Near-ultraviolet LED Research Articles

Ultrabroadband Long-Wavelength Near-Infrared MgIn2O4:Ni2+ Phosphor Synthesized via Sol-gel Combustion Method as a Light Source for Night Vision Imaging, Nonvisual Detection, and Anticounterfeiting Display.

Long-wavelength near-infrared (LWNIR) imaging technology has exciting application potential across various fields due to its ability of deeper penetration and unique properties related to its emission wavelength, when compared to short-wavelength near-infrared imaging. However, the limited availability of materials for LWNIR light sources, due to the lack of suitable host materials that constitute luminescence centers, has been a major challenge and technical obstacle in realizing such applications. Here, we developed MgIn2O4:Ni2+ phosphors with an antispinel structure and LWNIR luminescence properties through a sol-gel combustion method. Under excitation at 365 nm, its emission wavelength covers the range of 1000-2000 nm, with a peak emission at approximately 1520 nm, a full width at half-maximum of ∼340 nm, and an optimized photoluminescence quantum yield of ∼21.22%, when an optimal Ni2+ doping content of 1 mol % was used. Studies on the crystal structure of MgIn2O4 have shown that Ni2+ ions preferentially replace the lattice position occupied by Mg2+ ions in the [MgO6] octahedrons, which provides a crystal field microenvironment of weak strength to the Ni2+ luminescence centers and promotes their LWNIR emission with a large Stokes shift. A LWNIR pc-LED device was assembled using the optimized MgIn2O4:Ni2+ phosphor and a near-ultraviolet LED chip (@ 365 nm), and its potential applications, including NIR night vision imaging, nonvisual detection, and anticounterfeiting displays, were demonstrated. Our results show that the antispinel MgIn2O4:Ni2+ phosphor prepared by the sol-gel combustion method is a promising LWNIR luminescence material.

Cyan to orange color tunable emitting of Ca2GdHf2Al3O12: Ce3+, Mn2+ phosphors via energy transfer

Recently, the realization of spectrum-tunable single-phase phosphors has become the focus of phosphor-converted light-emitting diodes (pc-LEDs) research. Herein, a novel color-tunable Ca2GdHf2Al3O12 (CGHAO): Ce3+, Mn2+ single-phase phosphor was prepared. A thorough investigation was conducted into the crystal structure, site occupancy, morphology, luminescent characteristics, energy transfer (ET) process, and thermal quenching. These phosphors show broadband excitation between 200 and 460 nm matching commercially available near-ultraviolet LED chips (380–420 nm). The ratio of dopant ion concentration can be varied to achieve cyan-to-orange chromaticity adjustment due to the Ce3+ → Mn2+ effective ET. By confirming that the ET mechanism is controlled by dipole-quadrupole interaction with 42.64 % efficiency. In particular, under excitation at 405 nm, the optimal sample CGHAO: 5%Ce3+, 15%Mn2+ emits bright yellow light with an IQE of 84.35 %, and has good thermal (Ea = 0.271 eV) and chromatic stability (ΔE = 1.35 × 10−2 at 473K). Based on these findings, color-tunable CGHAO: Ce3+, Mn2+ phosphor could be considered potential candidates for pc-LEDs device applications.

Simultaneously boosting visible and near-infrared luminescence of Ho in Cs2AgInCl6 double perovskites via Bi3+ sensitization

The emergence of near-infrared (NIR) luminescence from lanthanide (Ln3+) ions doped lead-free Cs2AgInCl6 double perovskites (DPs) has garnered significant attention due to its extensive range of potential applications. Unfortunately, they still suffer from weak NIR emission due to the inherent wide band gap of DPs resulting in inefficient energy transfer from the host to Ln3+. Herein, Bi3+ can act as the sensitizer to activate Ho3+ emission and reduce the excitation energy in Cs2AgInCl6: Ho. Under 400 nm excitation, Bi3+ co-doped Cs2AgInCl6: Ho shows 39 times and 21 times higher emission intensity than Cs2AgInCl6: Ho in Visible and NIR. As a result, a photoluminescence quantum yield of 18.63 % ranging from 800 to 1600 nm can be obtained. Finally, by coupling a Bi-doped sample with a commercial near-ultraviolet LED chip (395 nm), a compact Vis-NIR ultrawideband light source was prepared, which has promising applications in multifunctional illumination and night vision.

