Terbium-doped ZnS quantum dots: Structural,morphological, optical,photoluminescence, and photocatalytic properties

Terbium (0, 2, and 4?at%)-doped ZnS quantum dots (QDs) were synthesized via a solvothermal method. The crystal structures of the synthesized QDs were determined to be zinc blend by X-ray powder diffraction (XRD) and Raman analyses. Transmission electron microscopy (TEM) studies revealed that particles with a mean size of 2–4?nm were formed. An X-ray photo electron spectroscopy (XPS) examination disclosed the existence of terbium with a trivalent state in the ZnS host lattice. The absorption bands of all QDs were located around 325?nm (3.81?eV) and were higher than that of the bulk ZnS band gap (3.67?eV), consistent with the quantum confinement effect. The photoluminescence spectra of the terbium-doped samples displayed five emission peaks at 467?nm (⁵D₄→⁷F₃), 491?nm (⁵D₄ → ⁷F₆), 460?nm (⁵D₄ – ⁷F₅), 484?nm (⁵D₄ – ⁷F₄), and 530?nm (⁵D₄ – ⁷F₃), respectively. The terbium-doped QDs exhibited a higher photocatalytic activity during the degradation of crystal violet dye under UV-light illumination compared to the undoped ZnS QDs. These interesting properties of terbium-doped ZnS QDs are potentially useful for both luminescent and photocatalysis applications.     

Cathodoluminescence and thermoluminescence of ZnB2O4:Eu3+ phosphors prepared via wet-chemical synthesis

In present work, a series of Eu doped zinc borate, ZnB₂O₄, phosphors prepared via wet chemical synthesis and their structural, surface morphology, cathodoluminescence (CL) and thermoluminescence (TL) properties have been studied. Phase purity and crystal structure of as-prepared samples are confirmed by X-ray diffraction measurements (XRD) and they were well consistent with PDF card No. 39-1126, indicating the formation of pure phase. The thermoluminescence (TL) behaviors of Eu activated ZnB₂O₄ host lattice are studied for various beta doses ranging from 0.1 to 10?Gy. The high-temperature peak of Eu activated sample located at 192?°C exhibited a linear dose response in the range of 0.1–10?Gy. Initial rise (IR) and peak shape (PS) methods were used to determine the activation energies of the trapping centres. The effects of the variable heating rate on TL behaviour of Eu activated ZnB₂O₄ were also studied. When excited using an electron beam induced light emission (i.e cathodoluminescence, CL) at room temperature (RT), the as-prepared phosphors generate reddish-orange color due to predominant emission peaks of Eu³⁺ ions located at 576–710?nm assigned to the ⁵D₀→⁷F_J (J=1,2,3, and 4) transitions. The maximum CL intensity for Eu³⁺ ions at 614?nm with transition ⁵D₀→⁷F₂ was reached Eu³⁺ concentration of 5?mol%; quenching occurred at higher concentrations. Strong emission peak for Eu³⁺ ions at 614?nm with transition ⁵D₀→⁷F₂ is observed. The CL experimental data indicate that ZnB₂O₄:Eu³⁺ phosphor as an orange-red emitting phosphor may be promising luminescence materials for the optoelectronic applications.     

Synthesis and characterization of cellulose ethers/Eu(III)

The coordination complexes of the crystalline structure of cellulose ethers/Eu(III) with fluorescence emission, viz CMC/Eu(III), MC/Eu(III), and HEC/Eu(III), were synthesized and characterized. Results showed the emission spectra of Eu³⁺ ions in these coordination compounds, which originates from electric dipole transition. The main emission peak at 615 nm generated from ⁵D₀→⁷F₂ transition of Eu³⁺ ions. Their absorption and excitation spectra were different, because the effect of the high polarity of water and having both hydrogen bond donor and acceptor properties on the excited molecule is different from the effect on the ground state of the molecule. Our study demonstrated that the Degree of Substitute (DS) of CMC could influence the fluorescence intensity (FI) of CMC/Eu(III). The emission intensity of CMC/Eu(III) varies with the DS of CMC. For example, when the DS of CMC was 0.89, the FS (fluorescent spectra) of solid CMC/Eu(III) displayed three emission peaks Eu(III): the strongest emission peak at 615 nm (⁵D₀→⁷F₂ transition) and other two weaker peaks at 583 nm (⁵D₀→⁷F₁ transition) and at 652 nm (⁵D₀→⁷F₃ transition), respectively. The concentration of Eu(III) could also affect the FI of these coordination complexes. The FI of the coordination complexes peaked at 615 nm all reached maximum when Eu³⁺ concentration was at 5% (wt/wt). © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 95: 743–747, 2005     

