Synthesis, structure and luminescent properties of yellow phosphor La3Si6N11:Ce3+ for high power white-LEDs

Accepted Manuscript Synthesis, structure and luminescent properties of yellow phosphor La3Si6N11:Ce for high power White-LEDs

3+

Du Fu, Zhuang Weidong, Liu Ronghui, Liu Yuanhong, Gao Wei, Zhang Xia, Xue Yuan, Hao Hongrui PII:

S1002-0721(17)30047-9

DOI:

10.1016/j.jre.2017.05.010

Reference:

JRE 38

To appear in:

Journal of Rare Earths

Received Date: 22 February 2017 Revised Date:

9 May 2017

Accepted Date: 15 May 2017

Please cite this article as: Fu D, Weidong Z, Ronghui L, Yuanhong L, Wei G, Xia Z, Yuan X, Hongrui 3+ H, Synthesis, structure and luminescent properties of yellow phosphor La3Si6N11:Ce for high power White-LEDs, Journal of Rare Earths (2017), doi: 10.1016/j.jre.2017.05.010. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Synthesis, structure and luminescent properties of yellow

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phosphor La3Si6N11:Ce3+ for high power White-LEDs DU Fu(杜甫), ZHUANG Weidong*(庄卫东), LIU Ronghui*(刘荣辉), LIU Yuanhong(刘元红), GAO Wei(高慰), ZHANG Xia(张霞), XUE Yuan(薛原), HAO Hongrui(郝红蕊)

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(National Engineering Research Center for Rare Earth Materials, General Research Institute for Nonferrous Metals, and Grirem Advanced Materials Co. Ltd., Beijing 100088, China)

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Abstract： ：A series of high phase purity blue light excitable yellow-emitting La3-xSi6N11:xCe3+ phosphors were synthesized by the high temperature solid state reactions method. The structure and luminescent properties were investigated. The phase structure was studied by means of X-ray diffraction, structures refinements and energy dispersive X-ray spectroscopy. The phosphors effectively excited by the light of 450 nm and show intense yellow emission at 535 nm with FWHM of 115 nm corresponding to the 5d→2F5/2 and 5d→2F7/2 transitions of Ce3+. In addition, the optimized La2.86Si6N11:0.14Ce3+ exhibits a weak thermal quenching, which remains 98.2% of the initial emission intensity when heated to 200 °C, the thermal quenching properties exhibit a modest decline when the temperature returned to room temperature. The above results indicate that La3Si6N11:Ce3+ can be regard as a high promising phosphor for applications in high power white-light LED.

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Keyword： ：high phase purity, La3Si6N11:Ce3+, small thermal quenching, high power white-light LED.

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White light-emitting diodes (w-LEDs) have attracted increasing attention as the next-generation general illumination and the automotive lighting applications due to its outstanding merits of energy savings and positive environmental effects promised by solid-state lighting[1,2,3,4]. Phosphor as an important component of LEDs device, directly determines the luminous efficiency and color rendering index (CRI) of the output light[5,6,7]. However, large numbers of phosphors have been extensively studied for use in low power w-LEDs under the blue or near-ultraviolet (nUV) excitation. Therefore, the study of high power white-light LED phosphor may be a new trend in the

future[8,9]. Presently, the main models for achieving white light-emitting diode (WLED) is combining blue LED chip with yellow-emitting YAG:Ce3+ phosphor[10,11]. Up to now, in the high-power LED field, the common strategy to obtain white light emission by combining YAG:Ce3+ phosphor with blue LED. However, the external quantum efficiency of YAG:Ce3+ declined more than 20% when the temperature reached at 200 °C[12]. Therefore, the cooling system of components must been designed large enough to against aging effect, and consequently the production cost increases exponentially. Under high energy density excitation, the phosphor also needs to be chemically and

Foundation item: Project supported by the National Basic Research Program of China (2014CB643801). * Corresponding author: ZHUANG Weidong LIU Ronghui (E-mail:[email protected], [email protected] Tel.:+86-10-82241180) DOI: 【此处保持为空即可】

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thermally quite stable in order to prevent degradation at high temperature (~200 °C). Therefore, it is an urgent need to seek for practical phosphors with high luminous efficacy and lower thermal quenching.

excited by blue LED chip. The high phase purity, well formed morphology and high crystallization can greatly improve external quantum efficiency and luminescent properties.