A Nickel-doped, lanthanum gallium germanate-based phosphor as an ultra-broadband short-wavelength infrared region emitter for optical imaging

Short-wavelength infrared region light source (SWIR, λ = 1000–2000 nm) is useful for optical imaging of biological tissues for lack of auto-fluorescence, low intrinsic absorption, and low scattering. Here, we report the synthesis, characterization, and mechanistic studies of a Ni2+-doped, lanthanum gallium germanate-based phosphor, La3Ga5GeO14:Ni2+, that emits SWIR with an ultra-broad bandwidth emission of 1000–2000 nm, a full-width at half maximum (FWHM) of 340 nm, which covers near infrared-II and III regions. The analysis of crystal structure suggests that Ga3+ ions in [GaO6] octahedron lattices are replaced by Ni2+ ions, possibly due to the similar radius shared by both ions. The observed ultra-broadband SWIR luminescence with large Stokes shift is most likely originated from the influence of a strong crystal field on the extent of the cleavage of energy levels of Ni2+ ions, which is supported by the charge density data of [GaO6] octahedra in La3Ga5GeO14 derived from state density functional calculation. A SWIR pc-LED was constructed by packaging the La3Ga5GeO14:Ni2+ phosphor with a near-ultraviolet LED chip, and its potential application in optical imaging of human bodies was explored.

Novel green/red oxyfluoride phosphors with excellent optical properties for near-UV excited white light emitting diode

Novel eye-sensitive Ba3Nb2O2F12(H2O)2:Tb3+ green and Ba3Nb2O2F12(H2O)2:Mn4+ red oxyfluoride phosphors with extremely strong absorption in the UV region were designed and synthesized by simple co-precipitation strategy. Particularly, Tb3+ ions were doped in this matrix for the first time, which greatly improves their absorption efficiency in the near ultraviolet region (367 nm) and emits sharp green light (544 nm). In addition, the Ba3Nb2O2F12(H2O)2:Mn4+ red phosphors have strong zero phonon line (ZPL) emission at 625 nm, which is conducive to improving the sensitivity of human eye and color purity. Meanwhile, the optical properties of the red phosphor are significantly enhanced via doping K+ cations as charge compensators. Crystal field environment and nephelauxetic effect of the as-prepared phosphors before and after K+ cation doping were systematically analyzed. Moreover, these synthesized red/green phosphors have good thermal stability and moisture resistance. Remarkably, the as-prepared Ba3Nb2O2F12(H2O)2:5%Mn4+ or K0.9Ba2.1Nb2O2F12(H2O)2:5%Mn4+ red phosphors can be directly mixed with the as-synthesized Ba3Nb2O2F12(H2O)2:13%Tb3+ green phosphor coating on 365 nm near-ultraviolet LED chip to package WLED devices with excellent electroluminescence performance. These findings are conducive to opening an avenue for screening the unique structure of optical materials.

Sol-gel combustion synthesis and near-infrared luminescence of Ni2+-doped MgAl2O4 spinel phosphor

