Tunable white light and energy transfer of Eu2+-Tb3+-Eu3+ tri-activated glasses synthesized in air

An ever increasing demand for white light-emitting diodes (W-LEDs) results in the gradual growth of research on functionalized luminescent glasses. In this paper, single-composition tunable white-emitting Eu²⁺-Tb³⁺-Eu³⁺ tri-activated glasses were synthesized by melt quenching method without additional reducing atmosphere. The coexistence of Eu²⁺ and Eu³⁺ was confirmed by ultraviolet-visible transmission spectra, photoluminescent spectra, fluorescence decay curves, and X-ray photoelectron spectroscopy. Tb³⁺ can act as bridge to connect Eu²⁺-Eu³⁺ luminescent centers by energy transfer. Tone-tunable white light can be achieved by coupling the emission centered at 412, 541, and 612 nm contributed from Eu²⁺, Tb³⁺, and Eu³⁺, respectively. By adjusting the relative content of Eu²⁺/Tb³⁺/Eu³⁺, ideal chromaticity coordinates of (0.33, 0.33) can be achieved under excitation of ultraviolet light. High thermal stability and tiny chromaticity shift were exhibited in samples. These results suggest that Eu²⁺-Tb³⁺-Eu³⁺ tri-activated glasses have great potential application in ultraviolet-driven W-LEDs.     

Structural and emission properties of Eu3+‐doped alkaline earth zinc‐phosphate glasses for white LED applications

Quaternary alkaline earth zinc‐phosphate glasses in molar composition (40 ? x)ZnO – 35P₂O₅ – 20RO – 5TiO₂ – xEu₂O₃ (where x=1 and R=Mg, Ca, Sr, and Ba) were prepared by melt quenching technique. These glasses were studied with respect to their thermal, structural, and photoluminescent properties. The maximum value of the glass transition temperature (T_g) was observed for BaO network modifier mixed glass and minimum was observed for MgO network modifier glass. All the glasses were found to be amorphous in nature. The FT‐IR suggested the glasses to be in pyrophosphate structure, which matches with the theoretical estimation of O/P atomic ratio and the maximum depolymerization was observed for glass mixed with BaO network modifier. The intense emission peak was observed at 613 nm (⁵D₀→⁷F₂) under excitation of 392 nm, which matches well with excitation of commercial n‐UV LED chips. The highest emission intensity and quantum efficiency was observed for the glass mixed with BaO network modifier. Based on these results, another set of glass samples was prepared with molar composition (40 ? x)ZnO – 35P₂O₅ – 20BaO – 5TiO₂ – xEu₂O₃ (x=3, 5, 7, and 9) to investigate the optimized emission intensity in these glasses. The glasses exhibited crystalline features along with amorphous nature and a drastic variation in asymmetric ratio at higher concentration (7 and 9 mol%) of Eu₂O₃. The color of emission also shifted from red to reddish orange with increase in the concentration of Eu₂O₃. These glasses are potential candidates to use as a red photoluminsecent component in the field of solid‐state lighting devices.     

Luminescence properties and tunable emission of Ag NCs in oxyfluoride glass through REF3 (RE = Y,La and Gd) doping

In this research, series of Ag nanoclusters (Ag NCs) contained oxyfluoride glasses were prepared using the melt quenching method, in which the REF₃ (RE = Y, La, and Gd) were selected as dopants to control their size distribution. The absorption, steady and time‐resolved PL spectra were carried to investigate the size dependent luminescence properties of Ag NCs. The spectral results indicated that the super broadband emission of Ag NCs is contributed by the spin‐allowed (blue side) and the spin‐forbidden (red side) transitions, respectively. Besides, the introduction of REF₃ (RE = Y, La, and Gd) can promote the formation of Ag NCs with different sizes and therefore modulated their luminescence properties. The maximum external quantum yields of Ag NCs with emissions at 430, 510, 520, and 570 nm were evaluated to be 24.6%, 40.7%, 56.3% and 30.7%, respectively, which can be obtained in SAg, SAgLa, SAgGd, and SAgY.     