For nearly a decade, rare-earth ion doped nitride phosphors have been explored widely because of the diversity of the photoluminescence emission and excitation, which can be excited effectively by blue or n-UV chip. On the other hand, most of nitrides exhibit high quantum efficiencies[7] or outstanding thermal quenching characteristics[13]. The basic frame of nitride phosphor is constructed with high dense (Si,Al)N4 tetrahedral, which forms a rigid skeleton structure and with nitrogen-rich environments. Basing on the principle of structure determining the properties, nitride phosphors show low thermal quenching property (this is termed the rigid skeleton structure) and the center of gravity for Ce3+/Eu2+ ions 5d energy levels were shifted to lower values (this due to the highly covalent bonding of Al-N/Si-N compared with Si-O or other oxygen-containing bond). Recently, blue light excitable yellow emitting Ce3+ activated La3Si6N11 (LSN) nitride phosphor strongly attracted attention because of its excellent physical, chemical stabilities, high quantum efficiencies and superior thermal quenching property than YAG:Ce3+ phosphor. The structure of LSN was first reported in 1995[14], but there is no research on the luminescent material because the pure phase can hardly be formed due to the harsh synthesis conditions[15,16,17,18]. The impurities consist mainly of the LaSi3N5 and lanthanide-involved (oxy) nitride such as La5Si3O12N:Ce3+, LaSiO2N:Ce3+. Moreover, photoluminescence efficiency within the excitation environment adopted for w-LEDs applications has not been improved, the best external quantum efficiency up to ∼42%, reported at present[17]. The impurities may be the one of the important reasons for the low luminescent efficiency of LSN:Ce3+.

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Experiment

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1.1 Preparation

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The La3-xSi6N11:xCe3+ (x = 0, 0.08, 0.10, 0.12, 0.14, 0.16, 0.18, 0.20) were synthesized by a high temperature solid state reaction. The stoichiometric amounts of LaN(Grirem Advanced Materials Co. Ltd., 4N), α-Si3N4:SN-E10, Ube Industries, 4N, CeN (Grirem Advanced Materials Co. Ltd., 4N) powers were weighed out and ground in an agate mortar. All processes should operate in a dry glove box with purity nitrogen due to the raw materials were easy to absorb water and oxygen. The mixed powder were put into molybdenum crucibles and calcined at 1000 °C for 4 h and raised to 1800 °C with heating rate of 10 °C/min, and held at that value for 5 h, the pressure of nitrogen gas maintained at 0.1 MPa. The chilled products were grounded for further measurements.

In present work, a series of high phase purity yellow-emitting La3-xSi6N11:xCe3+ were synthesized. The structure, thermal, luminescent properties of La3-xSi6N11:xCe3+ phosphors were also investigated in detail. The results demonstrated that La3-xSi6N11:xCe3+ phosphors exhibited two broad excitation bands with peaks at 360 nm and 455 nm, which can be effectively

1.2 Characterization

The phase purity of all samples were investigated by the X-ray powder diffraction (XRD, Advanced D8, Bruker, Cu Kα, λ=0.15418 nm) in the 2θ range of 10°-90°. Crystal structures refinements were performed by Rietveld method with the use of Fullprof software. The structure model was emerged by using the software VESTA. The energy dispersive X-ray spectroscopy (Tecnai G2 F20, FEI Co., Ltd., USA, accelerating voltage 200 kV) linked with HRTEM was used to assist the composition determination. The morphology of all samples was observed by using scanning electron microscopy (SEM, S4800, Hitachi, Japan). Measurement of photoluminescence properties were carried out on a UV-Vis spectrophotometer (Fluoromax-4, Edison, USA) using a 200 W Xe-lamp as the excitation source. All measurements were accomplished at room temperature. The temperature dependence external (η0) and internal (ηi) quantum efficiencies measurement were recorded by an intensified multichannel spectrometer (QE-2100, Otsuka electronics, Japan). A white BaSO4 powder was used as a standard reference for correction in the measurement

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Results and discussion

2.1 Phase component and crystal structure characterizations of La3-xSi6N11:xCe3+

(a)

(b)

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LSN:0.20Ce3+

calculated line raw date bragg positions differences

Intensity (a.u.)

