Engineering cation vacancies to improve the luminescence properties of Ca14Al10Zn6O35: Mn4+ phosphors for LED plant lamp

In the recent years, Mn⁴⁺-doped phosphors for indoor plant cultivation have received extensive concern owing to the far-red emission that can match well with the absorption spectra of plant pigments. Whereas, many Mn⁴⁺-doped phosphors still face some challenges such as poor light efficiency and low thermal stability. It is an effective way to resolve these problems via cation vacancies engineering. Herein, the Ca_14−xAl₁₀Zn_6−yO₃₅: Mn⁴⁺ phosphors are successfully synthesized by combustion method. The luminescence intensity of Ca_14−xAl₁₀Zn_6−yO₃₅: Mn⁴⁺ phosphor is enhanced through engineering Ca²⁺ and Zn²⁺ vacancies according to the charge compensation mechanism. The optimal content of each Ca²⁺ and Zn²⁺ vacancy is equal to be 0.3. Furthermore, the defect formation is accompanied with lattice distortion, which plays a vital role in driving the excited phonon traps to reduce the energy loss by non-radiation transitions. Therefore, the thermal stability of Ca_14−xAl₁₀Zn_6−yO₃₅: Mn⁴⁺ phosphor is also improved via engineering cation vacancies. In addition, the Ca_14−xAl₁₀Zn_6−yO₃₅: Mn⁴⁺ phosphors can be effectively excited by blue light and it exhibits far-red emission due to the Mn⁴⁺ spin-forbidden ²E → ⁴A₂ transition. The results suggest that the Ca_14−xAl₁₀Zn_6−yO₃₅: Mn⁴⁺ phosphors can have a tremendous potential in indoor plant cultivation.     

Deep-red-emitting SrLaLiTeO6: Mn4+ double perovskites: Correlation between Mn4+-O2− bonding and photoluminescence

Mn⁴⁺-activated deep red-emitting SrLaLiTeO₆ phosphors are investigated for indoor plant growth LED applications for the first time. The phosphors crystallize in monoclinic (P2₁/n) symmetry is isostructural with SrLaLiTeO₆ host. B-site substitution of Mn⁴⁺ ions is confirmed from the redshift of high energy phonon modes in both Raman and IR spectra. The phosphor exhibited a far-red emission centered at 696 nm corresponding to the ²E_g → ⁴A_2g spin-forbidden transition of the Mn⁴⁺ ions. Approximate crystal field parameters depict the weak influence of neighboring ligand fields on Mn⁴⁺ ions and the least covalence of Mn⁴⁺-ligand bonding compared to other double perovskite phosphors. Moreover, the phosphors exhibit excellent thermal stability with an activation energy of 0.23 eV. Phosphor parameters including CCT, color purity, and quantum yield are evaluated and their values meet the requirements of a red-emitting phosphor for LED applications. Furthermore, the PL emission spectrum of SrLaLiTeO₆: Mn⁴⁺ matches with the absorption spectrum of plant phytochromes denoting the prospects of this phosphor for indoor plant growth LED applications.     

Novel deep-red-emitting double-perovskite type Ca2ScNbO6:Mn4+ phosphor: Structure,spectral study,and improvement by NaF flux

A novel deep-red-emitting phosphor Ca₂ScNbO₆:Mn⁴⁺ is prepared via a high-temperature solid-state reaction and its luminescent properties are systematically investigated. The results show that Mn⁴⁺-activated Ca₂ScNbO₆ phosphors have broad absorption in ultraviolet region, and show bright deep-red emission at 692 nm. The optimal doping concentration, crystal-field strength, internal quantum efficiency, and mechanism of concentration and thermal quenching effects are discussed in detail. Moreover, NaF flux is screened out to improve both luminescent intensity and morphology of the phosphor. Finally, a red light-emitting diode (LED) lamp is fabricated with as-prepared Ca₂ScNbO₆:Mn⁴⁺ phosphors and a 365 nm LED chip. The electroluminescence spectra show a good overlapping with phytochrome P_R and P_FR absorbance. The results provided the as-synthesized Ca₂ScNbO₆:Mn⁴⁺ phosphors a great potential in plant growth lighting.     

