- Email: [email protected]
Temperature-dependent photoluminescence properties of (Ba,Sr)2SiO4:Eu2+ phosphors for white LEDs applications
Journal of Luminescence 151 (2014) 165–169
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Temperature-dependent photoluminescence properties of (Ba,Sr)2SiO4:Eu2 þ phosphors for white LEDs applications Qiyue Shao n, Huanyun Lin, Yan Dong, Jianqing Jiang Jiangsu Key Laboratory of Advanced Metallic Materials, School of Materials Science and Engineering, Southeast University, Nanjing 211189, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 14 August 2013 Received in revised form 27 December 2013 Accepted 19 February 2014 Available online 28 February 2014
(Ba,Sr)2SiO4: Eu2 þ phosphors with various Eu2 þ concentrations and Ba/Sr ratios were synthesized by solid-state reactions and their temperature-dependent photoluminescence properties were studied between 30 and 200 1C. Thermal quenching properties of (Ba1 xSrx)2SiO4: Eu2 þ phosphors barely depend on Eu2 þ concentrations, but exhibit a complex dependence on Ba/Sr ratios. With the increase of the Sr content, the luminescent thermostability is gradually improved for x o0.5 and then becomes worse as x 4 0.5. Considering changes in Stokes shifts, emission wavelengths and host lattice bandgaps for samples with various Ba/Sr ratios, this anomalous quenching behavior was discussed in terms of the configuration coordinate model and photo-ionization theory. & 2014 Elsevier B.V. All rights reserved.
Keywords: White LEDs (Ba,Sr)2SiO4:Eu2 þ phosphors Photoluminescence Thermal quenching
1. Introduction White LEDs are emerging as promising light sources to replace existing incandescent and fluorescent lamps, because of their numerous advantages such as higher energy efficiency, environment friendliness, longer life and reliability [1,2]. One widely used approach for producing white light is the combination of blue LED chips with yellow emitted phosphor materials, such as Y3Al5O12: Ce3 þ (YAG:Ce3 þ ). Therefore, phosphor properties significantly affect the luminous efficiency and color quality of white LEDs. For practical devices, phosphor particles are directly coated on the chip and the heat generated by the LED chip will result in the thermal quenching of the emission of phosphors. It is especially true for high-power packages of white LEDs, in which the LED junction temperature can be up to 150 1C [3,4]. The temperature of phosphor particles even exceeds this value due to their selfheating effect originated from Stokes losses of emissions [5]. As a consequence, LED phosphors should possess a good luminescent thermostability and maintain their quantum efficiency and spectral characteristics at elevated temperatures. (Ba,Sr)2SiO4:Eu2 þ orthosilicate phosphors have been studied since 1960 [6,7], but only recently have been the subject of intense investigation for use as conversion phosphors in both blue and UV LEDs, owing to their high quantum efficiency and suitable excitation/ emission bands that can also be tuned by changing Ba/Sr ratios [8–15]. A disadvantage of (Ba,Sr)2SiO4:Eu2 þ phosphors is the relatively strong luminescent quenching at elevated temperatures.
n
Corresponding author. Tel.: þ 86 25 52090630; fax: þ 86 25 52090634. E-mail address: [email protected] (Q. Shao).
http://dx.doi.org/10.1016/j.jlumin.2014.02.027 0022-2313 & 2014 Elsevier B.V. All rights reserved.
For example, the emission intensity of the Ba2SiO4:Eu2 þ phosphor declines by more than 50% at 150 1C compared to that at room temperature [6,16,17]. A prerequisite for improving their thermal stability is to clarify the basic mechanism behind the thermal quenching. Some studies have been performed to analyze the quenching mechanisms of the Eu2 þ emission in (Ba,Sr)2SiO4 host lattices. Kim et al. have attributed the thermal quenching of (Ba,Sr)2SiO4:Eu2 þ phosphors to thermally activated crossing between 5d and 4f energy levels in the configuration coordinate diagram, and pointed out that the Stokes shift and host lattice phonon energy had a strong effect on their thermal quenching properties [18]. Dorenbos has pointed out that the ionization of the 5d electron to conduction band states is the genuine quenching mechanism of M2SiO4:Eu2 þ (M¼ Sr and Ba) phosphors [19]. However, the experimental data indicated that quenching temperatures of (Ba,Sr)2SiO4:Eu2 þ phosphors show a nonlinear dependence on Ba/Sr ratios [6,17], and the reported quenching mechanisms cannot satisfactorily explain this anomalous quenching behavior. In addition, for 5d–4f emitters, such as Eu2 þ and Ce3 þ , the composition of hosts and the activator concentration have a significant effect on their thermal quenching properties. In this paper, effects of Eu2 þ concentrations and Ba/Sr ratios on temperature-dependent photoluminescence (PL) properties of (Ba,Sr)2SiO4:Eu2 þ phosphors were studied, and dominant quenching mechanisms were discussed based on the configuration coordinate model and photo-ionization theory.
2. Experimental (Ba,Sr)2SiO4:Eu2 þ phosphors were synthesized by typical solidstate reactions. The starting materials of BaCO3 (A. R.), SrCO3
166
Q. Shao et al. / Journal of Luminescence 151 (2014) 165–169
(A. R.), SiO2 (99.99%) and Eu2O3 (99.99%) were weighted in a stoichiometric ratio, and then mixed thoroughly along with a certain amount of BaF2 (A. R.) as flux. These homogeneous mixtures were placed in an alumina crucible and sintered at 1300 1C for 4 h. The samples were then reduced at 1200 1C for 1 h by using carbon powder as reductant to obtain the final products. The crystal structure of the final products was identified by powder X-ray diffraction (Shimadzu XD-3 A) with Cu Kα radiation. The diffusion reflection spectra of samples were measured by a Cary 5000 UV–vis–NIR spectrophotometer equipped with an internal diffuse reflectance accessory. Photoluminescence emission (PL) and excitation (PLE) properties were studied by a Hitachi F-7000 fluorescence spectrophotometer (with an R928F PMT). PL spectra at various temperatures were obtained with the help of a selfdesigned heating attachment, which mainly includes a resistively heated sample holder and a standard temperature controller. Fig. 2. Powder XRD patterns of (Ba1 xSrx)2SiO4:Eu2 þ phosphors with varying Sr2 þ concentrations (x).
