The transformation of Eu→Eu in SiO2-Al2O3-NaF-YF3: Eu glasses and glass ceramics

Transparent glasses and glass ceramics of SiO₂-Al₂O₃-NaF-YF₃: Eu³⁺ containing the YF₃ nano-crystals were prepared in air atmosphere and their spectroscopic properties are presented. The blue emission peaked at 433 nm shown in the spectra indicate the existence of Eu²⁺.The transformation of Eu³⁺→Eu²⁺ is related with the existence of F⁻, which is the key trigger for the transformation in air atmosphere. In addition, the emission intensities of Eu³⁺ and Eu²⁺ were also related with the concentrations of Eu³⁺. The emission intensities increased heavily with the increasing dopant of EuF₃ in both glasses and glass ceramics_. When the doped concentration of EuF₃ was higher than 0.5 mol.%, the concentration quenching effect occurred.     

Upconversion luminescence of Ho sensitized by Yb in transparent glass ceramic embedding BaYF5 nanocrystals

Transparent SiO₂-Al₂O₃-BaCO₃-YF₃-BaF₂ glass ceramics co-doped with Yb³⁺/Ho³⁺ ions were prepared by melt quenching and subsequent heating. X-ray diffraction and transmission electron microscopy observation revealed that BaYF₅ nanocrystals incorporated with Yb³⁺ and Ho³⁺ were precipitated homogeneously among the oxide glass matrix. Three upconversion emission bands centered at 483 nm, 545 nm and 645 nm, corresponding to the ⁵F₃ → ⁵I₈, ⁵S₂, ⁵F₄ → ⁵I₈ and ⁵F₅ → ⁵I₈ transitions of Ho³⁺ respectively, were detected under 976 nm excitation, ascribing to the efficient energy transfer from Yb³⁺ to Ho³⁺. The red emission is prevailing in the precursor glass, while the green one turns to be dominant in the glass ceramic.     

Synthesis and luminescence properties of YVO4:Eu nanocrystals grown in nanoporous glass

We report on a feasible method to synthesize luminescence nanocrystals in porous glass in this paper. Well dispersed YVO₄:Eu nanocrystals were proved being grown in nanoporous glass by XRD, micro-Raman spectra and HRTEM equipped with EDS. The YVO₄:Eu³⁺ nanocrystal grown in porous glass herein shows very different luminescence properties compared with single Eu-doped sample. By this method, intense red emission from high silica glass due to energy transfers VO₄³⁻ → Eu³⁺ was obtained. The results show that the reduction from Eu³⁺ to Eu²⁺ in porous glass impregnated with Eu³⁺ ions was avoided effectively.     

Low temperature synthesis and optical properties of CaTiO3 nanoparticles from Ca(NO3)2·4H2O and TiO2 nanocrystals

Choosing low-melting-point Ca(NO₃)₂·4H₂O and high-reactive-activity TiO₂ nanocrystals as the raw materials, a simple and cost-effective route was developed for the synthesis of CaTiO₃ nanoparticles at 600 °C, which is much lower than that (about 1350 °C) used in the conventional solid state reaction methods. X-ray diffraction, energy dispersive X-ray spectroscopy and field emission scanning electron microscopy revealed the formation of orthorhombic phase CaTiO₃ nanoparticles with oxygen-deficiency at the surface. UV-vis absorption spectrum of the as-obtained CaTiO₃ nanoparticles displayed an absorption peak centered at around 325 nm (3.8 eV), together with a tail at lower energy side. Room temperature photoluminescence spectrum of the as-obtained CaTiO₃ nanoparticles upon laser excitation at 325 nm demonstrated a strong and broad visible light emission ranging from about 527 to 568 nm, which may be originated from the surface states and defect levels.     

