Thermally stable and highly efficient red-emitting Eu3+-doped Cs3GdGe3O9 phosphors for WLEDs: non-concentration quenching and negative thermal expansion

Negative Thermal Expansion Materials as Anti-Thermal-Quenching Phosphor Matrixes: Status, Opportunities, and Challenges

Inorganic phosphor materials often face the common phenomenon of luminescence thermal quenching (TQ), which deteriorates their device performance and consequently limits their applicability for broad applications. Thus, exploring thermally stable and even anti-TQ phosphors is viable to meet the urgent requirements of lighting technology and many other luminescence-based applications. One of the emerging approaches devoted to solving the TQ issue of phosphors, especially at elevated temperatures, is to employ negative thermal expansion (NTE) properties occurring in some unique inorganic materials. The present Review focuses on the progress of exploring NTE-based inorganic phosphor materials that have demonstrated unusual negative TQ with enhancing upconversion and downshift luminescence upon elevating temperature. We have also provided a brief history of exploring NTE phosphors for thermally stable and enhanced emission along with the investigation methods and proposed mechanisms of these unusual phenomena. To summarize, we have further discussed some opportunities and challenges that NTE materials face as host matrixes for anti-TQ phosphors. Overall, the aim of this Review is to stimulate the exploration of new NTE-based inorganic phosphors, the correlation of their fundamental structural changes with varying temperature, and the investigation of their potential for broad applications.

Phosphate phosphors with anti-thermal quenching properties for urban ecological lighting

White light-emitting diode (LED) for night lighting disrupts photoperiod in plants, which affects the plant's photosynthesis. Therefore, it is necessary to find a new type of white LED with little effect on plant photosynthesis. In this study, a series of phosphate phosphors Ca9NaY2/3(PO4)7:Dy3+ (CNYP:Dy3+) were synthesized. Cation Li+ substitute Na+ were used to improve the luminescence properties of CNYP:Dy3+ phosphor. The CNYP:Dy3+ phosphor exhibits visible white light emission with emission peaks at 480 nm (blue light) and 570 nm (yellow light) excited by the near ultraviolet light 350 nm. The optimal concentration of Dy3+ was 0.10 mol, and the mechanism of concentration quenching was evaluated as energy migration among the nearest or next-nearest Dy3+. The substitution of Na+ by Li+ of CNYP:0.10Dy3+ improves the internal quantum efficiency from 30.24 % to 59.05 %, and presents good near-zero thermal quenching performance at 423 K. To assess the suitability of this phosphor for urban ecological lighting, the spectrum resemblance (SR) index between the electroluminescence spectrum of the prepared pc-LED and the absorption spectra of chlorophyll a and b was evaluated as 6.63 % and 18.61 %, respectively. This work exhibits a feasible scheme for the development of urban ecological lighting.

Luminescence-Monitored Progressive Chemical Pressure Implementation Realized through Successive Y3+ and Mg2+ Doping into Ca10.5(PO4)7:Eu2

Chemical pressure generated through ion doping into crystal lattices has been proven to be conducive to exploration of new matter, development of novel functionalities, and realization of unprecedented performances. However, studies are focusing on one-time doping, and there is a lack of both advanced investigations for multiple doping and sophisticated strategies to precisely and quantitatively track the gradual functionality evolution along with progressive chemical pressure implementation. Herein, high-valent Y3+ and equal-valent Mg2+ is successively doped to replace multiple Ca sites in Ca10.5(PO4)7:Eu2+. The luminescence evolution of Eu2+ serves as an optical probe, allowing step-by-step and atomic-level tracking of the site occupation of Y3+ and Mg2+, interassociation of Ca sites, and ultimately functionality improvement. The resulting Ca8MgY(PO4)7:Eu2+ displays a record-high relative sensitivity for optical thermometry. Utilization of the environment-sensitive emission of Eu2+ as a luminescent probe has offered a unique approach to monitoring structure-functionality evolution in vivo with atomic precision, which shall also be extended to optimization of other functionalities such as ferroelectricity, conductivity, thermoelectricity, and catalytic activity through precise control over atomic diffusion in other types of substances.

Well-Performed Green Phosphor BaY4Si5O17:Ce3+,Tb3+ with High Quantum Efficiency and Thermal Stability

For Tb3+-doped green phosphors, the energy transfer from Ce3+ to Tb3+ can largely enhance the absorption of excitation; however, obtaining phosphors that exhibit both high quantum efficiency and thermal stability continues to pose a significant challenge. Herein, we established a paradigm to achieve novel silicate BaY4Si5O17 (BYSO):Ce3+,Tb3+. The near-ultraviolet light efficiently excites the BYSO:Ce3+ material, causing it to emit light at a wavelength of 408 nm. The photoluminescence of BYSO:0.12Ce3+ exhibits a relatively small Stokes shift and a thermal stability of 89.8% of the 303 K emission intensity at 423 K (89.8%@423 K). The energy transfer (ET) from Ce3+ to Tb3+ ions can be readily constructed in BYSO:Ce3+,Tb3+ utilizing the overlap between the Ce3+ emission and the Tb3+ excitation. The ET efficiency from the Ce3+ to Tb3+ ions reached 83.8% at y = 1.2 and a maximum of 94.6%. Finally, the optimized phosphor BYSO:0.12Ce3+,1.2Tb3+ had an internal quantum efficiency of 94.4% and had excellent thermal stability (96.1%@423 K). Our work pointed out the avenue to novel green phosphors with high efficiency and thermal stability by choosing appropriate host and construct efficient ET.

Designing high thermally stable deep red phosphors based on low thermal expansion coefficients for optical applications

Mn4+-activated oxide phosphors with low cost and unique luminescent properties have been considered as a promising candidate for various optical applications, while the search for high thermal stable red-emitting phosphors is still a huge challenge. In our work, we find and unveil the relationship between luminescence thermal quenching behavior and thermal expansion coefficients (α/10-6 K-1) based on double-perovskite niobate phosphors Ca2LnNbO6:Mn4+ (Ln3+ = Y3+, Gd3+, La3+, or Lu3+). It can be concluded that the phosphors with low thermal expansion coefficients contribute to high thermal stability. Subsequently, Ca2LuNbO6:Mn4+ accomplishes accurate temperature testing and high-CRI white light-emitting diodes. Thus, a thermal expansion coefficient strategy is a new guide to select the appropriate substrate with high thermal stability for an Mn4+-activated emitter.

Regulating luminescence thermal enhancement in negative thermal expansion metal-organic frameworks

Overcoming thermal quenching is generally essential for the practical application of luminescent materials. It has been recently found that frameworks with negative thermal expansion (NTE) could be a promising candidate to engineer unconventional luminescence thermal enhancement. However, the mechanism through which luminescence thermal enhancement can be well tuned remains an open issue. In this work, enabled by altering ligands in a series of UiO-66 derived Eu-based metal-organic frameworks, it was revealed that the changes in the thermal expansion are closely related to luminescence thermal enhancement. The NTE of the aromatic ring part favors luminescence thermal enhancement, while contraction of the carboxylic acid part plays the opposite role. Modulation of functional groups in ligands can change the thermal vibration of aromatic rings and then achieve luminescence thermal enhancement in a wide temperature window. Our findings pave the way to manipulate the NTE and luminescence thermal enhancement based on ligand engineering.

