Effect of γ‐irradiation on spectral properties of undoped Y2SiO5 crystals

The absorption spectra of undoped Y₂SiO₅ crystals were studied before and after γ‐irradiation. After γ‐irradiation, the additional absorption peaks at 260‐270 and 320nm were observed in as‐grown and H₂‐annealed Y₂SiO₅ crystal, but it did not occur in air‐annealed Y₂SiO₅ crystal. These absorption peaks were attributed to F color centers and O^– hole centers, respectively. Owing to more oxygen vacancies and color centers in H₂‐annealed Y₂SiO₅ crystal than that in as‐grown Y₂SiO₅ crystal after γ‐irradiation, the additional absorption peaks were more intense in the former than that in the latter. With the irradiation dose increasing from 20 to 220kGy, the intensity of additional absorption peaks increased. © 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim     

Synthesis and photoluminescence of Eu3+-doped ZnO–Ga2O3–SiO2 nano-glass-ceramics

Transparent rare-earth Eu³⁺-doped ZnO–Ga₂O₃–SiO₂ nano-glass-ceramics were obtained by a sol–gel method. X-ray diffraction and transmission electron microscopy were used to characterize the as-synthesized materials. Results showed that ZnGa₂O₄ nanocrystals with the size of 5 nm were precipitated from ZnO–Ga₂O₃–SiO₂ system and dispersed in the SiO₂-based glass when the heat-treatment temperature was up to 800 °C. Photoluminescence characterization of Eu³⁺-doped ZnO–Ga₂O₃–SiO₂ nano-glass-ceramics was carried out and the results show that the as-synthesized material display intense emission at 615 nm belonging to ⁵D₀ → ⁷F₂ transition.     

Luminescence properties of Y3Al5O12:Eu3+-coated submicron SiO2 particles

Silica submicron spherical particles coated with an yttrium aluminum garnet (Y₃Al₅O₁₂, YAG) layer doped with Eu³⁺ were prepared by the sol–gel method. The structure and morphology of samples determined by the X-ray powder diffraction measurements and transmission electron microscope images, respectively, indicated that well-crystallized garnet nanocrystallites were formed with successive coating cycles. Similar trends were deduced from the evolution of the luminescence spectra. The ratio of integrated intensities of the ⁵D₀ → ⁷F₂ and ⁵D₀ → ⁷F₁ transitions was used to analyze the structural variations in the surroundings of the Eu³⁺ ion. The effect of coating was analyzed by comparing the luminescence properties of the Y₃Al₅O₁₂:Eu³⁺ nanocrystalline powders and composite Y₃Al₅O₁₂:Eu³⁺/SiO₂ materials.     

Energy transfer from Bi3+ sensitizing the luminescence of Eu3+ in clusters embedded into sol–gel silica glasses

Y³⁺(La³⁺), Eu³⁺ and Bi³⁺ ions co-doped sol–gel silica glasses were synthesized. Photoluminescence spectra show that there is energy transfer from Bi³⁺ ions to the emission band of Eu³⁺ ions. The co-dopants Y³⁺ or La³⁺ have strong effects on the local structure and luminescence of Eu³⁺ ions. For 0.5 mol% Eu³⁺ ions doped glasses, the co-doping of 1 mol% Bi³⁺ and 1 mol% Y³⁺ is the most appropriate for the sensitization from Bi³⁺ to Eu³⁺. The sensitization effectiveness from Bi³⁺ ions to Eu³⁺ ions was studied by changing the amount of Bi³⁺ and Y³⁺, and clusters containing rare earth ions and Bi³⁺ ions dominate the energy transfer processes. The comparison of luminescent R-values (the intensity ratio of ⁵D₀ → ⁷F₂/⁵D₀ → ⁷F₁ in Eu³⁺ ions) between glasses containing La³⁺ and containing Y³⁺ verifies the formation of clusters in sol–gel glasses. As a favorable configuration for energy transfer, the accurate design and synthesis of clusters-contained glasses may provide a new kind of luminescent materials.     