The luminescence and structural characteristics of Eu-doped NaBaPO4 phosphor

Eu³⁺-doped NaBaPO₄ was prepared by a high-temperature solid-state reaction. The phase formation was confirmed by X-ray powder diffraction measurements. The laser site-selective excitation and emission spectra have been investigated in the ⁵D₀ → ⁷F₀ region by using a pulsed, tunable and narrowband dye laser. The excitation spectra corresponding to the ⁷F₀ → ⁵D₀ transition consist of two transitions at 579.6 nm Eu(I) and 578.9 nm Eu(II), indicating the Eu³⁺ ions occupy two crystallographic sites of Ba²⁺ ions. The decay lifetimes of the two Eu³⁺ sites were measured. Two crystallographic sites for Eu³⁺ ions doped in NaBaPO₄ lattice were assigned from the luminescence characteristic and structure features. Meanwhile, the charge compensation mechanism of Eu³⁺ doping in NaBaPO₄ was discussed.     

The luminescence spectroscopy and thermal stability of red-emitting phosphor Ca9Eu(VO4)7

Eu-based vanadate Ca₉Eu(VO₄)₇ phosphor was synthesized by the solid state reaction method and was characterized by X-ray powder diffraction (XRD). The photoluminescence excitation and emission spectra, fluorescence decay curves and the dependence of luminescence intensity on temperature were investigated. The phosphor can be efficiently excited by near UV light to realize an intense red luminescence (614 nm) corresponding to the electric dipole transition ⁵D₀ → ⁷F₂ of Eu³⁺ ions. The crystallographic site-occupations of the Eu³⁺ ions in Ca₉Eu(VO₄)₇ were investigated by the site-selective excitation and emission spectra, and the fluorescence decay curves in the ⁵D₀ → ⁷F₀ region using a pulsed, tunable, narrowband dye laser. The red luminescence together with the thermal stability was discussed on the base of the Eu³⁺ site-distribution in Ca₉Eu(VO₄)₇ host.     

Difference in the Nature of Eu3+ Environment in Eu3+‐Doped BaTiO3 and BaSnO3

BaTiO₃ and BaSnO₃ samples doped with Eu³⁺ ions were prepared using glycine‐nitrate gel combustion method. Relative intensities and line shapes of magnetic dipole allowed ⁵D₀ → ⁷F₁ and electric dipole allowed ⁵D₀ → ⁷F₂ transitions of Eu³⁺ from the hosts, BaTiO₃ and BaSnO₃, are significantly different. Based on detailed structural investigations, it is confirmed that synthesizedBaTiO₃ sample is tetragonal with no center of symmetry around Ba²⁺ ions. Unlike this BaSnO₃ is cubic with centrosymmetric Ba²⁺ site. From X‐ray diffraction and experimentally obtained Judd–Ofelt parameters (Ω₂ and Ω₄ values), it is confirmed that in BaTiO₃ there is a decrease in the average Ba–O and Ba–Ba distances compared with that in BaSnO₃. This leads to higher Eu–O bond polarizability and adds to the distortion in its environment around Eu³⁺ in BaTiO₃:Eu compared with BaSnO₃:Eu. This is responsible for the observed difference in the luminescence properties.     