Development of a novel Eu2+ activated oxonitridosilicate cyan phosphor for enhancing the color quality of a violet-chip-based white LED

Cyan phosphors are urgently needed to fill the cyan gap and improve the spectral continuity of white light-emitting diodes (LEDs) to cater to the high demand for high-quality lighting. Here, a series of new Eu2+-activated La3Si6.5Al1.5N9.5O5.5 (LSANO) cyan phosphors were prepared, and their luminescence properties and color centers were analyzed through fluorescence spectral measurements from 7 K to 475 K. At 300 K, the photoluminescence excitation (PLE) spectrum monitored at 483 nm presents a broadband of 200-460 nm with a peak at 398 nm, matching well with commercial violet LED chips. When excited by 398 nm violet light, the photoluminescence emission (PL) spectrum of LSANO:0.01Eu2+ exhibits a cyan emission band at about 483 nm. At 7 K, the emission spectrum clearly shows an asymmetric emission band and the emission peak wavelength changes from 483 nm (300 K) to 500 nm (7 K), indicating that there are two possible color centers in the LSANO:Eu2+ phosphor. Moreover, the maximum emission value can be adjusted from 480 to 499 nm by adjusting the doping content of Eu2+. Finally, a violet-chip-based white LED with the optimized color quality of Ra = 91.4, Rf = 90.1, and Rg = 93.6 was fabricated by adding the prepared cyan phosphor, verifying the potential application of the prepared cyan phosphor LSANO:Eu2+ in high-quality white LEDs.

Regulating luminescence thermal quenching based on the synergistic effect of energy transfer and energy gap modulation

Thermal quenching is the core challenge that hinders the application of luminescent materials. Herein, a synergistic mechanism involving energy transfer and energy gap modulation is proposed based on the local crystal field regulation around sensitizers. The substitution of coordination cation V5+/P5+ weakens the crystal field strength of the sensitizer Bi3+, and the weakening of crystal field splitting causes an increase in the 3P1 energy level, thus increasing its energy gap. Compared with the YVO4:Bi3+,Eu3+ phosphor, the thermal stability of the YV0.25P0.75O4:Bi3+,Eu3+ phosphor is significantly improved, and the relative emission intensity of Eu3+ continuously increases with heating and reaches 1.24 times the original intensity at 523 K and does not show a decreasing trend in the studied temperature range. The anti-thermal quenching performance is mainly attributed to the increasing thermal quenching activation energy (ΔE) of the sensitizer by energy gap modulation, which enhances the energy transfer to compensate thermal quenching. Based on the thermal quenching characteristics of the materials, an optical thermometer is designed. The maximum relative sensitivity (Sr) and absolute sensitivity (Sa) are as high as 1.74% K-1 and 0.59 K-1, respectively, and the minimum temperature resolution reaches 0.288 K. The synergistic effect between energy gap modulation of the sensitizer and energy transfer enables the regulation of thermal quenching. Thus, this study provides a new strategy for exploiting high-performance luminescent materials.

Mn4+/Eu3+ co-doped fluoride toward a blue light-excited optical fiber thermometer

The ongoing development of ratiometric optical thermometry is mainly trapped in thermally coupled levels of rare-earth ions and inefficient ultraviolet excitation. Herein, a new-type multiple sharp line emitting, blue light-excited K2NaInF6:Mn4+, Eu3+ fluoride phosphor has been reported as a ratiometric thermometer. The f-f transition of Eu3+ paves a steady reference to a highly temperature sensitive Mn4+d-d transition and enables high relative sensitivity of 1.65% K-1 at 573 K. An optical fiber thermometry on a household oven with a relative standard deviation of 0.11% surpasses the standard of precision measurement, showing great potential in practical application. This discovery offers a highly sensitive neotype blue light-excitable ratiometric temperature sensor, that is Mn4+-doped fluoride, promoting practical applications of optical thermometry.

Tracking of structural defects induced by Eu-doping in β-Ag2MoO4: their influences on electrical properties

In this study, several methods were employed to investigate the electrical characteristics of β-Ag2MoO4 systems, both Eu-doped and undoped, synthesized using the microwave-assisted hydrothermal method. The focus extended to understanding how synthesis time influences material defects, with doping fixed at 1%. A systematic shift in the silver vacancy (VAg) concentration was observed within the doped β-Ag2MoO4 system. Specifically, this study demonstrated that the incorporation of Eu3+ into polycrystalline β-Ag2MoO4 initially increases the VAg concentration. However, as the synthesis time progresses, the VAg concentration decreases, resulting in alterations in the resulting electrical properties, arising from the intricate interplay between the number of grain boundaries and carrier density. By combining information obtained from photoluminescence, positron annihilation lifetime spectroscopy, and impedance spectroscopy, a comprehensive conduction mechanism was formulated, shedding light on both doped and undoped β-Ag2MoO4 systems.

Development of thermally stable red-emitting lead-free double-perovskite phosphors with an internal PLQY approaching 100

White light-emitting diode (WLEDs), acting as a new generation of solid-state lighting, play a critical role in energy conservation. Red-emitting phosphors with high efficiency could effectively improve the quality of WLED devices. In this report, Eu3+-doped Ca2ScTaO6 luminescent materials have been successfully synthesized by a high-temperature solid-state method. Its crystal structure was confirmed to be a monoclinic lead-free double-perovskite material system with the space group P21/n by the X-ray diffraction patterns. The strongest emission peak was about 614 nm distributed to the 5D0 → 7F2 electric-dipole transition. Additionally, the optimal doping concentration was found to be 40 mol%, and the concentration quenching mechanism is assigned to d-d interactions. The Ca2ScTaO6:Eu3+ phosphors exhibited an ultrahigh internal quantum yield (about 100%) with good thermal stability (81.5% at 423 K compared with the emission intensity at 303 K). Furthermore, a WLED with a suitable correlated color temperature (4201 K) and a color rendering index (87.62) was fabricated. The phosphor-based polydimethylsiloxane light-emitting flexible film exhibited good luminescence, which is suitable to be utilized in flexible displays. The obtained results suggest that the high-efficiency red-emitting Ca2ScTaO6:Eu3+ phosphors are promising commercial candidates for use in near-ultraviolet-excited WLEDs.

Highly Bright and Stable Lead-Free Double Perovskite White Light-Emitting Diodes

Lead-free double perovskites (DPs) are emerging highly stable emitters with efficient broadband self-trapped exciton (STE) photoluminescence (PL), but their low electroluminescent (EL) efficiency is a critical shortcoming. This work promotes the external quantum efficiency (EQE) and luminance of DP-based white light-emitting diode (wLED) with a normal device structure to 0.76% and 2793 cd m-2 via two modifications: This work prevents the formation of adverse metallic silver, spatially confined STE, and lowers local site symmetry in Cs2 Na0.4 Ag0.6 In0.97 Bi0.03 Cl6 DP by terbium doping; and this work develops a guest-host strategy to improve film morphology, reduce defect density, and increase carrier mobility. These alterations cause substantial increase in STE radiative recombination and charge injection efficiency of perovskite layer. Finally, pure white EL with ideal color coordinates of (0.328, 0.329) and a record-breaking optoelectronic performance is achieved by introducing additional green carbon dots in LED to fill the deficient green component.

Energy transfer and tunable emission in BaSrGd4O8:Bi3+,Eu3+ phosphors for warm WLED

In this work, a series of BaSrGd4O8:xBi3+ blue phosphors was synthesized employing the high-temperature solid-state method. Phase purity of the samples was verified by X-ray diffraction and Rietveld refinement. Time-resolved photoluminescence spectra revealed the existence of two distinct Bi sites. Subsequent optimization of dopant types and doping levels in the batch led to an almost twofold increase in quantum efficiency. The introduction of Eu3+ into the phosphors facilitated the construction of an energy transfer pathway. As the concentration of Eu3+ was increased, the emission color changed from blue to purple and finally to red. In addition, the thermal stability and potential applications of the phosphors were extensively investigated. Finally, two WLED devices were successfully fabricated with color rendering indices of 96.27 and 92.18, and correlated color temperatures of 5198 and 2475 K. This underscores the prospective application of these phosphors in the field of high-quality warm WLEDs.

