A new promising phosphor, Na3La2(BO3)3:Ln (Ln=Eu, Tb)

We present an efficient way to search a host for ultraviolet (UV) phosphor from UV nonlinear optical (NLO) materials. With the guidance, Na₃La₂(BO₃)₃ (NLBO), as a promising NLO material with a broad transparency range and high damage threshold, was adopted as a host material for the first time. The lanthanide ions (Tb³⁺ and Eu³⁺)-doped NLBO phosphors have been synthesized by solid-state reaction. Luminescent properties of the Ln-doped (Ln=Tb³⁺, Eu³⁺) sodium lanthanum borate were investigated under UV ray excitation. The emission spectrum was employed to probe the local environments of Eu³⁺ ions in NLBO crystal. For red phosphor, NLBO:Eu, the measured dominating emission peak was at 613 nm, which is attributed to ⁵D₀-⁷F₂ transition of Eu³⁺. The luminescence indicates that the local symmetry of Eu³⁺ in NLBO crystal lattice has no inversion center. Optimum Eu³⁺ concentration of NLBO:Eu³⁺ under UV excitation with 395 nm wavelength is about 30 mol%. The green phosphor, NLBO:Tb, showed bright green emission at 543 with 252 nm excited light. The measured concentration quenching curve demonstrated that the maximum concentration of Tb³⁺ in NLBO was about 20%. The luminescence mechanism of Ln-doped NLBO (Tb³⁺ and Eu³⁺) was analyzed. The relative high quenching concentration was also discussed.     

Mechanistic Insight into Energy-Transfer Dynamics and Color Tunability of Na4CaSi3O9:Tb3+,Eu3+ for Warm White LEDs

In this work, a latent energy-transfer process in traditional Eu³⁺,Tb³⁺-doped phosphors is proposed and a new class of Eu³⁺,Tb³⁺-doped Na₄CaSi₃O₉ (NCSO) phosphors is presented which is enabled by luminescence decay dynamics that optimize the electron-transfer energy process. Relative to other Eu³⁺,Tb³⁺-doped phosphors, the as-synthesized Eu³⁺,Tb³⁺-doped NCSO phosphors show improved large-scale tunable emission color from green to red upon UV excitation, controlled by the Tb³⁺/Eu³⁺ doping ratio. Detailed spectroscopic measurements in the vacuum ultraviolet (VUV)/UV/Vis region were used to determine the Eu³⁺–O²⁻ charge-transfer energy, 4f–5d transition energies, and the energies of 4f excited multiplets of Eu³⁺ and Tb³⁺ with different 4f^N electronic configurations. The Tb³⁺→Eu³⁺ energy-transfer pathway in the co-doped sample was systematically investigated, by employing luminescence decay dynamics analysis to elucidate the relevant energy-transfer mechanism in combination with the appropriate model simulation. To demonstrate their application potential, a prototype white-light-emitting diode (WLED) device was successfully fabricated by using the yellow luminescence NCSO:0.03Tb³⁺, 0.05Eu³⁺ phosphor with high thermal stability and a BaMgAl₁₀O₁₇:Eu²⁺ phosphor in combination with a near-UV chip. These findings open up a new avenue to realize and develop multifunctional high-performance phosphors by manipulating the energy-transfer process for practical applications.     

VUV spectroscopic properties of rare-earth (RE3+ = Sm3+, Eu3+, Tb3+, Dy3+) -activated layered borate Ba6Gd9B79O138

