Second-Sphere Polyhedron-Distortion-Induced Broadened and Red-Shifted Emission in Lu3(MgxAl2-x)(Al3-xSix)O12:Ce3+ for Warm WLED

Multiwavelength Excitation in Ho3+-Doped All-Inorganic Double Perovskites Achieved by Codoping Mn2+ for Warm-White LEDs and Plant Growth

Doping lanthanide ions is an efficient method to modify the optical properties of lead-free double-perovskite halides. However, most lanthanide-doped double perovskites show a low luminescence efficiency and require a high excitation energy. Here, we have successfully prepared a series of Ho3+-doped Cs2NaBiCl6 microcrystals through a simple hydrothermal method and obtained strong characteristic emissions of Ho3+ at 492 and 657 nm under low-energy excitation (449 nm). After codoping Mn2+, apart from the characteristic emissions from Ho3+ under 450 nm wavelength excitation, the orangish-red luminescence consisting of the emission band centered at 591 nm from Mn2+ and a sharp emission peak at 657 nm from Ho3+ is obtained under 355 nm UV light excitation. Photoluminescence (PL) emission and excitation spectra, along with the PL decay curves, confirm the existence of an energy-transfer channel from Cs2NaBiCl6 to Mn2+ and then from Mn2+ to Ho3+. The enhanced absorption efficiency (10.5 → 70.7%) suggests that the codoping of Mn2+ overcomes the low absorption efficiency caused by f-f forbidden transitions of Ho3+. Finally, the diverse luminescent performance within the Cs2NaBiCl6:Ho3+, Mn2+ phosphor is realized by altering the excitation wavelength, thereby enabling its application in warm-white-light-emitting diodes and plant growth in this work.

Ce3+-Doped Li2Ca5Gd(BO3)5 Phosphors with Multiple Luminescent Centers and High Pressure Sensitivity under Near UV Excitation

The performance of Ce3+-based phosphors under mechanical high pressures becomes attractive due to the potential application as a visual pressure sensor. Li2Ca5Gd(BO3)5 was selected as the host for the Ce3+ doping. Rietveld refinements reveal that rare earth cations occupy M1, M2, and M3 sites, and indeed, the photoluminescent spectra of Li2Ca5Gd1-xCex(BO3)5 (0.005 ≤ x ≤ 0.15) exhibit the characteristic of multiple activators, defined as CeI, CeII, and CeIII, with the maximal emission wavelength at ∼444, 419, and 378 nm, respectively. The optimal internal and external quantum efficiencies are 86.29% for x = 0.005 and 20.26% for x = 0.10, respectively, under the NUV excitation at 363 nm. In-situ high pressure emission spectra under 375 nm excitation exhibit an overall red-shift, and the linear pressure susceptibilities up to 6.7 GPa for CeI and CeII centers are -390 and -279 cm-1/GPa, respectively, which is probably the largest among Ce3+-doped oxides and oxysalts. Due to the above superiorities, Ce3+-doped LCGB possesses a high potential as a visual pressure sensor, and this is a successful study on the structure-property relationship of inorganic materials.

Recent Advances in Multi-Site Luminescent Materials: Design, Identification and Regulation

The development of novel phosphor materials with excellent performance and modification of their photoluminescence to meet the higher requirements for applications are the essential research subjects for luminescent materials. Multi-site luminescent materials with crystallographic sites for the activator ions that broaden the tunable range of luminescent spectra and even enhance the luminescent performance have attracted significant attention in the pursuit of high-quality luminescence for white light-emitting diodes. Here, we summarize multi-site luminescence characteristics based on the different kinds of host and activator ions, introduce the identifications of multi-site activator ions via optical analysis, provide a structural analysis and theoretical calculation methods, and introduce the regulation strategies and advance applications of multi-site phosphors. The review reveals the relationship between crystal structure and luminescent properties and discusses future opportunities for multi-site phosphors. This will provide guidance for the design and development of luminescent materials or other materials science.

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.