Limited energy migration and circumscribed multiphonon relaxation produced non-concentration quenching in a novel dazzling red-emitting phosphor

White LED applications are still constrained by extremely efficient narrow band red emitting phosphors. Meanwhile, the concentration quenching induced by energy migration is the main reason that limits the emission intensity of a red emitting phosphor. Therefore, developing a novel red emitting material with energy migration limitations seems necessary. Here, we proposed and realized the non-concentration quenching doping of Eu3+ ions in a Sr9Y2-2xW4O24:xEu3+ (0 ≤ x ≤ 1.0) phosphor for the first time by means of host preferential selection. By clearly investigating the crystal structure and luminescence kinetics, the long-distance between the nearby Eu3+ ions and the low phonon energy are the main reasons that suppress the energy migration and the cross-relaxation among Eu3+ ions. These advantages result in a high internal (90.47%) and external quantum efficiency (42.1%) of Sr9Eu2W4O24. With the help of the Judd-Ofelt theory and the large value of oscillator strength Ω2, Eu3+ ions are verified to occupy the non-symmetric lattice site with high color purity (94.4%). In addition, only 5.2% emission intensity loss at 140 °C can guarantee its application in LED devices. Moreover, the SYWO:Eu3+ phosphor has high thermal tolerance, high color stability, excellent moisture resistance and superior physical/chemical stability, and thus has broad practical spectral application prospects. The prepared WLED shows superior performance, and the calculated NTSC values are as high as 101.8% and 104.7%, respectively. For comparison, the optical performances of the Sr9Y2W4O24:Eu3+ phosphor outperform those of the standard commercial red phosphors, Y2O3:Eu3+ and Y2O2S:Eu3+, and almost match that of K2MnF6. These results may pave the way for fresh approaches to the study of high-performance Eu3+-activated phosphors.

High-Quantum-Efficiency Blue Phosphors with Superior Thermal Stability Derived from Eu3+-Doped Faujasite Y Zeolite

Blue phosphors of high efficiency and superior thermal stability constitute the critical component for achieving high-quality white light-emitting diodes (WLEDs). Herein, we report a highly efficient blue-emitting phosphor with superior thermal stability by heating Eu3+-doped Faujasite Y zeolite under a reducing atmosphere. The intensity and peak value of the phosphor are highly dependent on calcination temperature, and the intensity of PLE and PL spectra reaches a maximum at 1100 °C. Under the excitation of 360 nm, the phosphor shows a high quantum efficiency (90%) and thermal stability (the emission intensity at 423 K is about 125% of that at room temperature). WLEDs fabricated using this blue phosphor, a yellow Eu2+-SOD phosphor, and a commercially available red Sr2Si5N8:Eu2+ phosphor exhibit an excellent optical performance with a correlated color temperature of 4359 K and a color rendering index of 97. This work provides a new strategy for the synthesis of phosphors with high thermal stability and luminous efficiency.

Concentration quenching inhibition and fluorescence enhancement in Eu3+-doped molybdate red phosphors with two-phase mixing

Red phosphor plays a crucial role in improving the quality of white light illumination and backlight displays. However, significant challenges remain to enhance red emission intensity in different matrix materials. Herein, a class of two-phase mixing red phosphors of NaIn1-x(MoO4)2:xEu3+ (NIMO:xEu3+) has been successfully prepared by the traditional high-temperature solid-state reaction method. The coordination environment, phase structure, excitation and emission spectra, fluorescence kinetics, and temperature-dependent luminescence properties of the system have been studied comprehensively. It is worth mentioning that the red emission intensity continues to increase with the increased Eu3+ doping concentration, and the fluorescence lifetimes remain unchanged. These extraordinary phenomena mainly stem from the special concentration quenching mechanism in such two-phase mixing material, namely, the increased lattice interface barriers from Eu six-coordinated units and Eu eight-coordinated units can effectively block the non-radiation by enlarging the average distance between luminescent centers. The improved fluorescence thermal stability and suppressed non-radiative transition rate in NIMO:40%Eu3+ sample are further proving regulatory role of lattice interface barriers. In addition, a warm white light-emitting diode (LED) is successfully fabricated, exhibiting Commission Internationale de l'Eclairage (CIE) coordinates of (0.343, 0.335), a color rendering index (CRI) of 92.1, and a correlated color temperature (CCT) of 5013 K, showing significant application prospects for high-quality lighting devices.

Highly efficient near-infrared solid solution phosphors with excellent thermal stability and tunable spectra for pc-LED light sources toward NIR spectroscopy applications

Near-infrared (NIR) luminescent materials have attracted wide research interest due to their unique photophysical properties for designing NIR light-emitting diodes (NIR LEDs). Here, a series of Cr3+-activated NIR-emitting solid solution phosphors, Gd1-xLux(Al1-xScx)3(BO3)4:0.01Cr3+ (GLASB:Cr3+) (x = 0 to 0.5), are successfully synthesized via a cosubstitution approach. The GLASB:Cr3+ phosphors reveal extraordinary optical performance with a desirable high IQE of 93.6%, considerable broadened FWHM (from 128 nm to 196 nm) and redshift of 119 nm (747 → 866 nm) as the amount of [Lu3+-Sc3+] ion doping increases. Moreover, their photoluminescent thermal stability is substantially improved, maintaining 105.7% of the initial integral intensity up to 150 °C, namely zero-thermal-quenching. The NIR pc-LED fabricated using the GLASB:Cr3+ phosphor generates an NIR output power of 46 mW and an electro-optical efficiency of 37% at a 120 mA input current. Finally, the characteristic NIR emission of this phosphor can not only be utilized in the fields of night-vision technology and biometric identification, but also exhibits a perfect match with the absorption of the bacteriochlorophyll (BChl) and light-harvesting protein (LHP) of photosynthetic bacteria (PSB), presenting a high application prospect for improving PSB photosynthesis.

Suppressed Concentration Quenching Brightens Short-Wave Infrared Emitters

The brightness of doped luminescent materials is usually limited by the ubiquitous concentration quenching phenomenon resulting in an intractable tradeoff between internal quantum efficiency and excitation efficiency. Here, an intrinsic suppression of concentration quenching in sensitized luminescent systems, by exploiting the competitive relationship between light emitters and quenchers in trapping excitation energies from sensitizers, is reported. Although Cr3+ sensitizers and trivalent lanthanide (Ln3+ , Ln = Yb, Nd, and Er) emitters themselves are highly susceptible to concentration quenching, the unprecedentedly high-brightness luminescence of Cr3+ -Ln3+ systems is demonstrated in the short-wave infrared (SWIR) range employing high concentrations of Cr3+ , whereby a record photoelectric efficiency of 23% is achieved for SWIR phosphor-converted light-emitting diodes, which is about twice as high as those previously reported. The results underscore the beneficial role of emitters in terminating excitation energies, opening up a new dimension for developing efficient sensitized luminescent materials.

Investigation of the luminescence properties and energy transfer mechanisms in Gd3TaO7:Bi3+,Eu3+ phosphors for their potential application in full-spectrum WLEDs

In recent years, there has been increasing effort devoted to the development of single-phase white phosphors due to drawbacks such as severe reabsorption and color deviation in traditional white light-emitting diodes (WLEDs). A new feasible strategy has emerged for achieving white light emission through the Bi3+-Eu3+ energy transfer in suitable single-phase phosphors. Therefore, a series of Gd3TaO7:xBi3+ and Gd3TaO7:0.01Bi3+,yEu3+ phosphors were synthesized via a high-temperature solid-state method, and their properties were systematically characterized. In Gd3TaO7, Bi3+ occupies two kinds of Gd3+ site, resulting in two broad emission bands peaking at 427 nm and 500 nm respectively under ultraviolet (UV) excitation, which arise from 3P1 → 1S0 transitions. By adjusting the concentration of Eu3+ in Gd3TaO7:0.01Bi3+,yEu3+, effective energy transfer can occur between Bi3+ and Eu3+, thus enabling the regulation of green-white-red luminescence under 332 nm excitation and blue-white-red luminescence under 365 nm UV light irradiation. Upon stimulation with a 365 nm UV chip, Gd3TaO7:0.01Bi3+,0.02Eu3+ emits white light with CIE coordinates of (0.3509, 0.3202), a color temperature of 4629 K, and an impressive color rendering index of 87.96. The above results indicate the potential of Gd3TaO7:0.01Bi3+,yEu3+ phosphor as a viable candidate for WLED applications.