Accessing deep-red emission using chemical units cosubstituted LaTiSbO6:Mn4+ phosphor

Red light-emitting phosphors are important to the field of plant lighting. Therefore, it is necessary to further optimize phosphor materials. Herein, we synthesized a sequence of phosphor LaTiSbO₆:Mn⁴⁺ (LTS:Mn⁴⁺). Due to the ²E to ⁴A₂ transition, LTS:Mn⁴⁺ phosphors can emit red light in the range of 620–780 nm, with an emission peak at 687 nm. Chemical unit cosubstitution (substituting W⁶⁺ - Al³⁺/W⁶⁺ - Ga³⁺ for Ti⁴⁺ - Sb⁵⁺) was used as a method to enhance the luminescence properties of LTS:Mn⁴⁺. When the substitution ratio of W⁶⁺ - Ga³⁺ and W⁶⁺ - Al³⁺ reached 0.1% and 0.75%, respectively, the luminescence intensity increased to 204% and 182%. Using the LTS:Mn⁴⁺, W⁶⁺, Ga³⁺ phosphor and a 470 nm blue chip to fabricate a pc-LED device, the electroluminescence (EL) spectrum is well matched with the phytochrome absorption range. Therefore, the LTS:Mn⁴⁺ phosphor will be very promising for plant growth.     

Development of red-emitting Ba2LaSbO6:Mn4+ phosphors for latent fingerprint detection

Latent fingerprints provide crucial affirmations of identity in forensic science. However, they are microscopic. In this study, novel fluorescence materials, Ba₂LaSbO₆:Mn⁴⁺ (BLSO:Mn⁴⁺) phosphors, were developed by a sol–gel method for the fluorescence imaging of latent fingerprints. The structural properties of the phosphors were investigated by powder X-ray diffraction (XRD) and its Rietveld refinement analyses, and transmission electron microscopy and scanning electron microscopy techniques. The photoluminescence properties of the BLSO:Mn⁴⁺ phosphors were evaluated comprehensively by recording the emission, excitation, and decay curves. The BLSO:Mn⁴⁺ phosphors provide a high-intensity red emission at 677 nm under 350 nm excitation caused by the ²E_g→⁴A_2g transition of Mn⁴⁺. The optimum concentration of Mn⁴⁺ in the BLSO host was determined to be ~0.2 mol%. The calculated Commission International de L'Eclairage (CIE) chromaticity coordinates (0.716, 0.283) of the emission from the BLSO:Mn⁴⁺ phosphor are located in the pure red region of the CIE 1931 diagram. The red-emitting BLSO:0.2Mn⁴⁺ phosphor was used as a fluorescence imaging powder for visualizing latent fingerprints on various substrates with high resolution, high contrast, and high efficiency, as well as good selectivity.     

Enhanced luminescence properties of Li2MgTiO4: Mn4+, Ge4+ phosphor via single cation substitution for indoor plant cultivation

Red and far-red emitting phosphors have been widely used in phosphor-converted light emitting diode (pc-LED) devices to provide lighting for indoor plant growth, thus achieving desired product qualities. Among the many ways to optimize phosphors’ optical performance, cationic substitution is one of the most effective methods. In this study, red phosphors (Li₂MgTi_1-x-yO₄: xMn⁴⁺, yGe⁴⁺) were synthesized by high temperature solid state method and the optical performance of phosphors were improved with increasing Ge⁴⁺ constituents. In particular, luminescence intensity of Li₂MgTiO₄: 0.002Mn⁴⁺, 0.1Ge⁴⁺ increased by 152% under 468 nm excitation, and the thermostability of emission intensity increases from 22% (y = 0) to 43% (y = 0.1), which is about twice as much. Finally, pc-LED device was fabricated via the red phosphor Li₂MgTiO₄: 0.002Mn⁴⁺,0.1Ge⁴⁺ coated on a 470 nm ultraviolet chip. By changing the proportion of the phosphor, the electroluminescence spectra of pc-LED device could match well with the absorption regions of plant pigments. Therefore, Li₂MgTiO₄: 0.002Mn⁴⁺, 0.1Ge⁴⁺ phosphor has potential application in plant lighting. Furthermore, this work can offer some helpful references for improving luminescent efficiency by simply modulating the chemical composition.     

