Photoluminescence properties of Eu2+–Mn2+ codoped Ca-α-SiAlON phosphors

Photoluminescence properties of Eu2+–Mn2+ codoped Ca-α-SiAlON phosphors

Journal of Luminescence 132 (2012) 2561–2565 Contents lists available at SciVerse ScienceDirect Journal of Luminescence journal homepage: www.elsevi...

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Journal of Luminescence 132 (2012) 2561–2565

Contents lists available at SciVerse ScienceDirect

Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin

Photoluminescence properties of Eu2 þ –Mn2 þ codoped Ca-a-SiAlON phosphors Yu-qiang Zhang a,b,n, Xue-jian Liu a, Zheng-ren Huang a, Jian Chen a, Yan Yang a a b

Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China Graduate University of the Chinese Academy of Sciences, Beijing 100049, China

a r t i c l e i n f o

abstract

Article history: Received 12 January 2012 Received in revised form 10 April 2012 Accepted 18 May 2012 Available online 26 May 2012

Eu2 þ –Mn2 þ codoped Ca-a-SiAlON phosphors, Ca0.736  ySi9.6Al2.4O0.8N15.2:0.064 Eu2 þ , yMn2 þ , were firstly synthesized by the high temperature solid state reaction method. The effects of doped Eu2 þ and Eu2 þ –Mn2 þ concentrations on the photoluminescence properties of the as-prepared phosphors were investigated systematically. Powder X-ray diffraction shows that pure Ca-a-SiAlON phase is synthesized after sintering at 1700 1C for 2 h under 0.5 MPa N2 atmosphere. The excitation spectra of Eu2 þ -doped Ca-a-SiAlON phosphors are characterized by two dominant bands centered at 286 nm and 395 nm, respectively. The photoluminescent spectrum of Eu2 þ -doped Ca-a-SiAlON phosphor exhibits an intense emission band centered at 580 nm due to the allowed 4f 65d-4f 7 transition of Eu2 þ , showing that the phosphor is a good candidate for creating white light when coupled to a blue LED chip. The intensities of both excitation and emission spectra monotonously decrease with the increment of codoped Mn2 þ content (i.e. y value), indicating that energy transfer between Eu2 þ and Mn2 þ is inefficient in the case of Eu2 þ –Mn2 þ codoped Ca-a-SiAlON phosphors. & 2012 Elsevier B.V. All rights reserved.

Keywords: Photoluminescence Phosphors Energy transfer a-SiAlON

1. Introduction White light-emitting diodes (LED), being regarded as the next generation lighting source, have been focused for their excellent properties such as high efficiency, low power consumption, long lifetime, tunable color, and environmental friendliness [1–6]. So far, the most popular route to produce a white light LED is to package a blue chip (InGaN) and yellow phosphor (mostly YAG:Ce3 þ ) together [7]. However, LED in that pattern usually shows a low color rendering index (CRI) because of the lack of red component in the visible spectrum. Recently, rare-earth (RE)activated (oxy)nitrides such as CaAlSiN3:Eu2 þ [8], M2Si5N8: Eu2 þ (M ¼Ca, Sr, or Ba) [9], MSi2O2N2:Eu2 þ (M¼ Ca, Sr, or Ba) [10], LaSi3N5:Eu3 þ [11], AlN:Eu2 þ [12] and Eu2 þ - or Ce3 þ -doped a-SiAlONs [4,13–15] and b-SiAlONs [16] have attracted much attention as promising candidates for phosphors. Among the (oxy)nitride phosphors, Eu2 þ -doped Ca-a-SiAlON is generally considered as a new kind of yellow phosphor with high efficiency and chemical and thermal stability due to the strong covalence bonds of Si–N, Al–N, Si–O, and Al–O. Additionally, Si–Al–O–N solid solution has a bigger diversity in composition design, so Caa-SiAlON:Eu2 þ phosphors show a great prospect for white LED.

n Corresponding author at: Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China. Tel.: þ86 21 52411050; fax: þ 86 21 52413903. E-mail address: [email protected] (Y.-q. Zhang).

