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Thermal quenching of phosphor is an important challenge for its practical application in phosphor‐converted white light‐emitting diodes (pc‐WLEDs) and it usually becomes aggravated with the increase of activator concentration. Conversely, this work finds the thermal quenching of Eu2+ emission at 490 nm in Sr4Al14O25:Eu2+ does not follow this in the temperature range of 300 to 480 K, and the rate of it is even slowed down as the concentration of Eu2+ increases. However, at the same time, the experiment on three heating‐cooling cycles of Sr4Al14O25:Eu2+ reveals that the thermal degradation of Eu2+ emission becomes improved. Once Eu2+ ions are doped into Sr4Al14O25, they will prefer substituting for the 10‐ and 7‐coordinated strontium sites Sr1 and Sr2, respectively. The emission centers Eu1 and Eu2, therefore, appear. The abnormal phenomenon is perhaps partly due to the enhanced energy transfer from the emission center Eu1 at 407 nm to the one Eu2 at 490 nm. It is also found interesting that the introduction of AlN can enhance the emission of Sr4Al14O25:Eu2+ without leading to the deterioration of thermal degradation. In the end, a prototype of pc‐WLED was fabricated with Sr4Al14O25:Eu2+ to demonstrate the application of white lighting. This work is not only beneficial to the understanding of the relationship between concentration and thermal quenching, but also conducive to the design of the heavily doped phosphor for WLEDs with better resistance to thermal quenching. [ABSTRACT FROM AUTHOR]

Mechanoluminescence (ML) is a phenomenon upon external mechanical stimuli and it has found diverse applications such as stress sensing, structure health diagnosis, 3‐D signature, energy harvesting etc because of its unique properties of in situ and real‐time response to the stimuli. However, ML of most of the state‐of‐art phosphors primarily appears within the spectral range from ultraviolet to visible, which does not lie in the biological transparent windows, and, therefore, limits its applications in biological field. Here, we report a strong near infrared (NIR) ML from orthorhombic Cmcm perovskite Sr3Sn2O7: Nd3+, which is peaked at ~ 900 nm and located exactly within the first optical transparent window of tissues. The ML apparates after gradual discharge of the traps deeper than 0.73 eV triggered by the external mechanical stimuli and subsequently excitation of Nd3+ to the state of 4F3/2. The rechargable ML presents well repeatable linearity to loaded force at least up to 5000 N, and it can penetrate tissues easily that are thick up to 30 mm such as pigskin and the ceramic disk of hydroxy apatite which is the main constituent of bone. These results demonstrate the potential application for in situ biomechanical sensor. Meanwhile, by recording and processing these ML signals during signing on a pellet sample, for the first time, we provide a novel signature system based on NIR ML. This could raise the security level of existed signature anti‐countering to a higher level. [ABSTRACT FROM AUTHOR]

A novel Mn4+‐doped strontium lanthanum gallate red phosphor SrLaGaO4:Mn4+ has been successfully prepared via the conventional solid‐state reaction method. Phase purity, photoluminescence excitation/emission spectra, concentration quenching, decay curves, and temperature‐dependent photoluminescence have been investigated systematically. SrLaGaO4:Mn4+ phosphor exhibits broad excitation band from 250 to 600 nm and emits intense red light centered at 716 nm arising from spin‐forbidden transition, 2E → 4A2 of Mn4+. The optimal dopant concentration of Mn4+ is determined to be 0.2 mol%. Dipole‐dipole interaction is supposed to be the mechanism of concentration quenching. The crystal‐field strength Dq, the Racah parameters B and C, and the nephelauxetic ratio β1 of SrLaGaO4:Mn4+ have been calculated according to its luminescent spectra. Our systematic investigation on this new phosphor can provide a reference for the development of red‐emitting phosphor. [ABSTRACT FROM AUTHOR]

