Oxyfluoride Glass Ceramics Research Articles

Enhanced luminescent properties of Tb3+-doped transparent oxyfluoride glass-ceramics containing YF3 nanocrystals

Tb3+-doped transparent oxyfluoride glass-ceramics (GC) containing YF3 nanocrystals have been fabricated via a traditional melt-quenching route with appropriate thermal-treatment. The microstructural, luminescent and optical properties of the synthesized material were investigated systematically. The precipitated particles in the glass phase of GC540 are YF3 nanocrystals with a mean size of around 20 nm, which has been confirmed and estimated by XRD and TEM. Compared with the precursor glass (PG), the emission intensity at 545 nm (5D4→7F5) of YF3: Tb3+-doped GC is significantly enhanced (under ultraviolet and X-rays excitation) and the lifetime of GC is longer. It can be confirmed that the preferential enrichment of Tb3+ in the precipitated low-phonon-energy YF3 nanocrystals can effectively reduce non-radiative transitions. The obtained results confirm that such Tb3+-doped YF3 GC might be useful as a novel X-ray scintillating material for slow event detection.

Structure and spectroscopic properties of Tb3+/Sm3+ co-doped oxyfluoride glass ceramics containing LiYF4 nanocrystals

In this paper, transparent oxyfluoride glass ceramics containing LiYF4 nanocrystals were successfully synthesized by appropriate heat-treatment on the SiO2-Al2O3-YF3-LiF precursor glass. The average diameter of the LiYF4 nanocrystals is about 16 nm, which has been confirmed by XRD and TEM characterization. Compared with RE3+ (RE = Tb, Sm)-doped glasses, glass ceramics have shown stronger emission intensities due to most rare earth ions have entered LiYF4 nanocrystals with lower phonon energy. A combination of blue, green and orange-red emissions has emerged in Tb3+/Sm3+ co-doped glass ceramics, and white light emission could be realized by varying the proportions of Tb3+ and Sm3+ under UV light excitation. Furthermore, the concentration quenching effect and the Tb3+→Sm3+ energy transfer process were investigated in this paper. Our results indicate that the Tb3+/Sm3+ co-doped oxyfluoride glass-ceramics containing LiYF4 nanocrystals show great potential to achieve a white light emission.

Effect of annealing temperatures on luminescence properties in Ce3+-Yb3+ codoped oxyfluoride glass-ceramics for solar cells

An efficient near-infrared (NIR) down-conversion in Ce3+-Yb3+ codoped oxyfluoride glass ceramics (GC) have been investigated with different annealing temperature. With an 430 nm (Ce3+: 4f → 5d) excitation, the NIR emission at around 1000 nm (Yb3+: 2F5/2 → 2F7/2) was obtained which confirming efficient energy transfer between Ce3+ ions and Yb3+ ions. Subsequently, the possible mechanism of energy transfer was investigated and discussed. Finally, the energy conversion efficiency was measured which the Si solar cell covered by Ce3+-Yb3+ codoped oxyfluoride GC. The maximum efficiency which increased by 0.3% indicated that the Ce3+/Yb3+ sample had the potential applications for solar cells.

Crystallization mechanism in sol-gel oxyfluoride glass-ceramics

The crystallization mechanism of LaF3 in sol-gel oxyfluoride glass-ceramics in the system SiO2-LaF3 has been studied using DTA, XRD, FTIR, HRTEM and NMR. Based on classical calculations of non-isothermal DTA curves involving Ozawas's plots, a diffusion-controlled process should be responsible for the fluorides crystallization similarly to that obtained for melt-quenching oxyfluoride glass-ceramics. Therefore, an increase of the crystal size and fraction is expected when increasing the time and/or temperature of the treatment. Nevertheless, XRD, FIR, HRTEM and NMR demonstrated that neither bigger crystals nor higher crystal fraction are obtained for treatment times longer than 1 min at 550 °C. However, further heat treatment causes that the structural order of the crystals is lost. On the other hand, typical fluorides crystallization temperatures (~300 °C) in sol-gel materials are much lower than Tg of the matrix (~1130 °C) thus being very different to what observed for oxyfluoride glasses prepared by melt-quenching. The proposed crystallization mechanism is a fast process that occurs when crystallization temperature is reached. This behavior is explained as a chemical reaction from the xerogels, followed by the fast precipitation of the nanocrystals without energy barrier. The nanocrystals are unstable in the surrounding SiO2 matrix for aging at crystallization or higher temperatures. These treatments lead to a dissolution process, favored by the compositional gradient between matrix and crystals, which drive the material to reach the equilibrium.