0022-2313/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jlumin.2012.05.021

As a representative (oxo)nitridosilicate, a-SiAlON is isostructural to a-Si3N4 [17]. a-SiAlON is a solid solution of M–Si–Al–O–N (M¼ Ca, Sr, Ba, Li, Mg, and lanthanides except Eu, Ce, and La) with the general formula Mm/vSi12  (m þ n)Al(m þ n)OnN16  n in which m and n represents the number of Al–N bonds and Al–O bonds in the lattice respectively, v is valence of the cation Mv þ . The charge unbalance caused by the replacement of Si–N with Al–N is compensated by introducing the cation Mv þ . a-SiAlON has a hexagonal crystal structure with two holes in a unit cell and rareearth cations can be kept in the holes coordinated with seven N, O anions at three different M–(N, O) distances [18]. Both nephelauxetic and crystal field effects can be changed with the variation of the concentration or the kind of doped rare-earth or transition element. Correspondingly, the emission spectra of a-SiAlONbased phosphors can be tuned in a wider range. Song et al. [19] reported that K2Ca1  x  yP2O7:xEu2 þ , yMn2 þ emitted white light under UV excitation as a result of the combination of Eu2 þ and Mn2 þ emission bands. Ye et al. [20] also reported that energy transfer existed between Eu2 þ and Mn2 þ in MAl2Si2O8:Eu2 þ , Mn2 þ (M ¼Sr, Ba) phosphors. In the preliminary works on a-SiAlON based phosphors, much attention has been focused on the preparation and photoluminescence properties of single rare-earth element doped a-SiAlON; little has been paid to two elements codoped a-SiAlON. Xie et al. have prepared a series of oxynitride phosphors, for example, yellow phosphor Ca-a-SiAlON:Eu2 þ which absorbs UV–vis light and shows a single intense broadband emission at 583–605 nm

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[1,2]; green–yellow phosphor Li-a-SiAlON:Eu2 þ with an emission band ranging from 563 nm to 586 nm [13]; green phosphors b-SiAlON:Eu2 þ [16–21] and M-a-SiAlON:Yb2 þ (M ¼Ca, Li, Mg and Y) [14] with emission peaks at 535 nm and 549 nm, respectively; and rare-earth-doped Ca-a-SiAlON phosphors (RE ¼Ce, Sm, Dy) [4] were also reported. Based on our unpublished work, the optimized values of m and n to synthesize the pure phased Ca-a-SiAlON are 1.6 and 0.8. And then Eu2 þ -doped Ca-a-SiAlON phosphors according to the formula of Ca0.8Si9.6Al2.4O0.8N15.2 were synthesized and the effects of the doped Eu2 þ content on the luminescence were investigated in the present work. In addition, the effects of codoped Mn2 þ on the luminescence properties of Ca-a-SiAlON:Eu2 þ –Mn2 þ were studied, and the energy transfer mechanism between Eu2 þ and Mn2 þ was also explored.

