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 dodec-fluoride red phosphor Na3Li3In2F12:Mn4+ with excellent waterproof stability for WLEDs

Mn⁴⁺ activated fluoride red phosphors, as candidate red materials in white light-emitting diodes (WLEDs), have received widespread attention. However, the poor water stability limits their application. Herein, a novel dodec-fluoride red phosphor Na₃Li₃In₂F₁₂:Mn⁴⁺ with good waterproof stability was successfully synthesized by solvothermal method. The crystal structure, optical property, micro-morphology, element composition, waterproof property and thermal behavior of Na₃Li₃In₂F₁₂:Mn⁴⁺ phosphor were analyzed. Under the 468 nm blue light excitation, the Na₃Li₃In₂F₁₂:Mn⁴⁺ phosphor has narrow emission bands in the area of 590–680 nm. Compared with commercial red phosphor K₂SiF₆:Mn⁴⁺, the Na₃Li₃In₂F₁₂:Mn⁴⁺ phosphor possesses better waterproof stability. When soaked in water for 360 min, the PL intensity of the Na₃Li₃In₂F₁₂:Mn⁴⁺ phosphor remains at initial 80%. Finally, warm WLEDs with CRI of 87 and CCT of 3386 K have been fabricated using blue InGaN chip, YAG:Ce³⁺ yellow phosphor and Na₃Li₃In₂F₁₂:Mn⁴⁺ red phosphor.     

Zero-thermal-quenching of LiAl5O8: Eu2+, Mn2+ phosphors by energy transfer and defects engineering

The photoluminescence behavior of inorganic phosphors is generally influenced by thermal stability, which determines the luminescence efficiency of the corresponding devices. Here, a series of Eu²⁺, Mn²⁺ co-doped LiAl₅O₈ blue-green-emitting phosphors with thermal robust are successfully fabricated. The concentration-dependent emission spectra and the decay curves of the as-obtained LiAl₅O₈: Eu²⁺, Mn²⁺ samples manifest the occurrence of the energy transfer from Eu²⁺ to Mn²⁺ ions via dipole-dipole interaction, and the corresponding emitted colors are gradually modulated from blue to green under the excitation of 310 nm. Moreover, the zero-thermal-quenching luminescence is observed when the operation temperature is up to 423 K, which is attributed to the energy release from the trapping centers to emitting centers (Eu²⁺ and Mn²⁺) at high temperature. Furthermore, a warm white light-emitting diodes (WLEDs) device with correlated color temperature of 5061 K, a color rendering index of 80.6 and long-term stability is fabricated by combining UV LED chip (λ_ex = 310 nm), as-obtained LiAl₅O₈: Eu²⁺, Mn²⁺ phosphor, commercially available red phosphor and green phosphor. These results prove the potential application of the as-obtained LiAl₅O₈: Eu²⁺, Mn²⁺ phosphor for UV-pumped WLEDs devices.     

Preparation and optical properties of a novel double-perovskite phosphor,Ba2GdNbO6:Mn4+, for light-emitting diodes

Red phosphors serve an important function as red components of warm white light-emitting diodes (WLEDs). Given their remarkable luminescent properties and low cost, Mn⁴⁺-doped phosphors are attracting significant attention. In this study, the novel red phosphor Ba₂GdNbO₆:Mn⁴⁺ was synthesized through high-temperature solid-state reaction. The host Ba₂GdNbO₆ with a double-perovskite structure was investigated. Scanning electron microscopy and thermogravimetric analysis were performed to evaluate the structure and thermal stability of the phosphor, respectively. PLE and photoluminescence spectra were further used to study the luminescence properties of the phosphor. Moreover, crystal field strength and Racah parameters were calculated to estimate the nephelauxetic effect of Mn⁴⁺ on the Ba₂GdNbO₆ host lattice. Thermal quenching characteristics were also analyzed. The fabricated red-emitting LED revealed its potential application in WLEDs.     

Novel cyan-emitting KBaScSi2O7:Eu2+ phosphors with ultrahigh quantum efficiency and excellent thermal stability for WLEDs