3. Results 3.1. Crystalline phase First, effects of Eu2 þ and Sr2 þ contents on crystalline phases of (Ba,Sr)2SiO4:Eu2 þ were studied. Fig. 1 presents powder XRD patterns of Ba2SiO4:Eu2 þ phosphors with various Eu2 þ concentrations. As a reference, the XRD pattern of orthorhombic Ba2SiO4 from the Joint Committee on Powder Diffraction Standards (JCPDS) cards is also shown. XRD patterns of all the samples are found to agree well with the standard data of orthorhombic Ba2SiO4 (JCPDS no. 261403), indicating that the doped Eu2 þ ions do not generate any impurities or induce significant changes in the host structure. XRD patterns of (Ba1 xSrx)2 SiO4:Eu2 þ phosphors with various x values are shown in Fig. 2. The concentration of Eu2 þ ions is kept constant at 3 mol%. XRD patterns of Ba2SiO4:Eu2 þ (x ¼0) and Sr2SiO4:Eu2 þ (x ¼1.0) match well with the standard data of orthorhombic Ba2SiO4 (JCPDS no. 261403) and Sr2SiO4 (JCPDS no. 391256). The other samples are isostructured and show a solid solution between end members. No secondary phases can be detected and all the samples are single phases. It can be found that with the increase of the Sr content XRD peaks shift to higher
Fig. 3. PLE and PL (λex ¼ 370 nm) spectra of Ba2SiO4:Eu2 þ samples with various Eu2 þ concentrations. PLE spectra were recorded by monitoring their maximum emission wavelengths for each sample.
values of the diffraction angle, indicating that the lattice constants are decreased due to the fact that Sr2 þ ions are smaller than Ba2 þ ions. 3.2. PL and PLE spectra
Fig.1. Powder XRD patterns of Ba2SiO4:Eu2 þ concentrations.
samples with various Eu2 þ
Effects of Eu2 þ and Sr2 þ contents on room-temperature PL properties of (Ba,Sr)2SiO4:Eu2 þ were also studied. As an example, impacts of Eu2 þ concentrations on PL and PLE spectra of Ba2SiO4: Eu2 þ phosphors are shown in Fig. 3. At lower Eu2 þ concentrations, excitation spectra consist of two bands around 280 and 350 nm, respectively, which are mainly ascribed to 4f–5d transitions of Eu2 þ ions. The first excitation band may also be partly contributed by the host absorption [11]. At higher Eu2 þ concentrations, a broad excitation band extending from 250 to 450 nm can be observed and does not show any obvious structure. The emission band slightly shifts to a longer wavelength with an increase in the Eu2 þ concentration. The emission intensity reaches a maximum as the Eu2 þ concentration is equal to 3 mol%. The Sr substitution has significant effects on PL properties of (Ba,Sr)2SiO4:Eu2 þ phosphors. PLE and PL spectra of (Ba1 xSrx)2 SiO4: Eu2 þ with varying Sr2 þ concentrations (x) are shown in Fig.4. There are two nonequivalent Eu2 þ sites in (Ba,Sr)2SiO4:Eu2 þ phosphors, and therefore the PL spectra of Eu2 þ ions actually consist of two emission bands [16]. In the case of Sr2SiO4:Eu2 þ , one band is located around 486 nm, and another band is positioned around 555 nm.
Q. Shao et al. / Journal of Luminescence 151 (2014) 165–169
With the decrease of Sr concentrations, two emission bands are more overlapped. As shown in Fig. 4, with increasing Sr content the emission peaks of Eu2 þ ions shift to longer wavelengths and the emission intensities are decreased. Limited to the resolution of excitation spectra at room temperature, a rough estimation of Stokes shifts (SS) was made on the basis of the mirror-image relationship between excitation and emission spectra [20,21]. As shown in Fig. 4, Stokes shifts of Eu2 þ emissions increase with an increase in the Sr content.
3.3. Temperature-dependent PL properties Fig. 5 shows temperature-dependent emission spectra (λex ¼ 370 nm) of the typical sample Ba2SiO4:3%Eu2 þ . For ease of comparison, the excitation wavelength for temperature-dependent PL spectra measurements is fixed at 370 nm, which can effectively excite two emission bands of (Ba1 xSrx)2 SiO4:Eu2 þ phosphors. It can be found that with increasing temperature the peak intensity of Eu2 þ emission decreases rapidly. As shown in the inset of Fig. 5, the integrated intensity of Ba2SiO4:3%Eu2 þ at 150 1C only can remain 28% of that at room temperature. Compared to other commercial LED phosphors, such as Y3Al5O12:7%Ce3 þ (88% at 150 1C), Sr2Si5N8:10%Eu2 þ (84%) and CaAlSiN3:2%Eu2 þ (90%) [22–24], the thermal quenching of the Ba2SiO4:Eu2 þ phosphor is much more serious. The inset of Fig.5 also shows changes of the integrated intensity as a function of temperature during the cooling process. The emission intensity returns nearly to the same value after cooling down.
167
It can also be found that with increasing temperature the peak wavelength of Ba2SiO4:3%Eu2 þ slightly shifts to the blue side (Fig. 5). The similar blue-shift can be observed for other samples with various Ba/Sr ratios (not shown). As mentioned above, the PL spectra of Eu2 þ ions consist of two overlapped emission bands. Kim et al. have ascribed the blue-shift of Eu2 þ emission with increasing temperature to back tunneling from excited states of the low-energy emission band to excited states of the high-energy emission band by assistance of thermally active phonons [16]. Another possible reason for the blue-shift is that the low-energy emission band exhibits a more significant decrease in intensity at elevated temperatures than the high-energy emission band, due to different crystal field environments. Fig. 6 shows the influences of Eu2 þ concentrations on the thermal quenching behavior of Ba2SiO4:Eu2 þ and (Ba0.5Sr0.5)2SiO4: Eu2 þ phosphors. At a fixed Ba/Sr ratio, the reduction of emission intensity at each degree is almost same for samples with various Eu2 þ concentrations. Similar results can be obtained for other samples with different Ba/Sr ratios (not shown), and therefore we can conclude that the Eu2 þ concentration has a weak effect on the thermal quenching of (Ba,Sr)2SiO4:Eu2 þ phosphors. Ba/Sr ratios have a strong and complex effect on thermal quenching properties of (Ba,Sr)2SiO4:Eu2 þ phosphors. Fig. 7 shows temperature dependences of integrated intensities of (Ba1 xSrx)2 SiO4:Eu2 þ phosphors with various x values (λex ¼370 nm), and the Eu2 þ concentration is kept constant at 3 mol%. It is worth noting that effects of Ba/Sr ratios on the thermal quenching properties are similar under various excitation wavelengths between 250 and 460 nm. It can be found that the Ba/Sr ratio around 1:1 (x¼ 0.5) shows a better temperature stability than end members. As xo 0.5, the thermal quenching of (Ba1 xSrx)2SiO4:Eu2 þ phosphors is gradually suppressed with the increase of x. Inversely, as x4 0.5, the thermal quenching gradually becomes worse with the increase of x. Similar results were first reported in 1968 for the case under 254 nm light excitation [6] and were recently found for blue light excitation [17,25].
4. Discussion
Fig. 4. PLE and PL (λex ¼ 370 nm) spectra of (Ba1 xSrx)2SiO4:Eu2 þ phosphors with varying Sr2 þ concentrations (x). PLE spectra were recorded by monitoring their maximum emission wavelengths for each sample. Stokes shifts (SS) of phosphors are also presented.
Fig. 5. Temperature-dependent emission spectra (λex ¼ 370 nm) of the Ba2SiO4:3% Eu2 þ sample. The inset shows integrated intensities as a function of temperature for a heating-cooling cycle.
