Luminescence Properties Of Phosphors Research Articles

Influence of Chelants and Catalyst on Luminescence of CaAl<sub>2</sub>O<sub>4</sub>:Eu<sup>2+</sup> Phosphors for Optical Temperature Sensing Applications

This study synthesized CaAl2O4:Eu2+ blue phosphors using the sol-gel method. The effects of incorporating citric acid, poly (oxyethylene) (PEG), and HCl in the precursor on the luminescent properties of phosphors were investigated. No significant changes were observed in the photoluminescence (PL) spectra of the phosphors when different precursor solutions were used. However, the use of PEG and citric acid led to a noticeable decrease in the PL intensity. Notably, the use of HCl resulted in an increase in PL. In addition, upon heating the phosphors from room temperature to 110 °C, the PL spectrum remained unchanged, whereas the PL intensity decreased linearly with increasing temperature. This indicated the suitability of CaAl2O4:Eu2+ for optical temperature-sensing applications.

Optical characteristics, Judd-Ofelt analysis of enhanced luminescence by flux in thermally stable, novel Eu3+- doped BaZr(PO4)2 phosphor for indoor lighting applications

A series of novel single phased Eu3+ doped BaZr(PO4)2 phosphor were synthesized employing simple solid-state reaction route and studied the influence of different chlorides as fluxes (NH4Cl, KCl, LiCl and NaCl) on structural and photoluminescence properties. The structural characterization reveals pure phase formation of the as prepared phosphors with monoclinic crystal structure. The photoluminescence (PL) spectra recorded under 392 nm excitation depicts intense emission at 612 nm. Further, the concentration of Eu3+ ion was well tuned by carefully studying the PL spectra and the optimised amount of Eu3+ ion in BaZr(PO4)2 is found to be 3.0 mol%. The luminescence properties of phosphor with added aforementioned fluxes were also studied in detail. NH4Cl was found to be the best among all the fluxes taken and the photoluminescence intensity of phosphor gets enhanced by 1.43 times of without flux. The Judd-Ofelt theory was employed for determination of spectral parameters from photoluminescent emission spectra. The temperature dependent photoluminescent studies establishes stability of phosphor emission at operating temperatures. The CIE coordinates were evaluated that confirms the pure red emission under n-UV excitation. All the studies indicate the potential of the said phosphor in phosphor converted white light emitting diodes as red emitter and indoor lighting.

Bi3+/Mn4+ co-doped Y2MgTiO6 dual-emitting phosphors for plant growth lighting and temperature sensor

Novel Bi3+/Mn4+ co-doped Y2MgTiO6 dual-emitting phosphors were synthesized using a high-temperature solid-phase method. The crystal structure and luminescence properties of phosphors were systematically investigated by experimental characterization and theoretical calculation. Under excitation at 375 nm, Y2MgTiO6:Bi3+, Mn4+ phosphors exhibit two emission bands located at 410 nm and 696 nm (blue and red), which can well be matched with the absorption bands of chlorophylls a, b and phytochrome. The Y2MgTiO6: Bi3+, Mn4+ luminescence remains at 66.2 % intensity at 423 K and quantum yield is 52.06 %. Optical temperature properties of Y2MgTiO6:Bi3+, Mn4+ were investigated using fluorescence intensity ratio (FIR) analysis and were found to reach a maximum relative sensitivity of 2.93 % K−1 at 498 K. The study suggests that this material could be a potential option for indoor plant culture and optical temperature sensing.

Temperature- and pressure-dependent luminescence properties of K3AlF6: Cr3+ phosphor

Temperature and pressure are two effective methods to tune the structure and luminescence properties of phosphors. In this study, tetragonal K3AlF6: Cr3+ phosphors were synthesized through the solid-state method. Its structure stability and luminescence property were in situ studied under temperature and pressure using differential scanning calorimetry (DSC), Raman, and photoluminescence spectra. At ambient conditions, one broadband emission presented in the 1.3–1.9 eV range due to the 4T2→4A2 transition. Three phase transitions presented with increasing temperature to 400 °C, but no much change was observed for its luminescence property. In contrast, its structure was stable during compression, and no phase transition was observed up to 26.1 GPa. Furthermore, the crystal field around Cr3+ was enhanced under pressure, leading to its luminescence changing from broad 4T2→4A2 emission to narrow 2E→4A2 emission above 4.2 GPa. It was suggested that pressure was more effective for K3AlF6:5%Cr3+ phosphor. This result provides one effective method to modulate the luminescence property of Cr3+.

