Trans. Nonferrous Met. Soc. China 26(2016) 1670-1675

Luminescent properties of Lu₂MoO₆:Eu³⁺ red phosphor for solid state lighting

Li LI¹, Jun SHEN¹, Xian-ju ZHOU¹, Yu PAN¹, Wen-xuan CHANG¹, Qi-wei HE¹, Xian-tao WEI²

1. College of Science, Chongqing University of Posts and Telecommunications, Chongqing 400065, China;

2. Department of Physics, University of Science and Technology of China, Hefei 230026, China

Received 14 July 2015; accepted 3 March 2016

Abstract: The Eu³⁺ activated Lu₂MoO₆ phosphors were synthesized by high-temperature solid-state reaction method. The X-ray diffraction (XRD), excitation spectra, emission spectra and decay lifetime of the phosphors were measured to characterize the structure and luminescent properties. The XRD results show that all the prepared phosphors can be assigned to the monoclinic structure. The experimental results indicate efficient absorption of near ultraviolet light from the Mo⁶⁺-O^2- group followed by intensive emission in the visible spectral range. The optimal content of Eu³⁺ is 10% (mole fraction). The critical distance R_c and energy transfer mechanism were also discussed in detail. This red emitting material may be applied as a promising red phosphor for the near ultraviolet excited white light emitting diodes.

Key words: Lu₂MoO₆:Eu³⁺; luminescent properties; red phosphor

1 Introduction

White light-emitting diodes (WLED) have recently attracted great attention as promising candidates for next-generation lighting due to its low energy consumption, high efficiency, excellent reliability, long lifetime, etc [1,2]. In order to generate white light using LEDs, one approach is to combine a blue LED chip emitting at 465 nm with a broad band yellow Y₃Al₅O₁₂:Ce³⁺ (YAG:Ce³⁺) phosphor. However, it exhibits low color rendering index (CRI) due to lacking of red component in the emission spectra, which limits its application. Another approach is to combine a near-ultraviolet (NUV) InGaN-based LED chip (350-420 nm) with the red, green and blue phosphors. In this approach, the phosphors are excited by the NUV chips, and the tricolor emissions make an excellent white light, which usually offer better color rendering performance [3]. Unfortunately, the efficiency of currently commercially used red phosphor Y₂O₂S:Eu³⁺ is much lower than that of green and blue phosphors [4]. Therefore, more efforts should be devoted to searching for efficient red phosphors that can efficiently absorb light in the NUV region (350-420 nm).

Trivalent europium (Eu³⁺) ions activated phosphors are considered as ideal red sources for WLED because of the sharp emission lines in the red spectral region [5]. For applications in NUV-based WLED, the phosphors should be able to efficiently absorb the emission from the LED chips. However, the f-f excitation of Eu³⁺ ions is narrow in bandwidth and weak in intensity due to the parity forbidden nature of the f-f transitions, which leads to the low efficiency. In order to enhance light absorption, sensitizers are introduced to absorb the excitation light and transfer the excitation energy to the Eu³⁺ ions, which is a feasible method to overcome this shortage and obtain promising red phosphors [6].

The Eu³⁺-doped molybdates and tungstates were investigated because the MoO₄ and WO₄ groups can efficiently absorb ultraviolet light through the excitation of the Mo-O and W-O charge transfer states (CTS), respectively. This excitation is then followed by a transfer of the absorbed energy to Eu³⁺ ions for red emission [7,8]. Nevertheless, most of these materials exhibit low efficient absorption in the NUV region. Recently, much attention has been drawn to the Eu³⁺- doped molybdate and tungstate R₂MO₆ (R=Y, Gd, La; M=Mo, W) for their remarkable properties such as excellent thermal and chemical stabilities and a broad excitation band [9-12]. However, little attention has been attracted to investigate the Eu³⁺-doped Lu₂MoO₆ phosphors. LI et al [13] have synthesized the Lu₂MoO₆:Eu³⁺ phosphors by sol-gel method and investigated the structure and photoluminescence properties of such phosphors. However, the structural, luminescent properties and energy transfer mechanism of Eu³⁺-doped Lu₂MoO₆ obtained by solid state reaction method were not studied. In this work, Eu³⁺-doped Lu₂MoO₆ phosphors were synthesized using the high-temperature solid-state reaction method. The effect of Eu³⁺ molar concentration on the structure, luminescence and decay lifetime of the Lu₂MoO₆:Eu³⁺ phosphors was investigated. The energy transfer mechanism of Eu³⁺ in Lu₂MoO₆ host was also discussed in detail.