Design of a novel WLED structure based on the non-rare-earth Ca2Y(Nb,Sb)O6:Mn4+ materials

Herein, the novel white light-emitting diode (WLED) structures based on the Ca₂Y(Nb,Sb)O₆:Mn⁴⁺ materials were presented. The crystal structure, morphology, elemental composition, luminescent behavior, thermal stability, quantum yield, decay curve, and color purity of Ca₂YSbO₆:Mn⁴⁺ (CYS:Mn⁴⁺) and Ca₂YNbO₆:Mn⁴⁺ (CYN:Mn⁴⁺) phosphors were investigated and compared in detail. For these phosphors, the concentration quenching (CQ) mechanism is different. For CYS:Mn⁴⁺ phosphors, the dipole-dipole interaction dominates the CQ while the dipole-quadrupole interaction is dominant in CYN:Mn⁴⁺ phosphors. The emission intensity of the optimal CYN:Mn⁴⁺ phosphors was stronger than that of the CYS:Mn⁴⁺ phosphors. Meanwhile, the color purities of the optimal CYS:0.003Mn⁴⁺ and CYN:0.003Mn⁴⁺ phosphors were calculated to be as high as 96.40% and 96.91%, respectively. Eventually, it is interesting that the packaged novel WLED structure would improve the color rendering index and correlated color temperature values. From the above results, non-rare-earth Ca₂Y(Nb,Sb)O₆:Mn⁴⁺ materials with high color purities could be proposed for WLED devices.     

Synthesis and photoluminescence properties of Mn4+-doped magnetoplumbite-related aluminate X-type Ca2Mg2Al28O46 and W-type CaMg2Al16O27 red phosphors

Novel Mn⁴⁺-doped magnetoplumbite-related aluminate X-type Ca₂Mg₂Al₂₈O₄₆ and W-type CaMg₂Al₁₆O₂₇ red phosphors were synthesized by solid-state reaction, and we investigated their photoluminescence properties. X-type Ca₂Mg₂Al₂₈O₄₆:Mn⁴⁺ and W-type CaMg₂Al₁₆O₂₇:Mn⁴⁺ exhibited red photoluminescence, with peaks at 655 and 656 nm, arising from the spin-forbidden ²E→⁴A₂ transition of Mn⁴⁺ under near-ultraviolet and blue light excitation, respectively. Therefore, these red phosphors can be excited by near ultraviolet or blue LED light. The photoluminescence properties of these phosphors were similar because magnetoplumbite-related structures crystallize similarly, forming structures consisting of stacked S and R blocks. From these results, we confirmed that magnetoplumbite-related compounds can act as the host structure for Mn⁴⁺-doped phosphors.     

HF-free molten salt route for synthesis of highly efficient and water-resistant K2SiF6:Mn4+ for warm white LED

It has been one of the hot issues to prepare the red-emitting Mn⁴⁺-doped fluoride phosphors with highly efficient and waterproofness for warm white-light-emitting diodes (WLEDs) by the green and environmentally friendly method. Herein, we design a novel green molten salt route to synthesize K₂SiF₆:Mn⁴⁺ red powder using molten NH₄HF₂ salt instead of HF liquor as the reaction medium. The results show that KMnO₄ and MnF₂ could produce Mn⁴⁺ in NH₄HF₂ molten salt through a reduction reaction, and the resulting Mn⁴⁺-doped K₂SiF₆ exhibited a bright red emission peaked at 632 nm under blue light excitation. The luminescence intensity of the as-obtained product after immersing into water for 24 hours maintain nearly 100% of that before soaking and emission peak shape remains unchanged. The thermal stability of the sample was evaluated by temperature-dependent luminescence spectral intensity during heating and cooling. Furthermore, a warm white-light-emitting diodes (WLEDs) with an excellent color rendering index (Ra = 87.1), lower correlated color temperature (CCT = 3536K), and high luminous efficacy (116.99 lm·W⁻¹) was fabricated based on blue chip and K₂SiF₆:Mn⁴⁺ and commercial yellow phosphor (Y₃Al₅O₁₂:Ce³⁺).     

