Broadband near-infrared luminescence in a cubic pyrophosphate Al0.5Ta0.5P2O7:Cr3+ phosphor for multi-functional applications.

Near-infrared phosphor-converted light-emitting diodes (NIR pc-LEDs) are considered as next-generation of NIR light sources for spectroscopy. However, it is still a challenge to develop an inexpensive broadband NIR phosphor with relatively long-wavelength (λem > 800 nm) emission. In this work, an octahedral Al3+-containing pyrophosphate Al0.5Ta0.5P2O7 with a cubic structure was chosen as a host for Cr3+. Synthesizing this material indicates that this phosphor exhibits a broadband NIR emission peaking at 850 nm with a full width at half maximum (FWHM) of 155 nm under 465 nm excitation. The crystal structure, morphology, local structure, and photoluminescence properties of this material were investigated in detail. The results revealed a full understanding of this new material. A NIR pc-LED device fabricated by using this material combined with a 450 nm LED chip generates a NIR output power of 10.7 mW and a NIR photoelectric conversion efficiency of 3.4% under a 100 mA driving current, which shows the possibility of this material to be utilized in NIR pc-LED applications. Moreover, this material exhibits a linear relationship between emission intensity, decay time and temperature in a wide temperature range, implying that excellent multi-model temperature sensing applications can be expected.

Non-destructive food-quality analysis using near-infrared luminescence from Mg3Gd2Ge3O12:Cr3.

Rapid, non-destructive food-quality analysis using near-infrared (NIR) photoluminescence spectroscopy produced by phosphor-converted light-emitting diodes (pc-LEDs) has fascinating prospects for future food-safety monitoring. However, covering the energy window for organic molecular vibrations of interest in these applications requires NIR-emitting phosphors that are highly energy-efficient with ultra-broadband photoluminescence. This remains a materials design challenge. Here, a Cr3+-substituted garnet phosphor, Mg3Gd2Ge3O12, is found to possess a desired broadband NIR emission (λem = 815 nm, fwhm = 172 nm; 2513 cm-1) covering from 700 nm to 1200 nm with a photoluminescence quantum yield of 60.8% and absorption efficiency of 44.1% (λex = 450 nm). Fabricating a prototype NIR pc-LED device using the title material combined with a 455 nm emitting InGaN LED chip produces a NIR output power of 23.2 mW with photoelectric efficiency of 8.45% under a 100 mA driving current. This NIR light source is then used to demonstrate the quantitative detection of ethanol in solution. These results highlight the feasibility of this material for NIR spectroscopy and validate the prospects of using NIR pc-LEDs in food-quality analysis.

Understanding the broadband near-infrared luminescence in a highly distorted garnet Ca4HfGe3O12:Cr3+ phosphor.

Broadband near-infrared (NIR) spectroscopy generated from a phosphor-converted light-emitting diode (pc-LED) has multifunctional applications, including food-quality analysis, bio-medical and night-vision, stimulating the demand for developing various NIR phosphors with desired properties. Herein, we selected a highly distorted garnet Ca4HfGe3O12 as the host and explored the near-infrared luminescence of Cr3+. As expected, this material achieved a long-wavelength NIR emission and excellent absorption efficiency based on the effect of Jahn-Teller distortion. The synthesized Ca4HfGe3O12:Cr3+ phosphor exhibits a broadband NIR emission peaking at 840 nm with a full width at half maximum of 150 nm, and the absorption efficiency reaches 48.0%. However, the internal quantum efficiency of the 6 mol% Cr3+-doped sample was measured to be only 35.3% and the integral emission intensity at 373 K kept only 60.1% of the initial intensity. The possible reasons for the unsatisfactory internal quantum efficiency and thermal stability were systematically analyzed, which provided a comprehensive understanding of the relationship between the crystal structure and the luminescent properties of Cr3+-activated garnet-type phosphors. Nevertheless, the as-prepared NIR pc-LED device exhibits a NIR output of 16.52 mW with a NIR photoelectric conversion efficiency of 5.92% driven by 100 mA current, indicating the potential of this material for application in NIR pc-LED.

An efficient and thermally stable near-infrared phosphor derived from the Ln3ScInGa3O12:Cr3+ (Ln = La, Gd, Y, and Lu) garnet family.