It is known that the thermal degradation of host lattice and the thermal oxidation of activator ions can also lead to the reduction of emission intensity of phosphors at elevated temperatures. M2SiO4 (M ¼Ca, Sr, and Ba) silicates are normally amenable to be attacked by the moisture in air. The integrated intensity of (Ba1 xSrx)2SiO4:Eu2 þ phosphors in a heating–cooling cycle shows a reversible thermal quenching behavior (Fig. 5), indicating that the thermal degradation of host lattice and the Eu2 þ oxidation contribute little to the luminescent quenching of (Ba1 xSrx)2SiO4: Eu2 þ phosphors at elevated temperatures. Various thermal quenching paths have been proposed for 5d–4f emitters, mainly including the thermally activated crossing between 5d and 4f energy levels, temperature-dependent concentration quenching and thermally assisted photo-ionization [26]. It was reported that thermal quenching properties of Y3Al5O12:Ce3 þ and CaAlSiN3:Eu2 þ phosphors exhibit a strong dependence on activator concentrations and the pronounced thermal quenching occurs at higher activator concentrations, which can be attributed to the temperature-dependent concentration quenching effect [25–27]. The thermal quenching of (Ba, Sr)2SiO4:Eu2 þ phosphors is nearly independent of the Eu2 þ concentration (Fig. 6), implying that thermally activated concentration quenching is not the main cause of luminescent quenching at elevated temperatures. As shown in Fig. 7, the thermal quenching behavior of (Ba1 xSrx)2 SiO4:Eu2 þ phosphors exhibits a complex dependence on Ba/Sr ratios. This anomalous thermal quenching behavior cannot be satisfactorily
168
Q. Shao et al. / Journal of Luminescence 151 (2014) 165–169
Fig. 6. Temperature dependences of integrated emission intensities of (a) Ba2SiO4:Eu2 þ and (b) (Ba0.5Sr0.5)2SiO4:Eu2 þ phosphors with various Eu2 þ concentrations (λex ¼ 370 nm). The integrated intensity is normalized to that at 30 1C for each sample.
Fig. 7. Temperature dependences of integrated emission intensities of (Ba1 xSrx)2 SiO4:Eu2 þ phosphors normalized to 30 1C (λex ¼ 370 nm).
explained by the configuration coordinate model, where thermally assisted crossing between energy parabolas of the excited and the ground state leads to increased non-radiative decay at elevated temperatures. Based on the configuration diagram, the thermal quenching will appear at lower temperatures for larger Stokes shifts. As shown in Fig. 4, the Stokes shift of (Ba1 xSrx)2SiO4:Eu2 þ phosphors increases with the increase of the Sr content, which means that their thermal quenching properties should continuously become worse with increasing x values. Actually, quenching temperatures exhibit a nonlinear dependence on Ba/Sr ratios, indicating that the change in Stokes shift is not the only factor to affect the thermal quenching behavior of (Ba1 xSrx)2SiO4:Eu2 þ phosphors. It also implies that other thermally activated non-radiative decay paths should be considered. Changes of Ba/Sr ratios may lead to structure distortion with the formation of defects, and therefore induce the continuous decrease of room-temperature luminescent intensity with increasing Sr contents (Fig. 4). However, the formation of defects cannot explain the non-linear dependence of thermal quenching temperatures on Ba/Sr ratios. In addition, it is known that the thermally activated concentration quenching process is related to the density of structure defects [22,27]. Since the thermal quenching of (Ba,Sr)2SiO4:Eu2 þ phosphors with various Ba/Sr ratios is nearly independent of the Eu2 þ concentration, it is unlikely that increased energy migration to defects is responsible for the changes in thermal quenching. Baginskiy et al. thought direct tunneling between two nonequivalent Eu2 þ sites plays an important role in the thermal quenching process of (Ba,Sr)2SiO4:Eu2 þ phosphors [17]. Increased energy migration from high Eu2 þ excited states to low excited states at elevated temperatures promotes non-radiative relaxation and results in the decrease of quenching temperature of barium-
Fig. 8. (a) Diffuse reflection spectra and (b) absorption spectra of (Ba1 xSrx)2 SiO4 hosts.
richer phosphors [17]. However, this mechanism cannot explain the blue-shift of Eu2 þ emission with increased temperatures (Fig. 5). For 4f–5d emitters, the thermal activation from 5d states to the host lattice conduction band can also lead to the thermal quenching, which is usually described as photo-ionization or auto-ionization [4,26]. The photo-ionization process is strongly related to the energy barrier (EdC) of 5d states of rare earth ions to the bottom of the host conduction band. The EdC values can be determined by the host lattice bandgap, the 5d–4f transition energy, and the position of the 4f ground state within the bandgap. The host bandgap can be estimated by the optical absorption spectra. Fig. 8 gives the diffuse reflection spectra and the absorption spectra of (Ba1 xSrx)2SiO4 hosts without Eu2 þ doping. The absorption spectra F(R) were obtained from the diffuse reflection spectra by using the Kubelka–Munk function [11,28] FðRÞ ¼
ð1 RÞ2 ¼ K=S 2R
ð1Þ
R, K and S are the reflectivity, the absorption and scattering coefficients, respectively. It is clear that the host absorption edge of (Ba1 xSrx)2SiO4 shifts to higher energy with the increase of the Sr content. As shown in Fig. 4, Eu2 þ emission bands shift to longer wavelengths with increasing Sr content, indicative of the decrease of the 5d–4f transition energy. There is no experimental data about the position of the 4f ground state of Eu2 þ ions within the bandgap of (Ba1 xSrx)2SiO4 hosts in the literature. For materials with the similar structure, the cation substitution has a weak effect on the 4f ground state energy of rare earth ions relative to the host valence band [29]. Therefore, it is reasonable to assume that the position of the 8S7/2
Q. Shao et al. / Journal of Luminescence 151 (2014) 165–169
Conduction band
EdC
EdC 5d
EdC
5d
5d
4f Valence band Ba2SiO4:Eu2+
Conduction band
Conduction band
4f
4f Valence band
Valence band (Ba0.5Sr0.5)2SiO4
:Eu2+
Sr2SiO4:Eu2+
(Ba0.5Sr0.5)2SiO4:Eu2+
E Ba2SiO4:Eu2+
Sr2SiO4:Eu2+
r
r0 r1, r2, r3 2þ
Fig. 9. (a) Schematic of energy levels of Eu ions with the bandgap of (Ba1 xSrx)2SiO4; (b) configuration coordinate diagram of (Ba1 xSrx)2SiO4:Eu2 þ phosphors.
