Structure and photoluminescence properties of Ca0.99−xSrxAlSiN3:0.01Ce3+ solid solutions

J. Am. Ceram. Soc., 2019, 102(8), 4648-4658.

Nitride phosphors of Ca0.99−xSrxAl1.01Si0.99N3:0.01Ce3+ (0 ≤ ≤ 0.9) were synthesized by conventional solid-state method. XRD data analysis shows that all samples are single phase with CaAlSiN3-type structure. Under blue or near-ultraviolet (~400 nm) light excitation, the emission peak can be tuned largely from 615 to 568 nm by increasing Sr content, and the emission intensity is maximal at = 0.8. With the Sr content increase, the emission band blue-shifts due to the decreased crystal field splitting and the reduced centroid shift; while the thermal luminescence quenching resistance is almost unchanged. The quenching temperature (T50) is well above 500 K for all samples, which satisfies the requirement of commercial application. The quenching process is mainly attributed to the radiationless transitions by thermally activated crossover from the 5d excited state to 4f ground state in the configurational coordinate diagram. The luminescence properties show that the (Ca,Sr)AlSiN3:Ce3+ phosphors are very promising for use in blue and near-ultraviolet light excited white-light-emitting diodes.

Relationship of Stokes shift with composition and structure in Ce3+/Eu2+-doped inorganic compounds

J. Lumin., 2019, 212, 250-263.

Fig. 1. Energy level diagram showing the crystal field splitting εcfs, centroid shift…

The effect of chemical composition and structure on Stokes shift, an important luminescence property, has not been studied systematically. In this work, through statistical analysis of the structural and luminescent data of more than 60 compounds doped with Ce3+/Eu2+, the relationships between the Stokes shift, phonon energy, Huang-Rhys factor, chemical composition and structure of the compounds are revealed. Stokes shift has the positive correlation to the effective average coordination bond length Rav. It decreases in going through the halogenide series from fluorides to bromides, and the chalcogenides from oxides to selenides, respectively; while it is almost irrelevant of the anion types in the same period (nitrides, oxides, and fluorides). We also verify that the Stokes shift has no obvious correlation with coordination number of the cation. This paper provides a feasibility to predict the emission spectrum of Ce3+/Eu2+ doped inorganic compounds and discover the suitable host lattice for required phosphors.

Polyhedron transformation toward stable narrow‐band green phosphors for wide‐color‐gamut liquid crystal display

Adv. Funct. Mater., 2019, 29(30), 1901988.

A robust and stable narrow-band green emitter is recognized as a key enabler for wide-color-gamut liquid crystal display (LCD) backlights. Herein, an emerging rare earth silicate phosphor, RbNa(Li3SiO4)2:Eu2+ (RN:Eu2+) with exceptional optical properties and excellent thermal stability, is reported. The resulting RN:Eu2+ phosphor presents a narrow green emission band centered at 523 nm with a full width at half maximum of 41 nm and excellent thermal stability (102%@425 K of the integrated emission intensity at 300 K). RN:Eu2+ also shows a high quantum efficiency, an improved chemical stability, and a reduced Stokes shift owing to the modified local environment, in which [NaO8] cubes replace [LiO4] squares in RbLi(Li3SiO4)2:Eu2+ via polyhedron transformation. White light-emitting diode (wLED) devices with a wide color gamut (113% National Television System Committee (NTSC)) and high luminous efficacy (111.08 lm W−1) are obtained by combining RN:Eu2+ as the green emitter, K2SiF6:Mn4+ as the red emitter, and blue-emitting InGaN chips. Using these wLEDs as backlights, a prototype 20.5 in. LCD screen is fabricated, demonstrating the bright future of stable RN:Eu2+ for wide-color-gamut LCD backlight application.

Unraveling the mechanochemical synthesis and luminescence in MnII-based two-dimensional hybrid perovskite (C4H9NH3)2PbCl4

Sci. China Mater. 2019, 62, 1013–1022.

The mechanochemical route is a facile and fast way and has received much attention for developing versatile advanced functional materials. Herein, we reported a mechanochemical synthesis for incorporating divalent manganese ions (MnII) into a two-dimensional (2D) hybrid perovskite (C4H9NH3)2PbCl4. The mild external stimuli originating from the grinding at room temperature enabled the formation of MnII-doped 2D hybrid perovskites, and rapidly changed the luminescence characteristics. The photoluminescence analyses show that the violet and orange emissions are attributed to (C4H9NH3)2Pb1–xMnxCl4 band-edge emission and the T16A1 transition of Mn2+ resulting from an efficient energy transfer process, respectively. Site preference and distribution of the doped Mn2+ cations on the locations of Pb2+ were analyzed. The formation energy calculated by the density functional theory (DFT) indicates that the Mn2+ ions can rapidly enter the crystal lattice due to the unique 2D crystal structure of the hybrid perovskite. Such a case of mechanochemical synthesis for the 2D hybrid perovskite motivates many novel emerging materials and the related applications.

Enhanced Yellow Persistent Luminescence in Sr3SiO5:Eu2+ through Ge Incorporation

Inorg. Chem., 2019, 58, 13, 8694–8701.

