Near UV excited SrAl2O4:Dy3+ phosphors for white LED applications

doi:10.1016/j.matchemphys.2018.02.025

Materials Chemistry and Physics

Volume 211, 1 June 2018, Pages 181-191

https://doi.org/10.1016/j.matchemphys.2018.02.025 Get rights and content

Highlights

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SrAl₂O₄:Dy³⁺ phosphors were crystallized into monoclinic structure.
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Upon 350 nm excitation, the SrAl₂O₄:Dy³⁺ phosphors exhibit intense luminescence.
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The dipole-dipole interaction mechanism is responsible for luminescence quenching.
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The SrAl₂O₄:1.0 mol% Dy³⁺ phosphor is the best choice to design white LEDs.

Abstract

Strontium aluminate phosphors containing different concentrations of Dy³⁺ ions (SrAl₂O₄:Dy³⁺) were prepared by solid state reaction method by sintering at 1100 °C for 3 h. These phosphors were crystallized into monoclinic structure and well consistent with the JCPDS No. 34−0379. All these phosphors exhibit the characteristic emission transitions such as ⁴F_9/2 → ⁶H_15/2, ⁴F_9/2 → ⁶H_13/2 and ⁴F_9/2 → ⁶H_11/2 corresponding to blue (∼480 nm), yellow (∼572 nm) and red (∼663 nm) regions when excited at 350 nm near UV wavelength. Beyond 1.0 mol% concentration of Dy³⁺ ions, a luminescence quenching was noticed and it has been assigned to the energy transfer through dipole-dipole interactions at higher concentrations. Due to crystallization into nanophase, the SrAl₂O₄:Dy³⁺ phosphors exhibit novel and excellent luminescence properties at 350 nm excitation. The 1.0 mol% of Dy³⁺ -doped SrAl₂O₄ phosphor show a quantum yield of ∼9.52%. The experimental results suggest that the SrAl₂O₄:1.0 mol% Dy³⁺ phosphor could be the best choice to design white LEDs with bright and intense luminescence.

Graphical abstract

Introduction

Trivalent rare earth (RE³⁺) ions activated mono-phase nanophosphors attracted the researchers due to their strong absorption from VUV-to-NIR region [[1], [2], [3], [4]], high color-rendering index [5,6], environmental friendly nature [7,8], chemical stability and thermal behaviour [[9], [10], [11], [12]]. They play a vital role in different fields such as science & technology, industry and bio-medicine [[13], [14], [15]]. They find wide range of applications in lasers, flat panel displays, computers, smart-phones and light emitting devices (LEDs). The emission of white light has been achieved using blue GaN-based LED with yellow or YAG: Ce³⁺ or tricolor phosphors excited by UV radiation. The white LEDs designed by these methods show poor illumination and low color rendering index. Many efforts have been made to overcome these limitations to develop new and novel phosphors for white LED’s. However, there is a demand for novel and high fluorescence efficiency phosphor materials to meet the present fast growing smart technology [[16], [17], [18]].

Compared to the available phosphors, the SrAl₂O₄ show high luminescence efficiency and excellent optical properties owing to their strong absorption in UV region, long phosphorescence, excellent emission, high thermal stability, chemical durability and eco-friendly nature [[19], [20], [21], [22], [23]]. Diaz-Torres et al. [19] reported the enhancement in white light emission of SrAl₂O₄:Ce³⁺ phosphors by co-doping with Li⁺ ions synthesized by combustion technique. Green SrAl₂O₄:Eu²⁺, Dy³⁺ phosphors synthesized by microwave assisted co-precipitation route exhibits long afterglow photoluminescence (PL) after the removal of excitation source [20]. In order to investigate the luminescence and long lasting properties, Eu²⁺ and Dy³⁺ co-doped SrAl₂O₄ phosphor was synthesized via sol-gel method by L. Xiao et al. [22]. S. Chawla et al. [23] presented a broad yellow emitting SrAl₂O₄:Pr³⁺ phosphor with blue excitation for white LEDs.