ground state of Eu2 þ ions above the valence band remains constant with Sr substitution. In combination with the increase of host lattice bandgap and the reduction of 5d–4f transition energy of Eu2 þ ions with increasing Sr substitution, we can conclude that the EdC values of (Ba1 xSrx)2SiO4:Eu2 þ phosphors gradually increase with the increase of x values, as shown in Fig. 9(a). Based on changes of Stokes shifts and EdC values with Sr substitution, the nonlinear dependence of thermal quenching on Ba/Sr ratios can be ascribed to the combined action of the thermally activated crossing between 5d and 4f energy levels and photo-ionization. For higher Ba/Sr ratios (xo0.5), the energy barrier between the Eu2 þ 5d states and the host conduction band is lower, and therefore the thermally assisted photo-ionization process dominates the nonradiative decay paths of (Ba1 xSrx)2 SiO4:Eu2 þ phosphors. With increasing Sr substitution (xo0.5), the thermal quenching becomes weak due to the increase of EdC values. For the case of x4 0.5, the energy barrier EdC is higher while the displacement between 5d and 4f parabolas in the configuration diagram is larger, and therefore the thermally activated crossing between 5d and 4f states starts to dominate the thermal quenching of (Ba1 xSrx)2SiO4:Eu2 þ phosphors. As a result, their thermal quenching properties become worse with increasing Sr substitution due to the increase of Stokes shifts, as shown in Fig. 9(b). 5. Conclusion Different quenching models on the Eu2 þ emission in M2SiO4 (M ¼Sr and Ba) orthosilicates have been reported in the literature, however, cannot fully explain the thermal quenching behavior of (Ba,Sr)2SiO4:Eu2 þ phosphors. In this work, effects of Eu2 þ concentrations and Ba/Sr ratios on thermal quenching properties of (Ba,Sr)2SiO4:Eu2 þ phosphors were investigated and dominant quenching mechanisms were discussed. The emission intensity
169
of the phosphors shows a considerable decline at elevated temperatures, and this decline is nearly independent of Eu2 þ concentrations but exhibits strong and complex dependences on Ba/Sr ratios. The thermostability is highest at Sr content of x¼ 0.5 compared to end members of Ba2SiO4:Eu2 þ and Sr2SiO4:Eu2 þ . Based on changes of Stokes shifts, emission bands and absorption spectra with Sr substitution, this anomalous dependence of thermal quenching on Ba/Sr ratios can be ascribed to two competing processes of thermally activated crossing of energy parabolas of 5d and 4f states and photo-ionization. At higher Sr contents (x 40.5), the change in the Stokes shift is the dominant factor that affects the thermal quenching behavior of (Ba1 xSrx)2SiO4:Eu2 þ phosphors, while the photo-ionization process dominates at lower Sr contents (xo0.5). These results are relevant to the design, implementation and improvement of (Ba1 xSrx)2SiO4:Eu2 þ phosphors for their application in high-power white LED devices. The ideas and models on the thermal quenching of Eu2 þ emissions in (Ba1 xSrx)2SiO4 hosts can also be applied to other Eu2 þ -activated phosphors.
Acknowledgments This work was financially supported by the Natural Science Foundation of China (No. 51302038) and the Natural Science Foundation of Jiangsu Province of China (Nos. BK2011064 and BK2012346). References [1] E.F. Schubert, J.K. Kim, Science 308 (2005) 1274. [2] S. Pimputkar, J.S. Speck, S.P. DenBaars, S. Nakamura, Nat. Photonics 3 (2009) 180. [3] G. Gundiah, Y. Shimomura, N. Kijima, A.K. Cheetham, Chem. Phys. Lett. 455 (2008) 279. [4] P.F. Smet, A.B. Parmentier, D. Poelman, J. Electrochem. Soc. 158 (2011) R37. [5] B. Yan, N.T. Tran, J.P. You, F.G. Shi, IEEE Photonics Technol. Lett. 23 (2011) 555. [6] T.L. Barry, J. Electrochem. Soc. 115 (1968) 1181–1184. [7] G. Blasse, Philips Res. Rep. 24 (1969) A131. [8] M.A. Lim, J.K. Park, C.H. Kim, H.D. Park, J. Mater. Sci. Lett. 22 (2003) 1351. [9] J.K. Park, M.A. Lim, C.H. Kim, H.D. Park, J.T. Park, S.Y. Choi, Appl. Phys. Lett. 82 (2003) 683. [10] J.S. Kim, P.E. Jeon, J.C. Choi, H.L. Park, Solid State Commun. 133 (2005) 187. [11] M. Zhang, J. Wang, Q.H. Zhang, W.J. Ding, Q. Su, Mater. Res. Bull. 42 (2007) 33. [12] H. He, R.L. Fu, X.F. Song, D.L. Wang, J.K. Chen, J. Lumin. 128 (2008) 489. [13] W.H. Hsu, M.J. Sheng, M.S. Tsai, J. Alloys Compd. 467 (2009) 491. [14] X.G. Zhang, X.P. Tang, J.L. Zhang, M.L. Gong, J. Lumin. 130 (2010) 2288. [15] J.K. Han, M.E. Hannah, A. Piquette, G.A. Hirata, J.B. Talbot, K.C. Mishra, J. McKittrick, J. Lumin. 132 (2012) 106. [16] J.S. Kim, Y.H. Park, S.M. Kim, J.C. Choi, H.L. Park, Solid State Commun. 133 (2005) 445. [17] I. Baginskiy, R.S. Liu, C.L. Wang, R.T. Lin, Y.J. Yao, J. Electrochem. Soc. 158 (2011) P118. [18] J.S. Kim, Y.H. Park, J.C. Choi, H.L. Park, J. Electrochem. Soc. 152 (2005) H135. [19] P. Dorenbos, J. Phys.: Condens. Matter 17 (2005) 8103. [20] J. Ruan, R.J. Xie, N. Hirosaki, T. Takeda, J. Am. Ceram. Soc. 94 (2011) 536. [21] J.H. Ryu, Y.G. Park, H.S. Won, S.H. Kim, H. Suzuki, J.M. Lee, C. Yoon, M. Nazarov, D.Y. Noh, B. Tsukerblat, J. Electrochem. Soc. 155 (2008) J99. [22] Q.Y. Shao, Y. Dong, J.Q. Jiang, C. Liang, J.H. He, J. Lumin. 131 (2011) 1013. [23] C.W. Yeh, W.T. Chen, R.S. Liu, S.F. Hu, H.S. Sheu, J.M. Chen, H.T. Hintzen, J. Am. Chem. Soc. 134 (2012) 14108. [24] X.Q. Piao, K. Machida, T. Horikawa, H. Hanzawa, Y. Shimomura, N. Kijima, Chem. Mater. 19 (2007) 4592. [25] H. Yamamoto, Proc. SPIE 7598 (2010) 759808. [26] R.J. Xie, Y.Q. Li, N. Hirosaki, H. Yamamoto, Nitride Phosphors and Solid-State LightingTaylor & Francis, Boca Raton, 2011. [27] V. Bachmann, C. Ronda, A. Meijerink, Chem. Mater. 21 (2009) 2077. [28] G. Kortum, G. Schreyer, Z. Naturforsch. 11A (1957) 1018. [29] C.W. Thiel, H. Cruguel, H. Wu, Y. Sun, G.J. Lapeyre, R.L. Cone, R.W. Equall, R.M. Macfarlane, Phys. Rev. B 64 (2001) 085107.