Vacuum ultraviolet (VUV) spectroscopic properties of rare-earth RE³⁺- activated (RE³⁺ = Sm³⁺, Eu³⁺, Tb³⁺ and Dy³⁺) Ba₆Gd₉B₇₉O₁₃₈ borates (BGBO) are investigated. The strong absorption bands in the VUV range of un-doped and RE³⁺-activated BGBO were observed. The band range from 140 to 200 nm with a peak at about 173 nm results from the host lattice absorption. For Sm³⁺-activated BGBO, the charge transfer transition from O^2- to Sm³⁺ was observed at 202 nm. In addition, it exhibits bright red emission originating from the Sm³⁺ f-f transitions of ⁴G_5/2 → ⁶H_J (J = 5/2, 7/2 and 9/2). The O^2--Eu³⁺ charge transfer (CT) at 249 nm is observed in the excitation spectrum for Eu³⁺-doped BGBO. For Tb³⁺-activated BGBO, the broad bands around 208 and 230 nm are due to the spin-allowed and spin-forbidden f-d transitions of Tb³⁺, respectively. In addition, the absence of the f-d transitions of Sm³⁺ and Dy³⁺ in the excitation spectra probably due to the photo-ionization effect. It is demonstrated that there are energy transfers from the BGBO host lattice to the luminescent activators depending on the activators.     

Vacuum ultraviolet spectroscopic properties of rare earth (RE) (RE = Eu,Tb, Dy,Sm, Tm)-doped K2GdZr(PO4)3 phosphate

The luminescent characteristics of RE (RE³⁺ = Eu, Tb, Dy, Sm and Tm)-doped K₂GdZr(PO₄)₃ have been investigated. The band in the range of 130–157 nm in the VUV excitation spectra of these compounds is attributed to the host lattice or PO₄^3? group absorption and the band from 157 nm to 215 nm with the maximum at 188 nm is due to the O–Zr charge transfer transition. For Eu³⁺-doped sample, the relatively weak band of O^2?–Eu³⁺ charge transfer (CTB) at 222 nm is observed and for Tb³⁺-doped sample, the band at 223 nm is related to the 4f–5d spin-allowed transition of Tb³⁺. For Dy³⁺- and Sm³⁺-doped samples, the O^2?–Dy³⁺ and O^2?–Sm³⁺ CTBs have not been observed, probably due to the 2p electrons of oxygen tightly bound to the zirconium ion in the host lattice. In Tm³⁺-doped sample, the weak O^2?–Tm³⁺ CTB is located at 170 nm. It is observed that there is energy transfer between the host and the luminescent activators (e.g. Eu³⁺, Tb³⁺ and Sm³⁺) except for Tm³⁺.     

Tunable luminescence and energy transfer process between Tb3+ and Eu3+ in GYAG:Bi3+, Tb3+, Eu3+ phosphors

A series of Eu³⁺ ions co-doped (Gd_0.9Y_0.1)₃Al₅O₁₂:Bi³⁺, Tb³⁺ (GYAG) phosphors have been synthesized by means of solvothermal reaction method. The XRD pattern of GYAG phosphor sintered at 1500 °C confirms their garnet phase. The luminescence properties of these phosphors have been explored by analyzing their excitation and emission spectra along with their decay curves. The excitation spectra of the GYAG:Bi³⁺, Tb³⁺, Eu³⁺ phosphors consists of broad bands in the shorter wavelength region due to 4f⁸ → 4f⁷5d¹ transition of Tb³⁺ ions overlapped with 6s² → 6s¹6p¹ (¹S₀ → ³P₁) transition of Bi³⁺ ions and the charge transfer band of Eu³⁺–O^2?. The present phosphors exhibit green and red colors due to ⁵D₄ → ⁷F₅ transition of Tb³⁺ ions and ⁵D₀ → ⁷F₁ transition of Eu³⁺ ions, respectively. The emission was shifted from green to red color by co-doping with Eu³⁺ ions, which indicate that the energy transfer probability from Tb³⁺ to Eu³⁺ ions are dependent strongly on the concentration of Eu³⁺ ions.     

Uniform Ln (Eu, Tb) doped NaLa(WO4)2 nanocrystals: Synthesis, characterization, and optical properties

Uniform shuttle-like Ln³⁺ (Eu³⁺, Tb³⁺) doped NaLa(WO₄)₂ nanocrystals have been solvothermally synthesized, and the size of the nanocrystals could be easily controlled by adjusting the volume ratio of ethylene glycol (EG) to water. Doped with 5 mol% Eu³⁺ and Tb³⁺ ions, the NaLa(WO₄)₂ nanocrystals showed strong red and green emissions with lifetimes of 0.8 and 1.40 ms, respectively. A high quenching concentration of 15 mol% was observed in Eu³⁺-doped NaLa(WO₄)₂ nanocrystals and 35 mol% in Tb³⁺-doped NaLa(WO₄)₂ nanocrystals. The emission intensity measurements of Eu³⁺-doped NaLa(WO₄)₂ with different sizes indicated that the emission intensity of shuttles with length of 300 nm in average was stronger than that of shuttles with length of 900 nm in average, but was weaker than that of needles with length of 4 and 9 μm in average.     

