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[Display omitted] • Y 2 Mo 4 O 15 : x Yb3+/ y Ho3+ UC phosphors were converted from a new type of precursor. • The process of phase and morphology evolution was unveiled. • Y 2 Mo 4 O 15 : x Yb3+/ y Ho3+ exhibits strong deep-red (630–670 nm) emission under 978 nm laser pumping. • The optimal contents of Yb3+ and Ho3+ were determined. • The photon process of UC luminescence was explained in detail. Hydrothermal reaction of Y(NO 3) 3 ·6H 2 O and (NH 4) 6 Mo 7 O 24 ·4H 2 O at 180 °C for 24 h was performed in this work via systematically varying the solution pH (4–10) and Mo/Y molar ratio (R, 2–5), which produced a NH 4 Y(MoO 4) 2 ·0.19H 2 O new compound that yielded phase-pure Y 2 Mo 4 O 15 upon calcination at 500–700 °C. The pH/R window for the compound to crystallize was determined to be 6–7/3 and 6–9/4–5. The products were characterized in detail by XRD, FTIR, elemental analysis, SEM, TEM, STEM and TG to understand chemical composition and the courses of phase/morphology evolution. Applying the above synthesis strategy successfully produced Y 2 Mo 4 O 15 : x Yb3+, y Ho3+ phosphors that exhibit strong deep-red (630–670 nm, 5F 5 → 5I 8 transition) and weak green (530–560 nm, 5F 4 /5S 2 → 5I 8) emissions under 978 nm laser pumping, where the optimal Yb3+ and Ho3+ contents were determined to be x = 0.20 for y = 0.01 and y = 0.03 for x = 0.10, respectively. Spectral analysis indicated that the up-conversion luminescence occurred via a two-photon process, with the red/green intensity ratio and also the emission color dependent on Yb3+ and Ho3+ contents, which were explained by considering ground state excitation, energy transfer, non-radiative relaxation and cross relaxation. [ABSTRACT FROM AUTHOR]

We show the effects of the Zr4+-doping on the microstructural, optical, and luminescent properties of (Lu, Gd) 2 O 3 : Eu transparent ceramics in its as-sintered state and annealed state. Zr4+-dopant greatly enhanced the transparency of (Lu, Gd) 2 O 3 : Eu ceramic, giving Lu 0.42 Gd 0.5 Zr 0.03 Eu 0.05 O 1.5+δ ceramic with a transmittance of up to 80% at 600 nm. It is found that Zr4+-dopant dominates the luminescent properties of Eu ions in both as-sintered and annealed ceramics. Energy transfer from Eu2+ to Eu3+ was found by photoluminescence spectra, and the ratio of Eu2+/(Eu2++Eu3+) was affected by Zr4+ content in as-sintered ceramics. Moreover, the oxygen vacancy concentration increased with increasing Zr4+ content in as-sintered ceramics. The annealing process enhanced the intensity of all the excitation bands of 4f→4f transitions and the intensity of charge transfer band (CTB) decreased as the Zr4+ content increased in the annealed ceramics. This work highlights that Zr4+ doping is a potential way to improve the transmittance and red emission intensity of 4f→4f transition of Eu3+ in the (Lu, Gd) 2 O 3 : Eu ceramic for extended applications. [ABSTRACT FROM AUTHOR]