2. Experimental Eu2 þ -doped Ca-a-SiAlON (Ca-a-SiAlON:Eu2þ ) phosphors with the compositions of Ca0.8 xSi9.6Al2.4O0.8N15.2:xEu2 þ and Eu2þ – Mn2 þ codoped Ca-a-SiAlON (Ca-a-SiAlON:Eu2þ –Mn2 þ ) phosphors with the compositions of Ca0.736 ySi9.6Al2.4O0.8N15.2:0.064 Eu2þ , yMn2þ were prepared by the solid state reaction method from Si3N4 (SN-E10, Ube Industries, Japan), AlN (Tokuyama Corp., type H, Japan), CaCO3 (Sinopharm Chemical Reagent Co., Ltd, China), MnCO3 (Sinopharm Chemical Reagent Co., Ltd, China) and Eu2O3 (Sinopharm Chemical Reagent Co., Ltd, China). The source powders were firstly mixed and ground in an agate mortar sufficiently and then sifted through a 200-mesh screen. Then the well-distributed powders were put into BN crucible and synthesized by high temperature gaspressure sintering (GPS) at 1700 1C for 2 h under 5 atm of N2 (499.999%) atmosphere. After that, the as-prepared powders were cooled down to RT and then ground finely to get the final samples. The phase compositions of the products were characterized by powder X-ray diffraction (XRD, Bruker D8 Advance, Germany) operating at 40 kV and 40 mA and using Cu Ka radiation ˚ A step size of 0.021 was used with a scan speed (l ¼1.5406 A). of 101/min. The excitation and emission spectra were measured at room temperature using a fluorescent spectrophotometer (Fluorolog-4, Horiba Jobin Yvon, France) with a 150 W Ushio xenon short arc lamp.

Fig. 1. XRD patterns of Ca0.8  xSi9.6Al2.4O0.8N15.2:xEu2 þ at varied Eu2 þ concentrations (x).

3. Results and discussion Fig. 1 shows the XRD patterns of Ca0.8  xSi9.6Al2.4O0.8N15.2: xEu2 þ at varied Eu2 þ concentrations (x). It can be seen that all of the as-prepared products are composed of single phase Ca-aSiAlON and the diffraction peaks of the samples exhibit no shift with different x when the doped Eu2 þ concentration varies under 16 mol% (x¼0.128). Interestingly, the diffraction intensity of the sample reduces gradually with the increase of doped-Eu2 þ content, implying that Ca-a-SiAlON:Eu2 þ unit cell is distorted and Eu2 þ ions do not take the sites of Ca2 þ when more Ca2 þ are substituted by Eu2 þ [2]. Fig. 2(a) indicates the excitation spectra of the as-prepared Caa-SiAlON:Eu2 þ phosphors, Ca0.8  xSi9.6Al2.4O0.8N15.2:xEu2 þ , at varied Eu2 þ concentrations (x). Obviously, the excitation spectra of Ca-a-SiAlON:Eu2 þ are composed of two broad excitation bands, particularly 250–350 nm and 350–500 nm. The first excitation peak locates at 286 nm and stays at the same wavelength with the change of x value; and the other excitation peak shifts from 395 to 405 nm along with the increment of doped Eu2 þ concentration. The first excitation band centered at 286 nm is generated by the photo absorption of a-SiAlON lattice, which is

Fig. 2. Luminescence spectra of Ca0.8  xSi9.6Al2.4O0.8N15.2:xEu2 þ phosphors at varied Eu2 þ concentrations (x): (a) Excitation (lem ¼580 nm); (b) Emission (lex ¼ 416 nm).

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Fig. 3. Emission wavelength and relative intensity of Ca0.8  xSi9.6Al2.4O0.8N15.2: xEu2 þ phosphors as a function of Eu2 þ contents (x).