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Temperature-dependent photoluminescence properties of (Ba,Sr)2SiO4:Eu2 þ phosphors for white LEDs applications Qiyue Shao n, Huanyun Lin, Yan Dong, Jianqing Jiang Jiangsu Key Laboratory of Advanced Metallic Materials, School of Materials Science and Engineering, Southeast University, Nanjing 211189, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 14 August 2013 Received in revised form 27 December 2013 Accepted 19 February 2014 Available online 28 February 2014
(Ba,Sr)2SiO4: Eu2 þ phosphors with various Eu2 þ concentrations and Ba/Sr ratios were synthesized by solid-state reactions and their temperature-dependent photoluminescence properties were studied between 30 and 200 1C. Thermal quenching properties of (Ba1 xSrx)2SiO4: Eu2 þ phosphors barely depend on Eu2 þ concentrations, but exhibit a complex dependence on Ba/Sr ratios. With the increase of the Sr content, the luminescent thermostability is gradually improved for x o0.5 and then becomes worse as x 4 0.5. Considering changes in Stokes shifts, emission wavelengths and host lattice bandgaps for samples with various Ba/Sr ratios, this anomalous quenching behavior was discussed in terms of the configuration coordinate model and photo-ionization theory. & 2014 Elsevier B.V. All rights reserved.
Keywords: White LEDs (Ba,Sr)2SiO4:Eu2 þ phosphors Photoluminescence Thermal quenching
1. Introduction White LEDs are emerging as promising light sources to replace existing incandescent and fluorescent lamps, because of their numerous advantages such as higher energy efficiency, environment friendliness, longer life and reliability [1,2]. One widely used approach for producing white light is the combination of blue LED chips with yellow emitted phosphor materials, such as Y3Al5O12: Ce3 þ (YAG:Ce3 þ ). Therefore, phosphor properties significantly affect the luminous efficiency and color quality of white LEDs. For practical devices, phosphor particles are directly coated on the chip and the heat generated by the LED chip will result in the thermal quenching of the emission of phosphors. It is especially true for high-power packages of white LEDs, in which the LED junction temperature can be up to 150 1C [3,4]. The temperature of phosphor particles even exceeds this value due to their selfheating effect originated from Stokes losses of emissions [5]. As a consequence, LED phosphors should possess a good luminescent thermostability and maintain their quantum efficiency and spectral characteristics at elevated temperatures. (Ba,Sr)2SiO4:Eu2 þ orthosilicate phosphors have been studied since 1960 [6,7], but only recently have been the subject of intense investigation for use as conversion phosphors in both blue and UV LEDs, owing to their high quantum efficiency and suitable excitation/ emission bands that can also be tuned by changing Ba/Sr ratios [8–15]. A disadvantage of (Ba,Sr)2SiO4:Eu2 þ phosphors is the relatively strong luminescent quenching at elevated temperatures.
n
Corresponding author. Tel.: þ 86 25 52090630; fax: þ 86 25 52090634. E-mail address: [email protected] (Q. Shao).
http://dx.doi.org/10.1016/j.jlumin.2014.02.027 0022-2313 & 2014 Elsevier B.V. All rights reserved.
For example, the emission intensity of the Ba2SiO4:Eu2 þ phosphor declines by more than 50% at 150 1C compared to that at room temperature [6,16,17]. A prerequisite for improving their thermal stability is to clarify the basic mechanism behind the thermal quenching. Some studies have been performed to analyze the quenching mechanisms of the Eu2 þ emission in (Ba,Sr)2SiO4 host lattices. Kim et al. have attributed the thermal quenching of (Ba,Sr)2SiO4:Eu2 þ phosphors to thermally activated crossing between 5d and 4f energy levels in the configuration coordinate diagram, and pointed out that the Stokes shift and host lattice phonon energy had a strong effect on their thermal quenching properties [18]. Dorenbos has pointed out that the ionization of the 5d electron to conduction band states is the genuine quenching mechanism of M2SiO4:Eu2 þ (M¼ Sr and Ba) phosphors [19]. However, the experimental data indicated that quenching temperatures of (Ba,Sr)2SiO4:Eu2 þ phosphors show a nonlinear dependence on Ba/Sr ratios [6,17], and the reported quenching mechanisms cannot satisfactorily explain this anomalous quenching behavior. In addition, for 5d–4f emitters, such as Eu2 þ and Ce3 þ , the composition of hosts and the activator concentration have a significant effect on their thermal quenching properties. In this paper, effects of Eu2 þ concentrations and Ba/Sr ratios on temperature-dependent photoluminescence (PL) properties of (Ba,Sr)2SiO4:Eu2 þ phosphors were studied, and dominant quenching mechanisms were discussed based on the configuration coordinate model and photo-ionization theory.
2. Experimental (Ba,Sr)2SiO4:Eu2 þ phosphors were synthesized by typical solidstate reactions. The starting materials of BaCO3 (A. R.), SrCO3
166
Q. Shao et al. / Journal of Luminescence 151 (2014) 165–169
(A. R.), SiO2 (99.99%) and Eu2O3 (99.99%) were weighted in a stoichiometric ratio, and then mixed thoroughly along with a certain amount of BaF2 (A. R.) as flux. These homogeneous mixtures were placed in an alumina crucible and sintered at 1300 1C for 4 h. The samples were then reduced at 1200 1C for 1 h by using carbon powder as reductant to obtain the final products. The crystal structure of the final products was identified by powder X-ray diffraction (Shimadzu XD-3 A) with Cu Kα radiation. The diffusion reflection spectra of samples were measured by a Cary 5000 UV–vis–NIR spectrophotometer equipped with an internal diffuse reflectance accessory. Photoluminescence emission (PL) and excitation (PLE) properties were studied by a Hitachi F-7000 fluorescence spectrophotometer (with an R928F PMT). PL spectra at various temperatures were obtained with the help of a selfdesigned heating attachment, which mainly includes a resistively heated sample holder and a standard temperature controller. Fig. 2. Powder XRD patterns of (Ba1 xSrx)2SiO4:Eu2 þ phosphors with varying Sr2 þ concentrations (x).