Blue excitable red phosphor is the key component to improve the quality of lighting and display which is based on InGaN blue chips. Because of the potential in the area, Mn4+ red phosphors have recently got rising interests. However, most of them were found by trial and error. It remains very challenging to predict which kind of compound can stabilize Mn4+ and which wavelengths Mn4+ ions if they could survive in it will exhibit the excitation and emission at. Here, we first propose to use crystal field calculation on basis of exchange charge model to predict the energy levels of Mn4+ ion in germanate K2Ge4O9 since Mn4+ and Ge4+ are almost identical in size and charge, and the local field around Mn4+ will experience less distortion after substitution for Ge4+. The calculation shows the red emission peaking at 663 nm and the blue absorption of 4A2g (4F) → 4T2g (4F) in 450~470 nm, which matches better to blue chips than commercial phosphor 3.5MgO·0·5MgF2·GeO2:Mn4+. This inspires the synthesis of Mn-doped K2Ge4O9, the optical properties of which confirm the existence of Mn4+ and consist with the prediction. Comparison between theoretical and experimental results implies that no obvious preference of Mn4+ substitution over two different types of octahedral germanium sites, Ge1 and Ge2. Consequent systematic explorations have been made on the effects of flux content, preparation temperature, Mn content, and holding time to find the ways to enhance the Mn4+ luminescence for promotion of practical application. The results reveal the optimal sample can be made under much mild optimal condition (850°C for 4 h in air) with a quantum yield of >30% upon blue excitation of 460~470 nm. As temperature rises from 8 to 573 K, zero photon line (ZPL) emission redshifts along with gradual appearance of anti-Stokes side phonon bands due to the enhanced interaction of Mn4+ with host. This work demonstrates it possible to find Mn4+ red phosphors by guide of crystal field calculation. [ABSTRACT FROM AUTHOR]

For phosphor-converted warm white light-emitting diodes ( WLEDs), it is essential to find highly efficient red oxide phosphors, which are better chemically stable and benign to environment and can be prepared in a much milder condition. Here, we report a red phosphor LiNaGe4O9:Mn4+ with a quantum yield up to 78% after systematic optimization in synthesis temperature, dopant concentration of Mn4+, and sintering time. Best performance of the phosphor can be reached when it is synthesized in a mild reaction condition, that is, at 850°C for 3 h in air. The integrated emission intensity is more than four times stronger than commercial red phosphor 3.5MgO·0.5MgF2·GeO2:Mn4+ ( MFG:Mn4+) under a blue light excitation at 470 nm. Crystal structural analysis reveals that the high efficiency Mn4+ exhibits in the compound is mainly due to the well separation of GeO6 groups from each other by GeO4 tetrahedra in the neighborhood and the ideal substitution of octahedral Ge4+ site by Mn4+ in view of both size and charge matches. The high performance of the phosphor encourages us to apply the blue absorbing red phosphor to WLED, which is based on combination of a blue LED chip and YAG:Ce3+, and the warm perception WLED is therefore achieved with a color temperature of 3353 K. [ABSTRACT FROM AUTHOR]

Phosphors with an efficient yellow-emitting color play a crucial role in phosphor-converted white LEDs (pc-WLEDs), but popular yellow phosphors such as YAG:Ce or Eu2+-doped (oxy)nitrides cannot smoothly meet this seemingly simple requirement due to their strong absorptions in the visible range. Herein, we report a novel yellow-emitting LuVO4:Bi3+ phosphor that can solve this shortcoming. The emission from LuVO4:Bi3+ shows a peak at 576 nm with a quantum efficiency (QE) of up to 68 %, good resistance to thermal quenching ( T50 %=573 K), and no severe thermal degradation after heating-cooling cycles upon UV excitation. The yellow emission, as verified by X-ray photoelectron spectra (XPS), originates from the (3P0,3P1)→1S0 transitions of Bi3+. Increasing the temperature from 10 to 300 K produces a temperature-dependent energy-transfer process between VO43− groups and Bi3+, and further heating of the samples to 573 K intensifies the emission. However, it subsequently weakens, accompanied by blueshifts of the emission peaks. This abnormal anti-thermal quenching can be ascribed to temperature-dependent energy transfer from VO43− groups to Bi3+, a population redistribution between the excited states of 3P0 and 3P1 upon thermal stimulation, and discharge of electrons trapped in defects with a trap depth of 359 K. Device fabrication with the as-prepared phosphor LuVO4:Bi3+ has proved that it can act as a good yellow phosphor for pc-WLEDs. [ABSTRACT FROM AUTHOR]