not obviously influenced by the doped-Eu2 þ content. The second excitation band of Ca-a-SiAlON:Eu2 þ phosphor results from the 4f7-4f65d transition of doped-Eu2 þ coordinated with seven N/O anions [2]. With the increment of doped-Eu2 þ content, more Eu–N/O bonds enhance the crystal splitting effect of a-SiAlON, resulting in the shift of the second excitation peak, as shown in Fig. 2(a). Fig. 2(b) displays the emission spectra of Ca-aSiAlON:Eu2 þ phosphors at varied Eu2 þ concentration. It can be seen that the as-prepared Ca-a-SiAlON:Eu2 þ phosphors exhibit intense emission peaks centered at 580 nm due to the allowed 4f6 5d-4f 7 transition of doped-Eu2 þ ions, indicating that the phosphor is a good candidate for creating white light when coupled to a blue LED chip. In addition, the emission wavelength and intensity of the phosphor change as functions of doped-Eu2 þ content. Fig. 3 shows that the effects of the doped-Eu2 þ content (x) on the emission wavelength and intensity of the as-prepared Ca-aSiAlON:Eu2 þ phosphors. We can see that the intensity of the emission band reaches the top at x ¼0.064 with the center locating at 580 nm and decreases gradually above the critical content because of the concentration quenching effect. Meanwhile, the emission peak center of Ca-a-SiAlON:Eu2 þ phosphor shows a systematic shift from 570 nm to 584 nm with the increment of x value. The red-shift may be ascribed to some changes of the crystal field around Eu2 þ which cause the energy level of 5d electron orbit splitting into two components: the higher energy level t2g and the lower energy level eg [22]. The energy gap between t2g and eg becomes larger accompanying with the increase of doped-Eu2 þ content, and correspondingly the transition between eg levels of 5d to 4f gets easy. As a result, the emission peak shifts to the longer wavelength. Meanwhile, the lattice parameters of the Ca-a-SiAlON:Eu2 þ (a and c) functioning with the doped-Eu2 þ content are shown in Fig. 4. With the increment of doped-Eu2 þ concentration, the lattice parameters of both a and c increase monotonously from (7.800, 5.676) to (7.834, 5.698). In other words, the lattice volume of Ca-a-SiAlON:Eu2 þ expands when more Ca2 þ cations are substituted by larger Eu2 þ cations. Correspondingly, the Eu–N/O bond length is shortened and the crystal field effect around Eu2 þ is enhanced which result in the red-shift of Ca-a-SiAlON:Eu2 þ emission peak. Fig. 5 gives the XRD patterns of Ca0.736  ySi9.6Al2.4O0.8N15.2: 0.064Eu2 þ , yMn2 þ at varied Mn2 þ contents (y). Obviously, all of the samples consist of pure phase Ca-a-SiAlON without impurity regardless of the doped-Mn2 þ content. The diffraction peaks move towards the direction of larger 2y degree with the increase

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Fig. 4. Lattice parameters of Ca0.8  xSi9.6Al2.4O0.8N15.2:xEu2 þ phosphors as functions of Eu2 þ content (x).

Fig. 5. XRD patterns of Ca0.736  ySi9.6Al2.4O0.8N15.2:0.064Eu2 þ , yMn2 þ at varied Mn2 þ contents (y).

of y value, due to the shrinkage of the a-SiAlON lattice parameters that results from the replacement of Ca2 þ with smaller Mn2 þ . The above-mentioned experimental results illustrate that transition metal cation Mn2 þ can be codoped into Ca-a-SiAlON:Eu2 þ lattice by the replacement of Ca2 þ , without distinct lattice distortion and the appearance of any impurity phase. Fig. 6(a) shows the excitation spectra of the as-prepared Ca-a-SiAlON:Eu2 þ –Mn2 þ phosphors, Ca0.736  ySi9.6Al2.4O0.8N15.2: 0.064Eu2 þ , yMn2 þ , at varied Mn2 þ concentrations (y). The excitation spectra of Ca-a-SiAlON:Eu2 þ –Mn2 þ phosphors are very similar to that of Ca-a-SiAlON:Eu2 þ , as shown in Fig. 2(a). Fig. 6(b) exhibits the emission spectra of the as-prepared Ca-aSiAlON:Eu2 þ –Mn2 þ phosphors at varied Mn2 þ concentrations (y). The emission band consists of only one peak centered at 580 nm regardless of y value. Meanwhile, the emission intensity falls gradually with the introduction of Mn2 þ . According to the above-mentioned experimental results, it can be reasonably deduced that the photo energy does not effectively transfer from Eu2 þ to Mn2 þ in Ca-a-SiAlON:Eu2 þ –Mn2 þ phosphors. There are two possible explanations for this phenomenon: (i) Mn2 þ cannot be excited by the energy delivering from Eu2 þ , (ii) Mn2 þ is excited by the energy transferring from Eu2 þ , but the emission light of Mn2 þ is too faint to display in the apparent emission spectra.