Photonic properties of novel Yb3+ doped germanium-lead oxyfluoride glass-ceramics for laser cooling applications

In recent years, our research group has developed and studied new rare-earth doped materials for the promising technology of solid-state laser cooling, which is based on anti-stokes fluorescence. To the best of our knowledge, our group is the only one in Canada leading the research into the properties of nanoparticles, glasses and glass-ceramics for optical refrigeration applications. In the present work, optical properties of 50GeO2-30PbF2-18PbO-2YbF3 glass-ceramics for laser cooling are presented and discussed as a function of crystallization temperature. Spectroscopic results show that samples have near infrared photoluminescence emission due to the 2F5/2–2F7/2 Yb3+ transition, centered at ~1016 nm with an excitation wavelength of 920 nm or 1011 nm, and the highest photoluminescence emission efficiency occurs for heat-treatment for 5 h at 350°C. The internal photoluminescence quantum yield varies between 99% and 80%, depending on the temperature of heat-treatment, being the most efficient under 1011 nm excitation. The 2F5/2 lifetime increases from 1.472 to 1.970 ms for heat treatments at 330°C to 350°C, respectively, due to energy trapping and the low phonon energy of the nanocrystals. The sample temperature dependence was measured with a fiber Bragg grating sensor, as a function of input pump laser wavelength and processing temperature. These measurements show that the heating process approaches near zero for an excitation wavelength between 1020 and 1030 nm, which is an indication that phonons are removed effectivelly from the glass-ceramic materials, and they can be used for optical laser cooling applications. On the other hand, the temperature increase as a function of input laser power into samples remains constant between 920 and 980 nm wavelength excitation, a temperature variation of 36 K/W (temperature of 58°C/W) was attained under excitation at 950 nm, showing a possible use for biomedical applications to be explored.1)

Spectral conversion from green to red in Yb3+/Ho3+:Sr2GdF7 glass ceramics via Ce3+ doping

Ce3+ co-doped Sr2GdF7:Yb3+/Ho3+ transparent oxyfluoride glass ceramics were successfully fabricated through melt-quenching technique, which were further confirmed by intense diffraction peaks of X-ray diffraction, transmission electron microscopy, selected area electron diffraction, obvious Stark splitting of emission spectra and prolonged lifetimes of Ho3+ after crystallization. Under 980 nm excitation, green upconversion emission is dominant in Sr2GdF7:Yb3+/Ho3+ glass ceramics and is about 10,000 times stronger than that in precursor glass. Furthermore, upconversion emission tuning from green to red in Sr2GdF7:Yb3+/Ho3+ glass ceramics was obtained via codoping Ce3+ ions with different concentrations. Impressively, red to green fluorescence intensity ratio increases from 0.1 to 91, which is caused by two efficient cross-relaxation processes derived from Ho3+ and Ce3+ ions. The current study on the multicolor tuning behavior of Ce3+ doped Sr2GdF7:Yb3+/Ho3+ glass ceramics offers a potential application in color displays.