Defect regulation of a distinctive synthetic Eu2+-doped SiAlON S-phase towards controllable spectra and thermal quenching for white-light-emitting diode

Targeted control of the luminescent properties of phosphors for white-light-emitting diode (WLED) applications, particularly spectral position and thermal stability, is crucial. In this study, a newly developed Eu2+-doped sialon S-phase BaAl1+ySi5-yO2-y/2N7 with intrinsic O vacancy (Vo) defects was synthesised using a special raw material ratio with an O deficiency. A series of structure–performance characterisations and density functional theory (DFT) calculations were performed. It was found that the spectral position and thermal stability of the samples could be realised by defect regulation using two approaches: regulation of the Eu2+ content and Al/Si ratio. Consequently, the emitting colour of the samples could be adjusted from green to yellow, leading to a lower colour temperature and higher colour rendering in the assembled WLED. The abnormal thermal quenching effect can also be suppressed with less variation in the emission intensity and colour purity at high temperatures. The defect regulation process investigated in this study will hopefully offer novel ideas for improving the luminescence properties of phosphors for extensive WLED applications.

Effect of microwave heat-treatment on up-conversion luminescence of Er3+/Yb3+ co-doped NaYF4 oxy-fluoride glass-ceramics

The microscopic properties of fluoride grains are known to play critical roles in the up-conversion luminescence properties of phosphors carrying rare-earth-based luminescence centers. To study the effect of different heating methods during preparation on the microstructure and up-conversion luminescence performance of oxy-fluoride glass-ceramics, we synthesized Er3+/Yb3+ co-doped NaYF4 nanocrystals containing transparent glass-ceramics by microwave heat-treatment and the normally heating methods. We characterized the internal structure of glass-ceramics by X-ray diffraction, transmission electron microscopy, mid-infrared spectroscopy, and Raman spectroscopy. While glass-ceramics samples prepared by both methods have extremely high optical transmittance similar to that of a glass, the microwave heat-treatment promoted faster grain growth rate, the glass-ceramics sample had better crystallinity and much improved up-conversion luminescence intensity compared to the sample obtained by the normally heating method.

Emission enhancement in the luminescence performance of warm red light-emitting LiSrVO4 :Pr3+ phosphors.

Warm red-emitting praseodymium-doped LiSrVO4 phosphors were synthesized via solid-state reaction. The phase formation was verified using an X-ray diffraction study and the morphology was investigated using a scanning electron microscope study. The LiSrVO4 :Pr3+ phosphors emitted red light when exposed to ultraviolet light, indicating their possibility for use in warm white light-emitting diodes (WLEDs). Furthermore, the effect of charge compensators on the luminescence characteristics was addressed. The decay time was investigated using time-resolved photoluminescence. Furthermore, thermal quenching was analyzed through temperature-dependent photoluminescence spectra. Their sensitivity was calculated using temperature-dependent decay time analysis. The colour purity of the emitted light could be measured by photometric analysis. This comprehensive investigation provides a thorough understanding of the luminescence properties of phosphors for WLED applications.