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It is well known that the emission spectrum of Mn2 þ is a broad band, and the wavelength of emission peak is ascribed to the crystal structure of matrix ranging from green to deep red [23]. According to the Tanabe–Sugano diagram, the 4T1-6A1 transition of Mn2 þ emits green light at tetrahedral coordination (weak crystal field) and orange or red light at octahedral coordination (strong crystal field) [26]. In Ca-a-SiAlON, Mn2 þ is surrounded by seven N/O anions, meaning a strong crystal field. So Mn2 þ should emit orange or red light. However, the center of the emission spectra of Ca-a-SiAlON:Eu2 þ locates at about 580 nm, which is yellow light. Correspondingly, the energy of the emission light of Eu2 þ is not high enough to excite Mn2 þ is the explanation to the inefficient energy transfer from Eu2 þ to Mn2 þ in Ca-aSiAlON phosphor.

4. Conclusions Single phased Eu2 þ –Mn2 þ codoped Ca-a-SiAlON yellow phosphors were firstly synthesized by solid state reaction method. The solid solution phase is retained as Ca-a-SiAlON though the variation of the content of Eu2 þ and Mn2 þ . In the phosphors of Ca0.8 xSi9.6Al2.4O0.8N15.2:xEu2 þ , the strongest luminescence centered at 580 nm was obtained via appropriately adjusting Eu2 þ molar concentration (x) to 0.064. The emission intensity of Eu2 þ – Mn2 þ codoped Ca-a-SiAlON phosphor decreased monotonously as well as the increment of Mn2 þ content. The energy transfer mechanism between Eu2 þ and Mn2 þ belongs to electric multipole effect. With the advantage of modifying the luminescence intensity of phosphor, Ca-a-SiAlON:Eu2 þ –Mn2 þ was shown to be a promising candidate for white LED applications under blue light excitation.

Acknowledgments We are thankful for the financial support from the National Natural Science Fund of China (No. 51172263) and the Science and Technology Innovation Initiative of Shanghai Institute of Ceramics, Chinese Academy of Sciences (SICCAS). Fig. 6. Luminescence spectra of Ca0.736 ySi9.6Al2.4O0.8N15.2:0.064Eu2 þ , yMn2 þ at varied Mn2 þ contents (y): (a) Excitation (lem ¼ 580 nm); (b) Emission (lex ¼ 395 nm).

References

Generally, there are four kinds of energy transfer mechanism between the codoped cations in phosphors, including resorption, carriers transport, exciton migration, and resonance transfer [23]. The mechanism of resonance transfer contains both exchange effect and electric multipole effect [23]. Resorption and exciton migration mechanisms deliver the energy without the limitation of distance, and the carriers transport generally functions in semiconductors but fails in phosphors [23]. Consequently, the energy transfer between Eu2 þ and Mn2þ in the case of Ca-a-SiAlON:Eu2 þ –Mn2þ phosphors exclusively acts in the way of resonance transfer. According to the formula mentioned by L.G. van Uitert, C0 ¼V0  (4pR30/3)  1, in which R0 is the critical distance, V0 is the volume of unit cell and C0 is the critical concentration (at which the emission intensity of donordoped matrix decreases to the half) [24]. Ca-a-SiAlON belongs to the hexagonal crystal system with the lattice parameters of a, b, and c of 0.7852 nm, 0.7852 nm, and 0.5709 nm, respectively. The volume of Ca-a-SiAlON unit is 0.9145 nm3 and the C0 is 0.1 mol. Thus, R0 ¼1.2973 nm can be calculated. However, exchange effect needs a great overlap of the wave functions of cations, generally R0 o0.3– 0.4 nm [25]. Therefore, it is reasonably deduced that the energy transfer mechanism between Eu2 þ and Mn2 þ in Ca-a-SiAlON: Eu2 þ –Mn2 þ phosphors belongs to electric multipole effect.

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