High quantum efficiency and excellent color purity of red-emitting Eu3+-heavily doped Gd(BO2)3-Y3BO6-GdBO3 phosphors for NUV-pumped WLED applications

Eu3+-doped phosphors have been much attractive owing to their narrow-band red emission peak at 610-630 nm with high color purity; however, the weak and narrow absorption band in the NUV region limits their applications. Doping a higher amount of Eu3+ ions into a non-concentration quenching host could be key to enhancing the efficiency of the absorption value and emission intensity. Hence, the design of Eu3+-heavily doped phosphors with a suitable host lattice is key for applications. In this study, red-emitting Eu3+-doped Gd(BO2)3-Y3BO6-GdBO3 (GdYGd:Eu3+) phosphor with a high quantum efficiency of 58.4% and excellent color purity of 99.5% is reported for the first time. The phosphor is efficiently excited by NUV light at 394 nm and emits a strong red emission band in the 590-710 nm range, peaking at 612 nm. The optimal annealing temperature and Eu3+ doping content to obtain the strongest PL intensity are 1100 °C and 20 mol%, respectively. The optimized GdYGd:Eu3+ phosphor possesses a high activation energy of 0.319 eV and a lifetime of 1.14 ms. An illustration of phosphor-coated NUV LED with chromaticity coordinates (x = 0.5636,y = 0.2961) was successfully synthesized, demonstrating the great potential of GdYGd:Eu3+ phosphor for NUV-pumped WLED applications.

Light- and Chemical-Doping-Induced Magnetic Behavior of Eu Molecular Systems

Variable temperature electron paramagnetic resonance (VT-EPR) was used to investigate the role of the environment and oxidation states of several coordinated Eu compounds. We find that while Eu(III) chelating complexes are diamagnetic, simple chemical reduction results in the formation of paramagnetic species. In agreement with the distorted D3h symmetry of Eu molecular complexes investigated in this study, the EPR spectrum of reduced complexes showed axially symmetric signals (g⊥ = 2.001 and g∥ = 1.994) that were successfully simulated with two Eu isotopes with nuclear spin 5/2 (151Eu and 153Eu with 48% and 52% natural abundance, respectively) and nuclear g-factors 151Eu/153Eu = 2.27. Illumination of water-soluble complex Eu(dipic)3 at 4 K led to the ligand-to-metal charge transfer (LMCT) that resulted in the formation of Eu(II) in a rhombic environment (gx = 2.006, gy = 1.995, gz = 1.988). The existence of LMCT affects the luminescence of Eu(dipic)3, and pre-reduction of the complex to Eu(II)(dipic)3 reversibly reduces red luminescence with the appearance of a weak CT blue luminescence. Furthermore, encapsulation of a large portion of the dipic ligand with Cucurbit[7]uril, a pumpkin-shaped macrocycle, inhibited ligand-to-metal charge transfer, preventing the formation of Eu(II) upon illumination.

Novel Red-Emitting Eu3+-Doped Y2(WxMo1-xO4)3 Phosphor with High Conversion Efficiency for Lighting and Display Applications

In this study, a series of trivalent europium-doped tungstate and molybdate samples were synthesized using an improved sol-gel and high-temperature solid-state reaction method. The samples had different W/Mo ratios and were calcined at various temperatures ranging from 800 to 1000 °C. The effects of these variables on the crystal structure and photoluminescence characteristics of the samples were investigated. It was found that a doping concentration of 50% for europium yielded the best quantum efficiency based on previous research. The crystal structures were found to be dependent on the W/Mo ratio and calcination temperature. Samples with x ≤ 0.5 had a monoclinic lattice structure that did not change with calcination temperature. Samples with x > 0.75 had a tetragonal structure that remained unchanged with calcination temperature. However, samples with x = 0.75 had their crystal structure solely dependent on the calcination temperature. At 800-900 °C, the crystal structure was tetragonal, while at 1000 °C, it was monoclinic. Photoluminescence behavior was found to correlate with crystal structure and grain size. The tetragonal structure had significantly higher internal quantum efficiency than the monoclinic structure, and smaller grain size had higher internal quantum efficiency than larger grain size. External quantum efficiency initially increased with increasing grain size and then decreased. The highest external quantum efficiency was observed at a calcination temperature of 900 °C. These findings provide insight into the factors affecting the crystal structure and photoluminescence behavior in trivalent europium-doped tungstate and molybdate systems.

Lumino-structural properties of Dy3+ activated Na3Ba2LaNb10O30 phosphors with enhanced internal quantum yield for w-LEDs

With an intend to develop white light emitting phosphor, for w-LED application, a series of dysprosium (Dy³⁺) doped novel Na₃Ba₂LaNb₁₀O₃₀ phosphors were prepared using solid state reaction technique at 1300 °C. Their structural, morphological and vibrational spectroscopic analysis was performed. We illustrate the luminescence characteristics of the prepared phosphors for various Dy³⁺ ion doping concentration. The XRD analysis demonstrates that the prepared phosphors were in single phase, and of tetragonal tungsten bronze structure of the P4bm space group. The FE-SEM image reveals that the prepared phosphors contained irregular shaped both nano and micro particles. Under near-ultraviolet (n-UV) irradiation at 387 nm, the photoluminescence (PL) emission spectra shows three characteristic bands at 481 nm (blue), 575 nm (yellow) and 666 nm (red). Obtained optimized Dy³⁺ ion concentration for the prepared sample is 7.0 mol%, beyond which the concentration quenching begins. Bonding between Dy-O is covalent in nature as confirmed by bonding parameters and the Dexter theory revealed that the energy transfer among Dy³⁺ ions is dipole-diploe interaction. CIE chromaticity coordinates, CCT and color purity confirms the formation of warm white light emitting phosphors. Lifetime analysis demonstrates the longer decay time in the phosphors. The Internal Quantum Yield (IQE) and brightness (B) for the optimised phosphor is calculated as 45.35% and 11.41% respectively, which makes it a suitable phosphor for w-LED.

Multimodal Luminescent Low-Dimension Cs2ZrCl6:xSb3+ Crystals for White Light-Emitting Diodes and Information Encryption

Low-dimension perovskite materials have attracted wide attention due to their excellent optical properties and stability. Herein, Sb³⁺-doped Cs₂ZrCl₆ crystals are synthesized by a coprecipitation method in which Sb³⁺ ions partially replace Zr⁴⁺ ions. The Cs₂ZrCl₆:xSb³⁺ powder shows blue and orange-red emissions under a 254 and 365 nm light, respectively, due to the [ZrCl₆]^2- octahedron and [SbCl₆]^3- octahedron. The photoluminescence quantum yield (PLQY) of Cs₂ZrCl₆:xSb³⁺ (x = 0.1) crystals is up to 52.5%. According to experimental and computational results, the emission mechanism of the Cs₂ZrCl₆:xSb³⁺ crystals is proposed. On the one hand, a wide blue emission with a large Stokes shift is caused by the self-trapping excitons of [ZrCl₆]^2- octahedra under a 260 nm excitation. On the other hand, the luminescence mechanism of [SbCl₆]^3- octahedron is divided into two parts: ¹P₁ → ¹S₀ (490 nm) and ³P₁ → ¹S₀ (625 nm). The broad-band emission, high PLQY, and excellent stability endow the Cs₂ZrCl₆:xSb³⁺ powders with the potential for the fabrication of white light-emitting diodes (WLEDs). A WLED device is fabricated using a commercial 310 nm NUV chip, which shows a high color rendering index of 89.7 and a correlated color temperature of 5333 K. In addition, the synthesized Cs₂ZrCl₆:xSb³⁺ crystals can be also successfully used for information encryption. Our work will provide a deep understanding of the photophysical properties of Sb³⁺-doped perovskites and facilitate the development of Cs₂ZrCl₆:xSb³⁺ crystals in encrypting multilevel optical codes and WLEDs.