The Effect of Trivalent Ions on Luminescent Properties of SrAl2O4:Eu2+, Dy3+ Phosphors

Sr0.96Al2O4:Eu0.02, Dy0.02 green phosphors doped with La3+, Bi3+ and extra Dy3+ were successfully prepared by the solid-phase method. The crystal structure and luminescence properties of phosphors were characterized. The results showed that the doping of La3+ and Bi3+ and the continued increase of Dy3+ concentration in the SrAl2O4 lattice did not produce significant lattice distortion in the SrAl2O4 lattice, and the small amount of doping still maintained the presence of SrAl2O4. The photoluminescence intensity of both La3+ and Bi3+ doped samples showed an increasing and then decreasing trend within the doping amount, while the photoluminescence performance of Dy3+ doped samples did not show a decrease when increasing the same concentration of doping amount. The decay curves indicated that both La3+ and Bi3+ could decrease the afterglow time of SrAl2O4:Eu2+, Dy3+ phosphor, but increasing the concentration of Dy3+ effectively extends the afterglow time of SrAl2O4:Eu2+, Dy3+ phosphor.

Boosting up color tunability of Sr2Ga2GeO7 by energy transfer between Dy3+ and Eu3+ ions

A series of Sr2Ga2GeO7 (SGGO) phosphors doped with Dy3+ and/or Eu3+ were synthesized using solid-phase reaction. The structure and luminescence properties of phosphors were studied. The XRD, FT-IR, SEM and XPS analysis show that the doping with trivalent Dy and Eu have no effect on the structure of the phosphor, and the feature emission of Dy3+ or Eu3+ was observed by photoluminescence investigation. Based on the spectral overlapping of the Dy3+ emission and Eu3+ excitation, there is an energy transfer from Dy3+ to Eu3+ in the Sr2Ga2GeO7. Under excitation at 344 nm, with the increment of Eu3+ concentration, the Dy3+ emission intensity was decreased and the emission lifetime was shortened, whereas the lifetime of Eu3+ was increased, confirming the energy transfer from Dy3+ to Eu3+. The resulting tunable orange emission has potential applications in plant growth.

New compositions of double perovskite niobates with enhanced red luminescence

Red light (600–800 nm) plays an important role in the photosynthesis process of plants, therefore the development of red-light emitting phosphors for plant growth LED devices is of real interest from scientific and technological point of view. In this study, different compositions of niobate perovskites with the general formula LixMg2Nb0.992−yVyMn0.008O6 (x = 1–5, y = 0–0.25) were prepared by solid-state reaction. The influence of the lattice composition over the luminescent properties of phosphors was studied following two research directions. The first direction implies changes in the charge balance of the perovskite by varying the Li+ amount in the host lattice, while the second strategy produces lattice deformation through incorporation of various amounts of V5+at Nb5+ sites. The crystal structure, chemical composition, optical band gap, oxidation state of doping ions and luminescence properties of the samples are studied in detail. Under 483 nm excitation the photoluminescence spectra exhibit intense red emissions ascribed to the 2E→4A2 transition in Mn4+. It has been found that the addition of V5+ in Li4Mg2Nb0.992Mn0.008O6, intensifies the red emission even more with percentages up to 32 %.

Analysis of crystal-field effects on the energy levels of Mn4+ ions doped in different photoluminescent host lattices, exhibiting lowering of site symmetry: Exchange charge and superposition model calculations, for potential applications