Comprehensive studies of the Ag+ effect on borosilicate glass ceramics containing Ag nanoparticles and Er-doped hexagonal NaYF4 nanocrystals: morphology, structure, and 2.7 μm emission

AbstractIn this work, we have performed a comprehensive investigation of the Ag+ concentration effect on the morphological, thermal, structural, and mid-infrared emission properties of novel oxyfluoride borosilicate glasses and glass ceramics containing both Ag nanoparticles and erbium-doped hexagonal NaYF4 nanocrystals. The effect of Ag+ ions on the glass forming and crystallization processes was discussed in detail by glass structural analysis. It was found that the Ag+ concentration can affect the distribution of Na+ ion and bridge oxygen in boron-rich and silicon-rich phases, which induced the transformation between BO3 triangles and BO4 tetrahedra during crystallization process. In addition, there was a turning point when the doped Ag+ ion concentration reached its solubility in the borosilicate glass. Furthermore, the enhancement of the 2.7 μm emission and the reduction of the lifetime of the 4I13/2 level were observed both in glasses and in glass ceramics, and its origin was revealed by qualitative and quantitative analyses of the Er3+-Ag nanoparticles (localized electric field enhancement) and Er3+-Er3+ (nonradiative resonance energy transfer) interactions within glasses and glass ceramics. Moreover, the high lifetime of the 4I11/2 level (2.12 ms) and the peak emission cross section in 2.7 μm (6.8×10−21 cm2) suggested that the prepared glass ceramics have promising mid-infrared laser applications.

Luminescence of Er3+ doped oxyfluoride phosphate glasses and glass-ceramics

Glasses with the composition (75 NaPO3-(25-x) CaO-xCaF2) (in mol %) were prepared with 0.15 mol% of Er2O3. The effect of the glass composition and of heat treatment on the spectroscopic properties of the newly developed glasses is reported. With the progressive replacement of CaO by CaF2, the Er3+:4I13/2 lifetime and the intensity of the upconversion emission increase whereas the intensity of the emission at 1.5 μm decreases due to the decrease in the phonon energy in the as-prepared glasses. The glasses were heat treated at 20 °C above their respective glass transition temperature for 17 h to form nuclei and then at their crystallization temperature from 15min to 1 h to grow the nuclei into crystals. The heat treatment leads to the precipitation of crystalline phases, the composition of which depends upon the glass composition. As the Er3+:4I13/2 lifetime increases and the intensity of the upconversion increases for the glass with x = 0 after heat treatment, the Er3+ ions are expected to be incorporated into the phosphate-based crystals. However, as the shape of the emission band at 1.5 μm remains unchanged and the intensity of the upconversion decreases significantly after heat treatment of the glasses with x > 10, the crystals found in the glass-ceramics with x > 10 are thought to free of Er3+ ions. Although Er3+ ions entered in the CaF2 crystals precipitating in aluminosilicate glass, the Er3+ ions are believed to remain in the amorphous phosphate part of the glass-ceramic containing CaF2 crystals.

Crystalline phase detection in glass ceramics by EPR spectroscopy

The advances of EPR spectroscopy for the detection of activators as well as determining their local structure in the crystalline phase of glass ceramics is considered. The feasibility of d-element (Mn2+, Cu2+) and f-element (Gd3+, Eu2+) ion probes for the investigation of glass ceramics is discussed. In the case of Mn2+, the information is obtained from the EPR spectrum superhyperfine structure, for Gd3+ and Eu2+ probes—from the EPR spectrum fine structure, whereas for Cu2+ ions the changes in the EPR spectrum shape could be useful. The examples of EPR spectra of the above-mentioned probes in oxyfluoride glass ceramics are illustrated.

Optical Detection of Paramagnetic Centres in Activated Oxyfluoride Glass-Ceramics

Optical Detection of Paramagnetic Centres in Activated Oxyfluoride Glass-Ceramics

Optical Detection of Paramagnetic Centres in Activated Oxyfluoride Glass-Ceramics

Optical Detection of Paramagnetic Centres in Activated Oxyfluoride Glass-Ceramics

Broadening and enhancing 27 μm emission spectra in Er/Ho co-doped oxyfluoride germanosilicate glass ceramics by imparting multiple local structures to rare earth ions

Er/Ho co-doped oxyfluoride germanosilicate glass and glass ceramics are prepared and compared. The results indicate that the glass consists of SiO4 and GeO4 structural units, while the network of the glass ceramics consists of SiO4, GeO4, and GeO6 units together with NaYF4 nanocrystals. The presence of multiple local structures in glass ceramics creates a range of dipole environments, which is beneficial to the broadening of 2.7 μm emission. Two other reasons are attributed to the broadening of 2.7 μm emission in glass ceramics: the energy-level splitting of Er3+ and the enhancement of the Ho3+:I65→I75 transition in NaYF4 nanocrystals.

