability and hydrophobic stability of the phosphor were analyzed. Thanks to the strong ZPL, WLED device prepared with this phosphor can produce warm white light and show remarkable photoelectric properties, with a color rendering index (CRI, Ra) of up to 92.5 and a correlated color temperature (CCT, Tc) as low as 3944 K. The results indicate that the red phosphor BaNbF5.5(OH)1.5:Mn4+ has application possibilities in WLED illumination and display backlighting.

Crystal field optimization and fluorescence enhancement of a Mn4+-doped fluoride red phosphor with excellent stability induced by double-site metal ion replacement for warm WLED.

The effective double-site metal ion replacement strategy was adopted to optimize the crystal field environment of a Mn4+-activated fluoride phosphor. In this study, a series of K2yBa1-ySi1-xGexF6:Mn4+ phosphors with optimized fluorescence intensity, excellent water resistance, and outstanding thermal stability was synthesized. The composition adjustment includes two different types of ion substitution based on the BaSiF6:Mn4+ red phosphor: [Ge4+ → Si4+] and [K+ → Ba2+]. X-ray diffraction and theoretical analysis revealed that Ge4+ and K+ could be successfully introduced into BaSiF6:Mn4+ to form new solid solution K2yBa1-ySi1-xGexF6:Mn4+ phosphors. The emission intensity enhancement and slight wavelength shift were detected in different cation replacement procedures. Furthermore, K0.6Ba0.7Si0.5Ge0.5F6:Mn4+ with superior color stability performance possessed a negative thermal quenching phenomenon. Excellent water resistance was also found, which was more reliable than K2SiF6:Mn4+ commercial phosphor. A warm WLED with low correlated color temperature (CCT = 4000 K) and high color rendering index (Ra = 90.6) was successfully packaged by using K0.6Ba0.7Si0.5Ge0.5F6:Mn4+ as the red light component, and it also exhibited high stability for different currents. These findings demonstrate that the effective double-site metal ion replacement strategy can open up a new avenue for designing new Mn4+-doped fluoride phosphors to improve the optical properties of WLEDs.

Enhanced luminescence of Mn4+ equivalently doped fluoride phosphors by reducing defects and impurities

The Mn4+-equivalently doped fluoride has red emission with high color purity. Here, the luminescence of the Mn4+-equivalently doped fluoride K2TiF6:Mn4+ (KTF:Mn4+) is significantly enhanced by the NH4+ doping strategy with a 2.27 times increase in luminescence intensity and a slight enhancement in decay time. The mechanism of luminescence enhancement is attributed to lattice distortion caused after the substitution of K+ by NH4+, which improves the stability of Mn4+ and reduce the number of defects and impurities. The prototype WLED with high CRI (Ra = 90.4), the high luminous efficiency of 112 lm/W and low CCT (2836 K) are obtained by using the optimal K2TiF6:Mn4+,NH4+ (KTF:Mn4+, NH4+) as the red light component. Some important luminous performances of the prototype WLEDs barely varied as the drive current was increased from 20 to 100 mA. These results demonstrate that the Mn4+/ NH4+ co-doped KTF is well suited for the fabrication of highly stable WLEDs.

Optimized red luminescence of Mn4+ in fluorine phosphors with hetero-central ions by structural modification

Due to their unique optical properties, the fluoride phosphors activated by Mn4+ ions with red emissive properties are leading components in “warm” white light-emitting diodes (WLEDs). At the same time, it is still difficult to optimize their photoluminescence (PL) performance through the compositional regulation strategy. This paper has explored two novel Mn4+ doped phosphors with CsNaGe0.5Ti0.5F6 (CNGTF) and CsRbGeF6 (CRGF) as host lattices obtained under mild conditions. Their morphology, PL spectral characteristic, and thermal stability have been systematically evaluated compared to the identified phosphors CsNaGeF6:Mn4+ (CNGF:Mn) and CsNaTiF6:Mn4+ (CNTF:Mn). The PL intensity of Mn4+ in CsNaGexTi1-xF6 coincides strongly with the molar ratios of Ge4+/Ti4+ and the maximum value is obtained at Ge4+/Ti4+ = 6/4. The WLED device fabricated with CsNaGe0.5Ti0.5F6:Mn4+ phosphor exhibits a “warm” white light with superior performance. This work provides a comprehensive understanding of designing a Mn4+-doped fluoride phosphor with high efficiency through a structural modulation strategy, which paves a promising pathway to explore their potential application in daily room lighting.

Treatment with Na2SO3 alkaline reductant to restore luminescence intensity and improve the moisture resistance of deteriorated Mn4+-doped fluoride phosphors

Treatment with Na2SO3 alkaline reductant to restore luminescence intensity and improve the moisture resistance of deteriorated Mn4+-doped fluoride phosphors

High Water Resistance and Luminescent Thermal Stability of LiyNa(2-y)SiF6: Mn4+ Red-Emitting Phosphor Induced by Codoping of Li.

Mn4+-doped fluoride phosphors are efficient narrowband red-emitting phosphors for white light-emitting diodes (WLEDs) and backlight displays. However, erosion by moisture is the main obstacle that limits their application. In this work, LNSF:Mn4+ (Li0.06Na1.94Si0.94Mn0.06F6) with high quantum yield (QY), luminescent thermal stability, and waterproofness was synthesized using the H2O2-free reaction method at room temperature. Compared to NSF:Mn4+(Na2Mn0.06Si0.94F6), the QY value, luminescence thermal stability, and water resistance of LNSF:Mn4+ are obviously improved by codoping of Li+ because of the formation of charge-carrier transfer (CT) and rare-Mn4+ layer induced by codoping of Li+. The former produces the negative thermal quenching (NTQ) effect, which results in the improvement of the luminescent thermal stability. The latter can inhibit the hydrolysis of Mn4+ on the surface of the sample, which leads to the enhancement of waterproofness. The formation mechanism of the rare-Mn4+ layer is discussed. A prototype WLED emitting the ideal warm white light (CCT = 3173 K, Ra = 90.4) was assembled by coating a mixture of LNSF:Mn4+, yellow emitting phosphor (YAG:Ce3+), and epoxy resin on the blue light InGaN chip, indicating that the performance of the WLED can be improved by using LNSF:Mn4+.

Improvement of the luminescent thermal stability and water resistance of K2SiF6:Mn4+ by surface passivation

Mn4+-doped fluoride phosphors can solve the problem for lack of red emitting component in commercial white light-emitting diodes (WLEDs). However, its application is seriously hindered by its easy hydrolysis. Here, we propose to use sodium sulfite as a passivator to treat K2SiF6:Mn4+. After passivation, a Mn4+-rare K2SiF6 protective layer can be formed in situ on the surface of the phosphor, and lead to improved emission intensity, luminescent thermal stability and moisture resistance. When soaking in water for 6 h, the integrated fluorescent intensity of the passivated sample maintained 90.8% of the initial value, while the intensity of the un-passivated sample sharply decreased to 10.2% of the initial value. Mechanisms to improve the emission, water resistance and thermal stability of the luminescence are proposed and discussed. WLED was assembled with the passivated sample, and good performance of warm white light (CCT = 2963 K, Ra = 90.4) was obtained.