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As mentioned above, the purity of LSN is hard to obtained, which directly affects its luminescent properties. Therefore, it is necessary to study the phase structure. The phase purity and structural type of the as-prepared powder samples LSN and LSN:0.20Ce3+ were identified by XRD. As shown in Fig. 1(a), it is obvious that all the diffraction peaks of the samples were well indexed to the Standard Card No. 85-0113 of Ce3Si6N11, and no diffraction peaks of other impurity phases can be detected in the samples. It is worthy to note that the impurity phase LaSi3N5 successfully was avoided with this method. Moreover, Fig. 1(b) shows that the diffraction peaks were shifted to higher angle with doping Ce3+ ion, indicating the gradual shrink of unit cell. The shift of the diffraction position can be explained by the substitution of the smaller ionic radii of Ce3+ (1.03 Å) ion on the larger La3+ (1.06 Å) ion[19]. This suggests that LSN phosphors are effectively synthesized using complex LaN, Si3N4 and CeN nitrides as raw materials.

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cell parameters, atoms coordinates and residual factors are summarized in Table 1. The as-obtained goodness of fit parameters and cell parameters are Rp = 4.26%, Rwp = 6.57%, and lattice parameters of a = b = 10.2428(1) Å, c = 4.82516(1) Å, which is close to the theoretical value. The refinement parameters are reliable and further verify that the pure phase was formed. The crystallographic data of La3Si6N11 was reported in detail by Yamane, et al[20]. The crystal structure of La3Si6N11 belongs to tetragonal with space group P4bm (No. 100). The structure of LSN can be described as La atom layers and SiN4 ring layers, two types of rings SiN4 tetrahedra linking with each other through their nitrogen bonds, forming three-dimensional networks with large voids containing La, shown in Fig. 3(a). Fig. 3(b) shows there exist two La (La1/La2) sites with same coordination number and slightly different geometries in the host, which surrounded by the Si4N4 and Si8N8 rings, respectively, which hints that there are two sites for the activator Ce3+ to occupy, the luminescence properties will been discussing in the next part.

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of QE.

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LSN

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2θ/(°) Fig. 2 Rietveld refinement X-ray patterns of LSN:0.14Ce3+ sample.

PDF#85-0113

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Fig. 1 (a) XRD patterns of LSN and LSN:0.20Ce3+with the standard data PDF#85-0113, (b) the shift of bragg diffraction (38~41º). To further investigate the exactly crystal structure of LSN, Rietveld refinement of X-ray powder diffraction experiments were carried out and shown in Fig. 2. The structural parameters of Ce3Si6N11 were used as initial model in the Rietveld analysis. The refined unit

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The phase and the element distribution of La, Si and Ce in the LSN:0.20Ce3+ sample were also analyzed by the high-resolution transmission electron microscopy (HRTEM) and corresponding EDS analysis, as shown in Fig. 4. The sharp diffraction spots of the fast Fourier transform (FFT) images suggest the highly single-crystalline nature of LSN sample, as shown in Fig. 4(b). It could be seen that the continuous lattice fringe measurement with d spacing of 0.48 nm is in accordance with the structure of the compound by refinement, which could be assigned to the (001) plane. The EDS spectrum of LSN:0.20Ce3+ (Fig. 4c) shows few dissolution of nitrogen in the sample due to the relatively small atomic weight of nitrogen. The elemental composition of LSN:0.20Ce3+ were verified by EDS shows in Fig. 5. The atomic ratio of (La+Ce)/Si for different measuring points are displayed in Fig. 5, which are close to the theoretical ratio 0.5 (3:6, marked in blue dotted line) and further confirmed the purity of obtained samples.

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Fig. 4 (a) Transmission electron microscopy of the LSN, (b) HRTEM and FFT images of LSN, (c) Energy dispersive spectrum of LSN

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Occu. 0.50000 0.25000 1.00000 0.50000 1.00000 0.50000 1.00000 0.25000

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Silica Content (atomic %)

y 0.18011 0 0.21315 0.11754 0.21564 0.17528 0.71916 0

La3Si6N11 P4bm 10.2428 4.82516 90 2 4.26 6.57 10.6 z 0.62725 0.65326 0.12656 0.62015 0.44583 0.92607 0.00026 0.58300

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formula space group a = b (Å) c (Å) α = β = γ (deg) Z Rp (%) Rwp (%) χ2 site x atom 4c 0.31832 La(1) 2a 0 La(2) 8d 0.07641 Si(1) 4c 0.61560 Si(2) 8d 0.08482 N(1) 4c 0.65645 N(2) 8d 0 N(3) 2a 0.50000 N(4)

