Evolution of Yb environment and luminescence properties under ionizing irradiation in aluminoborosilicate glasses

Yb-doped aluminoborosilicate glasses were irradiated with 2.3 MeV electron and gamma rays at different doses ranging between 10⁴ and 2.6 × 10⁹ Gy and the local structure around the Yb³⁺ ions has been studied using a combination of NMR, EPR and photoluminescence spectroscopic techniques. The spectroscopic results indicate the presence of two distinct Yb³⁺ sites in these glasses and their relative fractions depend on both the Yb concentration and the irradiation dose. These two sites can be attributed to Yb³⁺ ions with and without Yb next-nearest neighbors. The evolution of the shape of the ⁷F_5/2 → ⁷F_7/2 infrared emission band under irradiation is explained by a preferential reduction of the Yb³⁺ ions with Yb next-nearest neighbors at high integrated doses (> 10⁸ Gy). This interpretation is supported by a strong decrease of the visible cooperative luminescence under irradiation due to the reduction into the Yb pairs. In other terms, it shows an interesting effect of ionizing irradiation on the Yb clusters. A low intensity emission band appears in irradiated samples at 390 nm that can be attributed to Yb²⁺. Finally, the decrease of the Yb³⁺ fluorescence lifetime observed under irradiation appears to be primarily due to electronic interaction of these ions with Non-Bridging Oxygen Hole Centre defects created by the ionizing radiation.     

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.     

Thermal and optical investigation of EuF3-doped lead fluorogermanate glasses

Europium-doped lead germanate and lead fluorogermanate glasses are studied by using differential thermal analysis, X-ray diffraction, photoluminescence and fluorescence lifetimes measurements of the ⁵D_j, j = 0, 1, 2 levels. PbF₂ addition increases the thermal stability of the lead germanate glass, while Eu³⁺ ions promote the crystallization of β-PbF₂:Eu³⁺ nano-crystals embedded in a glassy matrix. In the lead fluorogermanate glasses, Eu³⁺ ions exhibit a strong affinity for F⁻ ions although oxygen ions are much more numerous. It appears that luminescence concentration quenching is not important, while cross relaxation is very efficient in the glasses. The results allow to propose for these glasses a molecular model in which small fluorine rich island, incorporating the Eu³⁺ ions in low symmetry sites, are separated from each other by chains of germanate (GeO₄)⁴⁻ ions linked together.     

Site selective spectroscopy of Eu3+ in heavy-metal oxide glasses

In this work, we report time-resolved line-narrowed fluorescence spectra of the ⁵D₀ → ⁷F₀₋₆ transitions of Eu³⁺-doped 40Bi₂O₃–40PbO–10Ga₂O₃–10GeO₂ and 60GeO₂–25PbO–15Nb₂O₅ glasses obtained at 10 K by using different resonant excitation wavelengths along the ⁷F₀ → ⁵D₀ transition. The wavelength dependence of the ⁷F₁ splitting in both glasses suggests the existence of a broad distribution of Eu³⁺ centres possessing a variety of local field environments. The influence of the local coordination ions on the optical properties of Eu³⁺ has been studied by analysing the dependence of the crystal field (CF) parameters on the excitation energy along the ⁷F₀ → ⁵D₀ transition.     

Controlled chemical co-precipitation and solid phase synthesis,microstructure and photoluminescence of La3PO7:Eu3+ phosphors

Eu³⁺-doped oxy-phosphate (La₃PO₇:Eu³⁺) with monoclinic phase has been synthesized via solid phase and co-precipitation methods. The products present various regular morphologies after high temperature thermal treatment, such as bulk with holes, leaf-like flake, nano-rod and so on. All the phosphors exhibit the characteristic fluorescence of Eu ion, the electric dipole transition ⁵D₀→⁷F₂ of Eu³⁺ ions is dominant indicating that the sites of Eu³⁺ ions in La₃PO₇ have no the inversion center. Furthermore, the intensity of ⁵D₀→⁷F₂ increases due to the introducing of surfactant and increasing of the calcination temperature.     

