Synthesis, structure and photoluminescence properties of fine yellow-orange Ca-α-SiAlON: Eu2+ phosphors

Journal of Alloys and Compounds 541 (2012) 70–74

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Synthesis, structure and photoluminescence properties of fine yellow-orange Ca-a-SiAlON: Eu2+ phosphors Zhigang Yang, Yuhua Wang ⇑, Zhengyan Zhao Department of Material Science, School of Physics Science and Technology, Lanzhou University, Lanzhou 730000, China

a r t i c l e

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Article history: Received 13 March 2012 Received in revised form 20 June 2012 Accepted 20 June 2012 Available online 28 June 2012 Keywords: Phosphor Ca-a-SiAlON White LEDs Luminescence

a b s t r a c t Yellow-orange oxynitride Ca-a-SiAlON: Eu2+ phosphors with the compositions of Ca0.8Si9.2Al2.8O1.2N14.8: xEu2+ (x = 0–0.24) were obtained via carbothermal reduction and nitridation method. Crystallographic parameters of the refinement structure with triclinic space group P31c (159) were obtained by XRD refinement as a = b = 7.8859(3) Å, c = 5.7336(2) Å, V = 308.7915(3) Å3, Z = 1. The resulting phosphors can absorb light in the range of 300–500 nm efficiently and show a single intense broad emission band in the wavelength range of 500–700 nm with maximum intensity at 580–601 nm. The chromaticity coordinates of the synthesized Ca-a-SiAlON: Eu2+ phosphors can be tailored by simply controlling the activator concentration. All the results show that Ca-a-SiAlON: Eu2+ phosphors can be a candidate for warm white LEDs. Ó 2012 Elsevier B.V. All rights reserved.

1. Introduction White light-emitting diodes (w-LEDs) have attracted increasing attention as a light source for the next-generation general illumination owing to their reliability, high efficiency, and low energy consumption [1,2]. Currently, the most common approach for making w-LEDs is to combine a broadband yellow-emitting phosphor with a blue LED chip, the successful development of which depends on the availability of efficient blue-light excitable phosphors. The commercial yellow phosphor is a trivalent cerium activated yttrium– aluminum–garnet (YAG: Ce) [3–6]. Unfortunately, the output light of the w-LEDs using this phosphor cannot realize the desirable color balance for a true color rendition owing to the deficiency of red light component, causing low color rendering index (CRI) and high correlated color temperature (CCT), which limits the possible applications of w-LEDs in the fields such as room lighting [7]. In the last few years, to obtain the warm white light output of the fabricated w-LEDs, divalent or trivalent rear-earth doped matrices have been studied, like silicate [8,9], germanate [10], borate [11], oxyfluoride [12,13] and so on. Significantly, highly covalent Si3N4-based oxynitrides (SiAlONs) have been developed as potential hosts for w-LEDs applications [14–23], due to their high thermal, chemical stability and excellent photoluminescence properties. a-SiAlONs are solid solutions of M–Si–Al–O–N system, which have structures derived from a-Si3N4 with the general formula Mxv+Si12(m+n)Alm+nOnN16n (x = m/v and M is one of cations Li, Mg, Ca, Y, and some rare earths) in which m + n (Si–N) bonds are replaced by m (Al–N) and n (Al–O) ⇑ Corresponding author. Tel.: +86 931 8912772; fax: +86 931 8913554. E-mail address: [email protected] (Y. Wang). 0925-8388/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2012.06.107