Efficient red luminescence in Eu3+ doped CdSe/CdS all-inorganic quantum dots shows great potential for wLEDs

Complete inorganic quantum dots (QDs) CdSe/CdS:Eu³⁺ with full transmittance were proposed as red color converters for white light emitting diodes (wLEDs) using a facile one-step melt quenching method. TEM, XPS, and XRD were used to verify the successful nucleation of CdSe/CdS:Eu³⁺ QDs in silicate glass. The results indicated that the incorporation of Eu contributed to the nucleation of CdSe/CdS QDs in silicate glass, where the nucleation time of the CdSe/CdS:Eu³⁺ QDs rapidly decreased in 1 h compared with other inorganic QDs that took more than 15 h. CdSe/CdS:Eu³⁺ inorganic QDs exhibited bright and long-term stable red luminescence under both UV and blue light excitation; up to 53.5% quantum yield and 8.05 ms fluorescence lifetime were obtained by adjusting the Eu³⁺ concentration. Based on the luminescence performance and absorption spectra, a possible luminescence mechanism was proposed. Moreover, the application potential of the CdSe/CdS:Eu³⁺ QDs in wLEDs was studied by coupling the CdSe/CdS:Eu³⁺ QDs and commercial Intematix G2762 green phosphor on a InGaN blue LED chip. Warm white light (5217 K) with 89.5 CRI and 91.1 lm W^-1 luminous efficacy could be achieved. Additionally, 91% of the NTSC color gamut was obtained, demonstrating the great potential of the CdSe/CdS:Eu³⁺ inorganic QDs as a color converter for wLEDs.

Broadband-excited and green-red tunable emission in Eu2+-sensitized Ca8MnTb(PO4)7 phosphors induced by structural-confined cascade energy transfer

Novel green-red color-tunable Ca₈(Mg,Mn)Tb(PO₄)₇:Eu²⁺ phosphors have been synthesized via the traditional solid-state method. Since Tb³⁺/Mn²⁺ ions are the parent ions in the lattice, the structural confinement occurs when the sensitizer Eu²⁺ is introduced into the Ca₈(Mg,Mn)Tb(PO₄)₇:Eu²⁺ structure. The distance from Eu²⁺ to Tb³⁺/Mn²⁺ is confined in the 5 Å range, which induces a highly efficient energy transfer process. At Eu²⁺ 350 nm excitation, Ca₈MgTb(PO₄)₇:Eu²⁺ shows dominant Tb³⁺ green emission with almost-vanished Eu²⁺ emission. Red emission is clearly observed as Mn²⁺ ions doping into Ca₈MgTb(PO₄)₇:Eu²⁺, and color-tuning from green to red is realized by varying the Mn²⁺ contents. Eu²⁺-Tb³⁺-Mn²⁺ cascade energy transfer process is in effect due to short Eu²⁺-Tb³⁺/Mn²⁺ and Tb³⁺-Mn²⁺ distances, which is verified by PL and decay variations. Meanwhile, the Ca₈(Mg,Mn)Tb(PO₄)₇:Eu²⁺ phosphor indicates good thermal stability and maintained the 45% emission level at 150 °C, which demonstrates their potential applications in white light LEDs.

Broadband UV-Excitation and Red/Far-Red Emission Materials for Plant Growth: Tunable Spectrum Conversion in Eu3+,Mn4+ Co-doped LaAl0.7Ga0.3O3 Phosphors

Broadband ultraviolet (UV) excitation and red/far-red emission phosphors can effectively convert solar spectrum to enhance photosynthesis and promote morphogenesis in plants. Based on the above application requirements, Eu³⁺ single-doped LaAl_1-yGa_yO₃ solid solutions and Eu³⁺,Mn⁴⁺ codoped LaAl_0.7Ga_0.3O₃ phosphors were designed and synthesized in this work. The LaAl_0.7Ga_0.3O₃:0.05Eu³⁺ (LAG:Eu³⁺) phosphor exhibits a strong charge transfer band (CTB) excitation and characteristic ⁵D₀ → ⁷F₂ transition red emission (619 nm), which is very similar to the luminescence properties of Eu³⁺-organic ligand compound (EuL₃). Rietveld refinement studies further revealed that the cation substitution disturbs the site symmetry. The optimal Eu³⁺, Mn⁴⁺ co-doped LaAl_0.7Ga_0.3O₃ (LAG:Eu,Mn) phosphor possesses a dual-band excitation spectrum in broadband ultraviolet (UVA, UVB) area and a dual-band emission spectrum within red/far-red area. Under the sunlight radiation, the real-time spectrum of luminous laminated glasses fabricated by coating the LAG:Eu,Mn phosphor shows the percentage of radiant intensity in the red/far-red region significantly increases, suggesting that the phosphor can be a promising candidate for solar spectral conversion in plant cultivation. We believe this work provides a new idea for developing novel broadband ultraviolet excitation and red/far-red emission phosphors.

Highly-efficient Eu2+-activated Sr8Si4O12Cl8 cyan-emitting phosphors with zero-thermal quenching luminescence for versatile applications

To settle the problem of phosphors with unsatisfactory luminescence efficiency and serious thermal quenching, Eu²⁺-activated Sr₈Si₄O₁₂Cl₈ cyan-emitting phosphors were designed. Excited at 387 nm, a dazzling cyan emission located at 492 nm is observed in the resultant phosphors and its maximum intensity is obtained when the Eu²⁺ content is 4 mol%. Moreover, the zero-thermal quenching luminescence, even when the temperature is 503 K, the integrated emission intensity still maintains 106% of its starting value at 303 K, is realized in resultant phosphors because of the efficient energy transfer from defect levels to Eu²⁺, which is confirmed by the thermoluminescence spectrum. The electroluminescence spectrum of the packaged white light-emitting diode (white-LED) is detected and it is found to possess a high color rendering index (91.0), low correlated color temperature (4875 K) and a superior luminous efficiency (68.7 lm W^-1), implying that the developed phosphors can be adopted as cyan-emitting components to fulfill the cyan gap and realize a full spectrum white-LED. Furthermore, the cathodoluminescence (CL) performance of samples is also studied, in which its CL emission intensity is greatly impacted by the accelerating voltage and the filament current. Additionally, using the synthesized phosphors, various types of patterns are designed for use in information encryption. These achievements reveal that the Eu²⁺-activated Sr₈Si₄O₁₂Cl₈ phosphors are multifunctional cyan-emitting candidates for full spectrum white-LED, field-emission display and anti-counterfeiting applications.