Near-infrared phosphor converted light-emitting diode (NIR pc-LED) has many advantages over traditional NIR light sources, and has broad application prospects in night vision surveillance, non-destructive testing, bioimaging, and other fields. Phosphors are typically synthesized by the high-temperature solid-phase method as chunks or large granules, which need to be ground to small particles for practical application. This process introduces surface defects and reduces the luminous efficiency of phosphors. In this work, near-infrared phosphors composed of spinel MgAl2O4:Ni2+ with particle size less than 100 nm was synthesized in high-purity by sol-gel combustion method using nitrate salts with citric acid were used as raw materials. Under 365 nm excitation, MgAl2O4:Ni2+ phosphors emit broadband near-infrared luminescence of 1000–1600 nm with a width at half maximum of 237 nm. The optimal Ni2+ doping concentration was determined to be 1.5 mol% and the optimal sintering temperature was 1200 °C. The thermal activation energy of this phosphor was found to be 0.409 eV and the phosphor had good luminescence thermal stability. An NIR pc-LED device was assembled from the phosphor and commercially available near-ultraviolet LED, which showed a current-dependent increased of NIR luminescence up to 160 mA and an efficiency drop appeared with further increase of the current, suggesting a thermal dependent quenching may have occurred. On the other hand, the output of NIR luminescence increased nearly linearly with the increased of current when a 360 nm laser diode was used as the excitation light source, indicating the NIR phosphor has a very high fluorescence saturation threshold. Our study provides the utility for the sol-gel method in the preparation and the potential application of the nanoscale NIR-II phosphor materials.

High external quantum yield in near-infrared phosphor Bi2Ga4O9:Cr3+ excited by near-ultraviolet or blue light

The proliferation of portable and intelligent devices featuring Near-Infrared (NIR) light sources as a central component has resulted in a significant need for compact NIR light sources. The technique of NIR phosphor-converted light-emitting diodes (NIR-pc-LEDs) could offer a fresh viewpoint on NIR light sources. The performance of NIR-pc-LEDs is directly influenced by NIR phosphors. However, the development of NIR phosphors with high external quantum yield (EQY) NIR phosphors has been hampered. In this work, we report a mullite-structure NIR phosphor, Bi2Ga4O9: Cr3+ (BGO), which exhibits excellent NIR luminescence properties. The emission spectrum of the optimal doped sample BGO:0.06Cr3+ has two emission peaks at 711 and 763 nm. All samples can be efficiently excited by commercial 365 nm near-ultraviolet LED chips and 450 nm blue LED chips. The internal quantum yield (IQY) reaches an astonishing value of 89.13 % and an EQY of about 65.36 % at 365 nm excitation. Furthermore, BGO:0.06Cr3+ shows a high IQY (∼87.78 %) and EQY (∼41.64 %) upon 450 nm excitation. Moreover, concerning thermal stability, the integrated photoluminescence intensity remains at 82 % and 42 % of that at room temperature when measured at 373 K and 423 K, respectively. The capsuled NIR-pc-LED device can be utilized for blood veins imaging, food safety analysis, and information encryption. These results highlight the promising commercial prospects of BGO:0.06Cr3+ in the fields of bio-imaging and non-destructive testing, among others.

Engineering deep-red Al20B4O36:Cr3+ phosphors for photomorphogenesis

Cr3+ doped phosphors with deep-red luminescence hold great promise as artificial lighting sources for plants growth. Among them, α-Al2O3:Cr3+ is featured with an ultra-intense pure zero phonon line (ZPL) emission with negligible sidebands. Nonetheless, further enhancements in its luminescence thermal stability are required to fully unlock its potential for practical applications. To tackle this challenge, we propose that Al20B4O36 with a negative expansion coefficient could serve as a more favorable host materials for Cr3+ ion, thereby enhancing the luminescence thermal stability. Our first-principles calculations have unveiled similar electronic structures for α-Al2O3:Cr3+ and Al20B4O36:Cr3+, suggesting that the observed zero phonon R-line (2Eg) emission, arising from the 2Eg→4A2g transition in α-Al2O3:Cr3+, might also be present in Al20B4O36:Cr3+. Based on these findings, we prepared Al20B4O36:Cr3+ phosphors using a simple solid reaction method and confirmed that Al20B4O36:Cr3+ indeed exhibits a similar ultra-intense pure ZPL emission centered at 694 nm. Moreover, Al20B4O36:Cr3+ phosphors demonstrate remarkable properties including a high internal quantum efficiency (IQE) of 98.2 % and superior luminescence thermal stability compared to α-Al2O3:Cr3+. A phosphor converted light emitting diode (pc-LED) is developed by coating Al20B4O36:Cr3+ phosphor onto a near-ultraviolet LED chip. The lighting from this deep-red LED significantly impacts the photomorphogenesis of bean sprouts. Our study showcases a rational approach to design novel Cr3+ doped phosphors with enhanced luminescent properties, tailored for specific applications.

