Spectroscopic investigation of La7Ta3W4O30:Sm3+ orange-red phosphor for white LEDs

Yu Wang; Bin Deng; Yuee Ke; Shuang Shu; Rixin Liu; Ruijin Yu

doi:10.1016/j.arabjc.2020.03.018

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Original article

13 (

); 5581-5592

doi:

10.1016/j.arabjc.2020.03.018

Spectroscopic investigation of La₇Ta₃W₄O₃₀:Sm³⁺ orange-red phosphor for white LEDs

Yu Wang^{^a}, Bin Deng^{^b,^c,⁎}, Yuee Ke^{^a}, Shuang Shu^{^a}, Rixin Liu^{^a}, Ruijin Yu^{^a,⁎}

a College of Chemistry & Pharmacy, Northwest A&F University, Yangling, Shaanxi 712100, PR China

b College of Chemical Biological and Environmental Engineering, Xiangnan University, Chenzhou, Hunan 423043, PR China

c Hunan Provincial Key Laboratory of Xiangnan Rare-Precious Metals Compounds Research and Application, Chenzhou, Hunan 423043, PR China

⁎Corresponding authors at: College of Chemical Biological and Environmental Engineering, Xiangnan University, Chenzhou, Hunan 423043, PR China (B. Deng). ddhbyjs@126.com (Bin Deng), yuruijin@nwsuaf.edu.cn (Ruijin Yu)

Received: 2019-11-13, Accepted: 2020-3-22, Published: 2020-06-01

Disclaimer:
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.

Peer review under responsibility of King Saud University.

Abstract

The novel orange-red light emitting La₇Ta₃W₄O₃₀:xSm³⁺ (x = 0.005–0.20) phosphors were synthesized via the solid-state reaction method. The crystal structure, photoluminescence (PL) properties, optimum concentration, color purity, decay life, and thermal stability of the samples were systematically studied. Under the excitation of 404 nm, La₇Ta₃W₄O₃₀:Sm³⁺ emits intense orange-red light at 597 nm. The PL spectra of La₇Ta₃W₄O₃₀:Sm³⁺ phosphors are ascribed to the ⁴G_5/2 to ⁶H_J (J = 5/2, 7/2, 9/2, and 11/2) transitions of Sm³⁺ ions. The concentration quenching occurs at the doping level of 1 mol%. The quenching temperature is higher than 500 K. Finally, a white LED (w-LED) with the Commission Internationale de L'Eclairage (CIE) chromaticity coordinates of (0.312, 0.296) and good color rendering index (Ra) of 86 was fabricated. As a consequence, all the results suggest that the orange-red phosphors La₇Ta₃W₄O₃₀:Sm³⁺ have potential applications in w-LEDs structures.

Keywords

Luminescence

Tungstate

La7Ta3W4O30:Sm3+

Phosphor

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1 Introduction

At present, w-LEDs, as new ideal lighting sources, rapidly substitute the traditional fluorescent and incandescent lamps owing to its excellent advantages, such as environment-friendly, lower power consumption, robustness, durability, long operation time, high brightness and small size (Cao et al., 2019; Xu et al., 2013; Zhao et al., 2017; Min et al., 2017 ). Currently, the major way to manufacture w-LEDs is to merge the InGaN blue-emitting chip with yellow phosphor Y₃Al₅O₁₂:Ce³⁺ (YAG) (Tang et al., 2017; Atuchin et al., 2015; Ji et al., 2016; Galashov et al., 2017 ). Nonetheless, this product has some drawbacks, such as the high correlated color temperature (CCT) and low Ra, and these shortcomings make it difficult to develop the w-LEDs in the future (Wang et al., 2019). To resolve these problems, red-emitting phosphors can be applied to decrease CCT and improve the Ra value. As to commercial red phosphors of YVO₄:Eu³⁺, Y₂O₂S:Eu³⁺ and Y₂O₃:Eu³⁺, they absorb insufficiently in the NUV (near ultraviolet) region (Tang et al., 2014; Guo et al., 2008; Al-Mamun and Ishigaki, 2014 ). For some red nitride phosphors, such as CaAlSiN₃:Eu²⁺, Sr₂Si₅N₈:Eu²⁺, and Sr_xCa_1-_xAlSiN₃:Eu²⁺, the high pressure and high temperature conditions are needed for synthetic (Li et al., 2015; Wang et al., 2018; Zou et al., 2016 ). Hence, in w-LEDs production, it is significant to investigate new red phosphors.

In a large number of rare-earth ions doped phosphors, Sm³⁺ ions are considered to be among the most popular and effective doped ions due to its outstanding luminescence properties. Sm³⁺ ions exhibit intensity emission in reddish orange region due to its ⁴G_5/2 → ⁶H_J (J = 5/2, 7/2, 9/2, and 11/2) transitions in such phosphors Sr₂MgTeO₆:Sm³⁺, LiCa₃MgV₃O₁₂:Sm³⁺ and Y₁₋_x₋_yGd_xVO₄:ySm³⁺ (Zhao et al., 2019; Huang and Guo, 2018; Jaiswal et al., 2020 ).