Synthesis and Characterization of Electrospun Poly(ethylene oxide)/Europium-Doped Yttrium Orthovanadate (PEO/YVO4:Eu3+) Hybrid Nanofibers

Europium-doped yttrium orthovanadate/polyethylene oxide nanofibers were fabricated by firstly, synthesizing crystalline YVO₄:Eu³⁺ nanoparticles using an aqueous precipitation method followed by electrospinning of PEO/YVO₄:Eu³⁺ polymer composites. X-ray diffraction patterns showed that the nanoparticles exhibited well-defined peaks that were indexed as the tetragonal phase of YVO_4. No additional peaks of other phases were observed indicating that Eu³⁺ ions were effectively built into the YVO₄ host lattice. The photoluminescence spectra for the nanofibers showed peaks at 593, 615, 650, and 698 nm which was ascribed to the ⁵D_0? ⁷F₁, ⁵D_0? ⁷F₂, ⁵D_0? ⁷F₃ and ⁵D_0? ⁷F₄ transitions of Eu³⁺. Due to an efficient energy transfer from vanadate groups to Eu³⁺, the composite nanofibers showed a strong red emission under ultraviolet excitation characteristic of the red luminescence of the europium ion. The results demonstrate that this synthetic approach could prove to be viable for the fabrication of rare earth/polymer composite nanofibers intended for luminescent device applications.     

Structure and Luminescence Properties of Eu<Superscript>3+</Superscript>-Doped Cubic Mesoporous Silica Thin Films

Eu³⁺ ions-doped cubic mesoporous silica thin films with a thickness of about 205 nm were prepared on silicon and glass substrates using triblock copolymer as a structure-directing agent using sol–gel spin-coating and calcination processes. X-ray diffraction and transmission electron microscopy analysis show that the mesoporous silica thin films have a highly ordered body-centered cubic mesoporous structure. High Eu³⁺ ion loading and high temperature calcination do not destroy the ordered cubic mesoporous structure of the mesoporous silica thin films. Photoluminescence spectra show two characteristic emission peaks corresponding to the transitions of ⁵D₀-⁷F₁ and ⁵D₀-⁷F₂ of Eu³⁺ ions located in low symmetry sites in mesoporous silica thin films. With the Eu/Si molar ratio increasing to 3.41%, the luminescence intensity of the Eu³⁺ ions-doped mesoporous silica thin films increases linearly with increasing Eu³⁺ concentration.     

Excitation-dependent energy transfer and color tunability in Dy3+/Eu3+ co-doped multi-component borophosphate glasses

Using the melt-quench technique, potassium zinc borophosphate (KZnBP) glasses incorporated with Dy³⁺, Eu³⁺, and Dy³⁺/Eu³⁺ ions individually and combinedly were prepared, and their photoluminescence (PL)-related features were investigated. The KZnBP glass containing an optimized content of Dy³⁺ (0.5 mol%) is co-doped with Eu³⁺ in various contents, and the energy transfer (ET) process between them was studied at λ_exci = 349, 364, 387 (Dy³⁺), and 394 nm (Eu³⁺). The Dy³⁺/Eu³⁺ co-doped system, when excited with Dy³⁺ excitations has resulted in a significant decrease in the intensity of Dy³⁺ peaks observed at 480 nm (⁴F_9/2→⁶H_15/2, blue) and 574 nm (⁴F_9/2→⁶H_13/2, yellow), with simultaneous enhancement of the intensity of Eu³⁺ peaks at 591 nm (⁵D₀→⁷F₁, orange) and 617 nm (⁵D₀→⁷F₂, red). This trend is due to the efficient energy transfer from Dy³⁺ to Eu³⁺, indicating that Eu³⁺ ions were sensitized by Dy³⁺ ions. Dexter's theory and the Inokuti–Hirayama (I–H) model revealed that the dipole–dipole interaction is accountable for the energy transfer from Dy³⁺ to Eu³⁺ through energy-transfer channels [⁴F_9/2(Dy³⁺)+⁷F_1,2(Eu³⁺)→⁶H_15/2(Dy³⁺)+⁵D₂(Eu³⁺)] and [⁴F_9/2(Dy³⁺)+⁷F₀(Eu³⁺)→⁶H_13/2(Dy³⁺)+⁵D₀(Eu³⁺)]. The color coordinates of the Dy³⁺/Eu³⁺ co-doped glasses under various excitations fall within the white light emission spectrum, indicating their potential application in warm white LEDs.     