Optical thermometry of Sm3+ on laser‐induced local heating for precipitation of PbS quantum dots in glasses

Fluorescence intensity ratios (FIRs) of the 640 nm and 602 nm emissions from Sm³⁺ were recorded at various temperatures T to identify the temperature increases ΔT associated with laser‐induced local heating of Ag nanoparticles. The FIRs increased as intensities of the excitation beam from a 532‐nm continuous‐wave laser increased. Estimated T of the irradiated region increased to as high as 586°C when at laser irradiation of 1.5 W on the surface containing Ag nanoparticles. Local heating due to the surface plasmon resonance of Ag nanoparticles is a main reason for the ΔT that eventually leads to precipitation of PbS quantum dots in glasses.     

Molecular‐Like Ag Clusters and Eu3+ Co‐Sensitized Efficient Broadband Spectral Modification with Enhanced Yb3+ Emission

AgNO₃/EuF₃/YbF₃ tri‐doped oxyfluoride glass was prepared by a melt‐quenching method, in which a high‐efficient broadband spectral modification can be realized due to the simultaneous energy‐transfer processes of Eu³⁺→Yb³⁺, molecular‐like Ag (ML‐Ag) clusters→Yb³⁺, and ML‐Ag clusters→Eu³⁺→Yb³⁺. The spectral measurements indicated that besides the F‐center brought by the fluorides, the formation of the ML‐Ag clusters and the evolution of silver species within the glass matrix were also closely related to the introduction of Eu³⁺ and Yb³⁺ ions and which in return greatly affected the luminescence properties of these rare‐earth ions. As the UV‐visible irradiation in the wavelength region of 250–600 nm can be efficiently converted into near‐infrared emission around 1000 nm in the AgNO₃/EuF₃/YbF₃ tri‐doped glass, which thus has promising application in enhancing the photovoltaic conversion efficiency of the silicon solar cell.     

Experimental and theoretical studies on NIR luminescence of titanate-germanate glasses doped with Pr3+ and Tm3+ ions

Near-infrared (NIR) luminescence of Pr³⁺ and Tm³⁺ ions in titanate-germanate glasses has been studied for laser and fiber amplifier applications. The effect of the molar ratio GeO₂:TiO₂ (from 5:1 to 1:5) on spectroscopic properties of glass systems was studied by absorption, luminescence measurements, and theoretical calculations using the Judd–Ofelt theory. It was found that independent of the TiO₂ concentration, intense NIR emissions at 1.5 and 1.8 μm were observed for glasses doped with Pr³⁺ and Tm³⁺ ions, respectively. Moreover, several spectroscopic and NIR laser parameters for Pr³⁺ and Tm³⁺ ions, such as emission bandwidth, stimulated emission cross-section, quantum efficiency, gain bandwidth, and figure of merit, were determined. The results were discussed in detail and compared to the different laser glasses. Systematic investigations indicate that Pr³⁺-doped system with GeO₂:TiO₂ = 2:1 and Tm³⁺-doped glass with GeO₂:TiO₂ = 1:2 present profit laser parameters and could be successfully applied to NIR lasers and broadband optical amplifiers.     

Nd3+‐Doped Silicate Glasses as an Efficient Color Filter to Improve the Color Gamut of a White LED

Nd³⁺‐doped silicate glass (Nd‐glass) was employed as a color filter for a white LED based on red and green phosphor (RG‐LED), to manipulate the photoluminescence spectral shape and thus to provide a wider color gamut. The hypersensitive transition of Nd³⁺:⁴I_9/2→⁴G_5/2,²G_7/2 was adjusted via glass composition and Nd concentration, and improved absorbance as well as reduced the absorption bandwidth. The effective absorption of the Nd‐glass at ~580 nm reduced the spectral linewidth of the green and red emissions, improving the color reproduction range. The color gamut of the RG‐LED was improved from 75.3% to 81.6% NTSC by the introduction of Nd‐glass as a color filter. Reliability under high operating current and high temperature were also examined and discussed.     