Efficient and Thermally Stable Broad-Band Near-Infrared Emission in a KAlP2O7:Cr3+ Phosphor for Nondestructive Examination

Broad-band near-infrared (NIR) phosphors are essential to assembling portable NIR light sources for applications in spectroscopy technology. However, developing inexpensive, efficient, and thermally stable broad-band NIR phosphors remains a significant challenge. In this work, a phosphate, KAlP₂O₇, with a wide band gap and suitable electronic environment for Cr³⁺ equivalent substitution was selected as the host material. The synthesized KAlP₂O₇:Cr³⁺ material exhibits a broad-band emission covering 650-1100 nm with a peak centered at 790 nm and a full width at half-maximum (fwhm) of 120 nm under 450 nm excitation. The internal quantum efficiency (IQE) was determined to be 78.9%, and the emission intensity at 423 K still maintains 77% of that at room temperature, implying the high efficiency and excellent thermal stability of this material. Finally, a NIR phosphor-converted light-emitting diode (pc-LED) device was fabricated by using the as-prepared material combined with a 450 nm blue LED chip, which presents a high NIR output power of 32.1 mW and excellent photoelectric conversion efficiency of 11.4% under a drive current of 100 mA. Thus, this work not only provides an inexpensive broad-band NIR material with high performance for applications in NIR pc-LEDs but also highlights some strategies to explore this class of materials.

Remarkable structure and luminescence regulation of a Gd2LuAl5O12:Ce garnet phosphor with a Ca2+/Si4+ pair for high-quality w-WLED lighting

Doping Gd₂LuAl₅O₁₂:Ce with a Ca²⁺/Si⁴⁺ pair produced a series of Gd_1.97-xLuCa_xAl_5-xSi_xO₁₂:0.03Ce (x = 0.0-1.2) garnet new phosphors, and remarkable regulation of the crystal structure and Ce³⁺ luminescence was achieved. Rietveld refinement and spectral analysis revealed that greater incorporation of Ca²⁺/Si⁴⁺ led to a higher lattice rigidity by cell contraction, a narrower bandgap, a longer average bond length/less distortion of the [CeO₈] dodecahedron and decreased centroid shift/crystal field splitting of the Ce³⁺ 5d energy level. A blue-shifted 4f-5d₁ transition, a narrower emission band, improved thermal stability of luminescence and a shorter fluorescence lifetime were observed with increasing Ca²⁺/Si⁴⁺ content. Applying the yellowish-green-emitting Gd_0.77LuCa_1.2Al_3.8Si_1.2O₁₂:0.03Ce optimal phosphor (x = 1.2), together with a commercial CaAlSiN₃:Eu red phosphor, in 450 nm-excited LED lighting produced a low CCT of ∼3625 K and a high CRI/R9 of ∼95.2/94.8, indicating that the phosphor has application potential in high-performance w-WLED.

Multisite Cation Regulation of Broadband Cyan-Emitting (Ba1-xSrx)9Lu2Si6O24/Eu2+ Phosphors for Full-Spectrum wLEDs

Developing broadband cyan-emitting phosphors is an essential issue to achieve high-quality full-spectrum phosphor-converted white light-emitting diodes. Multisite cation regulation to modify the photoluminescence spectrum is a valid way to achieve broadband emission for phosphors. The Ba₉Lu₂Si₆O₂₄ lattice with various cation sites for activator ions is a preferred host for broadband emitting phosphors. The preferential crystallographic sites of Eu²⁺ in the Ba₉Lu₂Si₆O₂₄ lattice are identified based on the crystal field theory, crystal structure, and bond indices (such as NAC and SBOs) of the cations. Sr substitution in Ba₉Lu₂Si₆O₂₄/Eu²⁺ phosphor affects the location of Eu²⁺ activator ions, which is investigated via the first-principles density functional theory calculations, Rietveld refinement, and luminescence decay curves, and results in the modification of luminescence properties and thermal stability. The Sr-substituted (Ba_0.8Sr_0.2)₉Lu₂Si₆O₂₄/Eu²⁺ sample exhibits a broadband emission spectrum peaked at 471 and 518 nm with a large full width half maximum of 139 nm, covering blue-cyan-green regions, which can be an excellent candidate as broadband cyan-emitting phosphors for high-quality full-spectrum wLEDs.

Improvement of the thermal stability of bluish-cyan emitting phosphor Y2MgAl4SiO12:Eu2+ using substitution cations (Zn, Ca, Sr) for white LEDs

A series of bluish-cyan color adjustable phosphors Y2MgAl4SiO12:xEu2+(YMAS) and Y2(Mg1−y,My)Al4SiO12:Eu2+ (M = Zn, Ca, Sr) were synthesized by the solid-state method.