Owing to the conventional phosphor-converted white LEDs (pc-WLEDs) generally suffer from blue-green cavity, thus, developing an appropriate phosphors covering both the blue and green regions in their emission spectra are very urgent. Herein, a novel Sc silicate phosphor, KBaScSi₂O₇:Eu²⁺ (KBSS:Eu²⁺), has been successfully designed and prepared via a solid-state reaction. The crystal structure, luminescent properties, thermal quenching, quantum efficiency as well as its application in UV-pumped WLEDs have been investigated systematically. The KBSS:Eu²⁺ phosphor exhibits a strong and broad excitation band ranging from 290 to 450 nm, and gives a sufficient cyan emission of 488 nm with a full-width half-maximum (FWHM) of 70 nm, which filled the blue-green cavity. Importantly, the optimized KBSS:Eu²⁺ phosphor possesses an ultrahigh quantum efficiency (QE) up to 91.3% and an excellent thermal stability retaining 90% at 423 K with respect to that measured at room temperature. Finally, the as-fabricated UV-based WLEDs device, with only coupled the mixture of KBSS:Eu²⁺ cyan phosphor and CaAlSiN₃:Eu²⁺ red ones to a commercial 365nm UV chip, exhibits a satisfactory color-rendering index (Ra = 88.6), correlated color temperature (CCT = 3770K), and luminous efficiency (LE = 21 lm/W).     

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.     

Hydrophobic surface modification toward highly stable K2SiF6:Mn4+ phosphor for white light-emitting diodes

K₂SiF₆:Mn⁴⁺ phosphor is well known for its excellent red emission performance which is vital for improving the color rendering of white light-emitting diodes. However, the poor moisture resistance limits its application in optical devices. In this paper, K₂SiF₆:Mn⁴⁺ phosphor is coated with an inorganic hydrophobic protective layer to obtain good moisture resistance. Chemical vapor deposition method was used to decompose acetylene at high temperature, and the generated nanoscale carbon layer worked as a hydrophobic protective coating on the surface of the phosphor. Microstructure, compositions and properties of the synthesized K₂SiF₆:Mn⁴⁺@C phosphor were investigated in detail. It is found that most of the deposited carbon is coated on the surface of phosphor crystals in amorphous state. The carbon atoms are bonded with the fluorine element in K₂SiF₆:Mn⁴⁺ phosphor, forming carbon-fluorine (C–F) covalent bonds. The moisture resistance of K₂SiF₆:Mn⁴⁺@C phosphor is improved owing to the protection of the hydrophobic carbon. The relative emission intensity of K₂SiF₆:Mn⁴⁺@C phosphor could maintain 73% of the initial luminous intensity after immersing in the aqueous solution at room temperature for 8 h, whereas K₂SiF₆:Mn⁴⁺ phosphor without carbon coating was only 0.7% remaining of the initial value under the same conditions.     

Key role of Nb5+ in achieving water-resistant red emission in K2Ta1-xNbxF7:Mn4+ phosphors

Mn⁴⁺-activated fluoride is one of the most important red phosphors for white light-emitting diodes (WLEDs) with high color rendering index (CRI). Due to a lack of water resistance, their potential applications are limited. Although surface coating strategies improve the waterproof stability of fluoride red phosphors, they have downsides. It was found that Nb⁵⁺ plays an important role in improving the water resistance of Mn⁴⁺-activated oxyfluorides by preventing the hydrolysis of [MnF₆]^2-. In this work, the influence of Nb⁵⁺ on the waterproof stability of Mn⁴⁺-activated fluorides was explored. A set of synthesized K₂Ta_1-xNb_xF₇:Mn⁴⁺ phosphors exhibit tunable and superior water resistance. The photoluminescence (PL) intensity of the representative sample K₂Ta_0.6Nb_0.4F₇:5%Mn⁴⁺ remains nearly 100% of its initial value even after being immersed in water for 60 min, which is significantly higher than the commercial K₂SiF₆:Mn⁴⁺ red phosphor (8.7%). Our findings open up new possibilities for the development of waterproof fluoride red phosphors.     

A double-perovskite Sr2ZnWO6:Mn4+ deep red phosphor: Synthesis and luminescence properties

Novel double-perovskite Sr₂ZnWO₆:Mn⁴⁺(SZW:Mn⁴⁺) phosphor is synthesized by high-temperature solid-state reaction method in air. SZW:Mn⁴⁺ phosphor with excitation at 325 and 526 nm emits deep-red light, the chromaticity coordinate is (0.7315,0.2685), and the emission band peaking at ~702 nm within the range 640–760 nm is assigned to the ²E→⁴A₂ transition of Mn⁴⁺ ion. The influences of “Mn⁴⁺- ligand” bonding and crystal field strength to emission properties of Mn⁴⁺ ion are analyzed. The optimal Mn⁴⁺ ion concentration in SZW:Mn⁴⁺ phosphor is ~0.8 mol%. Lifetime of SZW:Mn⁴⁺ phosphor decreases from 554.77 to 401.35 μs with increasing Mn⁴⁺ ion concentration in the range of 0.2–1.0 mol%. The lifetime data and decay curves indicate that there is only a single type of Mn⁴⁺ ion luminescent center in SZW:Mn⁴⁺ phosphor. The luminous mechanism of SZW:Mn⁴⁺ phosphor is analyzed by Tanabe-Sugano energy level diagram of Mn⁴⁺ in the octahedron together with the simple energy level diagram. The experimental results are helpful to research the influences of the neighboring coordination environment around Mn⁴⁺ and host crystal structure to the luminescence properties of Mn⁴⁺ ion and to deeply understand other Mn⁴⁺-dopedmaterials.     

