Manipulation of Bi3+/In3+ Transmutation and Mn2+‐Doping Effect on the Structure and Optical Properties of Double Perovskite Cs2NaBi1‐xInxCl6

https://onlinelibrary.wiley.com/doi/abs/10.1002/adom.201801435

The halide double perovskite family represented by A2(B+,B3+)X6 can overcome the lead toxicity and enable generally large band gap engineering via B/B sites’ transmutation or exotic dopants to fulfill the emerging applications in the optoelectronic fields. Herein, the design and the experimental synthesis of a new family of Mn2+-doped Cs2NaBi1-xInxCl6 crystals with an intense orange-yellow emission band are reported, and the phase formation stability is discussed via a combined experimental–theoretical approach. Depending on the manipulation of Bi3+/In3+ combination, the band gap increases with In3+ content, and a subsequent evolution from indirect to direct band gap is verified. First-principles calculations and parity analyses indicate a parity forbidden effect on Cs2NaInCl6, and a combination effect of absorption on Cs2NaBi1-xInxCl6 from both Cs2NaBiCl6 and Cs2NaInCl6. The associated Mn2+-doped photoluminescence depending on the Bi3+/In3+ substitution is also addressed from the variation of the different Mn–Cl environment and neighboring-cation effect.

Effects of Neighboring Polyhedron Competition on the 5d Level of Ce3+ in Lanthanide Garnets

https://pubs.acs.org/doi/abs/10.1021/acs.jpcc.9b00395

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Herein, we provide a direct explanation for the reverse garnet effect based on polyhedron competition. On multisite substitution, the deformation of the dodecahedron which will accommodate Ce3+ meets suppression from a neighboring octahedron and tetrahedron. This makes the dodecahedral deformation nonisotropic. Further, it is found that the lowest 5d state of Ce3+ in garnet is closely related to the tetragonal distortion of the dodecahedron, which is characterized by a simplified cuboid model. Crystal-field calculations reveal how the edge-ratio of the cuboid affects energy levels. This model gives a satisfactory explanation for the reverse garnet effect and is helpful for seeking novel garnet-based luminescent materials.

Discovery of New Narrow‐Band Phosphors with the UCr4C4‐Related Type Structure by Alkali Cation Effect

https://onlinelibrary.wiley.com/doi/abs/10.1002/adom.201801631

Narrow-band luminescence materials used in white light-emitting diodes (WLEDs) have demonstrated a great potential to increase the maximum accessible color gamut, improve the color rendition, or enhance the visual luminous efficacy for illumination and display devices. So far, the discovery of narrow-band rare earth doped phosphors for emerging applications remains challenging owing to the limited systems in use and the broadening effect of 4f–5d transition. Here, two narrow-band cyan-emitting phosphors with the UCr4C4-related type structure, RbNa2K(Li3SiO4)4:Eu2+ (RNKLSO:Eu2+) and CsNa2K(Li3SiO4)4:Eu2+ (CNKLSO:Eu2+), are reported. The narrow-band emission results from the highly condensed network and a cube-like site for the activator (Eu2+). The emission peak assignments are investigated, and the variations of luminescence behavior with compositional changes of alkali cations are elaborated in detail. RNKLSO:8% Eu2+ (95%@250 °C of the integrated emission intensity at 25 °C) shows better thermal stability than that of CNKLSO:8% Eu2+ (79%@250 °C), which can be explained by the thermally activated crossover process represented in the configurational coordinate diagram. The optical properties of the as-fabricated WLEDs are studied and demonstrate potential with tunable properties for the full spectrum phosphor-converted LEDs. These findings in UCr4C4-type phases help shedding light on new avenues for fabricating new and totally unexpected narrow-emitting phosphors with versatile applications.

