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.

Controlling Cr3+/Cr4+ concentration in single-phase host toward tailored super-broad near-infrared luminescence for multifunctional applications

Mater. Chem. Today,2021,22,100555.https://doi.org/10.1016/j.mtchem.2021.100555

Full control on the valence of the active ions in solids to improve properties is the central topic of chemistry and materials. Cr3+ and Cr4+ ions generally emit wavelength-different near-infrared (NIR) light. Here, we have developed a chromium valence-controllable single-phase phosphor, Mg2GeO4:Cr3+,Cr4+ to achieve super-broad NIR luminescence. High Li+ content charge compensators can stabilize Cr3+ while high temperature sintering tends to facilitate the formation of Cr4+. Through fine adjusting the synthesis conditions, pure Cr3+ or Cr4+ luminescence can be obtained with peak emission locating at 935 and 1190 nm, respectively. Super-broad-band dual emission spanning from 650 to 1600 nm is realized via fully controlling the concentration ratio of Cr3+ to Cr4+ in single host. By measuring the transmission spectra of several foodstuffs illuminated by our phosphors, nondestructive analysis in food safety area can be realized. This study provides a new strategy for exploiting super-broad-band NIR luminescent materials.

Efficient Near‐infrared Pyroxene Phosphor LiInGe2O6:Cr3+ for NIR Spectroscopy Application

April 2021, Journal of the American Ceramic Society.https://doi.org/10.1111/jace.17856

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Broadband near-infrared phosphors are essential to realize nondestructive analysis in food industry and biomedical areas. Efficient long-wavelength (>830 nm) phosphors are strongly desired for practical applications. Herein, we demonstrate an efficient broadband NIR phosphor LiInGe2O6:Cr3+, which exhibits a broad NIR emission peaking at ~880 nm with a full width at half maximum of 172 nm upon 460 nm excitation. The internal/external quantum efficiencies of LiInGe2O6:Cr3+ are measured to be 81.2% and 39.8%, respectively. The absorption of the phosphor matches well with commercial blue LEDs. Using the fabricated phosphor converted LED illuminating human palm, distribution of blood vessels can be clearly recognized under a NIR camera. These results indicate that LiInGe2O6:Cr3+ is a promising candidate to be used in future non-destructive biological applications.

Color-Tunable Persistent Luminescence of Ca10M(PO4)7:Eu2+(M = Li, Na, and K) with a β-Ca3(PO4)2 -Type

Inorg. Chem. 2021, 60, 6, 3952–3960.https://doi.org/10.1021/acs.

Eu2+-activated Ca10M(PO4)7 (M = Li, Na, and K) phosphates have been explored extensively because of their tunable emission wavelengths and excellent luminescence performances. Herein, the persistent luminescence (PersL) properties of Eu2+-doped Ca10M(PO4)7 phosphors with a β-Ca3(PO4)2-type structure are reported. With the variation of alkali metal M from Li to Na and to K, the PersL color can be adjusted sequentially from yellow to white and to blue, and the persistent durations are prolonged significantly from about ∼61 s for Ca9.997Li(PO4)7:0.003Eu2+ to ∼1950 s for Ca9.999Na(PO4)7:0.001Eu2+ and to ∼7440 s for Ca9.9995K(PO4)7:0.0005Eu2+ at the threshold value (0.32 mcd/m2) after 254 nm irradiation. The trap depths are estimated according to the thermoluminescence glow curves with various heating rates. Comparing the thermoluminescence excitation and photoluminescence excitation spectra, it can be verified that there are two sources of ionized electrons in the charging process: one is excited from the valence band to the conduction band (CB) and the other is excited from the 4f ground state of Eu2+ to the higher 5d levels or directly to the CB. Finally, the PersL mechanism is proposed. This work is expected to motivate more research of Eu2+-doped phosphate-based PersL materials, as well as contributes to the understanding of the PersL mechanism of Eu2+-doped phosphors.