Full-visible-spectrum LED lighting by using a near-UV-excitable broadband blue-emitting Ca2LuHf2GaAl2O12:Ce3+ garnet phosphor

Highly efficient broadband blue-emitting phosphors are urgently needed for full-visible-spectrum white light-emitting diodes (WLEDs). Herein, we report a new highly luminescent garnet-type Ca2LuHf2GaAl2O12:Ce3+ (abbreviated as: CLHGAO:Ce3+) blue phosphor enabling full-visible-spectrum LED lighting. These CLHGAO:Ce3+ phosphors doped with different Ce3+ concentrations have been prepared by using a high-temperature solid-state reaction approach, and their phase purity, crystal structure, luminescent properties, CIE color coordinates, and quantum efficiency have been systematically studied. Importantly, CLHGAO:Ce3+ blue phosphors show a broadband excitation spectrum in the 300–450 nm spectral range peaking around 398 nm, matching well with the emission wavelengths of commercial near-ultraviolet LED chips (380–420 nm). Upon 398 nm excitation, the optimal CLHGAO:2%Ce3+ phosphor sample generates a bright broad blue emission band (emission peak: 472 nm; bandwidth: 84 nm) with high internal quantum efficiency of 79.2% and external quantum efficiency of 55.3%. Notably, the emission intensity of CLHGAO:2%Ce3+ phosphor is about 1.87 times higher compared to the commercial BaMgAl10O17:Eu2+ blue phosphor (emission peak: 451 nm; bandwidth: 53 nm). Finally, a white LED device is fabricated with a 395 nm near-UV LED chip with CLHGAO:2%Ce3+ blue phosphor and commercial (Ba,Sr)2SiO4:Eu2+ green phosphor as well as commercial CaAlSiN3:Eu2+ red phosphor, and the device demonstrates bright warm-white emission with an ultrahigh color rendering index (Ra = 95.1, R9 = 97.8, R12 = 88.4) and low correlated color temperature of 3540 K. These results show that as-developed blue-emitting CLHGAO:Ce3+ garnet phosphors with superior photoluminescence properties can potentially be used in phosphor-converted full-visible-spectrum WLEDs.

The effect of different AlGaN insertion layer thicknesses on the photoelectric properties of InGaN/AlGaN near UV light emitting diodes

In this paper, a near-ultraviolet LED structure was fabricated on a sapphire substrate by using metal–organic chemical vapor deposition with an undoped AlGaN insertion layer introduced between multiple quantum wells and electron blocking layers, and the effect of layer thickness on light-electric performance was investigated. Results of epitaxial structure characterization show that the introduction of an undoped AlGaN insertion layer almost does not affect the crystal quality and surface morphology of the LED epitaxial structure. Electroluminescence testing results indicate that as the insertion layer thickness increases, the device performance initially increases and then decreases. Furthermore, simulation analysis suggests that increasing the insertion layer thickness reduces the injection efficiency of carriers, lowers the radiative recombination rate and light emission performance of the active region's carriers. Additionally, the AlGaN insertion layer can effectively suppress the diffusion of Mg impurities, reduce the non-radiative recombination rate in the multiple quantum well active region, and thus improve the LED's light emission performance. Overall, under the competing effects of these two mechanisms, the optimal light emission performance for the near-ultraviolet LED is achieved when the insertion layer thickness is 1 nm.