3. Results 3.1. Crystalline phase First, effects of Eu2 þ and Sr2 þ contents on crystalline phases of (Ba,Sr)2SiO4:Eu2 þ were studied. Fig. 1 presents powder XRD patterns of Ba2SiO4:Eu2 þ phosphors with various Eu2 þ concentrations. As a reference, the XRD pattern of orthorhombic Ba2SiO4 from the Joint Committee on Powder Diffraction Standards (JCPDS) cards is also shown. XRD patterns of all the samples are found to agree well with the standard data of orthorhombic Ba2SiO4 (JCPDS no. 261403), indicating that the doped Eu2 þ ions do not generate any impurities or induce significant changes in the host structure. XRD patterns of (Ba1 xSrx)2 SiO4:Eu2 þ phosphors with various x values are shown in Fig. 2. The concentration of Eu2 þ ions is kept constant at 3 mol%. XRD patterns of Ba2SiO4:Eu2 þ (x ¼0) and Sr2SiO4:Eu2 þ (x ¼1.0) match well with the standard data of orthorhombic Ba2SiO4 (JCPDS no. 261403) and Sr2SiO4 (JCPDS no. 391256). The other samples are isostructured and show a solid solution between end members. No secondary phases can be detected and all the samples are single phases. It can be found that with the increase of the Sr content XRD peaks shift to higher
Fig. 3. PLE and PL (λex ¼ 370 nm) spectra of Ba2SiO4:Eu2 þ samples with various Eu2 þ concentrations. PLE spectra were recorded by monitoring their maximum emission wavelengths for each sample.
values of the diffraction angle, indicating that the lattice constants are decreased due to the fact that Sr2 þ ions are smaller than Ba2 þ ions. 3.2. PL and PLE spectra
Fig.1. Powder XRD patterns of Ba2SiO4:Eu2 þ concentrations.
samples with various Eu2 þ
Effects of Eu2 þ and Sr2 þ contents on room-temperature PL properties of (Ba,Sr)2SiO4:Eu2 þ were also studied. As an example, impacts of Eu2 þ concentrations on PL and PLE spectra of Ba2SiO4: Eu2 þ phosphors are shown in Fig. 3. At lower Eu2 þ concentrations, excitation spectra consist of two bands around 280 and 350 nm, respectively, which are mainly ascribed to 4f–5d transitions of Eu2 þ ions. The first excitation band may also be partly contributed by the host absorption [11]. At higher Eu2 þ concentrations, a broad excitation band extending from 250 to 450 nm can be observed and does not show any obvious structure. The emission band slightly shifts to a longer wavelength with an increase in the Eu2 þ concentration. The emission intensity reaches a maximum as the Eu2 þ concentration is equal to 3 mol%. The Sr substitution has significant effects on PL properties of (Ba,Sr)2SiO4:Eu2 þ phosphors. PLE and PL spectra of (Ba1 xSrx)2 SiO4: Eu2 þ with varying Sr2 þ concentrations (x) are shown in Fig.4. There are two nonequivalent Eu2 þ sites in (Ba,Sr)2SiO4:Eu2 þ phosphors, and therefore the PL spectra of Eu2 þ ions actually consist of two emission bands [16]. In the case of Sr2SiO4:Eu2 þ , one band is located around 486 nm, and another band is positioned around 555 nm.
Q. Shao et al. / Journal of Luminescence 151 (2014) 165–169
With the decrease of Sr concentrations, two emission bands are more overlapped. As shown in Fig. 4, with increasing Sr content the emission peaks of Eu2 þ ions shift to longer wavelengths and the emission intensities are decreased. Limited to the resolution of excitation spectra at room temperature, a rough estimation of Stokes shifts (SS) was made on the basis of the mirror-image relationship between excitation and emission spectra [20,21]. As shown in Fig. 4, Stokes shifts of Eu2 þ emissions increase with an increase in the Sr content.
3.3. Temperature-dependent PL properties Fig. 5 shows temperature-dependent emission spectra (λex ¼ 370 nm) of the typical sample Ba2SiO4:3%Eu2 þ . For ease of comparison, the excitation wavelength for temperature-dependent PL spectra measurements is fixed at 370 nm, which can effectively excite two emission bands of (Ba1 xSrx)2 SiO4:Eu2 þ phosphors. It can be found that with increasing temperature the peak intensity of Eu2 þ emission decreases rapidly. As shown in the inset of Fig. 5, the integrated intensity of Ba2SiO4:3%Eu2 þ at 150 1C only can remain 28% of that at room temperature. Compared to other commercial LED phosphors, such as Y3Al5O12:7%Ce3 þ (88% at 150 1C), Sr2Si5N8:10%Eu2 þ (84%) and CaAlSiN3:2%Eu2 þ (90%) [22–24], the thermal quenching of the Ba2SiO4:Eu2 þ phosphor is much more serious. The inset of Fig.5 also shows changes of the integrated intensity as a function of temperature during the cooling process. The emission intensity returns nearly to the same value after cooling down.
167
It can also be found that with increasing temperature the peak wavelength of Ba2SiO4:3%Eu2 þ slightly shifts to the blue side (Fig. 5). The similar blue-shift can be observed for other samples with various Ba/Sr ratios (not shown). As mentioned above, the PL spectra of Eu2 þ ions consist of two overlapped emission bands. Kim et al. have ascribed the blue-shift of Eu2 þ emission with increasing temperature to back tunneling from excited states of the low-energy emission band to excited states of the high-energy emission band by assistance of thermally active phonons [16]. Another possible reason for the blue-shift is that the low-energy emission band exhibits a more significant decrease in intensity at elevated temperatures than the high-energy emission band, due to different crystal field environments. Fig. 6 shows the influences of Eu2 þ concentrations on the thermal quenching behavior of Ba2SiO4:Eu2 þ and (Ba0.5Sr0.5)2SiO4: Eu2 þ phosphors. At a fixed Ba/Sr ratio, the reduction of emission intensity at each degree is almost same for samples with various Eu2 þ concentrations. Similar results can be obtained for other samples with different Ba/Sr ratios (not shown), and therefore we can conclude that the Eu2 þ concentration has a weak effect on the thermal quenching of (Ba,Sr)2SiO4:Eu2 þ phosphors. Ba/Sr ratios have a strong and complex effect on thermal quenching properties of (Ba,Sr)2SiO4:Eu2 þ phosphors. Fig. 7 shows temperature dependences of integrated intensities of (Ba1 xSrx)2 SiO4:Eu2 þ phosphors with various x values (λex ¼370 nm), and the Eu2 þ concentration is kept constant at 3 mol%. It is worth noting that effects of Ba/Sr ratios on the thermal quenching properties are similar under various excitation wavelengths between 250 and 460 nm. It can be found that the Ba/Sr ratio around 1:1 (x¼ 0.5) shows a better temperature stability than end members. As xo 0.5, the thermal quenching of (Ba1 xSrx)2SiO4:Eu2 þ phosphors is gradually suppressed with the increase of x. Inversely, as x4 0.5, the thermal quenching gradually becomes worse with the increase of x. Similar results were first reported in 1968 for the case under 254 nm light excitation [6] and were recently found for blue light excitation [17,25].
4. Discussion
Fig. 4. PLE and PL (λex ¼ 370 nm) spectra of (Ba1 xSrx)2SiO4:Eu2 þ phosphors with varying Sr2 þ concentrations (x). PLE spectra were recorded by monitoring their maximum emission wavelengths for each sample. Stokes shifts (SS) of phosphors are also presented.
Fig. 5. Temperature-dependent emission spectra (λex ¼ 370 nm) of the Ba2SiO4:3% Eu2 þ sample. The inset shows integrated intensities as a function of temperature for a heating-cooling cycle.
