Ag-Sensitized Yb3+ Emission in Glass-Ceramics

Abstract

Rare earth doped materials play a very important role in the development of many photonic devices, such as optical amplifiers and lasers, frequency converters, solar concentrators, up to quantum information storage devices. Among the rare earth ions, ytterbium is certainly one of the most frequently investigated and employed. The absorption and emission properties of Yb³⁺ ions are related to transitions between the two energy levels ²F_7/2 (ground state) and ²F_5/2 (excited state), involving photon energies around 1.26 eV (980 nm). Therefore, Yb³⁺ cannot directly absorb UV or visible light, and it is often used in combination with other rare earth ions like Pr³⁺, Tm³⁺, and Tb³⁺, which act as energy transfer centres. Nevertheless, even in those co-doped materials, the absorption bandwidth can be limited, and the cross section is small. In this paper, we report a broadband and efficient energy transfer process between Ag dimers/multimers and Yb³⁺ ions, which results in a strong PL emission around 980 nm under UV light excitation. Silica-zirconia (70% SiO₂-30% ZrO₂) glass-ceramic films doped by 4 mol.% Yb³⁺ ions and an additional 5 mol.% of Na₂O were prepared by sol-gel synthesis followed by a thermal annealing at 1000 °C. Ag introduction was then obtained by ion-exchange in a molten salt bath and the samples were subsequently annealed in air at 430 °C to induce the migration and aggregation of the metal. The structural, compositional, and optical properties were investigated, providing evidence for efficient broadband sensitization of the rare earth ions by energy transfer from Ag dimers/multimers, which could have important applications in different fields, such as PV solar cells and light-emitting near-infrared (NIR) devices.

Keywords: Ag nanoaggregates; Yb3+ ions; down-shifting; photonic microdevices; sol-gel.

Figure 1

RBS spectra for undoped silica-zirconia GC samples before and after Ag-exchange (GC0 and GC0-A). The simulated GC0-A spectrum and the specific Ag contribution are also shown, resulting in Ag concentrations decreasing from 2 mol.% (at surface) to 1.5 mol.% (inner part of the film).

Figure 2

XRD comparison between silica-zirconia-soda GC samples with or without Yb co-doping. Undoped GC samples contain tetragonal-phase zirconia nanocrystals (PDF 01-080-0784; ICSD 68589), while Yb doped samples have a fluorite-type cubic-phase zirconia nanocrystals (PDF 01-078-1309; ICSD 62462), attested by the different shape of the characteristic reflection peaks, especially those at 2θ ≈ 35.5° and 2θ ≈ 75°.

Figure 3

Photoluminescence emission by 280 nm excitation of undoped samples before Ag-exchange (GC0), after Ag-exchange (GC0-A), and after Ag-exchange and annealing (GC0-C). The substrate is also reported as a reference. The main contribution, peaked at 425 nm, is reasonably attributed to Ag⁺–Ag⁺ pairs. After 1 h annealing at 430 °C, the decreasing of the blue emission in favor of the red emission suggests the formation of Ag trimers and multimers.

Figure 4

Near-infrared photoluminescence emission of Yb doped samples before Ag-exchange (GC4), after Ag-exchange (GC4-A) and after Ag-exchange and annealing (GC4-C). It can be observed that 355 nm is a very weak excitation wavelength for Yb³⁺ ions, while it results in a strong PL emission in Ag-containing samples.