The high thermal stability of white emitting phosphor Mg2 Y2 Al2 Si2 O12 :Dy3+ , Eu3+ , La3+ /Lu3+ for white LEDs

A series of Mg₂ Y₂ Al₂ Si₂ O₁₂ :Dy³⁺ , Eu³⁺ were prepared by the solid-state method, and the phosphor can emit white light by tuning the ratio of Dy³⁺ /Eu³⁺ . The effects of La³⁺ /Lu³⁺ on the structure and luminescent properties of Mg₂ Y₂ Al₂ Si₂ O₁₂ :Dy³⁺ , Eu³⁺ were explored. Under the influence of bond length and twist, the luminescent intensity of the materials increased first and then decreased under the excitation of ultraviolet light. The lattice distortion of the trivalent cation La³⁺ substituted Mg₂ Y₂ Al₂ Si₂ O₁₂ :Dy³⁺ and Eu³⁺ phosphors are reduced, the symmetry of polyhedron occupied by the luminescent center is improved, and the thermal stability of the luminescent center is improved to a certain extent. The white light emitting diodes (LEDs) were fabricated by combining the 370 nm LED chip and Mg₂ Y₂ Al₂ Si₂ O₁₂ :Dy³⁺ , Eu³⁺ , La³⁺ (Mg₂ Y₂ Al₂ Si₂ O₁₂ :Dy³⁺ , Eu³⁺ , Lu³⁺ ) phosphor. The results show that Mg₂ Y₂ Al₂ Si₂ O₁₂ :Dy³⁺ , Eu³⁺ , La³⁺ /Lu³⁺ may be potential application in the region of white light emitting diodes.

Site-selective and cooperative doping of Gd3Al5O12:Ce garnets for structural stabilization and warm WLED lighting of low CCT and high CRI

Synergistic doping of the metastable Gd₃Al₅O₁₂:Ce garnet with a Ca²⁺/Hf⁴⁺ pair and Sc³⁺ to form Gd_2.97-xCa_xHf_xSc_yAl₃O₁₂:0.03Ce (x = 0.5-2.0, y = 0.0-1.5, x + y = 2.0) solid solution was conducted for the structural stabilization and photoluminescence manipulation. The site selection of Ca²⁺/Hf⁴⁺/Sc³⁺ dopants and the effects of doping on the crystal structure, local coordination, band structure and Ce³⁺ luminescence were revealed in detail with the results of XRD, Rietveld refinement, TEM, and UV-Vis/photoluminescence spectroscopy. A decrease in Ca²⁺/Hf⁴⁺ and an increase in the Sc³⁺ content were observed to shrink the lattice, widen the bandgap of the garnet host, red-shift the excitation/emission wavelength, broaden the emission band and shorten the fluorescence lifetime of Ce³⁺. The spectral changes were rationalized by considering the local coordination and crystal field splitting of the Ce³⁺ 5d energy level. Application of typical Gd_0.97Ca₂Hf₂Al₃O₁₂:0.03Ce (x = 2.0, y = 0) cyan and Gd_2.47Ca_0.5Hf_0.5Sc_1.5Al₃O₁₂:0.03Ce (x = 0.5, y = 1.5) greenish-yellow phosphors in w-WLED lighting produced low correlated color temperatures of ∼3842 and 3514 K, high color rendering indices of ∼88 and 93 and favorable luminous efficacies of ∼32.9 and 14.7 lm/W under the excitation of 395 nm n-UV and 450 nm blue LED chips, respectively.

Luminescence and energy transfer of warm white-emitting phosphor Mg2Y2Al2Si2O12:Dy3+,Eu3+ for white LEDs

A series of Mg₂Y₂Al₂Si₂O₁₂:Dy³⁺,Eu³⁺ phosphors were synthesized by the solid-state method. The luminescent properties and crystal structures of the Mg₂Y₂Al₂Si₂O₁₂:Dy³⁺,Eu³⁺ phosphors were analyzed. The XRD results show that the synthesized Mg₂Y₂Al₂Si₂O₁₂:Dy³⁺,Eu³⁺ phosphors are of pure phase. Mg₂Y₂Al₂Si₂O₁₂:Dy³⁺ can emit blue and yellow light under 367 nm light excitation; when doped with Eu³⁺, there is an obvious energy transfer from Dy³⁺ to Eu³⁺, and warm white light can be realized by adjusting the concentrations of Dy³⁺ and Eu³⁺ in Mg₂Y₂Al₂Si₂O₁₂. A warm white LED device was fabricated by combining Mg₂Y_1.88Al₂Si₂O₁₂:0.05Dy³⁺,0.07Eu³⁺ and a UV LED chip (370 nm) under a voltage of 3.2 V and current of 5 mA, the characteristics of the white LED being CIE = (0.4071, 0.3944), CCT = 3500 K and CRI = 91.3.

