Achieving High Quantum Efficiency Broadband NIR Mg4 Ta2 O9 :Cr3+ Phosphor Through Lithium-Ion Compensation

Regulation of the Crystal Structure and Luminescence Properties of a BaLa1.6ZnO5:0.4Eu3+ Phosphor by Gd3+-Mg2+ Codoping for Solar Energy Utilization

Converting solar UV to red/far-red (VTR/FR) light can boost crop photosynthesis and morphogenesis. Consequently, broadband ultraviolet excitation and red/far-red emission phosphors are important VTR/FR light-conversion auxiliaries (LCAs) in solar greenhouses and solar cells. In this study, we have designed and synthesized a series of solid solution BaLa2-x-yGdyZn1-zMgzO5:xEu3+ phosphors. A Gd3+-Mg2+ cosubstituted La3+-Zn2+ strategy has been employed to regulate the crystal structure and local crystal field strength of the solid solution. At a low level of Gd3+-Mg2+ doping, the phosphor exhibited a tetragonal symmetric (BaLa2ZnO5) structure and an I4/mcm space group. The Eu3+ 5D0 → 7F4 transition generated intense far-red (706 nm) luminescence. At higher doping levels, the crystal structure switched to an orthorhombic low-symmetry (BaGd2ZnO5) Pbnm space group and 5D0 → 7F2 transition with a red emission at 627 nm. The optimized tetragonal BaLa0.8Gd0.8Zn0.89Mg0.11O5:0.4Eu3+ phosphor exhibited effective UVA excitation with a far-red emission (706 nm) and high external/internal quantum efficiency (EQE/IQE = 46.3/88.7%). The optimized orthorhombic BaLa0.2Gd1.4Zn0.3Mg0.7O5:0.4Eu3+ phosphor exhibited UVB excitation with a red emission (627 nm) and EQE/IQE = 46.4/94.3%. The luminescent film laminated glasses (LFLGs) made with the two phosphors had good VTR/FR performance in sunlight, showing potential in greenhouse and solar cell applications.

W-Doped Cs2SnCl6 for Near-Infrared Emission

0D perovskite derivatives such as Cs2WCl6 and Cs2WOxCl6-x have been recently shown to emit near-infrared (NIR) radiation. The d-d electronic transition of W4+/W5+ yields an NIR emission. However, the close proximity of those ions can quench the photoluminescence via concentration quenching. To address this issue, here we dilute the emission centers by doping a small amount of W into the Cs2SnCl6 0D perovskite. The results suggest that the dopant centers are [WOCl5]2- replacing [SnCl6]2- octahedra in the host lattice. The optimal 3.3% W-doped Cs2SnCl6 exhibits NIR (965 nm) emission with over 52 times higher intensity compared to that of Cs2WOxCl6-x. The suppression of concentration quenching in W-doped Cs2SnCl6 also significantly alters its temperature-dependent (7-300 K) photoluminescence compared to that of Cs2WOxCl6-x. Finally, we demonstrated NIR phosphor-converted light-emitting diodes of W-doped Cs2SnCl6 showing an output power of 10.3 mW at 400 mA. This is the first report of W doping in 0D perovskites showing its potential as an NIR phosphor.

Structural Rigidity Control via Non-Primary Lattice Substitution toward Thermally Stable Cr3+-Doped Near-Infrared Phosphors for pc-LED Applications

The performance of near-infrared phosphor-converted light-emitting diodes (NIR pc-LEDs) is dependent on the performance of the phosphor applied to the LED surface; however, the challenges of low external quantum efficiency (EQE) and insufficient thermal stability of NIR phosphors remain. Herein, we propose a novel nonprimary lattice substitution strategy (Y3+ → Gd3+) to enhance the structural rigidity of Gd1-yAl3-x(BO3)4:xCr3+,yY3+ phosphor. Unlike conventional host-site substitutions, this approach induces compressive strain on the [Al/CrO6] octahedron via a Gd3+/Y3+ size mismatch, thereby increasing bond energy and suppressing electron-phonon coupling. The optimized phosphor emits NIR light in the range of 650-1000 nm under 430 nm excitation, with the thermal stability (@423 K) improving from 73.8% to 92.5%, and the EQE is effectively improved. A prototype NIR pc-LEDs fabricated with a 450 nm chip delivers 40.4 mW output power at 100 mA with 14.7% photoelectric efficiency, demonstrating ultralow quenching rate (<6% intensity loss after 30 days operation). The NIR pc-LEDs was used in butter lettuce cultivation experiments, and the results showed that the growth pattern of butter lettuce changed significantly and the biomass increased significantly (28.9%). In addition, the potential for application in organic detection was demonstrated. This work provides a lattice engineering route to design stable NIR phosphors for multifunctional photovoltaic applications.