Achieving highly efficient far-red emission from luminescence-ignorable Ca2MgTeO6:Mn4+ to Ca2-zLnzMgTe1-yWyO6:Mn4+ (Ln = La,Y, Gd) phosphors via solid solution and impurity doping strategies

The Mn⁴⁺-doped Ca2MgTeO6 (CMTO) far-red emitting phosphors with double perovskite-type structure were successfully synthesized. Upon near-ultraviolet (n-UV, 300 nm) light excitation, the as-prepared phosphors showed far-red light at 700 nm attributed to the ²E_g→⁴A_2g transition of Mn⁴⁺ ion. The doping concentration of the CMTO:xMn⁴⁺ samples was optimized to be 0.8 mol%. The relevant mechanism of concentration quenching was demonstrated as the dipole-dipole interaction. Furthermore, solid solution and impurity doping strategies were adopted to improve the far-red emission of the luminescence-ignorable CMTO:Mn⁴⁺ phosphor. Series of Ca₂MgTe_(1−y)W_yO₆:0.8 mol%Mn⁴⁺ (y = 0–100 mol%) solid solution and Ca_2−zLn_zMgTe_0.6W_0.4O₆:Mn⁴⁺ (Ln = La, Y, and Gd, z = 10 mol%) phosphors were synthesized through the above two strategies. The luminescence intensity of the optimal Ca_1.9Gd_0.1MgTe_0.6W_0.4O₆:Mn⁴⁺ phosphor was 13.7 times that of the CMTO:Mn⁴⁺ phosphor and 2.51 times that of red commercial phosphor K₂SiF₆:Mn⁴⁺. Notably, both CMTO:Mn⁴⁺ and Ca_1.9Gd_0.1MgTe_0.6W_0.4O₆:Mn⁴⁺ phosphors exhibited remarkable thermal stability compared with most Mn⁴⁺-doped phosphors. Finally, the highly efficient Ca_1.9Gd_0.1MgTe_0.6W_0.4O₆:Mn⁴⁺ phosphor was successfully applied in fabricating the warm white light diode (w-LED). This working along both lines strategy exhibited great potential for luminescence optimization of Mn⁴⁺-doped oxide 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⁺).     

Ionic selective occupation,lattice modulation and photoluminescence properties of garnet typed orange-yellow phosphors for white-light-emitting diodes

Orange-yellow luminescent Y_2-xSr_1-yAl₄SiO₁₂: xCe³⁺, yMn²⁺ (x = 0.04–0.06, y = 0.1–0.6, abbreviated as YSAS: xCe³⁺, yMn²⁺) phosphors with garnet structure were prepared by the high temperature solid-phase method. The matrix energy band is calculated by DFT, where YAG is 4.535 eV and YSAS is 4.874 eV. The Rietveld refinement of the XRD data indicates that Ce³⁺ mainly replaces the Y³⁺ sites, while Mn²⁺ selectively occupies. The Gaussian peaks of the fluorescence spectrum prove that Mn²⁺ occupies two luminescent centers, which are further determined by Van Uitert's empirical formula. The doping of Ce³⁺/Mn²⁺ units can effectively enhance the structural rigidity of the YAG host and promote the red-shift of the emission peak position of Mn²⁺. The fluorescence decay kinetics test proves that Ce³⁺ can transfer energy to Mn²⁺, and the transfer efficiency reaches 72.08% at y = 0.5. The thermal stability test of the phosphor shows the luminous intensity at 473 K still maintains 86% of the room temperature. The phosphor prepared in this experiment was combined with a blue light chip to make a w-LED device, and the performance showed Ra = 86.1, CCT = 5168 K. The above results show that the emission color of the garnet structure can be tuned by energy transfer, which is attractive for future applications.     

