Non-Linear Optical Properties of Er3+-Yb3+-Doped NaGdF4 Nanostructured Glass-Ceramics

Abstract

Transparent oxyfluoride glass-ceramics containing NaGdF₄ nanocrystals were prepared by melt-quenching and doped with Er³⁺ (0.5 mol%) and different amounts of Yb³⁺ (0-2 mol%). The selected dopant concentration the crystallization thermal treatments were chosen to obtain the most efficient visible up-conversion emissions, together with near infrared emissions. The crystal size increased with dopant content and treatment time. NaGdF₄ NCs with a size ranging 9-30 nm were obtained after heat treatments at T_g + 20-80 °C as confirmed by X-ray diffraction and high-resolution transmission electron microscopy. Energy dispersive X-ray analysis shows the incorporation of rare earth ions into the NaGdF₄ nanocrystals. Near-infrared emission spectra, together with the up-conversion emissions were measured. The optical characterization of the glass-ceramics clearly shows that Er³⁺ and Yb³⁺ ions are incorporated in the crystalline phase. Moreover, visible up-conversion emissions could be tuned by controlling the nanocrystals size through appropriated heat treatment, making possible a correlation between structural and optical properties.

Keywords: crystallization; nonlinear optical properties; oxyfluoride glass–ceramic; rare-earth.

Figure 1

Transparent samples of 0.5Er³⁺-doped and 0.5Er³⁺–2Yb³⁺-codoped 70Si7Gd glass and glass–ceramics (GCs) heat-treated at 550 °C-80 h.

Figure 2

(a) XRD patterns for undoped, Er³⁺-doped and Er³⁺–Yb³⁺-codoped 70Si7Gd GCs treated at 550 °C-80 h and (b) 580 °C-120 h; (c) XRD patterns of 0.5Er–2Yb-codoped 70Si7Gd GCs at different treatment temperatures for 80 h.

Figure 3

(a) TEM micrograph of 0.5Er³⁺–2Yb³⁺-codoped 70Si7Gd GCs treated at 580 °C-80 h. Inset, corresponding NCs size distribution; (b) HRTEM micrographs of the NCs with the different interplanar distances associated with the NaGdF4 structure.

Figure 4

(a) Scanning transmission microscopy-high angle annular dark field (STEM-HAADF) image of 0.5Er³⁺–2Yb³⁺-codoped 70Si–7Gd GCs treated at 580 °C-120 h used for EDX analysis; (b) corresponding EDXS analysis along with the line scan spectra of one of the nanocrystals (NCs).

Figure 5

Near-infrared spectra (NIR) in the 1000–1750 nm range obtained under excitation at 980 nm of (a) Er³⁺-doped and (b) Er³⁺–Yb³⁺-codoped 70Si7Gd glass and GCs treated at 550 °C for 80 h.

Figure 6

IR-absorption spectra of 0.5Er³⁺ and 0.5Er³⁺–2Yb³⁺ GCs treated at 580 °C for 80 h.

Figure 7

(a) Up-conversion emission spectra of 0.5 Er³⁺-doped and 0.5Er³⁺–2Yb³⁺-codoped 70Si7Gd glass and GCs treated at 550 °C for 80 h under excitation at 974 nm; (b) power dependence of UC emission intensity at 540 (green) and 650 (red) nm.

Figure 8

Energy level diagram of Er³⁺ and Yb³⁺ ions with the main emissions indicated by solid downwards-pointing arrows. Up-conversion (UC) mechanisms (energy transfer (ET), excited-state absorption (ESA) or energy-transfer up-conversion (ETU)) are also indicated.

Figure 9

(a) UC-emission spectra of 0.5Er³⁺–2Yb³⁺-codoped 70Si7Gd glass and GCs treated at different temperatures for 80 h under excitation at 974 nm; (b) CIE-standard chromaticity diagram showing up-conversion emissions of 0.5Er³⁺–2Yb³⁺-codoped 70Si7Gd glass and GCs treated at different temperatures for 80 h under excitation at 974 nm. Inset in (b) picture of 0.5Er³⁺–2Yb³⁺-codoped 70Si7Gd GCs under IR excitation at 974 nm showing an efficient visible green UC luminescence.