A Broadband Near-Infrared Phosphor Ca3Y2Ge3O12:Cr3+ with Garnet Structure

Journal of Alloys and Compounds, Volume 863, 15 May 2021, 158699.https://doi.org/10.1016/j.jallcom.

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Near-infrared phosphor (NIR) integrated light-emitting diode (LED) has ideal application prospects in the fields of food inspection and medical imaging. Herein, we have synthesized a garnet-type Ca3Y2Ge3O12:Cr3+ NIR phosphor using solid state reaction method. Broadband NIR emission ranging from 700 to 1100 nm with a peak located at 800 nm indicates a weak crystal field strength for Cr3+, which occupies the six-coordinated Y3+ site. Moreover, the phosphor has satisfactory luminous intensity showing 81%/10% internal/external quantum efficiency and excellent luminescence thermal stability. Our work provides an excellent NIR phosphor choice for NIR LED devices.

Site engineering strategy toward enhanced luminescence thermostability of a Cr3+-doped broadband NIR phosphor and its application

Graphical abstract: Site engineering strategy toward enhanced luminescence thermostability of a Cr3+-doped broadband NIR phosphor and its application
Efficient broadband near-infrared (NIR) light sources are urgently needed for emerging applications in medicine, food analysis, and others. Nevertheless, the performance is limited by luminescence efficiency and thermostability in state-of-the-art broadband NIR phosphors. Here we demonstrate an effective strategy for achieving efficient and thermostable broadband NIR emission by site engineering in a SrGa12O19–LaMgGa11O19:Cr3+ system. Due to the structure symmetry promotion, the end-member SrGa12O19:Cr3+ shows excellent luminescence thermostability, i.e., higher luminescence quenching temperature and more remarkable color stability compared with LaMgGa11O19:Cr3+. At 500 K, the integrated PL intensity remains 86.5% of that at 290 K. Fine local structure, photoluminescence spectra and luminescence decay curves together support that the optically activated Cr3+ centers are reduced due to site symmetry change in SrGa12O19:Cr3+, leading to the remarkable luminescence thermostability. Finally, we have fabricated NIR pc-LEDs with commercial blue-light-emitting InGaN chips (450 nm), showing that SrGa12O19:Cr3+ has a high external quantum efficiency of 45% and has great potential in high-power and efficient pc-LED applications.

Structural Confinement for Cr3+ Activators toward Efficient Near- Infrared Phosphors with Suppressed Concentration Quenching

Trivalent chromium ion-doped near-infrared (NIR) phosphors have been widely studied due to their tunable emission wavelengths and broad applications. High Cr3+ concentration can improve absorption efficiency but generally results in low emission intensity due to the concentration quenching effect. Herein, we report a series of efficient NIR phosphors with suppressed concentration quenching, Sr9M1–x(PO4)7:xCr3+ (M = Ga, Sc, In, and Lu), showing a broadband NIR emission ranging from 700 to 1100 nm peaking at 850 nm upon the 485 nm light excitation. The emission peak position is almost independent of the type of M ion and the Cr3+ dopant content, and the type of M ion has little influence on the luminescence thermal quenching, indicating that [MO6] octahedra are rigid enough to keep octahedral volumes and average M3+–O2– distances nearly constant owing to the formation of the framework structure on Cr3+ substitution. The NIR emission intensities monotonously increase with the Cr3+ content increasing from 0 to 80% with suppressed concentration quenching, the intensity of Sr9Cr(PO4)7 still maintains 84.23% of Sr9Ga0.2(PO4)7:0.8Cr3+ phosphor, and the thermal quenching behavior is slightly dependent on x; these effects can be attributed to the suppressed energy transfer due to the structural confinement effect. The optimal sample, Sr9Ga0.2(PO4)7:0.8Cr3+, has an internal/external quantum efficiency of 66.3%/29.9%. Finally, we fabricate a NIR phosphor-conversion light-emitting diode and demonstrate its applications in nondestructive examination and medical fields.