Luminescence properties of new red-emitting phosphor Li2Al2Si3O10:Eu3+ for near UV-based white LED

Luminescence properties of new red-emitting phosphor Li2Al2Si3O10:Eu3+ for near UV-based white LED

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Optical Materials xxx (xxxx) xxx

Contents lists available at ScienceDirect

Optical Materials journal homepage: http://www.elsevier.com/locate/optmat

Luminescence properties of new red-emitting phosphor Li2Al2Si3O10:Eu3þ for near UV-based white LED Bin Fan a, c, Jun Liu b, Weixin Zhou b, Limin Han c, * a

School of Chemistry and Chemical Engineering, Inner Mongolia University of Science and Technology, Baotou, 014010, China Intematix Photonics (Suzhou) Co.,Ltd., Suzhou, 215555, China c School of Chemical Engineering, Inner Mongolia University of Technology, Hohhot, 010051, China b

A R T I C L E I N F O

A B S T R A C T

Keywords: Luminescence Phosphors Optical materials and properties

Eu3þ doped Li2Al2Si3O10 red phosphor was synthesized by the conventional high-temperature solid-state method. The crystal structure, morphology, luminescence properties, thermal stability and quantum efficiency were characterized. The powders show average particle size of 8 μm. The optimum Eu3þ dopant concentration is 0.02 in Li2Al2Si3O10 host lattice. Luminescence quenching is attributed to electric dipole–dipole interaction and impurities. The emission intensity of the 5D0 → 7F2 transition (615 nm) is dominant upon the strongest excitation at 394 nm (7F0 → 5L6 transition). The Li2Al2Si3O10: 0.02Eu3þ phosphor can be excited by near-ultraviolet (n-UV) light and exhibits red emission with the CIE chromaticity coordinates of (0.6389, 0.3488) and color purity as high as 93%. The Li2Al2Si3O10: 0.02Eu3þ phosphor also possesses good thermal stability with quenching energy barrier of 1803 cm 1. The fluorescence lifetime and quantum efficiency are 0.388 ms and 31.5%, respectively. The results suggest that the red phosphor Li2Al2Si3O10:Eu3þ can be used for n-UV excited white LEDs.

1. Introduction Currently, white light-emitting diodes (WLEDs) are regarded as the next-generation light source due to high efficiency, long lifetime, energy savings and environment friendliness [1,2]. To obtain white light, the commercialized method is the combination of blue-emitting GaN chips (450–460 nm) and yellow-emitting phosphors Y3Al5O12: Ce3þ (YAG: Ce3þ). However, this method has a poor color rendering index (CRI, Ra < 80) and high corrected color temperature (CCT > 7000 K). In order to overcome these drawbacks, the n-UV based LEDs couple n-UV-LED chips with various primary colors from different phosphors and have gained much consideration in the WLEDs fabrication because they can deliver greater excitation energy than utilizing blue chips [3]. Among the phosphors, the red phosphors exhibit significant role to improve the CRI and reduce CCT, such as nitride, oxysulfide and fluoride red phosphors. However, nitride red phosphors need expensive raw materials and fastidious synthesis conditions. Oxysulfide and fluoride red phosphor have the low thermal stability and serious environmental pollution. Compared with them, oxide red phosphors is a proper option if they have an excitation band in the n-UV spectral region. Eu3þ ions as red activator are the most commonly used in phosphors due to the strong absorption in UV or n-UV region [4,5]. Especially,

when the Eu3þ ion occupies a non-centrosymmetric site in the host, the phosphors will emit pure red light coming from the 5D0 → 7F2 emission in the red spectral region. For example, the commercial red phosphor Y2O3:Eu3þ exhibits good luminescence performance, but it is unsuited to n-UV LED chip due to the strongest excitation band in the UV region (220–260 nm). Therefore, the feasible technique to improve the f-f excitation intensity of Eu3þ or shift charge transfer band (CTB) to the n-UV region. Recently, rare earth alkali based silicates are considered to be good host materials for rare-earth ions doped phosphors because of optical and stability properties, such as NaAlSiO4: Eu2þ, Ce3þ [6], BaZrSi3O9: Ce3þ [7], LiAlSiO4: Sm3þ [8]. Li2Al2Si3O10 belongs to alkali based silicates and it could be a good host for rare earth ions activated phosphors. However, up till now, Eu3þ-activated Li2Al2Si3O10 red phosphors have not been reported in detail. To find a novel red-emitting phosphor that is applied to n-UV LEDs, the red phosphor Li2Al2Si3O10: Eu3þ were prepared by the high-temperature solid-state reaction method. The luminescent properties and compared with commercial red phosphor phosphors were also investigated. It exhibited relatively strong absorption in the n-UV region and intense red emission around 615 nm with good color purity and high thermal stability.

