Journal of Alloys and Compounds 497 (2010) L21–L24
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Luminescent properties of Sr2 SiO4 : Eu2+ nanorods for near-UV white LED Chongfeng Guo ∗ , Yan Xu, Feng Lv, Xu Ding Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology, Hubei Province, Wuhan, 430074, China
a r t i c l e
i n f o
Article history: Received 4 January 2009 Received in revised form 1 March 2010 Accepted 2 March 2010 Available online 9 March 2010 PACS: 78.20.−e 78.55.−m
a b s t r a c t Sr2 SiO4 : Eu2+ phosphor was prepared by a modiﬁed sol–gel method. The structure and the composition of the phosphor nanorods were characterized by powder X-ray diffraction (XRD) and energy dispersion spectrum (EDS). Field-emission scanning electron microscopy (FE-SEM) and photo-luminescent spectra were used to characterize the shape and the luminescent properties of phosphor particles. Onedimensional Sr2 SiO4 : Eu2+ nanorods were obtained, which shows intense blue–green emission under the excitation of near ultraviolet (n-UV) light (275–425 nm). It indicates that Sr2 SiO4 : Eu2+ phosphors are promising blue-green candidates for the n-UV white LEDs. © 2010 Elsevier B.V. All rights reserved.
Keywords: Optical materials Optical properties Luminescence
1. Introduction White light-emitting diodes (w-LEDs) have wide application in displays and illumination due to their energy-saving, miniaturizing equipment and nonpollution. They are generally produced by blending multi-LEDs or combining a blue or near-ultraviolet (n-UV) LED chip with phosphors. It was estimated that the white LEDs produced through exciting multiphosphors by a n-UV LED chip would dominate the market in the near future for their high color rendering index (Ra) and tunable color temperature (Tc ). However, the dominating w-LEDs products in the current market are fabricated by the combination of blue LED chip with yellow-emitting phosphor YAG:Ce3+ [(Y1−a Gda )3 (Al1−b Gab )5 O12 :Ce3+ ], which offers unacceptable high color temperature (>4500 K) and low rendering index (<80) for general illumination and some special lighting. One of the obstacles that prevent the w-LEDs based on n-UV LED chip from popularizing is the lack of phosphor with high stability, therefore it is an urgent task to develop phosphors with high stability and intense absorption in n-UV spectral region [1–5]. Silicates are excellent hosts for n-UV white LEDs due to their high chemical–physical stability and various crystal structures. The emission color of Eu2+ -activated silicates can be controlled by changing the crystal ﬁeld in the host, which due to the fact that the emission of Eu2+ ion could vary from ultraviolet to red depending on the host lattice because of its parity-allowed 4f-5d transition [6,7]. Eu2+ -activated alkaline earth orthosilicates have been intensively
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studied. Results showed that the Sr2 SiO4 : Eu2+ phosphor exhibited the yellow-greenish emission, high quantum efﬁciency and strong absorption in n-UV region, which proves that Sr2 SiO4 : Eu2+ is a promising phosphor for white LEDs [8–10]. As we all known, the size distribution and the shape of phosphor particles have serious effect on the efﬁciency of w-LEDs, thus it is important for the phosphor used in w-LEDs to control the size and the shape of the phosphor particles. However, results of the previous references indicated that the most of Eu2+ -activated alkaline earth orthosilicates were synthesized by the conventional solid state reaction technique, which leads to the large size distribution and irregular shape of phosphor particles . In order to overcome these problems, in this paper, we prepared Sr2 SiO4 : Eu2+ phosphor nanorods by a modiﬁed sol–gel method and investigated its photoluminescent properties for potential application in near UV white LEDs. 2. Experimental The Sr2 SiO4 : Eu2+ phosphor nanorods were prepared by a modiﬁed sol–gel method. The starting chemicals include tetraethoxysilane (TEOS, 99 wt.%, AR), Eu2 O3 (99.99%), SrCO3 (AR), AgNO3 (AR.), NH4 OH (25 wt.%, AR), diluted HNO3 and ethanol (C2 H5 OH, AR). Firstly, the silica colloidal suspension was prepared by the wellknown Stöber method, in which TEOS was hydrolyzed in an ethanol medium with the addition of deionized water and ammonia . In our experimental, 3.5 ml of TEOS, 2.9 ml of de-ionized H2 O and 3.9 ml NH4 OH were mixed with absolute ethanol to ﬁx at 50 ml, and then the mixture was continuously stirred for half an hour at 50 ◦ C to obtain the required silica colloidal suspension A. Secondly, stoichiometric amount of raw materials Eu2 O3 were dissolved in dilute HNO3 under vigorous stirring, then a suitable volume of de-ionized water and stoichiometric amounts of SrCO3 were added to form the transparent solution B. Finally, the solution B was mixed with the silica colloidal suspension A, and the mixed solution was continuously stirred for half an hour, then the ﬁnal solution were kept at 60 ◦ C in water bath to get dried gel. After preheating the dried gel for 2 h at 600 ◦ C in air, the resulting samples were then ﬁred at 1300 ◦ C for 3 h in CO atmosphere to obtain the phosphor powders.