Eu3+ -activated red phosphor Ca3 YAl3 B4 O15 with low thermal quenching behaviour

This paper reports a sequence of a Ca₃ YAl₃ B₄ O₁₅ :xEu³⁺ red phosphor prepared using a high-temperature solid-state reaction. At the excitation of 396 nm, the samples emitted intense red emission centred at ~623 nm, which could be attributed to the ⁵ D₀ →⁷ F₂ transition of the Eu³⁺ ion. The results showed that the optimum Eu³⁺ doping concentration of Ca₃ YAl₃ B₄ O₁₅ :Eu³⁺ phosphor was x = 80 mol%, and the concentration quenching mechanism of Ca₃ YAl₃ B₄ O₁₅ :Eu³⁺ red phosphor belonged to the exchange coupling between Eu³⁺ ions. The Commission Internationale de l'éclairage (CIE) coordinates and colour purity of Ca₃ Y_0.2 Al₃ B₄ O₁₅ :0.8Eu³⁺ were calculated as (0.6375, 0.3476) and 95.5%, respectively. Moreover, the red emission of the obtained phosphor Ca₃ YAl₃ B₄ O₁₅ :0.8Eu³⁺ exhibited a low thermal quenching behaviour with an intensity retention rate of 92.85% at 150°C. The above results manifest that the Eu³⁺ -activated Ca₃ YAl₃ B₄ O₁₅ phosphor is predicted to be a promising red luminescent component for white light-emitting diodes.

Investigation of multicolor emitting Cs3GdGe3O9:Bi3+,Eu3+ phosphors via energy transfer for WLEDs

Bi³⁺/Eu³⁺ doped Cs₃GdGe₃O₉ luminescent materials were prepared by a solid-state reaction. The energy band and density of states of Cs₃GdGe₃O₉ were calculated by density functional theory. The Cs₃GdGe₃O₉ host presents a broadband emission peaking at 520 nm. Systemic measurement and analysis of luminescence properties were performed to confirm the energy transfer in Cs₃GdGe₃O₉:Bi³⁺,Eu³⁺. The multicolor modulated emission from blue (0.1678, 0.1568) to red (0.5931, 0.3251) can be achieved by varying the doping ratio of bismuth to europium. A white light-emitting diode (WLED) was produced by combining the Cs₃GdGe₃O₉:0.05Bi³⁺,0.1Eu³⁺ phosphor, a commercial green phosphor, and a 310 nm ultraviolet chip. The color rendering index of the WLED driven by 20 mA bias current is 89.6 with the CIE coordinates of (0.3520, 0.3626). The results reveal that the Cs₃GdGe₃O₉:Bi³⁺,Eu³⁺ phosphor is a potential material that can be used in multicolor tunable luminescence and WLEDs.

The synergism between self-activated and impurity-related emissions of LiCa3ZnV3O12: lattice distortion, energy transfer and temperature sensing effect

Some of the metal vanadates have special self-activated luminescence. In order to further enrich its luminous color, luminescent impurity ions can be introduced into its lattice. The interaction between the self-activated emission and the impurity-related emission remains to be studied. In this work, the synergism between the two kinds of emission in LiCa₃ZnV₃O₁₂ was explored from these three aspects: lattice distortion, energy transfer and temperature effect. Eu³⁺ ions replace Ca²⁺ ions in the lattice of LiCa₃ZnV₃O₁₂, leading to a lattice contraction of the LCZV host, which depresses the self-activating emission around 500 nm. The characteristic linear emissions of Eu³⁺ ions are also observed benefiting from the energy transfer from [VO₄]^3- to Eu³⁺. Since the temperature quenching effect is more sensitive for the self-activated emission than that for the Eu³⁺-related ones, the phosphor can be applied as a luminescent temperature sensor, with the absolute and relative temperature sensitivities of 0.012 K^-1 and 1.56% K^-1, respectively.

Room-Temperature Synthesis of Highly-Efficient Eu3+-Activated KGd2F7 Red-Emitting Nanoparticles for White Light-Emitting Diode

Luminescent materials with high thermal stability and quantum efficiency are extensively desired for indoor illumination. In this research, a series of Eu³⁺-activated KGd₂F₇ red-emitting nanoparticles were prepared at room temperature and their phase structure, morphology, luminescence properties, as well as thermal stability, have been studied in detail. Excited by 393 nm, the resultant nanoparticles emitted bright red emissions and its optimal status was realized when the Eu³⁺ content was 30 mol%, in which the concentration quenching mechanism was triggered by electric dipole-dipole interaction. Through theoretical analysis via the Judd-Ofelt theory, one knows that Eu³⁺ situates at the high symmetry sites in as-prepared nanoparticles. Moreover, the internal and extra quantum efficiencies of designed nanoparticles were dependent on Eu³⁺ content. Furthermore, the studied nanoparticles also had splendid thermal stability and the corresponding activation energy was 0.18 eV. Additionally, via employing the designed nanoparticles as red-emitting constituents, a warm white light-emitting diode (white-LED), which exhibits low correlated color temperature (4456 K), proper luminous efficiency (17.2 lm/W) and high color rendering index (88.3), was developed. Our findings illustrate that Eu³⁺-activated KGd₂F₇ nanoparticles with bright red emissions are able to be used to promote the performance of white-LED.

Recent Developments of Microscopic Study for Lanthanide and Manganese Doped Luminescent Materials

Luminescent materials are indispensable for applications in lighting, displays and photovoltaics, which can transfer, absorb, store and utilize light energy. Their performance is closely related with their size and morphologies, exact atomic arrangement, and local configuration about photofunctional centers. Advanced electron microscopy-based techniques have enabled the possibility to study nanostructures with atomic resolution. Especially, with the advanced micro-electro-mechanical systems, it is able to characterize the luminescent materials at the atomic scale under various environments, providing a deep understanding of the luminescent mechanism. Accordingly, this review summarizes the recent achievements of microscopic study to directly image the microstructure and local environment of activators in lanthanide and manganese (Ln/Mn²⁺ )-doped luminescent materials, including: 1) bulk materials, the typical systems are nitride/oxynitride phosphors; and 2) nanomaterials, such as nanocrystals (hexagonal-phase NaLnF₄ and perovskite) and 2D nanosheets (Ca₂ Ta₃ O₁₀ and MoS₂ ). Finally, the challenges and limitations are highlighted, and some possible solutions to facilitate the developments of advanced luminescent materials are provided.

Enhanced Photoluminescence of Gd3Al4GaO12: Cr3+ by Energy Transfers from Co-Doped Dy3

LEDs for plant lighting have attracted wide attention and phosphors with good stability and deep-red emission are urgently needed. Novel Cr3+ and Dy3+ co-doped Gd3Al4GaO12 garnet (GAGG) phosphors were successfully prepared through a conventional solid-state reaction. Using blue LEDs, a broadband deep-red emission at 650−850 nm was obtained due to the Cr3+ 4T2 → 4A2 transition. When the Cr3+ concentration was fixed to 0.1 mol, the crystal structure did not change with an increase in the Dy3+ doping concentration. The luminous intensity of the optimized GAGG:0.1Cr3+, 0.01Dy3+ was 1.4 times that of the single-doped GAGG:0.1Cr3+. Due to the energy transfer from Dy3+ to Cr3+, the internal quantum efficiency reached 86.7%. The energy transfer from Dy3+ to Cr3+ can be demonstrated through luminescence spectra and fluorescence decay. The excellent properties of the synthesized phosphor indicate promising applications in the agricultural industry.

Achieving stable photoluminescence by double thiacalix[4]arene-capping: the lanthanide-oxo cluster core matters

Luminescence stability is a critical consideration for applying phosphors in practical devices. In this work, we report two categories of double p-tert-butylthiacalix[4]arene (H₄TC4A) capped clusters that exhibit characteristic lanthanide luminescence. Specifically, {[Ln₄(μ₄-OH)(TC4A)₂(DMF)₆(CH₃OH)₃(HCOO)Cl₂]}·xCH₃OH (Ln = Eu (1), Tb (2); x = 0–1) with square-planar [Ln₄(μ₄-OH)] cluster cores and {[Ln₉(μ₅-OH)₂(μ₃-OH)₈(OCH₃) (TC4A)₂ (H₂O)₂₄Cl₉]}·xDMF (Ln = Gd (3), Tb (4), Dy (5); x = 2–6) with hourglass-like [Ln₉(μ₅-OH)₂(μ₃-OH)₈] cluster cores are synthesized and characterized. By comparing 2 and 4, we find that several critical luminescence properties (such as quantum efficiency and luminescence stabilities) depend directly on the cluster core structure. With the square-planar [Ln₄(μ₄-OH)] cluster cores, 2 demonstrates high quantum yield (∼65%) and excellent luminescence stability against moisture, high temperature, and UV-radiation. A white light-emitting diode (LED) with ultrahigh color quality is successfully fabricated by mixing 2 with commercial phosphors. These results imply that high quality phosphors might be achieved by exploiting the double thiacalix[4]arene-capping strategy, with an emphasis on the cluster core structure.