* Corresponding author. E-mail address: [email protected] (L. Han). https://doi.org/10.1016/j.optmat.2019.109499 Received 29 July 2019; Received in revised form 29 October 2019; Accepted 30 October 2019 0925-3467/© 2019 Elsevier B.V. All rights reserved.

Please cite this article as: Bin Fan, Optical Materials, https://doi.org/10.1016/j.optmat.2019.109499

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(ionic radius ¼ 0.42 Å) and Al3þ (ionic radius ¼ 0.39 Å) because they are close in ionic radius [9]. Considering the charge compensation mecha­ nism, there are some oxygenrelated defect centers or Liþ vacancies in the host. Since both ionic radius and charge are different indeed, lattice expansion will be obvious under high doping concentration. With Eu3þ ion concentration increasing, the XRD diffraction peak at 25� –26� shifts towards lower angles gradually as shown in the enlarged picture in Fig. 1. This result demonstrates the expansion of cell volume on the basis of Bragg equation. As a results, it is possible that the Eu3þ ions will take an interstitial position in lattice rather than replacing Liþ ions, where additional other peaks will be detected in the XRD pattern as shown in Fig. 1 (x ¼ 0.05). No matter what, the Eu3þ ions cannot occupy in the inversion centre. All the phosphors are single host with Eu3þ doping concentration below 0.045. Fig. 2(A) shows the emission intensity at 615 nm of Li2-xAl2Si3O10: xEu3þ (0.005 � x � 0.05) depending on the Eu3þ doping concentration (λex ¼ 611 nm). With the Eu3þ doping concentration increasing, the emission intensity presents an upward tendency at first, and reaches the maximum at x ¼ 0.02, and then decreases gradually. The luminescence intensity quenching can be explained by the defects introduced and the concentration quenching effect. When the Eu3þ ions replace Liþ ions

2. Experimental 2.1. Preparation A series of red phosphors Li2-xAl2Si3O10: Eu3þ (0.005 � x � 0.05) were prepared by the traditional high temperature solid-state method. The appropriate amounts of Li2CO3 (A.R.), Al2O3 (A.R.), SiO2 (A.R.) and Eu2O3 (99.99%) were weighed and ground together in an agate mortar with small amount of ethanol for 30min. The mixtures were transferred into a corundum crucible with a cover and sintered in a muffle furnace at 1150 � C for 4 h. After cooling to room temperature, grinding and washing, the target phosphors were obtained. 2.2. Characterization The X-ray diffraction (XRD) patterns were recorded using a Rigaku D/max-IIIB diffractometer with Cu Kα radiation (λ ¼ 0.15405 nm). The excitation and emission spectra were measured by a Hitachi F-4600 fluorescence spectrophotometer (Japan) equipped with a 150 W Xe lamp as the excitation source. The temperature-dependent luminescence properties were measured by the same spectrophotometer combined with a self-made heating controller. The morphology was observed with scanning electron microscopy (SEM) by FEI QUANTA 400. The particle size is measured by OMEC LS-POP(6) laser particle analyzer. The luminescence decay curve was recorded on an Edinburgh FLS 920 steady state and transient state fluorescence spectrometer. The sample was excited by an nF900 ns flash lamp with a pulse width of 1 ns and pulse repetition rate of 40–100 kHz. The internal quantum yields (QY) was obtained by Quantaurus-QY (C11347, Hamamatsu) with a 100 W Xe lamp. 3. Results and discussion In order to study the phase formation and purity of phosphors Li2xEu3þ, Fig. 1 shows the XRD patterns of phosphors Li23þ Al Si O : xEu (0.005 � x � 0.05). It can be found that all the XRD x 2 3 10 pattern agrees well with JCPDS (Joint Committee on Powder Diffraction Standards) file card number 25–1183 (Li2Al2Si3O10) except the phos­ phor of x ¼ 0.05. Other impurity peaks marked with the symbol “*” are observed when x ¼ 0.05. Based on the retrieved data, these impurity peaks may be diffraction peaks of LiAlSi2O6, LiAlSi3O8 and/or others, but the amount of these impurities does not exceed 1% of the total material. For Eu3þ doped Li2Al2Si3O10 samples, Eu3þ (ionic radius ¼ 0.95 Å) will replace Liþ (ionic radius ¼ 0.68 Å) rather than Si4þ xAl2Si3O10:

Fig. 2. Dependence of the emission intensity at 615 nm on the doping con­ centration of Eu3þ ions (0.005 � x � 0.05) (A) and curve of log x versus log (I/x) (0.02 � x � 0.045) (B) in Li2-xAl2Si3O10: xEu3þ phosphors excited by 394 nm.