In a general way, the selection of appropriate host materials are particularly significant for efficient luminescence of rare-earth ions. Rare-earth tungstates demonstrate high thermal and chemical stability, have good absorption in the NUV region and widely used in laser technology (Zhang et al., 2016; Atuchin et al., 2008, 2005 ). Hence, the preparation and properties of tungstate doped rare earth luminescent materials have aroused more and more interests. For instance, Y₆WO₁₂:Dy³⁺, Lu_2–_xSm_xWO₆ and KLaMgWO₆:Mn⁴⁺ can be pointed (Yu et al., 2014; Zhang et al., 2018; Liang et al., 2019 ). Recently, the columnar-perovskite-type La₇W₄M₃O₃₀ (M = Nb, Ta) tungstate has been found to be a member of the A₇B₇O₃₀ (A = La, B = Mo) family (Lacorre and Corbel, 2019). However, up to now, there were no reports on the optical properties of Sm³⁺ ions doped in La₇Ta₃W₄O₃₀ tungstate phosphor.

In this work, the novel orange-red emitting La₇Ta₃W₄O₃₀:xSm³⁺ (LTWO:xSm³⁺) phosphors were synthesized. The structural characterization, PL excitation (PLE) and PL properties, optimum concentrations, thermal stability and CIE chromaticity coordinates of the LTWO:xSm³⁺ phosphors were studied systematically. At final, a white LED with good Ra was fabricated by using LTWO:0.01Sm³⁺ phosphor.

2 Experimental procedure

2.1

2.1 Preparation

LTWO:xSm³⁺ (x = 0.005, 0.01, 0.02, 0.05, 0.10, 0.15 and 0.20) phosphors were prepared by the method of solid-state synthesis method. The raw materials were La₂O₃ (99.99%), WO₃ (99.8%), Ta₂O₅ (99.5%), and Sm₂O₃ (99.99%), which were from Sigma-Aldrich. The powder was weighed on the basis of the stoichiometric ratio and then ground appropriately in an agate mortar. Then, the mixtures were preheated at 700 °C for 2 h in the air, then reground and sintered at 1350 °C for 10 h in the air. Eventually, the phosphor powders of white color were obtained. The equation for this reaction is as follows: $7 (1 - x) {L a}_{2} O_{3} + 8 {W O}_{3} + 3 {T a}_{2} O_{5} + 7 x {S m}_{2} O_{3} \to 2 {L a}_{7 (1 - x)} {S m}_{7 x} {T a}_{3} W_{4} O_{30}$

2.2

2.2 Measurements

The X-ray diffraction (XRD) patterns were recorded by a Bruker D2 PHASER X-ray diffractometer with use of Cu Kα radiation (λ = 0.15405 nm) in the 2θ angle range of 10–70°. The morphology of LTWO:Sm³⁺ samples was measured by a Nova Nano SEM-450 field emission scanning electron microscopy (SEM). The size distribution was examined by a Malvern MS2000 laser particle size analyzer. The PL spectra at different temperatures (300–500 K) and the decay times were studied with the use of an Edinburgh FLS 980 spectrometer. The electroluminescence spectrum (EL) of the w-LED fabricated with LTWO:Sm³⁺ phosphor was examined by a fiber optic spectrometer Ocean Optics, USB 4000.

3 Results and discussion

3.1

3.1 Phase analysis

Fig. 1 (a) is the crystal structure of LTWO, which belongs to the trigonal crystal system, with R-3 (No.148) space group. The cell parameters are a = b = 17.0701(2) Å, c = 6.8851(1) Å, V = 1737.45(5) Å³, and Z = 3 (Goutenoire et al., 2005; La7Ta3W4O30 Crystal Structure). The new compound LTWO has the same structure type as La₇Mo₇O₃₀ and it belongs to the A₇B₇O₃₀ family. This type is a columnar-perovskite-type arrangement, which is formed by the isolated hexagonal perovskite building units (Goutenoire et al., 1999). In this structure, the 3b site is occupied by Ta(1), it coordinated with six oxygen atoms, and all the Ta(1)-O distances are equal to 1.9922 Å but the bond angles are different. Thus, the coordination polyhedron [Ta(1)O₆] is an almost regular octahedron as shown in Fig. 1 (b). The coordination of Ta(2)/W(1) is similar to Ta(1), and the coordination polyhedron [Ta(2)/W(1)O₆] is also an octahedron with six oxygen atoms, but with a strong distortion, the bond length is between 1.8824 and 2.2401 Å. The coordination polyhedron of La(1) and La(2) are different. La(1) occupies the 3a site which coordinates with twelve oxygen atoms and the coordination polyhedron [La(1)O₁₂] is a quasi-normal icosahedron with La-O distances ranging from 2.5619 to 2.7318 Å. La(2) is coordinated with nine oxygen atoms, and the coordination polyhedron [La(2)O₉] is more distorted, with the minimum bond length of 2.4231 Å³ and the maximum bond length of 2.8605 Å. The atom coordinates in LTWO structure are listed in Table 1. Moreover, with the purpose of further studying the substituted ions (La³⁺) in LTWO host lattices, the result can be estimated by this equation (Sun et al., 2018):