Preparation and properties of CdSiO3: Mn2+, Tb3+ phosphor

CdSiO₃: Mn²⁺, Tb³⁺ long-lasting phosphor was prepared by the conventional high temperature solid-state method. Effects of the content of Mn²⁺ and Tb³⁺ on the luminescent properties of phosphor CdSiO₃: Mn²⁺, Tb³⁺ were investigated by means of photoluminescence (PL) spectra, the afterglow intensity decay curves and the thermoluminescence (TL) spectra. It was found that when the Mn²⁺ and Tb³⁺ dopant-concentrations were 0.4 mol% and 0.8 mol% of Cd²⁺ ions in CdSiO₃, respectively, the luminescence of phosphor prepared had better luminescent property and longer afterglow time. Role of Tb³⁺ co-doped into CdSiO₃: Mn²⁺ matrix was discussed in this paper.     

Achieving thermally stable and anti-hydrolytic Sr2Si5N8:Eu2+ phosphor via a nanoscale carbon deposition strategy

Chemical stability of phosphors is critical to the efficiency and lifetime of the white light-emitting diodes. Therefore, many strategies have been adopted to improve the stability of phosphors. However, it is still lack of report on the improvement of thermal stability and hydrolysis resistance of phosphors by a single layer coating. Due to the high transmittance and high chemical inertness of graphene, it was coated on the surface of Sr₂Si₅N₈:Eu²⁺ phosphor by chemical vapor deposition, aiming to improve its thermal stability and hydrolysis resistance. The chemical composition and microstructure of the coating were characterized and analyzed. A nanoscale carbon layer was attached on the surface of Sr₂Si₅N₈:Eu²⁺ phosphor particles in an amorphous state. In coated Sr₂Si₅N₈:Eu²⁺ phosphor, the oxidation degree of Eu²⁺ to Eu³⁺ was significantly suppressed. At the same time, the surface of Sr₂Si₅N₈:Eu²⁺ particle turned from hydrophilic to hydrophobic after carbon coating, and consequently the hydrolysis resistance of Sr₂Si₅N₈:Eu²⁺ phosphor was greatly improved. After tests at 85 °C and 85% humidity for 200 h, the carbon coated Sr₂Si₅N₈:Eu²⁺ phosphor still maintained about 95% of its initial luminous intensity as compared with 35% of the uncoated. By observing the in-situ microstructure evolution of coated phosphor in air-water vapor environment, remained presence of the carbon layer even at 500 °C explained the excellent chemical stability of carbon coated Sr₂Si₅N₈:Eu²⁺ phosphor in complex environment. These results indicate that a nanoscale carbon layer can be used to provide superior thermal stability and hydrolysis resistance of (oxy) nitrides phosphors.     

Effects of HfO2 dopant on characteristics of Li2MgTiO4-based red phosphors: Thermal stability,photoluminescence intensity and quantum efficiency improvement

To produce natural and vivid color, the color rendering index of white light-emitting diodes (WLEDs) with single phosphors is usually lower than 70, which is problematic for LED applications. A commonly used method to resolve this issue is to enhance the red component of WLEDs. In the present study, Hf⁴⁺ and Mn⁴⁺ co-doped Li₂MgTiO₄ red phosphors are synthesized using a solid-state reaction method. When this red phosphor is excited at 397 and 468?nm, it exhibits weak reabsorption in the blue region and emits a broad and deep red emission band in the range of 640–750?nm, which is attributed to the ²E_g → ⁴A_2?g transition. With 5?mol% HfO₂ dopant, the photoluminescence intensity is enhanced by 1.45-fold and thermal stability is increased by 7.7%. Moreover, this red phosphor was applied to a red phosphor-in-glass (RPiG) optical device with a low-melting TeO₂-B₂O₃-ZnO-Na₂O-WO₃ glass system. In the RPiG melting process, Li₂MgTiO₄:Mn⁴⁺, Hf⁴⁺ red phosphor triggered neither a chemical reaction nor severe degradation, indicating good thermal stability. Li₂MgTiO₄:Mn⁴⁺, Hf⁴⁺ has potential as a red emission material for warm WLED applications.     

Tunable chromaticity and high color rendering index of WLEDs with CaAlSiN3:Eu2+ and YAG:Ce3+ dual phosphor-in-silica-glass

Phosphors-in-glass (PiG), which serves as a potential bi-replacement of both phosphors and organic encapsulants in high-power white light-emitting diodes (WLEDs), has captured much attention due to its high thermal stability and excellent luminescent properties. However, due to the high-temperature sensitivity and the chemical reactions between phosphors with glass matrix, a variety of phosphors, especially red phosphors could be hardly dispersed into the glass without thermal quenching and decomposition, which greatly limits the improvement of color rendering index and chromaticity tunability of the WLEDs. In this study, adopting the mesoporous silica (FDU-12) and commercial phosphors as raw materials, the phosphors-in-silica-glasses (PiSGs) embedded with red phosphor CaAlSiN₃:Eu²⁺ and yellow phosphor YAG:Ce³⁺ have been successfully prepared at low sintering temperature (950°C) and short preparation time (10 minutes) using spark plasma sintering. Owing to the well preservation of the originally emissive properties of the embedded phosphors, the warm WLEDs with tunable chromaticity and exhibited a superior performance with LE of 133 lm/W, CCT of 3970 K and CRI of 81 were fabricated by encapsulating the as-prepared PiSGs on the blue chips. Moreover, the PiSG composite exhibits a high thermal conductivity up to 1.6 W/m·K.     