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.     

Thermal stability, spectra and laser properties of Yb: lead-zinc-telluride oxide glasses

A series of new glasses of 70TeO₂-(20 − x) ZnO-xPbO − 5La₂O₃-2.5K₂O-2.5Na₂O (mol%) doped with Yb³⁺ is presented. Thermal stability, spectra and laser properties of Yb³⁺ ions have been measured. It found that 70TeO₂-15PbO-5ZnO-5La₂O₃-2.5K₂O-2.5Na₂O composition glass had fine stability ((T_x−T_g)>190 °C), high-stimulated emission cross-section of 1.25 pm² for the ²F_5/2 → ²F_7/2 transition and existed measured fluorescence lifetime of 0.94 ms and the broad fluorescence effective linewidth of 72 nm. Evaluated from the good potential laser parameters, this system glass is excellent for short pulse generation in diode pumped lasers, short pulse generation tunable lasers, high-peak power and high-average power lasers.     

Broadband infrared luminescence of Ni-doped silicate glass-ceramics containing lithium aluminate spinel nanocrystals

Transparent Ni²⁺-doped SiO₂-Al₂O₃-Ga₂O₃-Li₂O (LGAS) glass-ceramics embedding lithium aluminate spinel nanocrystals was prepared. After heat treatment, LiAl₅O₈ crystallite was precipitated in the glasses, and its size was about 3 nm. It was confirmed from the absorption spectra that the ligand environment of Ni²⁺ ions changed from the trigonal bi-pyramid fivefold sites in the as-made glass to the octahedral sites in the glass-ceramics. Upon excitation at 980 nm, broadband infrared luminescence centered at around 1250 nm with full width at half maximum (FWHM) more than 250 nm was observed originating from the ³T₂(³F) → ³A₂(³F) transition of Ni²⁺ in octahedral sites. The broadband near-infrared (NIR) emission from Ni²⁺-doped glass-ceramics can be as host materials for broadband optical amplifier.     

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.     

Reduction and luminescence of europium ions in glass ceramics containing SrF2 nanocrystals

Reduction of Eu³⁺ → Eu²⁺ and luminescence of europium (Eu) ions in glass ceramics containing SrF₂ nanocrystals have been investigated. The formation of SrF₂ nanocrystals in glass ceramics was confirmed by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Blue luminescence of the Eu²⁺ ions was observed in the Eu doped glass ceramics which were prepared by the heat treatment of the glass in air atmosphere. The double-exponential decay curves of ⁵D₀ state of Eu³⁺ in the Eu doped glass ceramics indicated that there were two different surroundings of the Eu ions in the glass ceramics.     

Studies of Er doped germanate-oxyfluoride and tellurium-germanate-oxyfluoride transparent glass-ceramics

An erbium doped germanate-oxyfluoride glass 60GeO₂ · 20PbO · 10PbF₂ · 10CdF₂ (GPOF) and a tellurium-germanate-oxyfluoride glass 30TeO₂ · 30GeO₂ · 20PbO · 10PbF₂ · 10CdF₂ (TGPOF) were prepared in the bulk form. By appropriate heat treatment of the as-prepared glasses above, transparent glass-ceramics were obtained with the formation of β-PbF₂ nanocrystals in the glass matrix confirmed by X-ray diffraction. Optical absorption and photoluminescence measurements were performed on as-prepared glass and glass-ceramics. The luminescence of Er³⁺ ions in transparent glass-ceramics revealed sub-band splitting generally seen in a crystal host. The intensity of red and near infrared luminescence significantly increased in transparent glass-ceramic compared to that in as-prepared glass. Two luminescence bands at 758 nm from ⁴F_7/2 → ⁴I_13/2 and at 817 nm from ²H_11/2 → ⁴I_13/2 transitions were observed from transparent glass-ceramic but cannot be seen from the corresponding as-prepared glass. These results are attributed to the change of ligand field of Er³⁺ ions and the decrease of effective phonon energy when Er³⁺ ions were incorporated into the precipitated β-PbF₂ nanocrystals.     