The trivalent green element of bismuth, when doped into different compounds, can produce multiple emissions in ultraviolet, blue, green or even yellow spectral regimes. The emission adjustability comes from the susceptibility of bismuth naked 6s electrons to the crystal field particularly surrounding Bi3+. However, this has never been observed in a single compound. In this work, we report that broadly tunable bismuth emission indeed occurs even within the same host (CaMoO4) when the local microenvironment around Bi3+ is modified by intentional introduction of alkali ions. As the radius of the selected alkali ion decreases, the emission regularly blue-shifts from 586 to 554 nm, and its intensity is enhanced greatly. For instance, the integrated intensity for the Li+ and Bi3+ codoped sample is 4.46 times stronger than the Bi3+ single-doped sample upon excitation at 300 nm. Low-temperature photoluminescence spectra surprisingly reveal that energy transfer can partially take place from the MoO42- group to Bi3+ at lower temperature when excited into the charge transfer state of the MoO42- group, however, this does not occur at higher temperature. The mechanism needs further study. Dynamic luminescence analysis between 10 and 300 K implies a population dependence of the excited states 3P0 and 3P1 on temperature that is responsible for the evolution of the Bi3+ emission lifetime with temperature. [ABSTRACT FROM AUTHOR]

SrB4O7 and SrB6O10 were considered as host materials for Bi2+ dopants with luminescence in the orange-to-red spectral range. Based on the predominance of tetrahedral anion groups, both compounds are highly efficient for stabilizing the Bi2+-ion over its trivalent form. SrB4O7: Bi2+ exhibits a broad blue excitation band while SrB6O10: Bi2+ can be excited in the yellow spectral range. These bands suggest high interest for application in light emitting diode devices such as InGaN (SrB4O7:Bi2+) and YAG:Ce3+ (SrB6O10:Bi2+) with improved color rendering index. Absorption due to strongly allowed 2P1/2→2S1/2 occurs at 245 and 286 nm, respectively. By substituting Sr2+ with other alkaline earth species, the parity selection rule for transitions to 2P3/2 (blue and yellow excitation) can be lifted to a large extent, in this way increasing the transition probability from both bands and, thus, improving quantum yield for visible excitation. Analyses of the temperature dependence of luminescence reveal superior performance for SrB4O7:Bi2+. [ABSTRACT FROM AUTHOR]

Here, a novel multifunctional phosphor of Li1−xGa5O8:xPr3+ is reported. The crystal structure, mechanoluminescence (ML), persistent luminescence (PersL), photoluminescence (PL) and photoluminescence excitation (PLE) are systematically studied. LiGa5O8 emits blue PersL peaked at 437 nm after irradiation by a 254 nm lamp for 10 s, and corresponding PersL can be observed for 30 min by naked eyes. In the same case, the PersL from Li0.995Ga5O8:0.005Pr3+ covers visible to near‐infrared lights (400–1000 nm) which can persist for 120 min. Thermoluminescence analysis shows Pr3+ doping not only increases the shallower trap (0.73 eV) concentration by at least 20 times but also helps the formation of deeper trap centers (0.87 eV). The differences between ML and PersL spectra reveal the selective effects of mechanical stimuli on the transitions of Pr3+ ions. In the end, the fabrication of an intelligent PersL quick response code (QR‐code) permits optical information write‐in after irradiation by a 254 nm lamp and then cryptical information of "SCUT" can be read out by thermal stimulation which shows the potential in information storage. Moreover, the biological tissues penetration experiments and cytotoxicity tests are carried out to display the potential of PersL in bioimaging. [ABSTRACT FROM AUTHOR]

Unusual concentration induced antithermal quenching of divalent bismuth ions occurs in heavily Bi2+-doped Sr2P2O7 phosphors along with a blueshift of the emission peak at higher temperature. [ABSTRACT FROM AUTHOR]