Precipitation Synthesis and Color‐Tailorable Luminescence of α‐Zr(HPO4)2: RE3+(RE = Eu,Tb) Nanosheet Phosphors

The rare earth (RE = Eu and Tb) ions‐doped α‐Zr(HPO₄)₂ (ZrP) nanosheet phosphors were synthesized by direct precipitation method, and their structures and photoluminescence properties were investigated. The results of X‐ray diffraction and scanning electron microscopy indicated that the systems of ZrP:RE³⁺ had similar nanosheet structure except with relatively larger interlayer spacing as compared with pure α‐ZrP. Under the excitation of UV light, the ZrP:RE³⁺ nanosheet phosphors showed red and green emission peaks corresponding to the ⁵D₀→⁷F₂ transition of Eu³⁺ and the ⁵D₄→⁷F₅ transition of Tb³⁺, respectively. After Eu³⁺ and Tb³⁺ were co‐doped in ZrP host, not only the red and green emission peaks were simultaneously observed, but also the luminescent intensity and fluorescence lifetimes of Tb³⁺ were gradually decreased with the increase in Eu³⁺‐doping concentration, which implied the energy transfer from Tb³⁺ to Eu³⁺ happened. It was deduced that the energy transfer from Tb³⁺ to Eu³⁺ occurred via exchange interaction. Through optimization to the samples, a nearly white‐light emission with the color coordinate (0.322, 0.263) was achieved under 377 nm excitation. The ZrP:RE³⁺ nanosheet phosphors may be a potential color‐tailorable candidate for fabricating optoelectronic devices such as electroluminescence panels.     

Effect of the Eu3+ Addition on Photoluminescence and Microstructure of ZnO–CdO–TeO2 Glasses

In this work, the role of europium doping of glasses formulated in the ternary system ZnO–CdO–TeO₂ is described. The Eu‐doped oxide glasses were prepared by the conventional melt‐quenching method and by using three different compositions. Structural studies reveal that there exists a good affinity between Cd and some rare earth (RE) ions to form the crystalline phase. The X‐ray diffraction (XRD) diagrams display that the structure of these glasses is amorphous and with the increase in CdO content and the compatibility of Eu³⁺, there is a tendency to form nanocrystals of CdTe₂O₅. The scanning electron microscopic (SEM) observation of their microstructure confirms the presence of phase separation. Differential thermal analysis (DTA) of these glasses showed small exothermic peaks noted around 450°C for the V2 glass and 480°C for V1 and V3 glasses, which could be attributed to the formation of these crystals. The infrared spectra showed a main absorption band around 800–600 cm^?1 corresponding to the Te–O stretching mode in TeO₄ and TeO₃ groups. By optical absorption (OA), the band gap (E_g) for each glass was determined; these values were 3.27, 3.14, and 3.3 eV for the V1–V3 glasses, respectively. Furthermore, the presence of Eu³⁺ was detected in the 370–470 nm short‐range wavelengths. The photoluminescence (PL) experiments of the glasses showed light emission due to the following transitions: ⁵D₀ → ⁷F₁, ⁵D₀ → ⁷F₂, ⁵D₀ → ⁷F₃, and ⁵D₀ → ⁷F₄.     