bonds [24]. Especially, the Eu2+ activated Ca-a-SiAlON is an excellent yellow ceramic phosphor for w-LEDs. For example, Ca-a-SiAlON: Eu2+ blending with b-SiAlON: Eu2+ (green) and CaAlSiN3: Eu2+ (red) embedded in a blue-LED chip, yielding w-LEDs with CRI of 80 of [25]. Furthermore, the CRI can be increased up to 96 when La1xCex(Si6zAlz)N10zOz (JEM: Ce3+, blue-green) phosphor is added to the above mixture phosphors combined with a UV–LED [25,26]. However, the synthesis of Ca-a-SiAlON are rather rigour through the traditional solid-state reaction and Gas Reduction Nitridation (GRN), because the former needs high-purity nitride starting material, high temperature and pressure; and the latter requires dangerous NH3–CH4 gas mixture as a reduction–nitridation agent [16,27]. Recently, a-SiAlONs have been prepared by selfpropagating high-temperature synthesis (SHS) and direct silicon nitridation method [21,28]. Direct synthesis of a-SiAlON powders by the carbothermal reduction and nitridation (CRN) method of oxide precursors has also been attempted, but the result is not ideal due to the concurrent SiC and carbon contamination in the products [29,30]. The SiC is caused by the reaction of SiO2 with carbon at high temperature. To obtain the pure phase of Ca-a-SiAlON: Eu2+ phosphor, Si3N4 was used to instead the SiO2 to avoid the formation of SiC in our work. Eu2+ doped Ca-a-SiAlON with the compositions of Ca0.8Si9.2Al2.8O1.2N14.8: xEu2+ are prepared by employing the carbothermal reduction and nitridation method, and their refinement structure and photoluminescence properties are investigated. 2. Experimental Stoichiometric amounts of CaCO3 (AR), Al2O3 (AR), Si3N4 (AR), Eu2O3 (4N), and high purity activated charcoal were mixed and placed in alumina crucibles positioned in an alumina tube furnace. Then the temperature was increased to

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1600 °C and maintained for 4 h under N2 gas flow. The as-prepared samples were heat-treated in air atmosphere at 650 °C for 2 h in order to reduce the carbon contamination. Phase assemblage of the product powders was analyzed by a Rigaku D/Max2400 X-ray diffractometer with Cu Ka radiation. Scanning electron microscope (SEM) images and energy-dispersive spectrometer (EDS) were taken on a Hitachi S-4800 scanning electron microscopy. The elements analysis has been done by using Vario EL and ICPAES, respectively. Reflectance spectra were measured on a PE lambda950 UV–vis spectrophotometer. The photoluminescence (PL) and excitation (PLE) spectra were obtained by a FLS-920T fluorescence spectrophotometer with Xe 900 (450 W xenon arc lamp) as the light source. The temperature-dependent luminescence measurements were carried out by the HORIBA JOBIN YVON Fluorlog-3 spectrofluorometer system starting from 20 to 250 °C in steps of about 30 °C with a heating rate of 100 °C/min.

3. Results and discussion The obtained Ca0.8Si9.2Al2.8O1.2N14.8: xEu2+ (0 6 x 6 0.24) series samples are all indexed by the Ca-a-SiAlON phase (JCPDS 330261). Structural refinement is further carried out by the Rietveld method, using the GSAS [31] program. The experimental, calculated, and difference XRD patterns are shown in Fig. 1. Ca0.8Si9.2 Al2.8O1.2N14.8: 0.15Eu2+ crystallizes as a triclinic structure (presented in the inset of Fig. 1) with a space group P31c (159) and lattice constants of a = b = 7.8859(3) Å, c = 5.7336(2) Å, V = 308.7915(3) Å3, Z = 1. All of the observed peaks satisfy the reflection condition, v2 = 1.64, Rp = 6.36%, and Rwp = 8.71%. Fig. 2 shows SEM images of the obtained Ca0.8Si9.2Al2.8O1.2N14.8: 0.14Eu2+ at various firing temperatures and an EDS spectrum of sample synthesized at 1600 °C. As verified in Fig. 2a–e, crystal morphology changes gradually with varying firing temperature. Fig. 2a and b show the initial reaction stage, and the dissolution-diffusion process already started. As firing temperature increased, the rodlike Ca-a-SiAlON crystals with the diameter of 0.2 lm are formed (in Fig. 2c and d). When the firing temperature up to 1600 °C, small rod-like crystals grown into large grains with the average diameter of 0.45 lm. The diameter distribution curve was given in inset of Fig. 2e, and almost 70% of rod-grains were greater than 0.35 lm. Hewett etc. [32] reported that the rod-like Ca-a-SiAlON grains fabricated via pressureless sintering may be a result of the preferential growth during densification(as seen in Fig. 2a–e). These processes can be proved by the XRD patterns shown in Fig. 3. At 1300 and 1400 °C, the main phase were Si3N4 and Ca2Al2SiO7, but we can find that the Ca-a-SiAlON already existed at 1400 °C by referring the JCPDS card. With increasing the firing temperature, the reaction acted more and the patterns of Si3N4 and Ca2Al2SiO7 disap-