A comparative investigation on upconversion luminescence in glass–ceramics containing LaF3 and CaF2 nanocrystals

Greater enhancement in upconversion (UC) luminescence was observed in the glass–ceramics (GCs) containing CaF2:Yb3+, Er3+ nanocrystals (NCs) compared with whose doped with LaF3:Yb3+, Er3+ NCs prepared by conventional melt-quenching technique followed by successive heat treatment. Our joint experimental and theoretical studies suggest the lager enhancement of UC emission in CaF2–GCs rather than of LaF3–GCs is associated with the difference in the microstructures, the crystal size, the local coordination environment around rare earth (RE) ions as well as the structural rigidity in the silicate region of the GCs. In addition, it was found that a more symmetric ligand field around RE ions in a smaller size crystal which distributed more homogeneously within the highest ordering silicate region favors stronger UC emission by facilitating the energy transfer process and minimizing the rate of non-radiative relaxation process. Our results may enrich our understanding about UC process in oxyfluoride GCs and provide a general picture for the photophysical processes that contribute to the UC in similar GCs systems.

Gd3+ doping induced enhanced upconversion luminescence in Er3+/Yb3+ co-doped transparent oxyfluoride glass ceramics containing NaYF4 nanocrystals

In this article, transparent oxyfluoride glass ceramics containing β-NaYF4 nanocrystals were successfully prepared via Gd3+ doping. Compared to conventional non-doped glasses, the thermal treatment temperature required for the precipitation of β-NaYF4 nanocrystals can be lowered with the doping of Gd3+. Furthermore, under the same thermal treatment condition, more β-NaYF4 nanocrystals were precipitated in Gd3+ doped ones, which greatly improves the luminescence efficiency of rare earth doped glass ceramics. Possible mechanism for the Gd3+ doping induced enhanced upconversion luminescence phenomenon was proposed, based on thorough structural and optical characterizations. The results revealed that the doping of Gd3+ ions could decrease the crystallization activation energy and promote the formation of Y-F-Na bonding, which helps the precipitation of β-NaYF4 nanocrystals. Consequently, a large enhancement in upconversion luminescence was achieved. Moreover, the strategy can be successfully applied to the development of other glass ceramic systems for various optoelectronic applications.

Study of spectroscopic and thermal characterization of TeO2-Ga2O3 oxyfluoride (TGOF) glasses and glass-ceramics activated by Pr3+ ions. The role of phonons

The glasses based on TeO2-Ga2O3 system as a promising group of heavy metal oxide glasses have been subject of great attention in the aspect of optoelectronics applications because of good transparency in the mid-infrared up to 6–7 µm and the possibility to incorporate a large amount of RE ions. The paper presents thermal stability, the UV–VIS and IR transmission and luminescence spectroscopy of the gallium-tellurite oxyfluoride glass. The base gallium-tellurite glass 0.74TeO2·0.15Ga2O3 was modified by addition of 11 mol% BaF2 and 1 mol% Pr6O11. DTA/DSC data showed that the glasses are characterized by the multi-stage crystallization. The luminescence spectra have been registered at various temperatures and resulting characteristics allowed to draw conclusions on the role of phonons in their evolution at different temperatures. To interpretations a kinetic analysis and the Bose-Einstein statistics were employed.

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.

Five-fold twinned β-PbF2 nanocrystals in oxyfluoride glass ceramics.