A high quantum yield red phosphor NaGdSiO4: Eu3+ with intense emissions from the 5D0→7F1,2 transition

Red phosphors with a high quantum yield and a lower thermal quenching are needed to improve the luminescence efficiency and the stability of phosphor-converted white light-emitting diodes (pc-WLEDs). We have designed a high quantum yield NaGdSiO₄ (NGSO) based phosphor with enhanced Eu³⁺ emissions of the ⁵D₀→⁷F₁ and ⁵D₀→⁷F₂ transitions. This design is based on the Eu³⁺ at both the inversion and non-inversion symmetry sites. In detail, we have studied the structure, morphology, and luminescence properties of NGSO: Eu³⁺ phosphors. Using a 394 nm UV excitation, a series of Eu³⁺ emissions of ⁵D₀→⁷F_J (0–4) transitions has been observed. The internal quantum efficiency (IQE) is 83.42% and the red color purity is 91.4%. These values are much higher than some reported results. The higher IQE and double intense ⁵D₀→⁷F₁ and ⁵D₀→⁷F₂ emissions might originate from an unusual structure disorder around Eu³⁺ ions in the NGSO lattice. The lifetime of the optimal phosphor NGSO: 0.5Eu³⁺ is about 2 ms, suitable for solid-state lighting. The intensities of the strong emissions at 595 and 624 nm of NGSO: 0.5Eu³⁺ at 150 °C is about 85% of that at 30 °C, demonstrating its excellent thermal stability. Furthermore, this red NGSO: 0.5Eu³⁺ phosphor was packaged into a warm pc-WLED, exhibiting a lower correlated color temperature (CCT) of 4222 K and a comparable color rendering index (CRI) of 86.7. These results show that this red phosphor could act as a red component of pc-WLEDs excited by the n-UV LED chip.     

Novel orange-emitting Ba2LaNbO6:Eu3+ nanophosphors for NUV-based WLEDs and photocatalytic water purification

Energy conservation and environmental safety are the key requirements in the modern world. We report novel orange-emitting double perovskite Ba₂LaNbO₆:Eu³⁺ (BLN:Eu³⁺) nanophosphor fabricated using a citrate sol-gel method for use in general illumination and photocatalysis. After annealing at 800?℃, the particles exhibited a nanorod-like morphology with monoclinic structure. The photoluminescence emission spectra exhibited an intense ⁵D₀→⁷F₁ transition at 594?nm and a moderate ⁵D₀→⁷F₂ transition at 615?nm, demonstrating that the Eu³⁺ ions occupied the La³⁺ sites with inversion symmetry. The optimal concentration of Eu³⁺ ions was found to be about 5?mol% for the BLN host lattice. Energy transfer from the NbO₆^7- octahedrons to the Eu³⁺ ions was clearly witnessed when the BLN:Eu³⁺ nanophosphors were excited with both the characteristic excitation bands of Eu³⁺ (⁷F₀→⁵L₆) and NbO₆^7- octahedrons at 392 and 380?nm, respectively. The thermal quenching temperature of 5?mol% Eu³⁺ ions doped BLN nanophosphors was found to be 183?℃, indicating that these nanophosphors are very stable at high temperatures. In addition, the dye removal efficiency of the proposed BLN nanophosphors was verified using Rhodamine B (RhB) dye as a model pollutant under UV irradiation. Compared to a commercial nano-ZnO catalyst, our synthesized BLN nanophosphors showed superior RhB de-colorization efficiency. Therefore, the proposed BLN:Eu³⁺ nanophosphors are promising multifunctional materials for photocatalysis and general lighting applications.     

Lead fluoride β-PbF2 nanocrystals containing Eu3+ and Tb3+ ions embedded in sol-gel materials: Thermal,structural and optical investigations