(1)

D_{r} = \frac{R_{1} (C N) - R_{2} (C N)}{R_{1} (C N)} \times 100 %

(a) The crystal structure of LTWO. (b) The coordination environment of La(1), La(2), Ta(1) and Ta(2)/ W(1) atoms.

Table 1 The crystal parameter of LTWO.

Atom	Site	Occupancy	x	y	z
La1	3a	1	0	0	0
Ta1	3b	1	0	0	0.5
La2	18f	1	0.7766(2)	−0.0173(2)	0.3443(4)
W1/Ta2	18f	0.66/0.33	0.1998(2)	0.0151(2)	0.1596(5)
O1	18f	1	0.2460(2)	0.1006(2)	0.3563(5)
O2	18f	1	0.2930(2)	0.0431(2)	−0.0074(5)
O3	18f	1	0.1691(2)	0.1118(2)	0.0454(5)
O4	18f	1	0.2044(3)	−0.0726(2)	0.3067(4)
O5	18f	1	0.0348(2)	0.1049(2)	0.3237(5)

In this equation, D_r is the radius percentage difference, R₁ and R₂ are the radius of the substituted ions and doped ions, respectively. CN is the coordination number. In this work, Sm³⁺ ions can replace La(1) and La(2), the coordination numbers are 12 and 9, respectively. When CN = 12, the radius of La(1) and Sm³⁺ ions are 1.032 Å and 0.958 Å, the D_r value is equal to 7.17% (Shannon, 1976). When CN = 9, the radius of La(2) and Sm³⁺ ions are 1.1 Å and 1.02 Å, the value of D_r is 7.27%. In both cases, the Dr Values are less than 30%, indicating that Sm³⁺ ions can replace La³⁺ ions without damaging the crystal structure, and the D_r value of La(1) is less than that of La(2), manifesting that Sm³⁺ ions occupy La(1) site first.

The XRD patterns of LTWO:xSm³⁺ (x = 0.005–0.20) phosphors are shown in Fig. 2 (a). The simulated pattern for LTWO host is obtained according to the study of F. Goutenoire et al. (Goutenoire et al., 2005). By comparison, it is clearly that the peaks of all samples fit well with the simulated pattern of LTWO. It also matches well with the La₇Mo₇O₃₀ standard card (PDF 89-4366), which indicated that the structure of LTWO is the same as La₇Mo₇O₃₀. But there are two small impure peaks in the XRD patterns, which are ascribed to La_0.33TaO₃ admixture (PDF 42-0061) (Trunov et al., 1968). Therefore, the experimental conditions will be further optimized. For the purpose of further studying the Sm³⁺ environment, the Rietveld refinement of the LTWO:0.01Sm³⁺ phosphor by GSAS software was carried out and the different pattern is presented in Fig. 2(b). As obtained by Rietveld analysis, the LTWO:0.01Sm³⁺ compound crystallized with unit cell parameters of a = b = 17.1125 Å, c = 6.9177 Å, V = 1754.376 Å³, and Z = 3 (R_wp = 15.19%, R_p = 9.30%). The appropriate R_wp and R_p values indicate that the Sm³⁺ ions have successfully replaced La³⁺ ions. Furthermore, the refinement parameters of the sample are listed in Table 2.

(a) The XRD patterns of LTWO:xSm3+ (x = 0.005–0.20) phosphors. (b) The Rietveld refinement of the LTWO:0.01Sm3+ phosphor.

Table 2 The Refinement parameters of the LTWO:0.01Sm³⁺ phosphor.