Up- and down-conversion luminescence in the oxyfluoride glass ceramics containing Ba2+1.5xYb1−xF7:Tb nanocrystals

Transparent oxyfluoride borosilicate glass ceramics containing Ba_2+1.5xYb_1−xF₇:Tb³⁺ nanocrystals were successfully prepared by a melt-quenching method with subsequent heat treatment. The precipitated crystalline phase in the glass matrix changed gradually from BaF₂ to Ba_2+1.5xYb_1−xF₇ with the increase of YbF₃ content, which was confirmed by the results of XRD, HRTEM and EDX measurements. The ultraviolet and visible up-conversion and near-infrared quantum cutting down-conversion emissions were observed and interpreted. These materials could be used to modify the solar spectrum and enhance the silicon solar cell efficiency by the up-conversion and down-conversion luminescence of Tb³⁺–Yb³⁺ couples in the oxyfluoride borosilicate glass ceramics.     

Plasmon-enhanced luminescence of YAG: Yb, Er nanopowders by Ag nanoparticles embedded in silicate glass

Solid-state field-assisted diffusion was used to prepare Ag nano-composite silicate glass. After positive diffusion process, small Ag atoms clusters were formed in the slides. The Ag atom clusters could aggregate into Ag nanoparticles (NPs) with bigger size after subsequent reverse diffusion process. YAG: Yb, Er nanopowders were screen-printed on the glass slides after positive and reverse diffusion, respectively, for up-conversion luminescence measurement. Almost no luminescence enhancement was observed for the slide after positive diffusion. Whereas, obvious enhancement was obtained for the slide after reverse diffusion, and the enhancement factor could reach about 26. The strong enhancement was due to the larger size of Ag NPs. The present work suggested a new promising method to enhance the luminescence of YAG: Yb, Er nanopowders.     

Synthesis of submicron-sized spherical Y2O3 powder for transparent YAG ceramics

Spherical monodispersed, submicron-sized Y₂O₃ powder was prepared via a homogeneous precipitation method using nitrate and urea as raw materials. The structure, phase evolution and morphology of Y₂O₃ precursor and the calcined powder were studied by FTIR, TG/DTA, XRD and SEM methods. The sphere size of the precursor was about 250 nm and that of Y₂O₃ powder calcined at 800 °C for 2 h was about 200-210 nm. With the spherical Y₂O₃ powder and a commercial Al₂O₃ ultrafine powder, high transparent YAG ceramics was fabricated by vacuum sintering at 1780 °C for 6 h through a solid-state reaction method. The in-line transmittances of the as-fabricated YAG ceramics at the wavelength of 1064 nm and 400 nm were 82.8% and 79.5%, respectively, which were much higher than that of the YAG ceramics with a commercial Y₂O₃ powder and a commercial Al₂O₃ ultrafine powder directly. The superior properties are attributed to the good morphology, dispersibility and uniform grain size of the as-prepared spherical Y₂O₃ powder, which matches that of the commercial Al₂O₃ powder.     

Enhancement of green emission for Al-doped YBO3:Tb

High-quality Y_1−x−yAl_xTb_yBO₃ (0 ≤ x ≤ 0.1, 0.04 ≤ y ≤ 0.16) phosphor powders with fine size, spherical and regular morphology, and non-agglomeration were successfully prepared by ultrasonic spray pyrolysis. The blue emission from the ⁵D₃→⁷F_J (J = 4, 5, and 6) transition for the Y_0.94−xAl_xTb_0.06BO₃ phosphor was quenched. The optimal concentration of Tb³⁺ was reduced by doping Al³⁺ into the Y_1−yTb_yBO₃ phosphor, i.e., y = 0.1 and 0.12 for Y_0.975−yAl_0.025Tb_yBO₃ and Y_1−yTb_yBO₃, respectively. The Al³⁺ doping was highly effective for improving the photoluminescence characteristics. The photoluminescence emission intensity of the Al³⁺-doped Y_0.915Al_0.025Tb_0.06BO₃ phosphor at 543 nm was about three times stronger than that of the Al³⁺-free Y_0.94Tb_0.06BO₃ phosphor.     