Nanocrystalline β-PbF₂ phase singly-doped with Eu³⁺ and Tb³⁺ ions have been successfully synthesized using sol-gel technique and subsequent heat-treatment of xerogels at 350 °C. Thermal behavior and structural properties of obtained materials were studied using thermogravimetric analysis (TG), differential scanning calorimetry (DSC) as well as FT-IR and Raman techniques. XRD results confirmed formation of β-PbF₂ nanocrystals embedded in silica amorphous hosts after annealing at 350 °C. Moreover, the photoluminescence properties have been investigated based on excitation and emission spectra as well as decay analysis from the ⁵D₀ (Eu³⁺) and the ⁵D₄ (Tb³⁺) excited states. The sharp intraconfigurational 4f⁶−4f⁶ and 4f⁸−4f⁸ emission transitions assigned as the ⁵D₀→⁷F_J (J=0–4) of Eu³⁺ and the ⁵D₄→⁷F_J (J=6-3) of Tb³⁺ bands, respectively, were registered. The most prominent bands in studied xerogels and glass-ceramic materials are related to the following electronic transitions: ⁵D₀→⁷F₁ (orange) and ⁵D₀→⁷F₂ (red) (Eu³⁺) as well as ⁵D₄→⁷F₅ (green) and ⁵D₄→⁷F₆ (blue) (Tb³⁺). Thus, the R/O (Eu³⁺) and G/B (Tb³⁺) luminescence intensity ratios were calculated and analyzed. Luminescence decay kinetic clearly indicated a presence of two different surroundings around Eu³⁺ and Tb³⁺ dopants in β-PbF₂-based glass-ceramic samples. In such singly-doped with Eu³⁺ and Tb³⁺ materials, the longer luminescence lifetimes (Eu³⁺: τ₁(⁵D₀)=0.90 ms, τ₂(⁵D₀)=5.15 ms; Tb³⁺: τ₁(⁵D₄)=0.48 ms, τ₂(⁵D₄)=4.01 ms) of an appropriate excited states were achieved in comparison to xerogel hosts (Eu³⁺: τ(⁵D₀)=0.38 ms; Tb³⁺: τ(⁵D₄)=0.49 ms). The obtained results indicate the incorporation of Eu³⁺ and Tb³⁺ ions into nanocrystalline phase during ceramization process.     

Anomalous preparation,intense 5D0→7F4 emission and adjustable double center emission of Eu3+ and Eu2+ codoped Ca2Al2SiO7

A series of Eu³⁺/Eu²⁺ codoped Ca₂Al₂SiO₇ were synthesized by traditional solid-state synthesis in reducing atmosphere. In this work, XRD powder diffraction proved that the obtained sample was pure. Photoluminescence properties are characterized by excitation, emission spectra and decay curves. Double center emission is achieved by adjusting excitation wavelength and concentration. Under the 394 nm excitation, the emission spectra Ca₂Al₂SiO₇: Eu phosphors exhibit two bands situated at blue emission of 4f5d-4f transition from Eu²⁺ ion and red emission of 4f-4f transition coming from Eu³⁺ ion. The red and yellow light can be obtained when the concentration of doped europium ions is at 0.5% and 1%, respectively. When the excitation wavelength was 394, 280 and 584 nm, the emission color change from yellow to blue, respectively. The bond energy theory explains Eu²⁺ and Eu³⁺ ion occupy Ca1 site in the Ca₂Al₂SiO₇ lattices. In addition, the spectra show that the abnormal intensity peaks of europium ion at 701 nm can be found. Analysis of the related intensity ⁵D₀-⁷F₂(618 nm) transition peak is similar to that of ⁵D₀-⁷F₄(701 nm) transition peak in the emission spectra with the Judd-Ofelt theory.     

Uniform colloidal spheres for RE3BO6 (RE = Eu-Yb,Y) and excitation-dependent luminescence of Y3BO6:Eu3+ red phosphor

Phosphor particles of spherical shape and uniform size are desired for high-definition displays to improve the resolution and the overall luminescent performance. However, the synthesis of RE₃BO₆ spherical particles is a considerable challenge in materials science. Here, uniform spheres of RE₃BO₆ (RE = Eu–Yb, Y) have been converted from their colloidal precursor spheres synthesized via homogeneous precipitation. The amorphous precursor spheres are solid particles with decreased boron going from the surfaces to the cores. Smaller particles were observed at decreased ionic radius from Eu³⁺ to Ho³⁺ (including Y³⁺), but particles with nearly unvaried sizes were observed by further decreasing the ionic radius from Ho³⁺ to Yb³⁺. They crystallized in monoclinic RE₃BO₆ at 900°C, with maintaining the spherical shape of precursors. However, the crystal growth and the densification toward the particle surfaces resulted in the formation of hollow spheres for smaller particles and core-shell structured spheres for larger particles. The parameters, a, b, and c, increase nearly monotonically with increasing the radius of rare earth ions. The uniform spheres of Y₃BO₆:Eu³⁺ exhibited a typical red emission at ~613 nm (⁵D₀ → ⁷F₂ electric dipole transition of Eu³⁺), with an intensity ratio I(⁵D₀ → ⁷F₂)/I(⁵D₀ → ⁷F₁) of ~3.5. The luminescence behavior of Y₃BO₆:Eu³⁺ phosphor is dependent on the excitation wavelength, which is closely related to the Eu³⁺ ions at different coordination sites. Driven by a 460-nm blue-LED chip, the Y₃BO₆:Eu³⁺ spheres exhibited a red emission with the CIE coordinates of (~0.65, ~0.35), indicating that they are an excellent red-emitting phosphor candidate for application in white-LEDs.     