Excitation-dependent photoluminescence is most frequently encountered in quantum dot systems and organic-inorganic metal halide hybrids. Here, we report a new paradigm for excitation-dependent luminescence by grafting picolinic acid (pa) onto the single-crystalline nanosheets of RE(OH) 2.94 (NO 3) 0.06 · n H 2 O (RE = Y/Tb, Y/Eu, Y/Tb/Eu). Grafting of pa through an interaction between COO− group of pa and RE3+ ions on the surface of nanosheets, did not change the morphology and the crystal structure of the original nanosheets, and the grafting content of pa reached the maximum value at R = 0.25 (R , the molar ratio of pa to nanosheets). Effective intramolecular energy transfer from coordinated ligands to the activated RE3+ ions, called the antenna effect, can give rise to an enhanced photoluminescence upon light irradiation. In this work, multi-modal energy transfer of pa → Eu3+, pa → Tb3+ → Eu3+, and the reconstructed Tb3+ → Eu3+ resulted from the antenna effect of pa contributed to the excitation-dependent luminescence behavior and greatly enhanced emission intensity. The luminescence reversibly undergoes repeated cycles by switching the excitation wavelength, indicating the grafted nanosheets are the potential materials of switched "on/off" optical sensors. The use of the energy transfer to the activated RE3+ ions through the antenna effect recommends a new paradigm for luminescence improvement and excitation-dependent luminescence. Unlabelled Image • Picolinic acid (pa) was grafted onto the rare earth hydroxynitrate nanosheets. • Grafting of pa did not change the morphology and the crystal structure. • The antenna effect of pa leaded to dramatically enhanced and excitation-dependent luminescence. • The grafted nanosheets are the potential materials of switched "on/off" optical sensors. [ABSTRACT FROM AUTHOR]

A series of well-dispersed (Ce 1- x Tb x)PO 4 (x = 0–1) microspheres, with an average diameter of ∼1.2 μm, have been originally prepared via two-phase interfacial precipitation and subsequent calcination. Characteristics of the as-synthesized and calcination products were investigated by the combined techniques of XRD, FE-SEM, TEM, FT-IR and TG-DSC. Under 275 nm excitation, tuning the emission color from blue (0.17, 0.02) to green (0.30, 0.56) was realized via varying the Tb3+ content and through Ce3+ → Tb3+ energy transfer. The optimal content of Tb3+ was found to be ∼15 at% (x = 0.15), and the energy transfer was determined to occur via electric dipole-dipole interaction. Moreover, both the emissions of Ce3+ and Tb3+ present gradually higher thermal stability with increasing Tb3+ content owing to enhanced energy transfer. The (Ce 0.85 Tb 0.15)PO 4 microspheres, which possess the highest absolute quantum efficiency (∼60.1%) across the series and excellent thermal stability (activation energy for thermal quenching of luminescence: ∼0.312 eV), may potentially serve as a single-phase green phosphor for solid-state lighting and field-emission display. Multi-color luminescent (Ce 1- x Tb x)PO 4 (x = 0–1) microspheres were successfully fabricated via gel/solution interfacial precipitation and subsequent calcination. Image 1 • (Ce 1- x Tb x)PO 4 (x = 0–1) microspheres were obtained via interfacial precipitation. • The formation mechanism of (Ce 1- x Tb x)PO 4 spheres was proposed. • Ce3+.→ Tb3+ energy transfer was analyzed to occur via dipole-dipole interaction • (Ce 0.85 Tb 0.15)PO 4 has favorable thermal stability for Tb3+ luminescence. [ABSTRACT FROM AUTHOR]