Tracing the origin of near-infrared emissions emanating from manganese (II)

The enduring enigma surrounding the near-infrared (NIR) emission of Mn2+ continues to ignite intense academic discussions. Numerous hypotheses have emerged from extensive research endeavors to explain this phenomenon, such as the formation of Mn2+-Mn2+ ion pairs, Mn2+ occupying cubically coordinated sites, as well as conjectures positing the involvement of Mn3+ oxidized from Mn2+ or defects. Despite these diverse and valuable insights, none of the hypotheses have yet achieved broad consensus. In this study, we have observed prolonged fluorescence lifetimes (~10 ms) for the NIR emissions of Mn2+ ions, hinting at these ions occupying the high-symmetry octahedral sites inherent to the garnet lattice. This inference is supported by the corroborating results from X-ray absorption fine structure analysis and first-principles calculations. The intense crystal field of octahedral sites, similar to that of AlO6, facilitates the splitting of d-d energy levels, thereby inducing a red-shift in the emission spectrum to the NIR region due to the transition 4T1(4G) → 6A1(6S) of isolated Mn2+. Our findings not only offer a plausible rationale for the NIR emission exhibited by other Mn2+-activated garnet phosphors but also pave a definitive route towards understanding the fundamental mechanisms responsible for the NIR emission of Mn2+ ions.

Lithiation-Driven Phase Engineering Unlocking Broadband NIR Emission in Cr-Doped Zinc Tantalate

Structural phase evolution is among the most powerful tools for tuning material properties, enabling advancements in catalysis, dielectrics, optoelectronics, and photoluminescence. Such an evolution can significantly enhance the near-infrared (NIR) emission properties of Cr3+-doped phosphors. Herein, we present, to the best of our knowledge, the first observation of lithiation-induced continuous structural phase evolution in ZnTa2O6 phosphors, driven by Li+ incorporation. This evolution proceeds systematically from orthorhombic ZnTa2O6 (Pbcn) to tetragonal ZnTa2O6 (P42/mnm) and ultimately to trigonal (Li0.5Zn0.5)TaO3 (R3c) as the Li+ content increases. When doped with Cr3+, the NIR emission peak exhibits a progressive blue shift, moving from 949 to 885 nm and eventually to 862 nm, in tandem with the phase evolution. This phase evolution also yields significant enhancements in photoluminescent intensity, internal quantum yield (IQY), and photoluminescence thermal stability. Our findings establish a new paradigm for designing highly efficient ultra-broadband NIR phosphors and offer a foundation for developing tantalate-based materials with versatile functionalities, including improved dielectric properties.

Large-scale preparation of Sb3+-activated hybrid metal halides with efficient tunable emission from visible to near-infrared regions for advanced photonic applications

Zero-dimensional metal halides with diverse structures and rich photophysical properties have been reported. However, achieving multimode dynamic luminescence and efficient near-infrared (NIR) emission under blue light excitation in a single system is a great challenge. Herein, Sb3+-doped hybrid Cd(II) halides were synthesized by a large scale synthesis process at room temperature. Compared with the poor emission of (C12H28N)2CdX4 (C12H28N = tetrapropylammonium; X = Cl and Br) and single steady-state visible light emission of (C12H28N)2SbX5, (C12H28N)2CdX4:Sb3+ exhibits efficient tunable emission from visible to NIR regions. More specifically, (C12H28N)2CdCl4:Sb3+ exhibits distinct excitation wavelength-dependent luminescence characteristics, which can change from green to white and orange emission. Parallelly, halogen substitution can regulate the optical properties of Sb3+-doped (C12H28N)2CdCl4-xBrx (x = 0-1), which enables the excitation and emission bands to exhibit a significant redshift. Thus, the efficient broad NIR emission upon 450 nm excitation was realized in (C12H28N)2CdBr4:Sb3+. In addition, we demonstrated the use of (C12H28N)2CdCl4:Sb3+ phosphors in solid state lighting, and an advanced NIR light source was fabricated by coating (C12H28N)2CdBr4:Sb3+ on a commercial blue chip (450 nm), which exhibits the most advanced photoelectric efficiency (14.67%) and output power (32.84 mW) in hybrid metal halides. Finally, we also demonstrated the use of Sb3+-activated phosphors in four-level fluorescence anti-counterfeiting and information encryption.

Constructing Broadband Near-Infrared Garnet Emitters CaGd2Ga4SiO12:Cr3+ with Unity Quantum Efficiency and High Thermal Stability for Versatile Applications

The pursuit of broadband near-infrared (NIR) phosphors for next-generation smart NIR light sources has garnered extensive interest. However, developing phosphors efficiently excitable by blue light to produce thermally stable and highly efficient broadband NIR emission surpassing 830 nm remains a formidable challenge. Herein, a novel CaGd2Ga4SiO12 garnet is reported, designed through a structure reconstruction approach to host Cr3+ ions for developing a high-performance broadband NIR phosphor. By strategically introducing Jahn-Teller distortion at the octahedral sites via chemical pressure, Cr3+ is endowed with a super-broadband NIR emission spanning 600-1300 nm centered at 837 nm. The full-width at half maximum (FWHM) varies from 187 to 223 nm across Cr3+ doping concentrations, with the highest internal quantum efficiency (IQE) of 99.01%. Remarkable luminescence thermal stability (90.37%@423 K) is bolstered by a weak electron-phonon coupling (EPC) effect and trap-mediated energy compensation, a result of the heterovalent ion substitutions in dodecahedral and tetrahedral sites. Furthermore, a prototype broadband NIR phosphor-converted light-emitting diode (pc-LED) is fabricated, delivering a substantial NIR output power of 287.7 mW at 1100 mA and a power conversion efficiency (PCE) of 24.4% at 30 mA, enabling impressive performance in versatile applications, including component analysis, non-destructive testing, NIR imaging, and night vision.