Formula	La₇Ta₃W₄O₃₀:0.01Sm³⁺
Space group	R-3 (No.148)
Cell parameters	a = 17.1125 Å
	b = 17.1125 Å
	c = 6.9177 Å
	α = β = 90°
	γ = 120°
	Z = 3
Cell Volume (Å)	1754.376
χ²	7.818
R_wp	15.19%
R_p	9.30%

The particle sizes and surface morphology are significant for the phosphor properties. The SEM micrographs of the samples with doping concentrations of 2 and 20 mol%, which are monitored at the magnification of 20,000 times are shown in Fig. 3(a) and (b), respectively. It is obvious that the samples are mainly composed of partly coalescent particles and the particle diameters are very small and evenly distributed. It could be mentioned that such particle morphology is formed due to intense grain interdiffusion and it is typically observed in oxide compounds subjected to high temperature treatment (Atuchin et al., 2008; Lim et al., 2015). Comparing the panels, it can be concluded that the Sm³⁺ ion concentration has little effect on the particle size. The particle size distribution in LTWO:0.20Sm³⁺ is presented in Fig. 3(c). The highest peak value of the particle size distribution is 21.678 µm, indicating the particle size of samples is mainly distributed around 21 µm. Therefore, this sample has the potential to be used in industrial w-LEDs.

(a) and (b) The SEM micrographs of LTWO:0.01Sm3+ and LTWO:0.20Sm3+. (c) The particle size distribution of LTWO:0.20Sm3+.

In Fig. 4, the optical band gap (E_g) determination in the LTWO and LTWO:0.01Sm³⁺ compounds is shown. The samples have a strong absorption between 200 and 350 nm. Based on the relation between (αhν)² and hν, the E_g value can be obtained by this formula (Devi et al., 2019):

(2)

{(α h ν)}^{1 / n} = A (h ν - E g)

where α is the absorption coefficient, hν means the photon energy, A is constant and n = 0.5 for direct transition. The E_g values of LTWO and LTWO:0.01Sm³⁺ are 3.44 and 3.38 eV, respectively.

The relationship between the (αhν)2 and photon energy for LTWO and LTWO:0.01Sm3+.

3.2

3.2 Optical properties

The excitation spectrum of LTWO:0.01Sm³⁺, as monitored at 597 nm at room temperature, is shown in Fig. 5. It is mainly composed of two components. The first component is a broad band of CTB (charge transfer band) between 200 and 350 nm. By Gaussian fitting, it is be found that the wide band consists of two broads (a) and (b), which are due to the electron transition of O^2–-Sm³⁺ and O^2–-W⁶⁺, respectively (Li et al., 2017, 2011). The second component is a set of several sharp peaks appeared between 350 and 500 nm by the 4f-4f transitions of Sm³⁺ ions, which from the ground state ⁶H_5/2 to the excited states ⁴D_3/2, ⁴D_1/2, ⁴F_7/2, (⁶P,⁴P)_5/2, ⁴G_9/2, ⁴I_13/2 and ⁴I_11/2 at 361, 376, 404, 419, 439, 463 and 480 nm, respectively (Deng et al., 2015). Among these peaks, the highest one is located at 404 nm, manifesting that the samples can sufficiently be excited by the NUV chips and have a potential in w-LED manufacturing.

Excitation spectrum of LTWO:0.01Sm3+ (λem = 597 nm).

In Fig. 6(a), the emission spectrum is shown for LTWO:0.01Sm³⁺ phosphor excited at 404 nm, which revealed four obvious peaks between 500 and 750 nm. Distinctly, the peak at 597 nm, owing to ⁴G_5/2 → ⁶H_7/2 transition, has the highest intensity. Simultaneously, other characteristic peaks are centered at 561, 644 and 703 nm, which are in line with the 4f-4f electronic forbidden transitions of Sm³⁺ ions from the ⁴G_5/2 level to ⁶H_J (J = 5/2, 9/2 and 11/2) levels (Guo et al., 2017; Atuchin et al., 2015, 2017 ). Among these peaks, the first peak at 561 nm is of MD (magnetic-dipole) natured, and the highest peak at 597 nm is combined MD and ED (electric-dipole) transition. The third peak at 644 nm is due to ED transition, which is sensitive to the crystal field. Based on Judd–Ofelt theory, the comparison of ED/MD ratio is a good way to judge the environment around the Sm³⁺ ions in LTWO host. The stronger the ED transition, the greater the asymmetry property, and vice versa. In this study, the ED transition (644 nm) is significantly stronger than the MD transition (561 nm) that indicates the asymmetry nature of Sm³⁺ ions in LTWO lattice. In Fig. 6(b), the Sm³⁺ energy level diagram, illustrating the Sm³⁺ ions energy transfer process in the LTWO host lattice, is given. Under the 404 nm excitation, Sm³⁺ ions at ⁶H_5/2 ground state level absorb photon energy, and the electrons excite to ⁴F_7/2 level, and, then the electrons of Sm³⁺ ions relax without radiation to the ⁴G_5/2 level. At final, the radiative transition of Sm³⁺ ions from ⁴G_5/2 level to ⁶H_J (J = 5/2, 7/2, 9/2 and 11/2) levels are corresponding to four sharp peaks at 561, 597, 644 and 703 nm, respectively.

(a) Emission spectrum of LTWO:0.01Sm3+ (λex = 404 nm). (b) The Sm3+ energy level diagram.