Fig. 2. SEM images of samples obtained at (a) 1300, (b) 1400, (c) 1500, (d) 1550 and (e) 1600 °C, and EDS spectrum of sample synthesized at 1600 °C. The inset shows grain size distribution chart.

Fig. 3. XRD patterns of the obtained samples at 1300, 1400, 1500, 1550 and 1600 °C.

Fig. 1. Rietveld refinement plot for Ca0.8Si9.2Al2.8O1.2N14.8: 0.14Eu2+. Observed (crosses), calculated (line), and difference profile of the X-ray powder diffraction are plotted on the same scale. Bragg peaks are indicated by vertical bars. The inset shows structural view along the direction parallel to the c axis.

peared gradually. At 1600 °C, the pure phase of Ca-a-SiAlON was achieved and the intensities of XRD patterns became stronger, which is consistent with the rod-like Ca-a-SiAlON grains growing up. The typical EDS spectrum analysis indicates that the product have a chemical composition of Si, Al, Ca, O and N. The atom ratio of Si: Al and N: O are approximate 3.28 and 12.26, respectively, which agrees well with that calculated by formula Ca0.8Si9.2 Al2.8O1.2N14.8. In addition, to confirm the element compositions of the sample, the nitrogen content is analyzed using an elemental analyzer (Vario EL, Germany). The nitrogen content (weight %) obtained from the host sample was about 34.99%, in accordance

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with that in the formula Ca0.8Si9.2Al2.8O1.2N14.8 (34.93%). The molar ratio of Ca: Al was analyzed to be about 0.29 (versus nominal 0.28) by inductively coupled plasma-atomic emission spectrometry (ICPAES, IRIS Advantage ER/S, USA). All of these confirmed the formula Ca0.8Si9.2Al2.8O1.2N14.8. Fig. 4 shows the PLE spectra of synthesized samples with various Eu2+ concentrations of x = 0.10–0.20. The PLE spectra are broad ranging from UV to visible spectral region peaked at 300 and 396 nm, which is consistent with the reflection spectrum. (Fig. 4 inset) According to Ref. [34], the undoped sample shows a strong absorption in the range of 200–250 nm caused by the electronic transition in the host matrix (Ca-a-SiAlON). In the Eu2+-doped samples, strong absorption bands are presented from 300 to 500 nm region, which is assigned to 4f7 ? 4f65d transitions of Eu2+. Fig. 5 shows the PL spectra of the Ca0.8Si9.2Al2.8O1.2N14.8: xEu2+ samples with a single intense broad emission band at 580601 nm, and no detectable line emissions attributable to the 4f intraconfigurational transition of Eu3+ were observed. This emission band is attributable to the allowed 4f65d ? 4f7 transition of Eu2+ [14]. It also apparently indicates that the emission intensity of heat-treated sample is higher than that of as-prepared sample (Fig. 5 inset). The main reason could be that the residual carbon has been effectively removed after heat-treatment, which absorbs both excitation and emission light and affects the emission intensity. The carbon content was also analyzed using the elemental analyzer (Vario EL, Germany). For Ca0.8Si9.2Al2.8O1.2N14.8: 0.14Eu2+, the carbon content decreased from about 2.62 to 0.69% by the heat-treatment. The dependence of luminescence intensity on the Eu2+ concentration of heat-treated sample is given in Fig. 6. The emission intensity is maximized at the Eu2+ concentration around x = 0.14. Concentration quenching occurs when the Eu2+ concentration is beyond x = 0.14. The concentration quenching is mainly caused by the energy transfer among Eu2+, which increases as the concentration of Eu2+ increases [34]. In many cases, concentration quenching is due to energy transfer from one activator to another until energy sink in the lattice is reached [35]. To understand the energy-transfer mechanism of Ca0.8Si9.2Al2.8O1.2N14.8: xEu2+, it is useful to know the critical distance Rc for energy transfer between two activated ions such as Eu2+. In this study, the Dexter formula [36] that represents the transfer of the electric dipole–dipole interaction is used to dealing with symmetry-allowed transitions. Blasse [35] and Grabmaier [37] modified and suggested the following formula for Rc,