Abstract Eulytite-type Ba 3 La 1- x (PO 4) 3 : x Ce3+ (x = 0–0.10) and Ba 3- y La 0.97 (PO 4) 3 :0.03Ce3+, y Mn2+ (y = 0.05–0.30) phosphors were first synthesized via gel-combustion and subsequent calcination in an Ar/H 2 atmosphere at 1250 °C. Photoluminescence properties of the phosphors were thoroughly studied, including excitation, emission, emission color, thermal stability and the process of Ce3+ → Mn2+ energy transfer. The Ba 3 La(PO 4) 3 : x Ce3+, y Mn2+ phosphor series simultaneously exhibited the near-ultraviolet emission of Ce3+ (∼370 nm, 4 f 05 d 1 → 4 f 15 d 0 transition) and the orange-red emission of Mn2+ (∼591–605 nm, 4T 1 → 6A 1 transition) upon exciting the Ce3+ ions with ∼276 nm UV light. Owing to Ce3+ → Mn2+ energy transfer most probably occurring via dipole-dipole interaction, gradually enhanced Mn2+ while weakened Ce3+ emission was resulted at a higher Mn2+ content, with which the emission color can be adjusted from blue through reddish-purple and ultimately to orange red. The efficiency of the energy transfer was analyzed to reach its maximum of ∼43.2% at y = 0.30. It was shown that the Ba 3 La 1- x (PO 4) 3 : x Ce3+ and Ba 3- y La 0.97 (PO 4) 3 :0.03Ce3+, y Mn2+ phosphor series both have favorable thermal stabilities, whose activation energy for thermal quenching of luminescence was calculated to be ∼0.28 ± 0.02 eV for Ce3+ in the former while ∼0.22 ± 0.01 eV for Ce3+ and ∼0.21 ± 0.01 eV for Mn2+ in the latter system. Increasing temperature did not alter the emission wavelength of Ce3+ but gradually blue-shifted that of Mn2+ from ∼602 to 583 nm with increasing temperature from 25 to 200 °C. Graphical abstract Image 1 Highlights • Gel-combustion produced Ce3+ and Ce3+/Mn2+ doped Ba 3 La(PO 4) 3 phosphors. • Photoluminescence properties of the phosphors were thoroughly analyzed. • The mechanism and efficiency of Ce3+.→Mn2+ energy transfer were determined. • Favorable thermal stabilities were demonstrated for the phosphors. [ABSTRACT FROM AUTHOR]

Graphical abstract Gd3+ mediated morphology tailoring and selective energy transfer were found in white-light emitting (Y,Gd)PO 4 :Dy3+ microspheres. Highlights • Well-dispersed and one micron-sized spheres of tetragonal (Y,Gd)PO 4 :Dy3+ have been facilely synthesized. • Incorporation of Gd3+ resulted in greatly enhanced luminescence intensity. • Morphology tailoring and selective energy transfer are mediated by Gd3+ doping. • (Y,Gd)PO 4 :Dy3+ microspheres exhibited a white-light emission. Abstract Well-dispersed and one micron-sized spheres of tetragonal Y/Gd/Dy ternary phosphate have been facilely synthesized via microwave-assisted homogeneous precipitation. The spheres are polycrystalline, and the particle size gradually decreases with increasing Gd3+ content. Excessive doping of Gd3+ leads to the appearance of hexagonal-structured rods on particle surface. The Y/Gd/Dy ternary phosphor exhibits two predominant emission peaks, with a blue one at ∼483 nm and a yellow one at ∼575 nm, thus yielding a white light-emitting color. Mainly due to the efficient energy transfer, incorporation of Gd3+ resulted in enhanced luminescence intensity, with nearly a 1000% increase at 575 nm. Because the selective energy transfer from Gd3+ to the 4F 9/2 →6H 13/2 transition of Dy3+ takes the dominant role, more remarkable enhanced intensity at ∼575 nm than that at ∼483 nm was observed. Enhanced emission intensity, shorter lifetimes, and almost the same CIE coordinates were observed at a higher calcination temperature. [ABSTRACT FROM AUTHOR]