Synthesis and optical properties of novel red phosphors YNbTiO6:Eu3+ with highly enhanced brightness by Li+ doping

Highly luminescent euxenite phased YNbTiO₆:Eu³⁺ and Li⁺-doped YNbTiO₆:Eu³⁺ red phosphors have been prepared through a facile sol–gel combustion process and investigated for the first time. The introduction of Li⁺ ions into YNbTiO₆:Eu³⁺ is able to result in significant changes of the crystallinity and particle size, and bring a clear red-shift of absorption edge. A dominant red emission peak at 611 nm due to the ⁵D₀ → ⁷F₂ transition of Eu³⁺ was observed from photoluminescence spectra of the YNbTiO₆:Eu³⁺ and Li⁺-doped YNbTiO₆:Eu³⁺ phosphors. In particular, the emission intensity of the optimal Li⁺-doped YNbTiO₆:Eu³⁺ was examined to be close to 400% of commercial Y₂O₃:Eu³⁺ phosphor. The mechanism of the enhanced emission by Li⁺ doping was discussed.     

Structure-dependent luminescence properties of Eu3+-doped CsBPO4 (B = Mg,Zn)

Eu³⁺-doped monophosphate CsBPO₄ (B = Mg, Zn) was synthesized by conventional solid state reaction. The crystal phase formations and morphology of the phosphates were analyzed by X-ray powder diffraction (XRD) and scanning electron microscope (SEM) measurements, respectively. The luminescence properties, including the photoluminescence excitation and emission spectra, decay curve (lifetime), the color coordinates and the internal quantum efficiency were investigated. Two Eu³⁺-doped monophosphates display very distinct luminescence properties. CsMgPO₄:Eu³⁺ presents the dominant reddish-orange emission from magnetic dipole transition ⁵D₀ → ⁷F₁ and a long luminescence lifetime. In contrast, CsZnPO₄:Eu³⁺ has a pure red color with the dominant induced electric-dipole transition ⁵D₀ → ⁷F₂ and a fast decay. Different results on the luminescence features of CsBPO₄ (B = Mg, Zn):Eu³⁺ were discussed on the base of crystal structure. Eu³⁺ has structure-dependent transitions due to the special microstructure occupied in a given host.     

Lanthanide(III)‐Based Multicolor Luminescent Hybrid Gel for Amine Sensing

Bridged polysilsesquioxanes (BPs) show great potential in the development of lanthanide‐based luminescent materials, owing to their capacity to loading lanthanide complexes with high concentration and their flexible processability. A novel BP precursor, consisting of a C ₃‐symmetrical benzene central core moiety, capable of sensitizing the luminescence of Eu³⁺ and Tb³⁺ is reported. Tunable, full‐color luminescent gels were facilely prepared by mixing the as‐synthesized precursor and Ln³⁺ ions in appropriate solvents. By either changing the Eu³⁺/Tb³⁺ molar ratio or altering the excitation wavelength, the emission colors of the final gels can be finely tuned. Additionally, the yellow‐colored emissive gel with a molar ratio of Eu³⁺ to Tb³⁺ of 0.5 can be used as an effective ratiometric luminescent sensor for distinguishing amines with lower pK _a (<5) from those with higher pK _a (>9).     

