Unveiling the luminescence property of Li2MgGeO4:Mn4+ featuring the tetrahedral crystallographic-site occupancy of Mn4

Owing to the occupying tendency of Mn4+ at octahedral sites, doping Mn4+ activators in tetrahedral structures poses challenges and hence is seldom reported. In this work, tetrahedrally sited Mn4+ phosphors were studied. By combining X-ray diffraction (XRD) data with Rietveld refinement analysis, the location of Mn4+ was determined. It was found that by adding excessive raw MgO, the phosphor synthesis temperature can be improved, enhancing the crystallinity of the crystal and thus improving the emission performance of the phosphor. In addition, excessive raw MgO forms a second phase in an LMGO matrix, which does not change the doping site for Mn4+. The Tanabe-Sugano diagram of Mn4+ in the tetrahedral field and the energy-level diagram of these phosphors were constructed for the first time, and the excitation and emission mechanisms are discussed in detail. With 1.2-fold excess of raw MgO, the prepared sample (LMGO-Mn-1.2) shows the best luminescence, demonstrating red emissions peaked at 656 nm and affording an emission intensity enhancement of over 50 times compared to a stoichiometric LMGO:Mn4+ system. At 150 °C, LMGO-Mn-1.2 keeps 90% emission intensity compared to that at room temperature. Finally, a high-efficiency warm white light-emitting diode was built. This work provides new insights into the study of Mn4+-activated phosphors in a tetrahedron crystal field.

Improvement in luminescent properties and thermo-optical conversion mechanism of Na2SiF6:Mn4+,K+@GQDs

Herein, a series of NSF:0.05Mn⁴⁺,0.04K⁺@GQD (NSF: Na₂SiF₆, GQDs: Cl-containing graphene quantum dot) phosphors was prepared. Double enhancement effects on the luminescent intensity and thermal stability triggered by the GQD coating were observed for the optimal sample as follows: (a) its PL intensity was 1.72 times that of the uncoated control sample and (b) its luminescent thermal stability was greatly enhanced, with integrated PL intensities of 120, 150 and 180 °C to 179.7%, 175.8%, and 119.3% of the initial value at 25 °C, respectively. It is proposed that the above-mentioned behaviors involve a change in some of the thermal energy into light energy via a phonon-induced mechanism. The thermal stability analysis results showed that the optimal sample is suitable for application in high-power WLEDs. Specifically, warm white light with a low correlated color temperature, high luminescent efficiency and high color rendering index was obtained from the prototype WLEDs using the optimal sample as a red-emitting component.

Improved Moisture-Resistant and Luminescence Properties of a Red Phosphor Based on Dodec-fluoride K3RbGe2F12:Mn4+ through Surface Modification

Mn⁴⁺-activated red-emitting fluoride phosphors are essential for white light-emitting diodes (WLEDs) with desirable color rendition index (CRI) because of their unique and efficient luminescence characteristics. Herein, we synthesized a novel Mn⁴⁺-activated dodec-fluoride phosphor K₃RbGe₂F₁₂:Mn⁴⁺ (KRGF:Mn) through a facile ionic exchange method at room temperature. A surface-modified strategy using weak reducing agents such as oxalic acid and citric acid is proposed to improve the moisture-resistance ability of KRGF:Mn phosphor dramatically, and the possible mechanism of surface modification has been investigated. A shell formed on the surface of the KRGF:Mn phosphor reduces the concentration of Mn⁴⁺ on the surface, which can prevent the internal KRGF:Mn group hydrolysis by the external moisture and effectively decreased the probability of energy migration to surface defects, thereby increasing both the emission efficiency and the moisture-resistance ability of KRGF:Mn. More interestingly, the KRGF:Mn phosphor is quenched after soaking in water for 72 h but recovered to the initial brightness after soaking in the modifier solutions for 2 min. This work fabricates a new efficient red phosphor KRGF:Mn for application in warm WLEDs and provides insight into the mechanism of the strategy to improve the moisture resistance of the stability of Mn⁴⁺ through surface modification.

