Efficiency-Tunable Single-Component White-Light Emission Realized in Hybrid Halides Through Metal Co-Occupation

ACS Appl. Mater. Interfaces 2021, 13, 25, 29835–29842 https://pubs.acs.org/doi/abs/10.1021/acsami.1c07636

Organic–inorganic hybrid metal halides have attracted widespread attention as emerging optoelectronic materials, especially in solid-state lighting, where they can be used as single-component white-light phosphors for white light-emitting diodes. Herein, we have successfully synthesized a zero-dimensional (0D) organic–inorganic hybrid mixed-metal halide (Bmpip)2PbxSn1–xBr4 (0 < x < 1, Bmpip+ = 1-butyl-1-methyl-piperidinium, C10H22N+) that crystallizes in a monoclinic system in the C2/c space group. Pb2+ and Sn2+ form a four-coordinate seesaw structure separated by organic cations forming a 0D structure. For different excitation wavelengths, (Bmpip)2PbxSn1–xBr4 (0 < x < 1) exhibits double-peaked emission at 470 and 670 nm. The emission color of (Bmpip)2PbxSn1–xBr4 can be easily tuned from orange-red to blue by adjusting the Pb/Sn molar ratio or excitation wavelength. Representatively, (Bmpip)2Pb0.16Sn0.84Br4 exhibits approximately white-light emission with high photoluminescence quantum yield up to 39%. Interestingly, the color of (Bmpip)2PbxSn1–xBr4 can also be easily tuned by temperature, promising its potential for application in temperature measurement and indication. Phosphor-converted light-emitting diodes are fabricated by combining (Bmpip)2PbxSn1–xBr4 and 365 nm near-UV LED chips and exhibit high-quality light output.

Lead-Free Broadband Orange-Emitting Zero-Dimensional Hybrid (PMA)3InBr6 with Direct Band Gap

Inorg. Chem., 2019, 58, 22, 15602–15609. https://pubs.acs.org/doi/abs/10.1021/acs.inorgchem.9b02669

Low-dimensional organic–inorganic hybrid metal halides have emerged as broadband light emitters for phosphor-converted white light-emitting diodes (WLEDs). Herein, we report a new zero-dimensional (0-D) lead-free metal halide (PMA)3InBr6 [PMA+: (C6H5CH2NH3)+] that crystallizes in the monoclinic system with P21/c space group. The structure consists of slightly distorted [InBr6]3– octahedra surrounded by organic PMA+ cations. The direct band gap characteristic of (PMA)3InBr6 was demonstrated by density functional theory calculation, and its relatively wide band gap of 3.78 eV was experimentally determined. Upon 365 nm ultraviolet light excitation, (PMA)3InBr6 exhibited strong broadband orange luminescence with a full-width at half-maximum of ∼132 nm resulting from self-trapped exciton emission, and the photoluminescence quantum yield was determined to be ∼35%. A WLED fabricated by combining the orange-emitting (PMA)3InBr6, a green phosphor Ba2SiO4:Eu2+, and a blue phosphor BaMgAl10O17:Eu2+ exhibited a high color-rendering index of 87.0. Our findings indicate that the organic–inorganic hybrid (PMA)3InBr6 may have potential for luminescence-based applications.

Luminescent thermal stability and electronic structure of narrow-band green-emitting Sr-Sialon: Eu2+ phosphors for LED/LCD backlights

J. Alloys Compd., 2019, 805, 1246-1253. https://www.sciencedirect.com/science/article/abs/pii/S092583881932715X

Stable and high-efficiency narrow-band green phosphor is a key component for wide color gamut liquid crystal display (LCD) backlights. In this paper, narrow-band green-emitting Sr3-3xSi13Al3O2N21:3xEu2+ (0.001 ≤ x ≤ 0.09) (Sr-Sialon:Eu2+) phosphor with a full-width at half maximum of 66 nm has been successfully synthesized by using the solid-state reaction method. All the samples are the pure phase with Sr3Si13Al3O2N21-type structure. Their emission band maximum can be tuned from 495 to 523 nm by increasing Eu2+ content. The compound with x = 0.03 possesses the highest luminescence intensity with the peak position around 510 nm. Luminescent thermal stability gets better with Eu2+ concentration decreasing. The integrated intensity of the sample with x = 0.01 at 425 K remains about 80% of the intensity at room temperature. The host referred binding energy (HRBE) and vacuum referred binding energy (VRBE) schemes are constructed to further explain its luminescent thermal quenching mechanism. White light-emitting-diode (w-LED) device using optimized Sr2.91Si13Al3O2N21:0.09Eu2+ phosphor demonstrates its potential application for LCD backlights.

Tolerance factor and phase stability of the garnet structure

Acta Cryst., 2019. C75, 1353-1358. https://scripts.iucr.org/cgi-bin/paper?qp3034

We introduce a structural descriptor, the tolerance factor, for the prediction and systematic description of the phase stability with the garnet structure. Like the tolerance factor widely adopted for the perovskite structure, it is a com­positional parameter derived from the geometrical relationship between multi-type polyhedra in the garnet structure, and the calculation only needs the information of the ionic radius. A survey of the tolerance factor over 130 garnet-type com­pounds reveals that the data points are scattered in a narrow range. The tolerance factor is helpful in understanding the crystal chemistry of some garnet-type com­pounds and could serve as a guide for predicting the stability of the garnet phase. The correlation between the tolerance factor and the garnet-phase stability could be utilized by machine learning or high-throughput screening methods in material design and discovery.

Selecting nitride host for Yb3+ toward near-infrared emission with low-energy charge transfer band

Yb3þ-doped phosphors have characteristic near-infrared (NIR) emissions, but their applications in
phosphor-converted light-emitting-diodes (pc-LEDs) and Si solar cells are limited due to their mismatching
excitation spectra. Here, we selected nitride La3Si6N11 (LSN) as host material to achieve Yb3þ
NIR emission upon low-energy charge transfer (CT) excitation. The obtained phosphor LSN:Yb3þ has a
broad CT excitation band ranging from 250 to 500 nm and narrowband NIR emissions ranging from 950
to 1100 nm centered at 983 nm. On the basis of spectral data, the vacuum referred binding energies
(VRBE) schemes are constructed to locate energy levels of all lanthanide ions in LSN. We also fabricated
NIR pc-LED device using 395 nm LED chip to demonstrate the potential applications of LSN:Yb3þ
phosphors.

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

J. Am. Ceram. Soc., 2019, 102(8), 4648-4658. https://ceramics.onlinelibrary.wiley.com/doi/abs/10.1111/jace.16337


Nitride phosphors of Ca0.99−xSrxAl1.01Si0.99N3:0.01Ce3+ (0 ≤ x ≤ 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 x = 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. https://www.sciencedirect.com/science/article/abs/pii/S0022231319301425

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. https://onlinelibrary.wiley.com/doi/abs/10.1002/adfm.201901988

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. https://link.springer.com/article/10.1007/s40843-018-9404-4

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 T1→6A1 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. https://pubs.acs.org/doi/abs/10.1021/acs.inorgchem.9b01020

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.