Preparation and luminescent properties of Eu-doped transparent glass-ceramics containing SrF2 nanocrystals

Transparent glass-ceramics containing SrF₂ nanocrystals were fabricated by melt-quenching and subsequent heating of glass with a composition of 50SiO₂–10Al₂O₃–20ZnF₂–20SrF₂. X-ray diffractometry, transmission electron microscopy, and energy dispersive spectroscopy were used to investigated the microstructure of the SrF₂ glass-ceramics. Results show that SrF₂ nanocrystals were homogeneously precipitated among the aluminosilicate glass matrix, and the mean size of the SrF₂ nanocrystals was about 20 nm, and Eu³⁺ ions partition mainly into the precipitated SrF₂ nanocrystals after crystallization. The glass-ceramics exhibited intense red emission corresponding to the ⁵D₀ → ⁷F_J (J = 0–4) transitions of Eu³⁺ ions under 393 nm excitation. A significant Eu³⁺ luminescence enhancement by a factor of about nine times was observed after crystallization. Besides, the obvious stark splitting emissions, the low forced electric dipole ⁵D₀ → ⁷ F₂ transition, and the long decay lifetimes of Eu³⁺ ions also revealed the partition of Eu³⁺ ions into low phonon energy SrF₂ nanocrystals. Our results indicate the SrF₂ based fluorosilicate glass-ceramics is an excellent host for trivalent lanthanide ion doping and may find applications in photonics.     

Luminescence properties and energy transfer of Eu/Tb ions codoped aluminoborosilicate glasses

Eu/Tb codoped aluminoborosilicate glasses were fabricated by high temperature melting-quenched technique and their luminescence properties were investigated by excitation and emission spectra. Under 376 nm excitation, blue, green and red emission bands were simultaneously observed at 425 nm, 485 nm, 540 nm and 611 nm, respectively. The broad blue emission band centered at 425 nm was originated from the reduced Eu²⁺ ions, which were reduced from Eu³⁺ ions at high temperature in an ambient atmosphere and the reduction process may be related with the optical basicity of glass matrix. A complex bright white light emission was obtained for 0.5 mol% Eu₂O₃, 0.5 mol% Tb₂O₃ codoped aluminoborosilicate glass with CIE-X = 0.31 and CIE-Y = 0.33. The energy transfer among Eu³⁺, Eu²⁺ and Tb³⁺ ions was also discussed.     

Highly enhanced f–f transitions of Eu3+ in La2O3 phosphor via citric acid and poly (ethylene glycol) precursor route

The luminescent material europium-activated La₂O₃ have been prepared by the citric acid and poly (ethylene glycol) (PEG) precursor route. Their structures and optical properties were characterized by FT-IR spectrum, X-ray diffraction (XRD), thermogravimetry-differential thermal analysis (TG-DTA), UV–vis spectroscopy, and photoluminescence (PL) spectra, respectively. The results show considerable enhancement of the photoluminescence, especially the Eu³⁺ f–f transition excitation lines and the charge transfer band (CT). The samples can exhibit strong red emission centered at 626 nm excited at either the CT band (300 nm) or the Eu³⁺ f–f transition (396 nm), suggesting the potential application as the red phosphors for ultraviolet light-emitting diodes (LEDs), which can be attributed to the ⁵D₀→⁷F₂ transition of Eu³⁺. The remarkable enhancement of color purity of red emission and the concentration quenching of Eu³⁺ in La₂O₃ were also observed with increasing Eu³⁺ doped concentration.     

Rare earth centers properties and electron trapping in SnO2 thin films produced by sol–gel route

Some very relevant optical, electrical, and structural properties of SnO₂ doped with rare-earth ions Er³⁺ and Eu³⁺ are presented. Films are produced by the sol–gel-dip coating process, and may be described as a combination of nanoscopic dimension crystallites (about 3–10 nm) with their respective intergrain potential barriers. The Er³⁺ and Eu³⁺ ions are expected to act as acceptors in SnO_2, which is a natural n-type conductor, inducing a high degree of charge compensation. Electron trapping and emission spectra data are presented and are rather distinct, depending on the location of the rare-earth impurity. This behavior allows the identification of two distinct centers: located either in the SnO₂ lattice or segregated at the particles surface. Based on a model for thermally activated cross-section defects, the difference between the capture energy of the photo-excited electron and the intergrain potential barrier is evaluated, leading to distinct values for high and low symmetry sites. A higher distortion in the lattice of undoped SnO₂ and SnO₂:Eu (1 at.%) was evaluated from Rietveld refinements of X-ray diffraction data. This was confirmed by Raman spectra, which are associated with the particles size and disorder. By comparing the samples with the same doping concentration, it was found that this disorder is higher in Eu-doped SnO₂ than in Er-doped SnO₂, which is in agreement with a higher energy for the lattice relaxation in the trapping process by Eu³⁺ centers.     