Herein, we presented a comparative analysis of crystal-field (CF) effects on the energy levels of Mn4+ ions doped in the various host lattices exhibiting site symmetry from: triclinic: Y2MgTiO6, La2MgTiO6, Ca2YTaO6, Ca2YSbO6; trigonal: Ba2LaSbO6, LaTiSbO6; tetragonal: Ba2GdTaO6; to octahedral: Li2MgTiO4, Ba2GdSbO6, Ba2YSbO6. For modelling CF parameters (CFPs), we employed exchange charge model (ECM) using one model parameter (G), and superposition model (SPM) using four model parameters. A modified crystallographic axis system (CAS*) was adopted in ECM and SPM calculations for low symmetry hosts. The modelled CFPs served as input for crystal-field analysis/microscopic spin-Hamiltonian program to calculate the Mn4+ energy levels and match them with photoluminescence spectra of Mn4+-doped crystals. The 2nd-rank ECM/CFPs vary with G depending on distortions of the metal (M)-ligand (L) MLn polyhedron. The 4th-rank ECM/CFPs depend predominantly on the exchange charge effects. The ECM yields seemingly a better agreement between the observed and calculated energies than the SPM one. The disparity between the CFP sets calculated using SPM and ECM approaches in CAS*, while yielding same CFP strength, poses dilemmas. To solve these dilemmas, we employed the triclinic standardization of CFP sets, thus uncovering intricate low symmetry aspects inherent in ECM/CFPs, hitherto not realized. This method proved very useful for meaningful analysis and interpretation of CFP sets. Our CFP modelling illustrates importance of well-defined axis systems and adequate treatment of the actual triclinic (and monoclinic) site symmetry of dopant ions, instead of octahedral symmetry (usually adopted by experimentalists), in modelling of CFPs for triclinic symmetry cases. The relations between the Racah parameters, B and C, and 2Eg → 4A2g peak emission energy of Mn4+ ions are extended and suitably modified for triclinic symmetry. These modifications may be significant in engineering the local CF environment by suitably choosing the host crystals and dopant ions to improve the luminescence properties of phosphors for practical applications. The influence of dopant metal ions on the local structure and thus spectroscopic properties of doped crystals was also explored.

A novel Cr3+-doped stannate far red phosphor for plant lighting: structure evolution, broad-narrow spectrum tuning and application prospect

Chemical unit cosubstitution is an effective strategy to regulate the luminescence properties of phosphors, and it has attracted huge attention in the last few years. Following the design principles of the match of effective ion radius, atomic valence equilibrium and stoichiometric substitution, the local structure of crystal can be well customized, which is beneficial to develop high-performance luminescent materials. In this work, Mg2+-Sn4+ sites were cosubstituted by Zn2+-Ti4+ ion pairs in Mg2SnO4: Cr3+, respectively, and the emission spectra was successfully serially tuned from broad-band to narrow-band along with a unique variation trend of blue shift first and then slightly red shift. The photoluminescence excitation (PLE), photoluminescence (PL) spectra, lifetime decay, Raman spectra and electron paramagnetic resonance (EPR) were used to analyze the mechanism of luminescence tuning. Based on the manipulation of local structure, the as-observed distinct luminescence tuning can be well understood by the crystal field theory, nephelauxetic effect and changed Cr3+ capacity in this phosphor. Moreover, the adjusted narrow-band far-red emission spectra peak at 732 nm, matching the characteristic absorption of plant phytochrome PFR, has broad application prospects in the field of plant growth lighting. In addition, light-regulated dwarf potted tomato experiments were carried out to verify the application of far-red light in plant growth.

Activators lattice migration strategy customized for tunable luminescence of Ce3+ doped β-Ca3(PO4)2

The luminescent properties of phosphors are determined by the electronic configuration of their activators and the crystal environment in which they are embedded. By manipulating the site-selective occupancy of activators, we can coordinately control the local crystal field and electron cloud distribution to master the movement of emission wavelength and obtain a color-tunable phosphor. The system of β-Ca3(PO4)2 is an excellent carrier for crystal-site engineering to modify luminescent performance. It has five distinct crystal sites and a strong photoluminescence response. However, due to low doping concentration and structural complexity, the specific occupation of Ce remains unknown. We discussed the specific position of Ce in this study and proposed a novel activators lattice migration strategy for controlling the relative strength of crystal field splitting (CFS) and the nephelauxetic effect (NE) to control luminescence properties. By combining X-ray Absorption Fine Structure (XAFS) with DFT calculation, we predicted the local environment of each crystal lattice and migrated Ce3+ from M(3) to a strengthened crystal field site with a lower coordination number, shorter bonding bond length, and greater polyhedral distortion, M(5), resulting in phosphors with tunable luminescence emission. Relevant parameters ΔD(Ce3+) were identified for dealing with interference caused by the electronegativity difference during a structural modification. Some potential applications emerged as a supplement and an intriguing QR-code and response program for epidemic prevention were demonstrated. This structure–activity relationship-based strategy provides new inspiration for designing luminescent materials with tunable emission and a broader range of applications.

