Hybrid Metal Halides with Multiple Photoluminescence Centers

Angewandte Chemie., 2019, 58(51), 18670-18675. https://onlinelibrary.wiley.com/doi/abs/10.1002/anie.201911419

Very little is known about the realm of solid-state metal halide compounds comprising two or more halometalate anions. Such compounds would be of great interest if their optical and electronic properties could be rationally designed. Herein, we report a new example of metal halide cluster-assembled compound (C9NH20)9[Pb3Br11](MnBr4)2, featuring distinctly different anionic polyhedra, namely, a rare lead halide cluster [Pb3Br11]5− and [MnBr4]2−. In accordance with its multinary zero-dimensional (0D) structure, this compound is found to contain two distinct emission centers, 565 nm and 528 nm, resulting from the formation of self-trapped excitons and 4T1–6A1 transition of Mn2+ ions, respectively. Based on the high durability of (C9NH20)9[Pb3Br11](MnBr4)2 upon light and heat, as well as high photoluminescence quantum yield (PLQY) of 49.8 % under 450 nm blue light excitation, white light-emitting diodes (WLEDs) are fabricated, showcasing its potential in backlight application.

Single‐Component White‐Light Emission in 2D Hybrid Perovskites with Hybridized Halogen Atoms

Adv. Opt. Mater., 2019, 7(24), 1901335. https://onlinelibrary.wiley.com/doi/abs/10.1002/adom.201901335

With single-component photoinduced white-light (WL) emission, low-dimensional hybrid halide perovskites have emerged as a new generation of luminescent materials; however, the effect of halogens on the intrinsic light emissions and the corresponding mechanisms is still unknown. Herein, the investigation of a family of two-dimensional (2D) hybrid perovskites R2PbBr4−xClx (R = BA+, PMA+, PEA+; x = 0, 1, 2, 3, 4) highlights the influence of halogens on intrinsic emission to reveal the dependence of the photoluminescence on the nature and contribution of the halogens. Ultrabroad emissions covering the entire visible-light region are achieved in the halogen hybrid systems with the stoichiometry of R2PbBr2Cl2 (R = BA+, PMA+, PEA+), showing their potential as single-component WL phosphors in solid-state lighting devices. The origin of the WL emissions is the synergistic recombination emission of free excitons and self-trapped excitons. The ratio of halogens (Br/Cl) is confirmed to be a critical factor to fine-tune the intrinsic emission properties. This work provides a feasible strategy to achieve single-component WL emission in 2D hybrid perovskites, and proposes a method for regulating halogen contents for optimizing luminescent properties.

Sb3+ Doping-Induced Triplet Self-Trapped Excitons Emission in Lead-Free Cs2SnCl6 Nanocrystals

J. Phys. Chem. Lett., 2019, 10, 23, 7439–7444 https://pubs.acs.org/doi/abs/10.1021/acs.jpclett.9b03035

Doped halide perovskite nanocrystals (NCs) have opened new opportunities for the emerging optical and optoelectronic applications. Here, we describe a hot-injection synthesis of all-inorganic lead-free Cs2SnCl6 and Sb3+ doped Cs2SnCl6 NCs. Cs2SnCl6 NCs present a blue emission peak at 438 nm, whereas a new broad-band emission peak appears at 615 nm for the Sb3+ doped NCs. Comparative structural and spectral characterizations of Sb3+ doped Cs2SnCl6 NCs with micrometer-sized undoped and Sb3+ doped crystals show that the formation of broad-band orange emission is originted from triplet self-trapped excitons, attributed to the 3Pn–1S0 transitions (n = 0, 1, 2). Our results in Sb3+ doped Cs2SnCl6 materials provide insights into the machanisms of doping-induced emission centers, and it extends the existing knowledge of optical properties of doped halide NCs for further studies.

Optically Modulated Ultra-Broad-Band Warm White Emission in Mn2+-Doped (C6H18N2O2)PbBr4 Hybrid Metal Halide Phosphor

Chem. Mater., 2019, 31, 15, 5788–5795. https://pubs.acs.org/doi/abs/10.1021/acs.chemmater.9b01864