Synthesis and Luminescence Properties of CsPbX3@Uio-67 Composites toward Stable Photoluminescence Convertors

https://pubs.acs.org/doi/abs/10.1021/acs.inorgchem.8b03295

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All-inorganic halide perovskite (CsPbX3, X = Cl, Br, or I) nanocrystals (NCs) have been widely studied due to their outstanding optoelectronic properties. However, some inevitable factors like light, heat, and moisture affected the stability of CsPbX3 NCs and further limited their practical application. In this work, the stability of all-inorganic halide perovskite NCs can be improved by integrating them in the stable Zr-based metal–organic frameworks (Uio-67). Compared to pristine perovskite NCs, typical CsPbBr3@Uio-67 composites display a stable photoluminescence property that can be maintained for 30 days under ambient atmospheric conditions. Due to the proposed confinement effects of CsPbX3 NCs coordinated with the pore structures of Uio-67, the related structural model of CsPbX3@Uio-67 composites was elucidated. White LED device was further fabricated by combining CsPbBr3@Uio-67 composites and commercial K2SiF6:Mn4+ red phosphors with a blue-emitting chip, which demonstrated a wide color gamut (138% of National Television Standards Committee color space). The strategy on encapsulation of CsPbX3 NCs into Uio-67 will open up a stable platform for optoelectronic applications.

Luminescent perovskites: recent advances in theory and experiments

https://pubs.rsc.org/en/content/articlehtml/2019/qi/c9qi00777f

image file: c9qi00777f-f1.tif

Perovskites form an important and enormous class of inorganic compounds. Recently, perovskite materials have attracted extensive research interest owing to their excellent optoelectronic properties. Deep insights into the relationships between the crystal structure, electronic structure and properties play an important role in the development of new functional materials and high-performance devices. In this review, after a brief introduction, we first discuss the crystal structure and crystal chemistry of perovskites according to their three classes: standard perovskites, low-dimensional perovskites and perovskite-like halides. Next, the electronic structure and luminescence from different physical origins are presented. Then, we present a survey on the design, synthesis and luminescence properties of different perovskites, including halide perovskites, oxide perovskites, and lanthanide- or transition metal-doped perovskites, also including dimension-different perovskites (3D, 2D, 1D and quantum dots). We also summarize the strategies for improving the photoluminescence quantum yield (PLQY) and chemical stability, including by surface passivation, encapsulation and doping. Finally, we review their applications and give a brief outlook.

Li substituent tuning of LED phosphors with enhanced efficiency, tunable photoluminescence, and improved thermal stability

https://www.science.org/doi/10.1126/sciadv.aav0363?intcmp=trendmd-adv&

Solid-state phosphor-converted white light-emitting diodes (pc-WLEDs) are currently revolutionizing the lighting industry. To advance the technology, phosphors with high efficiency, tunable photoluminescence, and high thermal stability are required. Here, we demonstrate that a simple lithium incorporation in NaAlSiO4:Eu system enables the simultaneous fulfillment of the three criteria. The Li substitution at Al sites beside Na sites in NaAlSiO4:Eu leads to an enhanced emission intensity/efficiency owing to an effective Eu3+ to Eu2+ reduction, an emission color tuning from yellow to green by tuning the occupation of different Eu sites, and an improvement of luminescence thermal stability as a result of the interplay with Li-related defects. A pc-WLED using the Li-codoped NaAlSiO4:Eu as a green component exhibits improved performance. The phosphors with multiple activator sites can facilitate the positive synergistic effect on luminescence properties.

Green persistent luminescence and the electronic structure of β-Sialon: Eu 2+

https://pubs.rsc.org/en/content/articlelanding/2019/tc/c9tc03833g/unauth

Graphical abstract: Green persistent luminescence and the electronic structure of β-Sialon:Eu2+

Divalent europium doped aluminum silicate oxy-nitride (β-Sialon:Eu2+) has been widely used in backlights for liquid-crystal displays due to its outstanding green emission properties. Herein, the persistent luminescence (PersL) performance and electronic structure of β-Sialon:Eu2+ with the general formula Eu0.015Si5.5Al0.485O0.515N7.485 are first reported. The PersL duration is observed to be 400 s after 254 nm irradiation. By virtue of density functional theory (DFT) calculations, we verify that the trap levels responsible for PersL are impurity levels induced by Si–O bonds located below the bottom of the conduction band (CB) on random substitution of Al–O for Si–N in β-Si3N4. The trap depth and density are estimated through experimental data. The charging process for PersL is clarified by the thermoluminescence excitation (TLE) spectrum. The electronic structure diagrams (host referred binding energy, HRBE and vacuum referred binding energy scheme, VRBE) of β-Sialon:Eu2+ are constructed to deeply understand the PersL mechanism and luminescence behavior. We propose a novel strategy to construct the HRBE schemes, i.e. using the onset energy of the thermoluminescence excitation (TLE) spectrum as the energy difference between the 4f ground state and the bottom of the CB to pinpoint the 4f energy level location of Eu2+. This work would allow more rational design of luminescent materials.