Rare-earth-free Li5La3Ta2O12:Mn4+ deep-red-emitting phosphor: Synthesis and photoluminescence properties

Li₅La₃Ta₂O₁₂:Mn⁴⁺ (LLTO:Mn⁴⁺) phosphors are prepared in air via high-temperature solid-state method and investigated for their crystal structures and luminescence properties. LLTO:Mn⁴⁺ phosphor under excitation at 314 nm shows deep-red emission peaking at 714 nm due to the ²E→⁴A₂ transition of Mn⁴⁺ ion. The excitation bands in the range 220 - 570 nm are attributed to the Mn⁴⁺ - O^2- charge-transfer band and the ⁴A_2g→⁴T_1g, ²T_2g, and ⁴T_2g transitions of Mn⁴⁺, respectively. The optimal Mn⁴⁺ ion concentration is ~0.4 mol%. The concentration quenching mechanism in LLTO:Mn⁴⁺ phosphor is electric dipole-dipole interaction. The luminous mechanism and temperature quenching phenomenon are explained by the Tanabe-Sugano energy level diagram and the configurational coordinate diagram of Mn⁴⁺ in the octahedron, respectively. The experimental results indicate that LLTO:Mn⁴⁺ phosphor has a potential application prospect as candidate of deep-red component in light-emitting diode (LED) lighting.     

Luminescence enhancement of Mn4+-activated double-perovskite phosphors for LEDs through a co-replacement strategy

Phosphors doped with Mn⁴⁺ ions have strong emission in the red and far-red light regions and are therefore used as red phosphors for indoor plant cultivation light-emitting diodes (LEDs) and white LEDs (w-LEDs). This paper introduces La₂Mg(Mg_1/3Ta_2/3)O₆: Mn⁴⁺ (Mg₂La₃TaO₉: Mn⁴⁺) red phosphors prepared by conventional high-temperature solid-phase method. The broad excitation band of Mg₂La₃TaO₉: Mn⁴⁺ phosphor is effectively excited by ultraviolet and blue light in the range of 250–600 nm, with the emission of 707 nm centered on far-red light. The phosphor has a high color purity of 99.07% and an internal quantum efficiency of 59.87%. To further enhance the performance of the phosphor, a cation substitution method was adopted in this paper to synthesize La₂Mg(Al_1/2Ta_1/2)O₆: Mn⁴⁺ phosphor by replacing [1/3Mg²⁺–2/3Ta⁵⁺] in La₂Mg(Mg_1/3Ta_2/3)O₆: Mn⁴⁺ with [1/2Al³⁺–1/2Ta⁵⁺]. The luminescence intensity and thermal stability of the samples were enhanced. The emission spectrum of the Mg₂La₃TaO₉: Mn⁴⁺ samples matched well with the phytochrome P_FR (phytochrome that absorbs far-red light) and is suitable for the preparation of LEDs for indoor plant cultivation. The concentration quenching effect of the samples was investigated, the main mechanism of which is the electric dipole–dipole interaction. Red LEDs and w-LEDs devices were prepared with the synthesized phosphors that produce light stably at different currents. The w-LEDs have a correlated color temperature of 5310 K and a color rendering index of 80.1. Therefore, these samples are expected to be used as red components for w-LEDs.     