Stable and Highly Efficient Near-Infrared Emission Achieved in Spinel Blocks

Developing efficient and stable near-infrared emitters related to Cr3+-pairs for advanced optoelectronic devices remains a challenge due to concentration quenching effects and unclear luminescence mechanisms. In this study, Cr3+ ions are incorporated into a matrix structure consisting of ZnAl₂O₄ spinel units separated by 11.312 Å, effectively restricting energy transfer between luminescent centers and alleviating quenching effects. Computational analysis identifies the lattice positions of isolated Cr3+ ions and Cr3+-pairs at different doping levels, providing insights into their spatial distribution and local structural environments. Photoluminescence measurements reveals a Cr3+-concentration-dependent emission broadening, with a Cr3+-pair emission band peak at 750 nm, while detailed spectral analysis further clarified the energy level structure of Cr3+-pairs for the first time. Enhanced material performance is achieved through flux-assisted synthesis, reaching a high external quantum efficiency of 58.3%. Consequently, the assembled pc-LEDs exhibit minimal efficiency roll-off and achieve a high output of 183 mW at 650 mA, demonstrating their potential in near-infrared light sources and night vision technology application.

Near-infrared downshifting luminescence of Ca2LaTaO6:Nd3+/Yb3+/K+ phosphors and their applications: solar cells and anti-counterfeiting

A series of Nd3+/Yb3+ co-doped Ca2LaTaO6 (CLTO) phosphors are synthesized by a high temperature solid phase method. Structural characterization confirms the successful incorporation of Nd3+ and Yb3+ ions into the CLTO host lattice. The photoluminescence excitation (PLE) spectra and photoluminescence (PL) spectra of CLTO:Nd3+ and CLTO:Nd3+/Yb3+ are investigated in detail. Under the excitation of ultraviolet (UV) and visible (VIS) light, the CLTO:Nd3+/Yb3+ phosphor emits broadband near infrared (NIR) luminescence. In particular, the luminescence intensity of the Yb3+ ion is increased through an energy transfer (ET) process from Nd3+ to Yb3+. The luminescence mechanism of the CLTO:Nd3+/Yb3+ sample is analyzed based on the decay lifetime and PL spectra. In addition, the NIR emission intensity of Yb3+ ions is also enhanced by doping K+ ions. The broadband luminescence (850-1050 nm) of the CLTO:Nd3+/Yb3+/K+ phosphor has good application in solar cells and anti-counterfeiting.

Interstitial Oxygen-Driven Far-Red/Near-Infrared Emission and Efficiency Enhancement via Heterovalent Cation Substitution in Ca3WO6 Phosphors

In this work, Ca3WO6 (CWO) phosphors were successfully synthesized using a high-temperature solid-state method, exhibiting an anomalous far-red/near-infrared (FR-NIR) emission centered at 685 nm. The origin of this FR-NIR emission is confirmed through Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), density functional theory (DFT) calculations, and heterovalent cationic substitution (Y3+/Na+ → Ca2+). These analyses indicate that interstitial oxygen (Oi) defects within the lattice are primarily responsible for the FR-NIR emission. The heterovalent substitution of Y3+ for Ca2+ increases the concentration of Oi, significantly enhancing the FR-NIR emission intensity. This results in an increase in the internal quantum efficiency (IQE) increasing from 12.8 to 90.9%, realizing an efficient FR-NIR emission from self-activated phosphors. Furthermore, CWO phosphors demonstrate a unique dual-band emission characterized by a blue emission at 435 nm and an FR-NIR emission at 685 nm. This dual-band emission endows CWO phosphors with multifunctional applications in plant growth lighting, nondestructive testing, and night vision fields.

Rare-Metal-Free Ultrabroadband Near-Infrared Phosphors

Trivalent chromium (Cr3+) is an attractive near-infrared (NIR) emitter, but its ultrabroadband NIR emission is limited to host crystals containing large amounts of rare-metal elements and usually suffers from low internal quantum efficiency (IQE) and poor thermal stability. Here, a class of high-performance, rare-metal-free ultrabroadband NIR phosphors, are reported by revealing that weak-field Cr3+ centers featuring broadband NIR emission with near-unity IQEs are intrinsic, though in trace quantities, to Cr3+ doped MgAl2O4 spinel (MAS) and its derivatives well-known for their narrowband far-red emission. It is shown that such weak-field Cr3+ centers stem from cation inversion ubiquitous in spinel compounds, and their quantity can be increased simply by superstoichiometric Al2O3/Ga2O3. Then SiO2 is introduced into Al2O3-excess MAS to break the inversion symmetry of Cr3+ centers for greatly improving the probabilities of their otherwise parity-forbidden 3d-3d transitions. The as-fabricated phosphor-converted light-emitting diodes are capable of emitting ultrabroadband NIR light with high photoelectric efficiency (16.0%) and optical power (180.8 mW), and excellent spectral stability, which apparently outperforms existing state-of-the-art devices.