{Ln₄} cores outperform {Ln₉} cores in achieving stable photoluminescence from double thiacalix[4]arene-capped lanthanide-oxo clusters.

Tuning of the thermal quenching performance of Bi3+-doped scheelite Ca(Mo/W)O4 solid solution phosphors

The utilization of phosphor materials has always been a significant challenge in terms of improving thermal quenching performance. In this work, the thermal quenching performance tuning mechanism which establishes the band gap and thermal quenching correlation patterns is proposed. The crystal field splitting energy D_q was decreased by changing the surrounding crystal lattice environment of Bi³⁺ through a solid solution replacement, and the thermal quenching activation energy ΔE of Bi³⁺ was tuned from 0.117 eV to 0.182 eV accordingly. At 423 K, the luminous intensity increases from 0.101 to 0.396 of the preliminary intensity at 303 K with increasing substitution. In addition, the band gap value of Bi³⁺ calculated by diffuse reflectance spectroscopy increased from 4.40 eV to 4.72 eV, which corresponds to a linear positive correlation between the band gap and the thermal quenching properties. Furthermore, a monophase white-emitting phosphor with good thermal stability was prepared by constructing a Bi³⁺-Eu³⁺ co-doping system. In particular, the relative sensitivity of S_r for temperature measurement applications reached 3.17% K^-1 based on the double-luminescence fluorescence intensity ratio. Thus, this modulation scheme can be used as a reference for the design of various phosphor materials with tunable thermal quenching properties in the future.

Environment-Dependent Eu2+-Activated Ba3CaK(PO4)3 toward White-Light Emission by Chemical Cosubstitution of the (BO3)3- Anion Group

Investigating the phosphors doped with single activators in a single component to realize white-light emission is urgently desired for phosphor-converted white-light-emitting diodes. In this work, on the basis of the chemical unit cosubstitution strategy, the new borophosphate phosphors Ba₃CaK(PO₄)_3-x(BO₃)_x:0.02Eu²⁺ with a mixed anion group were prepared. Coupling structure refinement and photoluminescence analyses, Ba₃CaK(PO₄)_3-x(BO₃)_x consists of five different cationic sites with different coordination environments, with Eu²⁺ occupying the three sites for Ba²⁺. In the process of partial substituting (BO₃)^3- for (PO₄)^3-, because of the greatly distorted coordination field generated from the difference in the geometric configurations between the two anion groups, a red shift and broadening of the emission bands occurs, resulting in a color-adjustable emission from blue to white. A phosphor-converted light-emitting diode has been successfully fabricated with the incorporation of an as-prepared Ba₃CaK(PO₄)_2.6(BO₃)_0.4:0.02Eu²⁺ phosphor and a 405 nm near-ultraviolet chip, which exhibits Commission International de I'Eclairage chromaticity coordinates of (0.31, 0.37) and a correlated color temperature of 6295 K. As demonstrated in the present work, an approach adopted from phosphate to borophosphate is conducted to develop high-quality phosphors.

A Triple-Doped Phosphor Strategy for the Conversion of Cool and Warm White Light

This work proposes a new type of Eu²⁺, Ce³⁺, Mn²⁺ codoped strategy that can be adapted to both ultraviolet (UV) and blue chips to achieve high-quality white light illumination. Primarily, the target sample was confirmed by X-ray diffraction (XRD) and Rietveld refinement, and the surface morphology and element distribution were observed by scanning electron microscopy (SEM). Second, the energy transfer behavior and mechanism were determined by studying double-doped samples. Lu₂Mg₂Al₂Si₂O₁₂: Eu²⁺,Ce³⁺ (LMAS: Eu²⁺,Ce³⁺) can realize an emission color adjustment from blue to yellow. The emission color of LMAS: Ce³⁺,Mn²⁺ can be adjusted from light yellow to orange yellow. Afterward, the triple-doped sample exhibits full-spectrum emission under the excitation at 365 nm, and yellow emission under the excitation at 450 nm. When combined with a 365 nm chip, the obtained light-emitting diode (LED) devices can achieve warm white light with a color rendering index (Ra) of 96.6, light emission (LE) of 1.79 lm/W, and correlated color temperature (CCT) of 4874 K. When this phosphor was combined with a 460 nm chip, cold white light with Ra = 70, LE = 13.57 lm/W, and CCT = 5782 K can be achieved. Finally, according to the properties of the phosphor, a conceptual diagram of a new type parallel device was designed, which can easily and effectively realize the conversion of cold and warm white light. This work provides a new idea for the design of single-substrate white light phosphor and proposes a new parallel device concept, which is expected to be applied in the field of lighting.

High-Efficiency Continuous-Luminescence-Controllable Performance and Antithermal Quenching in Bi3+-Activated Phosphors

Recently, Bi³⁺-activated phosphors have been widely researched for phosphor-converted light-emitting diode (pc-LED) applications. Herein, novel full-spectrum A₃BO₇:Bi³⁺ (A = Gd, La; B = Sb, Nb) phosphors with a luminescence-tunable performance were achieved by a chemical substitution strategy. In the La₃SbO₇ host material, a new luminescent center was introduced, with Gd³⁺ replacing La³⁺. The photoluminescence (PL) spectra show a large blue shift from 520 to 445 nm, thus achieving regulation from green to blue lights. Moreover, a series of solid solution-phase phosphors La₃Sb_1-xNb_xO₇:Bi³⁺ were prepared by replacing Sb with Nb, and a PL spectral tunability from green (520 nm) to orange-red (592 nm) was realized. Temperature-dependent PL spectra show that La_3-xGd_xSbO₇:Bi³⁺ phosphors have excellent thermal stability. Upon 350 nm excitation, the PL intensity of La_3-xGd_xSbO₇:Bi³⁺ phosphors at 150 °C remained at more than 93% at room temperature. With Gd³⁺ doping, the thermal stability gradually improved, and LaGd₂SbO₇:0.03Bi³⁺ represents splendid antithermal quenching (135.2% at 150 °C). Finally, a full-visible spectrum for pc-LED with a high color-rendering index (Ra = 94.4) was obtained. These results indicated that chemical substitution is an effective strategy to adjust the PL of Bi³⁺, which is of great significance in white-light illumination and accurate plant lighting.

Thermal Enhancement of Luminescence for Negative Thermal Expansion in Molecular Materials

Overcoming thermal quenching is an essential issue in the practical application of luminescent materials. Herein, we found that negative thermal expansion (NTE) can achieve the thermal enhancement of luminescence in molecular materials based on three metal-organic frameworks CuX-bpy (X = Cl, Br, I; bpy = 4,4'-bipyridine). All complexes exhibit NTE on the c-axis, and the strongest NTE leads to a contraction of the Cu...Cu distance in CuCl-bpy, which further intensifies the luminescence emission. This phenomenon indicates the existence of thermally enhanced charge transfer. Moreover, the origin of the distinction in charge transfer attributed to the different valence states of the copper is investigated through the combined studies of X-ray photoelectron spectroscopy, X-ray absorption near-edge structure, and density functional theory calculations. This research provides a new approach to modulating the luminescence thermal enhancement by NTE.