C. Guo et al. / Journal of Alloys and Compounds 497 (2010) L21–L24
Fig. 1. XRD patterns of standard JCPDS 39-1256 (a) and Sr1.99 Eu0.01 SiO4 phosphors (b).
The X-ray diffractions (XRD) of the samples were examined on a X’ Pert PRO (PANalytical B.V.) powder diffract-meter with Cu-K␣( = 1.54065 Å) radiation to identify the crystal phase. The grain morphology and the grain size of the samples were inspected using a ﬁeld-emission scanning electron microscope (FESEM, Sirion 200, FEI) and a transmission electron microscope (TEM, Tecnai G2-20, FEI). The photoluminescence excitation (PLE) and photoluminescence (PL) spectra measurements were carried out using a JASCO FP-6500 ﬂuorescence spectrophotometer equipped with a 150 W xenon lamp as the excitation source (excitation slit width = 1 nm; emission width = 3 nm). All measurements were performed at room temperature.
3. Results and discussion The powder XRD patterns of all samples prepared in our experiments were measured, and the XRD patterns of all as-prepared phosphors are similar. Fig. 1 gives the crystal structure of the Sr2 SiO4 : (0.01) Eu2+ sample and the standard XRD (JCPDS ﬁle 391256) patterns of ␣ phase (Fig. 1a). Comparing two XRD patterns (Fig. 1a and b), all of diffraction peaks of samples match well with those of the standard patterns, which indicate that all the samples are conﬁrmed to be the ␣ phase in our experiments. Fig. 2a shows the typical SEM image of Sr1.99 Eu0.01 SiO4 phosphor, and it is clearly observed that the particles have a rod-like morphology with the diameter of about 10 nm and the length of 0.5–1 m, which makes the phosphor have high surface area. It is found that the phosphors with higher surface-area have better thermal conductivity when excited with high-energy photons  and are favorable to the lifetime of UV GaN-based white-LED. In order to further examine the composition of nanorods, the energy
dispersion spectrum (EDS) coupled with FE-SEM was also used to characterize the nanorods. Fig. 2b shows that the EDS spectrum of the nanorods was composed of O, Si and Sr elements. The C element originates from the carbon ﬁlm sample holder, but no Eu elements were checked due to its trace concentration. In addition, the atomic ratio of the Sr:Si:O is close to 2:1:4, which conﬁrms that the composition of nanorods is Sr2 SiO4 . As seen in Fig. 2a, most of the nanorods aggregated like a ﬂower, but the single nanorods could not been seen clearly. Therefore, the as-synthesized samples were dispersed in an ultrasonic bath with ethanol for 15 min, then the sample were characterized by FE-SEM and TEM images shown in Fig. 3a and b, respectively. As shown in Fig. 3a, the diameters of the nanorods were not uniform. The nanorods seem to be formed by coupling short rods or particles, and the junctures could be clearly observed. Fig. 3b shows the TEM image of the dispersed samples, which is also conﬁrmed our results and no single nanoparticles were found. However, the growth process of the nanorods was not clear, and a series of further experiments are being undergone to investigate the effects of precursors, reaction time and reaction temperature. As we all know, the luminescent properties of phosphor are ﬁrst determined by the structure of the host. It is known that there are two alkaline earth strontium sites those are equally distributed in the Sr2 SiO4 lattice . One of the sites has ten coordinations and the other is surrounded by nine oxygen ions, and they are designated as Sr(I) and Sr(II), respectively. As the Eu2+ doped Sr2 SiO4 host, Eu2+ ions occupy the sites of Sr2+ , hence, it is reasonable to predict that there are two emission bands since Eu2+ ions enter into two different sites with different crystal ﬁeld environments. According to the results of a recent papers dealing with the twoemission behavior in the phosphor with two different cation sites, the loose site accommodating Eu2+ activators corresponds to a high energy (short-wavelength) emission peak and vice versa [15,16]. In addition, Lakshminarasimhan and Varadaraju  also calculated the position in energy of emission of Eu2+ in Sr2 SiO4 host using the empirical relation given by van Uitert to predict the position of emission of Eu2+ and Ce3+ in different host lattices. The calculated results also conﬁrmed that the shorter-wavelength emission is ascribed to Eu2+ on a Sr(I) site, whereas the longer-wavelength emission is attributed to Eu2+ on a Sr(II) site . The PL and PLE spectra for phosphor Sr1.99 Eu0.01 SiO4 were presented in Fig. 4a and b. As expected, the PL spectra of phosphor under different wavelength excitation exhibit two emission bands those are assigned to the f–d transitions of Eu2+ ions entered into two different cation sites in the host. The PL spectrum of Sr1.99 Eu0.01 SiO4 under the excitation of 323 nm was displayed in Fig. 4a, and two broad bands were observed. The shorterwavelength blue emission centered at 474 nm dominates the emission band resulting from the 5d–4f transition of Eu2+ in sites Sr(I). The PLE spectrum monitored at emission 474 nm consists of
Fig. 2. Typical FE-SEM images of Sr1.99 Eu0.01 SiO4 phosphor particles (a) and the EDS patterns of Sr1.99 Eu0.01 SiO4 nanorods (b).
