Optical properties of silica sol-gel materials singly- and doubly-doped with Eu3+and Gd3+ ions

Optical Properties of Transparent Rare-Earth Doped Sol-Gel Derived Nano-Glass Ceramics

Rare-earth doped oxyfluoride glass ceramics represent a new generation of tailorable optical materials with high potential for optical-related applications such as optical amplifiers, optical waveguides, and white LEDs. Their key features are related to the high transparency and remarkable luminescence properties, while keeping the thermal and chemical advantages of oxide glasses. Sol-gel chemistry offers a flexible synthesis approach with several advantages, such as lower processing temperature, the ability to control the purity and homogeneity of the final materials on a molecular level, and the large compositional flexibility. The review will be focused on optical properties of sol-gel derived nano-glass ceramics related to the RE-doped luminescent nanocrystals (fluorides, chlorides, oxychlorides, etc.) such as photoluminescence, up-conversion luminescence, thermoluminescence and how these properties are influenced by their specific processing, mostly focusing on the findings from our group and similar ones in the literature, along with a discussion of perspectives, potential challenges, and future development directions.

Influence of Oxide Glass Modifiers on the Structural and Spectroscopic Properties of Phosphate Glasses for Visible and Near-Infrared Photonic Applications

The effect of oxide modifiers on multiple properties (structural and spectroscopic) of phosphate glasses with molar composition 60P₂O₅-(10-x)Ga₂O₃-30MO-xEu₂O₃ and 60P₂O₅-(10-y)Ga₂O₃-30MO-yEr₂O₃ (where M = Ca, Sr, Ba; x = 0, 0.5; y = 0, 1) were systematically examined and discussed. The local structure of systems was evidenced by the infrared (IR-ATR) and Raman spectroscopic techniques. The spectroscopic behaviors of the studied glass systems were determined based on analysis of recorded spectra (excitation and emission) as well as luminescence decay curves. Intense red and near-infrared emissions (1.5 μm) were observed for samples doped with Eu³⁺ and Er³⁺ ions, respectively. It was found that the value of fluorescence intensity ratio R/O related to ⁵D₀→⁷F₂ (red) and ⁵D₀→⁷F₁ (orange) transition of Eu³⁺ ions depends on the oxide modifiers MO in the glass host. However, no clear influence of glass modifiers on the luminescence linewidth (FWHM) was observed for phosphate systems doped with Er³⁺ ions. Moreover, the ⁵D₀ and ⁴I_13/2 luminescence lifetimes of Eu³⁺ and Er³⁺ ions increase with the increasing ionic radius of M²⁺ (M = Ca, Sr, Ba) in the host matrix. The obtained results suggest the applicability of the phosphate glasses with oxide modifiers as potential red and near-infrared photoluminescent materials in photonic devices.

Energy transfer phenomenon of Gd3+ to excited ground state of Eu3+ ions in Li2O-BaO-Gd2O3-SiO2-Eu2O3 glasses

Li₂O-BaO-Gd₂O₃-SiO₂ glasses with different concentration of Eu³⁺ ions were developed by the traditional melt quenching technique and characterized via FTIR, absorption, excitation, emission and CIE color coordinates analysis for visible red emission application. The FTIR shows strong band at position 740 cm^-1 which is attributed to Si-O-Si symmetric stretching mode. Density and molar volume of LBGSEu glasses increases with Eu³⁺ ions concentration. The covalent nature of bond between the Eu³⁺ ions and surrounding ligands was confirmed from the bonding parameter (δ). From absorption spectra JO-parameters and oscillator strength are evaluated for LBGSEu6 glass. From JO-parameters, Eu³⁺ ions have asymmetric coordination environment and stronger covalency. The phonon line PSB (22,522 cm^-1) confirm the phonon energy ≈971 cm^-1, that corresponds to the energy of one phonon associated with maximum energy of the vibrational mode couple to Eu³⁺ ions. Under 275 nm and 393 nm excitation, intense red emission was observed at 613 nm, we observe efficient energy transfer phenomena from Gd³⁺ → Eu³⁺ in these glasses. Increasing trend of I_R with increasing concentration of Eu₂O₃ indicates the asymmetric environment around Eu³⁺ ions in LBGS. Moreover, from JO analysis, LBGS glasses have high capability for red laser device with high lasing power and energy extraction ratio. The fluorescence lifetimes show decreasing trend in lifetime with increasing concentration of Eu₂O₃ is due to radiative transition. From CIE color coordinate, the CIE color coordinates of LBGSEu6 glass fall in reddish region close to orange region and can be useful for optical display devices.