It is known that the thermal degradation of host lattice and the thermal oxidation of activator ions can also lead to the reduction of emission intensity of phosphors at elevated temperatures. M2SiO4 (M ¼Ca, Sr, and Ba) silicates are normally amenable to be attacked by the moisture in air. The integrated intensity of (Ba1 xSrx)2SiO4:Eu2 þ phosphors in a heating–cooling cycle shows a reversible thermal quenching behavior (Fig. 5), indicating that the thermal degradation of host lattice and the Eu2 þ oxidation contribute little to the luminescent quenching of (Ba1 xSrx)2SiO4: Eu2 þ phosphors at elevated temperatures. Various thermal quenching paths have been proposed for 5d–4f emitters, mainly including the thermally activated crossing between 5d and 4f energy levels, temperature-dependent concentration quenching and thermally assisted photo-ionization [26]. It was reported that thermal quenching properties of Y3Al5O12:Ce3 þ and CaAlSiN3:Eu2 þ phosphors exhibit a strong dependence on activator concentrations and the pronounced thermal quenching occurs at higher activator concentrations, which can be attributed to the temperature-dependent concentration quenching effect [25–27]. The thermal quenching of (Ba, Sr)2SiO4:Eu2 þ phosphors is nearly independent of the Eu2 þ concentration (Fig. 6), implying that thermally activated concentration quenching is not the main cause of luminescent quenching at elevated temperatures. As shown in Fig. 7, the thermal quenching behavior of (Ba1 xSrx)2 SiO4:Eu2 þ phosphors exhibits a complex dependence on Ba/Sr ratios. This anomalous thermal quenching behavior cannot be satisfactorily
168
Q. Shao et al. / Journal of Luminescence 151 (2014) 165–169
Fig. 6. Temperature dependences of integrated emission intensities of (a) Ba2SiO4:Eu2 þ and (b) (Ba0.5Sr0.5)2SiO4:Eu2 þ phosphors with various Eu2 þ concentrations (λex ¼ 370 nm). The integrated intensity is normalized to that at 30 1C for each sample.
Fig. 7. Temperature dependences of integrated emission intensities of (Ba1 xSrx)2 SiO4:Eu2 þ phosphors normalized to 30 1C (λex ¼ 370 nm).
explained by the configuration coordinate model, where thermally assisted crossing between energy parabolas of the excited and the ground state leads to increased non-radiative decay at elevated temperatures. Based on the configuration diagram, the thermal quenching will appear at lower temperatures for larger Stokes shifts. As shown in Fig. 4, the Stokes shift of (Ba1 xSrx)2SiO4:Eu2 þ phosphors increases with the increase of the Sr content, which means that their thermal quenching properties should continuously become worse with increasing x values. Actually, quenching temperatures exhibit a nonlinear dependence on Ba/Sr ratios, indicating that the change in Stokes shift is not the only factor to affect the thermal quenching behavior of (Ba1 xSrx)2SiO4:Eu2 þ phosphors. It also implies that other thermally activated non-radiative decay paths should be considered. Changes of Ba/Sr ratios may lead to structure distortion with the formation of defects, and therefore induce the continuous decrease of room-temperature luminescent intensity with increasing Sr contents (Fig. 4). However, the formation of defects cannot explain the non-linear dependence of thermal quenching temperatures on Ba/Sr ratios. In addition, it is known that the thermally activated concentration quenching process is related to the density of structure defects [22,27]. Since the thermal quenching of (Ba,Sr)2SiO4:Eu2 þ phosphors with various Ba/Sr ratios is nearly independent of the Eu2 þ concentration, it is unlikely that increased energy migration to defects is responsible for the changes in thermal quenching. Baginskiy et al. thought direct tunneling between two nonequivalent Eu2 þ sites plays an important role in the thermal quenching process of (Ba,Sr)2SiO4:Eu2 þ phosphors [17]. Increased energy migration from high Eu2 þ excited states to low excited states at elevated temperatures promotes non-radiative relaxation and results in the decrease of quenching temperature of barium-
Fig. 8. (a) Diffuse reflection spectra and (b) absorption spectra of (Ba1 xSrx)2 SiO4 hosts.
richer phosphors [17]. However, this mechanism cannot explain the blue-shift of Eu2 þ emission with increased temperatures (Fig. 5). For 4f–5d emitters, the thermal activation from 5d states to the host lattice conduction band can also lead to the thermal quenching, which is usually described as photo-ionization or auto-ionization [4,26]. The photo-ionization process is strongly related to the energy barrier (EdC) of 5d states of rare earth ions to the bottom of the host conduction band. The EdC values can be determined by the host lattice bandgap, the 5d–4f transition energy, and the position of the 4f ground state within the bandgap. The host bandgap can be estimated by the optical absorption spectra. Fig. 8 gives the diffuse reflection spectra and the absorption spectra of (Ba1 xSrx)2SiO4 hosts without Eu2 þ doping. The absorption spectra F(R) were obtained from the diffuse reflection spectra by using the Kubelka–Munk function [11,28] FðRÞ ¼
ð1 RÞ2 ¼ K=S 2R
ð1Þ
R, K and S are the reflectivity, the absorption and scattering coefficients, respectively. It is clear that the host absorption edge of (Ba1 xSrx)2SiO4 shifts to higher energy with the increase of the Sr content. As shown in Fig. 4, Eu2 þ emission bands shift to longer wavelengths with increasing Sr content, indicative of the decrease of the 5d–4f transition energy. There is no experimental data about the position of the 4f ground state of Eu2 þ ions within the bandgap of (Ba1 xSrx)2SiO4 hosts in the literature. For materials with the similar structure, the cation substitution has a weak effect on the 4f ground state energy of rare earth ions relative to the host valence band [29]. Therefore, it is reasonable to assume that the position of the 8S7/2
Q. Shao et al. / Journal of Luminescence 151 (2014) 165–169
Conduction band
EdC
EdC 5d
EdC
5d
5d
4f Valence band Ba2SiO4:Eu2+
Conduction band
Conduction band
4f
4f Valence band
Valence band (Ba0.5Sr0.5)2SiO4
:Eu2+
Sr2SiO4:Eu2+
(Ba0.5Sr0.5)2SiO4:Eu2+
E Ba2SiO4:Eu2+
Sr2SiO4:Eu2+
r
r0 r1, r2, r3 2þ
Fig. 9. (a) Schematic of energy levels of Eu ions with the bandgap of (Ba1 xSrx)2SiO4; (b) configuration coordinate diagram of (Ba1 xSrx)2SiO4:Eu2 þ phosphors.