Effect of oxygen vacancy on photoluminescence properties of Dy3+ doped K2BaCa(PO4)2 phosphor and remote luminescence multi-layer for white light

Dy3+ ions singly-doped phosphors, emerging as single-component white light-emitting phosphors, have been regarded as a promising candidate for white light generation under ultraviolet (UV) or near-ultraviolet (NUV) excitation. Here, a series of Dy3+ doped K2BaCa(PO4)2 phosphors with single phase was successfully synthesized by solid-state reaction method. Under NUV excitation, K2BaCa(PO4)2: Dy3+ phosphors exhibits the desire blue emission at 490 nm and yellow emission at 580 nm, attributed to the 4F9/2 → 6H15/2 and 4F9/2 → 6H13/2 transitions of Dy3+ ions, respectively. Interestingly, the broad emission peak centered at 458 nm can also be found and assigned to the oxygen vacancy in K2BaCa(PO4)2 host lattice. Significantly, the emission intensity of the phosphor is greatly enhanced to 4.1 times higher than that of the sample without oxygen vacancy due to the energy transfer from oxygen vacancy to Dy3+ ions. K2BaCa(PO4)2: 0.03Dy3+ phosphor with high quantum yield of 76.9% exhibits good thermal stability with the relative emission intensity of 95.7% and 88.6% at 323 K and 373 K, respectively. Subsequently, the white light with correlated color temperature (CCT) of 4521 K and color rendering index (CRI) of 79 can be achieved for the optimized K2BaCa(PO4)2: 0.03Dy3+ phosphor packaged with NUV LED chip (365 nm). Further, the remote luminescence multi-layer based on K2BaCa(PO4)2: 0.03Dy3+ phosphors is fabricated on glass substrate by screen printing technology and exhibit better heat dissipation performance than that of the conventional NUV LEDs, which can be a promising candidate of single-component phosphors for the manufacture of white LEDs with adjustable remote luminescent layers.

Systemic development of Mn4+-activated double perovskite red phosphors for plant growth applications

Developing efficient and stable red phosphor has always been considered as important academic research of LED plant-growth lamps. In this study, Mn4+-activated double perovskite red phosphors, A2BB’O6:Mn4+ (A = Sr, Ba; B = Lu, Gd; B’ = Nb, Ta), were successfully synthesized by solid-phase method. To elucidate the structure-activity relationship between the double perovskite structure and the luminescence of Mn4+, the crystal structure, electronic structure and luminescence properties of Sr2LuNbO6:Mn4+ were first studied in detail using X-ray powder diffraction data, density functional theory calculations and spectroscopy data. The coordination environment of Mn4+ was analyzed by investigating the crystal-field parameters, Racah parameters and nephelauxetic parameter. Then, the effect of structural evolution on the luminescence of Mn4+ was investigated by regulating the local coordination environment of Mn4+ through cationic modification. The results demonstrate that Mn4+ enters the octahedral lattice site with a strong crystal field and covalent environment, which can be excited by a near-ultraviolet LED chip to emit narrow-band far-red light in the double perovskite oxides. Moreover, the Mn–O bond length, octahedral distortion and crystal-system type all affect the luminescence of Mn4+. This systemic development provides some guidance for the exploration of novel Mn4+-activated double perovskite red phosphors.

Luminescence investigation of Eu2+/Ce3+/Mn2+ doped Y2Mg2Al2Si2O12 phosphors

To investigate the energy transfers and tunable luminescence, Eu2+/Ce3+/Mn2+-doped Y2Mg2Al2Si2O12 phosphors were synthesized by sintering at a temperature of 1400 °C. The phase and luminescence characteristics were investigated subsequently. The XRD results indicate that the synthesized phosphors have the pure phase and garnet structure. Eu2+/Ce3+-, Eu2+/Mn2+- and Eu2+/Ce3+/Mn2+-doped phosphors show tunable luminescence because of the energy transfer processes from Eu2+ to Ce3+ and Mn2+. White lights with different correlated color temperatures are generated by the synthesized phosphors. The dependence of the emission intensity on temperature shows that the thermal quenching temperature of the Y1.955Eu0.030Ce0·015Mg1·985Mn0·015Al2Si2O12 phosphor is higher than 175 °C. Our results demonstrate that the synthesized phosphors can be applied to near-ultraviolet LED chip-excited white-light-emitting diodes.