Mn4+-related photoemission enhancement via energy transfer in La2MgGeO6:Dy3+, Mn4+ phosphor for plant growth light-emitting diodes

A double perovskite-type substrate of La₂MgGeO₆ (LMGO) was successfully synthesized via a high-temperature solid-state reaction method and was codoped with Mn⁴⁺ and Dy³⁺ to form a new deep-red phosphor (LMGO:Mn⁴⁺,Dy³⁺) for artificial plant growth light-emitting diodes (LEDs). This extraordinary phosphor can exhibit strong far-red emission with a maximum peak at 708 nm between 650 and 750 nm, which can be ascribed to the ²E_g → ²A_2 g spin-forbidden transition of Mn⁴⁺. The X-ray diffraction (XRD) patterns and high-resolution transmission electron microscopy (HRTEM) clarified that the La³⁺ sites in the host were partly replaced by Dy³⁺ ions. Moreover, we discovered energy transfers from Dy³⁺ to Mn⁴⁺ by directly observing the significant overlap of the excitation spectrum of Mn⁴⁺ and the emission spectrum of Dy³⁺ as well as the systematic relative decline and growth of the emission bands of Dy³⁺ and Mn⁴⁺, respectively. With the increase in the activator (Mn⁴⁺) concentration, the relationship between the luminescence decay time and the energy transfer efficiency of the sensitizer (Dy³⁺) was studied in detail. Finally, an LED device was fabricated using a 460 nm blue chip, and the as-obtained far-red emitting LMGO:Mn⁴⁺,Dy³⁺ phosphors for Wedelia chinensis cultivation. As expected, the as-fabricated plant growth LED-treated Wedelia chinensis cultured in the artificial climate box with overhead LEDs demonstrated that after 28 days of irradiation, the average plant growth rate and the total chlorophyll content were better than those of specimens cultured using the commercial R-B LED lamps, indicating that the as-prepared phosphor could have a potential application in the agricultural industry.     

An efficient far-red emission Sr2InSbO6:Mn4+, M (M = Li+, Na+, and K+) phosphors for plant cultivation LEDs

High-efficiency and far-red light phosphors based on Mn⁴⁺-doped inorganic luminescence materials are beneficial to plant cultivation. However, Mn⁴⁺-doped oxide phosphors have a common problem of low quantum efficiency. Alkali metal ion codoping can effectively improve the luminescence properties of Mn⁴⁺-activated oxide phosphors. Herein, a series of Sr₂InSbO₆:Mn⁴⁺, M (SISO:Mn⁴⁺, M) (M = Li⁺, Na⁺, and K⁺) far-red-emitting phosphors codoped alkali metal ions were first synthesized. Density functional theory calculation indicated that SISO is a kind of indirect bandgap material with a bandgap of ∼1.60 eV. The SISO:Mn⁴⁺ samples showed a far-red light at 698 nm upon 365 nm, which perfectly matched the absorption spectrum of the far-red-phytochrome (Pfr) of plants. The doping concentration of the SISO:Mn⁴⁺ samples was optimized to be 0.006 mol. The concentration quenching mechanism was defined as dipole–dipole interaction by combining the Dexter theory and the Inokuti–Hirayama model. Optimizing the sintering temperature and codoped with alkali metal ions (Li⁺, Na⁺, and K⁺) could improve the luminescent intensity of SISO:Mn⁴⁺. The optimum sintering temperature was 1300°C. The internal quantum efficiencies of SISO:0.006Mn⁴⁺ and SISO:0.006Mn⁴⁺, 0.006Li⁺ phosphors are 22.67% and 60.56%, respectively. SISO:Mn⁴⁺, Li⁺ phosphors-based plant growth light-emitting diodes (LEDs) demonstrate excellent optical stability and long lifetime. Thus, these phosphors are promising candidates for plant cultivation LEDs.     