A broadband near-infrared (NIR) light source based on a phosphor-converted light-emitting diode (pc-LED) has attracted increasing interest to be used in non-destructive examination, security-monitoring and medical diagnosis fields, which stimulates the exploration of NIR phosphors with high performance. Herein, a series of Cr3+-activated garnet Ln3ScInGa3O12:Cr3+ (Ln = La, Gd, Y, and Lu) phosphors were reported, allowing an emission peak ranging from 726 to 822 nm. Among them, Y3ScInGa3O12:Cr3+ with an optimized Cr3+-doping concentration of 6 mol% exhibits a high internal quantum efficiency (IQE = 83.1%) and excellent absorption efficiency (AE = 44.2%) under 450 nm blue light excitation, enabling an external quantum efficiency as high as 36.7%. Moreover, this material can maintain 93.0% of the initial intensity when heated up to 423 K, implying outstanding thermal stability. Finally, a prototype NIR pc-LED device was fabricated by coating the optimized phosphor on a 455 nm LED chip, which generates a broadband NIR emission with a peak located at 765 nm and a full width at half maximum of 127 nm. The NIR output power and NIR photoelectric conversion efficiency of this device were found to be 38.01 mW and 11.0%, respectively, under 100 mA driving current, demonstrating the feasibility of this material to be applied in NIR pc-LEDs.

Producing Tunable Broadband Near-Infrared Emission through Co-Substitution in (Ga1-xMgx)(Ga1-xGex)O3:Cr3.

Broadband near-infrared (NIR) phosphors are in high demand for creating "smart" NIR phosphor-converted light-emitting diode (pc-LED) sources. In this work, a series of Cr3+-substituted NIR-emitting materials with highly efficient, broad, tunable emission spectra are achieved by modifying the simple oxide Ga2O3 using [Mg2+-Ge4+] and [Ga3+-Ga3+] co-unit substitution. The results show that the emission peak can be shifted from 726 to 830 nm while maintaining a constant excitation peak in the blue light region, enabling extensive application. The optical properties stem from changes in the Cr3+ crystal field environment upon substitution. Intriguingly, the temperature-dependent photoluminescence emission peak position shows virtually no change in the [Mg2+-Ge4+] co-substituted materials. This abnormal phenomenon is found to be a comprehensive embodiment of a weakening crystal field environment (red-shift) as the temperature increases and reduced local structure distortion (blue-shift) with increasing temperature. The high quantum yield, NIR emission, and net-zero emission shift as a function of temperature make this phosphor class optimal for device incorporation. As a result, their performance was studied by coating the phosphor on a 450 nm emitting LED chip. The fabricated device demonstrates an excellent NIR output power and NIR photoelectric conversion efficiency. This study provides a series of efficient, tunable, broadband NIR materials for spectroscopy applications and contributes to the basic foundation of Cr3+-activated NIR phosphors.

Efficient and thermally stable broadband near-infrared emission in a garnet Gd3In2Ga3O12:Cr3+ phosphor.

Compact broadband near-infrared (NIR) light sources generated by phosphor-converted light-emitting diodes (pc-LEDs) have attracted considerable interest in emerging smart NIR spectroscopy applications. However, discovering a highly efficient and thermally stable broadband NIR phosphor still remains a significant challenge. Here, we report a new efficient garnet phosphor, Gd3In2Ga3O12:Cr3+, which has a broadband emission peaking at 780 nm with a full-width at half maximum (FWHM) of 124 nm. The optimized Cr3+-doping concentration of this material is particularly high (9 mol%), resulting in a high quantum yield and absorption efficiency of 85.3% and 49.1%, respectively. Moreover, 87.7% of the initial emission intensity can be retained when heating up to 150 °C, demonstrating the excellent thermal stability of this material. Fabricating a prototype NIR device by using the as-prepared material in combination with a blue LED chip, an excellent NIR output power (33.7 mW) with a NIR photoelectronic conversion efficiency of 12.0% can be achieved under a 100 mA driving current. These results indicate that the Gd3In2Ga3O12:Cr3+ phosphor may have great potential for broadband NIR pc-LED applications.

Achieving a tunable and ultra-broadband near-infrared emission in the Ga2-2xZnxGexO3:Cr3+ phosphor.

Near-infrared (NIR) spectroscopy (700-1100 nm) based on a phosphor-converted light-emitting diode (pc-LED) has multi-functional applications in bio-imaging, night-vision, and food quality analysis, simulating the development of efficient and ultra-broadband NIR phosphors. Herein, a series of tunable and ultra-broadband NIR phosphor Ga2-2xZnxGexO3:Cr3+ was successfully produced by a [Zn2+-Ge4+] unit co-substituting a [Ga3+-Ga3+] unit in Ga2O3:Cr3+. With the increasing amount of [Zn2+-Ge4+] incorporation, the emission peak can be tuned from 726 to 808 nm, and the maximum FWHM can be extended from 126 to 190 nm. However, the excitation peak position remained nearly unchanged in the blue light region, enabling this material to perfectly match the InGaN blue LED chip. Although the gradually enhanced electron-lattice coupling and low energy barrier for thermal quenching caused by this co-substitution lead to a decrease in the photoluminescence quantum yield (PLQY) and thermal stability, respectively, the absolute values of these two criteria can be well maintained. Finally, to evaluate the practical applications, prototype NIR pc-LEDs were fabricated using these materials combined with 450 nm LED chips. Under the same conditions, the NIR output power and photoelectronic conversion efficiency of these devices are superior to those fabricated using the well-known ScBO3:Cr3+ phosphor. These results demonstrate that this series of materials with tunable and ultra-broadband NIR emissions has great potential for NIR pc-LED applications.

