Broadband Short-Wave Infrared-Emitting MgGa2O4:Cr3+, Ni2+ Phosphor with Near-Unity Internal Quantum Efficiency and High Thermal Stability for Light-Emitting Diode Applications

Non-destructive prediction and pixel-level visualization of polysaccharide-based properties in ancient paper using SWNIR hyperspectral imaging and machine learning

Ancient documents and artworks are invaluable cultural heritage artworks that require careful preservation. Traditional methods for assessing their physical and chemical properties-such as tearing index, tensile index, water absorption, and pH-are often destructive, risking irreversible damage. This study introduces a novel, non-destructive approach using Short-Wave Near-Infrared (SWNIR) hyperspectral imaging (HSI) combined with advanced machine learning models. By integrating spectral preprocessing, feature selection, and machine learning techniques-including Back Propagation Neural Networks (BPNN), Long Short-Term Memory Networks (LSTM), and Convolutional Neural Networks (CNN)-with Sparrow Search Algorithm (SSA) optimization and Gray Level Co-occurrence Matrix (GLCM) texture feature extraction, the resulting SSA-BP-UVE-GLCM model achieved high predictive accuracy (R2 ≥ 0.98). This framework enables precise, pixel-level predictions of paper properties, influenced by polysaccharides like cellulose, offering a non-invasive analysis that supports targeted restoration strategies and advances the conservation of cultural heritage. The findings enhance non-invasive testing and classification methods for polysaccharide-based materials, providing a foundation for further exploration of environmental impacts on artwork integrity using sophisticated machine learning algorithms.

Ni2+ in Distorted Octahedral Geometry toward High-Efficiency NIR-II/III emitter

Ni2+ activated phosphor has attracted wide attention because it can be radiated in near-infrared (NIR) II-III region; however, the low external quantum yield (EQY) hinders its further application (most are below 8 %). In this work, distortion of crystallographic site strategy is proposed to enhance EQY. LaTiTaO6: 0.004Ni2+ exhibits an NIR emitting spectrum centered at 1450 nm with an EQY of ~9.00 %, which has been the highest EQY of reported Ni2+ single-doped phosphor. Distortion degrees of the octahedral geometry in LaTiTaO6 and LaTiTaO6: 0.004Ni2+ changing from 0.038 to 0.047 result in the absorption efficiency of 38.6 % due to the breaking of parity forbidden of the d-d transition, further improving the EQY. Besides, LaTiTaO6: 0.004Ni2+ mixed with LaTiTaO6: 0.04Cr3+ phosphors have demonstrated their application for spectral analysis. This work provides a new idea for constructing efficient NIR-II to NIR-III phosphors.

Ultrabroad Near Infrared Emitting Perovskites

Phosphor converted light emitting diodes (pc-LEDs) have revolutionized solid-state white lighting by replacing energy-inefficient filament-based incandescent lamps. However, such a pc-LED emitting ultrabroad near-infrared (NIR) radiations still remains a challenge, primarily because of the lack of ultrabroad NIR emitting phosphors. To address this issue, we have prepared 2.5 % W4+-doped and 2.8 % Mo4+-doped Cs2Na0.95Ag0.05BiCl6 perovskites emitting ultrabroad NIR radiation with unprecedented spectral widths of 434 and 468 nm, respectively. Upon band-edge excitation, the soft lattice of the host exhibits broad self-trapped exciton (STE) emission covering NIR-I (700 nm), which then nonradiatively excites the dopants. The π ${\pi }$ -donor ligand Cl- reduces the energy of dopant d-d transitions emitting NIR-II with a peak at ~950 nm. Vibronic coupling broadens the dopant emission. The large spin-orbit coupling and local structural distortion might possibly enhance the dopant emission intensity, leading to an overall NIR photoluminescence quantum yield ~40 %. The composite of our ultrabroad NIR phosphors with biodegradable polymer polylactic acid could be processed into free-standing films and 3D printed structures. Large (170 × ${\times }$ 170 m m 2 ${m{m}^{2}}$ ), robust, and thermally stable 3D printed pc-LED panels emit ultrabroad NIR radiation, demonstrating NIR imaging applications.