Influence of Bi3+ as a sensitizer and SiO2 shell coating as a protecting layer towards the enhancement of red emission in LnVO4: Bi3+, Eu3+ @ SiO2 (Ln=Gd,Y and Gd/Y) powder phosphors for optical display devices

Enhanced red luminescence in LnVO₄: Bi³⁺, Eu³⁺ @ SiO₂ phosphors has been improved mainly in three stages by investigating the effects of: (i) host composition (Gd, Y and Gd/Y), (ii) co-doping Bi³⁺ as a sensitizer and finally (iii) SiO₂ shell coating. XRD data revealed that the produced phosphors possess crystalline, pure phase with tetragonal structure. Silica coating on phosphor particles have been characterized by SEM/EDAX, TEM, PL and with the presence Si–O–Si, Si–O vibrational modes from the FT-IR spectra. Absorption band edges due to VO₄^3?, shifted to higher wavelength with Bi-concentration, owing to the presence of Bi–O bond in addition to V–O. The emission intensities of ⁵D₀→⁷F₂ transition are stronger than ⁵D₀→⁷F₁; indicating the lower inversion symmetry near Eu³⁺, ions. Red emission intensity due to the efficient energy transfer from VO₄^3? to Eu³⁺ via Bi³⁺ ions in Y_0.949VO₄: Bi³⁺_0.001, Eu³⁺_0.05 phosphor was improved significantly, i.e. 1.6 times compared to Y_0.95VO₄: Eu³⁺_0.05. This was further enhanced 2.25 times by SiO₂ shell coating. Thus, Y_0.949VO₄: Bi³⁺_0.001, Eu³⁺_0.05 @ SiO₂ are suggested to be a promising red phosphor for application in display devices or lighting.     

Synthesis of Eu-doped hydroxyapatite whiskers and fabrication of phosphor layer via electrophoretic deposition process

Highly biocompatible and efficiently luminescent whiskers of the hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂, HAp) doped with various concentrations (0-5 at.%) of europium were prepared by hydrothermal synthesis and the Eu-doped Hap-coating layers onto the surface of titanium substrate were fabricated by the electrophoretic deposition (EPD) process for fluorescent probe application. The maximum doping concentration of Eu accommodating into the host lattice of HAp was detected as ~1.5 at.% and all the hydrothermally synthesized Eu-doped HAp whiskers were found to have high crystallinity and orientation growth along the c-axis by X-ray diffraction (XRD) identification. The valence of the doped Eu was identified as trivalent and divalent coexistence at a concentration percentage of Eu³⁺: Eu²⁺ = 78%: 22% by X-ray photoelectron spectroscopy (XPS) spectra. The replacement site of the doped Eu ions in the crystal structure of HAp host was clarified by Rietveld refinement. The whisker morphology of the hydrothermally synthesized particle was demonstrated by field-emission scanning electron microscopy (FE-SEM) observation and their component elements were analyzed by energy dispersive X-ray (EDX) mapping. The photoluminescence (PL) emissions of the Eu-doped HAp whiskers and fabricated their coating layers were both revealed mainly at ~615 nm (⁵D₀ → ⁷F₂) and ~697 nm (⁵D₀ → ⁷F₄), which is a wavelength that easily transmitting through living system for biological imaging. The PL emission are falling in the region of reddish orange and belonging to color temperature below 1500 K. Decay time and internal and external quantum efficiencies (QEs) were also measured to reveal them depending on the doping concentration of Eu. The hydrothermally prepared Eu-doped HAp whiskers would be aimed at biomedical application, due to their promising fluorescent function of probe for in vivo imaging in medical diagnose by utilizing the superior biocompatibility of the HAp host and highly efficient luminescent property of the Eu activator.     