MYAS:xDy³⁺,yEu³⁺ can produce warm white light with CIE chromaticity coordinates of (0.4071, 0.3944) and relative color temperature of 3500 K.

Improvement of thermal stability and photoluminescence in Mg2Y2Al2Si2O12:Ce3+ by the cation substitution of Ca2+, Sr2+ and Ba2+ ions

The luminescence of Ce³⁺ in Mg₂Y₂Al₂Si₂O₁₂ (MgYAlSiO₆) can be controlled by substituting with Ca²⁺, Sr²⁺, and Ba²⁺ ions. The materials show blue shifts in their emission spectra that become highly evident with the increase in the ionic radius, and this phenomenon is the result of the combined action of the nephelauxetic effect and crystal field splitting. Due to the introduction of Ca²⁺, Sr²⁺, and Ba²⁺, the distortion of the crystal decreases and the structural rigidity and stability increase, improving the quantum efficiency and temperature stability of Mg₂Y_1.93Al₂Si₂O₁₂:0.07Ce³⁺. Mg₂Y_1.93Al₂Si₂O₁₂:0.07Ce³⁺,Ca²⁺ is more stable than Mg₂Y_1.93Al₂Si₂O₁₂:0.07Ce³⁺,Sr²⁺/Ba²⁺ because the radius of Sr²⁺/Ba²⁺ is larger than that of Mg²⁺. The Mg_1.9Y_1.93Al₂Si₂O₁₂:0.07Ce³⁺,0.1Ca²⁺ fluorescent powder and blue chip were packaged to obtain warm white light-emitting diodes (LEDs) with luminous efficiency of 60 lm W^-1. These results show that the Mg₂Y₂Al₂Si₂O₁₂:Ce³⁺ phosphor has potential application value in white LEDs.

Broadband emission Gd3Sc2Al3O12:Ce3+ transparent ceramics with a high color rendering index for high-power white LEDs/LDs

The discovery of single structure Ce³⁺ doped garnet transparent ceramics (TCs) with a broad full width at half maximum (FWHM) is essential to realize a high CRI for high-power white light emitting diodes (LEDs) and laser diodes (LDs). In this work, by utilizing the ion substitution engineering strategy, pure phase Gd₃Sc₂Al₃O₁₂:Ce³⁺ (GSAG:Ce) TC with a broad FWHM of 132.4 nm and a high CRI value of 80.7 was fabricated through the vacuum sintering technique for the first time. The optimized in-line transmittance of TCs was 58.4% @ 800 nm. Notably, the GSAG:Ce TCs exhibited a remarkable red shift from 546 nm to 582 nm, with a high internal quantum efficiency (IQE) of 46.91%. The degraded thermal stability in Ce:GSAG TCs was observed compared with that of Ce:YAG TC, owing to the narrowed band gap of GSAG. Additionally, remote excitation white LEDs/LDs were constructed by combining GSAG:Ce TCs with blue LED chips or laser sources. A tunable color hue from yellow to shinning white was achieved in white LEDs, whereas the acquired CRI and CCT of the white LDs were 69.5 and 7766 K, respectively. This work provides a new perspective to develop TCs with high CRI for their real applications in high-power white LEDs/LDs.

Thermal stability of nitride phosphors for light-emitting diodes

Improving thermal stability of nitride phosphors has become an important material challenge. Our review describes three thermal phenomena, lists strategies for enhancing thermal stability of nitride phosphors, and discusses prospects in the future.

Garnet phosphors for white-light-emitting diodes: modification and calculation

Herein, we outline our recent research progress in the modification and the first-principles calculations of garnet phosphors which should be helpful in guiding scientists to discover novel garnet phosphors for emerging applications.