Light‐induced electrons suppressed by Eu3+ ions doped in Ca11.94−xSrxAl14O33 caged phosphors for LED and FEDs

Control of light‐induced electron generation is of vital importance for the application of caged phosphors. For Eu‐doped Ca_11.94?xSr_xAl₁₄O₃₃ caged phosphors, the suppressed effect of strontium doping on the light‐induced electrons is observed compared to the europium‐free Ca_11.94?xSr_xAl₁₄O₃₃ phosphors. In the presence of europium ions, Sr doping will promote the reduction of Eu³⁺ to Eu²⁺. The Rietveld refinement suggests that unit cell volumes of the Ca_11.94?xSr_xAl₁₄O₃₃:Eu_0.06 samples are expanded when Ca²⁺ ions are replaced by Sr²⁺ ions. The absorption and FTIR transmittance spectra confirm that the competitive reaction of encaged O^2? anions with H₂ is suppressed. For the sample (x=0.48), the higher thermal activation energy (~0.40 eV) for luminescence quenching can be attributed to the more rigid framework structure after Sr doping. For Ca_11.94?xSr_xAl₁₄O₃₃:Eu_0.06 phosphors, their emission colours are tuned from red to purple upon 254 nm excitation and from pink to blue under electron beam excitation through Sr substitution. The insight gained from this work may have a significant guiding to design new phosphors for LED and FEDs and novel nanocaged mutifunctional materials.     

Optical properties of Cu ions‐doped ZnSe quantum dots in silicate glasses

In order to alleviate the effect of the surface defects on the emission properties of quantum dots, copper ions‐doped ZnSe quantum dots (QDs) in the glasses are prepared using melt‐quenching and subsequent thermal annealing methods. For glasses without copper doping, tunable band‐edge emission from ZnSe QDs is achieved. For glasses with copper doping, efficient energy transfer from ZnSe QDs to copper ions is observed, and efficient broad band emission from copper ions is realized at the expense of the band‐edge emission of ZnSe QDs. Absorption spectra, size‐dependent broad‐band emission spectra and electron spin resonance spectra show the cupric ions are doped into the ZnSe QDs. Results reported here shows that doping of transition‐metal ions into semiconductor QDs in glasses is promising for development of high efficient luminescent glasses.     

Role of Nd3+ ions on the nucleation and growth of PbS quantum dots (QDs) in silicate glasses

The influence of Nd₂O₃ addition on the precipitation kinetics of lead chalcogenide (PbS) quantum dots (QDs) in silicate glasses was investigated. Energy dispersive X‐ray spectroscopy (EDS) indicated that the Nd³⁺ ions are preferentially located inside the PbS QDs rather than in the glass matrix. Changes in diameter (D) of PbS QDs exhibited smaller time dependencies (i.e., D≈t^{0.270‐0.286}) than that predicted by the classical Lifshitz–Slyozov–Wagner (LSW) theory. This is due to the limited concentrations of Pb²⁺ and S^2? ions and the large diffusion distance inside the glass matrix. In addition, extended X‐ray absorption fine structure (EXAFS) results indicated that the formation of PbS QDs was retarded due to the presence of Nd₂O₃ in the glasses, as the large NdO_x polyhedra interrupt the diffusion of Pb²⁺ and S^2? ions. We believe that these Nd³⁺ ions are primarily located in PbS QDs in the form of Nd–O clusters, and that the PbS QDs are built on top of these clusters.     

Thermal effects of Er3+/Yb3+‐doped NaYF4 phosphor induced by 980/1510 nm laser diode irradiation

The thermal effects of Er/Yb‐doped NaYF₄ phosphor induced by 980/1510 nm laser diode irradiation were intuitively and contrastively investigated using an infrared thermal imaging technology with real‐time online monitoring. The Yb³⁺/Er³⁺ codoped materials have strong thermal effects and high‐temperature elevation under 980 nm irradiation. However, the severe thermal effects of materials with higher Er³⁺ ion doping concentration are remarkably attributed to the cross relaxation between the Er³⁺ ions under 980 nm irradiation. The energy transfer between Er³⁺ and Yb³⁺ ions in Er³⁺/Yb³⁺‐codoped materials also contributes to the thermal effects under 1510 nm laser diode irradiation. Under the same testing conditions, the temperature elevation ?T of samples induced by 1510 nm laser diode irradiation is lower than that induced by 980 nm laser diode irradiation. The temperature rising rate and elevation ?T value of samples depend on the ion doping concentration and power density of the laser diode excitation. The internal temperature of the samples exhibits deep temperature gradient under 980/1510 nm laser diode irradiation. By comparing the two kinds of thermometry methods, the temperature value calculated by fluorescence intensity ratio is almost similar to that obtained through infrared thermal imaging technology under higher excitation power pumping.     