Deep red fluoride dots-in-nanoparticles for high color quality micro white light-emitting diodes

In this study, a novel nanostructure of fluoride red emitting phosphor is synthesized via soft templates. K₂SiF₆:Mn⁴⁺ nanocrystals in the range of 3-5 nm diameter are found inside the porous K₂SiF₆:Mn⁴⁺ nanoparticle hosts, forming unique dots-in-nanoparticles (d-NPs) structures with controlled optical properties. The porous K₂SiF₆:Mn⁴⁺ d-NPs exhibit a sharp and deep red emission with an excellent quantum yield of ∼95.9%, and ultra-high color purity with the corresponding x and y in the CIE chromaticity coordinates are 0.7102 and 0.2870, respectively. Moreover, this nanophosphor possesses good thermal stability in range of 300 K-500 K, under light excitation of 455 nm. The K₂SiF₆:Mn⁴⁺ d-NPs are covered onto a surface of 100×100 µm² blue-yellow In_xGa_1-xN nanowire light-emitting diode (LED) to make warm white LEDs (WLEDs). The fabricated WLEDs present an excellent color rendering index of ∼95.4 and a low correlated color temperature of ∼3649 K. Porous K₂SiF₆:Mn⁴⁺ d-NPs are suggested as a potential red component for high color quality micro WLED applications.

Mn4+-activated Li3Mg2SbO6 as an ultrabright fluoride-free red-emitting phosphor for warm white light-emitting diodes

In this paper, we report on highly efficient Mn⁴⁺-activated double perovskite Li₃Mg₂SbO₆ (LMS) red-emitting phosphors. These LMS:Mn⁴⁺ phosphors can be efficiently excited over a broad wavelength band from 235 nm to 600 nm peaking at 344 nm and 469 nm, and exhibited an intense red emission band with a range from 600 nm to 800 nm centered around 651 nm. The optimal Mn⁴⁺ doping concentration of LMS:Mn⁴⁺ was 0.6 mol% and its internal quantum efficiency can reach as high as 83%. Besides, the thermal quenching effect on the optical property was also analyzed. Finally, a warm white light-emitting diode (WLED) lamp was fabricated by using a 454 nm InGaN blue LED chip combined with a blend of YAG:Ce³⁺ yellow phosphors and the as-prepared LMS:0.6% Mn⁴⁺ red phosphors, which showed bright white light with CIE chromaticity coordinates (0.4093, 0.3725), correlated color temperature (CCT = 3254 K), color rendering index (CRI = 81) and luminous efficacy (LE = 87 lm/W).

Mn⁴⁺-activated Li₃Mg₂SbO₆ red-emitting phosphor with internal quantum efficiency as high as 83% was developed for blue-pumped warm-white light-emitting diodes.

Red-emitting K3HF2WO2F4:Mn4+ for application in warm-white phosphor-converted LEDs – optical properties and magnetic resonance characterization

A novel efficient red-emitting Mn⁴⁺ phosphor of composition K₃HF₂MO₂F₄:Mn⁴⁺ (M = Mo, W).

Optimizing and adjusting the photoluminescence of Mn4+-doped fluoride phosphors via forming composite particles

Nowadays, Mn⁴⁺-doped red phosphors are widely used in white LEDs to improve the color rendering index and decrease the correlated color temperature.