The emission spectra of LTWO:xSm³⁺ (x = 0.005–0.20) phosphors are shown in Fig. 7(a). It is evident that the characteristic peaks of all phosphors are of similar shape except for the intensity, and no wavelength shift is detected at high dopant concentrations. With the doping level increases, the emission intensity first increases. When the doping content is higher than 1%, the emission intensity gradually decreases owing to concentration quenching, as shown in the upper right corner of Fig. 7(a). The luminescence intensity quenching is resulted from the energy transfer between activators. The critical transfer distance (R_c) can be calculated by this formula (Xia and Liu, 2012):

(3)

R_{c} \approx 2 {(\frac{3 V}{4 π x_{c} N})}^{1 / 3}

where x_c stands the optimum concentration, N means the number of substitutable ions, V points the cell volume. In this work, V = 1754.376 Å³, x_c = 0.01 and N = 3, and the value of R_c is equal to 48 Å. This result is greater than 5 Å, indicating that the main cause of concentration quenching is the multipole-multipole interaction.

(a) Emission spectra of several LTWO:xSm3+ (x = 0.005–0.20) phosphors (λex = 404 nm). (Inset: The normalized intensity of the emission spectra). (b) The lgI/x ∼ lgx diagram of LTWO:xSm3+ (x = 0.005–0.20) phosphors.

Based on the theory of Dexter, the multipole interaction can be judged by the change of emission intensity (I) between the emission level and the multipole interaction. The value of I can be obtained by this equation (Chen et al., 2016):

(4)

\frac{I}{x} = K {[1 + β {(x)}^{Q / 3}]}^{\begin{matrix} - 1 \end{matrix}}

where x points the doping concentration of Sm³⁺ ions, β and K are constants. Q value is controlled by the mechanism of dipole-dipole (6), dipole-quadrupole (8) or quadrupole-quadrupole (10) interaction. The multipole interaction can also be judged by this equation (Tian et al., 2011):

(5)

l o g (\frac{I}{x}) = A - \frac{Q}{3} \log (x), A = \log (K) - \log (β)

The relation between lgI/x and lgx of LTWO:Sm³⁺ phosphors is displayed in Fig. 7(b), which indicates that the slope of lgI/x to lgx is −2.06. Afterward, Q is equal to 6.18, which is close to 6. This result manifests that the dipole-dipole interaction is the main mechanism of concentration quenching of Sm³⁺ ions emission in LTWO host.

3.3

3.3 CIE chromaticity coordinate and color purity

The CIE color coordinates of LTWO:xSm³⁺ (x = 0.005–0.20) phosphors can be calculated on the basis of PL spectra, and the formula is as follows (Robertson, 1968):

(6)

\{\begin{matrix} X = \int_{500}^{750} P (λ) \bar{x} (λ) d λ \\ Y = \int_{500}^{750} P (λ) \bar{y} (λ) d λ \\ Z = \int_{500}^{750} P (λ) \bar{z} (λ) d λ \end{matrix}

(In this study, the integral is obtained by summing in the range of 500–750 nm with the unit of 1 nm). Where X, Y, and Z are the tristimulus values that can be acquired from the P(λ). P(λ) means the PL spectral intensity at wavelength λ. $\bar{x}$ , $\bar{y}$ , and $\bar{z}$ are the color-matching functions. The CIE color coordinates of LTWO:Sm³⁺ phosphors are shown in Fig. 8(a). It is clear that all the coordinate points of phosphors are located at the boundary of the CIE graph, suggesting the samples have good color purity. Furthermore, the color purity can be calculated by this formula (Seeta Rama Raju et al., 2011):

(7)

C o l o r p u r i t y = \frac{\sqrt{{(x_{s} - x_{i})}^{2} + {(y_{s} - y_{i})}^{2}}}{\sqrt{{(x_{d} - x_{i})}^{2} + {(y_{d} - y_{i})}^{2}}} \times 100 %

(a) The CIE chromaticity graph of LTWO:xSm3+ (x = 0.005–0.20) phosphor. (b) The movement of color coordinates x and y of LTWO:xSm3+ (x = 0.005–0.20).

In this equation, $(x_{i}, y_{i})$ are the color coordinates of the optical source point (0.333, 0.333). $(x_{s}, y_{s})$ are the color coordinates value of the phosphor. $(x_{d}, y_{d})$ are the CIE color coordinates of the extension of the line that connects $(x_{s}, y_{s})$ to $(x_{i}, y_{i})$ to the boundary. For the LTWO:0.01Sm³⁺ phosphor, $(x_{s}, y_{s})$ are (0.590, 0.409), $(x_{d}, y_{d})$ are (0.590, 0.410). The obtained color purity is 99.9% by fitting the formula. The color purity and the CIE color coordinates of other doping concentrations are given in Table 3. In Fig. 8(b), the extension of Fig. 8(a) is shown, reflecting the color coordinates movement of LTWO:xSm³⁺ (x = 0.005–0.20) phosphors. In the process of gradually increasing the doping concentration, the x-coordinate shift is Δx = −0.043 and the y-coordinate shift is Δy = 0.048. The Δx and Δy values are small, indicating that the color purity is less affected by Sm³⁺ doping concentration. Therefore, the LTWO:Sm³⁺ phosphors have a potential to replace the common red phosphors in the w-LEDs application.

