Heng Guo

Ce3+ and Tb3+ doped Ca3Gd(AlO)3(BO3)4 phosphors: synthesis, tunable photoluminescence, thermal stability, and potential application in white LEDs

Novel blue-green-emitting Ca3Gd(AlO)3(BO3)4:Ce3+,Tb3+ phosphors were successfully synthesized via traditional high temperature solid reaction method. X-ray diffraction, luminescence spectroscopy, fluorescence decay time and fluorescent thermal stability tests have been used to characterize the as-prepared samples. The energy transfer from Ce3+ to Tb3+ ions in the Ca3Gd(AlO)3(BO3)4 host has been demonstrated to be by dipole–dipole interaction, and the energy transfer efficiency reached as high as 83.6% for Ca3Gd0.39(AlO)3(BO3)4:0.01Ce3+,0.6Tb3+. The critical distance was calculated to be 9.44 A according to the concentration quenching method. The emission colour of the obtained phosphors can be tuned appropriately from deep blue (0.169, 0.067) to green (0.347, 0.494) through increasing the doping concentrations of Tb3+. Moreover, the Ca3Gd0.39(AlO)3(BO3)4:0.01Ce3+,0.6Tb3+ phosphor possessed excellent thermal stability at high temperature, and the emission intensity at 423 K was about 87% of that at 303 K. Finally, the fabricated prototype LED device with a BaMgAl10O7:Eu2+ blue phosphor, CaAlSiN3:Eu2+ red phosphor, Ca3Gd0.39(AlO)3(BO3)4:0.01Ce3+,0.6Tb3+ green phosphor and 365 nm-emitting InGaN chip exhibited bright warm white light. The current study shows that Ca3Gd0.39(AlO)3(BO3)4:0.01Ce3+,0.6Tb3+ can be used as a potential green phosphor for white LEDs.

Novel Mn4+-activated LiLaMgWO6 far-red emitting phosphors: high photoluminescence efficiency, good thermal stability, and potential applications in plant cultivation LEDs

Double perovskite-based LiLaMgWO6:Mn4+ (LLMW:Mn4+) red phosphors were synthesized by traditional solid-state route under high temperature, and they showed bright far-red emission under excitation of 344 nm. The crystal structure, luminescence performance, internal quantum efficiency, fluorescence decay lifetimes, and thermal stability were investigated in detail. All samples exhibited far-red emissions around 713 nm due to the 2Eg → 4A2g transition of Mn4+ under excitation of near-ultraviolet and blue light, and the optimal doping concentration of Mn4+ was about 0.7 mol%. The CIE chromaticity coordinates of the LLMW:0.7% Mn4+ sample were (0.7253, 0.2746), and they were located at the border of the chromaticity diagram, indicating that the phosphors had high color purity. Furthermore, the internal quantum efficiency of LLMW:0.7% Mn4+ phosphors reached up to 69.1%, which was relatively higher than those of the reported Mn4+-doped red phosphors. Moreover, the sample displayed good thermal stability; the emission intensity of LLMW:0.7% Mn4+ phosphors at 423 K was 49% of the initial value at 303 K, while the activation energy was 0.39 eV. Importantly, there was a broad spectral overlap between the emission band of LLMW:Mn4+ phosphors and the absorption band of phytochrome PFR under near-ultraviolet light. All of these properties and phenomena illustrate that the LLMW:Mn4+ phosphors are potential far-red phosphors for applications in plant cultivation LEDs.

Synthesis and photoluminescence properties of Eu3+-activated LiCa3ZnV3O12 white-emitting phosphors

Single-component white-emitting phosphors are highly promising for applications in phosphor-converted white light-emitting diodes. In this paper, novel single-phase LiCa3(1−x)ZnV3O12:Eu3+ (x = 0–0.05) phosphors with tunable white emissions were prepared by a conventional solid-state reaction. The LiCa3ZnV3O12 (x = 0) phosphor showed an efficient self-activated bluish-green emission due to the V5+–O2− charge transfer transition of the [VO4]3− groups, and possessed an intense broad excitation spectrum in the 250–400 nm wavelength range. Together with the [VO4]3− emission, the red emission of Eu3+ ions was also observed in LiCa3(1−x)ZnV3O12:Eu3+ phosphors. The energy transfer from the [VO4]3− groups to the Eu3+ ions was studied. Importantly, the emission colors of LiCa3(1−x)ZnV3O12:Eu3+ phosphors varied from greenish-blue to whitish and then to red with increasing Eu3+ content and the white-light emission was realized in the single-phase phosphor of LiCa3ZnV3O12:0.003Eu3+ by combining the [VO4]3−-emission and the Eu3+-emission. The energy-transfer efficiency from [VO4]3− groups to Eu3+ ions in the LiCa3ZnV3O12:0.003Eu3+ sample was determined to be about 52% and the internal quantum efficiency of the LiCa3ZnV3O12:0.003Eu3+ phosphor was found to be about 41.5%. In addition, the CIE chromaticity coordinates of LiCa3ZnV3O12:0.003Eu3+ were (x = 0.3374, y = 0.3596), and the correlated color temperature was estimated to be about 5311 K.

