Emission tunability and local environment in europium-doped OH⁻-free calcium aluminosilicate glasses for artificial lighting applications

Highlights

• Eu²⁺-doped OH⁻ free calcium aluminosilicate glass as a new source for white lighting.

• Correlation between emission tunability and local environment of europium ions.

• Significant reduction of Eu³⁺ to Eu²⁺ by melting the glasses under vacuum atmosphere.

• Broad, intense and tunable luminescence ranging from blue to red.

Abstract

The relationship between emission tunability and the local environment of europium ions in OH⁻-free calcium aluminosilicate glasses was investigated, focusing on the development of devices for artificial lighting. Significant conversion of Eu³⁺ to Eu²⁺ was obtained by means of melting the glasses under a vacuum atmosphere and controlling the silica content, resulting in broad, intense, and tunable luminescence ranging from blue to red. Electron spin resonance and X-ray absorption near edge structure measurements enabled correlation of the luminescence behavior of the material with the Eu²⁺/Eu³⁺ concentration ratio and changes in the surrounding ions' crystal field. The coordinates of the CIE 1931 chromaticity diagram were calculated from the spectra, and the contour maps showed that the light emitted from Eu²⁺ presented broad bands and enhanced color tuning, ranging from reddish-orange to blue. The results showed that these Eu doped glasses can be used for tunable white lighting by combining matrix composition and the adjustment of the pumping wavelength.

Introduction

A major challenge in the development of a new generation of artificial lighting is the tailoring of phosphor materials that possess efficient and tunable emissions across the whole range of the visible spectrum [1], [2], [3], [4], [5], [6], [7]. Europium and cerium are currently the most widely used phosphors in devices for lighting and displays, and both can coexist in several oxidation states in the host matrices. In addition, their emissions are known to be strongly dependent on the surrounding ions' crystal field [2].

There are several factors that determine the luminescence properties of a material, such as the phonon energy, the presence of impurities, the nature and intensity of the surrounding crystal field, the oxidation states of the sensitizer ions, the synthesis procedure, and the morphology and geometry of the final material. From the point of view of materials science, tailoring of the surrounding ions' crystal field is essential in order to control the luminescence behavior. Consequently, many different materials have been developed for solid-state lighting, including crystals [2], ceramics [8], polymers [3], [9], and glasses [1], [7], [10], [11], [12]. Specifically, the amorphous nature of glasses means that their broad luminescence properties are influenced by the average local crystal field surrounding the dopant ions. Selection of the matrix composition, the phosphor doping level, and the melting procedure are important ways of obtaining the desired emission characteristics.

We recently proposed the use of OH⁻-free low-silica calcium aluminosilicate (LSCAS) glass as an alternative luminescent material for tunable white lighting, due to its intense and broad emission spectrum in the visible region. These studies employed systems doped with Ce [7], [12], Eu [10], and Ce–Eu [11]. The results revealed a significant influence of melting under a vacuum atmosphere on the production of glasses with ions in reduced oxidation states and with minimal presence of OH⁻ in their structures. This LSCAS glass is known to have superior thermo-mechanical properties, good chemical resistance, transparency from the UV–visible to the near infrared (up to 5 μm), reduced oxidation states for some dopant ions, and efficient laser emissions [13], [14], [15], [16], [17], [18].

The certification of visible light sources for artificial lighting can employ parameters such as the correlated color temperature (CCT), the color rendering index (CRI), and the distance from the Planckian locus to the (u', v') color coordinates. The latter is known as Du'v' and describes how close the tested light source is in relation to the ideal lighting. These parameters, which characterize the emission, can be used to evaluate the melting procedure and the changes in composition, in order to optimize the glass luminescence. To this end, knowledge of the oxidation states of the sensitizer ions and the nature of the surrounding crystal field are important for tailoring the intended luminescent source.

Given the above considerations, this work therefore involved the development of a set of OH⁻-free Eu-doped aluminosilicate glasses with different silica contents. Investigation was made of the influence of the surrounding ions' crystal field and their oxidation states on the luminescence properties. The techniques used for characterization of the samples were electron spin resonance (ESR), X-ray absorption near edge structure (XANES), and photoluminescence (PL).