Under suitable excitation, the Dy³⁺ ions doped phosphors emit blue, yellow and red emissions corresponding to ⁴F_9/2 → ⁶H_15/2, ⁴F_9/2 → ⁶H_13/2 and ⁴F_9/2 → ⁶H_11/2 transitions, respectively. Among the available Dy³⁺ ions activated phosphors for white LED applications [21,24,25], the SrAl₂O₄:Dy³⁺ phosphors are of interest due to their excellent spectral properties and high luminescence efficiency. Sahu et al. [26] reported the photoluminescence and mechanoluminescence properties of 1.0 mol% of Dy³⁺ -doped SrAl₂O₄ phosphor only for white light generation. The present research work explores the concentration (0 ≤ x ≤ 5.0 mol%) dependent luminescence, color purity, quantum yield and fluorescence decay of SrAl₂O₄:Dy³⁺ phosphors. The possible reasons for the luminescence quenching of Dy³⁺ ions at higher concentrations was discussed in detail.

Section snippets

Materials and method

Different concentrations of Dy³⁺ ions (x = 0, 0.1, 0.5, 1.0, 2.0 and 5.0 mol%) doped SrAl₂O₄ phosphors were synthesized by solid state reaction method. High-purity SrCO₃ (99%), Al₂O₃ (99.9%), and Dy₂O₃ (99.99%) were used as starting chemicals. Stoichiometric amounts of these chemicals were mixed thoroughly in the presence of acetone using a pestle and agate mortar. About 1.0 wt% of B₂O₃ was added as flux to the homogeneous mixtures to promote the decomposition of SrCO₃. These mixtures were heat

Structure and morphology

The XRD profiles of SrAl₂O₄: xDy³⁺ (0 ≤ x ≤ 5.0 mol%) phosphors are well consistent to JCPDS No. 34-0379. The addition of Dy³⁺ impurity in small amounts (i.e., x ≤ 5.0%) has no effect on the monoclinic structure of SrAl₂O₄:Dy³⁺ phosphors. However, the intensity of XRD peaks decrease with the increase of Dy³⁺ ion concentration with peak positions remains unaffected. The XRD profiles of undoped and Dy³⁺ -doped SrAl₂O₄ phosphors are illustrated in Fig. 1. The average crystallite size of SrAl₂O₄:Dy

Conclusions

The SrAl₂O₄:Dy³⁺ phosphors have been synthesized by solid state reaction method by sintering at 1100 °C for 3 h. These phosphors are crystallized into monoclinic SrAl₂O₄ structure and well consistent to the JCPDS No. 34−0379. Upon 350 nm near UV excitation, the SrAl₂O₄:Dy³⁺ phosphors exhibit ⁴F_9/2 → ⁶H_15/2 (480 nm), ⁴F_9/2 → ⁶H_13/2 (572 nm) and ⁴F_9/2 → ⁶H_11/2 (663 nm) transitions. Beyond 1.0 mol% concentration, the Dy³⁺ ions show luminescence quenching due to the ET among the excited Dy³⁺ ions

Acknowledgement

The author is thankful to the DST and the SAIF, IIT Madras, Chennai – 600036 for providing FT-IR analysis.

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Citation Excerpt :
Fig. 2(b) illustrates the XRD pattern of SAO. The intensity and position of the primary peak of SAO were consistent with those of the standard data (JCPDS card number 34–0379) [25]. This proves that the material consists of a SrAl2O4 phase.
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Near UV excited SrAl2O4:Dy3+ phosphors for white LED applications

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Materials and method

Structure and morphology

Conclusions

Acknowledgement

Mater. Chem. Phys.

Opt. Mater.

J. Solid State Chem.

J. Alloy. Compd.

J. Lumin.

Curr. Appl. Phys.

Solid State Sci.

Mater. Sci. Semicond. Process.

Mater. Chem. Phys.

Mater. Res. Bull.

J. Alloy. Compd.

J. Lumin.

Mater. Res. Bull.

Mater. Lett.

Solid State Sci.

J. Lumin.

Mater. Sci. Eng. B

J. Lumin.

Ceram. Int.

Ceram. Int.

Opt. Mater.

J. Rare Earths

J. Lumin.

Mater. Chem. Phys.

Near UV excited SrAl₂O₄:Dy³⁺ phosphors for white LED applications