Growth phase diagram and X-ray excited luminescence properties of NaLuF4:Tb3+ nanoparticles

X-rays are energy sources exhibiting extended penetration depths, and they have attracted increasing attention in industry and for clinical application. With the rapid development of nanomaterials and X-ray excited luminescent nanoparticles (XLNPs), new modalities for bioimaging and cancer therapy have been developed, such as X-ray luminescent computed tomography (XLCT) and X-ray excited photodynamic therapy (X-PDT). To meet the requirements of biomedical applications, XLNPs must exhibit high luminescence intensities, appropriate size distributions (less than100 nm) and negligible cytotoxicity. Due to the optical properties associated with f-electrons, rear earth (RE) elements are highly suitable for creating XLNPs. NaREF₄ nanoparticles (NPs) have been shown to be suitable hosts with high luminescence intensities, controllable sizes, and biocompatibility for X-ray-based biomedical applications. Syntheses of NaLuF₄ NPs doped with rare earth elements for upconversion applications have been systematically studied. However, for X-ray excited applications, the doping levels of the NPs must be totally different, which greatly affects the morphologies and sizes of the NaLuF₄ NPs. Thus, in this paper, nucleation, phase transitions, morphologies and sizes, and luminescence properties of Tb³⁺-doped NaLuF₄ NPs were systematically studied. OA-capped NaLuF₄:Tb³⁺ NPs were synthesized via coprecipitation processes with different reaction temperatures and reaction times to study the nucleation mechanism systematically, and the morphologies, size distributions and crystal phases were characterized with TEM and XRD. The morphologies, size distributions and crystal phases of these NPs were seriously influenced by the reaction temperature and reaction time. At 295 ℃, the NP sizes increased with prolonged reaction time, and the crystalline phase was a mixture of cubic and hexagonal phases. At 300 ℃ and 310 ℃, the pure hexagonal phase was obtained after 20 min and 35 min reaction times, respectively. The luminescence strengths of these NPs were associated with the particle sizes, crystalline phases, and Tb³⁺ doping levels. Stronger luminescence was achieved with larger particle sizes and purer hexagonal crystal phases. In addition, the 15 % doping level for Tb³⁺ provided the maximum luminescence intensity. The present work provides insights into the mechanism of NaLuF₄:Tb³⁺ nanocrystal growth.     

Crystal structure of Eu-doped M3MgSi2O8 (M: Ba, Sr, Ca) compounds and their emission properties

Emission properties of Eu²⁺-doped M₃MgSi₂O₈ (M: Ba, Sr, Ca) are discussed in terms of the crystal structure. When Ba²⁺ ions account for over one third of M²⁺ ions, M₃MgSi₂O₈ crystallizes in glaserite-type trigonal structure, while Ba-free compounds crystallize in merwinite-type monoclinic structure. Under UV excitation, the Eu²⁺-doped glaserite-type compounds exhibit an intense blue emission assigned to 5d-4f electron transition at about 435 nm, regardless of the molar ratio of Ba²⁺, Sr²⁺ and Ca²⁺ ions. By contrast, the Eu²⁺-doped merwinite-type compounds show an emission color sensitive to the ratio. A detailed analysis of the emission spectra reveals that the emission chromaticity for the Eu²⁺-doped M₃MgSi₂O₈ is composed of two emission peaks reflecting two different sites accommodating M²⁺ ion.     

Energy transfer and heat-treatment effect of photoluminescence in Eu-doped TbPO4 nanowires

We have successfully synthesized Eu³⁺-doped TbPO₄ nanowires, which are orderly organized to form bundle-like structure. A thermal treatment up to 600 °C does not modify the size, shape and structure of as-synthesized sample. Due to the energy overlap between Tb³⁺ and Eu³⁺, an efficient energy transfer occurs from Tb³⁺ to Eu³⁺. The effects of Eu³⁺ concentration and thermal treatment on the luminescent properties of Eu³⁺ are investigated. The increase of Eu³⁺ concentration leads to the increase of the energy transfer efficiency from Tb³⁺ to Eu³⁺, but also enhances the probability of the interaction between neighboring Eu³⁺, which results in the concentration quenching. With the heat-treatment, the luminescence of Eu³⁺ presents an obvious increase, but almost no change for the luminescence of Tb³⁺. This difference is explained based on the TGA, DTA, and fluorescent decay dynamics analyses.     