Design and simulation of a GST-based metasurface with strong and switchable circular dichroism.

Circular dichroism (CD) is required in the applications of biological detection, analytical chemistry, etc. Here, we numerically demonstrated large-range switchable CD by controlling the phase change of Ge2Sb2Te5 (GST) in a zigzag array. At the amorphous state of GST (a-GST), the strong and dual-waveband CD effects are realized via the selective excitations of electric, magnetic, and toroidal resonances. With the transition from a-GST to crystalline state GST, CD strengths are tailored dynamically in large ranges. In detail, the CD magnitudes change by about 0.93 and the modulation depths exceed 94% at dual wavebands. The strong CD effects and large-range switch capability in the GST-based metasurfaces will boost the development of active chiroptical devices.

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.

Efficient and Tunable Luminescence in Ga2-xInxO3:Cr3+ for Near-Infrared Imaging.

Broadband near-infrared (NIR) emitting materials are in great demand as next-generation smart NIR light sources. In this work, a Cr3+-substituted phosphor capable of efficiently converting visible to NIR light is developed through the solid solution, Ga2-xInxO3:Cr3+ (0 ≤ x ≤ 0.5). The compounds were prepared using high-temperature solid-state synthesis, and the crystal and electronic structure, morphology, site preference, and photoluminescence properties are studied. The photoluminescence results demonstrate a high quantum yield (88%) and impressive absorption efficiency (50%) when x = 0.4. The NIR emission is tunable across a wide range (713-820 nm) depending on the value of x. Moreover, fabricating a prototype of a phosphor-converted NIR light-emitting diode (LED) device using 450 nm LED and the [(Ga1.57Cr0.03)In0.4]O3 phosphor showed an output power that reached 40.4 mW with a photoelectric conversion efficiency of 25% driven by a current of 60 mA, while the resulting device was able to identify damaged produce that was undetectable using visible light. These results demonstrate the outstanding potential of this phosphor for NIR LED imaging applications.

Thermally Robust and Color-Tunable Blue-Green-Emitting BaMgSi4O10:Eu2+,Mn2+ Phosphor for Warm-White LEDs.

The dual emission produced from Mn2+ when codoped with rare earth ions like Eu2+ or Ce3+ in inorganic compounds makes these materials attractive as efficient, color-tunable phosphors for warm-white solid-state lighting. Here, a series of efficient blue-green-emitting BaMgSi4O10:Eu2+,Mn2+ phosphors with thermally robust, tunable luminescence are reported. Steady-state and time-resolved photoluminescence spectroscopy reveal that Eu2+ and Mn2+ each occupy a single crystallographic site and confirm that energy transfer occurs from Eu2+ to Mn2+. The internal and external quantum efficiency of BaMgSi4O10:Eu2+,Mn2+ can reach as high as 69.0 and 47.5%, respectively, upon 360 nm excitation. Moreover, this phosphor possesses nearly zero-thermal quenching up to 440 K due to thermally induced electron detrapping. A fabricated UV-excited white LED device incorporating the blue-green-emitting BaMgSi4O10:Eu2+,Mn2+ and the red-emitting Sr2Si5N8:Eu2+ phosphors exhibits an excellent CRI of 94.3 with a correlated color temperature of 3967 K. These results prove the potential applications of Eu2+,Mn2+ codoped BaMgSi4O10 phosphor for generating warm-white light.

First-Principles Study on Self-Activated Luminescence and 4f → 5d Transitions of Ce3+ in M5(PO4)3X (M = Sr, Ba; X = Cl, Br).