MxLa1–xSiO2–yNz (M = Ca/Sr/Ba): Elucidating and Tuning the Structure and Eu2+ Local Environments to Develop Full-Visible Spectrum Phosphors

The local environments of rare-earth activators have profound effects on the luminescent properties of phosphors. Here, we elucidate the crystal structure of the LaSiO2N phosphor host using a combination of density functional theory calculations and synchrotron X-ray diffraction. We determine that LaSiO2N crystallizes in the monoclinic C2/c instead of the hexagonal P6̅c2 space group. To improve the luminescence performance, divalent cations M (M = Ca/Sr/Ba) were introduced into LaSiO2N to eliminate Eu3+. A family of apatite M1+xLa4–xSi3O13–x/2:Eu2+ (x ∼ 1.5, M = Ca/Sr/Ba) phosphors was further developed with unprecedented ultra-broadband (290 nm) emission spectra and excellent thermal stability. Detailed local environment investigations reveal that the formation of oxygen vacancies within and beyond the first-shell environment of Eu2+ is responsible for the redshift and broadening of the emission spectra via geometrical alteration of the Eu2+ local environment. This work provides new insights for understanding and optimizing the luminescence of rare-earth phosphors.

Near-UV excited green-emission enhancement by efficient energy transfer in Na1.8Mg0.9Si1.1O4:Ce3+,Tb3+ phosphor for solid-state lighting applications

Luminescence properties of phosphor play a key role in the performance of white light-emitting-diodes (LEDs). The Tb3+ emission under near-UV excitation is usually very weak due to f-f spin-forbidden transitions. In this paper, we report that Tb3+ emission under near-UV excitation enhances up successfully in Ce3+,Tb3+ co-doped Na1.8Mg0.9Si1.1O4 (NMSO) phosphor, as a result of efficient energy transfer from Ce3+ to Tb3+. The newly-prepared NMSO:Ce3+,Tb3+ exhibits a strong green-emission line at λmax = 545 nm under 340 nm excitation, and the quantum efficiency is 0.82. The maximum energy transfer efficiency is up to 92.3%. The good thermal quenching resistance, high color-rendering index Ra (91.5) and low CCT (4752 K) of the packaged white LED demonstrate that energy transfer of rare earth phosphors is an effective strategy to target tailored optical performance. NMSO:Ce3+,Tb3+ phosphor has potential applications in near-UV chip excited white LEDs as a green-emitting component.

High quantum efficiency and luminescence properties of far-red Sr3NaTaO6: Mn4+, Ba2+ phosphor for application in plant growth lighting LEDs

Far-red emitting Sr3NaTaO6: Mn4+ and Sr3NaTaO6: Mn4+, Ba2+ phosphors were successfully synthesized by high-temperature solid-state method for the first time. The crystal structure and luminescence properties of phosphors were studied by X-ray diffraction (XRD), photoluminescence (PL), thermal stability, UV–vis spectroscopy and CIE color coordinate in detail. The Sr3NaTaO6: Mn4+ phosphor exhibits an intense emission peak at 692 nm under the excitation of 365 nm, which matches well with the absorption spectra of plant pigments of chlorophyll a and b, phytochrome PR and PFR. Besides, by co-doping of Ba2+ ions, the emission intensity of Sr3NaTaO6: Mn4+ phosphors were enhanced 1.1–1.5 times and an internal quantum efficiency (IQE) of 75.1% was obtained. Finally, a LED device was fabricated with the Sr3NaTaO6: 0.3%Mn4+, 0.45%Ba2+ phosphor and showed an effective promotion on the average stem length, fresh weight, dry weight and chlorophyll content of wheat. The result shows that Sr3NaTaO6: 0.3%Mn4+, 0.45%Ba2+ phosphor may have a potential application in the fields of plant growth lighting LEDs.