Table 3 The color purity and the CIE color coordinates of LTWO:xSm³⁺ (x = 0.005–0.20) phosphors.

Concentration × (mol)	CIE (x, y)	Color purity
0.005	(0.591, 0.408)	99.9%
0.01	(0.590, 0.409)	99.9%
0.02	(0.590, 0.409)	99.9%
0.05	(0.588, 0.411)	99.9%
0.10	(0.584, 0.415)	99.9%
0.15	(0.565, 0.433)	99.6%
0.20	(0.548, 0.449)	99.3%

3.4

3.4 Luminescence decay curves

The decay curves of LTWO:xSm³⁺ (x = 0.005, 0.01, and 0.02) phosphors, which are monitored at 597 nm and excited under 404 nm, are shown in Fig. 9. The decay curves can be well fitted by this formula (Huang et al., 2018):

(8)

I (t) = I 0 + A e x p (\frac{- t}{τ})

The decay curves of LTWO:xSm3+ (x = 0.005, 0.01, and 0.02) phosphors.

In this equation, I(t) is the luminescence intensity at time t. I₀ is system noise which exactly is the background of the fluorescence intensity (Zhang et al., 2018). A is a constant and τ is the decay lifetime. When the Sm³⁺ ions contents are 0.5; 1 and 2 mol%, the lifetime of Sm³⁺ ions in LTWO samples are 1.095, 0.993, and 0.824 ms, respectively. With the increase of doped Sm³⁺ ions, the critical distance decreases gradually, which makes the decay time shorten gradually.

3.5

3.5 Thermal stability

The phosphor thermal stability is an essential factor for w-LEDs application. Good phosphors are required to have good thermal stability to avoid thermal quenching at high temperature. The PL spectra of LTWO:0.01Sm³⁺ recorded from 300 to 500 K under 404 nm excitation are presented in Fig. 10(a). It is clear that all spectra have the same peak shape and position. With the temperature increase, the emission intensity of the phosphor decreases gradually. The emission intensity of the sample is 85% of the initial intensity at normal working temperature (420 K) of LED. T_0.5 is the quenching temperature while the emission intensity reduced to 50% of the initial intensity. It can be observed from the inset in Fig. 10 (a) that the T_0.5 of LTWO:0.01Sm³⁺ phosphor is greater than 500 K and the luminescence intensity decreased slightly. In Fig. 10 (b), the contour map of the strongest emission peak changing with temperature is shown. With the temperature increasing, the intensity of the emission peak at 597 nm decreases slowly.

(a) The PL spectra of LTWO:0.01Sm3+ detected from 300 to 500 K under 404 nm excitation. Iinset: The relationship between PL intensity and temperature). (b) The intensity of the strongest peak (597 nm) changing with temperature of LTWO:0.01Sm3+ phosphor.

The activation energy (E_a) of LTWO:Sm³⁺ can be obtained by the Arrhenius formula (Qiao et al., 2019):

(9)

I (T) = \frac{I_{0}}{1+c exp (- E_{a} / kT)}

where I(T) represents the Sm³⁺ ions emission intensity at temperature T, I₀ is the initial luminescence intensity, c is a constant, and k is the Boltzmann constant (8.62 × 10⁻⁵ eV/K) (Tian et al., 2013). The relation of ln[(I₀/I)⁻¹] on 1/T is shown in Fig. 11 (a), and the E_a is determined to 0.28 eV. The thermal quenching mechanism of LTWO:Sm³⁺ phosphors is presented in Fig. 11 (b). Under the excitation of 404 nm, the electron transitions from the ⁶H_5/2 ground state to the ⁴F_7/2 excited state, afterward decreasing to the ⁴G_5/2 level without radiation (process ①), and then returns to the ground state through the radiation transition. As the temperature increases, some electrons at the ⁴G_5/2 level overcome the barrier of E_a through phonon-electron coupling to reach the junction between the ⁴G_5/2 level and CTB (process ②). Subsequently, these electrons will supply to CTB and relax to the ⁶H_5/2 ground state without radiation (process ③). The higher the temperature, the more electrons overcome the E_a to reach the junction, afterward the emission peak intensity of Sm³⁺ ion decreases gradually and the thermal quenching occurs. Furthermore, the LTWO:Sm³⁺ phosphors have a higher E_a (0.28 eV) comparing with those of Ba₂Y₃(SiO₄)₃F:Sm³⁺ (0.13 eV) and Li₃Gd_2.85Te₂O₁₂:Eu³⁺ (0.22 eV), and, so it has good thermal stability (Yu et al., 2015; Deng et al., 2017). All the results demonstrate that the LTWO:Sm³⁺ phosphors have wonderful thermal stability and good potential to be applied in w-LEDs manufacturing.