Photodarkening mechanisms of Pr3+ singly doped and Pr3+/Ce3+ co-doped silicate glasses and fibers

In this work, we revealed the possible mechanisms of the photodarkening in Pr³⁺ ions singly doped and Pr³⁺/Ce³⁺ co-doped silicate glasses and fibers induced by X-ray and 488-nm laser radiations and studied the role of Ce³⁺ in increasing radiation resistance in Pr³⁺-doped silicate glasses and fibers. The absorption, emission, electron paramagnetic resonance (EPR), radiation induced attenuation spectra, and X-ray photoelectron spectroscopy (XPS) of Pr³⁺ singly doped and Pr³⁺/Ce³⁺ co-doped silicate glasses before and after X-ray radiation were measured and analyzed. The fluorescence intensity and photoinduced attenuation of Pr³⁺ singly doped and Pr³⁺/Ce³⁺ co-doped silicate fibers at visible wavelengths pumped by 488-nm laser were measured and analyzed. The influence of Ce³⁺ ions co-doping on the spectroscopic properties of Pr³⁺ ions as well as the radiation-induced defects in silicate glasses was studied. Results demonstrate that both X-ray and 488-nm laser radiations will induce photodamage in Pr³⁺ ions-doped silicate glasses and fibers. Co-doping Ce³⁺ (by up to 1 mol%) is efficient to suppress the darkening induced by both X-ray and 488-nm laser radiations without influence on the luminescence behavior of Pr³⁺ ions in silicate glasses and fibers. Our studies demonstrate the promising potential of Pr³⁺/Ce³⁺ co-doped silicate glasses for visible lasing applications.     

Paramagnetic,NIR‐luminescent Nd3+‐ and Gd3+‐doped fluorapatite as contrast agent for multimodal biomedical imaging

Combining near infrared (NIR) luminescence and magnetic resonance (MR) contrasts in a crystal host is highly desirable for contrast agents in biomedical imaging technology, as it will enable multimodal imaging processes. In the present work, biocompatible luminescent and paramagnetic fluorapatite (FAp) nanoparticles were prepared via doping with neodymium (Nd³⁺) and gadolinium (Gd³⁺), respectively. While Nd³⁺‐doped FAp (Nd:FAp) exhibits dopant concentration‐dependent photoluminescence (PL) in the NIR spectral region, Gd³⁺‐doped FAp (Gd:FAp) shows paramagnetic behavior and strong transverse relaxation effects resulting in MR contrastive properties. Remarkably, multimodal co‐doped FAp (Nd:Gd:FAp) nanoparticles combine both properties in 1 single crystal enabling luminescence as well as MR contrast.     

Role of Al3+ on tuning optical properties of Ce3+‐activated borosilicate scintillating glasses prepared in air

A colorless Ce³⁺‐activated borosilicate scintillating glass enriched with Gd₂O₃ is successfully synthesized in air atmosphere for the first time. The full replacement of 10 mol% BaO by Al₂O₃, and the partial substitution of 3 mol% SiO₂ by Si₃N₄ in the designed glass composition are crucial for this success. The role of Al³⁺ on tuning the optical properties of Ce³⁺‐activated borosilicate scintillating glass synthesized in air are analyzed by optical transmittance, X‐ray absorption near edge spectroscopy (XANES) spectra, photoluminescence (PL) and radioluminescence (RL) spectra. The results suggest that the stable Ce⁴⁺ ions can be effectively reduced to stable Ce³⁺ ions by the full replacement of BaO by Al₂O₃, and both the PL andRL intensity of the designed borosilicate scintillating glass are enhanced by a factor of 6.7 and 5.2, respectively. The integral RL intensity of the synthesized Ce³⁺‐activated borosilicate scintillating glass is ~17.2%BGO, with a light output of about 1180 ph/MeV. The strategy of substituting BaO by Al₂O₃ will trigger more scientific and technological considerations in designing novel fast scintillating glasses.     

