Spectral and thermal properties of Dy3+-doped NaGdTiO4 phosphors

doi:10.1016/j.jallcom.2011.12.072

Journal of Alloys and Compounds

Volume 517, 15 March 2012, Pages 170-175

https://doi.org/10.1016/j.jallcom.2011.12.072 Get rights and content

Abstract

Dy³⁺-doped NaGdTiO₄ phosphors were synthesized by a solid-state reaction method. The crystal structure, spectral properties and fluorescence quenching of the phosphors were systematically studied by means of X-ray diffraction (XRD) and spectroscopy. It was found that the phosphors can be effectively excited by 281 nm ultraviolet light, and intense white light emission was observed. The white light was generated by mixing blue (483 nm) and yellow (578 nm) emissions corresponding to the transitions from ⁴F_9/2 to ⁶H_15/2 and ⁶H_13/2 levels of Dy³⁺. The electric dipole–dipole interaction between Dy³⁺ ions was identified as the main mechanism for the concentration dependent fluorescence quenching of ⁴F_9/2 level. The CIE color coordinates of the phosphors were calculated to be (x = 0.3345, y = 0.3535) in the white region under 281 nm excitation, which is very close to the E point (energy equal point, x = 0.3333, y = 0.3333). In addition, the thermal quenching behavior of ⁴F_9/2 level of Dy³⁺ was also discussed, and the crossover effect was confirmed to be the dominant physical mechanism responsible for the fluorescence temperature quenching of Dy³⁺ in NaGdTiO₄ host.

Highlights

► Dy³⁺-doped NaGdTiO₄ phosphors were synthesized by a solid-state reaction method. ► The phosphors can be effectively excited by 281 nm ultraviolet light. ► Intense white light emission was observed. ► Energy transfer between Dy³⁺ was confirmed as electric dipole-dipole interaction. ► The crossover effect was responsible for the fluorescence temperature quenching.

Introduction

In recent years, white light emitting diodes (WLEDs) have received lots of attention driven by their potential applications in the fields of panel display, field-emission display and solid-state lighting (SSL) owing to their superior properties, such as energy saving, reliability and safety [1], [2], [3], [4], [5], [6], [7]. White light can be generated in several manners, and the most common route for obtaining white light is to combine the blue light from GaN-based LED and the yellow light from YAG:Ce³⁺ phosphor excited by GaN LED. However, such WLEDs have a few problems discussed in previous reports [8], [9], [10]. To solve such problems, considerable efforts have been devoted to research on the rare earth (RE) ions doped luminescent materials, since RE ions possess abundant energy levels from which multicolor emissions covering ultraviolet (UV), visible and infrared wavelength region can be achieved.

Additionally, thermal stability is one of the most important characteristics for phosphors especially the one applied in phosphor converted WLEDs, since the WLEDs may operate at high temperature, then the light output and color rendering index are easily deteriorated. Therefore, the development of a suitable phosphor for WLEDs and examine its fluorescence thermal stability is necessary.

Trivalent dysprosium ion (Dy³⁺) as a promising activator for white light emitting materials has been extensively studied due to its peculiar blue and yellow emission bands in the emission spectrum [11], [12], [13], [14], [15], [16], [17]. The blue band centered at around 484 nm corresponds to the ⁴F_9/2 → ⁶H_15/2 transition, and the yellow band located at around 575 nm corresponds to the hypersensitive transition ⁴F_9/2 → ⁶H_13/2. The crystal field environment of Dy³⁺ has remarkable influences on the intensity of yellow emission, but has little effect on that of the blue emission. Therefore, it is possible to obtain white light emission from Dy³⁺-activated luminescent materials by changing the intensity ratio of yellow to blue emissions (Y/B) via changing the matrix or adjusting the host compositions [18].

In this paper, titanate was chosen as the host due to its special properties, such as low cost, easy preparation, excellent thermal and chemical stabilities, and especially the strong absorption in the near-UV region and effective energy transfer from host lattice to the activator [19]. Dy³⁺-doped NaGdTiO₄ phosphors were synthesized by a solid-state reaction method. Intense white light was obtained under 281 nm UV light excitation. The energy transfer, fluorescence quenching, chromatic properties and thermal quenching behavior of the phosphors were systematically discussed.

Section snippets

Sample preparation

Dy³⁺-doped NaGdTiO₄ phosphors were prepared by a solid-state reaction method in air atmosphere. The starting materials of Na₂CO₃, Gd₂O₃, TiO₂ and Dy₂O₃ powders were weighed according to certain stoichiometric ratios. The Dy³⁺ doping concentrations were 0.2, 0.6, 1, 3, 5, 7, 9, 12 and 15%, which are the molar percentage of Gd³⁺ replaced by Dy³⁺. The weighed starting materials were well mixed and then calcined at 1000 °C for 4 h. A detailed preparation procedure could be found elsewhere in Ref. [20]

Structural properties

In order to identify the crystal structure of the obtained samples, XRD measurements for all the samples with various Dy³⁺ concentrations were carried out, and very similar diffraction pattern for each sample was observed. Fig. 1 presents the normalized XRD patterns of the as-synthesized NaGdTiO₄ phosphors doped with 0.2, 3 and 15 mol% Dy³⁺ as a representative, together with the standard pattern of orthorhombic structure NaGdTiO₄ (JCPDS 86-0830). As can be seen, all of the observed diffraction

Conclusions

Dy³⁺-doped NaGdTiO₄ phosphors were successfully synthesized by a solid-state reaction method. The crystal structure of the resultants was characterized by means of XRD. From spectroscopic studies, it was found that the energy transfer from host NaGdTiO₄ to doping Dy³⁺ is very efficient, and intense blue and yellow emissions can be obtained upon UV excitation corresponding to the host absorption band. Chromatic analysis on the phosphors indicates that white light could be achieved from the Dy³⁺

Acknowledgments

This work was supported by NSFC (National Natural Science Foundation of China, Grant Nos. 50972021, 11104023, 61078061 and 11104024), Scientific Research Foundation for Doctoral program of Liaoning Province of China (No. 20111032, 20111031), China Postdoctoral Science Foundation (No. 20110491539), and Fundamental Research Funds for the Central Universities.

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Journal of Alloys and Compounds

Abstract

Highlights

Introduction

Section snippets

Sample preparation

Structural properties

Conclusions

Acknowledgments

References (35)

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