Fig. 5. The emission spectra of Ca0.8Si9.2Al2.8O1.2N14.8: xEu2+ samples. The inset shows the emission spectra of typical sample with x = 0.14, as-prepared and after heat-treatment at 650 °C.

Fig. 6. Dependence of emission intensity of Ca0.8Si9.2Al2.8O1.2N14.8: xEu2+ s phosphors on the concentration of Eu2+ ion.

R6c ¼ 0:63  1028 

QA 4

E

Z

F S ðEÞF A ðEÞdE

ð1Þ

where QA = 4.8  1016fd is the absorption cross section of Eu2+, fd  0.02 is the electric-dipole oscillator strength for Eu2+, R F S ðEÞF A ðEÞdE represents the spectral overlap between the normalized shapes of Eu2+ emission FS(E) and Eu2+ excitation FA(E), and E (in electron volts) is the energy of maximum spectral overlap. Therefore, the Rc of energy transfer is calculated to be about 17.1 Å. Based on the calculated critical distance and the spectral overlap of the excitation and emission bands, it can be confirmed that multipolar interaction and radiation reabsorption are responsible dominantly for the energy transfer between two Eu2+ [33]. Blasse suggested that the critical distance (Rc) of energy transfer can also be calculated by the critical concentration of the activator ion [35],

Rc ¼ 2

Fig. 4. The excitation spectra of Ca0.8Si9.2Al2.8O1.2N14.8: xEu2+ samples. The inset shows diffuse reflectance spectra of un-doped and typical samples with x = 0.14.



3V 4 p xc N

1=3 ð2Þ

where, xc is critical concentration, N is the number of ions which will be replaced by the activator in the unit cell, and V is the volume of the unit cell. For Ca-a-SiAlON host, when N = 1, xc = 0.14, and V = 308.7915(3) Å3, the obtained Rc value is 16.3 Å, which agrees approximately with that obtained by using the spectral overlap method. Another phenomenon relating to the Eu2+ concentration

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Fig. 7. Relative emission intensity as a function of temperature and the variation of the emission spectra with temperature for Ca-a-Sialon: Eu2+ (inset).

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processes [37–41], by which the luminescence intensity/quantum efficiency is quenched due to an increased 5d electron concentration of Eu2+. As the relative emission intensities are above 94.8% and 97.6%, respectively, at 150 and 250 °C. In this work, the thermal stability is better than other Ref. [33], in which the emission intensity remains at about 80% of that measured at room temperature. The reason could be that crystal defects which were harmful for luminescence reduced by thermal treatment at 650 °C. Ca-aSialon: 0.14Eu2+ are certainly suitable for use at high temperatures, in particular for white LED applications. The inset shows the emission spectrum of the samples measured at different temperatures. The position of the emission peak of Eu2+ is nearly kept at 585 nm from room temperature to 250 °C. And the shift of the emission band is very limited when the temperature raises, indicative of stable chromaticity coordinates of Ca-a-Sialon: Eu2+ phosphors. It is believed that no shift in color point is due to the rigid crystal structure of the a-sialon host lattice built up on (Si,Al)–(O,N) tetrahedral networks. The stable chromaticity of a-sialon is of importance for reducing the color point shift of w-LEDs. In addition, the spectral broadening is observed with increasing temperature, which is likely due to the increase in the electron–phonon interaction with temperature. Fig. 8 shows the chromaticity coordinates of Ca0.8Si9.2 Al2.8O1.2N14.8: xEu2+ (x = 0.06–0.24), which is on the Commission Internationale de l’Eclairage 1931 diagram. Bright yellow-orange emissions can be obtained by Ca-a-SiAlON: 0.14Eu2+ under 365 nm excitation, shown in Fig. 8. Their chromaticity coordinates can be varied from (0.500, 0.494) to (0.541, 0.456) by solely controlling the doping concentration of Eu2+ with the fixed host lattice composition. This enables the fabrication of bi-chromatic warm w-LEDs possessing broad correlated color temperatures (CCT) in the range of 3270–2260 K, covering the ‘‘warm white’’ (Japanese Industrial Standard Z9112, class WW) to ‘‘incandescent lamp’’ (class L), in contrast with the conventional YAG: Ce3+ phosphor, by which only the ‘‘white’’ light (class W, CCT 4200 K) can be attainable. 4. Conclusions