Achieving an Improved NIR Performance of Ca4-xSc2xZr1-xGe3O12:Cr3+ via [Sc3+-Sc3+]→[Ca2+-Zr4+]

High-performance near-infrared (NIR) light sources are highly sought after in advanced spectroscopy techniques, driving the development of NIR phosphor-converted light-emitting diodes (pc-LEDs). Escalating the luminescence intensity and thermal stability of NIR-emitting phosphors, which is a core component of NIR pc-LEDs, is of paramount importance. Herein, chemical unit cosubstitution and cosolvent addition tactics were implemented to simultaneously boost the NIR luminescence performance of the synthesized phosphors. The replacement of [Sc3+-Sc3+] for [Ca2+-Zr4+] in Ca4ZrGe3O12:Cr3+ likely reduces the antisite defects and offers a more rigid crystal structure. As a result, the emitting intensity is reinforced significantly, along with a remarkable improvement in thermal stability, acquiring 65% of the initial luminescence at 423 K compared with 42% for the primal sample. Moreover, the introduction of H3BO3 further enhances NIR luminescence while maintaining a favorable thermal resistance. The encapsulated NIR pc-LED carries an impressive output power of 71.8 mW at 300 mA and a conversion efficiency up to 15.6% at 10 mA. The practical presentations in food checking, imaging, and detection manifest that Ca3.5ScZr0.5Ge3O12:Cr3+,H3BO3 is a promising material for spectroscopy-based technologies.

Ultrabroadband Visible-Near-Infrared Phosphor CaY2Mg2Ge3O12:Ce3+/Cr3+ and Its Temperature Sensing

With the wide application of phosphor conversion light-emitting diode (pc-LED) in nondestructive testing, spectral illumination, near-infrared (NIR) spectroscopy, agricultural medical care, and other fields, the ultrabroadband visible-NIR tunable emission phosphor is considered as the most promising fluorescent material. In this paper, Ce3+/Cr3+ single-doped and codoped CaY2Mg2Ge3O12 phosphors were successfully prepared by using the high-temperature solid-state method. Ce3+ and Cr3+ codoped phosphors show ultrabroadband visible-NIR emission from 480 to 900 nm with two emission peaks at visible 556 nm and infrared 758 nm. Interestingly, energy transfer and occupation competition are shown in the Ce3+/Cr3+ codoped CaY2Mg2Ge3O12. Additionally, the luminescence of Ce3+/Cr3+ codoped CaY2Mg2Ge3O12 is adjusted with the doping ion concentration or ambient temperature changes. The energy transfer mechanism of Ce3+-Cr3+ is studied according to Dexter's formula, and a dipole-dipole interaction is determined. Furthermore, its superior sensitivity with Sr of 1.331%K-1 at 298 K was calculated by using the fluorescence intensity ratio. The pc-LED device was fabricated by combining a commercial 460 nm blue LED chip with CaY2Mg2Ge3O12:Ce3+, Cr3+ phosphors, producing yellow-white light and broadband NIR light, indicating great potential application in night vision pc-LEDs.

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.

Phosphor-in-Ceramic-Converted Laser-Driven Near-Infrared Light Sources for Multiple Intelligent Spectroscopy Applications

Ultrabright broadband near-infrared (NIR) phosphor-converted laser diode (pc-LD) as a light source is increasingly essential for improving the sensitivity and spatial resolution of intelligent NIR spectroscopy technologies. However, the performance of NIR pc-LD is greatly hindered by the low external quantum efficiency (EQE) and poor thermal resistance of phosphor materials. Herein, a highly stable phosphor-in-ceramic (PiC) film deposited on high thermal conductivity substrate, in which the NIR-emitting Ca3MgHfGe3O12:Cr3+ phosphor is incorporated into a glass-crystallized Ca3Ga2Ge3O12 ceramic matrix, along with the formation of a new type PiC composite material with high efficiency, absorbance, and thermal conductivity, is designed and prepared. The obtained PiC exhibits an impressive EQE of 57.7%, high thermal conductivity of 17.1 W m-1 K-1, and the PiC wheel demonstrates a broadband NIR emission exceeding 5 W when excited by a 450 nm laser. Finally, a groundbreaking electrically driven pc-LD device based on PiC achieves 1.6 W of NIR output, enabling multiple intelligent spectroscopy applications in archaeology and night vision imaging. This work paves the way for advancing broadband NIR light sources in a diverse range of photonic applications.

Sodium Assists Controlled Synthesis of Cubic Rare-Earth Oxyfluorides Nanocrystals for Information Encryption and Near-Infrared-IIb Bioimaging

Rare-earth oxyfluoride (REOF) colloidal nanocrystals (NCs) suffer from a low photoluminescence efficiency due to their small size with poor crystallinity and a detrimental surface quenching effect. Herein, we introduce an innovative approach that involves doping sodium ions into REOF NCs to produce monodisperse, size-controllable, well-crystallized, and highly luminescent colloidal REOF core/shell NCs. The Na+ doping allows for successfully synthesizing the cubic REOF NCs with a tunable size from 6 to 30 nm. Further fabrication of the core/shell NCs doped with Na+ results in enhancements up to 1062 (Ho3+), 1140 (Er3+), and 2212 (Tm3+) folds in upconversion luminescence and 17.7 folds (Er3+) in downconversion luminescence compared to that of core/shell NCs without doping Na+ ions. These NCs were subsequently developed into multicolor luminescent inks, demonstrating significant potential application for information security, and used for near-infrared-IIb (NIR-IIb) (1500-1700 nm) in vivo imaging, which exhibits a high-resolution in vivo dynamic imaging capability with a signal-to-noise ratio of 5.28. These results present the way to the controlled synthesis of efficient luminescent cubic LuOF: RE3+/LuOF core/shell NCs, expanding the toolkit of rare-earth doped NCs in diverse applications such as advanced encoding encryption, varied fluorescence imaging, and biomedicine.