Predictable tendency of Bi NIR emission in Bi‐doped magnesium aluminosilicate laser glasses

Because of superbroad luminescence in the range of near infrared (NIR), Bi‐doped glasses and fibers have received more attentions recently for the applications in super broadband optical fiber amplifiers or new wavelength lasers. As the luminescence comes from the transitions between naked 6p orbitals of bismuth, it is very susceptible to slight changes of local field around Bi. Therefore, it is always very challenging to predict NIR emission of bismuth in advance. Here, we found bismuth NIR emission shows predictable tendency in ternary glass system of MgO–Al₂O₃–SiO₂. The emission peak shifts red along the content of magnesium upon the excitation of 484 nm, which follows a single exponential growth equation. In the meantime, the full width at half maximum (FWHM) is broadened while the lifetime keeps decreasing. Glass structure analysis on basis of FTIR, ²⁷Al NMR, ²⁹Si NMR spectra reveals that these changes correlate to integrity of glass network, the increased disorder of local field around bismuth and the enhanced interaction between bismuth and host, which are perhaps due to the linear increase of nonbridging oxygen, and the enhanced Si–O asymmetric stretching vibrations along with magnesium, respectively. Electron probe microanalysis shows good homogeneity of Si, Al, Mg, Bi, and O distribution within the samples, and yoyo experiments of heating and cooling between 30°C and 300°C reveal the good resistance of such doped glasses to thermal degradation. This makes the glasses promising in applications of fiber devices even under extreme condition such as at higher temperature. The finding in this work should be helpful for the design of Bi‐doped laser glasses in future.     

Superdense Tb3+-activated borogermanate-tellurite scintillating glasses

Novel Tb³⁺-activated borogermanate-tellurite scintillating glasses with a maximum density of 7.15 g/cm³ aimed at detection of high-energy rays were prepared by a melt-quenching method for the first time. The concentration-dependent optical properties including transmittance, photoluminescence, luminescence dynamic behaviors, and X-ray excited luminescence in the as-prepared Tb³⁺-activated borogermanate-tellurite glasses were studied. The optimal content of Tb₂O₃ in the superdense borogermanate-tellurite glasses is revealed to be 7 mol% under both 275 nm ultraviolet light and X-ray excitation. The integral scintillation efficiency of Tb³⁺-activated borogermanate-tellurite scintillating glass is about 33.71% of the standard Bi₄Ge₃O₁₂ (BGO) scintillating crystal.     

Intense upconversion luminescence and energy‐transfer mechanism of Ho3+/Yb3+ co‐doped SrLu2O4 phosphor

A series of novel SrLu₂O₄: x Ho³⁺, y Yb³⁺ phosphors (x=0.005‐0.05, y=0.1‐0.6) were synthesized by a simple solid‐state reaction method. The phase purity, morphology, and upconversion luminescence were measured by X‐ray diffraction (XRD), scanning electron microscopy (SEM), and photoluminescence (PL) spectroscopy. The doping concentrations and sintering temperature were optimized to be x=0.01, y=0.5 and T=1400°C to obtain the strongest emission intensity. Under 980 nm laser diode excitation, the SrLu₂O₄:Ho³⁺, Yb³⁺ phosphors exhibit intense green upconversion (UC) emission band centered at 541 nm (⁵F₄,⁵S₂→⁵I₈) and weak red emission peaked at 673 nm (⁵F₅→⁵I₈). Under different pump‐power excitation, the UC luminescence can be finely tuned from yellow‐green to green light region to some extent. Based on energy level diagram, the energy‐transfer mechanisms are investigated in detail according to the analysis of pump‐power dependence and luminescence decay curves. The energy‐transfer mechanisms for green and red UC emissions can be determined to be two‐photon absorption processes. Compared with commercial NaYF₄:Er³⁺, Yb³⁺ and common Y₂O₃:Ho³⁺, Yb³⁺ phosphors, the SrLu_1.49Ho_0.01Yb_0.5O₄ sample shows good color monochromaticity and relatively high UC luminescence intensity. The results imply that SrLu₂O₄:Ho³⁺, Yb³⁺ can be a good candidate for green UC material in display fields.     