Intense luminescence of amorphous Eu2O3 prepared by aqueous sol–gel method

Amorphous Eu₂O₃ was prepared by an aqueous sol–gel method. Emission due to the ⁵D₀ → ⁷F_J (J = 0, 1, 2) transitions of Eu³⁺ ions were observed. The dominant transition was the ⁵D₀ → ⁷F₂ red emission of Eu³⁺. The properties of the as-prepared samples were different with changes in the annealing temperature. To investigate the luminescence properties of the amorphous Eu₂O₃, the temperature-dependent photoluminescence (PL) spectra of samples annealed at 600 °C were measured in the temperature range 77–300 K. PL peak positions were unchanged with the change of temperature.     

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.     

Role of nickel ion coordination on spectroscopic and dielectric properties of ZnF2-As2O3-TeO2:NiO glass system

20ZnF₂-30As₂O₃-(50 − x)TeO₂:xNiO (0 ≤ x ≤ 2.0) glasses were synthesized. The glasses were characterized by X-ray diffraction, scanning electron microscopy, EDS and DSC techniques. A variety of properties, i.e. optical absorption, infrared, magnetic susceptibilities and dielectric properties (constant ?′, loss tan δ, a.c. conductivity σ_ac over a wide range of frequency and temperature) of these glasses have been carried out. The analysis of results of all these studies has indicated that the nickel ions occupy both octahedral and tetrahedral positions and the gradual increase of NiO content in the glass matrix causes growing proportions of Ni²⁺ ions that occupy octahedral positions. The luminescence spectra of these glasses have exhibited a broad emission band in region 1200-1450 nm identified due to ³T₂(3F) → ³A₂(3F) octahedral transition of Ni²⁺ ions. The luminescence efficiency and cross section have been found to be the highest for the glass containing highest concentration of NiO. Finally it is concluded that the higher the concentration of octahedrally positioned Ni²⁺ ions, the higher is the luminescence efficiency.     

Enhanced luminescent properties of Tb3+ ions in transparent glass ceramics containing BaGdF5 nanocrystals

Tb³⁺ doped transparent oxyfluoride glass ceramics containing BaGdF₅ nanocrystals were prepared. The transmission spectra, photoluminescence spectra, decay time and X-ray excited luminescence spectra of the Tb³⁺-doped glass and glass ceramics were investigated. Energy transfer from Gd³⁺ to Tb³⁺ has been observed in the glass and glass ceramics. The emission intensity of green band (⁵D₄ → ⁷F_J) of the Tb³⁺-doped glass ceramics is enhanced compared with that of the glass under ultraviolet and X-rays, which could be attributed to that the generation of BaGdF₅ nanocrystals with low phonon energy reduces the non-radiative transitions.     

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.     

Energy transfer between Tm and Er ions in a TeO2-based glass pumped at diode laser wavelength

In this paper we investigate the energy transfer processes in Tm³⁺/Er³⁺ doped telluride glass pumped at the commercial diode laser pump wavelength ∼800 nm. Tailoring the rare-earths content in the glass matrix, seven main energy transfer channels within the doping range considered were identified. A 6-fold enhancement of the Er³⁺ visible frequency upconversion fluorescence at ∼660 nm is observed due to the inclusion of Tm³⁺ ions. This is evidence of the relevant contribution of the route Er₁(⁴I_11/2) + Er₂(⁴I_13/2) → Er₁(⁴I_15/2) + Er₂(⁴F_9/2) to the process. Energy migration among pumped ⁴I_9/2 level reducing the efficiency of the upconversion emission rate (³H_11/2, ⁴S_3/2, and ⁴F_9/2) is observed for Er³⁺ above 1.5 wt%. The rate equations regarding the observed energy transfer routes are determined and a qualitative analysis of the observed processes is reported.     