Realize short-wave infrared luminescence in NaScP2O7:Cr3+,Yb3+ phosphor: Spectral and energy transfer

Short-wave infrared emitting phosphors have extensive applications for spectroscopy technology. The near-infrared phosphor NaScP2O7:Cr3+ that we present in this work has a full width at half maximum (FWHM) of approximately 196 nm, which ranges from 700 to 1200 nm. To achieve efficient short-wave infrared, Yb3+ ions were co-doped. The NaScP2O7:Cr3+,Yb3+ material emitted infrared bands with peaks at 970 and 1003 nm upon excitation at450 nm. Benefitting from energy transfer (ET), the light in the 900-1200 nm from Yb3+ is effectively enhanced. Photoluminescence spectra, thermal quenching, and decay curves of Cr3+/Yb3+ single and codoped NaScP2O7 were investigated. An internal quantum yield of 29.6 % wasdemonstrated by the optimized phosphor NaScP2O7:Cr3+,Yb3+. Furthermore, The final fabrication of the short-wave infrared pc-LED was done through the combination of a blue-emitting chip and NaScP2O7:Cr3+,Yb3+ phosphor, thereby showing great promise for real implementations.

Hydrofluoric Acid-Free Broadband Near-Infrared Phosphors K2LiMF6:Cr3+ with Zero-Thermal Quenching: Structure, Luminescence, and Application

Near-infrared (NIR) phosphor-converted light-emitting diodes (pc-LEDs) are considered promising light sources for night vision, food analysis, biomedicine, and plant growth. Yet, the application potential of this technology is vulnerable to the function degradation of the phosphors used, such as thermal quenching, which needs to be addressed urgently. Herein, the NIR phosphors K2LiMF6:Cr3+ (M = Al, Ga, In) with a cubic double-perovskite structure synthesized by a green hydrofluoric acid-free hydrothermal method exhibit outstanding thermal stability. Under 450 nm excitation, the as-synthesized K2LiMF6:Cr3+ phosphors all exhibited broadband NIR emission covering 650-1000 nm peaking at 755-780 nm. The prepared K2LiAlF6:Cr3+ phosphor shows a unique zero-thermal quenching performance (I423 K/I298 K = 102%). The comprehensive effects of a wide band gap, large thermal energy barrier, weak electron-phonon coupling effect, and high structural rigidity are responsible for the suppression of thermal quenching in this material. The output power of the NIR pc-LED device reached 285 mW at 100 mA. This series of phosphors has promise in night vision and bioimaging applications.

Efficient and Near-Zero Thermal Quenching Cr3+-Doped Garnet-Type Phosphor for High-Performance Near-Infrared Light-Emitting Diode Applications

In recent years, near-infrared (NIR) phosphors have attracted great research interest due to their unique physical properties and broad application prospects. However, obtaining NIR phosphors with both high quantum efficiency and excellent thermal stability remains a great challenge. In this study, novel NIR Ca3Mg2ZrGe3O12:Cr3+ phosphors were successfully prepared using a high-temperature solid-phase method, and the structure and luminescent properties of the material were systematically investigated. Ca3Mg2ZrGe3O12:0.01Cr3+ emits NIR light in the range of 600 to 900 nm with a peak at 758 nm and a half-height width of 89 nm under the excitation of 457 nm blue light. NIR luminescence shows considerable quantum efficiency, and the internal quantum efficiency of the optimized sample is up to 68.7%. Remarkably, the Ca3Mg2ZrGe3O12:0.01Cr3+ phosphor exhibits a near-zero thermal quenching behavior, and the luminescence intensity of the sample at 250 °C maintains 92% of its intensity at room temperature. The mechanism of high thermal stability has been elucidated by calculating the Huang Kun factor and activation energy. Finally, NIR pc-LED devices prepared from Ca3Mg2ZrGe3O12:0.01Cr3+ phosphor with commercial blue LED chips have good performance, proving that this Ca3Mg2ZrGe3O12:0.01Cr3+ NIR phosphor has potential applications in night vision and biomedical imaging.