Dual color tuning in Ce3+-doped oxyfluoride ceramic phosphor plate for white LED application

Ceramic phosphor plates of cerium (Ce³⁺)-doped oxyfluoride were fabricated by the solid-state reaction method. These phosphors exhibit efficient emission, with the novel feature of color tuning by varying both the doping concentration and excitation wavelength. As the Ce³⁺ concentration increases, the excitation spectrum broadens by a factor of 1.6, and the excitation peak wavelength shifts from 390 to 435 nm, and there is a variation in excitation energy of ~ 10%. Luminescence spectrum of low Ce³⁺ concentration samples is tuned from blue to green with the change of excitation wavelength. The emission peak exhibits a shift of 58 nm into the red spectral region, varying the Ce³⁺ concentration from 0.05 to 0.1 mol% ; whereas this shift is only 6 nm when Ce³⁺ content changes from 0.25 to 1 mol%. Photoluminescence (PL) quantum yield has achieved 76%. The crystal structure was examined using X-ray diffraction to explain its possible influence on the redshift luminescence. A proof of concept of white LED was constructed using a 450 nm blue LED chip with an oxyfluoride phosphor plate, showing a luminous efficacy (LE) of 64 lm/W with a color rendering index of 74.     

A Single‐Phase Phosphor NaLa9 (GeO4)6O2: Tm3+, Dy3+ for Near Ultraviolet‐White LED and Field‐Emission Display

Single‐phase white‐light‐emitting phosphors NaLa_9(1?x?y) (GeO₄)₆O₂: xTm³⁺, yDy³⁺ (NLGO: xTm³⁺, yDy³⁺) have been synthesized by a traditional solid‐state reaction method. The powder X‐ray diffraction (XRD), photoluminescence (PL), PL excitation (PLE) spectra, fluorescence decay curves, chromaticity coordinates, correlated color temperature (CCT), and the cathodoluminescence (CL) properties of the obtained phosphors are measured and discussed in detail. It is discovered that the series samples could be color‐tunable (from blue to yellow) by tuning the doping content of Dy³⁺ with a fixed Tm³⁺ content excited at 357 nm and white light (0.341, 0.324) could be obtained with the CCT of 5079 K. A NLGO: 0.01Tm³⁺, 0.02Dy³⁺ is studied carefully as representative. The main emissions of Tm³⁺ (453 nm, ¹D₂–³F₄) and Dy³⁺ (478 nm, ⁴F_9/2–⁶H_15/2; 572 nm, ⁴F_9/2–⁶H_13/2) make it emit white light with good thermal stability (67% of the initial till 523 K). The energy transfer from Tm³⁺ to Dy³⁺ is noticed and further research has been done to explain the enhancement of Dy³⁺ emission and the excellent thermal stability. It also keeps stable under continuous electron bombardment with high intensity. All of these indicate that it could be a suitable candidate for white‐emitting phosphor applied for near ultraviolet‐white light‐emitting diode (NUV‐WLED) and field‐emission display (FED).     

Effect of BaF2 Content on Luminescence of Rare‐Earth Ions in Borate and Germanate Glasses

Rare‐earth‐doped oxyfluoride germanate and borate glasses were synthesized and next studied using spectroscopic methods. Influence of fluoride modifier on luminescence properties of rare earths in different glass hosts was examined. The excitation and emission spectra of Pr³⁺ and Er³⁺ ions in the studied glasses were registered. The emission spectra of Pr³⁺ ions in germanate and borate glasses are quite different and depend strongly on the glass host. In samples doped with Er³⁺ ions emission bands located around 1530 nm corresponding to the main ⁴I_13/2→⁴I_15/2 laser transition were registered, independently of the glass host. Quite long‐lived near‐infrared luminescence of Er³⁺ ions was observed for germanate glasses with low BaF₂ content, while in borate glass systems influence of barium fluoride on luminescence lifetimes is not so evident. The Judd–Ofelt calculations were used in order to determine quantum efficiencies of excited states of rare‐earth ions in germanate and borate glasses.