The red persistent luminescence of (Sr,Ca)AlSiN3:Eu2+ and mechanism different to SrAl2O4:Eu2+,Dy3+

Journal of Luminescence, April 2019, 208

(Sr,Ca)AlSiN3:Eu²⁺ phosphors have been widely used in phosphor-converted white light emitting diodes. Herein, we reported the strong red persistent luminescence in (Sr,Ca)AlSiN3:Eu²⁺ under UV light excitation. The Sr0.8Ca0.2AlSiN3:0.15% Eu²⁺ shows the strongest red persistent luminescence with a peak emission wavelength at ~628 nm and a persistent time of ~9600 s at the 0.32 mcd/m² threshold value. A new persistent luminescence mechanism, which is different to that of SrAl2O4:Eu²⁺,Dy³⁺, was proposed by comparing the thermoluminescence excitation spectrum (TLES) and the photoluminescence excitation spectrum (PLES) of Sr0.8Ca0.2AlSiN3:0.15% Eu²⁺. The electrons are directly excited from 4f ground states to the conduction band or from valence band to conduction band in (Sr,Ca)AlSiN3:Eu²⁺; while, in SrAl2O4:Eu²⁺,Dy³⁺, they are first excited to 5d level and then stimulated thermal process to the conduction band. The effect of Eu²⁺ concentration on red persistent luminescence in (Sr,Ca)AlSiN3:Eu²⁺ were discussed. The proposed mechanism of persistent luminescence can help us to design and find new persistent luminescence materials.

Enhanced Persistence Properties through Modifying the Trap Depth and Density in Y3Al2Ga3O12:Ce3+,Yb3+ Phosphor by Co-doping B3+

Journal of Alloys and Compounds, Volume 770, 5 January 2019, Pages 1069-1077

To fabricate white-light-emitting diodes (white LEDs) with high color-rendering index or full light spectrum emission, the discovery of more efficient deep-red emitting phosphor materials is essential. In this paper, we have synthesized a series of Sr2-2xEu2xSi5N8 (0 ≤ x ≤ 1) solid-solution compounds, and have systemically investigated effects of full-range Eu concentration on their luminescence. Their emission band maximum can be largely tuned from 610 to 725 nm by increasing Eu content. Reabsorption at low Eu2+ concentration while both the energy transfer and Stocks shift at high Eu2+ concentration account for this large spectral red-shift. Luminescent thermal quenching performance gets worse with Eu2+ concentration increasing. The compound with x = 0.15 possesses the best crystallinity and the highest luminescence intensity with the peak position around 660 nm, and still maintains 88.5% room-temperature intensity at 400 K, indicating that great potential for the application as a deep-red phosphor.

Effects of full-range Eu concentration on Sr2-2xEu2xSi5N8 phosphors: A deep-red emission and luminescent thermal quenching

Journal of Alloys and Compounds, Volume 770, 5 January 2019, Pages 1069-1077

To fabricate white-light-emitting diodes (white LEDs) with high color-rendering index or full light spectrum emission, the discovery of more efficient deep-red emitting phosphor materials is essential. In this paper, we have synthesized a series of Sr2-2xEu2xSi5N8 (0 ≤ x ≤ 1) solid-solution compounds, and have systemically investigated effects of full-range Eu concentration on their luminescence. Their emission band maximum can be largely tuned from 610 to 725 nm by increasing Eu content. Reabsorption at low Eu2+ concentration while both the energy transfer and Stocks shift at high Eu2+ concentration account for this large spectral red-shift. Luminescent thermal quenching performance gets worse with Eu2+ concentration increasing. The compound with x = 0.15 possesses the best crystallinity and the highest luminescence intensity with the peak position around 660 nm, and still maintains 88.5% room-temperature intensity at 400 K, indicating that great potential for the application as a deep-red phosphor.