Tunable light emission and similarities with garnet structure of Ce-doped LSCAS glass for white-light devices

Abstract

In this paper, we report results concerning tunable light emission and color temperature in cerium-doped low-silica-calcium-alumino-silicate (LSCAS) glass for smart white-light devices. Spectroscopic results, analyzed using the CIE 1931 x–y chromatic diagram, show that this glass presents two broad emission bands centered at 475 and 540 nm, whose intensities can be tuned by the excitation wavelength. Moreover, the same emission can be achieved from a color temperature range from 3200 to 10,000 K, with a color-rendering index (CRI) of around 75% obtained by changing the optical path length of the sample. Our new phosphor LSCAS glass, which is a unique system that exhibits tunable yellow emission, combines all qualities for white-light devices.

Introduction

Recently, much effort has been dedicated to develop luminescent materials able to produce a new generation of white light (WL) for light sources and components used in electronic display devices [1]. White-light-emitting diodes (WLEDs) and many materials doped with luminescent ions have been studied as potential WL generators. Among those materials with luminescent ions, the most used are oxides doped with rare-earth ions like Sm³⁺, Pr³⁺, Tb³⁺, Dy³⁺, Eu³⁺ and Eu²⁺ and co-doped with Ce³⁺ ions as activators [2], [3], [4], [5], [6], [7]. In particular, the Ce³⁺ ion is interesting for phosphors due to both short lifetime (about 50 ns) and a broad emission band centered between 350 and 550 nm that is associated to the allowed 5d → 4f electronic transition [8]. One material that has been extensively studied is the phosphor Ce³⁺:YAG, which presents a broad yellow luminescence when excited with sources emitting in the range between 410 and 480 nm. This system is very interesting because the radiation of the GaN-based UV-blue LED can used simultaneously for excitation and to be added to the yellow emission of the Ce³⁺:YAG, with an appropriate intensity, yielding WL [9], [10], [11]. Although crystals are interesting for WL generation, they have some disadvantages: their production is expensive, difficult to grow and demand high Ce³⁺ concentrations (∼2 at.%), leading to fast luminescence quenching. Another drawback of Ce³⁺-doped crystals is the low color-rendering index (CRI), which is due to its very weak emission intensities in the red region [10]. There are few Ce³⁺-doped crystals emitting in the yellow region, and, until recently, those reported in the literature exhibit a garnet structure [10], [11], [12], [13]. For other Ce³⁺-doped materials, the emissions are located in the UV or blue spectral regions.

In the last few years, we have focused our attention on the study of the optical properties of OH⁻ free rare-earth and metal-transition-doped low-silica-calcium-aluminosilicate (LSCAS) glass. This glass exhibits phonon energy of approximately 800 cm⁻¹, lower than those of silicate glasses [14]. When co-doped with Er³⁺/Yb³⁺, it presents high emission rates in the mid infrared, near 2.8 μm [15]. Additionally, laser emission at 1.07 and 1.37 μm were observed when this glass was doped with Nd³⁺ [16] and Yb³⁺ [17]. More recently, a long lifetime (∼2.0 ms at 77 K and 170 μs at room temperature) and a broad emission band of 190 nm (4237 cm⁻¹) centered at 637 nm (15698 cm⁻¹) were found in a Ti³⁺-doped LSCAS glass [18], which also shows high values of both gain cross section (∼4.7 × 10⁻¹⁹ cm²) and luminescence quantum efficiency (∼70%) [19] comparable to the values for Ti³⁺:sapphire crystals. Our most recent achievement in this host glass was to show that, when doped with cerium, it can produce smart WL combining a glass phosphor with light-emitting diodes (LEDs) at 405 nm, resulting in an emission close to ideal WL [20].

In this work, we deal with results concerning the possibility of tunable emission in Ce³⁺:LSCAS glass in two ways: by changing the excitation wavelength within the violet and blue regions or by changing the re-absorption by varying the sample thickness. A structural interpretation was proposed to explain this glass particularity, which indicates that the LSCAS glass system is able to change its color temperature in accordance to the occasion. This is very interesting for the regulation of the circadian rhythms of humans, which is a great advantage for this material when compared with crystals. This matter of circadian rhythms will be a new direction of research for commercial luminophosphors of interest for artificial lights.