Oxyfluoride glass ceramics (GCs) doped with trivalent lanthanide ions (Ln3+) have been prepared using a conventional melting–quenching method and studied by X-ray diffraction (XRD). β-PbF2 nanocrystals (NCs) doped with Ln3+ ions (β-PbF2:Ln3+) in GCs were released from the GCs by etching off the glass matrix. β-PbF2:Ln3+ NCs can be clearly observed by eliminating the influence of the glass matrix. The nanotwinned structures of β-PbF2:Ln3+ NCs, including two-fold twinned NCs and five-fold twinned NCs, were examined using high-resolution transmission electron microscopy (HRTEM). The five-fold twinned phenomenon in metal fluorides with strong ionicity in oxyfluoride GCs is reported for the first time. Based on detailed analysis of the twinned NC structure, the twinning mechanism of β-PbF2:Ln3+ NCs was proposed. Ln3+ ions and ‘sublattice melting’ of fluorine ions (F−) in β-PbF2 play extremely important roles in the formation of five-fold twinned β-PbF2:Ln3+ NCs. The nanotwinned structures reported here may have far-reaching significance with respect to the further application of oxyfluoride glass ceramics doped with rare-earth elements and NC fabrication.

Phase evolution of KLaF4 nanocrystals and their effects on the photoluminescence of Nd3+ doped transparent oxyfluoride glass-ceramics

Quantity of cubic (α-phase) and hexagonal (β-phase) KLaF4 nanocrystals (NCs) tailored by doping.

Upconversion luminescence of Er3 +/Yb3 + and their role in the stabilization of cubic NaLaF4 nanocrystals in transparent oxyfluoride glass ceramics

Novel Er3+ and Yb3+ doped transparent oxyfluoride glass ceramics containing NaLaF4 nanocrystals were prepared from oxyfluoride glasses with general composition of Na2O-NaF-LaF3-Al2O3-SiO2. The crystallization processes, changes in morphology and upconversion luminescence were analyzed. The heat treatment of the precursor glasses promoted the formation of hexagonal NaLaF4 and an unidentified fluorite type compound. An enhancement of upconversion luminescence with the introduction of YbF3 in the glass ceramics was detected and indicated an efficient energy transfer from Yb3+ to Er3+. The local environment of Er3+ in hexagonal NaLaF4 and the fluorite phase was analyzed using site-selective spectroscopy and compared with polycrystalline ceramics. The incorporation of Er3+ ions in three non-equivalent cationic sites in the crystalline lattice of hexagonal NaLaF4 nanocrystals was observed. The local environment of erbium ions in the fluorite phase was found to be similar to α-NaYF4 suggesting the formation of Er3+ and Yb3+ stabilized α-NaLaF4.

Luminescence and energy transfer in Dy3+/Eu3+ co-doped aluminosilicate oxyfluoride glasses and glass-ceramics

A series of oxyfluoride glasses with the composition SiO2–CaF2–Al2O3–CaO, doped with Dy2O3 (0.5–1 mol%) and Eu2O3 (0–4 mol%), have been prepared using the melt quenching method. The glass-ceramics have been obtained by heating the as-made glasses at 680 °C and 750 °C for 1 h. The glasses and the glass-ceramics were investigated using photoluminescence techniques in the near-UV and visible ranges, and differential thermal analysis (DTA) and x-ray diffraction (XRD) measurements were performed. The samples doped with 0.5 mol% Dy3+ were observed to have the highest luminescence intensity under 453 nm excitation compared to the samples doped with 1 mol% Dy3+ and the co-doped samples. The increased luminescence intensity could be caused by energy transfer between rare-earth (RE) ions and cross-relaxation of Dy3+ ions. Energy transfer from Dy3+ to Eu3+ ions in the glasses and the glass-ceramics was confirmed by excitation spectra and luminescence decay kinetics. The average luminescence lifetimes of the Dy3+ emission band at 575 nm under 453 nm excitation decrease with the addition of Eu3+ ions. Energy transfer in the glasses and the glass-ceramics was estimated to be in the range from 11% to 68%, depending on the RE ion concentration.