Development of a ratiometric time-resolved luminescence sensor for pH based on lanthanide complexes

Time-resolved luminescence bioassay technique using lanthanide complexes as luminescent probes/sensors has shown great utilities in clinical diagnostics and biotechnology discoveries. In this work, a novel terpyridine polyacid derivative that can form highly stable complexes with lanthanide ions in aqueous media, (4′-hydroxy-2,2′:6′,2′′-terpyridine-6,6′′-diyl) bis(methylenenitrilo) tetrakis(acetic acid) (HTTA), was designed and synthesized for developing time-resolved luminescence pH sensors based on its Eu³⁺ and Tb³⁺ complexes. The luminescence characterization results reveal that the luminescence intensity of HTTA–Eu³⁺ is strongly dependent on the pH values in weakly acidic to neutral media (pK_a = 5.8, pH 4.8–7.5), while that of HTTA–Tb³⁺ is pH-independent. This unique luminescence response allows the mixture of HTTA–Eu³⁺ and HTTA–Tb³⁺ (the HTTA–Eu³⁺/Tb³⁺ mixture) to be used as a ratiometric luminescence sensor for the time-resolved luminescence detection of pH with the intensity ratio of its Tb³⁺ emission at 540 nm to its Eu³⁺ emission at 610 nm, I_540 nm/I_610 nm, as a signal. Moreover, the UV absorption spectrum changes of the HTTA–Eu³⁺/Tb³⁺ mixture at different pHs (pH 4.0–7.0) also display a ratiometric response to the pH changes with the ratio of absorbance at 290 nm to that at 325 nm, A_290 nm/A_325 nm, as a signal. This feature enables the HTTA–Eu³⁺/Tb³⁺ mixture to have an additional function for the pH detection with the absorption spectrometry technique. For loading the complexes into the living cells, the acetoxymethyl ester of HTTA was synthesized and used for loading HTTA–Eu³⁺ and HTTA–Tb³⁺ into the cultured HeLa cells. The luminescence imaging results demonstrated the practical utility of the new sensor for the time-resolved luminescence cell imaging application.     

Homogeneous one-dimensional structured Tb(OH)3:Eu nanorods: Hydrothermal synthesis, energy transfer, and tunable luminescence properties

Nearly monodisperse, homogeneous and well-defined one-dimensional Tb_(1−x)(OH)₃:xEu³⁺ (x=0-3 mol%) nanorods have been prepared through hydrothermal method. The size of the Tb(OH)₃:Eu³⁺ rods could be modulated from nano- to micro-scale by using different amount of ammonia solution. They present highly crystallinity in spite of the moderate reaction temperature. Under ultraviolet excitation into the f→f transition of Tb³⁺ at 382 nm, Tb(OH)₃ samples show the characteristic emission of Tb³⁺ corresponding to ⁵D₄→⁷F_{6, 5, 4, 3} transitions; whereas Tb(OH)₃:Eu³⁺ samples mainly exhibit the characteristic emission of Eu³⁺ corresponding to ⁵D₀→⁷F_{1, 2, 4} transitions due to an efficient energy transfer occurs from Tb³⁺ to Eu³⁺. The increase of Eu³⁺ concentration leads to the increase of the energy transfer efficiency from Tb³⁺ to Eu³⁺. The PL colors of Tb(OH)₃:xEu³⁺ phosphors can be easily tuned from green, yellow, orange, to red by changing the doping concentration (x) of Eu³⁺.     

Luminescent properties of Tb3+ and Gd3+ ions doped aluminosilicate oxyfluoride glasses

Tb³⁺ and Gd³⁺ ions doped lithium–barium–aluminosilicate oxyfluoride glasses have been prepared. The transmission, emission and excitation spectra were measured. It has been found that those Tb³⁺-doped lithium–barium–aluminosilicate oxyfluoride glasses exhibit good UV-excited luminescence. The luminescence intensity of Tb³⁺ ion increases for those (Tb³⁺, Gd³⁺)-codoped glasses. Energy transfer process from Gd³⁺ ion to Tb³⁺ ion is indicated.     