Finding new low-dimensional metal halides with broad-band emission is attracting interest in single-component phosphor for white light-emitting diodes (WLEDs). The full-spectrum white light still remains a challenge as found in the two-dimensional hybrid material (C6H18N2O2)PbBr4 exhibiting the intrinsic free exciton (FE) and broad-band self-trap exciton (STE) emission upon 365 nm ultraviolet excitation, and a combined strategy has been proposed through doping the Mn2+ ions enabling a superposition of multiple emission centers toward the ultra-broad-band warm white light. The occupation of Mn2+ in (C6H18N2O2)PbBr4 has been discussed, and optical investigations verify that the warm white-light emission of Mn2+-doped (C6H18N2O2)PbBr4 originates from the coupling effects of the FE, STEs, and the 4T1–6A1 transition of the doped Mn2+. When the concentration of Mn2+ is 5%, the emission spectrum of the phosphor covers all visible-light areas with a full width at half maximum (FWHM) of about 230 nm. The high Ra (84.9) and warm light CCT (3577 K) values of the as-fabricated WLED lamp demonstrate that (C6H18N2O2)Pb1–xMnxBr4 can be promising as single-component white-light phosphor in solid-state lighting. Our work could provide a new understanding and perspective about hybrid metal halides for designing superior phosphor toward single-component white emission.

Tuning of the Compositions and Multiple Activator Sites toward Single-Phased White Emission in (Ca9–xSrx)MgK(PO4)7:Eu2+ Phosphors for Solid-State Lighting

Inorg. Chem. 2019, 58, 8, 5006–5012 https://pubs.acs.org/doi/abs/10.1021/acs.inorgchem.9b00028

Manipulating the distribution of rare earth activators in multiple cations’ sites of phosphor materials is an essential step to obtain tunable emission for the phosphor-converted white-light-emitting diodes (pc-WLEDs). However, it remains the challenge to realize the photoluminescence tuning in the single-phased phosphor with single activator, due to the uncertain location of doped ions and adjustable crystallographic sites. Herein we reported the β-Ca3(PO4)2-type solid solution phosphors (Ca8.98–xSrx)MgK(PO4)7:2%Eu2+ (x = 0–8.98) and the effects of replacing Ca2+ by Sr2+ ions on the phase structures and color-tunable emission were investigated in detail. Tunable color emission has been realized by manipulating the redistribution of Eu2+ ions among different cation sites with adjustable chemical environment, and the related mechanism on the local structures has been discussed. The high Ra (85) and low color temperature (CCT) (4465 K) values of the as-fabricated WLEDs lamp indicate that (Ca4.98Sr4)MgK(PO4)7:2%Eu2+ can act as a promising white-emitting phosphor for single-phased pc-WLEDs. This work provides a new insight into the tuning of the compositions and multiple activator sites toward single-phased white emission.

Broad-Band Emission in a Zero-Dimensional Hybrid Organic [PbBr6] Trimer with Intrinsic Vacancies

J. Phys. Chem. Lett. 2019, 10, 6, 1337–1341 https://pubs.acs.org/doi/abs/10.1021/acs.jpclett.9b00238

The understanding of broad-band emission mechanisms on low-dimensional metal halides is an urgent need for the design principle of these materials and their photoluminescence tuning. Herein, a new zero-dimensional (0D) organic–inorganic hybrid material (C9NH20)6Pb3Br12 has been discovered, in which face-sharing PbBr6 trimer clusters crystallize with organic cations (C9NH20+), forming periodic structure with 0D blocks. Broad-band green emission peaking at about 522 nm was observed for this material, with a full width at half-maximum (fwhm) of 134 nm. The emission was attributed to excitons trapped at controlled intrinsic vacancies, and this is the new example in 0D metal halides, also confirmed by spectroscopy analysis and first-principles calculations. Discovery of the single-crystalline hybrid material and observation of defect-induced luminescence extend the scope of bulk 0D materials and understanding of photophysical properties for optoelectronic applications.

A review on the Eu2+ doped β-Ca3 (PO4) 2-type phosphors and the sites occupancy for photoluminescence tuning

Opt. Mater.X., 2019, 1. https://www.sciencedirect.com/science/article/pii/S2590147819300099

β-Ca3(PO4)2-type phosphors have received much attention due to their ability for heterovalent substitution of Ca2+ by different cations to form the new phases, and their abundant crystallographic sites for the doped activator, such as Eu2+, to tune the photoluminescence. Thus, these phosphors have great potential on the applications in white light-emitting diodes (WLEDs) for their tunable emission. Accordingly, there is increasing interest in the discovery of new β-Ca3(PO4)2-type phosphors for WLEDs and the deep understanding on the mechanisms responsible for occupation by Eu2+ ions of particular sites in the host lattice, so that the modulation of emission color and intensity can be controlled. In this review, we summarized the structural construction of β-Ca3(PO4)2-type compounds based on such a mineral-inspired prototype evolution perspective. Then we reviewed the recent research on the luminescence properties and sites occupancy of Eu2+ ions in different β-Ca3(PO4)2-type phosphors. Finally, combining with the current advances, we proposed the research prospects and future work of β-Ca3(PO4)2-type phosphors.

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.