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Fig. 3 Projection of the crystal structure of LSN, and the local coordination polyhedron of La1 and La2 Table 1 The results of structure refinement of La3Si6N11

points

42 40 38 36 34

La:Si theoretical ratio = 0.5

32 30 28 26

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Lanthanum Content (atomic %)

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Fig. 5 EDS elemental La and Si composition of the LSN sample. Generally, the luminous efficiency of the phosphor largely depends on the particle morphology, size, crystallinty and crystal structure. The morphology of the LSN and LSN:0.14Ce3+ are investigated. As shown in Fig. 6, the particles have uniform smooth, sharp, well crystallinity morphology and narrow size distribution with average diameter of about 20 µm. Normally, nearly spherical particles, well dispersed and crystallinity of the powder are beneficial to the luminous efficiency of phosphor. The above analysis shows that the luminous efficiency of LSN:Ce3+ may be improved by pure nitride synthesis and suitable in practical terms in the fabrication of white LEDs devices.

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raw materials. The PL spectra of La3-xSi6N11:xCe3+ with different Ce3+ dopant concentrations are shown in Fig. 7(b). The phosphors emit intense yellow emissions peaking at about 535 nm with a full width at half maximum (FWHM) of 110 nm under the excitation of 450 nm. The optimal emission intensity is obtained when x reached 0.14, which named critical concentration (xc) (in inset of Fig. 7b). The concentration quenching mainly attributed to the nonradiative energy transfer among Ce3+. The critical energy transfer distance (Rc) can be calculated using the following relation[22].

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 3V  Rc = 2    4π xc N 

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2.2 Photoluminescence properties of 3+ La3-xSi6N11:xCe (x = 0.08, 0.10, 0.12, 0.14, 0.16, 0.18, 0.20)

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The excitation and emission spectra of La3-xSi6N11:xCe3+ are shown in Fig. 7. The excitation spectra show a strong absorption ranging from 400 nm to 500 nm, which could be efficiently excited in the blue region. This makes it very suitable for using in solid state lighting in combination with commercial blue LEDs. Meanwhile, there are no changes in the shape and position of the excitation spectra, indicating no significant change of the structure with different Ce3+ doping contents. Unlike literature reports[15,17], the emission peak of 543nm is much stronger than that of 585nm, the same phenomenon occurs when doping other elements through a lot of experiments which we have done, such as Al3+, Ge4+, Lu3+. The refinement analysis revealed that Al3+ were easier to be doped into Si(1) site than Si(2) site causing the preferential occupation of Ce3+ in the La(1) site, enhancing the relative intensity of emission spectra in the range of 580~700 nm. This work is discussed in detail in our previous work[21]. This different phenomenon may be attributed to impurities of

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Where V is the volume of the unit cell, xc is the critical concentration, N is the number of cations in the unit cell that can be substituted by the activator ions. In this host, V = 505.9 Å3, xc= 0.14 and N = 6, the critical distance is calculated to be 10.46 Å. It is known that the nonradiative energy transfers would take place with three mechanisms: exchange interaction, radiation reabsorption or multipolar interaction. Base on the above results, it can be inferred that there is no mechanism of exchange interaction because the value of Rc (10.46 Å) is longer than the forbidden transition (the Rc is typically 5 Å). The PLE and PL do not overlap well[23], which suggests that radiation re-absorption have no effect on nonradiative energy transfers. Therefore, we can infer that the nonradiative energy transfers among Ce3+ occurs via multipolar interaction based on the Dexter theory[24]. Additionally, the PL spectra shows that there is a slight red shift from 532 nm ( x = 0.08) to 535 nm (x = 0.14) of the peaking, ascribing the ionic similar radius of Ce3+ ion and La3+ ion, the crystal field strength surrounding Ce3+ ion also remains little change when La3+ ion is substituted by Ce3+ ion.

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Fig. 6 SEM patterns obtained from LSN and LSN:0.14Ce3+ phosphor.

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Wavelength (nm) Fig. 7 Excitation spectra of LSN:xCe3+ (x = 0.08, 0.10, 0.12, 0.14, 0.16, 0.18, 0.20) monitoring at 535 nm(a) and emission spectra under 450 nm blue light excitation.