(Tb 1 -x Ce x ) 3 Al 5 O 12 yellow phosphors (0≤x≤0.05) were calcined from their coprecipitated carbonate precursors, and the effects of the temperature and atmosphere (air and H 2 ) of calcination on the sequence of phase evolution and the characteristics of the powders were investigated in detail. The activation energy for the crystallite growth during calcination was estimated to be ~39 kJ/mol. The powder calcined at 1000 °C showed good reactivity and was sintered into a ceramic plate of ~97% dense (average grain size: ~1.3 µm) in a H 2 /Ar gas mixture at the relatively low temperature of 1500 °C. The phosphors simultaneously exhibit the 4f 8 →4f 7 5d 1 , 7 F 6 → 5 D 3 and 7 F 6 → 5 D 4 excitations of Tb 3+ and the 4f 1 →5d 1 excitation of Ce 3+ when monitoring the yellow emission of Ce 3+ at 560 nm, suggesting the presence of efficient Tb 3+ →Ce 3+ energy migration. The optimal Ce 3+ content for luminescence was found to be x =0.015 and 0.01 under the direct excitation of Ce 3+ and through Tb 3+ →Ce 3+ energy transfer, respectively, and concentration quenching of luminescence was analyzed to be resulted from exchange interaction. Luminescence features of the phosphors, including excitation, emission, quantum yield, fluorescence lifetime, color coordinates and color temperature, were thoroughly investigated against the processing temperature and Ce 3+ content, with an in-depth discussion on the process of energy transfer among the optically active Tb 3+ and Ce 3+ ions. The materials may find application in blue-light excited white LEDs. [ABSTRACT FROM AUTHOR]

The Gd 3 Al 5 O 12 :Tb/Ce (GdAG:Tb/Ce) garnet solutions effectively stabilized by Lu 3+ have been achieved by calcining their precursor at 1300 °C. Detailed characterizations are given to the materials in terms of XRD, FE-SEM, BET, PL/PLE, and fluorescence decay analysis. The occurrence of Gd 3+ and Tb 3+ transitions from the photoluminescence excitation spectrum monitoring the Ce 3+ yellow emission strongly confirmed the efficient Gd 3+ → Ce 3+ and Tb 3+ → Ce 3+ energy transfer. The [(Gd 0.8 Lu 0.2 ) 0.99− x Ce 0.01 Tb x ]AG ( x = 0–0.1) phosphors with good dispersion and uniform particle size exhibit various luminescent properties under different excitation wavelength of 275, 338, and 457 nm, respectively. The photoluminescence comparison indicated that owing to the Gd 3+ → Ce 3+ and Tb 3+ → Ce 3+ energy transfer, the best luminescent phosphor [(Gd 0.8 Lu 0.2 ) 0.89 Ce 0.01 Tb 0.1 ]AG is almost identical to the well-known YAG:Ce, higher than LuAG:Ce in emission intensity, and has a substantially red-shifted emission band that is desired for warm-white lighting. The Tb 3+ → Ce 3+ energy transfer was suggested to be electric multipolar interactions, and the processes of energy migration among the optically active Gd 3+ , Tb 3+ , and Ce 3+ ions were discussed in detail. Fluorescence decay analysis found the lifetime for the Ce 3+ emission hardly changes with the Tb 3+ incorporation. The [(Gd 0.8 Lu 0.2 ) 0.99− x Ce 0.01 Tb x ]AG garnets developed in this work may serve as a new type of phosphor that hopefully meets the requirements of various lighting, optical display, and scintillation applications. [ABSTRACT FROM AUTHOR]

[(Gd 0.8 Lu 0.2 ) 0.9− x Tb 0.1 Eu x ] 3 Al 5 O 12 ( x = 0–0.1) garnet phosphors have been calcined from their coprecipitated carbonate precursors at 1300 °C. Detailed materials characterizations are done via XRD, FE-SEM, BET, particle sizing, PL/PLE, and fluorescence decay analyses. The phosphors, with excellent dispersion and uniform particle size and shape, simultaneously exhibit the characteristic Tb 3+ and Eu 3+ emissions with the strongest peaks located at 545 nm (green) and 592 nm (orange red) under the optimal excitation wavelength of 275 nm, which correspond to the 5 D 4 → 7 F 5 and 5 D 0 → 7 F 1 transitions of Tb 3+ and Eu 3+ , respectively. The emission intensities of both Tb 3+ and Eu 3+ remarkably vary with increasing Eu 3+ incorporation, and, as a consequence, the emission color can be readily tuned from approximately green to orange-red. At the optimal Eu 3+ concentration of x = 0.03, the energy transfer efficiency was calculated to be ∼83.2% and the mechanism of energy transfer was analyzed to be electric dipole–dipole interactions. The processes of energy migration among the optically active Gd 3+ , Tb 3+ , and Eu 3+ ions are discussed in detail. Fluorescence decay analysis found rapidly decreasing lifetime for the Tb 3+ emission, conforming to the Tb 3+ → Eu 3+ energy transfer, and the probability of energy transfer is calculated. The [(Gd 0.8 Lu 0.2 ) 0.9− x Tb 0.1 Eu x ] 3 Al 5 O 12 solid-solutions developed in this work may serve as a new type of phosphor that hopefully meets the requirements of various lighting, optical display, and scintillation applications. [ABSTRACT FROM AUTHOR]