Luminescence of borogermanate glasses activated by Er and Yb ions

Glasses of the 25Ln₂O₃-25B₂O₃-50GeO₂ composition (mol%) where Ln = (1 − x − y) La, xEr, yYb, with an addition of Al₂O₃ have been obtained and their luminescent characteristics examined. Probabilities of spontaneous emission, peak sections of the induced radiation and quantum yields of luminescence corresponding to the ²F_5/2 → ²F_7/2 transition of Yb³⁺ ions and the ⁴I_13/2 → ⁴I_15/2 transition of Er³⁺ ions have been defined. Quantum yield of Yb³⁺ luminescence for glasses with low Yb₂O₃ concentration reaches values closed to 100%. The luminescence spectrum of Er³⁺ ions exhibits a broad peak at about 1530 nm with effective width more than 80 nm when excited by irradiation at λ = 977 nm. Spontaneous emission probability and peak stimulated radiation section for Er³⁺ luminescence band ⁴I_13/2 → ⁴I_15/2 were determined to be equal to 175 s⁻¹ and 4.9 × 10⁻²¹ cm² respectively. Effective quenching of both rare-earth activators by oscillations with ν ≈ 2630 and 2270 cm⁻¹ was found. These oscillators, most likely, represent OH-groups connected by a hydrogen bond with non-bridging oxygen atoms in the borogermanate matrix.     

The effect of PbF2 content on the microstructure and upconversion luminescence of Er-doped SiO2-PbF2-PbO glass ceramics

The Er³⁺ doped transparent oxyfluoride glass ceramics were obtained by appropriate heat treatment of the precursor glasses with composition (mol%) 50SiO₂-xPbF₂-(50 − x)PbO-0.5ErF₃. The microstructure and optical properties of the glasses and glass ceramics were determined by differential scanning calorimetry (DSC), X-ray diffraction (XRD), absorption spectra and luminescence spectra. The intensity of upconversion luminescence significantly increased in glass ceramics compared to that in precursor glass. The emission bands centered around 660 nm (⁴F_9/2 → ⁴I_15/2) and 410 nm (²H_9/2 → ⁴I_15/2) were simultaneously observed in glass ceramics but cannot be seen in the corresponding precursor glass. The influence of different PbF₂ content on the microstructure and upconversion luminescence of the samples was analyzed in detail. The results indicated that with the increase of PbF₂ content, the Ω₂ was almost the same and the ratios of red to green upconversion luminescence decreased in glass ceramics.     

Spectroscopic studies of lead arsenate glasses doped with nickel oxide

Lead arsenate glasses containing different concentrations of NiO ranging from 0 to 1 mol% (in steps of 0.2 mol%) were prepared. The samples were characterized by X-ray diffraction and differential thermal analysis. A number of studies viz., optical absorption, thermoluminescence, magnetic susceptibilities and IR spectra, have been carried out on these glasses. The bands observed in the optical absorption spectra of the glasses have been analyzed using Tanabe-Sugano diagrams for d⁸ ion; the analysis indicates the presence of Ni²⁺ ions in both tetrahedral and octahedral positions. The increase in the concentration of NiO in the glass matrix shows a gradual transformation of nickel ions from tetrahedral positions to octahedral positions in the glass network. The thermoluminescence light output of the X-ray irradiated glasses has exhibited a glow peak at 353 K (with increasing intensity with an increase in the concentration of NiO) in addition to the conventional peak due to the recombination of electron-hole centers; this glow peak is identified due to ³T₂ → ³A₂ emission transition of octahedral Ni²⁺ ions. The value of the magnetic moment evaluated from the measured magnetic susceptibility show a decreasing trend from 3.84 to 3.10 μ_B with the increase in the concentration of NiO. All these studies indicate an increasing presence of octahedral (lasing) Ni²⁺ ions in these glasses with the increase in the concentration of NiO.