Synthesis and characterization of Ca3Lu(GaO)3(BO3)4:Ce3+,Tb3+ phosphors: tunable-color emissions, energy transfer, and thermal stability

Blue-green dual-emitting phosphors Ca3Lu(GaO)3(BO3)4:Ce3+,Tb3+ (CLGB:Ce3+,Tb3+) were synthesized via a traditional solid-state reaction method. The phase of the phosphors was characterized by X-ray diffraction and the luminescence properties were investigated using the excitation and emission spectra, decay curves, temperature-dependent emission spectra, CIE chromaticity coordinates, and the internal quantum efficiency. Under 345 nm UV light excitation, Ce3+ singly doped CLGB phosphors presented intense blue light in the 350–550 nm wavelength region with a maximum peak at 400 nm. In sharp contrast, CLGB:Ce3+,Tb3+ phosphors showed both the blue and green emission wavelengths of Ce3+ and Tb3+ ions, respectively. The overall emission colors can be tuned from blue (0.164, 0.042) to green (0.331, 0.485) via increasing the concentration of Tb3+ ions, due to the energy transfer (ET) from Ce3+ ions to Tb3+ ions. The optimal doping concentration of Tb3+ ions in CLGB:Ce3+,Tb3+ phosphors was found to be 40 mol%. The mechanism of the ET from the Ce3+ to Tb3+ ions was demonstrated to be electric quadrupole–quadrupole interaction. The CLGB:0.04Ce3+,0.40Tb3+ sample possessed a high IQE of 54.2% and excellent thermal stability with an activation energy of 0.3142 eV when excited at 345 nm. The integrated emission intensity of CLGB:0.04Ce3+,0.40Tb3+ at 423 K was found to be about 74% of that at 303 K. Finally, under 300 mA driven current, the fabricated prototype white light-emitting diode showed CIE chromaticity coordinates of (0.3996, 0.3856) and high color rending index of 81.2. Considering all the above characteristics, the obtained CLGB:Ce3+,Tb3+ phosphors can be a type of multicolor emitting phosphor for application in white light-emitting diodes.

Novel Eu3+-activated Ba2Y5B5O17 red-emitting phosphors for white LEDs: high color purity, high quantum efficiency and excellent thermal stability

Eu3+-activated Ba2Y5B5O17 (Ba2Y5−xEuxB5O17; x = 0.1–1) red-emitting phosphors were synthesized by the conventional high temperature solid-state reaction method in an air atmosphere. Powder X-ray diffraction (XRD) analysis confirmed the pure phase formation of the as-synthesized phosphors. Morphological studies were performed using field emission-scanning electron microscopy (FE-SEM). The photoluminescence spectra, lifetimes, color coordinates and internal quantum efficiency (IQE) as well as the temperature-dependent emission spectra were investigated systematically. Upon 396 nm excitation, Ba2Y5−xEuxB5O17 showed red emission peaking at 616 nm which was attributed to the 5D0 → 7F2 electric dipole transition of Eu3+ ions. Meanwhile, the influences of different concentrations of Eu3+ ions on the PL intensity were also discussed. The optimum concentration of Eu3+ ions in the Ba2Y5−xEuxB5O17 phosphors was found to be x = 0.8. The concentration quenching mechanism was attributed to the dipole–dipole interaction and the critical distance (Rc) for energy transfer among Eu3+ ions was determined to be 5.64 A. The asymmetry ratio [(5D0 → 7F2)/(5D0 → 7F1)] of Ba2Y4.2Eu0.8B5O17 phosphors was calculated to be 3.82. The fluorescence decay lifetimes were also determined for Ba2Y5−xEuxB5O17 phosphors. In addition, the CIE color coordinates of the Ba2Y4.2Eu0.8B5O17 phosphors (x = 0.653, y = 0.345) were found to be very close to the National Television System Committee (NTSC) standard values (x = 0.670, y = 0.330) of red emission and also showed high color purity (∼94.3%). The corresponding internal quantum efficiency of the Ba2Y4.2Eu0.8B5O17 sample was measured to be 47.2%. Furthermore, the as-synthesized phosphors exhibited good thermal stability with an activation energy of 0.282 eV. The above results revealed that the red emitting Ba2Y4.2Eu0.8B5O17 phosphors could be potential candidates for application in near-UV excited white light emitting diodes.