ground state of Eu2 þ ions above the valence band remains constant with Sr substitution. In combination with the increase of host lattice bandgap and the reduction of 5d–4f transition energy of Eu2 þ ions with increasing Sr substitution, we can conclude that the EdC values of (Ba1 xSrx)2SiO4:Eu2 þ phosphors gradually increase with the increase of x values, as shown in Fig. 9(a). Based on changes of Stokes shifts and EdC values with Sr substitution, the nonlinear dependence of thermal quenching on Ba/Sr ratios can be ascribed to the combined action of the thermally activated crossing between 5d and 4f energy levels and photo-ionization. For higher Ba/Sr ratios (xo0.5), the energy barrier between the Eu2 þ 5d states and the host conduction band is lower, and therefore the thermally assisted photo-ionization process dominates the nonradiative decay paths of (Ba1 xSrx)2 SiO4:Eu2 þ phosphors. With increasing Sr substitution (xo0.5), the thermal quenching becomes weak due to the increase of EdC values. For the case of x4 0.5, the energy barrier EdC is higher while the displacement between 5d and 4f parabolas in the configuration diagram is larger, and therefore the thermally activated crossing between 5d and 4f states starts to dominate the thermal quenching of (Ba1 xSrx)2SiO4:Eu2 þ phosphors. As a result, their thermal quenching properties become worse with increasing Sr substitution due to the increase of Stokes shifts, as shown in Fig. 9(b). 5. Conclusion Different quenching models on the Eu2 þ emission in M2SiO4 (M ¼Sr and Ba) orthosilicates have been reported in the literature, however, cannot fully explain the thermal quenching behavior of (Ba,Sr)2SiO4:Eu2 þ phosphors. In this work, effects of Eu2 þ concentrations and Ba/Sr ratios on thermal quenching properties of (Ba,Sr)2SiO4:Eu2 þ phosphors were investigated and dominant quenching mechanisms were discussed. The emission intensity
169
of the phosphors shows a considerable decline at elevated temperatures, and this decline is nearly independent of Eu2 þ concentrations but exhibits strong and complex dependences on Ba/Sr ratios. The thermostability is highest at Sr content of x¼ 0.5 compared to end members of Ba2SiO4:Eu2 þ and Sr2SiO4:Eu2 þ . Based on changes of Stokes shifts, emission bands and absorption spectra with Sr substitution, this anomalous dependence of thermal quenching on Ba/Sr ratios can be ascribed to two competing processes of thermally activated crossing of energy parabolas of 5d and 4f states and photo-ionization. At higher Sr contents (x 40.5), the change in the Stokes shift is the dominant factor that affects the thermal quenching behavior of (Ba1 xSrx)2SiO4:Eu2 þ phosphors, while the photo-ionization process dominates at lower Sr contents (xo0.5). These results are relevant to the design, implementation and improvement of (Ba1 xSrx)2SiO4:Eu2 þ phosphors for their application in high-power white LED devices. The ideas and models on the thermal quenching of Eu2 þ emissions in (Ba1 xSrx)2SiO4 hosts can also be applied to other Eu2 þ -activated phosphors.
Acknowledgments This work was financially supported by the Natural Science Foundation of China (No. 51302038) and the Natural Science Foundation of Jiangsu Province of China (Nos. BK2011064 and BK2012346). References [1] E.F. Schubert, J.K. Kim, Science 308 (2005) 1274. [2] S. Pimputkar, J.S. Speck, S.P. DenBaars, S. Nakamura, Nat. Photonics 3 (2009) 180. [3] G. Gundiah, Y. Shimomura, N. Kijima, A.K. Cheetham, Chem. Phys. Lett. 455 (2008) 279. [4] P.F. Smet, A.B. Parmentier, D. Poelman, J. Electrochem. Soc. 158 (2011) R37. [5] B. Yan, N.T. Tran, J.P. You, F.G. Shi, IEEE Photonics Technol. Lett. 23 (2011) 555. [6] T.L. Barry, J. Electrochem. Soc. 115 (1968) 1181–1184. [7] G. Blasse, Philips Res. Rep. 24 (1969) A131. [8] M.A. Lim, J.K. Park, C.H. Kim, H.D. Park, J. Mater. Sci. Lett. 22 (2003) 1351. [9] J.K. Park, M.A. Lim, C.H. Kim, H.D. Park, J.T. Park, S.Y. Choi, Appl. Phys. Lett. 82 (2003) 683. [10] J.S. Kim, P.E. Jeon, J.C. Choi, H.L. Park, Solid State Commun. 133 (2005) 187. [11] M. Zhang, J. Wang, Q.H. Zhang, W.J. Ding, Q. Su, Mater. Res. Bull. 42 (2007) 33. [12] H. He, R.L. Fu, X.F. Song, D.L. Wang, J.K. Chen, J. Lumin. 128 (2008) 489. [13] W.H. Hsu, M.J. Sheng, M.S. Tsai, J. Alloys Compd. 467 (2009) 491. [14] X.G. Zhang, X.P. Tang, J.L. Zhang, M.L. Gong, J. Lumin. 130 (2010) 2288. [15] J.K. Han, M.E. Hannah, A. Piquette, G.A. Hirata, J.B. Talbot, K.C. Mishra, J. McKittrick, J. Lumin. 132 (2012) 106. [16] J.S. Kim, Y.H. Park, S.M. Kim, J.C. Choi, H.L. Park, Solid State Commun. 133 (2005) 445. [17] I. Baginskiy, R.S. Liu, C.L. Wang, R.T. Lin, Y.J. Yao, J. Electrochem. Soc. 158 (2011) P118. [18] J.S. Kim, Y.H. Park, J.C. Choi, H.L. Park, J. Electrochem. Soc. 152 (2005) H135. [19] P. Dorenbos, J. Phys.: Condens. Matter 17 (2005) 8103. [20] J. Ruan, R.J. Xie, N. Hirosaki, T. Takeda, J. Am. Ceram. Soc. 94 (2011) 536. [21] J.H. Ryu, Y.G. Park, H.S. Won, S.H. Kim, H. Suzuki, J.M. Lee, C. Yoon, M. Nazarov, D.Y. Noh, B. Tsukerblat, J. Electrochem. Soc. 155 (2008) J99. [22] Q.Y. Shao, Y. Dong, J.Q. Jiang, C. Liang, J.H. He, J. Lumin. 131 (2011) 1013. [23] C.W. Yeh, W.T. Chen, R.S. Liu, S.F. Hu, H.S. Sheu, J.M. Chen, H.T. Hintzen, J. Am. Chem. Soc. 134 (2012) 14108. [24] X.Q. Piao, K. Machida, T. Horikawa, H. Hanzawa, Y. Shimomura, N. Kijima, Chem. Mater. 19 (2007) 4592. [25] H. Yamamoto, Proc. SPIE 7598 (2010) 759808. [26] R.J. Xie, Y.Q. Li, N. Hirosaki, H. Yamamoto, Nitride Phosphors and Solid-State LightingTaylor & Francis, Boca Raton, 2011. [27] V. Bachmann, C. Ronda, A. Meijerink, Chem. Mater. 21 (2009) 2077. [28] G. Kortum, G. Schreyer, Z. Naturforsch. 11A (1957) 1018. [29] C.W. Thiel, H. Cruguel, H. Wu, Y. Sun, G.J. Lapeyre, R.L. Cone, R.W. Equall, R.M. Macfarlane, Phys. Rev. B 64 (2001) 085107.