A novel Cs2NbOF5:Mn4+ oxyfluoride red phosphor for light-emitting diode devices

The addition of a red-emitting phosphor to YAG:Ce3+-based white light-emitting diodes (WLEDs) greatly facilitates their applications in the field of high-color-rendering-index warm solid-state lighting. It is highly desirable to develop a red phosphor with satisfactory spectral features and low synthesis cost. In this study, a novel non-rare-earth and nonequivalent doping type of Cs2NbOF5:Mn4+ oxyfluoride red-emitting phosphor with high luminous efficiency was obtained via a facile room-temperature co-precipitation method, and its morphology and luminescent properties were investigated in detail. The Cs2NbOF5:Mn4+ phosphor with micro-rod-like morphology exhibited broad band absorption at blue light region (∼474 nm) and narrow bandwidth emissions at red region (∼633 nm). The color purity of the Cs2NbOF5:Mn4+ phosphor was calculated to be about 99%, and the internal quantum yield (QY) under 474 nm excitation was 63.4%. The concentration quenching of Mn4+ in Cs2NbOF5 matrix was mainly due to dipole-dipole interactions, and the activation energy of temperature quenching was calculated to be ∼0.2610 eV. The demonstration of a blue InGaN LED chip in combination with a blend of newly developed Cs2NbOF5:Mn4+ red phosphor and YAG:Ce3+ yellow phosphor greatly decreased the correlated color temperature (CCT) from 6255 to 3517 K while significantly improving the color rendering index (CRI) from 72.5 to 87.5. It deserves to be mentioned that the brand-new matrix to phosphor in the present study can be extended to various niobium/tantalum oxyfluoride series, which is very helpful for developing and designing new red phosphors.

Highly Regular, Uniform K3ScF6:Mn4+ Phosphors: Facile Synthesis, Microstructures, Photoluminescence Properties, and Application in Light-Emitting Diode Devices

A new generation of red phosphors of complex fluoride matrices activated with Mn⁴⁺ has gained a broad interest in getting high color quality and low color temperature of solid-state white light-emitting diodes (WLEDs). However, besides their instability toward moisture, the extremely irregular and nonuniform morphologies of these phosphors have limited their practical industry applications. In the present study, a novel type of K₃ScF₆:Mn⁴⁺ red phosphor with highly regular, uniform, and high color purity was obtained successfully through a facile coprecipitation route under mild conditions. The crystal structure was identified with aids of the powder X-ray diffraction, Rietveld refinement, and density functional theory calculations. The prototype crystallizes in the space group Fm3 m with a cubic structure, and the lattice parameters are fitted well to be a = b = c = 8.4859(8) Å and V = 611.074(2) ų. The Mn⁴⁺ ions occupy Sc³⁺ sites and locate at the centers of the distorted ScF₆ octahedrons. A wide band gap of approximately 6.15 eV can provide sufficient space to accommodate impurity energy levels. Unlike most other Mn⁴⁺ ion-activated fluoride phosphors, the as-prepared K₃ScF₆:Mn⁴⁺ phosphors demonstrate highly uniform and regular morphologies with shapes transforming from cube to octahedron with increasing Mn⁴⁺ ion concentration. Under blue light excitation, the as-prepared K₃ScF₆:Mn⁴⁺ sample exhibits intense sharp-line red fluorescence (the strongest peak located at 631 nm) with high color purity. An excellent recovery in luminescence upon heating and cooling processes implies high stability of K₃ScF₆:Mn⁴⁺. Furthermore, a warm WLED fabricated with blue GaN chips merged with the mixture of K₃ScF₆:Mn⁴⁺ and the well-known commercial YAG:Ce³⁺ yellow phosphors exhibits wonderful color quality with lower correlated color temperature (3250 K) and higher color-rendering index ( R_a = 86.4). These results suggest that the K₃ScF₆:Mn⁴⁺ phosphor possesses stupendous potentiality for practical applications.

Highly Stable K2SiF6:Mn4+@K2SiF6 Composite Phosphor with Narrow Red Emission for White LEDs