C. Guo et al. / Journal of Alloys and Compounds 497 (2010) L21–L24
Fig. 3. FE-SEM (a) and TEM (b) images of the as-synthesized Sr1.99 Eu0.01 SiO4 nanorods after dispersing them in an ultrasonic bath for 15 min in ethanol.
an intense absorption band that is associated with the crystal-ﬁeld splitting of 5d states of Eu2+ in Sr(II) sites. Shown in Fig. 4b is the PL spectrum of the sample under the excitation of 380 nm, and there are also two broad emission bands centered at about 475 and 535 nm those are close to the calculated wavelengths in Ref. , however the band centered at longer-wavelength 535 nm from the Eu2+ in the site Sr(I) dominates the emission. The PLE spectrum of sample monitored at 535 nm shows a strong absorption in near UV region, ranging from 225 to 475 nm, which matches well with the commercial available near-UV LED chips. Therefore, Eu2+ doped Sr2 SiO4 is an excellent phosphor candidate for n-UV white LEDs. The characteristic of this phosphor is its potential application in n-UV white LEDs, thus we paid more attention to the emission of phosphor under the excitation of near ultraviolet light. Photoluminescence spectra (ex = 380 nm) of Sr2−x Eux SiO4 phosphors with different Eu2+ contents were shown in Fig. 5. The PL intensity initially increases with the increasing of Eu2+ contents, reaching a maximum at x = 0.01 mol. A further increase of the Eu2+ contents leads to a decrease in the PL intensity, which is due to the occurrence of energy migration between Eu2+ in different sites in the lattice, resulting in the concentration quenching . Seen in Fig. 4, it is observed that there is a spectral overlap between the excitation spectrum of Eu2+ (II) and the emission spectrum of Eu2+ (I), which indicates the resonant energy transfer of Eu2+ (I) → Eu2+ (II) is expected. Therefore, the decrease of the emission intensity from the nonradiative energy transfer between Eu2+ ions in Sr2 SiO4 should be dominated by electric multipole–multipole interaction .
Fig. 4. PL (dash line) and PLE (solid line) spectra of Sr1.99 Eu0.01 SiO4 phosphors: (a) ex = 323 nm, em = 474 nm; (b) ex = 380 nm, em = 535 nm.
As the concentration of Eu2+ increases, the interatomic distance between Eu2+ ions is shortened and then energy transfer occurs with high probability. The critical distance (Rc ) of the energy transfer between the same sorts of activators Eu2+ in Sr2−x Eux SiO4 , the activator is introduced solely on Z ions (Sr) sites, could be estimated according to the following formula :
Rc = 2
here c is the critical concentration, N is the number of Z ions in the unit cell and V is the volume of the unit cell, then there is one activator ion per V/c N on the average. By taking the experimental and analytic values of V, N and c (391.20 Å3 , 8, 0.01, respectively ), the critical transfer distance of Eu2+ in Sr2 SiO4 phosphors is found to be about 21 Å. There is an obvious redshift for the two emission bands with an increase of Eu2+ concentration, which may be attributed to the changes in the crystal ﬁeld resulting from the increase of Eu2+ contents. In addition, it is observed that the PL intensity from Eu2+ located at the Sr(I)sites decreases and the relative intensity of Eu2+ from the Sr(II) increases with the increasing of the Eu2+ concentrations, which is due to the fact that the dopant Eu2+ ions usually prefer the loose environment of Sr(I) site , so that the emission peak locates at the high energy region of spectrum in the low Eu2+ ion concentration region.
Fig. 5. PL spectra (ex = 380 nm) of Sr2−x Eux SiO4 phosphors with different Eu2+ concentrations.
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4. Conclusions One dimensional Sr2 SiO4 : Eu2+ nanorods were synthesized by a modiﬁed sol–gel method, and their structure, shape and luminescent properties have been investigated. The as-prepared phosphors exhibit rod-like morphology and broad intense absorption ranging from 275 to 425 nm, which makes this phosphor an excellent blue–green phosphor candidate for UV white LEDs. For this phosphor, the optimal composition is Sr1.99 Eu0.01 SiO4 , and the concentration quenching occurs as the concentration of Eu2+ beyond 0.01 mol. Acknowledgements The authors wish to thank Analytical and Testing Center of Huazhong University of Science and Technology for providing the facilities to carry out measurements. National Natural Science Foundation of China (No. 50802031) and the Ph.D. Programs Foundation of Ministry of Education of China (No. 20070487076) supported this work. References  M.S. Wang, S.P. Guo, Y. Li, L.Z. Cai, J.P. Zou, G. Xu, W.W. Zhou, F.K. Zheng, G.C. Guo, J. Am. Chem. Soc. 131 (2009) 13572–13573.
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