Broadband Shortwave Infrared-Emitting Cr3+- and Ni2+-Codoped Y3Al2Ga3O12 Phosphor with Excellent Thermal Stability for Multifunctional Applications

Shortwave infrared (SWIR)-emitting materials have emerged as superior light sources with increasing demand for potential applications in noninvasive analysis, night vision illumination, and medical diagnosis. For developing next-generation SWIR phosphor-converted light-emitting diodes (pc-LEDs), the scarcity of intense blue-light-pumped broadband SWIR luminescent materials and poor thermal stability of current Ni2+-activated phosphors are the ongoing challenges. Here, a blue-light-excitable (440 nm) Y3Al2Ga3O12:Cr3+,Ni2+ phosphor with ultrawide SWIR emission centered at ∼1430 nm (FWHM ∼264 nm) is reported. The efficient Cr3+ → Ni2+ energy transfer is exploited for a remarkable enhancement of 18-fold in SWIR emission under blue-light excitation. Significantly, the excellent thermal stability (90% at 440 K) of the SWIR band is obtained in the codoped phosphor which is the best among reported SWIR phosphors. Besides, the experimental techniques (XANES and EXAFS) and density functional theory calculations are employed to investigate the local structure and propose [GaO6] octahedrons as favorable occupation sites for Ni2+/Cr3+ ions. A SWIR pc-LED is fabricated using a blue chip as a proof of concept for invisible illumination technology, nondestructive spectroscopic analysis, anticounterfeiting, and imaging applications. This work offers effective insights into energy transfer-assisted SWIR enhancement and presents a thermally robust Cr3+-Ni2+-codoped phosphor for designing future mini-SWIR pc-LEDs for spectroscopy applications.

Mechanistic Insights into Dual NIR Emission from Cr-Doped Sc2O3 via First-Principles Calculations

The emission of Cr-doped Sc2O3 (space group Ia3̅, No. 206) phosphor features a broad band in NIR-I (700-1000 nm) and another in NIR-II (1000-1700 nm), which is significant for spectral analysis and medical applications. Although Sc2O3 has two 6-coordinated Sc sites─the nearly octahedral site with S6 point-group symmetry (S6 site) and the highly distorted site with C2 symmetry (C2 site)─the origin of the dual-band emission remains widely debated. In this study, we performed first-principles calculations to investigate the properties of Cr and Ni dopants in Sc2O3, including preference in site occupation, valence state, ligand field strength, Stokes shift, and line shape. Our calibrated calculations conclusively determined that the NIR-I emission peak at 840 nm is due to Cr3+ at the S6 site. However, the broad NIR-II emission peaking at around 1280 nm cannot be attributed to Crq (q = +2, +3, +4) at either site, suggesting the presence of possible trace impurities and phases. Ni2+ ions at octahedral sites exhibit a narrow peak width and long lifetime, which contradict reported experimental observations. The Cr4+ ions at a tetrahedral site, similar to that in the phase of Sc2O3 with the space group Pna21 (No. 33), show the most consistent line shape and emission decay rates with experimental data. A systematic first-principles approach incorporating the line shape calculation can be useful to resolve the issues in identifying luminescent centers in systems involving intrinsic defects and dopants.

Unveiling Suppressed Concentration Quenching Enhanced Broadband Near-Infrared Emitters

Transition metal ions are exceptional emitters of broadband near-infrared (near-IR) luminescence, enabling various applications in photonics and optics; however, their weak absorption and excitation bands often limit conversion performance. Among potential sensitizers, Cr3+ stands out, allowing the excitation of Ni2+ with broadband visible-near-IR light while preserving luminescence properties. Nevertheless, concentration-induced quenching in doped luminescent solids fundamentally undermines brightness due to a trade-off between internal quantum efficiency and excitation energy dynamics. In this study, we demonstrate unprecedented brightness in the broadband near-IR photoluminescence (PL) of the Cr3+-Ni2+ system, where single, heavily doped Cr3+ exhibits minimal PL, and tuning Ni2+ concentration offsets the competitive relationship between the light emitter and quencher. By coupling InGaN light-emitting diodes (LEDs) with an ultrabroadband near-IR PL phosphor, we achieve phosphor-converted LEDs with a record output power of 41.5 mW at 100 mA and a photoelectric efficiency of 19.53% at 20 mA, nearly doubling previous reports. Our findings unveil intrinsic suppression of concentration quenching and eliminate competing energy sinks by emphasizing the dominant role of emitters in downshifting excitation energies, thus opening new avenues for the design of bright, sensitized luminescent materials.