Efficient and tunable Mn2+ sensitized luminescence via energy transfer of a novel red phosphor Ca19Mn2(PO4)14: Eu2+ for white LED

The exploration of efficient and high-purity red phosphors is an urgent need in LED development. Due to the compact and compositional-tunable structure of whitlockite compound, manganese-based Ca₁₉Mn₂(PO₄)₁₄ is chosen as phosphor host for Eu²⁺ sensitization. Rietveld refinement, steady-state spectra, decay lifetime analysis and temperature-dependent emission spectra were investigated and clearly discussed. Under 360 nm excitation, Ca₁₉Mn₂(PO₄)₁₄: Eu²⁺ shows a strong Mn²⁺ sensitized emission at 655 nm with FWHM of 82 nm, benefiting from the short-distance-induced high-efficient Eu² -Mn²⁺ energy transfer. Emission engineering of Ca₁₉Mn₂(PO₄)₁₄: Eu²⁺ is achieved by Sr²⁺ co-doping, leading to both tunable peak wavelength (ranging from 650 to 610 nm) and improved intensity (130% of original value). Moreover, Ca₁₉Mn₂(PO₄)₁₄: Eu²⁺ exhibits a promising thermal stability where only 40% of emission intensity is lost at 200 °C. Finally, we explored the working performance of the fabricated RGB phosphor-converted white LED. The present work indicates that Ca₁₉Mn₂(PO₄)₁₄: Eu²⁺ phosphor is of great potential as a promising and efficient red phosphor in phosphor-converted white LED.     

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.     

Mn4+-activated KLaMgWO6: A new high-efficiency far-red phosphor for indoor plant growth LEDs

Novel Mn⁴⁺-activated KLaMgWO₆ red phosphors with different Mn⁴⁺ concentrations were successfully synthesized via a high-temperature solid-state reaction method. The phase formation, microstructure, photoluminescence properties, decay lifetimes and internal quantum efficiency were discussed to analyze the properties of the as-prepared phosphors. The samples belonged to monoclinic crystal system with enough WO₆ octahedrons that provided suitable sites for Mn⁴⁺ ions. Upon the excitation of 348?nm, KLaMgWO₆:Mn⁴⁺ phosphors gave bright far-red emission around 696?nm due to the ²E_g→⁴A_2g transition of Mn⁴⁺ ions. The critical concentration of Mn⁴⁺ was 0.6?mol% and the concentration quenching mechanism belonged to electric multipolar interaction. Besides, the CIE chromaticity coordinates of the KLaMgWO₆:0.6%Mn⁴⁺ phosphor were (0.7205, 0.2794) which located in deep red range, and its color purity reached up to 96.6%. The KLaMgWO₆:0.6%Mn⁴⁺ sample also exhibited high internal quantum efficiency of 43%. All of the admirable optical properties indicate that the KLaMgWO₆:Mn⁴⁺ phosphors can be applied to indoor plant growth illumination.     

Enhanced luminescence and energy transfer performance of double perovskite structure Gd2MgTiO6:Bi3+, Mn4+ phosphor for indoor plant growth LED lighting

Deep-red light emitting phosphors are widely used in LEDs for indoor plant growth because of the critical role played by red light in plant growth. The luminescence properties of deep-red phosphors are still not well understood at present. An energy transfer strategy is a common and effective method to improve luminescence properties. In principle, the energy transfer process may occur when the sensitizer's emission spectra overlap with the activator's excitation spectra. In this work, Bi³⁺ and Mn⁴⁺ were incorporated into the matrix of Gd₂MgTiO₆ as sensitisers and activators, respectively. Mn⁴⁺ ions tend to occupy the [TiO₆] octahedral site and the Bi³⁺ ions are expected to substituted in the site of Gd³⁺. The energy transfer process from Bi³⁺ to Mn⁴⁺ was realised and the photoluminescence (PL) intensity of Mn⁴⁺ increased with the doping content of Bi³⁺. Upon excitation at 375 nm, the PL intensity of Mn⁴⁺ increased to 116.4% when the doping concentration of Bi³⁺ reached 0.3%. Finally, the pc-LED devices were prepared by a Gd₂MgTiO₆:Bi³⁺, Mn⁴⁺ phosphor. The high red luminescence indicated that this phosphor has potential applications in indoor LED lighting.     

