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A potential high color purity and thermally stable red-emitting phosphor based on Tb3+ and Eu3+ co-doped sodium yttrium borate: Synthesis and luminescence spectroscopic characterization

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Highlights

  • A series of novel narrow-band red emitting NYB:Tb3+, Eu3+ phosphors was synthesized.

  • An optimum doping concentration was chosen based on comprehensive luminescence study.

  • The phosphor with highest efficiency also demonstrated excellent thermal stability.

  • Highest PLQY of 84% under 395 nm excitation.

  • CRI of fabricated pc-WLED is up to 85.

Abstract

A series of Tb3+ and Eu3+ single- and co-doped Na3Y(BO3)2 (NYB) was prepared by the solid-state reaction method and characterized their structural, thermal and photoluminescence properties while focusing on the development of efficient narrow-band red-emitting phosphors for near-UV chip-based phosphor-converted white LED (pc-WLED). Luminescence spectroscopic and decay kinetics studies of NYB:Tb3+, Eu3+ sample series demonstrated an occurrence of energy transfer from Tb3+ to Eu3+ with its efficiency dependent on Eu3+ concentration and temperature. The highest quantum yield of 83.5% was observed for NYB:5.0%Tb3+, 40%Eu3+ composition with excellent thermal stability. The phosphor was finally tested for the use as a red phosphor component in pc-WLED, which demonstrated a high color rendering index of 84.7. The phosphor is a good modelling composition for the development of wider family of narrow-band red-emitting phosphors based on Tb3+/Eu3+ co-doped inorganic oxides for application in pc-WLED.

Graphical abstract

A novel Tb3+ and Eu3+ co-doped Na3Y(BO3)2 phosphor with the highest quantum yield and energy transfer efficiency as well as the results of testing the optimum composition of Na3Y(BO3)2:5.0%Tb3+, 40%Eu3+ for its practical performance in terms of electro-luminescence are presented.

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Introduction

White light emitting diodes (WLEDs) have become a new generation of light source due to many advantages such as energy saving, environment-friendliness, small volume, and long lifetime. Nowadays, they are widely used in solid-state lighting, optical sensing devices, displays, etc. [[1], [2], [3]]. The most popular commercial phosphor-converted WLED (pc-WLED) is based on combination of a Y3Al5O12:Ce3+ (YAG:Ce3+) yellow phosphor and a InGaN chip emitting blue light. However, the YAG:Ce3+-based WLEDs lack a red light component and limits their applications for indoor lighting that requires warm WLEDs. In particular, this kind of pc-WLED is characterized by relatively high correlated color temperature (CCT) (>4500 K) and low color rendering index (CRI) (Ra < 80) [4].

In this background, plenty of works have been conducted towards the development of narrow-band red-emitting phosphors in the last few years. Pust et al. investigated many novel Eu2+-doped phosphors, such as Sr[LiAl3N4]:Eu2+ and Sr[Li2Al2O2N2]:Eu2+, which exhibit efficient narrow-band red emissions [5,6]. However, in comparison with Mn4+ or trivalent lanthanides (such as Eu3+), which demonstrate narrow emission lines due to spin-forbidden 2Eg4A2g or 4fn→4fn transitions, respectively, Eu2+ doped in most nitrides shows broad 5d→4f emission band that obviously cannot satisfy the requirements for narrow-band red-emitting phosphors in pc-WLEDs. Therefore, significant attention has been paid to Mn4+-doped inorganic phosphors [7,8]. Thus, a good progress has been achieved in study of Mn4+-doped fluorides, which exhibit strong absorption ranging from 300 to 500 nm and red emission near 625 nm [[9], [10], [11], [12]]. However, many challenges including low thermal stability and humidity resistance still restrict practical application of the fluoride-based phosphors in pc-WLED [[13], [14], [15]]. Mn4+-doped oxides are thermally stable and humidity resistant; however, their emissions lie in the deep-red range at around 680 nm, where the sensitivity of human eye is very low [16]. Meanwhile, Eu3+ characterized by a red emission at around 620 nm arising from the 5D07F2 transition is a very popular dopant for red-emitting phosphors applied in traditional lighting devices and CRTs illuminated by 254 nm Hg emission (Y2O3:Eu3+) or by electron irradiation (Y2O2S:Eu3+) [17]. The high efficiency of the Eu3+-activated phosphors is caused by strong absorption at around 254 nm either due to Eu–O charge transfer (CT) or band-to-band transitions in the host. One of the primary drawbacks of Eu3+ as an activator for red-emitting phosphors in pc-WLEDs consists in a small absorption coefficient of Eu3+ in the near-UV range resulting from parity-forbidden character of electric-dipole 4f→4f transitions [18]. An effective strategy enabling to enhance the absorption coefficient of a Eu3+-based phosphor in the near-UV range is co-doping with a sensitizer that is characterized by a strong absorption in the near-UV range and capable to efficiently transfer energy to Eu3+. To this end, a number of studies on sensitization of Eu3+ have been conducted [[19], [20], [21]]. One of the most popular scheme is based on energy transfer (ET) from Ce3+ to Eu3+ through Tb3+ [22]. Here, Ce3+ provides a strong absorption in the near-UV range due to parity-allowed transitions from 4f to 5d state. However, peculiarities of Ce3+ and Eu3+ electronic structure can lead to the formation of CT state between Ce and Eu, that in turn results in delocalisation of Ce3+ 5d excited state energy through CT [23]. To resolve the issues, a core-shell-shell structure was proposed to isolate Ce3+ from Eu3+ and to noticeably improve the quantum efficiency [24].