Energy-Transfer-Enhanced Cr3+/Ni2+ Co-doped Broadband Near-Infrared Phosphor for Fluorescence Thermometers and Near-Infrared Window Imaging

Here, near-infrared broad dual-band emission phosphors were achieved through energy transfer between Cr3+ and Ni2+ ions in the β-Ga2O3 host. All samples co-doped with Cr3+ and Ni2+ exhibit dual-band emission covering 600-1700 nm under 430 nm excitation. Thanks to the doping of Cr3+ ions, the emission intensity of Ga2O3:Cr3+, Ni2+ phosphors has increased by about 2.4 times and the internal quantum efficiency has increased by 83.2% compared to Ga2O3:Ni2+ phosphors. Meanwhile, when the fluorescence lifetime was monitored at 745 nm, an efficient energy transfer between Cr3+ and Ni2+ ions in the β-Ga2O3 host was verified. Due to the significant differences in the emission temperature-sensitive properties of Cr3+ and Ni2+ ions, a thermometer was designed utilizing fluorescence intensity ratio technology, achieving a maximum relative sensitivity of 5.26% K-1, which surpasses most optical temperature measurement phosphors. This suggests that Ga2O3:Cr3+, Ni2+ samples hold promise as potential candidates for optical thermometer materials. Additionally, the broadband near-infrared emission of the Ga2O3:Cr3+, Ni2+ sample has been investigated for potential applications in component analysis and night vision, demonstrating its versatility for multifunctional applications.

Achieving Ultrahigh Thermal Stability in Cr3+-Activated Garnet Phosphors through Electron Migration between Thermally Coupled Levels

Recently, Cr3+-activated near-infrared (NIR) phosphors have received much more attention due to their excellent photoluminescence (PL) properties. However, most of them suffer from poor thermal stability which limits further application. Herein, a novel Lu2CaGa4SnO12:Cr3+ phosphor with broadband NIR emission (λem = 750 nm) is synthesized successfully. Despite the good luminescence property, its PL intensity decreases obviously with temperature (I425 K = 79%). To improve the thermal stability, a series of Lu2+xCa1-xGa4+xSn1-xO12:Cr3+ (x = 0-1.0) solid solutions with tunable thermal quenching performance have been designed. It is found that the fluorescence intensity ratio (FIR) of 4T2 → 4A2 to 2E → 4A2 [I(4T2)/I(2E)] transitions (i.e. electron occupation) decreases monotonously with increasing [Lu3+-Ga3+] co-substitution, resulting from a strengthened crystal field strength and increased energy difference between 4T2 and 2E energy levels. Benefiting from the various thermal population and energy difference Δ', the PL thermal quenching behavior of Lu2+xCa1-xGa4+xSn1-xO12:Cr3+ can be adjusted easily, and the corresponding mechanism is explored in detail. Most notably, the emission intensity of Lu2+xCa1-xGa4+xSn1-xO12:Cr3+ at 425 K can reach up to 142% compared with that at 300 K, which may be the best for Cr3+-activated NIR phosphors. This work may provide an alternative path for the development of thermally stable broadband NIR phosphors.