(a) The activation energy photograph of LTWO:0.01Sm3+ phosphor for the thermal quenching process. (b) Thermal quenching mechanism of Sm3+ in LTWO host.

3.6

3.6 Fabrication of w-LED

For the purpose of investigating the application of LTWO:Sm³⁺ phosphors, a w-LED was fabricated by merging a NUV 407 nm chip with BaMgAl₁₀O₁₇:Eu²⁺ (BAM, blue phosphor), (Ba, Sr)₂SiO₄:Eu²⁺ (green phosphor) and the synthesized LTWO:0.01Sm³⁺ (orange-red phosphor). The EL spectrum of the w-LED is shown in Fig. 12(a). Several main peaks of the w-LED are located at 560, 595, 641, and 702 nm, which are basically consistent with previous spectroscopic data. The fabricated w-LED has high Ra (86) and good CCT (6818 K) that are comparable with the parameters of those of w-LED structures based on different phosphors (Xia et al., 2013; Ji et al., 2014; Leaño et al., 2016 ). Ra plays an important role in the application of w-LEDs, which is the average value of the special color rendering index from R1 to R8. R9 is the saturated red special color rendering index. R1 to R14 are special color rendering indexes, representing the color rendering performance of different colors (Wang et al., 2017). Fig. 12(b), the Ra value histogram is given for the fabricated w-LED and the commercial w-LED (blue chip + YAG phosphor), it can be observed that the fabricated w-LED has higher Ra (86) and R9 (94) than Ra (80) and R9 (14) of the commercial w-LED (Zhang et al., 2008). These results indicate that the fabricated w-LED has stronger white light rendering performance than the commercial w-LED. The LTWO:Sm³⁺ phosphors can help to overcome the lack of red light in traditional commercial w-LEDs.

(a) The EL spectrum of the w-LED fabricated by a NUV 407 nm chip with BAM, (Ba, Sr)2SiO4:Eu2+ and LTWO:0.01Sm3+. (b) The Ra histogram of the w-LED manufactured by combining NUV chip with LTWO:0.01Sm3+ phosphor and the commercial w-LED made by blue chip and YAG phosphor.

The CIE chromaticity coordinates and actual pictures of LTWO:0.01Sm³⁺ phosphor and the fabricated w-LED are shown in Fig. 13. The CIE chromaticity coordinates of LTWO:0.01Sm³⁺ are (0.590, 0.409), which fall in the orange-red region. The CIE chromaticity coordinates of the fabricated w-LED are (0.312, 0.296), which are in the centre of the white region. The real sample of LTWO:0.01Sm³⁺ phosphor emits bright orange-red light under the exposure of ultraviolet lamp. The fabricated w-LED shows a dazzling white light driven by a current of 300 mA and a voltage of 3.2 V. Therefore, all the results manifest that the LTWO:Sm³⁺ phosphors can be well applied in w-LEDs manufacture.

The CIE chromaticity coordinates and actual pictures of LTWO:0.01Sm3+ phosphor and the fabricated w-LED.

4 Conclusion

In summary, the novel orange-red light emitting LTWO:Sm³⁺ phosphors were successfully prepared via the solid-state reaction. The Rietveld refinement manifested that the samples belong to the trigonal crystal structure of space group R-3 (No.148), and Sm³⁺ ions replace La³⁺ ions in the LTWO lattice. The PLE spectrum demonstrates that the LTWO:Sm³⁺ phosphors can be availably excited by the O^2–-Sm³⁺ CTB, O^2–-W⁶⁺ CTB, and NUV light. The LTWO:Sm³⁺ phosphors emit intense orange-red light at 597 nm under 404 nm excitation, owing to the ⁴G_5/2→⁶H_7/2 transition. The quenching concentration of LTWO:Sm³⁺ phosphors is approximately 1 mol%. The main quenching mechanism of Sm³⁺ ions in LTWO host lattice is the dipole-dipole interaction. The quenching temperature is higher than 500 K, and the E_a is as high as 0.28 eV. By merging a NUV 407 nm chip with BAM blue phosphor, (Ba, Sr)₂SiO₄:Eu²⁺ green phosphor and LTWO:0.01Sm³⁺ orange-red phosphor, a w-LED with the CIE chromaticity coordinates of (0.312, 0.296) and Ra of 86 was obtained. All the results suggest that the orange-red LTWO:Sm³⁺ phosphors have the potential value to replace the commercial red phosphors in w-LEDs application.