2 Experimental

2.1 Sample preparation

The Lu₂MoO₆:Eu³⁺ phosphors were synthesized by the high-temperature solid-state method. The starting materials are analytical reagent (AR) grade molybdenum trioxide (MoO₃), lutetium oxide (Lu₂O₃) (99.99%) and europium oxide (Eu₂O₃) (99.99%). The stoichiometric amounts of reactants were ground thoroughly in an agate mortar and preheated at 600 °C for 1 h. Subsequently, the products were removed from the muffle furnace, cooled, finely ground and sintered at 1200 °C for 4 h in air. Finally, the products were cooled to room temperature and then ground into white powder to form the final products.

2.2 Characterization

The crystal structures were analyzed by X-ray diffractometer (Persee, XD-2, Beijing Purkinje General Instrument Co., Ltd, Beijing, China) with Cu K_α radiation (l=0.15406 nm). The excitation, emission spectra and the decay curves were measured by FLSP920 (Edinburgh Instrument Ltd, Livingston, UK) fluorescence spectrophotometer equipped with 450 W xenon lamp or a pulse xenon lamp as light sources and Shimidazu R9287 (Hamamatsu Photonics K.K., Hamamatsu, Japan) photomultiplier (200-900 nm) along with a liquid nitrogen-cooled InGaAs (Hamamatsu Photonics K.K.) (800-1700 nm) as the detectors. All spectra were collected at room temperature under identical experimental conditions so that the emission intensities of the samples with different Eu³⁺ doping concentrations can be compared.

3 Results and discussion

3.1 Structure analysis of Lu₂MoO₆:Eu³⁺ phosphors

Figure 1 shows unit cell of Lu₂MoO₆ drawn with VESTA [14]. This material is crystallized in the I2/a space group (No.15). The Lu³⁺ ions occupy three nonequivalent crystallographic sites, namely, 4e, 4e (with C₂  site symmetry) and 8f (with C₁  site symmetry), and all are coordinated to eight O atoms, whereas the Mo atoms are coordinated to five O atoms with four of which at a short distance and the other at a long distance [15].

Fig. 1 One unit cell of Lu₂MoO₆ with 8-fold coordination of Lu³⁺ ions

Figure 2 shows the XRD patterns of Lu₂MoO₆ phosphors doped with xEu³⁺ (x=1%, 3%, 5%, 10%, 20%, 40%). It indicates that all the diffraction peaks coincide well with the data from the JCPDS card No. 25–0972 (Tm₂MoO₆), and Lu₂MoO₆ can be assigned to the monoclinic structure [13]. No additional peaks of other phases have been found, indicating that the Eu³⁺ ions are effectively doped into the host lattice. Additionally, the diffraction peaks of Lu₂MoO₆:Eu³⁺ samples are found to shift a little to lower angles with the increase of Eu³⁺ concentration, this is because the radius of Eu³⁺ (0.95 ) is larger than that of Lu³⁺ (0.85 ) in Lu₂MoO₆ host. When Lu³⁺ is substituted by Eu³⁺, the interplanar distance d increases, the diffraction angles decrease according to the Bragg equation: 2dsinθ=λ, where d is the interplanar distance, θ is diffraction angle of peak, and λ is the X-ray wavelength (0.15405 nm). Therefore, it also demonstrates that Eu³⁺ ions are effectively doped into the Lu₂MoO₆ host to replace Lu³⁺ sites.