Near-Infrared Luminescent Materials Incorporating Rare Earth/Transition Metal Ions: From Materials to Applications

The spotlight has shifted to near-infrared (NIR) luminescent materials emitting beyond 1000 nm, with growing interest due to their unique characteristics. The ability of NIR-II emission (1000-1700 nm) to penetrate deeply and transmit independently positions these NIR luminescent materials for applications in optical-communication devices, bioimaging, and photodetectors. The combination of rare earth metals/transition metals with a variety of matrix materials provides a new platform for creating new chemical and physical properties for materials science and device applications. In this review, the recent advancements in NIR emission activated by rare earth and transition metal ions are summarized and their role in applications spanning bioimaging, sensing, and optoelectronics is illustrated. It started with various synthesis techniques and explored how rare earths/transition metals can be skillfully incorporated into various matrixes, thereby endowing them with unique characteristics. The discussion to strategies of enhancing excitation absorption and emission efficiency, spotlighting innovations like dye sensitization and surface plasmon resonance effects is then extended. Subsequently, a significant focus is placed on functionalization strategies and their applications. Finally, a comprehensive analysis of the challenges and proposed strategies for rare earth/transition metal ion-doped near-infrared luminescent materials, summarizing the insights of each section is provided.

Two-Site Occupation for Constructing Double Perovskite BaLaMgNbO6:Cr3+ Ultrabroadband NIR Phosphors

Given the escalating significance of near-infrared (NIR) spectroscopy across industries, agriculture, and various domains, there is an imminent need to address the development of a novel generation of intelligent NIR light sources. Here, a series of Cr3+-doped BaLaMgNbO6 (BLMN) ultrabroadband NIR phosphor with a coverage range of 650-1300 nm were developed. The emission peak locates at 830 nm with a full width at half maximum of 210 nm. This ultrabroadband emission originates from the 4T2→4A2 transition of Cr3+ and the simultaneous occupation of [MgO6] and [NbO6] octahedral sites confirmed by low photoluminescence spectra (77-250 K), time-resolved photoluminescence spectra, and electron paramagnetic resonance spectra. The fluxing strategy improves the luminescence intensity and thermal stability of BLMN:0.02Cr3+ phosphors. The internal quantum efficiency (IQE) is 51%, external quantum efficiency (EQE) can reach 33%, and thermal stability can be maintained at 60%@100 °C. Finally, we successfully demonstrated the application of BLMN:Cr3+ ultrabroadband in the qualitative analysis of organic matter and food freshness detection.

Strategy of Charge Compensation for High-Performance Ni2+-Activated MgAl2O4 Spinel Near-Infrared Phosphor Synthesis via the Sol-Gel Combustion Method

Near-infrared (NIR) phosphor conversion light-emitting diodes (pc-LEDs) have great application potential as NIR light sources in many fields such as food analysis, night vision illumination, and bioimaging for noninvasive medical diagnosis. In general, phosphors synthesized by a high-temperature solid-phase method have large particle sizes and have to be processed to fine powders by a grinding process, which may introduce surface defects and lower the luminous efficiency. Here, we report a sol-gel sintering method with ammonium nitrate and citric acid as the sacrificing agents to synthesize high purity, nanosized (less than 50 nm) Zr4+/Ni2+ codoped MgAl2O4 spinel NIR phosphors, in which MgAl2O4 spinel is the matrix, Ni2+ is the luminous center, and Zr4+ acts as the charge compensator. We systematically characterized the crystal structures and NIR luminescence properties of the Ni2+-doped MgAl2O4 and the Zr4+/Ni2+ codoped MgAl2O4. Under 390 nm light excitation, the emission spectrum of the Ni2+-doped MgAl2O4 phosphor covers 900-1600 nm, the half-peak width is 251 nm, and the peak position is located at 1230 nm. We demonstrated that by incorporating small amounts of Zr4+ as the charge compensator, the NIR emission intensity of the Zr4+/Ni2+ codoped MgAl2O4 nanosized phosphor was doubled over that of the Ni2+-doped MgAl2O4 phosphor. The optimal content of the charge compensator was 2 mol %. More importantly, the inclusion of Zr4+ led to a NIR phosphor with improved thermal stability in luminous properties, and the luminous intensity measured at 100 °C was 33.83% of that measured at room temperature (20 °C). This study demonstrates that NIR phosphor nanomaterials with high-purity and enhanced optical properties can be designed and synthesized through the charge compensation strategy by a sol-gel sintering method.