Characterization of Eu3+-doped fluorophosphate glasses for red emission

Fluorescence spectra and decay curves of the ⁵D₀ level for different concentrations of Eu³⁺ (4f⁶) ions in K–Ba–Al fluorophosphate glasses have been measured at room temperature and are analyzed. The Judd–Ofelt intensity parameters Ω₂ and Ω₄ have been determined from the intensity ratios of emission peaks corresponding to ⁵D₀ → ⁷F_J (J = 2 and 4) to ⁵D₀ → ⁷F₁ transitions for 1.0 mol% glass. The intensity parameters thus obtained are in turn used to calculate the radiative properties of the fluorescent levels of Eu³⁺ ions. Second and fourth rank crystal-field parameters have been evaluated by assuming a C_2V site symmetry for the local environment of Eu³⁺ ions to estimate the crystal-field strength experienced by Eu³⁺ ions in the present host. The decay profiles of the ⁵D₀ → ⁷F₂ transition of Eu³⁺ ions in the present glasses are found to be single exponential for all the studied Eu³⁺ ion concentrations. A marginal increase in lifetime of the ⁵D₀ level has been noticed with Eu³⁺ ion concentration up to 2.0 mol% and then the lifetime marginally decreases for higher Eu³⁺ ion concentrations.     

Luminescence behaviors of the Eu2+ ions in the GeS2-Ga2S3-CsCl chalcohalide glasses

In the paper, we report the luminescence behaviors of Eu²⁺-doped chalcohalide glasses. Composition of glass matrix is 37.5GeS₂-22.5Ga₂S₃-40CsCl (mol%) with the concentration of Eu²⁺ in the range of 0.1–0.25 (mol%). The optimal Eu²⁺ doping concentration was 0.15 mol% at which the glass exhibited an enhanced emission peaking at 434 nm with a full width at half maximum (FWHM) about 80 nm with the excitation at 395 nm.     

Color centers and charge state change in Ce:YAG crystals grown by temperature gradient techniques

Color centers and impurity defects of Ce:YAG crystals grown in reduction atmosphere by temperature gradient techniques have been investigated by means of gamma irradiation and thermal treatments. Four absorption bands associated with color centers or impurity defects at 235, 255, 294 and 370 nm were observed in as-grown crystals. Changes in optical intensity of the 235 and 370 nm bands after gamma irradiation indicate that they are associated with F⁺-type color center. Charge state change processes of Fe³⁺ impurity and Ce³⁺ ions take place in the irradiation process. The variations of Ce³⁺ ions concentration clearly indicate that Ce⁴⁺ ions exist in Ce:YAG crystals and gamma irradiations could increase the concentration of Ce³⁺ ions. Annealing treatments and the changes in optical density suggest that a heterovalent impurity ion associated with the 294 nm band seems to be present in the crystals.     

Influence of Ag nanoparticles on luminescent performance of SiO2:Tb3 + nanomaterials

Tb^3 + single-doped SiO₂ (SiO₂:Tb^3 +) and Tb^3 +, Ag co-doped SiO₂ (SiO₂:Tb^3 +–Ag) nanostructured luminescent materials were prepared by a modified Stöber method. Their microstructure and optical property were investigated using scanning electron microscopy, ultraviolet visible absorption and photoluminescence spectrophotometry. Results show that the samples are composed of well-dispersed near-spherical particles with a diameter about 50 nm, the highest fluorescence intensity is obtained when the doping concentration of Tb^3 + is 4.86 mol%, the corresponding internal quantum efficiency is 13.6% and the external quantum efficiency is 8.2%. The experimental results indicate that Ag nanoparticles can greatly enhance the light absorption at 226 nm and the light emission at 543 nm of SiO₂:Tb^3 +–Ag, and the fluorescence lifetime reduces with increasing Ag concentration in SiO₂:Tb^3 +–Ag. Additionally, the present results indicate that fluorescence enhancement is attributed to the local field enhancement and the increased radiative decay rates induced by Ag nanoparticles.     

Luminescent nano-composites generated from a spray

The spray-pyrolysis (SP) synthesis technique has been employed to obtain SiO₂:Eu³⁺ and γ-AlOOH:Eu³⁺. It was possible to obtain sub-micrometric spherical particles of SiO₂ with luminescent Eu³⁺ ions bonded to the silica surface or embedded in amorphous silica beads, by controlling the synthesis and annealing process. Boehmite γ-AlOOH doped with Eu³⁺ nanoparticles were synthesized by SP at moderate temperature (200 °C) with Eu³⁺ ions bonded to the surface hydroxyls of the boehmite nanocrystals. Luminescent nano-composites were obtained by controlled reaction of γ-AlOOH:Eu³⁺ nanocrystals with ASN (asparagine). In these nano-composites, the Eu³⁺ are held at the surface of the boehmite nanocrystals and partially shielded from interactions with additional luminescence quenchers (hydroxyl groups, water molecules).     