Fig. 2 X-ray diffraction patterns of Lu₂MoO₆:xEu³⁺ phosphor contrasted with standard pattern of JCPDS 25-0972

3.2 Photoluminescent properties and luminescence decay lifetime of Lu₂MoO₆:Eu³⁺ phosphors

Figure 3 shows the emission spectra of Lu₂MoO₆:xEu³⁺ phosphors under 363 nm excitation as a function of the Eu³⁺ concentration. The emission spectra of the Lu₂MoO₆:x%Eu³⁺ phosphors with similar shape consist of sharp lines with wavelength from 550 to 720 nm. The dominant red emission centered at 610 nm is assigned to the ⁵D₀→⁷F₂ transition of Eu³⁺ ions, while other weak emission located at 578 nm, 585 nm, 657 nm and 706 nm are attributed to the following transitions: ⁵D₀→⁷F_0, ⁵D₀→⁷F_1, ⁵D₀→⁷F₃ and ⁵D₀→⁷F₄, respectively. It is well-known that Eu³⁺ is an excellent probe ion, because the ⁵D₀→⁷F₂ transition (electric dipole allowed) is very hypersensitive to the chemical environment, while the ⁵D₀→⁷F₁ transition (magnetic dipole allowed) is insensitive to the environment. According to the Judd-Ofelt theory, the magnetic dipole transition is allowed. The electric dipole transition is permitted when the Eu³⁺ occupies the crystallographic sites without inversion symmetry [16]. Consequently, when the Eu³⁺ ions occupy the inversion center sites, the ⁵D₀→⁷F₁ transition should be relatively strong, while the ⁵D₀→⁷F₂ transition is very weak. The above experimental results indicate that the Eu³⁺ mainly occupies the nonequivalent crystallographic sites (both the C₁ and C₂) without inversion symmetry in the Lu₂MoO₆ lattice, which is consistent with the reported crystal structure of Lu₂MoO₆.

Fig. 3 Emission spectra of Lu₂MoO₆:xEu³⁺ phosphors (Inset represents emission intensity as function of Eu³⁺ concentration)

From the inset in Fig. 3, it is noticed that the emission intensity of 610 nm first increases and approaches a maximum at a doping concentration of 10% and then decreases with the increase of Eu³⁺ concentration. With the increase of Eu³⁺ concentration, the luminescence centers of Eu³⁺ increase, which results in increasing the emission intensity of Eu³⁺. Whereas, when Eu³⁺ concentration is very high, the concentration quenching occurs, thus reducing emission intensity. The optimal doping concentration of Eu³⁺ is 10%.

In order to obtain the concentration quenching mechanism of Eu³⁺ under high doping concentration, the emission intensity of ⁵D₀→⁷F₂ transition (610 nm) is integrated under 363 nm according to the DEXTER theory [17,18], and its relationship with doping Eu³⁺ concentration (x) can be expressed as

                                      (2)

where s stands for the series of electronic multidipole interaction, s=3, 6, 8, 10 denote the exchange interactions of ions, dipole-dipole, dipole-quadrupole, quadrupole-quadrupole interactions, respectively, and b is a constant [19]. The value of s can be deduced from the slope -s/3 of the linear line in Fig. 4, which plots lg(I/x) vs lg x on a logarithmic scale of I/x. The slope -s/3 can be obtained as -2.1038 by fitting the data. Subsequently, the value of s is calculated as approximately 6. Thus, the results indicate that the concentration quenching mechanism of Eu³⁺ emission in the Lu₂MoO₆ host is attributed to the dipole-dipole interaction.