Fig. 1. XRD patterns of phosphors Li2-xAl2Si3O10: xEu3þ (0.005 � x � 0.05). 2

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with no isovalent charge, a distortion in the Eu3þ site can be induced in order to compensate the electronic charge deficiency. With the Eu3þ concentration increasing, structural defects will increase and impurities are formed according to the result of XRD. They are not good for improving luminescence intensity. The concentration quenching phe­ nomena can be explained by Blasse and Grabmaier equations as follows [10]: Rc ¼ 2[(3V)/(4πxeZ)]1/3, where V is the volume of the unit cell (V ¼ 130.02 Å3), xe is the critical concentration of activator ion (xe ¼ 0.02), and Z is the number of cations in the unit cell (Z ¼ 0.6). Rc among Eu3þions value is calculated to be 27.45 Å, which is higher than 5 Å. Therefore, electric multipolar interaction is involved in energy transfer. The multipole interactions can be judged by the equation [11, 12]: I/x ¼ K[1þβ(x)θ/3] 1, where I/x is the emission intensity per acti­ vator concentration, x represents the activator concentration beyond the quenching concentration (0.02 � x � 0.045). To investigate the con­ centration quenching phenomena, the Li2Al2Si3O10: 0.05Eu3þ phosphor should not be considered because it is not single host. K and β are constants. θ ¼ 6, 8, and 10 correspond to electric dipole-dipole (d-d), dipole-quadrupole (d-q), quadrupole-quadrupole (q-q) interactions, respectively. After the logarithm, the slope of the approximately linear relation between lg(I/x) and lg(x) is found to be -θ/3. As shown in Fig. 2 (B), θ/3 is about 1.935. θ value is calculated to be 5.8, closing to 6. Hence, in the single phase Li2Al2Si3O10: Eu3þ phosphor, electric dipole-dipole interaction is the dominated mechanism for concentration quenching. Based on above results, the optimum Eu3þ doped concen­ tration is 0.02 in the Li2Al2Si3O10: Eu3þ phosphors. Fig. 3 (a) shows the SEM image of the phosphor Li2Al2Si3O10: 0.02Eu3þ. It can be seen that the phosphor exhibits irregular morphology with different diameter ranging from 2 μm to 15 μm. The particles show good dispersion with slight agglomeration. Fig. 3(b) shows the particle size distribution of the phosphor Li2Al2Si3O10: 0.02Eu3þ The mean grain size is about 8 μm. Therefore, this phosphor can meet the chip coating requirements and can be used in LED package. Fig. 4 presents the excitation spectra of phosphor Li2Al2Si3O10: 0.02Eu3þ (λem ¼ 615 nm) and Y2O3: Eu3þ (commercial red phosphor, λem ¼ 611 nm). The excitation spectrum of Li2Al2Si3O10: 0.02Eu3þ con­ sists of a broad band in the wavelength range 250–350 nm and some sharp peaks from 300 to 550 nm. The former can be attributed to elec­ tron transfer from the filled 2p orbit of O2 ions to the partially filled 4f orbit of Eu3þ ion, named charge transfer band (CTB). The latter are attributed to the Eu3þ ions 4f-4f electronic transition, such as 7F0 → 5H3 (319 nm), 5D4 (362 nm), 5G2 (382 nm), 5L6 (394,400 nm), 5D3 (413 nm), 5 D2 (463, 466, 470 nm) and 5D1 (526, 531, 536 nm). The strongest excitation line at 394 nm in the n-UV region fits well with the emission wavelength of a commercial n-UV LED chip (395 nm). Compared with the commercial red phosphor Y2O3: Eu3þ (blue line in Fig. 4), the

Fig. 4. Excitation spectra of phosphors Li2Al2Si3O10: λem ¼ 615 nm) and Y2O3: Eu3þ (b: λem ¼ 611 nm).