Abstract: Eu3+ doped Gd4Al2O9 (GdAM), GdAlO3 (GdAP), and Gd3Al5O12 (GdAG, containing 10at% of Lu3+ for lattice stabilization) have been developed in this work as efficient red-emitting phosphors. With coprecipitated carbonate precursors, phase evolution studies found minimum processing temperatures of ~1000, 1100, and 1300°C for the three phosphors to crystallize as pure phases, respectively. Compared with their yttrium aluminate counterparts, the gadolinium-based phosphors exhibit red-shifted O2−–Eu3+ charge transfer excitation band (CTB) centers due to the lower electronegativity of Gd3+ and appreciably higher quantum yields of photoluminescence owing to the occurrence of efficient Gd3+→Eu3+energy transfer. The optimal Eu3+ contents were determined to be ~7.5at% for GdAM and 5.0at% for both GdAP and GdAG, and concentration quenching of luminescence was suggested to be due to exchange interactions. Fluorescence decay analysis found a shorter lifetime for the phosphor powder processed at a higher temperature or with a higher Eu3+ content, and the underlying mechanism was discussed. Fluorescence lifetimes were also compared between the yttrium and gadolinium phosphor systems for the dominant emissions. [Copyright &y& Elsevier]

Abstract: (Y0.95− x Gd x Eu0.05)2O3 red phosphors (x =0.25–0.95), in both the nanoplate and rounded nanoparticle forms, have been derived from their layered rare-earth hydroxide (LRH) precursors of (Y0.95− x Gd x Eu0.05)2(OH)5NO3·nH2O synthesized via hydrothermal reactions under neutral pH. Crystal structures and physical properties of the materials are investigated in detail by the combined techniques of XRD, TG, BET, FE–SEM, TEM, and PLE/PL spectroscopies. The LRH precursors crystallize as nanoplates and a higher Gd3+ content produces thinner crystals (down to ∼5nm). Calcining the LRHs at 800°C directly yields (Y0.95− x Gd x Eu0.05)2O3 phosphor nanoplates in the solid solution form. Red-shifting of the charge transfer excitation band (CTB) from ∼250 to 256nm was observed with increasing Gd3+. Exciting the phosphors with the peak wavelengths of the CT bands yielded successively weaker Eu3+ emissions despite the decreasing specific surface area of the powder, which was ascribed to the lowered exposure of the close packed (222) facets of the oxide crystals. Nonradiative energy transfer from Gd3+ to Eu3+ was observed upon exciting the phosphors with the 8S7/2–6I J transition of Gd3+ at 276nm, and successively stronger Eu3+ emissions were observed at a higher Gd3+ content. The LRHs nanoplates completely collapse at 1000°C or above to form rounded nanoparticles and a shortened luminescence lifetime and improved internal/external quantum efficiencies were observed at a higher calcination temperature. [Copyright &y& Elsevier]