Highly efficient Ce: Lu(Mg,Al)2(Si,Al)3O12 phosphor ceramics for high-power white LEDs/LDs

Lu₃Al₅O₁₂: Ce³⁺ (LuAG: Ce³⁺) phosphor ceramics (PCs) with high quantum efficiency and excellent thermal stability are incredibly promising color converters for high-power white light emitting diodes (LEDs)/ laser diodes (LDs) lighting. However, the greenish emission of LuAG:Ce³⁺ PCs does not allow to reach white light emission upon pumping by a blue LED/ LD without an additional red luminescent material. In this work, a series of (Ce_0.003Lu_0.997)₃(MgxAl_1-2xSix)₅O₁₂ (LCMASG) (x = 0-0.15) PCs were fabricated by solid state reaction method. Impressively, the as-prepared PCs exhibited a distinct red-shift (513→538nm) and a 17% increase of the color index (CRI) of high-power white LED(58.4→70.4). Particularly, Ce: Lu(Mg, Al)₂(Si, Al)₃O₁₂ PC with 15 at.% substitution concentration showed only 8% luminescent intensity loss at 150 °C and high internal quantum efficiency (IQE) of 82%, exhibiting desirable optical thermal stability. By combining with a 460 nm blue chip or a 455 nm laser source, white LED/LD devices based on the LCMASG PCs in a remote excitation mode were constructed. The optimized luminous efficiency of Ce: Lu(Mg, Al)₂(Si, Al)₃O₁₂ PC with 15 at.% Mg²⁺/Si⁴⁺ doping up to 176.4 lm/W was obtained as the power density of the blue laser increased to 6.52 W/mm². Also, a 4053K CCT of the warm white light emission was realized. Therefore, this work proves that the LCMASG PCs are promising to serve as color converters for high power LEDs/LDs lighting in the future.

Utilizing the energy transfer mechanism to realize color tunable luminescence

Bi³⁺/Eu³⁺ co-doped phosphors can realize multi-color luminescence by adjusting the concentration ratio, which makes it possible to manually control the emission color. On this basis, a series of Bi³⁺/Eu³⁺ co-doped phosphors SrLaGa₃O₇:xBi³⁺,yEu³⁺ were prepared. The existence of energy transfer from Bi³⁺ to Eu³⁺ was verified by spectral analysis. The emission spectra of SrLaGa₃O₇:xBi³⁺ show a wide-band peaking at 472 nm. For SrLaGa₃O₇:xBi³⁺,yEu³⁺, the Bi³⁺ → Eu³⁺ energy transfer occurs and a series of sharp emitting peaks of Eu³⁺ can be observed simultaneously. The relative luminescence intensity of Bi³⁺ and Eu³⁺ can be modulated by changing the relative concentrations of Bi³⁺ and Eu³⁺. Using this mechanism, the color tunable luminescence of SrLaGa₃O₇:xBi³⁺,yEu³⁺ from cyan, through white to orange, and finally to red is realized. By using a 320 nm UV chip and SLG:0.06Bi³⁺,0.07Eu³⁺ white phosphor, a white light-emitting diode (WLED) lamp was fabricated with chromaticity coordinates of (0.3199, 0.3083) and a color rendering index R_a of 82. This indicates that the prepared sample is a very promising candidate in the LED field.

Synthesis of the Red-Emitting (Ba, Ca)2ScAlO5:Eu3+ Phosphors with Photoluminescence Properties

The light-emitting diodes (LED) are regarded as one of the most promising devices for inexpensive and widely used illumination; in particular, they are highly dependent on the development of red-emitting phosphors. Herein, we developed two types of red-emitting (Ba, Ca)2ScAlO5:Eu3+ multiple excitations phosphors (λex = 255-465 nm) via freeze-drying followed by calcination. Powder X-ray diffraction and NMR results point out that they have hexagonal space group P63/mmc (194), and the structural framework is composed of multi-coordinated Al3+-O2- polyhedron and Sc3+-O2- polyhedron. In addition, the valence state of europium (Eu3+) is confirmed by X-ray photoelectron spectroscopy characterization. Investigation on the photoluminescence properties showed that the photoluminescence process of (Ba, Ca)2ScAlO5:Eu3+ is attributable to the charge transfer band of Eu-O and abundant spectral terms of Eu3+. The α-(Ba, Ca)2ScAlO5:Eu3+ and β-(Ba, Ca)2ScAlO5:Eu3+ exhibited red emission under 465 and 395 nm excitation, respectively. The PL spectra and decay curves explain the intrinsic photoluminescence mechanism. The strongest emission peaks of the red-emitting α-(Ba, Ca)2ScAlO5:Eu3+ and β-(Ba, Ca)2ScAlO5:Eu3+ phosphors are at 615 and 619 nm, respectively, exhibiting a high fluorescence of 64 and 67% under the temperature of 423 K (150 °C). Further exploration of the red-emitting phosphors would provide a variety of choices for the design of red LEDs and white LEDs for the solid-state lighting system.

Thermally boosted upconversion and downshifting luminescence in Sc2(MoO4)3:Yb/Er with two-dimensional negative thermal expansion

Rare earth (RE³⁺)-doped phosphors generally suffer from thermal quenching, in which their photoluminescence (PL) intensities decrease at high temperatures. Herein, we report a class of unique two-dimensional negative-thermal-expansion phosphor of Sc₂(MoO₄)₃:Yb/Er. By virtue of the reduced distances between sensitizers and emitters as well as confined energy migration with increasing the temperature, a 45-fold enhancement of green upconversion (UC) luminescence and a 450-fold enhancement of near-infrared downshifting (DS) luminescence of Er³⁺ are achieved upon raising the temperature from 298 to 773 K. The thermally boosted UC and DS luminescence mechanism is systematically investigated through in situ temperature-dependent Raman spectroscopy, synchrotron X-ray diffraction and PL dynamics. Moreover, the luminescence lifetime of ⁴I_13/2 of Er³⁺ in Sc₂(MoO₄)₃:Yb/Er displays a strong temperature dependence, enabling luminescence thermometry with the highest relative sensitivity of 12.3%/K at 298 K and low temperature uncertainty of 0.11 K at 623 K. These findings may gain a vital insight into the design of negative-thermal-expansion RE³⁺-doped phosphors for versatile applications.

Rare-earth doped phosphors with negative thermal expansion (NTE) may display thermally-enhanced emission, but their performance is generally limited. Here the authors report thermally-boosted green upconversion luminescence and near-infrared downshifting luminescence in Sc₂(MoO₄)₃:Yb/Er phosphors with two-dimensional NTE, and their application in temperature sensing.

Intense Luminescence and Good Thermal Stability in a Mn2+-Activated Mg-Based Phosphor with Self-Reduction

White light-emitting diodes provide widespread applications in lighting, electronic equipment, and high-tech displays. However, thermal quenching effect severely limits their practical application. Here, we developed an orange-red phosphor β-KMg(PO₃)₃:Mn²⁺, which emits bright orange-red light when excited by ultraviolet light without the energy transfer of sensitizer, owing to the strong crystal field provided by β-KMg(PO₃)₃ for Mn²⁺. The self-reduction of Mn⁴⁺ → Mn²⁺ and good thermal stability have been realized in an ambient atmosphere. The defect types were verified by X-ray photoelectron spectroscopy, and cationic vacancy plays a significant role in the self-reduction of Mn⁴⁺ → Mn²⁺. Furthermore, the properties of the trap energy levels were studied by thermoluminescence. The recombination luminescence of the detrapped carriers released from the deep trap levels at high temperatures suppresses the luminescence loss of thermal quenching. Moreover, the trap energy levels play an important role in the mechanoluminescence of β-KMg(PO₃)₃:Mn²⁺. This work emphasizes the significance of the defects in the luminescent characteristics and opens up a new approach for the development of advanced optical functional materials.