The origin of the self-activated luminescence in the apatite-type M5(PO4)3X (MPOX; M = Sr or Ba; X = Cl or Br) samples and the spectral assignment for cerium-doped Sr5(PO4)3Cl (SPOC) phosphors are determined from first-principles methods combined with hybrid density functional theory (DFT) calculations, using the standard PBE0 hybrid functional, with wave function-based embedded-cluster ab initio calculations (at the CASSCF/CASPT2/RASSI-SO level). Electronic structure calculations are performed in order to accurately derive the band gaps of the hosts, the locations of impurity states in the energy bands that are caused by native defects and doped Ce3+ ions, and the charge-compensation mechanisms of aliovalent doping. The calculations of defect formation energies under O-poor conditions demonstrate that the native defects are easily generated in the undoped MPOX samples prepared under reducing atmospheres, from which thermodynamic and optical transition energy levels, as well as the corresponding energies, are derived in order to interpret the luminescence mechanisms of the undoped MPOX as previously reported. Our calculations reveal that the self-activated luminescence is mainly attributed to the optical transitions of the excitons bound to the oxygen vacancies (VO), along with their transformation of the charge states 0 ↔ 1+. Furthermore, the eight excitation bands observed in the synchrotron radiation excitation spectra of SPOC: Ce3+, Na+ phosphors are successfully assigned according to the ab initio calculated energies and relative oscillator strengths of the 4f1 → 5d1-5 transitions for the Ce3+ ions at both the Sr(1) and Sr(2) sites in the host. It is hoped that the feasible first-principles approaches in this work are applied in order to explore the origins of the luminescence in undoped and lanthanide-doped phosphors, complementing the experiments from the perspective of chemical compositions and the microstructures of materials.

Combining experiment and computation to elucidate the optical properties of Ce3+ in Ba5Si8O21.

Complex alkaline earth silicates have been extensively studied as rare-earth substituted phosphor hosts for use in solid-state lighting. One of the biggest challenges facing the development of new phosphors is understanding the relationship between the observed optical properties and the crystal structure. Fortunately, recent improvements in characterization techniques combined with advances in computational methodologies provide the research tools necessary to conduct a comprehensive analysis of these systems. In this work, a new Ce3+ substituted phosphor is developed using Ba5Si8O21 as the host crystal structure. The compound is evaluated using a combination of experimental and computational methods and shows Ba5Si8O21:Ce3+ adopts a monoclinic crystal structure that was confirmed through Rietveld refinement of high-resolution synchrotron powder X-ray diffraction data. Photoluminescence spectroscopy reveals a broad-band blue emission centered at ∼440 nm with an absolute quantum yield of ∼45% under ultraviolet light excitation (λex = 340 nm). This phosphor also shows a minimal chromaticity-drift but with moderate thermal quenching of the emission peak at elevated temperatures. The modest optical response of this phase is believed to stem from a combination of intrinsic structural complexity and the formation of defects because of the aliovalent rare-earth substitution. Finally, computational modeling provides essential insight into the site preference and energy level distribution of Ce3+ in this compound. These results highlight the importance of using experiment and computation in tandem to interpret the relationship between observed optical properties and the crystal structures of all rare-earth substituted complex phosphors.

Synthesis, electronic structures, and photoluminescence properties of an efficient and thermally stable red-emitting phosphor Ca3ZrSi2O9:Eu3+,Bi3+ for deep UV-LEDs.

A series of red-emitting Ca3ZrSi2O9:Eu3+,xBi3+ phosphors was synthesized using a conventional high temperature solid-state reaction method, for the purpose of promoting the emission efficiency of Eu3+ in a Ca3ZrSi2O9 host. The site preference of Bi3+ and Eu3+ in the Ca3ZrSi2O9 host was evaluated by formation energy. The effects of Bi3+ on electronic structure, luminescent properties, and related mechanisms were investigated. The inner quantum yield of the optimized sample increased to 72.9% (x = 0.08) from 34.6% (x = 0) at 300 nm ultraviolet light excitation. The optimized sample (x = 0.08) also showed excellent thermal stability, and typically, 84.2% of the initial emission intensity was maintained when the temperature increased to 150 °C from 25 °C, which is much higher than that without Bi3+ doping (70.1%). The mechanisms of emission properties and thermal stability enhancement, as well as the redshift of the charge transfer band (CTB) induced by Bi3+ doping in the Ca3ZrSi2O9:Eu3+ phosphor, were discussed. This study elucidates the photoluminescence properties of Bi3+-doped Ca3ZrSi2O9:Eu3+ phosphor, and indicates that it is a promising luminescent material that can be used in ultraviolet light-emitting diodes.

Origin of Spectral Blue Shift of Lu3+-Codoped YAG:Ce3+ Phosphor: First-Principles Study.

Lu3+, with the smallest ionic radii in lanthanide ions, is an important and beneficial cation for tuning spectrum shifting toward a longer wavelength by ion substitution in many phosphors for solid-state lighting. However, in the Lu3+-substituted garnet system, the phosphor always has smaller lattice parameters and exhibits a shorter emission wavelength than other garnet phosphors. The mechanism of such a spectral blue shift induced by the Lu3+-codoped garnet phosphor is still unclear. In this study, the local and electronic structures of Lu3+-codoped and Lu3+-undoped YAG:Ce3+ phosphor have been studied by first-principles calculation to reveal the origin of the spectral blue shift. Our results provide a full explanation of the experimental data and the methodology, which is useful to understand and design garnet phosphors with tunable emission characteristics.