Well-crystallized (Y0.97-x Tb0.03Eux )2(OH)5NO3 ⋅ nH2O (x = 0-0.03) layered rare-earth hydroxide (LRH) nanoflakes of a pure high-hydration phase have been produced by autoclaving from the nitrate/NH4OH reaction system under the optimized conditions of 100 °C and pH ∼7.0. The flakes were then converted into (Y0.97-xTb0.03Eux)2O3 phosphor nanoplates with color-tunable photoluminescence. Detailed structural characterizations confirmed that LRH solid solutions contained NO3- anions intercalated between the layers. Characteristic Tb3+ and Eu3+ emissions were detected in the ternary LRHs by selectively exciting the two types of activators, and the energy transfer from Tb3+ to Eu3+ was observed. Annealing the LRHs at 1100 °C produced cubic-lattice (Y0.97-xTb0.03Eux)2O3 solid-solution nanoplates with exposed 222 facets. Multicolor, intensity-adjustable luminescence was attained by varying the excitation wavelength from ∼ 249 nm (the charge transfer excitation band of Eu3+) to 278 nm (the 4f8-4f75d¹ transition of Tb3+). Unitizing the efficient Tb3+ to Eu3+ energy transfer, the emission color of (Y0.97-xTb0.03Eux)2O3 was tuned from approximately green to yellowish-orange by varying the Eu3+/Tb3+ ratio. At the optimal Eu3+ content of x = 0.01, the efficiency of energy transfer was ∼91% and the transfer mechanism was suggested to be electric multipole interactions. The phosphor nanoplates developed in this work may be incorporated in luminescent films and find various lighting and display applications. [ABSTRACT FROM AUTHOR]

Abstract: This paper reports the synthesis and luminescence properties of rare earth doped BiPO4:Ce,Tb nanorods which were prepared via a solvothermal method. The nanorods are of hexagonal structure and they have intense luminescence. In Ce3+/Tb3+ co-doped nanorods, there is energy transfer from Ce3+ to Tb3+ which enhances the luminescence of Tb3+. These materials may find applications in solid state lighting and radiation detection. [Copyright &y& Elsevier]

• Air-calcined TbAG:Eu phosphor exhibits red luminescence of Eu3+ (∼575–725 nm) mostly via Tb3+ → Eu3+ energy transfer. • The optimal Eu3+ content of air-calcined TbAG:Eu is about 3 at.%. • ∼63% of the Eu in (Tb 0.97 Eu 0.03)AG was reduced to Eu2+ by calcination in hydrogen at 1500 °C. • Hydrogen-reduced TbAG:Eu exhibits cyan luminescence, with chromaticity coordinates of around (0.286, 0.381), by Tb3+, Eu3+ and Eu2+. • Forward and backward energy transfer among Tb3+, Eu2+ and Eu3+ was suggested to occur via electron exchange interaction. Calcination of coprecipitated precursors at 1500 °C in air produced (Tb 1- x Eu x) 3 Al 5 O 12 garnet phosphors (x = 0.0025-0.05) that exhibit dominant excitations via 4f8 → 4f75d1 (E 1-4) and 7F 6 → 5D 3,4 transitions of Tb3+ and negligible excitation of Eu3+. Exciting the phosphors with 275 nm light (E 2 transition of Tb3+) produced the typical 5D 0 → 7FJ red emissions of Eu3+. Annealing the x = 0.03 sample in hydrogen at 1500 °C reduced ∼62.8% of Eu3+ to Eu2+ and yielded a cyan phosphor that simultaneously exhibits the luminescence of Eu3+ (red), Tb3+ (green) and Eu2+ (broad-band, 400–725 nm) under 327 nm excitation (E 1 transition of Tb3+). The (Tb 0.97 Eu 0.03)AG optimal phosphors calcined in air and hydrogen were assayed to have external/internal quantum efficiencies (in percentage) of ∼23.2/24.2 and 18.6/23.7, respectively. Luminescence features of the two types of phosphors were investigated in detail, and the energy transfer among Tb3+, Eu2+ and Eu3+ ions was discussed. [ABSTRACT FROM AUTHOR]