Fluorescence property investigations on Er-doped oxyfluoride glass ceramics containing LaF3 nanocrystals

The fluorescence properties of oxyfluoride glass ceramics containing nanosized LaF₃ crystals with different ErF₃ doping level were investigated. The spectroscopy analysis indicated that a dominant fraction of ErF₃ had been incorporated into the crystal phase. Broad 1.5 μm emission spectra with full width at half maximum (FWHM) value ranging from about 40–100 nm were obtained, which increased accordingly with ErF₃ doping level. Noteworthily, intense green and red upconversion emissions from samples with high ErF₃ doping level were observed when excited even with a 30 mW diode laser at 976 nm. An overall increment of the upconversion emissions intensity, and, a relative increase in intensity of the red emission with respect to that of the green one were also identified with increasing ErF₃ concentration. The possible upconversion mechanisms were proposed.     

Synthesis and upconversion luminescence properties of Yb/Er codoped BaGd2(MoO4)4 powder

Synthesis and upconversion luminescence properties of the new BaGd₂(MoO₄)₄:Yb³⁺,Er³⁺ phosphor were reported in this paper. The phosphor powder was obtained by the traditional high temperature solid-state method, and its phase structure was characterized by the XRD pattern. Based on the upconversion luminescence properties studies, it is found that, under 980 nm semiconductor laser excitation, BaGd₂(MoO₄)₄:Yb³⁺,Er³⁺ phosphor exhibits intense green upconversion luminescence, which is ascribed to ²H_11/2 → ⁴I_15/2 and ⁴S_3/2 → ⁴I_15/2 transition of Er³⁺. While the observed much weaker red emission is due to the non-radiative relaxation process of ⁴S_3/2 → ⁴F_9/2 and ⁴F_9/2 → ⁴I_15/2 transition originating from the same Er³⁺. The concentration quenching effects for both Yb³⁺ and Er³⁺ were found, and the optimum doping concentrations of 0.5 mol% Yb³⁺ and 0.08 mol% Er³⁺ in the new BaGd₂(MoO₄)₄ Gd³⁺ host were established.     

Facile synthesis of water-soluble YF3 and YF3: Ln nanocrystals

Water-soluble lanthanide-doped YF₃ nanocrystals with cubic structure were successfully synthesized for the first time using a simple solvothermal method with ethanol as solvent at 160 °C for 12 h. SEM and TEM results demonstrated that the obtained nanocrystals have an irregular shape and an average size of below 30 nm. Tb³⁺, Tb³⁺/Ce³⁺ co-doped YF₃ nanocrystals were also prepared and their photoluminescence properties were investigated. The luminescent intensity of the Tb³⁺ for the Tb³⁺/Ce³⁺ co-doped YF₃ nanocrystals is about ten times higher than that of the Tb³⁺ doped YF₃ nanocrystals. The products were characterized by XRD, SEM and TEM. Owing to their water-soluble properties and high processability, extensive applications may be found.     

Role of organic molecules on upconversion luminescence of LaF3 nanoparticles

Yb and Er codoped LaF₃ nanocrystals were synthesized and studied. The upconversion luminescence properties of nanocrystals capped with different ligands are mainly dependent on the ligands, especially for the red emission which is sensitive to the nonradiative relaxation. The chelation between the ligands and rare earth ions can affect the morphology and fluorescent properties of samples. The chelating ligands will reduce the nonradiative quenching by isolating the RE ions from surrounding environment. So the upconversion luminescence properties of the samples vary correspondingly.     