Direct Formation of Luminescent Fine Crystals Based on (Y,Eu)TiNbO6 Complete Solid Solution with High Crystallinity

Luminescent fine crystals of Y_1.00?xEu_xTiNbO₆, x = 0–1.00 with high crystallinity were directly synthesized by mild hydrothermal method from weakly basic precursor solution mixtures of YCl₃, EuCl₃, TiOSO₄, and NbCl₅. Orthorhombic aeschynite‐type crystals in the range of 0.5–2.0 μm consisting of crystallites with 33–91 nm based on a complete solid solution in the YTiNbO₆–EuTiNbO₆ system were hydrothermally formed at 240°C for 5 h. A single phase of as‐prepared aeschynite‐type structure was maintained in all the (Y,Eu)TiNbO₆ solid solution after heating at temperatures up to 1000°C for 1 h in air. In the range of composition x ≤ 0.6, the as‐prepared aeschynite‐type (Y,Eu)TiNbO₆ solid solutions transformed to a single phase of euxenite structure after heat treatment at temperatures higher than 1100°C–1200°C. The as‐prepared (Y,Eu)TiNbO₆ fine crystals in the range of composition x = 0.75–0.90 showed the strongest luminescence in the red spectral region: strong red (⁵D₀→⁷F₂ transition of Eu³⁺) and weak orange light (⁵D₀→⁷F₁) line spectra among the as‐prepared and heat‐treated samples. Another wet chemical synthesis route confirmed the advantage in directly synthesizing the (Y,Eu)TiNbO₆ crystals through this hydrothermal method because heating at 1200°C for 1 h in air was necessary for obtaining crystalline (Y,Eu)TiNbO₆ with sufficient luminescence intensity in a composition Y_0.70Eu_0.30TiNbO₆ from amorphous powders that were formed via co‐precipitation method.     

Gel-combustion assisted synthesis of eulytite-type Sr3Y(PO4)3 as a single host for narrow-band Eu3+ and broad-band Eu2+ emissions

A series of eulytite-type Sr₃Y_1-x(PO₄)₃:xEu³⁺ (x = 0–0.13) and Sr_3-yY(PO₄)₃:yEu²⁺ (y = 0–0.10) phosphors were successfully synthesized via gel-combustion and subsequent calcination in O₂ and Ar/H₂ atmospheres at 1250 °C, respectively. Detailed crystal structure analysis via Rietveld refinement showed that the phosphors were crystallized in the cubic system (space group I-43d, No. 220), in which the Eu³⁺ and Eu²⁺ activators reside at the Y³⁺ and Sr²⁺ sites, respectively. The trivalent Eu³⁺ ions (CN = 6) exhibited typical narrow-band luminescence via intra-4f⁶ transitions, with the red emission at ~ 615 nm being dominant (⁵D₀ → ⁷F₂ transition, FWHM = 15.9 ± 0.2 nm). The divalent Eu²⁺ ions (CN = 6 and 9) showed broad-band luminescence ranging from light-blue to blue via 4f⁶5d¹ → 4f⁷ transitions (FWHM = 115 ± 2 nm). The optimal Eu³⁺ and Eu²⁺ concentrations were determined to be 10 at% (x = 0.10) and 7 at% (y = 0.07), respectively, and the mechanisms of concentration quenching were discussed. The excitation/emission properties, fluorescence decay kinetics, CIE chromaticity, and particularly the rarely addressed thermal stability of the phosphors were investigated in detail.