Fig. 8. Chromatic coordinates of Ca–SiAlON: Eu2+ phosphors. Inset shows photograph of Ca–SiAlON: 0.14Eu2+ irradiated under 365 nm UV lamp box.

is the redshift of the broad emission band with increasing Eu2+ concentration, as seen in Fig. 4. This may be due to some changes in the crystal field around Eu2+ which causes the splitting of 5d electrons. The probability of the energy transfer from the Eu2+ at higher levels of 5d to lower 5d levels increases with increasing Eu2+ concentration [14], which makes it possible that higher Eu2+ concentration lowers the emission energy of transfer from 5d excited state to 4f ground state, and hence shifts the emission to longer wavelength. Fig. 7 plots the relative emission intensity of Ca-a-Sialon: 0.14Eu2+ as a function of temperature using a YAG: Ce3+ (P46-Y3) phosphor as a benchmark. The quenching temperature Tq (the temperature at which the emission intensity is half of the initial intensity at room temperature 25 °C) is above 250 °C and is much higher than that of YAG: Ce3+ (P46-Y3, Tq = 230 °C). With increasing temperature, in particular at higher Eu2+ concentrations, more thermal energy is available to excite electrons from the lowest 5d states of Eu2+ to the bottom of the conduction band of Ca-a-Sialon and then to be ionized, namely the thermal ionization

The promising yellow-orange oxynitride phosphors Ca-a-SiAlON with compositions of Ca0.8Si9.2Al2.8O1.2N14.8: xEu2+ (0 6 x 6 0.24) have been successfully obtained by CRN, a simple and facile reduction–nitridation method. The products are pure phase and well-crystallized, and the rod-like grains are observed in Ca-a-SiAlON prepared by CRN method for the first time. The series of Eu2+ doped samples show flexibility of chromaticity control, enabling the creation of a broad range of warm-white light. The emission intensity of 585 nm at 250 °C remains at 97.6% of that measured at room temperature. Our results reveals that Ca-a-SiAlON: Eu2+ is a good candidate for w-LEDs. Acknowledgment This work is supported by National Science Foundation for Distinguished Yong Scholars (Grant No. 50925206). References [1] S. Nakamura, G. Fasol, Springer, Berlin (1997). [2] Y. Narukawa, J. Narita, T. Sakamoto, T. Yamada, H. Narimatsu, M. Sano, T. Mukai, Phys. State Solidi (a) 204 (2007) 2087–2093. [3] K. Bando, K. Sakano, Y. Noguchi, Y. Shimizu, J. Light Visual Environ. 22 (1998) 2–5. [4] V. Bachmann, C. Ronda, A. Meijerink, Chem. Mater. 21 (2009) 2077–2084. [5] C. Zhao, D. Zhu, M.X. Ma, T. Han, M.J. Tu, J. Alloys Compd. 523 (2012) 151–154. [6] X.J. Wang, G.H. Zhou, H.L. Zhang, H.L. Li, Z.J. Zhang, Z. Sun, J. Alloys Compd. 519 (2012) 149–155. [7] M. Yamada, T. Naitou, K. Izuno, H. Tamaki, Y. Murazaki, M. Kameshima, T. Mukai, Jpn. Appl. Phys. 42 (2003) 20–23.

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