0.02Eu3þ

(a:

Li2Al2Si3O10: 0.02Eu3þ phosphor can match the n-UV LED chips better. Fig. 5 exhibits emission spectra of phosphors Li2Al2Si3O10: 0.02Eu3þ and Y2O3: Eu3þ (commercial red phosphor) under 394 nm excitation. The characteristic emission peaks located at 575–585 nm, 585–605 nm, 605–640 nm, 640–670 nm and 685–720 nm are corresponding to the 5 D0 → 7F0, 7F1, 7F2, 7F3 and 7F4 electronic transitions of Eu3þ, respec­ tively. In the Eu3þ ions activated Li2Al2Si3O10 phosphor, the emission peak at 615 nm (5D0 → 7F2) is found to be strongest in intensity, sug­ gesting the doped Eu3þ ions locate at a noncentrosymmetry site without inversion centre. The synthesized phosphor Li2Al2Si3O10: 0.02Eu3þ emission intensity is higher than that of commercial phosphor Y2O3: Eu3þ under 394 nm excitation, indicating the phosphor Li2Al2Si3O10: Eu3þ is sufficient for practical applications in the n-UV LED chips. In addition, all the emis­ sion spectrum curve and the strongest emission peak are different for Li2Al2Si3O10: 0.02Eu3þ and Y2O3:Eu3þ phosphors due to host difference. The Commission International de ‘Eclairage (CIE) chromaticity co­ ordinates and the color purity of the phosphors will be different. On the basis of the emission spectra, Fig. 6 shows the CIE chromaticity co­ ordinates diagram of phosphors Li2Al2Si3O10: 0.02Eu3þ and Y2O3: Eu3þ as well as ideal red light [(0.6687, 0.3307)]. The CIE chromaticity co­ ordinate of phosphor Li2Al2Si3O10: Eu3þ is (0.6389, 0.3488), which lo­ cates in the red region and is more close to that of ideal red light

Fig. 3. SEM image (a) and particle size distribution (b) of Li2Al2Si3O10: 0.02Eu3þ. 3

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purity of Li2Al2Si3O10: Eu3þ phosphor is calculated to be as high as 93%. Meanwhile, the inset in Fig. 6 exhibits the luminescence photograph of the Li2Al2Si3O10: Eu3þ phosphor under the 394 nm n-UV lamp excita­ tion. Obviously, the Li2Al2Si3O10 phosphor can emit bright red light. All the results indicate that the Li2Al2Si3O10: Eu3þ phosphor has high emission intensity, good CIE chromaticity and good color purity. Thus, it can be appropriately applied in the n-UV LEDs as the red components. Fig. 7 shows the decay curve of Li2Al2Si3O10: 0.02Eu3þ (λex ¼ 394 nm, λem ¼ 615 nm). It can be seen that the decay curve can be fitted by a single exponential function equation: I(t) ¼ I0exp(-t/τ), where I(t) and I0 are the luminescent intensities at time t and initial intensity, respectively. τ is the decay time. This exponential fitting well (R2 ¼ 0.99864) clarifies that the emissions originate from one cationic site. The fluorescence lifetime τ is calculated to be around 0.388 ms, including that this phosphor is suitable for n-UV LED applications with short response time. The temperature stability of the luminescent property is one of the most important influence parameters for phosphor actual application during long-term operation. To investigate the luminescent temperature-dependent behavior, Fig. 8 depicts the normalized emis­ sion intensities of Li2Al2Si3O10: 0.02Eu3þ phosphor as a function of temperature ranging from 25 to 250 � C. There is no obvious change in temperature-dependent emission spectral shape. The emission in­ tensities exhibit a continuous decreasing tendency with increased tem­ perature owing to thermal quenching. Obviously, the emission intensity dropped to 85% of its initial value at 100 � C and 59% at 200 � C, indi­ cating its good thermal stability. To clarify the thermal quenching behavior, the quenching energy barrier (ΔE) for the thermal quenching process must be calculated by the Arrhenius equation [14]: I0/IT ¼ 1 þ Ce( ΔE/kT). Where, I0 the initial intensity, IT is the intensity at given temperature T, C is a constant, k is the Boltzmann’s constant (8.625 � 10 5 eV/K). Fig. 9 represents plots of ln[(I0/IT)-1] versus 1/(kT) for Li2Al2Si3O10: 0.02Eu3þ phosphor. After performing linear regression, the temperature quenching process com­ plies well with the Arrhenius-type quenching model and the obtained ΔE value is 1803 cm 1 (0.2237 eV) based on the slope of linear fitting. In addition, CIE chromaticity coordinates of Li2Al2Si3O10: 0.02Eu3þ phos­ phor at 150 � C, and 200 � C are (0.6379, 0.3485) and (0.6375, 0.3481), respectively, indicating that this phosphor has excellent thermal sta­ bility and good color stability at high temperature of ~200 � C. The internal QE defined as ratio of the number of emitted photons to the number of absorbed photons is also an important parameter for phosphor. Fig. 10 shows the excitation line of white BaSO4 powder and

Fig. 5. Emission spectra of phosphors (a) Li2Al2Si3O10: 0.02Eu3þ and (b) Y2O3: Eu3þ (λex ¼ 394 nm).

