Spectroscopic properties and simulation of white-light in Dy³⁺-doped silicate glass

Abstract

Spectroscopic properties of various concentrations Dy³⁺-doped silicate glasses were characterized by excitation and emission spectra. The optimal doping concentration of Dy³⁺ ions was found to be 3.0 wt%, and the nature of resonance energy transfer was confirmed to be electric dipole–dipole interaction according to Huang’s rule. Simulation of white-light for these glasses was also performed by varying the excitation wavelength. The results show that the white-light luminescence color could be tuned to various wavelength excitations, and the present silicate glass is more suitable for generation of white-light for blue LED chips.

Introduction

As a potential candidate for replacement conventional incandescent and fluorescent lamps, white-light emitting diodes (w-LEDs) has attracted intensive attention in recent years due to the advantages of long lifetime, saving energy, high efficiency, reliability, and its environmental-friendly characteristics [1], [2], [3]. At present, the common way for assemble w-LEDs is combining an ultraviolet (UV) or blue chip with down-converted phosphors [2]. The first w-LEDs was commercialized by combining a GaN-based blue chip with yellow YAG:Ce phosphors in 1996 [4]. However, the lack of red component in YAG:Ce phosphors leads to low color-redering index (CRI, Ra in the 60–75 range). As may be expected, red enhanced YAG:Ce or a small amount of red phosphors was introduced to improve the CRI to the acceptable range (Ra > 80) and increased the light conversion [5], [6]. In addition to the blue LEDs plus yellow approach, three-band w-LEDs were also proposed to achieve by the combination of the blue GaN-based LED with green and red emitted phosphors or the pumping of tri-color (red, green and blue) phosphors with ultraviolet (UV) or violet LED [3]. Three-band w-LEDs maintain a very high CRI (Ra > 90) and were believed to offer the greatest potential for high efficiency solid state light [7]. For excellent CRI, both methods need efficient red phosphors that should have the excitation wavelength matching with the emission wavelength of blue LEDs (440–470 nm) or the UV/violet LEDs (350–420 nm). So far, most of these w-LEDs were limited to use as back light sources of liquid crystals displays for handy phones or digital cameras in view of low luminous flux from one w-LED [8]. Up to now, many studies have been carried out to obtain enough brightness for general lighting, in which useful method is to increase output power of LED chips. However, this also increases the chip temperature, which may cause a deterioration of the resin, which is used to fix the powder phosphors onto the LED chip, and decrease the luminous efficiency and lifetime [9].

The versatility of glasses regarding the possibility of a wide doping concentration and the narrow lines emission spectra of the lanthanide ions could be considered as a promising alternative approach since the first simulation of white-light in borate glass [10]. Compared with phosphors, glasses have more advantages such as lower production cost, simpler manufacture procedure, and free from halo effect [10], [11], [12], [13], [14], [15], [16], [17], [18], [19]. On the other hand, the visible luminescence of Dy³⁺ (4f⁹) ion mainly consists of two intense bands in the blue (470–500 nm) and yellow (570–600 nm) regions, which are associated with the ⁴F_9/2 → ⁶H_15/2 and ⁴F_9/2 → ⁶H_15/2 transitions, respectively. The latter one is a hypersensitive transition, which is strongly influenced by the environment. At a suitable yellow to blue (Y/B) intensity ratio, Dy³⁺ ions will emit white-light. Thus, luminescent materials doped/codoped with Dy³⁺ ions are usually used to the generation of white-light both in glasses [11], [12], [13], [14], [15], [16] and phosphors [20], [21], [22]. These studies on glasses have emphasized on luminescent behavior and generation of white-light in Dy³⁺-doped, Dy–Ce-, Dy–Eu-, or Dy–Tm-codoped aluminasilicate [12], borosilicate [16], phosphate glass [11] and oxyfluoride [13], [14], [15] glasses. To the best of our knowledge, little work is reported in the conventional silicate glass. In the present work, spectroscopic properties and simulation of white-light in various concentrations Dy³⁺-doped silicate glasses were investigated in details, here silicate glass is chosen as the host glass because of its high chemical stability and low cost.