Acknowledgments

The work was supported by the Natural Science Foundation of Shaanxi Province (Grant No. 2018JM5055), the Fundamental Research Funds for the Central Universities (2452019076), the Hunan Provincial Key Laboratory of Xiangnan Rare-Precious Metals Compounds and Applications (2019XGJSKFJJ01), the Construction Program of the key discipline in Hunan Province, the Projects of the Education Department of Hunan Province (No.18A465), the Science and Technology Plan Project of Chenzhou city (jsyf2017014) and Undergraduate Innovation Fund of Northwest A&F University, China (201910712037).

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Introduction

Experimental procedure

Preparation
Measurements

Results and discussion

Phase analysis
Optical properties
CIE chromaticity coordinate and color purity
Luminescence decay curves
Thermal stability
Fabrication of w-LED

Conclusion

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[1] Al-Mamun S.A., Ishigaki T., . Influence of hydrogen peroxide addition on photoluminescence of Y₂O₃:Eu³⁺ nanophosphors prepared by laser ablation in water. J. Am. Ceram. Soc.. 2014;97:1083-1090.
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[2] Atuchin V.V., Kesler V.G., Maklakova N.Y., Pokrovsky L.D., . Core level spectroscopy and RHEED analysis of KGd(WO₄)₂ surface. Solid State Commun.. 2005;133:347-351.
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[3] Atuchin V.V., Gavrilova T.A., Grivel J.C., Kesler V.G., . Electronic structure of layered titanate Nd₂Ti₂O₇. Surf. Sci.. 2008;602:3095-3099.
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[4] Atuchin V.V., Pokrovsky L.D., Khyzhun O.Y., Sinelnichenko A.K., Ramana C.V., . Surface crystallography and electronic structure of potassium yttrium tungstate. J. Appl. Phys.. 2008;104 033518
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[5] Atuchin V.V., Beisel N.F., Galashov E.N., Mandrik E.M., Molokeev M.S., Yelisseyev A.P., Yusuf A.A., Xia Z., . Pressure-stimulated synthesis and luminescence properties of microcrystalline (Lu, Y)₃Al₅O₁₂:Ce³⁺ garnet phosphors. ACS Appl. Mat. Interfaces. 2015;7:26235-26243.
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[6] Atuchin V.V., Aleksandrovsky A., Chimitova O., Diao C.-P., Gavrilova T., Kesler V., Molokeev M., Krylov A., Bazarov B., Bazarova J., . Electronic structure of β-RbSm (MoO₄)₂ and chemical bonding in molybdates. Dalton Trans.. 2015;44:1805-1815.
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[7] Atuchin V.V., Aleksandrovsky A.S., Molokeev M.S., Krylov A.S., Oreshonkov A.S., Zhou D., . Structural and spectroscopic properties of self-activated monoclinic molybdate BaSm₂(MoO₄)₄. J. Alloys Compd.. 2017;729:843-849.
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[8] Cao R., Chen T., Ren Y., Chen T., Ao H., Li W., Zheng G., . Synthesis and photoluminescence properties of Ca₂LaTaO₆:Mn⁴⁺ phosphor for plant growth LEDs. J. Alloys Compd.. 2019;780:749-755.
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[9] Chen X., Lv F., Ma Y., Zhang Y., . Preparation and spectroscopic investigation of novel NaAlP₂O₇:Eu²⁺ phosphors for white LEDs. J. Alloys Compd.. 2016;680:20-25.
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[10] Deng H., Zhao Z., Wang J., Hei Z., Li M., Noh H.M., Jeong J.H., Yu R., . Photoluminescence properties of a new orange-red emitting Sm³⁺ doped Y₂Mo₄O₁₅ phosphor. J. Solid State Chem.. 2015;228:110-116.
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[11] Deng H., Gao Z., Xue N., Jeong J.H., Yu R., . A novel Eu³⁺-doped garnet-type tellurate red-emitting phosphor with high thermal stability and color purity. J. Lumin.. 2017;192:684-689.
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[12] Devi S., Dalal M., Dalal J., Hooda A., Khatkar A., Taxak V.B., Khatkar S.P., . Near-ultraviolet excited down-conversion Sm³⁺-doped Ba₅Zn₄Gd₈O₂₁ reddish-orange emitting nano-diametric rods for white LEDs. Ceram. Int.. 2019;45:7397-7406.
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[13] Galashov E.N., Atuchin V.V., Gavrilova T.A., Korolkov I.V., Mandrik Y.M., Yelisseyev A.P., Xia Z., . Synthesis of Y₃Al₅O₁₂:Ce³⁺ phosphor in the Y₂O₃-Al metal-CeO₂ ternary system. J. Mater. Sci.. 2017;52:13033-13039.
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