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

Abstract

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.

Keywords: nanocrystal; optical properties; oxyfluoride glass-ceramics; sol-gel.

Figure 1

TGA/DTA curve of the MgF₂ gel obtained by drying the sol at 80 °C. Figure modified from Figure 4 of [22].

Figure 2

PL spectra recorded on Eu³⁺-doped xerogel (dotted curve) and GC (solid curve) using λex. = 394 nm [35].

Figure 3

(a) XRD patterns of 95SiO₂-5LaF₃ GCs at different temperatures and (b) CIR standard chromaticity showing the up-conversion emission for GCs co-doped with 0.3Yb³⁺, 0.1Ho³⁺ and 0.1 Tm³⁺ [53].

Figure 4

(a) XRD patterns of 90SiO₂-10YF₃ Ln³⁺-doped GCs heat treated at 650 and 675 °C together with the JCPDS-YF₃ and (b) Up-conversion emission spectrum of GCs co-doped with 1.5 Yb³⁺ and 0.1 Tm³⁺ heat-treated at 675 °C [60].

Figure 5

HRTEM image of Eu³⁺ doped SiO₂-GdF₃ GC film heat = treated at 400 °C [64].

Figure 6

Up-conversion emission spectra of 95SiO₂-5KYF₄ GCs co-doped with Yb³⁺-Er³⁺-Tm³⁺ heat-treated at 700 °C [76].

Figure 7

(a) TEM and (b) HRTEM images of 95SiO₂-5BaGd_(1−x)Eu_xF₅. Inset show power spectrum (FFT pattern) and filtered higher-contrasted red and blue square nanoparticles [86].

Figure 8

(a) DTA in air of (100 − x)SiO₂-xLaF₃ (x = 10–40 mol %) bulk samples prepared with TEOS; (b) DTA and TG in argon atmosphere of 80SiO₂-20LaF₃ bulk samples prepared with TEOS; (c) DTA in air of 80SiO₂-20LaF₃ bulk samples with TEOS and TEOS/MTES. All measurements were performed using a heating rate of 10 °C/min.

Figure 9

XRD of 80SiO₂-20LaF₃ GC treated at 550 °C for 1 min performed at the synchrotron SpLine BM25B of the ESRF.

Figure 10

HRTEM of 80SiO₂-20LaF₃ (a) thin film and (b) self-supported layer prepared using TEOS/MTES and treated at 550 °C for 1 min. The corresponding crystal size distributions are also shown.

Figure 11

¹⁹F MAS-NMR spectra of 80SiO₂-20LaF₃ xerogel and GC treated at 550 °C for 1 min. The spectrum of pure LaF₃ crystal powder is also given for comparison. Stars indicate spinning sidebands.

Figure 12

FTIR of 80SiO₂-20LaF₃ xerogel and GC self-supported layer treated at 550 °C for 1 min and 1 h.

Figure 13

PL (a) emission and (b) excitation spectra of 80SiO₂-20LaF₃ bulk xerogel and GC treated at 650 °C for 3 h. All spectra were recorded at 9 K.

Figure 14

DTA (red) and TG (blue) curve of 90SiO₂-10NaGdF₄ bulk sample acquired in air using a heating rate of 10 °C/min.

Figure 15

XRD of (100 − x)SiO₂-xGdF₃ (x = 10 and 20 mol %) GC treated at 550 °C for 1 min performed at the synchrotron SpLine BM25 of the ESRF.

Figure 16

XRD of 80SiO₂-20NaGdF₄ GC treated at 600 °C for 1 and 120 h. The measurements were performed at the synchrotron SpLine BM25 of the ESRF.

Figure 17

HRTEM of 80SiO₂-20GdF₃ self-supported layer treated at 550 °C for 1 min.

Figure 18

FTIR of 80SiO₂-20GdF₃ xerogel and GC treated at 550 °C for 1 min.

Figure 19

XAS (a) spectra and (b) derivate of Eu₂O₃ and EuF₃ reference samples. The values shown in (b) refer to the maximum of the derivate curves.

Figure 20

XAS (a) spectra and (b) derivate of 0.5Eu³⁺-doped 80SiO₂-20LaF₃ xerogel and GC samples treated at 550 °C for 1 min up to 8 h. The values shown in (b) refer to the maximum of the derivate curves.

Figure 21

Emission spectra of Eu³⁺ and Gd³⁺ ions in xerogel (bottom) and GC (top) treated at 550 °C-1 min under excitation of Eu³⁺ at 393 nm (red spectra) and of Gd³⁺ at 273 nm (blue spectra).