Fig. 4 Logarithmic plots for emission intensity per activator Eu³⁺ ion (I/x) as function of Eu³⁺ concentration (x) in Lu₂MoO₆:Eu³⁺ phosphors

Moreover, in terms of the Blasse’s concentration quenching theory in inorganic phosphors [20], the critical distance R_c in Lu₂MoO₆:%Eu³⁺ phosphors can be estimated from the following equation:

                          (1)

where V is the volume of the unit cell of Lu₂MoO₆ (V=879.749 ³), x_c is the critical concentration of Eu³⁺ (x_c=0.1), and N is the number of host cations per unit cell (N=8) [13]. From Eq. (1), it can be calculated that the value of R_c is approximately equal to 12.8 . The results show the different values estimated from the other Eu³⁺-doped phosphors [21,22], which emphasizes the influence of crystal structure on the luminescent properties of the Eu³⁺ ions. From the presented experimental results, it is seen that the Lu₂MoO₆:Eu³⁺ phosphors exhibit a strong red light with high color purity, which can be utilized to improve the color rendering property of white LEDs.

Figure 5 shows the excitation spectra of Lu₂MoO₆:xEu³⁺ phosphors monitoring the ⁵D₀→⁷F₂ emission at 610 nm. As shown in Fig. 5, the excitation spectra Lu₂MoO₆:xEu³⁺ phosphors all have the same profile, while the intensity declines with the increase of the Eu³⁺ concentration. The excitation spectra consist of two parts: the narrow excitation peaks between 450 nm and 550 nm and the intense broad band ranging from 250 nm to 440 nm. The broad band centered at 363 nm is attributed to the overlap of the Mo⁶⁺-O^2- and Eu³⁺-O² charge transfer states [23]. The narrow peaks at 462 nm and 530 nm correspond to the ⁷F₀→⁵D₂ and ⁷F₀→⁵D₁ transitions of Eu³⁺, respectively. Therefore, the Lu₂MoO₆:Eu³⁺ phosphors can effectively absorb the emission in the 370-400 nm range from near-ultraviolet LED chips, which may be a potential candidate for application in near-ultraviolet excited WLED. Additionally, the observation of the Mo⁶⁺-O^2- charge transfer state in the excitation spectra of Eu³⁺ indicates that energy transfer from the MoO₆^6- groups to the emitting Eu³⁺ ions is present.

Fig. 5 Excitation spectra of Lu₂MoO₆:xEu³⁺ phosphors

Fig. 6 Luminescence decay curves of Eu³⁺ (l_ex=363 nm, l_em= 610 nm) in Lu₂MoO₆:xEu³⁺ phosphors

Figure 6 shows the luminescence decay curves of red emission (610 nm) for Lu₂MoO₆:xEu³⁺ phosphors under 363 nm excitation. The decay curves of phosphors show nonexponential characteristics and a mean decay lifetime (τ_m) can be evaluated by

                           (3)

where I(t) is the luminescence intensity at the time t, and I₀ represents the initial intensity at t=0 [24]. The calculated lifetimes are shown in the inset in Fig. 6. When the concentration of Eu³⁺(x) increases from 0 to 5%, there is no apparent change in the decay curve. As x reaches 10%, 20% and 40%, the faster decay was observed.

The emission spectra of the optimal Lu₂MoO₆: 10%Eu³⁺ and commercial Y₂O₂S:5%Eu³⁺ phosphors under 363 nm excitation are shown in Fig. 7. It is obvious that both the spectra show the characteristic emission of the Eu³⁺ ions. However, the spectral distributions are different as a result of different site symmetry for the Eu³⁺ ions in these two host lattices [5]. Moreover, the emission intensity of Lu₂MoO₆:10%Eu³⁺ under 363 nm excitation is about 2.2 times higher than that of Y₂O₂S:5%Eu³⁺ by comparing the two emission spectra. This result suggests that the Lu₂MoO₆:Eu³⁺ phosphors are potential red phosphors for near ultraviolet based solid state lighting.