Abstract Image
The Sr3SiO5:Eu2+ phosphor has attracted considerable attention for applications in white LEDs owing to its highly efficient yellow emission under violet-blue excitation. We report herein an enhancement of yellow persistent luminescence in Sr3SiO5:Eu2+ through Ge incorporation. The strongest persistent luminescence intensity is observed for Sr3(Si1–xGex)O5:Eu2+ with x = 0.005 with a peak emission wavelength at ∼580 nm and a persistent time of ∼7000 s at the 0.32 mcd/m2 threshold value after UV radiation. A combination of thermoluminescence measurements and density functional theory (DFT) calculations reveals that the afterglow enhancement is due to a significant increase in the number of oxygen vacancies that act as electron trapping centers with appropriate trap depths. This investigation is anticipated to encourage more exploration of GeSi substitution to design and improve Si-containing persistent phosphors with superior functionalities.

Recent advances in solid-state LED phosphors with thermally stable luminescence

J. Rare. Earth., 2019, 37(6), 565-572.

Image 1
Phosphor-converted white light-emitting diode (LED) lighting has gained tremendous achievements since the invention of the InGaN blue LED by Nakamura et al., who won the Nobel Physics Prize in 2014. By far, a significant challenge comes from the thermal quenching (TQ) behavior of the present LED phosphors during the high-power LED operation or the updated laser lighting. But systematic research or review on the luminescence quenching character and/or how to realize thermally stable luminescence are lacking. Since TQ is an inherent property of phosphors, it can be diminished by different approaches. This review proceeds from the mechanism of TQ, summarizes previous researches on improving the thermal stability of LED phosphors and also discusses future research opportunities in this field. The developments of the phosphors with properties of high luminance and thermal stability, as well as the improved strategies involved, will benefit the basic researches and applications in high power lighting or high-luminance laser lighting.

Structural Indicator to Characterize the Crystal-Field Splitting of Ce3+ in Garnets

J. Phys. Chem. C., 2020, 124, 1, 870–873.

The structure–property relationship is a hot research topic in both chemistry and material science. A structural descriptor has significant applications in machine learning and high-throughput screening for rapid estimations of material properties. In this work, we report a simple structural indicator to characterize the crystal-field splitting of Ce3+ ions in the garnet structure. The indicator is the interplanar distance of the octahedron, which is calculated as 2/√3 | x + y + z | · L, where L stands for the cell parameter and (x, y, z) are the oxygen coordinates in a specific form. The indicator value is correlated to the crystal-field splitting of Ce3+ in the lanthanide aluminum/gallium garnets and is able to reproduce the reverse garnet effect. By inspecting the polyhedral competition effect in the garnet structure, this indicator is found to be correlated to the tetragonal distortion of the dodecahedron, which determines the crystal-field splitting.

Lead‐Free Perovskite Derivative Cs2SnCl6−xBrx Single Crystals for Narrowband Photodetectors

Adv. Opt. Mater. 2019, 7(10).

Lead-free and stable Sn halide perovskites demonstrate tremendous potential in the field of optoelectronic devices. Here, the structure and optical properties of the “defect” perovskites Cs2SnCl6−xBrx are reported, as well as their use as photodetector materials. Millimeter-sized Cs2SnCl6−xBrx single crystals are grown by the hydrothermal method, with the body color continuously changing from transparent to yellow and finally to dark red. Narrowband single-crystal photodetectors using Cs2SnCl6−xBrx crystals are presented, which show a high detectivity of ≈2.71 × 1010 Jones, with narrowband photodetection (full-width at half-maximum ≈45 nm) and high ion diffusion barriers. Moreover, the response spectra are continuously tuned from near violet to orange depending on the variation of the bandgap of the single crystals by changing the halide compositions. The strong surface charge recombination of the excess carriers near the crystal surfaces produced by short wavelength light elucidates the narrowband photodetection behavior. This work provides a new paradigm in the design of lead-free, stable, and high-performance perovskite derivatives for optoelectronics applications.

Emerging ultra-narrow-band cyan-emitting phosphor for white LEDs with enhanced color rendition

Light Sci Appl., 8, 38 (2019).

Phosphor-converted white LEDs rely on combining a blue-emitting InGaN chip with yellow and red-emitting luminescent materials. The discovery of cyan-emitting (470–500 nm) phosphors is a challenge to compensate for the spectral gap and produce full-spectrum white light. Na0.5K0.5Li3SiO4:Eu2+ (NKLSO:Eu2+) phosphor was developed with impressive properties, providing cyan emission at 486 nm with a narrow full width at half maximum (FWHM) of only 20.7 nm, and good thermal stability with an integrated emission loss of only 7% at 150 °C. The ultra-narrow-band cyan emission results from the high-symmetry cation sites, leading to almost ideal cubic coordination for UCr4C4-type compounds. NKLSO:Eu2+ phosphor allows the valley between the blue and yellow emission peaks in the white LED device to be filled, and the color-rendering index can be enhanced from 86 to 95.2, suggesting great applications in full-spectrum white LEDs.

Design Optimization of Lead-Free Perovskite Cs2AgInCl6:Bi Nanocrystals with 11.4% Photoluminescence Quantum Yield

Chem. Mater. 2019, 31, 9, 3333–3339.

Lead-free halide double perovskites continue to draw increasing attention in view of their nontoxicity and stability compared to lead-based perovskites. Herein, we optimized the colloidal synthesis of undoped and Bi-doped Cs2AgInCl6 nanocrystals (NCs) via the facile hot-injection process using nontoxic and available precursors. The synthesis conditions including temperature, ligands, and hydrochloric acid are investigated to boost the photoluminescence quantum yield (PLQY) of Cs2AgInCl6:Bi NCs with the trace amount of Bi doping. The broad-band orange emission peaking at 580 nm is observed with the PLQY as high as ∼11.4%, and the related luminescence mechanism has been discussed. This work provides a reliable avenue toward the optimization of optical properties for emerging lead-free halide perovskite NCs with enhanced PLQY.