An efficient ET from Tb3+ to Eu3+ has also been reported to be realized in many host lattices that allowed to propose phosphors with enhanced absorption in the near-UV range [[25], [26], [27]]. The NYB host has gained increasing attention as a new host lattice for phosphors in recent years while it is known to have excellent chemical stability and has the advantage of a low cost of raw materials and a low temperature of synthesis. This type of compounds Na2Ln(BO)3 (Ln = Y, Gd) with isostructure was first reported by Zhang etc. [28]. In 2012, luminescence properties of Eu3+ and Tb3+ singly doped Na2Ln(BO)3 (Ln = Y, Gd) were reported [29,30]. Later in 2015, energy transfer from Ce3+ to Tb3+, Dy3+ and Eu3+ in Na3Y(BO3)2 (NYB) was investigated, efficient energy transfer from Ce3+ to Tb3+ and Dy3+ was observed, but energy transfer from Ce3+ to Eu3+ was absent [31]. In 2016, the VUV spectroscopic properties of Tb3+ doped Na2Ln(BO)3 (Ln = Y, Gd) and energy transfer from host to Tb3+ through Gd3+ were reported by Shi etc. [32]. Furthermore, the 4fn-15d positions of trivalent lanthanides ions in NYB were obtained [33]. However, no work has been ever reported on the topic of energy transfer from Tb3+ to Eu3+ in NYB. Therefore, we found it inspiring to expand the research on Tb3+ and Eu3+ co-doped NYB. To evaluate the potentials of NYB:Tb3+,Eu3+ for red phosphor applications, temperature-dependent spectroscopic properties and decay kinetics of Tb3+ and Eu3+ single- and co-doped NYB samples have been investigated. A quantum yield of the prepared phosphor with optimal dopant concentration has also been measured. Finally, pc-WLED containing NYB:Tb3+,Eu3+ as a red phosphor component has been fabricated and tested for evaluating its performance.

Section snippets

Sample preparation

A series of Tb3+/Eu3+ single- and co-doped NYB samples was prepared by a solid-state reaction method [28]. The raw materials, Na2CO3 (A.R.), Eu2O3 (99.99%), Tb4O7 (99.99%), Y2O3 (99.99%) and H3BO3 (A.R.) were weighed according to stoichiometric proportion, then ground in an agate mortar to obtain a homogeneous mixture. The mixtures were presintered in corundum crucibles at 873 K for 12 h, then naturally cooled down to room temperature and ground. The powders were sintered again at 1103 K for

XRD patterns and crystal structure

XRD patterns for selected samples of NYB:5%Tb3+, x%Eu3+ (x = 2, 30, 50) are displayed in Fig. 1-a. The positions of diffraction peaks of the samples with doping content of up to 30%Eu3+ are compatible with ICDD data on NYB (PDF card No. 53–1131) (see Fig. 1-a, upper panel). Increase of Eu3+ content results in appearance of new diffraction peaks originating from difference in ionic radii of Eu3+ (1.01 Å) and Y3+ (0.96 Å). Because a standard PDF card for Na3Eu(BO3)2 could not be found in the

Conclusions

A series of Tb3+ and Eu3+ single-doped and co-doped Na3Y(BO3)2 phosphors was successfully synthesized using the high-temperature solid state reaction. ET from Tb3+ to Eu3+ with efficiency approaching 80% was revealed with the electric dipole interaction as a dominant mechanism. The phosphor of NYB:5.0%Tb3+,40%Eu3+ demonstrated the highest PLQY of about 83.5% under intracenter Eu3+ excitation at 395 nm, while that obtained for Tb3+ excitation at 353 nm was just 33.2%. The NYB:5.0%Tb3+,40%Eu3+

Credit author statement

Qiufeng Shi and Tingyu Wang: Conceptualization, Methodology, Data curation, Writing – original draft. Tingyu Wang: Experiment. Qiufeng Shi: Supervision. Qiufeng Shi, Konstantin V. Ivanovskikh, Caie Cui, Ping Huang, Lei Wang and Haijie Guo: Writing- Reviewing and Editing

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was financially supported by the Natural Science Foundation of Shanxi Province (201801D121020 and 201801D221132) and Key Research and Development (R&D) Projects of Shanxi Province (201903D121097). K.V.I. acknowledges partial support from the Ministry of Science and Higher Education of the Russian Federation (Project No. FEUZ-2020-0060).

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