Poor water resistance and nongreen synthesis remain great challenges for commercial narrow red-emitting phosphor A₂MF₆:Mn⁴⁺ (A = alkali metal ion; M = Si, Ge, Ti) for solid-state lighting and display. We develop here a simple and green growth route to synthesize homogeneous red-emitting composite phosphor K₂SiF₆:Mn⁴⁺@K₂SiF₆ (KSFM@KSF) with excellent water resistance and high efficiency without the usage of toxic and volatile hydrogen fluoride solution. After immersing into water for 6 h, the as-obtained water-resistant products maintain 76% of the original emission intensity, whereas the emission intensity of non-water-resistant ones steeply drops down to 11%. A remarkable result is that after having kept at 85% humidity and at 85 °C for 504 h (21 days), the emission intensity of the as-obtained water-resistant products is at 80-90%, from its initial value, which is 2-3 times higher than 30-40% for the non-water-resistant products. The surface deactivation-enabled growth mechanism for these phosphors was proposed and investigated in detail. We found that nontoxic H₃PO₄/H₂O₂ aqueous solution promotes the releasing and decomposition of the surface [MnF₆]^2- ions and the transformation of the KSFM surface to KSF, which finally contributes to the homogeneous KSFM@KSF composite structure. This composite structure strategy was also successfully used to treat KSFM phosphor prepared by other methods. We believe that the results obtained in the present paper will open the pathway for the large-scale environmentally friendly synthesis of the excellent antimoisture narrow red-emitting A₂MF₆:Mn⁴⁺ phosphor to be used for white light-emitting diode applications.

Synthesis and improved photoluminescence of a novel red phosphor LiSrGaF6:Mn4+for applications in warm WLEDs

A novel red-emitting phosphor LiSrGaF₆:Mn⁴⁺was synthesized by a facile cation exchange method. The maximum luminescence intensity was obtained by using Sr(OH)₂as the strontium source and with 0.17 wt% NH·H₂O ammonia in the reaction system.

Room-temperature synthesis, controllable morphology and optical characteristics of narrow-band red phosphor K2LiGaF6:Mn4+

The white light-emitting diodes fabricated by mixing GaN chips with yellow phosphor (Y₃Al₅O₁₂:Ce³⁺) and red phosphors (K₂LiGaF₆:Mn⁴⁺) exhibit good performance with lower color temperature and higher color rendering index.

A novel red phosphor of seven-coordinated Mn4+ ion-doped tridecafluorodizirconate Na5Zr2F13 for warm WLEDs

A novel red phosphor, Na₅Zr₂F₁₃:Mn⁴⁺, exhibits a strong zero phonon line at 616 nm, which is attributed to the low symmetry of Mn⁴⁺ ions occupied in a seven-coordinated environment. A warm white light has been produced by a WLED fabricated with the red phosphor Na₅Zr₂F₁₃:Mn⁴⁺.

Synthesis and improved photoluminescence of hexagonal crystals of Li2ZrF6:Mn4+ for warm WLED application

A novel red phosphor Li₂ZrF₆:Mn⁴⁺ being promising for application in WLEDs has been synthesized for the first time by the ion-exchange method at room temperature, and its luminescence intensity has been optimized.

Hydrothermal synthesis of narrow-band red emitting K2NaAlF6:Mn4+phosphor for warm-white LED applications

The truncated octahedron shaped K₂NaAlF₆:Mn⁴⁺was preparedviahydrothermal route, which emits red light under blue light radiation. Warm WLEDs fabricated by the obtained red phosphor show lower color temperature and higher color rendering index.

Synthesis and photoluminescence of Mn4+ activated ternary-alkaline fluoride K2NaGaF6 red phosphor for warm-white LED application

A novel ternary-alkaline red emitting fluoride phosphor K₂NaGaF₆:Mn⁴⁺ was successfully synthesized. The crystal structure, morphology, electronic band structure and luminescence properties of K₂NaGaF₆:Mn⁴⁺ phosphors were investigated in detail.

Abnormal site occupancy and high performance in warm WLEDs of a novel red phosphor, NaHF2:Mn4+, synthesized at room temperature

A novel red phosphor, NaHF₂:Mn⁴⁺, was obtainedviasubstituting Na⁺located at the center of the octahedron with Mn⁴⁺. This work provides a novel strategy to develop novel Mn⁴⁺doped red phosphors with controlled luminescence properties by substituting for alkali metal ions.