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

Abstract

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.

Keywords: fluorides; glass ceramic; luminescence; nanocrystals; rare-earth; sol-gel.

Figure 1

The XRD patterns of SiO₂–LaF₃ gel and glass ceramics obtained after calcination at different temperatures. (Reproduced from reference [5].)

Figure 2

TEM images of the SiO₂–YF₃ xerogel sample calcinated at 400 °C (left) showing the congeries (right) (reproduced from reference [12]).

Figure 3

Thermal analysis of Eu-doped SiO₂–BaF₂ xerogel showing the TG (red) and DSC (black) curves (reproduced from reference [8]).

Figure 4

Photoluminescence spectra recorded on Eu³⁺-doped xerogel (dotted curve) and glass ceramic (solid curve) using 394 nm excitation wavelength (modified from reference [19]); the inset shows the Eu³⁺ energy levels diagram.

Figure 5

Photoluminescence spectra of Eu (1%)-doped 95SiO₂–5BaF₂ nano-glass ceramic (prepared in ref. [8]) recorded under 394 and 464 nm excitation.

Figure 6

The green (²H_11/2, ⁴S_3/2 → ⁴I_15/2) and red (⁴F_9/2 → ⁴I_15/2) Er³⁺ up-conversion luminescences excited at 980 nm in various Yb³⁺/Er³⁺-doped oxyfluoride glass ceramics [32] and the energy level schemes of Yb³⁺ and Er³⁺ with the main energy transfer processes.

Figure 7

Thermoluminescence mechanism (left) (reproduced from reference [34]); and normalized glow curves recorded after X-ray irradiation at room temperature of Eu³⁺ doped SiO₂–BaF₂ dried xerogel and Eu³⁺ doped glass ceramic (prepared in ref. [8]) (right).

Figure 8

The PSB spectrum associated to the ⁵F₀ → ⁵D₂ transition of Eu³⁺-ion (at 461 nm), recorded in glass–ceramic (solid curve) and the fit with Gaussian-type curves (dashed curves); the abscissa is taken as the energy shift from the pure electronic transition (PET) peak ⁵F₀ → ⁵D₂ at 461 nm. (Modified from reference [15].)

Figure 9

Photoluminescence spectra recorded on SrF₂:Eu/Tb@SiO₂ glass ceramics after under 368 nm excitation wavelength; blue broad emission of SiO₂ is accompanied by the green light peaks of Tb³⁺ and red light peaks of Eu³⁺ (reproduced from reference [41]).

Figure 10

TL curves recorded on undoped (dotted line) and RE³⁺-doped glass ceramics (solid lines) after X-ray irradiation at room temperature (reproduced from reference [44]); the energy levels scheme of lanthanides in YPO₄ (reproduced from reference [34]).

Figure 11

Up-conversion emission spectra of Er³⁺-doped SiO₂–PbF₂ glass ceramic under 980 nm IR light pumping at different powers; the inset shows emission spectrum excited at 378 nm (reproduced from reference [47]).

Figure 12

Excitation spectra of Eu³⁺-doped LaF₃–SiO₂ glass ceramics recorded at indicated wavelengths; the spectra of 89.9SiO₂–10LaF₃–0.1EuF₃ glassy sample and SiO₂:Eu³⁺ sol–gel glass detected at 590 nm are also included (reproduced from reference [49]).

Figure 13

Up-conversion luminescence spectra of 95SiO₂–5LaF₃:0.1Er³⁺ glass ceramics recorded at room temperature under 980 nm IR light pumping with different pumping powers from 10 to 90mW (left) and the energy level schemes of Er³⁺ with the main energy transfer processes (right); reproduced from reference [52].

Figure 14

Photoluminescence spectra recorded in Eu³⁺ doped SiO₂–GdF₃ glass ceramic (left) and Tb³⁺ doped SiO₂–GdF₃ glass ceramic excited at Gd³⁺ or Eu³⁺/Tb³⁺ excitation wavelength peaks (reproduced from reference [63]).

Figure 15

X-ray diffraction patterns of the Yb/Er co-doped SiO₂–GdF₃ xerogel (curve a) and glass ceramics undoped (curve b) and Li (1%) co-doped (c) [65]; the PDF file of orthorhombic GdF₃ is shown for comparison.

Figure 16

Normalized emission spectra of the ⁴F_3/2 → ⁴I_11/2 transition obtained under excitation at 786 (black) and 792 nm (red) of the GC-0.95 sample doped with 0.1 mol% of Nd³⁺ corresponding to Nd³⁺ ions in LaF₃ and NaLaF₄ crystalline phases, respectively [68].

Figure 17

Emission spectra of 0.3Ce³⁺/0.3Tb³⁺/0.6Eu³⁺ (mol %) triply-doped SiO₂–KYF₄ glass ceramics under UV excitation of Ce³⁺ by comparison to the direct excitation of Eu³⁺ and Tb³⁺ ions, showing the Eu³⁺ and Tb³⁺ luminescence peaks (reproduced from reference [71]).