Effects of yttrium codoping on fluorescence lifetimes of Er3+ ions in SiO2–Al2O3 sol–gel glasses

In order to find a new glass host and optimize erbium doping for IR glass optical amplifiers in photonic applications, a study on the optimization of the emission of erbium ions in the SiO₂–Al₂O₃ glass by codoping with Y₂O₃ is performed. It is first attempted to make a new sol–gel glass host based on SiO₂, Al₂O₃, and Y₂O₃ doped with Er³⁺ ions of the composition (1−x)SiO₂–xAl₂O₃–yY₂O₃:0.65Er₂O₃ (in mol%), x varies from 0 to 65, and y from 0 to 4. The optimal proportion in mol% of SiO₂ and Al₂O₃ for the Er³⁺ emission (at a fixed optimal concentration of 0.65) was 65 – 35. The effect of Y₂O₃ content on photoluminescence, decay curve profiles and lifetime of the ⁴I_13/2 level of Er³⁺ in SiO₂–Al₂O₃ glass is observed. The largest quantum efficiency and the higher emission intensity are observed in the sample with 65Al₂O₃ and 4Y₂O₃. The emission intensity at 1530 nm is two times higher than in glasses without Y₂O₃. A shift of 3 nm to shorter wavelengths is observed. The emission spectral profiles are flatter and broader for the glasses containing Al and Y (bandwidth of 59.5 nm). The decay curves show strong difference profiles for the different samples. The increase of the lifetime value τ (about ms) of the ⁴I_13/2 level of Er³⁺ in the SiO₂–Al₂O₃ with the Y₂O₃ is discussed.     

Spectroscopy and crystal-field analysis of Cr4+-doped transparent silicate glass-ceramics

We report transparent Cr⁴⁺-doped SiO₂–Al₂O₃–ZnO–Li₂O–K₂O glass-ceramics with broadband infrared luminescence. After heart-treatment, Li₂ZnSiO₄ crystallite was precipitated in the glasses, and its average size increased with increasing heat-treatment temperature. Racah parameters of Cr⁴⁺–Li₂ZnSiO₄ glass-ceramics have been calculated, and it was confirmed from absorption spectra that the energy levels of Cr⁴⁺-doped glass-ceramics are close to the cross point ¹E and ³T states. No infrared emission was detected in the as-made glass samples, but the broadband infrared emission centered at 1210 nm with the full width at half maximum (FWHM) of more than 250 nm was observed by exciting the glass-ceramics with excitation of an 808 nm laser diode. In order to analyze the located crystal field of Cr⁴⁺ ions, the emission spectra are fitted by multi-peak Gauss fitting. It is seen that the fluorescence spectra are fitted into two Gaussian bands at around 1195 and 1263 nm with band widths of 208 and 278 nm, respectively. The two Gaussian bands at around 1195 and 1263 nm have about the same decay rate, and hence they would probably originate from the same luminescent centers. The observed infrared emission could be attributed to Cr⁴⁺ ions at low-field sites in Li₂ZnSiO₄ glass-ceramics.     

The luminescence of Pr3+ ions doped in Y4Al2O9 by sol–gel synthesis

The Pr³⁺-doped Y₄Al₂O₉ powders were synthesized by sol–gel method. Powder X-ray diffraction and SEM techniques were used to check for Y₄Al₂O₉ powders. The Li⁺ co-doping with Pr³⁺ has an influence on the sintering temperature and morphology of the Y₄Al₂O₉ powders produced from the gel. The emission spectra under different excitations, e.g., the 488 nm line of an argon-ion laser, X-ray and UV light, were investigated. The luminescence intensity of Y₄Al₂O₉:Pr³⁺ could be increased with Li⁺ co-doping. Luminescence properties of Pr³⁺ ions in the two samples have some difference. In the Y₄Al₂O₉:Pr³⁺, the emission at 490 nm from ³P₀ is dominant, while, the Y₄Al₂O₉:(Pr³⁺ + Li⁺) system was characterized by a red emission at 607 and 610 nm corresponding to the ¹D₂ → ³H₄ inner transition of Pr³⁺ ions; and these two emissions show different excitation band from the 4f5d state.     