Synthesis of Gd3PO7:Eu nanospheres via a facile combustion method and optical properties

Eu³⁺-doped Gd₃PO₇ nanospheres with an average diameter of ∼300 nm and a narrow size distribution have been prepared by a facile combustion method and structurally characterized by X-ray diffraction and field emission scanning electron microscopy. The luminescent properties were systemically studied by the measurement of excitation/emission spectra, and emission spectra under different temperatures, as well as by photostability. The strong red-emission intensity peaking at 614 nm originates the ⁵D₀→⁷F₂ transition and is observed under 254-nm irradiation, indicating that Eu³⁺ ions in Gd₃PO₇ mainly occupied non-centrosymmetry sites. The CIE1931 XY chromaticity coordinates of Gd₃PO₇:Eu³⁺ nanospheres are (x=0.654, y=0.345) in the red area, which is near the National Television Standard Committee standard chromaticity coordinates for red. Thus, Gd₃PO₇:Eu³⁺ nanospheres may be potential red-emitting phosphors for PDP and Xe-based mercury-free lamps.     

Synthesis,characterization, and photoluminescence of SrBPO5:R,Na+ (R = Eu3+, Tb3+) phosphor for solid state lighting

A series of phosphors SrBPO₅:R,Na⁺ (R = Eu³⁺, Tb³⁺) were prepared by high-temperature solid-state synthesis, and their phase purity, morphology, IR spectra, and UV-Vis photoluminescence properties were investigated. The f-f transitions of Eu³⁺ and Tb³⁺ ions in the host lattice were assigned and discussed. The excitation and emission spectra indicate that SrBPO₅:Eu³⁺,Na⁺ and SrBPO₅:Tb³⁺,Na⁺ can be effectively excited by ultraviolet (394 and 370 nm), and exhibit reddish orange emission and yellowish green emission, respectively. The influence of the doping concentration on the relative emission intensity of Eu³⁺/Tb³⁺ was investigated, and the critical distance R_c was estimated in term of the concentration quenching data. The present study suggests SrBPO₅:R,Na⁺ (R = Eu³⁺, Tb³⁺) phosphor can be a potential candidate as an UV-convertible phosphor for white light-emitting diodes (LEDs).     

UV-VUV-excited photoluminescence of RE-activated CaLaP3O10 (RE=Eu,Tb)

Monazite-type polyphosphate CaLaP₃O₁₀ was synthesized by solid-state reaction at 1000 °C and their photoluminescence of Eu³⁺ and Tb³⁺ in CaLaP₃O₁₀ under ultraviolet (UV) and vacuum-ultraviolet (VUV) excitation were evaluated for the first time. The emission spectra of CaLaP₃O₁₀:Eu³⁺showed that Eu³⁺ are in a site with inversion symmetry because the magnetic dipole transition ⁵D₀-⁷F₁ was the strongest both upon 254 and 147 nm excitation. Monitored at 621 nm the excitation spectra consisted of host absorption bands, charge transfer band of Eu-O and the intraconfiguration 4f⁶ transition of Eu³⁺. Green phosphor CaLaP₃O₁₀:Tb³⁺exhibited better color purity when excited by 147 nm than that excited by 254 nm. With monitored at 542 nm the host absorption bands of CaLaP₃O₁₀:Tb³⁺ were also observed. Besides the host absorption bands there were strong f-d and weak f-f transitions of Tb³⁺.     

Photoluminescence properties of rare earths (Eu3+, Tb3+, Dy3+ and Tm3+) activated NaInW2O8 wolframite host lattice

The photoluminescence (PL) studies on NaIn_1?xRE_xW₂O₈, with RE=Eu³⁺, Tb³⁺, Dy³⁺ and Tm³⁺ phases have shown that the relative contribution of the host lattice and of the intra-f–f emission of the activators to the PL varies with the nature of the rare earth cation. In the case of Dy³⁺ and Tm³⁺ activators, with yellow and blue emission, respectively, the energy transfer from host to the activator plays a major role. In contrast for Eu³⁺, with intense red emission, the host absorption is less pronounced and the intra-f–f transitions of the Eu³⁺ ions play a major role, whereas for Tb³⁺ intra-f–f transitions are only observed, giving rise to green emission.