Achieving a Balance of Good Quantum Efficiency and Thermal Stability in the Y2CaScAl3GeO12:Cr3+ Broadband Phosphor for Multiple NIR Spectroscopy Applications

Near-infrared phosphor-converted light emitting diodes (NIR pc-LEDs) are considered as desirable NIR light sources to satisfy current needs owing to their numerous remarkable features. Nevertheless, as an essential component, previously reported NIR phosphors with broadband emission often suffer from inferior efficiency or thermal stability, therefore restricting their use and promotion. Herein, a novel Cr3+-doped garnet phosphor Y2CaScAl3GeO12:Cr3+ (YCSAG:Cr3+) is developed via regulating the near-neighbor coordination polyhedron. Under the excitation of blue light, it exhibits a broadband NIR emission peaking near 800 nm with a full width at half-maximum (fwhm) exceeding 150 nm, owing to the increased structural distortion of the octahedron. Particularly, due to the enhanced local structural rigidity induced by lattice shrinkage, the optimal sample achieves a balance of high internal quantum efficiency (IQE) of approximately 83% and thermal stability of approximately 90% at 393 K, facilitating its practical application as an NIR light source. Eventually, using the typical YCSAG:0.04Cr3+ phosphor and 450 nm blue LED chip, a high-performance NIR pc-LED device has been manufactured, demonstrating potential applications in anticounterfeiting and night vision.

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.

Bioimaging and prospects of night pearls-based persistence phosphors in cancer diagnostics

Inorganic persistent phosphors feature great potential for cancer diagnosis due to the long luminescence lifetime, low background scattering, and minimal autofluorescence. With the prominent advantages of near-infrared light, such as deep penetration, high resolution, low autofluorescence, and tissue absorption, persistent phosphors can be used for deep bioimaging. We focus on highlighting inorganic persistent phosphors, emphasizing the synthesis methods and applications in cancer diagnostics. Typical synthetic methods such as the high-temperature solid state, thermal decomposition, hydrothermal/solvothermal, and template methods are proposed to obtain small-size phosphors for biological organisms. The luminescence mechanisms of inorganic persistent phosphors with different excitation are discussed and effective matrixes including galliumate, germanium, aluminate, and fluoride are explored. Finally, the current directions where inorganic persistent phosphors can continue to be optimized and how to further overcome the challenges in cancer diagnosis are summarized.

Realizing Ultrabroadband NIR-II Emission and Wide-Range Wavelength Tuning in Cr4+-activated ABO2 (A = Li, Na; B = Al, Ga) Phosphors

Cr4+-activated phosphors are important candidate materials for NIR-II light sources, but providing a suitable lattice coordination environment for Cr4+ and achieving long wavelength broadband emission remains a challenge. In this work, a series of Cr4+-activated ABO2 (A = Li, Na; B = Al, Ga) phosphors were successfully prepared. Due to the presence of only tetrahedral coordination structures available for Cr4+ to occupy in the matrix crystal ABO2, the valence state and luminescence stability of Cr4+ are effectively guaranteed. Through the cation substitution design of A-site (Na → Li) and B-site (Ga → Al), the [BO4] tetrahedron is distorted and expanded, which degrades the symmetry of the Cr4+ coordination crystal field. Consequently, the central wavelength of the Cr4+ emission peak is tuned from 1280 to 1430 nm, and the fwhm is significantly extended from 257 to 355 nm. Thebroadband NIR-II light sources constructed with LiAlO2: 0.03Cr4+ and NaGaO2: 0.03Cr4+ phosphors verify their important potential applications in nondestructive testing and biological imaging.

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.

Unlocking Cr3+-Cr3+ Coupling in Spinel: Ultrabroadband Near-Infrared Emission beyond 900 nm with High Efficiency and Thermal Stability

Broadband near-infrared (NIR) phosphor-converted light-emitting diodes (pc-LEDs) hold promising potential as next-generation compact, portable, and intelligent NIR light sources. Nonetheless, the lack of high-performance broadband NIR phosphors with an emission peak beyond 900 nm has severely hindered the development and widespread application of NIR pc-LEDs. This study presents a strategy for precise control of energy-state coupling in spinel solid solutions composed of MgxZn1-xGa2O4 to tune the NIR emissions of Cr3+ activators. By combining crystal field engineering and heavy doping, the Cr3+-Cr3+ ion pair emission from the 4T2 state is unlocked, giving rise to unusual broadband NIR emission spanning 650 and 1400 nm with an emission maximum of 913 nm and a full width at half-maximum (fwhm) of 213 nm. Under an optimal Mg/Zn ratio of 4:1, the sample achieves record-breaking performance, including high internal and external quantum efficiency (IQE = 83.9% and EQE = 35.7%) and excellent thermal stability (I423 K/I298 K = 75.8%). Encapsulating the as-obtained phosphors into prototype pc-LEDs yields an overwhelming NIR output power of 124.2 mW at a driving current of 840 mA and a photoelectric conversion efficiency (PCE) of 10.5% at 30 mA, rendering high performance in NIR imaging applications.

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.