Rationalized crystal structure augmented highly efficient far-red-emitting double perovskite niobate phosphor for indoor plant growth LED applications

Since the inception of white light-emitting diodes (white LEDs), phosphor-coated LEDs have been in focus owing to the tremendous advantageous that they offer in assorted applications ranging from biomedical to automotive applications, including plant growth. While, plant growth LEDs have been bustling the commercial market, there is ample scope to commercialize the promising far-red (FR) emitting LEDs for potential growth of plants in indoor farming. Herein, a facile and efficient strategy was used to develop Mn4+ (Nb site)- and Sm3+ (La site)- ions-doped Ca2LaNbO6 (CLNO:Mn4+/Sm3+) phosphors with outstanding photoluminescence (PL) properties. The PL properties of CLNO:Mn4+ and CLNO:Sm3+ phosphors showed FR emissions at 650 and 697 nm, respectively. When Mn4+/Sm3+ ions were co-doped in a CLNO host lattice, Rietveld refinement revealed alterations in bond lengths of the CLNO crystal structure, which enhanced the phosphor’s thermal stability and photostability and caused the creation of effective pathways for efficient energy transfer from Sm3+ to Mn4+ ions. Furthermore, the reverse strategy, where the sensitizer (Sm3+) concentration was changed instead of activator (Mn4+) concentration, was used to achieve brilliant FR emission from the CLNO:Mn4+/Sm3+ phosphors and the dominant emission was perceived for the CLNO:0.2Mn4+/1.5Sm3+ phosphor. The internal quantum efficiency of the CLNO:0.2Mn4+/1.5Sm3+ phosphor was increased from 28% (CLNO:0.2Mn4+) to 56%. An LED fabricated by coating a CLNO:Mn4+/Sm3+ phosphor layer on a near-ultraviolet LED chip exhibited excellent FR emission with high color purity and with chromaticity coordinates of (0.733, 0.267). The results obtained for the CLNO:0.2Mn4+/1.5Sm3+ phosphor along with the characteristics of fabricated LED showed the high potential of the phosphor for use in promoting indoor plant growth.

Luminescent properties and energy transfer mechanism from Tb3+ to Eu3+ in single-phase color-adjustable Sr3MgSi2O8:Eu3+,Tb3+ phosphor prepared by the sol-gel method

A series of single-phase color-adjustable Sr3MgSi2O8:Eu3+,2Tb3+ phosphors were synthesized with the application of sol-gel method. The crystal structure, luminescence characteristics, fluorescence lifetime, energy transfer mechanism and thermal stability of phosphors were investigated. The results show that the emission spectrum of Sr3−x−yMgSi2O8:xEu3+,yTb3+ compose both the 5D4→7F5 transition of Tb3+ and the 5D0→7F1, 5D0→7F2 transition of Eu3+ at the excitation wavelength of 238 nm. The optimal doping concentration of Tb3+ was determined to be 2% when concentration quenching effect was comprehensively considered. On the basis of the energy transfer of Tb3+→Eu3+, the emission peak intensity of Eu3+ increases and that of Tb3+ decreases with the rising Eu3+ concentration. The unidirectional energy transfer efficiency of Tb3+→Eu3+ reaches 89.4% when the Eu3+ doping concentration is 8%. On the basis of Dexter's theory, the major mechanism of energy transfer lies in the electric dipoles interaction between Tb3+ and Eu3+. With an activation energy of 0.309 eV, Sr2.95MgSi2O8:3%Eu3+,2%Tb3+ phosphors enjoys good thermal stability. Its color coordinate diagram indicates that the luminous colors of phosphors can be customized from red to orange, then to green with the changing doping concentrations of Eu3+ and Tb3+. Based on Sr3MgSi2O8:Eu3+,Tb3+ phosphors has a good matchability with ultraviolet and near-ultraviolet LED chips, so it can be considered that Sr3MgSi2O8:Eu3+,Tb3+ phosphors have good application potential in the field of ultraviolet and near-ultraviolet white light emitting diodes.