Preparation and optical study of CdS nanocrystals embedded phosphate glass

A new phosphate glass system with CdS nanocrystals dispersed in glass matrix was investigated. The phosphate glass composition with good stability has been used for preparation of CdS doped glasses. The CdS in the range of 0.5-7.0% has been doped into this glass composition. Effect of CdS content on the optical and other properties has been investigated. The optical characterization of the glass samples showed that with increasing concentration of CdS, there was a red shift in transmission cut-off of the glasses. From the transmission cut-off of each glass sample, the band gap of the CdS nanocrystals embedded glass was calculated. The band gap of CdS particles embedded glass was observed in the range of 3.1-4.1 eV. The present system is compared with CdS nanocrystals doped in silica based glass system. In the phosphate glass system, the UV transmission cut-off's are not sharp and the optical transmittance decreases with increasing CdS content in contrast to silica glass system. The reason for such behavior has been discussed in the present investigation. TEM of the CdS doped phosphate glasses showed CdS particle size in the range of 5-7 nm for lower concentration of CdS and 10-100 nm for higher concentration of CdS. The nanocrystals are non-uniform in size but uniformly dispersed in glass matrix.     

Enhanced luminescence by charge compensation in red-emitting Eu-activated Ca3Sr3(VO4)4

The effects of charge compensation on the luminescence behavior of a red-emitting phosphor, Ca₃Sr₃(VO₄)₄:Eu³⁺, were investigated. It has been observed that charge compensated by monovalent ions, especially Na⁺, shows greatly enhanced red emission under ultraviolet excitation. It is found that Na₂CO₃ addition acts as a fluxing agent and plays a role in charge compensation, which clearly improves the emission intensity of Eu³⁺-activated Ca₃Sr₃(VO₄)₄. Enhanced emission intensity of the corresponding charge compensated phosphors under ultraviolet radiation may find application in the production of red phosphors for white light-emitting diodes.     

Enhanced photoluminescence of Gd0.94(P1−xVx)O4:Eu0.06 red-emitting (0 ≤ x ≤ 1.0) phosphors

High-quality Gd_0.94(P_1−xV_x)O₄:Eu_0.06 (0 ≤ x ≤ 1.0) powders having small size, spherical morphology, smooth surface, and nonaggregation are synthesized by the ultrasonic spray pyrolysis. The complex host composition, Gd_0.94(P_1−xV_x)O₄:Eu_0.06 (x ≥ 0.5), shows a single phase with the tetragonal xenotime structure. The introduction of V⁵⁺ ions in the P⁵⁺ lattice yields the deviation from the centrosymmetry of Eu³⁺ ions. By increasing the V⁵⁺ content, the emission intensity corresponding to the ⁵D₀ → ⁷F₁ transition decreases. On the other hand, the emission intensity corresponding to the ⁵D₀ → ⁷F₂ transition increases up to x = 0.5, and then decreases upon further increasing the V⁵⁺ content. The emission intensity, peaking at 620 nm, of complex composition Gd_0.94(P_0.5V_0.5)O₄:Eu_0.06 is approximately five and two times stronger than that of the Gd_0.94PO₄:Eu_0.06 and Gd_0.94VO₄:Eu_0.06, respectively. It is believed that the introduction of the complex composition Gd_0.94(P_1−xV_x)O₄:Eu_0.06 is highly effective for improving the emission properties.     

The UV and VUV luminescence properties of the phosphor Mg2GeO4:Tb

A novel green phosphor Mg₂GeO₄:Tb³⁺ with pure phase was prepared by the solid state reaction. The luminescence properties were investigated in detail. The diffusion reflection spectra of the undoped and Tb³⁺ doped Mg₂GeO₄ phosphors were recorded, the result reveals that there is an absorption band superposition of the host material and Tb³⁺ ion. The study on the excitation and diffusion spectra shows that there is an effective energy transfer from the host material to Tb³⁺ ion. Under 277 and 172 nm excitation, the phosphor presents predominant green emission at 543 and 547 nm respectively. The excitation intensity at 172 nm is about 1.8 times of that at 272 nm. The promising luminescence properties make it a candidate for application in Plasma Display Panel.     