Thermal stability and quantum efficiency improvement of Cr3+-activated garnet phosphors via regulating A/B sites for near-infrared LED applications

The discovery of phosphors with highly efficient broadband near-infrared emission is urgent for constructing NIR sources for various applications. Herein, we synthesized a series of near-infrared emitting garnet-type (A3B2C3O12) Lu2-xCaAl3.99Cr0.01SiO12:xGd/La and Lu2CaAl3.99-yCr0.01SiO12:ySc/Ga phosphors and systematically investigated the effect of A/B-site substitution on the crystal structure and luminescence properties. Structural and optical analyses revealed that the A/B-site substitution weakened the crystal field strength, further enhancing the broadband emission of the allowed 4T2 → 4A2transition and diminishing the narrow-peak emission of the forbidden 2E → 4A2 transition. As expected, NIR phosphors, exemplified by Lu1.7CaAl3.99Cr0.01SiO12:0.3Gd and Lu2CaAl3.49Cr0.01SiO12:0.5Sc, showed outstanding thermal stabilities at 423 K (150 °C) registering values of 103.02% and 94.91%, with high quantum efficiencies of 80.48% and 85.01%, respectively. In addition, pc-LEDs with broadband NIR output and good optoelectronic properties have been realized, demonstrating the great potential of broadband NIR pc-LEDs for applications. This work not only provides a series of high-efficiency phosphors for NIR pc-LED applications, but also provides a systematic idea and an efficient method to improve the luminescence performance of garnet-type phosphors.

High Output Power and High Quantum Efficiency in Novel NIR Phosphor MgAlGa0.7 B0.3 O4 :Cr3+ with Profound FWHM Variation

There is strong demand for ultraefficient near-infrared (NIR) phosphors with adjustable emission properties for next-generation intelligent NIR light sources. Designing phosphors with large full-width at half-maximum (FWHM) variations is challenging. In this study, novel near-ultraviolet light-emitting diode (LED)-excited NIR phosphors, MgAlGa0.7 B0.3 O4 :Cr3+ (MAGBO:Cr3+ ), with three emission centers achieve ultra-narrowband (FWHM = 29 nm) to ultra-broadband (FWHM = 260 nm) emission with increasing Cr3+ concentration. Gaussian fitting and decay time analysis reveal the alteration in the FWHM, which is attributed to the energy transfer occurring between the three emission centers. The distinct thermal quenching behaviors of the three emission centers are revealed through the temperature-dependent decay times. The ultra-broadband NIR phosphor MAGBO:0.05Cr3+ exhibits high thermal stability (85%, 425 K) and exceptional external quantum efficiency of 68.5%. An NIR phosphor-converted LED (pc-LED) is fabricated using MAGBO:0.05Cr3+ phosphor, exhibiting a remarkable NIR output power of 136 mW at 600 mA in ultra-broadband NIR pc-LEDs. This study describes the preparation of highly efficient phosphors and provides a further understanding of the tunable FWHM, which is vital for high-performance NIR phosphors with versatile applications.

Broadband short-wave near-infrared-emitting phosphor MgNb2O6:Cr3+ for pc-LED applications

Broadband short-wave near-infrared (NIR) phosphor-converted light-emitting diodes (pc-LEDs) have been attracting keen interest for miniature NIR spectroscopy, while still lacking sufficient novel broadband NIR-emitting phosphors. Herein, we report a novel MgNb2O6:Cr3+ polycrystalline phosphor with a broad NIR emission band centered at 970 nm and a large full-width at half-maximum of approximately 155 nm under excitation of bluish-green light at around 515 nm. The optimized phosphor MgNb2O6:1%Cr3+ features a high internal quantum efficiency (IQE) of ∼85.5% and a moderate external QE of 25.2%. The fluorescence properties determined by two distorted hexa-coordination octahedral sites (i.e. [MgO6] and [NbO6]), low crystal field strength (Dq/B ∼ 1.65), and Cr3+-doping concentration were systematically investigated for comprehensive understanding of photophysical mechanisms. Besides, this broadband NIR phosphor MgNb2O6:Cr3+ exhibits a moderate thermal quenching of 21.4%@373 K for pc-LED application. An NIR pc-LED self-built by combining the optimal phosphor with a commercial cyan of ∼515 nm exhibits an NIR output power increase from 3.19 to 11.38 mW as the drive current is varied from 40 to 220 mA. With the help of this prototype pc-LED device, multiple applications were successfully performed to clearly recognize blood vessel distributions in the human finger, penetrate a plastic cap, and distinguish multi-color text. Undoubtedly, further development of such broadband short-wave NIR-emitting phosphors will make novel pc-LED devices for significant applications in biomedical imaging, nondestructive safety detection, intelligent identification, etc.

Luminescence-Based Infrared Thermal Sensors: Comprehensive Insights

Recent chronological breakthroughs in materials innovation, their fabrication, and structural designs for disparate applications have paved transformational ways to subversively digitalize infrared (IR) thermal imaging sensors from traditional to smart. The noninvasive IR thermal imaging sensors are at the cutting edge of developments, exploiting the abilities of nanomaterials to acquire arbitrary, targeted, and tunable responses suitable for integration with host materials and devices, intimately disintegrate variegated signals from the target onto depiction without any discomfort, eliminating motional artifacts and collects precise physiological and physiochemical information in natural contexts. Highlighting several typical examples from recent literature, this review article summarizes an accessible, critical, and authoritative summary of an emerging class of advancement in the modalities of nano and micro-scale materials and devices, their fabrication designs and applications in infrared thermal sensors. Introduction is begun covering the importance of IR sensors, followed by a survey on sensing capabilities of various nano and micro structural materials, their design architects, and then culminating an overview of their diverse application swaths. The review concludes with a stimulating frontier debate on the opportunities, difficulties, and future approaches in the vibrant sector of infrared thermal imaging sensors.