Improvement in the Output Power of Near-Ultraviolet LEDs of p-GaN Nanorods through SiO2 Nanosphere Mask Lithography with the Dip-Coating Method.

In this paper, the conditions of the dip-coating method of SiO2 nanospheres are optimized, and a neatly arranged single-layer SiO2 array is obtained. On this basis, a “top-down” inductively coupled plasma (ICP) technique is used to etch the p-GaN layer to prepare a periodic triangular nanopore array. After the etching is completed, the compressive stress in the epitaxial wafer sample is released to a certain extent. Then, die processing is performed on the etched LED epitaxial wafer samples. The LED chip with an etching depth of 150 nm has the highest overall luminous efficiency. Under a 100 mA injection current, the light output power (LOP) of the etched 150 nm sample is 23.61% higher than that of the original unetched sample.

Turning on the luminescence of Sr5(PO4)3Cl: Eu2+ by doping of La, Gd and Lu ions for full-spectrum lighting application

Blue phosphor Sr5−x-yRy(PO4)3Cl:xEu2+ (R = La, Gd, Lu) was prepared via solid-state reaction process, in which R3+ replaces part of Sr2+ ions in the matrix to benefit Eu2+ luminescence. The change of Eu2+/Eu3+ values were studied by XPS, and the results shown that the incorporation of R3+ in the matrix resulted in the values increase of Eu2+/Eu3+. And substitute R3+ non-equivalently for Sr2+, the modified Sr5−x-yRy(PO4)3Cl:xEu2+ phosphors show better luminescence intensity and thermal stability. The emission intensity of Sr4.72Lu0.2(PO4)3Cl:0.08Eu2+ is 1.97 times of Sr4.95(PO4)3Cl:0.05Eu2+. Moreover, the thermal quenching of Eu2+ photoluminescence could be reduced. Compared to the commercial BAM:Eu2+ phosphor, the stabilities of all Sr4.72R0.2(PO4)3Cl:0.08Eu2+(R=La3+, Gd3+ and Lu3+) phosphors are better than that of BAM:Eu2+ at the temperatures below 100 °C. The luminescent intensity of Sr4.72Lu0.2(PO4)3Cl:0.08Eu2+ maintains 95% while BAM:Eu2+ is only 88% of the initial intensity at 150 °C. By combining Sr4.72Lu0.2(PO4)3Cl:0.08Eu2+, Y3(Al,Ga)5O12:Ce3+ and CaAlSiN3:Eu2+ with 400 nm near-ultraviolet LED chip, a white-light-emitting diode was obtained. The fabricated WLED device with CIE color coordinates x = 0.3896, y = 0.3788, low CCT (3775 K) and high Ra (95.1), has enormous potential application in full-spectrum lighting field.

Solvatochromic and solid-state emissive azlactone-based AIEE-active organic dye: Synthesis, photophysical properties and color-conversion LED application

