J.J. Velázquez

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.

Oxyfluoride glass–ceramic fibers doped with Nd3+: structural and optical characterization

Transparent oxyfluoride glass–ceramic fibers containing LaF3 nanocrystals have been drawn using a single crucible method and crystallization after an appropriate heat treatment. Optical fibers have been obtained a posteriori through the deposition of SiO2 cladding prepared by sol–gel and deposited by dip-coating. Detailed thermal and structural characterization performed by DTA, XRD, HRTEM and SAXS showed the good reproducibility of the technology. Phase separation, due to fluorine immiscibility in an oxide glass matrix, initiates the crystallization. The crystallization mechanism is a diffusion-controlled process and the local compositional changes of the glass matrix around the nanocrystals limit the crystal size to 10–20 nm depending on the treatment conditions. The optical characterization demonstrated the light propagation into the glass–ceramic core and the possibility to selectively excite Nd3+ ions in the fluoride nanocrystals with a corresponding increase of the luminescence efficiency.

Site-resolved emission of Nd3+-doped oxyfluoride nano glass-ceramics

Transparent oxyfluoride glass-ceramics obtained by the adequate heat treatment of Nd3+-doped glass with composition SiO2-Al2O3-Na2O-LaF3 are investigated. X-ray diffraction (XRD) and high resolution transmission electron microscopy (HR-TEM) show that the precipitated nanocrystals are LaF3 with a crystal size between 9-12 nm. Furthermore, energy dispersive X-ray (EDX) analysis shows the incorporation of Nd3+ ions into the LaF3 nanocrystals. Site-selective and time-resolved emission and excitation spectra of the 4F3/2 and 4F5/2 states, allows to unambiguously isolate the emission of Nd3+ ions in LaF3 nanocrystals which shows well defined spectra, similar to those obtained for pure LaF3 crystal.

Chapter 16 Performance of Nd3+ As Structural Probe of Rare-Earth Distribution in Transparent Nanostructured Glass-Ceramics

Nanostructured glass-ceramics obtained by the adequate heat treatment of a Nd3+-doped glass with composition SiO2-Al2O3-Na2O-LaF3 have been investigated by several techniques. X-ray diffraction (XRD) and high resolution transmission electron microscopy (HR-TEM) show that the precipitated nanocrystals are LaF3 with a crystal size between 9 and 13 nm. Furthermore, energy dispersive X-ray (EDX) analysis shows the incorporation of Nd3+ ions into the LaF3 nano-crystals. A detailed optical characterization clearly shows that Nd3+ ions in glass-ceramics are incorporated in both crystalline and amorphous phases. Steady-state and time-resolved site-selective laser spectroscopies allow to isolate unambiguously the emission of Nd3+ ions in LaF3 nanocrystals which shows extremely well defined spectra, similar to those obtained for pure LaF3 crystal.

Transparent SiO2-GdF3 sol–gel nano-glass ceramics for optical applications

AbstractTransparent oxyfluoride nano-glass-ceramics (GCs) containing GdF3 nanocrystals undoped and doped with 0.5 Eu3+ (mol%) were obtained by a novel sol–gel method after sintering at temperatures such low as 550 °C. X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM) show the precipitation of GdF3 nanocrystals with size between 7 and 10 nm, depending on the crystalline phase (hexagonal or orthorhombic) and the heating time. Fourier transform infrared spectroscopy (FTIR) analysis allows following the system evolution during the heat treatment showing the decomposition of trifluoroacetic acid (TFA), used as fluorine precursor, together with the formation of fluoride lattice bonding. Energy dispersive X-ray (EDX) analysis confirms the incorporation of the RE ions in the fluoride nanocrystals in the GCs. The ions incorporation on the GdF3 crystals is also supported by optical characterisation. Photoluminescence measurements result in a well resolved structure together with a narrowing of the Eu3+ emission and excitation spectra in the GCs compared to the xerogel. Moreover, the asymmetry ratio between the electric dipole transition (5D0→7F2) to the magnetic dipole transition (5D0→7F1) is reduced in GCs, indicating that Eu3+ ions are incorporated in the GdF3 crystalline phases. Moreover, Gd3+→Eu3+ energy transfer with enhancement of the energy transfer efficiency was observed in the GCs, further supported by fluorescence decay curves. HighlightsEu3+ doped SiO2-GdF3 GCs with 20 mol% of crystalline phase has been successfully obtained by sol–gel method.The use of methyl triethoxysilane allows obtaining crack-free GCs samples and reduces the hydroxyl groups.Energy transfer with enhancement of efficiency was observed from Gd3+ to Eu3+ in the nanocrystals.

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

Transparent oxyfluoride glass-ceramics (GCs) were prepared starting from the glass composition 70SiO2–7Al2O3–16K2O–7LaF3 (mol%) doped with four different Nd3+ concentrations (0.1–2 mol%). The glass-ceramics were prepared by heat treatment above the corresponding glass transition temperatures (Tg + 10–100 °C). Depending on the Nd3+ concentration, cubic (α-phase) and hexagonal (β-phase) KLaF4 nanocrystals (NCs) precipitated from the starting glass. By X-ray diffraction (XRD) and high resolution transmission electron microscopy (HR-TEM), the crystallization of the cubic structure was observed for all compositions, while β-KLaF4 nanocrystals were mostly detected for the glass-ceramic samples doped with 0.5 mol% NdF3. This noteworthy result is confirmed by site-selective laser spectroscopy. The presence of α-KLaF4 and β-KLaF4 crystalline phases is unambiguously identified from the emission and excitation spectra and lifetime measurements of the 4F3/2 state of Nd3+ ions. The spectroscopic results confirm those obtained with XRD and HR-TEM techniques: α-KLaF4 NCs are found to be present for all NdF3 concentrations whereas β-KLaF4 NCs are predominant in the glass-ceramic samples doped with 0.5 mol%.

Site-selective luminescence of Nd 3+ doped transparent oxyfluoride nano glass-ceramics

In this work, transparent oxyfluoride glass-ceramics obtained by the adequate heat treatment of Nd³⁺-doped glass with composition SiO<inf>2</inf>-Al<inf>2</inf>O<inf>3</inf>-Na<inf>2</inf>O-LaF<inf>3</inf> are investigated. X-ray diffraction (XRD) and high resolution transmission electron microscopy (HR-TEM) show that the precipitated nanocrystals are LaF<inf>3</inf> with a crystal size between 9 – 12 nm. Site-selective and time-resolved emission and excitation spectra of the ⁴F<inf>3/2</inf> and ⁴F<inf>5/2</inf> states, allow to unambiguously isolate the emission of Nd³⁺ ions in LaF<inf>3</inf> nanocrystals which shows extremely well defined spectra, similar to those obtained for pure LaF<inf>3</inf> crystal. As the Nd³⁺ concentration is increased beyond 0.1 mol%, the luminescence quenching of lifetime for Nd³⁺ ions inside LaF<inf>3</inf> crystals is observed to be stronger than for those dispersed in the glass matrix.

Spectroscopic probe of rare-earth distribution in transparent oxyfluoride glass-ceramics

The growing field of photonics demands the design of new rare-earth (RE)-based optical materials for their use in optical telecommunications, solid state lasers, three-dimensional full-color displays, solar cells, and others [1]. Oxyfluoride glass-ceramics are promising RE hosting candidates since they combine the good chemical and mechanical stability of oxide glass matrixes with the good optical properties of low phonon energy fluoride nano-crystals [2].