A facile route to synthesize luminescent YVO4:Eu3+ porous nanoplates

Porous nanoplates made of yttrium orthovanadate doped with europeum (YVO₄:Eu³⁺) have been successfully synthesized via a chemical co-precipitation method using NH₄VO₃, Y₂O₃, Eu₂O₃ and ethylene glycol. To investigate the effect of temperature on the pore size and morphology of the final product, the as-synthesized YVO₄:Eu³⁺ nanoplates were subjected to heat treatment at 300 and 600 °C for 2 h. The obtained samples were characterized by X-ray diffractometion, Fourier-transform infrared spectroscopy, transmission electron microscopy, high-resolution transmission electron microscopy and photoluminescence spectroscopy. The experimental results showed that the luminescent properties were significantly enhanced with increasing pore size of the YVO₄:Eu³⁺ porous nanoplates.     

Spectral analysis of a novel Eu2+ activated SiO2–BaO glass ceramics phosphor under UV-LED excitation

A novel Eu²⁺ activated 60SiO₂–40BaO (mol%) glass ceramics phosphor was prepared and the optical properties were investigated. X-ray diffraction (XRD) and Raman spectra confirmed the formation of Ba₂Si₃O₈ nano-crystals in the glass matrix. The Eu²⁺ activated glass ceramics exhibited broad emission band centered at 518 nm due to the 4f⁶5d¹→4f⁷ transition of Eu²⁺. Compared with the glass, the emission intensity of Eu²⁺ activated glass ceramics was much stronger, and the peak wavelength shifted toward shorter wavelength. The photoluminescence excitation (PLE) spectra of the glass ceramics showed an overlap with the main emission region of an ultraviolet light-emitting diode (UV-LED). According to the photoluminescence (PL) spectra, the CIE chromaticity coordinates of the Eu²⁺ activated glass and glass ceramics were calculated. The results indicated that the Eu²⁺ activated glass ceramics containing Ba₂Si₃O₈ nano-crystals can be used as a potential green emitting phosphor under UV-LED excitation.     

Spectroscopic properties of Co2+ ions in La2O3–MgO–Al2O3–SiO2 glass-ceramics

Co²⁺-doped La₂O₃–MgO–Al₂O₃–SiO₂ (LMAS) glass-ceramics was synthesized by conventional method. The microstructure of LMAS GCs heat-treated at 760 °C/12 h + 930 °C/4 h was characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The spectroscopic properties of Co²⁺-doped LMAS GCs were studied by absorption spectrum, excitation spectrum, and temperature dependent luminescence spectra. XRD results revealed the sizes of MgAl₂O₄ crystalline phases are about 9.1 ± 1.5 nm. The three peaks in the visible absorption band of LMAS GCs at 549 nm, 585 nm and 626 nm are connected with the transitions from ⁴A₂ level to ²A₁/²T₂(²G), ⁴T₁(⁴P) and ²E/²T₁(²G) levels, respectively, and excitations into them emit the radiation at around 666 nm. The luminescence intensity increased with temperature increasing from 10 K to 150 K, while it weakened with temperature increasing from 150 K to 350 K. These features were explained by the effects of two competing mechanisms.     

Time-resolved fluorescence measurements on Eu3+- and Eu2+-doped glasses

Various fluoride, phosphate and borosilicate glasses with known properties and global structure have been doped with Eu³⁺ (4f⁶) at doping concentrations between 10¹⁸ and 10²¹ cm^?3. By applying reductive melting conditions Eu³⁺ could partially be transformed to Eu²⁺ (4f⁷). Four fluoroaluminate glasses with varying phosphate content between 0 and 20 mol% (FPx), a pure phosphate glass (P100) and two borosilicate glasses with low (DURAN^®-like) and high optical basicity (NBS1) have been used for these investigations. The time-resolved fluorescence in the visible range has been studied for both ions. Although static and dynamic fluorescence of Eu³⁺ and Eu²⁺ are dramatically different (f–f-transitions for Eu³⁺; d–f-transitions for Eu²⁺), glass structure changes have a similar influence on the dynamic fluorescence behavior of both ions. A strongly ionic surrounding due to fluorine ligands as in fluoroaluminate glass samples provides the longest fluorescence lifetime (about 7 ms for Eu³⁺; about 1.3 μs for Eu²⁺). Increasing phosphate content decreases the fluorescence lifetime due to more oxygen ligands. Interesting differences have been found for the two borosilicate glasses due to the difference in their optical basicity (Na₂O/B₂O₃ ratio). Measurements indicate a homogeneous distribution of europium ions in most FP samples. NBS1 measurements suggest that two different local europium sites are formed. For Duran-like samples only one specific europium site was found, although these samples show phase separation at high doping concentrations into a SiO₂-rich phase and borate- and europium-rich droplets. Fluorescence quenching due to energy transfer from Eu²⁺ to Eu³⁺ could be found for co-doped samples; Eu³⁺-doped samples show no fluorescence quenching.