Synthesis,Structure, and Performance of Efficient Red Phosphor LiNaGe4O9:Mn4+ and Its Application in Warm WLEDs

For phosphor‐converted warm white light‐emitting diodes (WLEDs), it is essential to find highly efficient red oxide phosphors, which are better chemically stable and benign to environment and can be prepared in a much milder condition. Here, we report a red phosphor LiNaGe₄O₉:Mn⁴⁺ with a quantum yield up to 78% after systematic optimization in synthesis temperature, dopant concentration of Mn⁴⁺, and sintering time. Best performance of the phosphor can be reached when it is synthesized in a mild reaction condition, that is, at 850°C for 3 h in air. The integrated emission intensity is more than four times stronger than commercial red phosphor 3.5MgO·0.5MgF₂·GeO₂:Mn⁴⁺ (MFG:Mn⁴⁺) under a blue light excitation at 470 nm. Crystal structural analysis reveals that the high efficiency Mn⁴⁺ exhibits in the compound is mainly due to the well separation of GeO₆ groups from each other by GeO₄ tetrahedra in the neighborhood and the ideal substitution of octahedral Ge⁴⁺ site by Mn⁴⁺ in view of both size and charge matches. The high performance of the phosphor encourages us to apply the blue absorbing red phosphor to WLED, which is based on combination of a blue LED chip and YAG:Ce³⁺, and the warm perception WLED is therefore achieved with a color temperature of 3353 K.     

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.     

Comparative analysis on the photoluminescence properties of Cs2BF6:Mn4+ (B = Ge,Si, Zr,Ti) red phosphors for WLEDs

A series of Cs₂BF₆:Mn⁴⁺ (B = Ge, Si, Ti, Zr) red phosphors were synthesized by a precipitation-cation exchange route. The phase purity, morphology, and constituent were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Optical properties were investigated by photoluminescence (PL) spectra and high-resolution PL. Temperature-dependent PL examination at the range of both 273-573 K and 10-300 K was performed to investigate the emission mechanism of Mn⁴⁺ in these fluorides. The intensity for both zero-phonon lines (ZPLs) and vibration coupled emission of Mn⁴⁺ in these four systems with different crystal structures was investigated systematically. These phosphors present bright red emission under blue light (467 nm) illumination, among which Cs₂GeF₆:0.1Mn⁴⁺ shows the highest emission intensity with ultrahigh quantum efficiency of 94%. The white light-emitting diodes (WLEDs) fabricated with this sample, blue InGaN chips and commercial YAG:Ce³⁺ phosphor exhibited high luminous efficacy beyond 100 lm/w with high color rendering index (~88.6) and low color temperature (~3684 K).     

Experimental evaluation of the luminescence performance of fired clay brick coated with SrAl2O4:Eu/Dy phosphor

This study evaluates the luminescence performance of fired clay bricks coated with SrAl₂O₄:Eu/Dy phosphor. To do so, SrAl₂O₄:Eu/Dy phosphor was first produced using the traditional solid-state reaction synthesis technique. The prepared phosphor was then used for coating fired clay bricks to analyze the luminescence performance via spectral analysis, decay characteristics, and microstructure of the bricks. The results reveal that excitation and emission spectra of the phosphor coated bricks range from 200 to 480 nm and 455 to 650 nm, respectively, suggesting that the phosphor coated bricks have the capacity of absorbing light with a wide range of wavelengths. The peak wavelength projected at 511 nm in the emission spectrum is achieved, which indicates 4f⁶5 d¹-4f⁷ transition of Europium (Eu²⁺). The repeated excitation and deexcitation of Eu²⁺ by using hole traps and trap levels offered by Dysprosium (Dy³⁺), exist between the ground and the excited state of Eu²⁺ leads to luminescent phenomenon. Moreover, the decay characteristics has revealed that phosphor coated bricks can emit light for a considerable amount of time (>8.5 min) upon the removal of the excitation source. The results reveal that phosphor coated bricks has the potential of increasing energy efficiency of residential and commercial buildings.