Energy transfer and luminescence study of Dy3+ doped zinc-aluminoborosilicate glasses for white light emission

Highlights

• Various concentrations of Dy³⁺-doped SiO₂–B₂O₃–Al₂O₃–NaF–ZnO glasses were synthesized.

• Mechanical properties of these glasses were studied along with experimental analysis.

• Optical bandgap and Urbach energy of these glasses were calculated.

• Energy transfer mechanism and radiative parameters were studied.

• All Dy³⁺- doped glasses CIE (x, y), and CCT values represent the bright white light region.

Abstract

A series of Dy³⁺ doped aluminoborosilicate glasses with general formula 20SiO₂-(40-x) B₂O₃–10Al₂O₃–20NaF₂–10ZnO-xDy₂O₃ (x = 0, 0.1, 0.5, 1.0. 1.5, 2.0 and 2.5 mol %) were prepared by melt-quenching method. The FTIR analysis confirms the presence of stretching and bending vibrations of BO₄, BO₃ and SiO₄ units in the prepared glasses. DTA results show that Tg is decreasing with addition of Dy³⁺ ions in the glass system. Theoretically calculated mechanical properties such as elastic modulus, bulk modulus, shear modulus and Poisson's ratio suggest the increase in mechanical stability of glasses with dysprosium addition. Also experimental analysis carried out using Vicker's microhardness suggests glass stability with applied loads. Absorption spectrum shows twelve bands that exist due to transition from ⁶H_15/2 level to different excited levels. Nephelauxetic ratio and bonding parameter calculated shows decreasing ionicity of glasses with increase in Dy³⁺ ions. Judd-Oflet parameter calculated for all the glasses follow the trend Ω₆ > Ω₄ > Ω₄. Luminescence study shows three emission peaks having transitions from ⁴F_9/2 → ⁶H_15/2 (blue), ⁴F_9/2 → ⁶H_13/2 (yellow) and ⁴F_9/2 → ⁶H_11/2 (red). Radiative parameters calculated suggest higher stimulated emission cross-section for present glasses having ⁴F_9/2 → ⁶H_13/2 transition. The decay measurement for all the glass samples were recorded with an excitation at 350 nm and monitoring emission at 575 nm corresponding to the ⁴F_9/2 → ⁶H_13/2 transition and decay curves were fitted to bi-exponential fit. The CIE colour chromaticity coordinates were determined using CIE chromaticity diagram and the values were found to be in close proximity with the standard white light (0.33. 0.33) for all the glasses. Further colour correlated temperature values were found to lie in the near bright white region with CCT around ~4000 K.

Introduction

Solid state lighting devices that use semiconductor light-emitting diodes especially rare earth doped glasses for white light emission have drawn vast attention in recent times. White light emitting diodes (W-LEDs) are receiving great importance due to their longer lifetime, consumption of low power (2–17 W of electricity which is equal to one-third of its counterparts such as incandescent and CFL bulbs) and also high efficiency [1,2]. A well-known method for white light production based on solid state lighting (SSL) technology is made possible with the use of phosphor powders. Most of the commercially available LED bulbs use one or more phosphor powders that are coated over a blue/UV led chips that act as optical activation source. Both the phosphor material and LED chip were fixed firmly with the use of an organic resin, which acts as an encapsulating material for the chip and the powder. However, the coating of phosphor powder may not be uniform in some cases and the yellowing and carbonizing of the organic resin can lead to serious decrease in optical transmittance because of chemical bond breakage [3]. Also the high junction temperature of LED will degrade the emission efficiency, sometimes fades away giving yellowish colour light rather than white light as time goes on. The two important parameter for white light emissions such as colour rendering index (CRI) and colour correlated temperature (CCT) produced by phosphor method has low CRI (Ra< 80) and high CCT (>6000 K) [4]. To overcome this problem inorganic glasses are considered as right choice for phosphor powders.

Inorganic glasses in general are prepared by considering the glass formers like borate, silicate or phosphate as raw materials. Combination of any of these two glass formers can also be possible in most cases. Borosilicate glasses are of technological interest rather than phosphate glasses due to their lower thermal expansion, good chemical resistance, higher dielectric strength and higher mechanical stability [5,6]. Introduction of alkali or alkaline earth metals (say, Na) to borate combined silica glass system causes formation of non-bridging oxygens (NBOs) that decreases the connectivity of melts. Metal fluorides help in reducing the melting temperature of the raw materials and the high phonon energy of boron. Alumina oxide (Al₂O₃) is mostly preferred candidate for boro-silicate glasses as it partially replaces cations (Si) and thereby gives mechanical strength to the glasses [7]. On the other hand zinc oxide (ZnO) further helps in reducing the melting temperature of raw materials. Also due to its non-hygroscopic nature it gives good transparency to glasses and serves as a good candidate for borosilicate matrix [8]. Hence, with aforementioned literatures the host matrix for present work includes the following components B₂O₃–SiO₂–Al₂O₃–NaF–ZnO as raw materials. However luminescence effect of the glasses can be well understood with proper dopant added to this matrix. Among the known rare earth ions, dysprosium oxide (Dy₂O₃) plays a vital role for white light emission as it can be easily excited using UV or blue LED chips similar to phosphor method. Having several excitation bands in the wavelength range of 320–480 nm and also two important emission bands in the blue (482 nm) and yellow (575 nm) visible region, Dy³⁺ ions are one step ahead among other rare earths and can also be used as co-dopant also [9]. The emission intensity of the blue and yellow emissions in Dy³⁺ doped glasses can be tuned by altering the dysprosium concentration in the parent glass. Obtaining yellow to blue (Y/B) intensity ratio equal to one that satisfies the condition for pure white light emission i.e. the x and y chromaticity coordinates acquiring the values as 0.33 and 0.33 is still in a process of high demand for many researchers throughout.

Moreover few of the recent reports on white light emission is given as follows: (i) White light from NaO–BaO–Bi₂O₃–SiO₂ glass system doped with Dy³⁺ studied by Kaewjaeng et al. [10] posses CIE coordinates at 0.39 and 0.42 lying in warm white light region; (ii) Khan et al. [11] obtained the white light from Dy₂O₃ doped Li₂O–BaO-Gd₂O₃–SiO₂ with CCT value at 3621 K; (iii) Vijayakumar et al. [12] observed the white light emission from Dy³⁺ doped Boro-phosphate glasses having CCT value at 3943 K; (iv) Dy³⁺ doped lead borosilicate glasses for white light emission described by Reddi Babu et al. [13] shows the x and y coordinates at 0.35 and 0.35 thus suitable for white light generation; (v) Vijaya Babu et al. [14] studied the luminescence properties of Dy³⁺ doped alkali lead alumino borosilicate glasses with chromaticity coordinates 0.34 and 0.39 and CCT values at 5271 suggesting its suitability in white light emission; (vi) Lodi et al. [15] made an approach to obtain white light emission from Dy³⁺ doped calcium boroaluminate glasses by mixing the blue LED emission with Dy glass emission having chromaticity coordinates 0.38 and 0.41 and CCT around 4000 K. These literatures signify the use of dysprosium as a dopant material and also its easy incorporation into different host matrix.

Thus in our present work we have tried the white light emission from zinc aluminoborosilicate (ZABS) glasses doped with Dy³⁺ ions. Additionally structural, thermal and mechanical properties of these glasses were discussed.