• Overview recent advances on synthesis, morphology and afterglow performance of NIR LPPs. • Summarize the reported NIR LPPs and their primary application achievements. • Highlight the rational design of NIR LPPs in the biological window (650–1800 nm). Long persistent phosphors (LPPs) can maintain luminescence for an appreciable time after terminating the excitation light, which can be compared to an optical battery. This intriguing optical phenomenon has attracted a lot of attention in numerous areas, such as safety display, biomedicine and data-storage technologies. Owing to the efficient elimination of autofluorescence interference from biological samples and the ultra-long near-infrared (NIR) afterglow emission, NIR LPPs show deeper tissue penetration depth offering a bright prospect for various bioapplications, such as in vivo biosensing, bioimaging, and imaging-guided therapy. In this review, we focus on the recent developments in the aspect of synthesis procedures, design and applications of NIR LPPs and highlight the rational design of various kinds of transition metal and rare earth ions-doped NIR LPPs in the first to third tissue transparency window. Moreover, some remaining challenges and further efforts towards this rapidly expanding area are also envisioned at the end of this review. [ABSTRACT FROM AUTHOR]

A novel Mn4+‐doped strontium lanthanum gallate red phosphor SrLaGaO4:Mn4+ has been successfully prepared via the conventional solid‐state reaction method. Phase purity, photoluminescence excitation/emission spectra, concentration quenching, decay curves, and temperature‐dependent photoluminescence have been investigated systematically. SrLaGaO4:Mn4+ phosphor exhibits broad excitation band from 250 to 600 nm and emits intense red light centered at 716 nm arising from spin‐forbidden transition, 2E → 4A2 of Mn4+. The optimal dopant concentration of Mn4+ is determined to be 0.2 mol%. Dipole‐dipole interaction is supposed to be the mechanism of concentration quenching. The crystal‐field strength Dq, the Racah parameters B and C, and the nephelauxetic ratio β1 of SrLaGaO4:Mn4+ have been calculated according to its luminescent spectra. Our systematic investigation on this new phosphor can provide a reference for the development of red‐emitting phosphor. [ABSTRACT FROM AUTHOR]

Abstract Thermal quenching is a common phenomenon hindering commercial applications of phosphors, and the relationship between matrix structure and thermal quenching is not yet clear especially for the red phosphor SrAl 12 O 19 :Mn4+. Here, we determined the energy levels of three types of Mn4+ sites in SrAl 12 O 19 crystal structure based on the exchange-charge-model crystal field calculations and analyzed the Mn4+ ions preferential occupation combined with chemical bond covalence. The phosphor suffers from serious thermal quenching when the luminescence monitoring from 8 to 500 K was recorded. The combination of theoretical calculations and experimental verification also reveals that the symmetry and spatial distribution of octahedral sites for Mn4+ are crucial to better resistance to thermal quenching. This can be helpful to search for novel Mn4+ doped red phosphor with better resistance to thermal impact in the future. [ABSTRACT FROM AUTHOR]

Blue excitable red phosphor is the key component to improve the quality of lighting and display which is based on InGaN blue chips. Because of the potential in the area, Mn4+ red phosphors have recently got rising interests. However, most of them were found by trial and error. It remains very challenging to predict which kind of compound can stabilize Mn4+ and which wavelengths Mn4+ ions if they could survive in it will exhibit the excitation and emission at. Here, we first propose to use crystal field calculation on basis of exchange charge model to predict the energy levels of Mn4+ ion in germanate K2Ge4O9 since Mn4+ and Ge4+ are almost identical in size and charge, and the local field around Mn4+ will experience less distortion after substitution for Ge4+. The calculation shows the red emission peaking at 663 nm and the blue absorption of 4A2g (4F) → 4T2g (4F) in 450~470 nm, which matches better to blue chips than commercial phosphor 3.5MgO·0·5MgF2·GeO2:Mn4+. This inspires the synthesis of Mn-doped K2Ge4O9, the optical properties of which confirm the existence of Mn4+ and consist with the prediction. Comparison between theoretical and experimental results implies that no obvious preference of Mn4+ substitution over two different types of octahedral germanium sites, Ge1 and Ge2. Consequent systematic explorations have been made on the effects of flux content, preparation temperature, Mn content, and holding time to find the ways to enhance the Mn4+ luminescence for promotion of practical application. The results reveal the optimal sample can be made under much mild optimal condition (850°C for 4 h in air) with a quantum yield of >30% upon blue excitation of 460~470 nm. As temperature rises from 8 to 573 K, zero photon line (ZPL) emission redshifts along with gradual appearance of anti-Stokes side phonon bands due to the enhanced interaction of Mn4+ with host. This work demonstrates it possible to find Mn4+ red phosphors by guide of crystal field calculation. [ABSTRACT FROM AUTHOR]