Here, we report one-pot synthesis of a new azlactone-based dye bearing isoxazole moiety as a donor group (i.e., Dye-Iso), which displays fluorescence emission both in solution and solid-state, and a systematic study of its photophysical properties. The analyses of steady-state photoluminescence (PL) spectroscopy measurements reveal that Dye-Iso exhibits positive solvatochromic behavior accompanied by the decreasing trend in PL quantum yield (QY), as the polarity of solvent increases. In addition, time-resolved fluorescence (TRF) spectroscopy shows that the PL lifetime is shortening and overall effective nonradiative decay rate becomes accelerated with increasing polarity of the medium, which explain well the decrease in PL QY. These findings strongly suggest that the dipole-dipole interactions between the dye molecule having intramolecular charge transfer (ICT) characteristic and solvent are responsible for the redshift in PL emission with the increase in solvent polarity. Furthermore, the electronically ground- and excited-state dipole moments were determined by using linear correlation methods having different solvent polarity functions. The results depict that the Dye-Iso has a larger excited-state dipole moment as against that in the ground-state, which reflects the ICT mechanism. The spectroscopic studies show that the Dye-Iso exhibits aggregation-induced emission enhancement (AIEE) under aggregate formation and is AIEE-active compound. The PL QYs of the Dye-Iso in aggregation- and solid-state were also found to be higher in comparison with those in solutions, which has been elucidated by the increase in the radiative decay rate due to restriction of intramolecular motions (RIM). Finally, we have demonstrated that the AIEE-active Dye-Iso can be used as a color-conversion material for solid-state lighting applications by employing that on a commercial near-ultraviolet LED.

The Effect of Package Structure on the Light Extraction Efficiency of Near-Ultraviolet LED

The near-ultraviolet high-power LED, with five package structures, is designed and fabricated. The efficiency of electric conversion to light (EECL) of various package structures is measured, and the EECL is used to characterize the light extraction efficiency (LEE) of the package structure. By analyzing the Fresnel loss (FNL) and total inner reflection loss (TIRL) of light at different interfaces, the experimental results are explained qualitatively. When approximate spherical lens is used, filling silica gel between chip and lens can improve the LEE. The EECL of the device can reach 66.84%, which is higher than that of bare chip by 55.10%. In planar packaging, the EECL of the device is lower than that of the bare chip, whether or not it is filled with silica gel. After filling with silica gel, a new TIRL is produced at the final interface of the device, and the EECL of the device is even lower, which is only 37.14%. The experimental results show that when the LED chip adopts graphic substrate and surface microstructure to improve the LEE, the traditional coating of silica gel (or epoxy) and other materials on the chip surface may not improve the LEE of the device. If the introduction of the silica gel layer leads to a new total inner reflection interface, it will result in a significant decrease in LEE of the device. When a layer, such as silica gel, does not result in a new total reflection interface, the FNL and TIRL of the chip surface can be effectively reduced, and the LEE of the LED device can be improved.

Site occupancy and tunable photoluminescence properties of Eu2+-Activated Ba3Sc(BO3)3 phosphors for white light emitting diodes

A series of emission-tunable Ba3-xSc(BO3)3: xEu2+ (0 ≤ x ≤ 0.15) and Ba2.97-yGdySc1-yMgy(BO3)3: 0.03Eu2+ (0 ≤ y ≤ 0.15) phosphors were synthesized by a solid-state method. Structure analysis of X-ray powder diffraction (XRD) and Rietveld refinement show that Eu2+ ions occupy randomly at Ba sites of hexagonal lattice with space group P63cm. Photoluminescence emission spectra and the decay curves of phosphors demonstrated the existence of multiple Eu2+ emitting centers. Eu2+ ions occupy at four Ba sites of Ba3-xSc(BO3)3: xEu2+ (0.03 ≤ x ≤ 0.15) emit orange light (λem = 670 nm and 560 nm) through 370 nm UV excitation. The broader absorption band, ranging from 220 nm to 550 nm, matches well with the near-ultraviolet LED chips. Furthermore, color tuning from orange to red is successfully achieve by the double doping of Gd3+-Ba2+ and Mg2+-Sc3+ in Ba2.97Sc(BO3)3: 0.03Eu2+. Our results indicate that red emission Ba3Sc(BO3)3: Eu2+ phosphors is an ideal candidate for the application in white light-emitting diodes.