Fig. 6. CIE chromaticity coordinates diagram of Y2O3: Eu3þ, Li2Al2Si3O10: 0.02Eu3þ and ideal red light [(0.6687, 0.3307)]. Inset represents the lumines­ cence photograph of the Li2Al2Si3O10: 0.02Eu3þ phosphor under 395 nm n-UV lamp excitation. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

[(0.6687, 0.3307)] [13] compared with phosphor Y2O3: Eu3þ (0.6138, 0.3823). Besides, the color purity can be calculated by using the following equation: qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ðx xi Þ2 þ ðy yi Þ2 color ​ purity ¼ qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi � 100% ðxd xi Þ2 þ ðyd yi Þ2 where (x, y), (xi, yi) and (xd, yd) refer to the CIE chromaticity coordinates of the Li2Al2Si3O10: Eu3þ phosphor, white illumination and dominate wavelength, respectively. In this work, (x, y) ¼ (0.6389, 0.3488), (xi, yi) ¼ (0.3333, 0.3333), and (xd, yd) ¼ (0.6801, 0.3194), and thus the color

Fig. 7. Decay curve 615 nm (λex ¼ 394 nm). 4

of

Li2Al2Si3O10:

0.02Eu3þ

monitored

at

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Fig. 8. Emission spectra of Li2Al2Si3O10: 0.02Eu3þ phosphor for various tem­ peratures (λex ¼ 394 nm); the inset shows normalized emission intensities as a function of temperature ranging from 25 to 250 � C.

Fig. 10. Excitation line of BaSO4 and emission spectrum of Li2Al2Si3O10: 0.02Eu3þ phosphor collected by using an integrating sphere (λex ¼ 394 nm). Inset shows a magnification of the emission spectrum.

range from 300 nm to 550 nm with the strongest excitation peak at 394 nm, indicating a suitable n-UV LED chip phosphor is obtained. The emission spectrum displays the characteristic 5D0→7FJ (J ¼ 0–4) tran­ sitions of Eu3þ by near ultraviolet light excitation at 394 nm. Compared Eu3þ, the with the commercial red phosphor Y2O3: 3þ Li2Al2Si3O10:0.02Eu phosphor is better applied in the n-UV LED chips with high emission intensity, good CIE chromaticity of (0.6389, 0.3488) and good color purity of 93%. The Li2Al2Si3O10: 0.02Eu3þ phosphor shows a good thermal stability and good color stability at high tem­ perature with quenching energy barrier of 1803 cm 1, fluorescence lifetime of 0.388 ms and internal QE of 31.5%. Consequently, the results indicates that the red phosphor Li2Al2Si3O10: Eu3þ is promising red components for n-UV converted LEDs devices. 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.

Fig. 9. Plots fitted for thermal-quenching energy.

Acknowledgments

emission spectrum of Li2Al2Si3O10: 0.02Eu3þ phosphor collected by using an integrating sphere (λex ¼ 394 nm). On the basis of the data of Fig. 10, the internal QE value can be calculated by the formula R Ls R ,where, LS is the emission spectrum of the sam­ [15]:ηQE ¼ R ER

This work was supported by the Natural Science Fundation Com­ mittee of Inner Mongolia (2018MS02002).

Es

References

ple; ES and ER represent the integrated intensities of spectra of Li2Al2 Si3O10: 0.02Eu3þ phosphor and BaSO4 powder, respectively. The internal QE is calculated to be 31.5% under excited at 394 nm, indi­ cating that the Li2Al2Si3O10:0.02Eu3þ phosphor can act as a potential red-emitting n-UV convertible phosphor for white LEDs.

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4. Conclusions All the phosphors Li2Al2Si3O10: Eu3þ are synthesized by the tradi­ tional high temperature solid state reaction. The phosphors are single host with Eu3þ doping concentration below 0.045. The average particle size of powder is around 8 μm. The optimum Eu3þ doping concentration for obtaining maximum luminescent intensity at 615 nm is 0.02. The concentration quenching is identified to the electric dipole–dipole interaction. The excitation spectrum shows a series of excitation peaks 5

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