Tunable Color of Eu2+/Mn2+ Coactivated Na5Ca2Al(PO4)4 via Energy Transfer

Eu²⁺ and Eu²⁺/Mn²⁺‐activated Na₅Ca₂Al(PO₄)₄ phosphors have been synthesized by the combustion method. X‐ray powder diffraction profiles, luminescence spectra, chromaticity variation, and energy transfer of Na₅Ca₂Al(PO₄)₄:Eu²⁺, Mn²⁺ were investigated as a function of the Eu²⁺ and Mn²⁺ concentrations in Na₅Ca₂Al(PO₄)₄. The Na₅Ca₂Al(PO₄)₄:Eu²⁺,Mn²⁺ phosphors can be effectively excited at wavelength ranging from 300 to 430 nm, which matches well with that for near‐ultraviolet (UV) light‐emitting diode (LED) chips. Under excitation at 354 nm, Na₅Ca₂Al(PO₄)₄:Eu²⁺,Mn²⁺ not only exhibits blue‐green emission band attributed to 4f⁶5d¹→4f⁷ of Eu²⁺ but also gives an orange emission band attributed to ⁴T₁→⁶A₁ of Mn²⁺. The emission color of the phosphor can be systematically tuned from blue‐green through white and eventually to orange by adjusting the relative content of Eu²⁺ and Mn²⁺ through the principle of energy transfer. The results indicated that Na₅Ca₂Al(PO₄)₄:Eu²⁺, Mn²⁺ may serve as a potential color‐tunable phosphor for near UV white‐light LED.     

Surface modification of K2TiF6:Mn4+ phosphor with SrF2 coating to enhance water resistance

K₂TiF₆:Mn⁴⁺ is an attractive narrow-band red-emitting phosphor for warm white light-emitting diodes (LEDs). Nevertheless, the hexafluoride phosphor is liable to deliquesce in moist environments, which leads to a sharp deterioration performance of luminescence. Surface modification of K₂TiF₆:Mn⁴⁺ phosphor with SrF₂ coating has been introduced, with the aid of KHF₂ transition layer to moderate the lattice mismatch. The reaction mechanism is discussed in detail, as so as the influence of SrF₂ coating on the luminescence intensity. The SrF₂ coating is able to prevent the hydrolysis of internal [MnF₆]²⁻ group; thereby, the luminescence intensity retains over 90% of initial value after being immersed in distilled water for 2 h. The LED devices fabricated with commercial Y₃Al₅O₁₂:Ce³⁺ and as-modified K₂TiF₆:Mn⁴⁺ phosphors exhibit bright white light with tunable chromaticity coordinate, correlated color temperature, and color rendering index. It enlightens a convenient method to enhance the moisture resistance of Mn⁴⁺ doped fluoride phosphors for commercial application in the field of white LEDs.     

Broadband near-infrared emission enhancement in K2Ga2Sn6O16:Cr3+ phosphor by electron-lattice coupling regulation

Cr³⁺-doped phosphors have recently gained attention for their application in broadband near-infrared phosphor-converted light-emitting diodes (pc-LEDs), but generally exhibit low efficiency. In this work, K₂Ga₂Sn₆O₁₆:Cr³⁺ (KGSO:Cr) phosphor was designed and synthesized. The experimental results show that the Cr³⁺-doped phosphor exhibited broadband emissivity in the range 650-1300 nm, with a full width at half maximum (FWHM) of approximately 220-230 nm excited by a wavelength of 450 nm. With the co-doping of Gd³⁺ ions, the internal quantum efficiency (IQE) of the KGSO:Cr phosphor increased from 34% to 48%. The Gd³⁺ ions acted neither as activators nor sensitizers, but to justify the crystal field environment for efficient Cr³⁺ ions broad emission. The Huang-Rhys factor decreased as the co-doping of Gd³⁺ ions increased, demonstrating that the nonradiative transitions were suppressed. An efficient strategy for enhancing the luminescence properties of Cr³⁺ ions is proposed for the first time. The Gd³⁺–co-doped KGSO:Cr phosphor is a promising candidate for broadband NIR pc-LEDs.