The broadband emission of Cr3+-doped CaY2Mg2Ge3O12 and its applications for NIR detectors

A phosphor-converted light-emitting diode (pc-LED) is a prime light source in smart broadband near-infrared (NIR) spectroscopy. The performance of NIR pc-LEDs crucially depends on the employed NIR luminescent materials. In this study, we synthesized a novel high-efficiency broadband NIR phosphor, CaY2Mg2Ge3O12:Cr3+ (CYMG:Cr3+). Under 450 nm excitation, CYMG:Cr3+ exhibited remarkable broadband NIR emission from 650 to 900 nm with a full width at half maximum (FWHM) of 115 nm. Within the CYMG lattice, the Cr3+ ion occupies Ca/Y sites in the dodecahedron Ca/YO8 and Mg sites in the octahedron MgO6, giving rise to two distinct Cr3+ luminescence centers. Remarkably, the emission at 100 °C remained at 92% of its room temperature intensity and 81% at 150 °C, showcasing its exceptional thermal stability. The internal quantum efficiency (IQE) reached an impressive 81.1%, with an activation energy ΔE of 0.324 eV. Furthermore, we integrated the CYMG:Cr3+ phosphor with a commercial 450 nm blue chip to fabricate a micro NIR pc-LED, which exhibited stable NIR emission across different driving currents, with a NIR output power of 49.65 mW@400 mA and a photoelectric conversion efficiency of 10.52% at 20 mA. All findings highlight CYMG:Cr3+ as a stable and efficient broadband luminescent material for high-performance NIR LEDs.

Realization of Broadband Near-Infrared Emission with High Thermal Stability in YGa3(BO3)4: Cr3+ Borate Phosphor

As a key material for phosphor-converted light-emitting diodes (pc-LEDs) applications, broadband near-infrared (NIR) phosphors currently face poor thermal stability issues. In this work, we synthesized a broadband near-infrared phosphor YGa3(BO3)4: Cr3+ (YGBO: Cr3+) with a high thermal stability. The YGBO: Cr3+ sample exhibits a broadband near-infrared emission centered at 770 nm with a full width at half-maximum (fwhm) of 2130 cm-1 under blue light excitation. Benefiting from the borate host crystal's strong structural rigidity, wide optical band gap, and weak electron-phonon coupling strength, YGBO: Cr3+ demonstrates strong luminescence thermal stability, and the corresponding luminescence intensity can maintain 80% at 150 °C compared to room temperature. Furthermore, we fabricated a pc-LED device using a blue light chip and YGBO: Cr3+ phosphor, and confirmed its application potential as a near-infrared light source in the spectral analysis of fruit freshness.

A novel broadband Ba3Ca4(BO3)3(SiO4)Cl:Mn4+ near-infrared phosphor with a special pseudo-octahedral Mn4+ coordination structure

A pseudo-octahedral coordination structure of Mn4+ has been innovatively designed, which has realized the maximum red shift and the widest full width at half-maximum (FWHM) of Mn4+ emission so far, not only extending the emission wavelength of Mn4+ to the near-infrared (NIR) region, but also effectively broadening its bandwidth. In the Ba3Ca4(BO3)3(SiO4)Cl:Mn4+ (BCBSC:Mn4+) phosphor, the [Mn/Ca1O9] polyhedron contains one [Mn/Ca1O6] octahedron, which constitutes the pseudo-octahedral coordination structure of Mn4+. The BCBSC:Mn4+ phosphor can be excited at 362 nm and 470 nm and exhibits a broadband NIR emission centered at ∼756 nm with a super-wide range from 650 nm to 1100 nm. The FWHM can reach ∼90 nm. In addition, the internal quantum efficiency (IQE) of the BCBSC:0.01Mn4+ phosphor is 69.7%. The unique luminescence characteristics of BCBSC:Mn4+ phosphors are explored using experimental data and first principles calculation. The significant redshift, the abnormal broadband emission, and the high luminous efficiency are closely related to the special highly distorted [Mn/Ca1O6] pseudo-octahedral coordination environment. The results contribute to comprehending the mechanism of the broadband NIR emission of Mn4+ activated phosphors and broaden the research ideas of developing high-performance Mn4+ doped phosphors for NIR phosphor-converted light-emission diode applications.

Suppressed Concentration Quenching Brightens Short-Wave Infrared Emitters

The brightness of doped luminescent materials is usually limited by the ubiquitous concentration quenching phenomenon resulting in an intractable tradeoff between internal quantum efficiency and excitation efficiency. Here, an intrinsic suppression of concentration quenching in sensitized luminescent systems, by exploiting the competitive relationship between light emitters and quenchers in trapping excitation energies from sensitizers, is reported. Although Cr3+ sensitizers and trivalent lanthanide (Ln3+ , Ln = Yb, Nd, and Er) emitters themselves are highly susceptible to concentration quenching, the unprecedentedly high-brightness luminescence of Cr3+ -Ln3+ systems is demonstrated in the short-wave infrared (SWIR) range employing high concentrations of Cr3+ , whereby a record photoelectric efficiency of 23% is achieved for SWIR phosphor-converted light-emitting diodes, which is about twice as high as those previously reported. The results underscore the beneficial role of emitters in terminating excitation energies, opening up a new dimension for developing efficient sensitized luminescent materials.