Hydrothermal synthesis of ZnGa2O4 nanophosphors with high internal quantum efficiency for near-infrared pc-LEDs

NIR luminescent materials have garnered widespread attention because of their exceptional properties, with high tissue penetration, low absorption and high signal-to-noise ratio in the field of optical imaging. However, producing nanophosphors with high quantum yields of emitting infrared light with wavelengths above 1000 nm remains a significant challenge. Here, we prepared a nanoscale ZnGa2O4:xCr3+,yNi2+ phosphor with good luminescence performance in near-infrared emission, which was synthesized via a hydrothermal method and subsequent calcination process. By co-doping with Cr3+ and Ni2+, the ZnGa2O4 phosphor shows a strong broadband emission of 1100-1600 nm in the second near-infrared (NIR-II) region, owing to the energy transfer from Cr3+ to Ni2+ with an efficiency up to 90%. Meanwhile, a near-infrared phosphor-conversion LED (NIR pc-LED) device is fabricated based on the ZnGa2O4:0.8%Cr3+,0.4%Ni2+ nanophosphor, which has under 100 mA input current, an output power of 23.99 mW, and a photoelectric conversion efficiency of 7.53%, and can be effectively applied in imaging and non-destructive testing. Additionally, the intensity ratio of INi/ICr of ZnGa2O4:0.8% Cr3+,0.4%Ni2+ with its high sensitivity value of 4.21% K-1 at 453 K under 410 nm excitation, indicates its potential for thermometry application.

Near-Infrared Nanophosphors Based on CuInSe2 Quantum Dots with Near-Unity Photoluminescence Quantum Yield for Micro-LEDs Applications

Highly efficient near-infrared (NIR) luminescent nanomaterials are urgently required for portable mini or micro phosphors-converted light-emitting diodes (pc-LEDs). However, most existing NIR-emitting phosphors are generally restricted by their low photoluminescence (PL) quantum yield (QY) or large particle size. Herein, a kind of highly efficient NIR nanophosphors is developed based on copper indium selenide quantum dots (CISe QDs). The PL peak of these QDs can be exquisitely manipulated from 750 to 1150 nm by altering the stoichiometry of Cu/In and doping with Zn2+ . Their absolute PLQY can be significantly improved from 28.6% to 92.8% via coating a ZnSe shell. By combining the phosphors with a commercial blue chip, an NIR pc-LED is fabricated with remarkable photostability and a record-high radiant flux of 88.7 mW@350 mA among the Pb/Cd-free QDs-based NIR pc-LEDs. Particularly, such QDs-based nanophosphors acted as excellent luminescence converter for NIR micro-LEDs with microarray diameters below 5 µm, which significantly exceeds the resolutions of current commercial inkjet display pixels. The findings may open new avenues for the exploration of highly efficient NIR micro-LEDs in a variety of applications.

Boosting broadband short-wave infrared emission to achieve near-unity quantum efficiency via bridging Cr3+-Ni2+ in spinel solid-solutions towards light-emitting diode applications

Recently, short-wave infrared (SWIR) phosphor-converted light-emitting diodes (pc-LEDs) have garnered increased attention due to their widespread applications in night vision, biological imaging, and non-destructive testing. Nevertheless, the currently used SWIR phosphors suffer from poor thermal stability and low quantum efficiency. In this study, a finely tuned spinel-based solid solution, Mg0.5Zn0.5Ga2O4, is prepared to host Ni2+ to induce SWIR emission. Cr3+ is codoped as a sensitizer to bridge Cr3+ and Ni2+ ions, significantly enhancing blue light absorption and facilitating energy transfer (ET) to Ni2+ ions. The champion SWIR phosphor exhibits a broadband emission centered at 1304 nm with a full width at half maximum (FWHM) of 250 nm, achieving a near-unity internal quantum efficiency (IQE = 97.7%) and a good thermal stability (70.7%@423 K). The fabricated SWIR pc-LED device delivers a high SWIR output power of 39.9 mW at 360 mA, enabling its application in non-destructive imaging and testing.