Thermally stable La2LiSbO6:Mn4+,Mg2+ far-red emitting phosphors with over 90% internal quantum efficiency for plant growth LEDs

In this paper, we reported on the high-efficiency and thermally-stable La2LiSbO6:Mn4+,Mg2+ (LLS:Mn4+,Mg2+) far-red emitting phosphors. Under 338 nm excitation, the composition-optimized LLS:0.3%Mn4+,1.6%Mg2+ phosphors which were made up of [SbO6], [LiO6], and [LaO8] polyhedrons, showed intense far-red emissions peaking at 712 nm (2Eg → 4A2g transition) with internal quantum efficiency as high as 92%. The LLS:0.3%Mn4+,1.6%Mg2+ phosphors also exhibited high thermal stability, and the emission intensity at 423 K only reduced by 42% compared with its initial value at 303 K. The far-red light-emitting device has also been made by using the LLS:0.3%Mn4+,1.6%Mg2+ phosphors and a 365 nm emitting InGaN chip, which can emit far-red light that is visible to the naked eye. Importantly, the emission spectrum of the LLS:0.3%Mn4+,1.6%Mg2+ phosphors can match well with the absorption spectrum of phytochrome PFR, indicating the potential of these phosphors to be used in plant growth light-emitting diodes.

Synthesis, structure, and luminescence characteristics of far-red emitting Mn4+-activated LaScO3 perovskite phosphors for plant growth

Far-red emitting phosphors LaScO3:Mn4+ were successfully synthesized via a high-temperature solid-state reaction method. The X-ray powder diffraction confirmed that the pure-phase LaScO3:Mn4+ phosphors had formed. Under 398 nm excitation, the LaScO3:Mn4+ phosphors emitted far red light within the range of 650–800 nm peaking at 703 nm (14 225 cm−1) due to the 2Eg → 4A2g transition, which was close to the spectral absorption center of phytochrome PFR located at around 730 nm. The optimal doping concentration and luminescence concentration quenching mechanism of LaScO3:Mn4+ phosphors was found to be 0.001 and electric dipole–dipole interaction, respectively. And the CIE chromaticity coordinates of the LaScO3:0.001Mn4+ phosphor were (0.7324, 0.2676). The decay lifetimes of the LaScO3:Mn4+ phosphors gradually decreased from 0.149 to 0.126 ms when the Mn4+ doping concentration increased from 0.05 to 0.9 mol%. Crystal field analysis showed that the Mn4+ ions experienced a strong crystal field in the LaScO3 host. The research conducted on the LaScO3:Mn4+ phosphors illustrated their potential application in plant lighting to control or regulate plant growth.

Far-red-emitting double-perovskite CaLaMgSbO6:Mn4+ phosphors with high photoluminescence efficiency and thermal stability for indoor plant cultivation LEDs

A series of Mn4+-activated CaLaMgSbO6 far-red-emitting phosphors were synthesized by a solid-state reaction route and the microstructure and optical characterizations were investigated in detail. Upon excitation at 370 and 469 nm, the samples showed intense far-red emission at about 708 nm originating from the 2Eg → 4A2g transition and the optimal Mn4+ concentration was 0.7 mol%. The as-prepared phosphors also exhibited excellent internal quantum efficiency (88%) and high thermal stability. The emission intensity at room temperature dropped to 54% when the temperature rose to 423 K and the activation energy was 0.34 eV. The outstanding optical properties and the fact that the emission band of the obtained phosphors had a broad overlap with the absorption band of phytochrome PFR demonstrated that the CaLaMgSbO6:Mn4+ phosphors may be promising potential spectral converters for applying to indoor plant cultivation light-emitting diodes.