Thermoluminescence properties of La3+ , Gd3+ ,Dy3+ and Ho3+ co-doped MgAl2 Si2 O8 :Mn4

Thermoluminescence properties of Mn⁴⁺ -doped and La³⁺ , Gd³⁺ , Dy³⁺ and Ho³⁺ co-doped MgAl₂ Si₂ O₈ host phosphors were investigated in detailed and reported for the first time. The phosphors were exposed to β- and α-irradiation and the glow curves appeared at 260-262 ± 2°C and 245-252 ± 2°C, respectively. Thermoluminescence analysis show that the main glow peaks actually consisted of three distinct peaks (P₁ , P₂ and P₃ ). In addition, all the phosphors had a main peak, and many satellite peaks also appeared up to 200°C. Moreover, it was observed that La³⁺ , Gd³⁺ , Dy³⁺ and Ho³⁺ dopants in MgAl₂ Si₂ O₈ Mn⁴⁺ phosphor can change the thermoluminescence peak intensities, but they did not cause any new glow peak. In addition, when α-irradiation was used, the glow peaks were shift to a lower temperature region. Kinetic parameters of P₁ , P₂ and P₃ were calculated, and it was found that P₁ and P₃ had first-order kinetics and P₂ had general-order kinetic parameters. In addition, it was found that when the easy and convenient annealing procedure was applied to the phosphorus, the thermoluminescence signal was measured as stable over many repetitions. The dose sensitivity, reproducibility and fading features of the powdered phosphors were investigated and computerized glow curve deconvolution analysis was also performed. Results showed that thermoluminescence properties of these materials were good but the fading reliabilities were poor and therefore the sintered or single crystal forms should be preferred for their better fading properties.

Transparent Glass-Ceramics Produced by Sol-Gel: A Suitable Alternative for Photonic Materials

Transparent glass-ceramics have shown interesting optical properties for several photonic applications. In particular, compositions based on oxide glass matrices with fluoride crystals embedded inside, known as oxyfluoride glass-ceramics, have gained increasing interest in the last few decades. Melt-quenching is still the most used method to prepare these materials but sol-gel has been indicated as a suitable alternative. Many papers have been published since the end of the 1990s, when these materials were prepared by sol-gel for the first time, thus a review of the achievements obtained so far is necessary. In the first part of this paper, a review of transparent sol-gel glass-ceramics is made focusing mainly on oxyfluoride compositions. Many interesting optical results have been obtained but very little innovation of synthesis and processing is found with respect to pioneering papers published 20 years ago. In the second part we describe the improvements in synthesis and processing obtained by the authors during the last five years. The main achievements are the preparation of oxyfluoride glass-ceramics with a much higher fluoride crystal fraction, at least double that reported up to now, and the first synthesis of NaGdF₄ glass-ceramics. Moreover, a new SiO₂ precursor was introduced in the synthesis, allowing for a reduction in the treatment temperature and favoring hydroxyl group removal. Interesting optical properties demonstrated the incorporation of dopant ions in the fluoride crystals, thus obtaining crystal-like spectra along with higher efficiencies with respect to xerogels, and hence demonstrating that these materials are a suitable alternative for photonic applications.