Variable temperature persistent luminescence properties of phosphors with continuous traps

A yellow long persistent luminescence (LPL) phosphor of Sr3Ga4O9:Bi3+ were synthesized by a simple solid-state method. The luminescence properties have been investigated by photoluminescence (PL) spectra, LPL spectra, afterglow decay curves and thermoluminescence (TL) curves. A series of the excitation time and decay time dependent TL experiments have been conducted, and the trap characteristics of the phosphor were studied. The LPL can be detected more than 10 h, which is due to the fact that the feasible trap depth ensures the slow release of charge carriers. The results of LPL properties at variable temperature show that the phosphor has a broad application prospect in high-temperature environments.

Local Structure Modulation-Induced Highly Efficient Red-Emitting Ba2Gd1-xYxNbO6:Mn4+ Phosphors for Warm WLEDs.

Modulating the crystal field environment around the emitting ions is an effective strategy to improve the luminescence performance of the practical effective phosphor materials. Here, smaller Y3+ ions are introduced into substituting the Gd3+ sites in Ba2GdNbO6:Mn4+ phosphor to modify the optical properties, including the enhanced luminescence intensity, redshift, and longer lifetime of the Mn4+ ions. The substitution of smaller Y3+ ions leads to lattice contraction and then strengthens pressure on the local structure, enhances lattice rigidity, and suppresses nonradiative transition. Moreover, the prototype phosphor-converted light-emitting diode (LED) demonstrates a continuous change photoelectric performance with a correlated color temperature of 4883-7876 K and a color rendering index of 64.1-83.2, suggesting that it can be one of the most prospective fluorescent materials applied as a warm red component for white LEDss. Thus, the smaller ion partial substitution can provide a concise approach to modulate the crystal field environment around the emitting ions for excellent luminescence properties of phosphors toward the modern artificial light.

Ce3+ luminescence, near-UV excitation enhancement of Tb3+ emission via energy transfer in Y4Zn4(SiO4)5:Ce3+,Tb3+ phosphor for white LED application

The luminescence property of phosphors plays an important role in the performance of phosphor-converted white light-emitting-diode (pc-WLED). In this paper, we report a simple energy-transfer (ET) strategy to enhance Tb3+ emission under near-UV excitation in Y4Zn4(SiO4)5 (YZS) doped with Ce3+/Tb3+ phosphor. The luminescence properties of YZS:Ce3+ based on crystal structure of YZS were investigated. The ET mechanism from Ce3+ to Tb3+ was also discussed in detail. The color of emitted light can be adjusted from blue to green by introducing Tb3+ ions into YZS:Ce3+. More importantly, the Tb3+ emissions under near-UV excitation are enhanced evidently, and the excitation spectrum for the Ce3+,Tb3+ co-doped phosphor matches well with the emission of near-UV chip. The selected green phosphor and commercial red, blue phosphors were coated on a 365-nm chip to fabricate a WLED device. The WLED shows an outstanding performance (Ra = 93.5 and CCT = 4769 K), indicating that YZS:Ce3+,Tb3+ phosphors have potential applications in near-UV pc-WLED as a green-emitting component.

Design, Preparation, and Mechanical Property Study of Polylactic Acid/Ca2BO3Cl: Eu2+, Dy3+ Composite Material for 3D Printing.

To break through the simplification of three-dimensional printing (3DP) materials and realize the application of long-persistent phosphors in more fields, polylactic acid doped with Ca2BO3Cl: Eu2+, Dy3+ was prepared in this study. The structure of the mixtures was analyzed and determined by infrared spectroscopy. The luminescence properties of phosphors and the composites were studied by fluorescence spectra and afterglow decay curve measurements. The yellow light of the mixtures could be attributed to the 5d-4f energy level transition of Eu2+. After the excitation of 254 nm ultraviolet lamp, the luminance and duration of the composites could be clearly observed. The mechanical properties of the composite filaments were tested, including maximal force and elasticity modulus. In particular, the influence of humidity on mechanical properties was analyzed in detail. The prepared composite filaments were printed into hollow dodecahedrons and presented in this study. Therefore, the composites had great properties and might be expected to put into practical applications of 3DP technology.