Figure 8 shows the CIE (Commission Internationale de L’Eclairage) chromaticity diagram of Lu₂MoO₆: 10%Eu³⁺ phosphor under 363 nm excitation [25]. The Lu₂MoO₆:10%Eu³⁺ phosphors exhibit strong red emission. The corresponding chromaticity coordinates were calculated to be (0.65, 0.35), which are marked with Point 1 in the red luminescent region of Fig. 8. Expected values of the commercial Y₂O₂S:Eu³⁺ phosphors and the red phosphors are (0.62, 0.35) and (0.67, 0.33), which are shown with Points 2 and 3, respectively [26]. Based on our results, we suggest that Lu₂MoO₆:Eu³⁺ phosphors have met the commercial requirement in solid-state lighting.

Fig. 7 Emission spectra of Lu₂MoO₆:10%Eu³⁺ and Y₂O₂S: 5%Eu³⁺ phosphors under 363 nm excitation

Fig. 8 CIE chromaticity diagrams of Lu₂MoO₆:10%Eu³⁺ phosphors under 363 nm excitation

3.3 Energy transfer mechanism

According to the investigation above, the energy level diagrams involved in the energy transfer process from the host to Eu³⁺ in Lu₂MoO₆ are presented in Fig. 9. Under ultra-violet light excitation, the MoO₆^6- group is excited owing to charge transfer transition from the O 2p states in the host’s valence band to the Mo 4d states in the host’s conduction band. The absorption energy can be transferred to the Eu³⁺ ions followed by red emission from the ⁵D₀^→7F_J transitions as shown in Fig. 3, because the resonant nonradiative energy transfer will occur, as illustrated in Fig. 9.

Fig. 9 Energy level diagram of MoO₆^6- and Eu³⁺ with energy transfer process

4 Conclusions

1) The Eu³⁺ activated Lu₂MoO₆ phosphors were synthesized by the high-temperature solid-state reaction method. The XRD results indicate that Lu₂MoO₆ can be assigned to the monoclinic structure.

2) The excitation and emission spectra show that Lu₂MoO₆:Eu³⁺ phosphors can be effectively excited by the near ultraviolet light (363 nm) and exhibit a strong red emission at 610 nm. The optimal Eu³⁺-doped concentration and critical distance are 10% and 12.8 , respectively. The concentration quenching mechanism of Eu³⁺ emission in the Lu₂MoO₆ host is ascribed to the dipole-dipole interaction.

3) The experimental spectroscopic results presented in this work show the Lu₂MoO₆:Eu³⁺ phosphor as an excellent candidate for getting red emission for near ultraviolet based solid state lighting.

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用于固态照明的红色荧光粉Lu₂MoO₆:Eu³⁺的发光性质

李 丽¹，沈 君¹，周贤菊¹，潘 雨¹，常文轩¹，何琪伟¹，韦先涛²

1. 重庆邮电大学 理学院，重庆 400065；

2. 中国科学技术大学 物理系，合肥 230026

摘  要：采用高温固相法制得Eu³⁺掺杂的Lu₂MoO₆荧光粉，通过X射线衍射(XRD)及激发、发射光谱和衰减寿命等手段对样品的结构和发光性质进行了表征。XRD结果表明：制备的荧光粉均为单斜结构。实验结果表明该样品在可见光谱范围内能够被近紫外光有效地吸收，该吸收来自Mo⁶⁺–O^2-吸收带。在掺杂10% Eu³⁺的情况下，发光最强。详细地研究最佳临界距离R_c和能量机制。Lu₂MoO₆:Eu³⁺ 红色荧光粉是一种可应用于近紫外激发白光LED用的新型红色荧光粉。

关键词：Lu₂MoO₆:Eu³⁺；发光性质；红色荧光粉

(Edited by Yun-bin HE)

Foundation item: Project (11404047) supported by the National Natural Science Foundation of China; Projects (CSTC2015jcyjA50005, CSTC2014JCYJA50034) supported by the Natural Science Foundation Project of Chongqing, China; Project (KJ1500412, KJ1500409) supported by Scientific and Technological Research Program of Chongqing Municipal Education Commission, China

Corresponding author: Li LI; Tel: +86-23-62471721; E-mail: lilic@cqupt.edu.cn

DOI: 10.1016/S1003-6326(16)64276-0