• An extraordinary absorption efficiency of 73.21% was discovered from SrZnSO:0.03Bi3+. • SrZnSO:Bi3+ exhibits a wide near-ultraviolet excitation band and an ideal blue emission band. • The relationship between the excitation peak of Bi3+-related phosphors and the polyhedron size were summarized. • A outstanding CRI (93.4) and low CCT (4051 K) were achieved from the fabricated WLEDs. [Display omitted] Trivalent bismuth (Bi3+) activated blue-emitting materials have attracted considerable attention because they effectively avoid the reabsorption demerits compared to traditional rare earth activators. Unfortunately, these materials are often plagued by mismatch to the near-ultraviolet (n-UV) chip and short blue emission peak as well as low n-UV absorption efficiency. In this work, we successfully designed a blue-emitting phosphor SrZnSO:Bi3+ with a wide n-UV excitation band ranging from 310 to 400 nm and an ideal blue emission band centered at 460 nm. Furthermore, SrZnSO:0.03Bi3+ phosphor exhibits an extraordinary absorption efficiency of 73.21% ascribed to the intense absorption of the matrix and Bi3+ in the n-UV region. The relationship between the excitation peak position of Bi3+-related phosphors and the size of the polyhedron were also summarized. A white light-emitting diode (WLED) was fabricated using a 370 nm n-UV chip, the composition-optimized sample SrZnSO:0.03Bi3+ and the commercial phosphors. Interestingly, the temperature-sensitive luminescence behavior is beneficial to the performance improvement of the WLED. The WLED demonstrates a warm white light whose Commission Internationale de L′Eclairage (CIE) coordinates are (0.3817, 0.3885), outstanding color rendering index of 93.4 and low correlated color temperature of 4051 K at current of 80 mA. The study suggests that SrZnSO:Bi3+ blue emitting phosphor has great potential in LED-based applications. Also inspires researchers to explore Bi3+-related phosphors with low energy excitation band and high absorption efficiency to develop high-quality n-UV pumped WLEDs. [ABSTRACT FROM AUTHOR]

• A new type of deep-red-emitting LaTiSbO 6 :Mn4+ phosphor has been developed. • The specific spectroscopic properties have been studied in detail. • Phosphor-in-glass engineer enhances the performance of LaTiSbO 6 :Mn4+. • Possible application of LaTiSbO 6 :Mn4+ and its topo-architecture has been suggested. Increasingly rigorous temperature detection applied to characterize physico-chemical state has stimulated thriving demands for contactless optical thermometry, which must overcome obstacles of complex preparation, low sensitivity, poor stability and inflexible design. Herein, a new optical thermal-sensitive LaTiSbO 6 :Mn4+ and its upgraded engineering frameworks are firstly constructed and systematically analyzed. The title phosphor exhibits specific single-band narrow emission (FWHM = ~31 nm) in deep-red region centered at 683 nm and superior thermal-sensitivity (S R = 2.75% K−1, 298–418 K) based on reliable fluorescence lifetime mode. Outstanding robustness of the phosphor concluded from aging experiment highlights the prerequisites of its practical duration and functional independence in diversified architecture. Through phosphor-in-glass (PiG) strategy, robust products are controllably organized without ascertainable interfacial reaction between introduced particles and glass matrix, yielding intriguing improved performance (S R = 3.01% K−1) inherited from LaTiSbO 6 :Mn4+. Moreover, fabricated heterogeneous PiG architecture, spatially confining LaTiSbO 6 :Mn4+ and YAG:Ce3+ to block unfavorable energy-transfer depletion, facilely accomplish dual-mode thermometry without significant mutual interference, integrating the Mn4+-lifetime and highly recyclable fluorescence intensity ratio (FIR) from the Ce–Mn non-thermally-coupled system. This work not only suggests LaTiSbO 6 :Mn4+ as a promising candidate, but also expands new horizons with the topological composite pathway toward rational designing and perfecting multi-mode thermometer or other versatile constructions. [ABSTRACT FROM AUTHOR]