Investigation of near-infrared-to-ultraviolet upconversion luminescence of Tm doped NaYF4 phosphors by Yb codoping

2 mol% Tm³⁺ doped NaYF₄ phosphors with 0–98 mol% Yb³⁺ codoping were synthesized by sol–gel method. The phase transition from the mixture of hexagonal and cubic phases to single cubic phase of Tm³⁺–Yb³⁺:NaYF₄ phosphors was investigated with increasing of Yb³⁺ concentration. Near-infrared, red, blue, violet and ultraviolet upconversion emissions of Tm³⁺ were observed from the Tm³⁺–Yb³⁺:NaYF₄ phosphors under 976 nm laser diode excitation, with the strongest near-infrared to ultraviolet emissions at 20 mol% Yb³⁺ codoping. The violet and blue emissions for the ¹D₂ → ³F₄ and ¹G₄ → ³H₆ transitions of Tm³⁺ can be tuned by varying Yb³⁺ codoping concentration, which was elucidated using steady-state equations. The intensity ratio of red emissions for the ³F₂ → ³H₆ and ³F₃→³H₆ transitions of Tm³⁺ was strongly related to the Yb³⁺ codoping concentration and temperature, implying a potential application of Tm³⁺–Yb³⁺:NaYF₄ phosphors for optical temperature sensing.     

Vacuum ultraviolet excited photoluminescence properties of novel Na3Y9O3(BO3)8:Eu phosphor

The novel vacuum ultraviolet (VUV) excited Na₃Y₉O₃(BO₃)₈:Eu³⁺ red phosphor was synthesized and the photoluminescence (PL) properties were investigated. The phosphor showed strong VUV PL intensity, large quenching concentration (40 mol%) and good chromaticity (0.649, 0.351). The Eu³⁺-O²⁻ charge transition (CT) was observed to be at a higher energy (232 nm, 5.35 eV). The host absorption at 127-166 nm was broad and strong when monitoring the Eu³⁺ emission, which indicated that energy transfer from the host-lattice to the Eu³⁺ ions was efficient in Na₃Y₉O₃(BO₃)₈:Eu³⁺. These excellent VUV PL properties were revealed to be correlated with the unique isolated layer-type structure of Na₃Y₉O₃(BO₃)₈ host. The results showed that the Na₃Y₉O₃(BO₃)₈:Eu³⁺ would be a good candidate for VUV-excited red phosphor.     

Fabrication and properties of highly transparent Er:YAG ceramics

Highly transparent Er:YAG ceramics with different Er doping concentrations were fabricated by a reactive sintering method under vacuum. The optical properties and the microstructures of the Er:YAG ceramics were investigated. For 3 mm thickness samples, the in-line transmittances of the as-fabricated Er:YAG ceramics at the wavelength of 1100 nm and 400 nm were about 84% and 82%, respectively. The micrograph of the Er:YAG transparent ceramics exhibited a pore-free structure and the average grain size was about 10 μm. The grain boundary of the ceramics was clean and no secondary phase was detected. The absorption and emission spectra, the fluorescence decay traces of the Er:YAG ceramics were measured and discussed. The ceramics obtained may have potential use for eye-safe solid-state lasers partly replacing Er:YAG single crystals.     

Synthesis and spectral analysis of Yb/Tm/Ho-doped Na0.5Gd0.5WO4 phosphor to achieve white upconversion luminescence

Yb³⁺/Tm³⁺/Ho³⁺-doped Na_0.5Gd_0.5WO₄ phosphors were synthesized by the high-temperature solid-state method. Bright white luminescence upon 980 nm near-infrared excitation can be observed for the sample at the optimum chemical composition of Na_0.5Gd_0.5WO₄:10%Yb³⁺/1%Tm³⁺/0.4%Ho³⁺, which is produced via an upconversion (UC) process by tuning the dopant ions concentration. The measured white light consists of the blue, green, and red UC emissions which correspond to the transitions ¹G₄ → ³H₆ of Tm³⁺, ⁵F₄(⁵S₂) → ⁵I₈, and ⁵F₅ → ⁵I₈ of Ho³⁺ ions, respectively. The calculated color coordinates display that white light can be achieved in a wide range of dopant concentrations. The UC mechanisms were also proposed based on their spectral and pumping power dependence analyses.