2.3 Temperature-dependent photoluminescence properties and CIE chromaticity

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For the application of w-LEDs, especially in high-power lighting fields, the excellent thermal stability of phosphors is one of important indicator because it has a strong influence on the light output and color rendering index[25]. Temperature-dependent PL spectra of LSN:0.14Ce3+ under the excitation of 450 nm were investigated and are shown in Fig. 8. The emission intensities of LSN:0.14Ce3+ decrease slightly (the emission intensities at 200°C remains 98.2%) when the ambient temperature increased to 200 °C. Besides, the emission band shows a little red-shifted with raising the temperature. And similar red-shifts were also observed in CaSiAlN3:Ce3+, which may be caused by the structural relaxation causing larger Stokes shift at high temperature[7]. Moreover, to further evaluate the luminous efficiency of LSN:0.14Ce3+, tempera-

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λem = 535 nm

ture-dependent of external quantum efficiency were recorded from 50 to 200 °C(red line) for heating and then from 200 to 50 °C(blue line) for cooling under air, and the measured intensities are plotted in the Fig. 8(b). The quantum efficiency decreased from 78.2% to 76.9% by further heating to 200 °C, whereas it recovered the initial value(77.82%) upon cooling, without showing any tendency of degradation in air. Generally, the quantum efficiency of phosphor was mainly determined by the morphology or crystalline and the total number of photons released. According to the temperature curve as we provided, high temperature and long holding time are beneficial to improve the crystallinity of the particles, as shown in Fig. 6. In addition, impurities have a large effect on luminous efficiency, ascribing to the redistribution of Ce3+ by introduction impurity ions. A lot of experiments with same phenomenon we have done, the mechanism analysis were carried out in the article[21]. The excellent thermal stability of LSN may be due to the crystal structure of the high dense SiN4 tetrahedral[26,27]. To further clarify the thermal quenching stability, the temperature-dependent emission intensity can be described in following equation[28]:

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Intensity (a.u.)

x = 0.08 x = 0.10 x = 0.12 x = 0.14 x = 0.16 x = 0.18 x = 0.20

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I (T ) =

I0  E  1 + A exp  - a   kT 

Where the I0 is the initial PL intensity and I (T) is the intensity at the testing temperature, k is the Boltzmann's constant and Ea is the activation energy. The activation energy of thermal quenching properties is estimated to be 0.62 eV by the Arrhenius equation, which is in accordance with the theoretical calculation[29]. These results indicate that this yellow phosphor exhibits promising photoluminescence properties, and can be potentially used to create white light when coupled with a blue LED.

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LSN:0.14Ce

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Fig. 9 Color coordinate of LSN:0.14Ce3+ under 460 nm in the CIE chromaticity diagram(a digital image of the phosphor under natural light revealing an intense yellow light is shown in the inset).

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heating curve cooling curve

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Fig. 8 Temperature dependence of the emission intensity for LSN:0.14Ce3+ sample.

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The Commission International de L’Eclairage (CIE) chromaticity coordinates of LSN:0.14Ce3+ upon 460 nm blue lamp excitation is shown in Fig. 9. The color coordinates are calculated to be (x = 0.4225, y = 0.5528), indicating this phosphor may serve as a yellow emitting component for w-LED application. The inset in Fig. 9 shows the digital photograph of LSN:0.14Ce3+, an intense yellow emission can be observed. The above results illustrate that La3Si6N11:Ce3+ would be a competitive yellow phosphor for high power white-light LED applications.

Conclusions

In this study, a series of high phase purity La3-xSi6N11:xCe3+ phosphors with intense yellow emission were successfully synthesized by solid-state reaction at 1800 °C The crystal structure, luminescence properties, and thermal quenching property have been investigated. The excitation spectra show a strong absorption ranging from 400 nm to 500 nm, which could be efficiently excited in the blue region. The optimal emission intensity is obtained when x reached 0.14. La2.86Si6N11:0.14Ce3+ exhibits a small thermal quenching, which remains 98.2% of the initial emission intensity measured at 200 °C. The heating and cooling curve indicate small tendency of degradation in high temperature. Excellent photoluminescence properties and high external quantum efficiency indicate that LSN:Ce3+ can be considered as practical phosphor for high power white-light LED applications.

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nitride phosphors: Neutral excitation, Stokes shift, and