Luminescent properties of green- or red-emitting Eu2+-doped Sr3Al2O6 for LED

Luminescent properties of green- or red-emitting Eu2+-doped Sr3Al2O6 for LED

Journal of Luminescence 131 (2011) 2463–2467 Contents lists available at ScienceDirect Journal of Luminescence journal homepage: www.elsevier.com/lo...

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Journal of Luminescence 131 (2011) 2463–2467

Contents lists available at ScienceDirect

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

Luminescent properties of green- or red-emitting Eu2 þ -doped Sr3Al2O6 for LED Jilin Zhang, Xinguo Zhang, Jianxin Shi n, Menglian Gong n Key Laboratory of Bioinorganic and Synthetic Chemistry of Ministry of Education, State Key Laboratory of Optoelectronic Materials and Technologies, School of Chemistry and Chemical Engineering, Sun Yat-Sen University, Guangzhou 510275, China

a r t i c l e i n f o

abstract

Article history: Received 22 June 2010 Received in revised form 19 May 2011 Accepted 30 May 2011 Available online 13 June 2011

Eu2 þ -doped Sr3Al2O6 (Sr3  xEuxAl2O6) was synthesized by a solid-state reaction under either H2 and N2 atmosphere or CO atmosphere. When H2 was used as the reducing agent, the phosphor exhibited green emission under near UV excitation, while CO was used as the reducing agent, the phosphor mainly showed red emission under blue light excitation. Both emissions belong to the d–f transition of Eu2 þ ion. The relationship between the emission wavelengths and the occupation of Eu2 þ at different crystallographic sites was studied. The preferential substitution of Eu2 þ into different Sr2 þ cites at different reaction periods and the substitution rates under different atmospheres were discussed. Finally, green-emitting and red-emitting LEDs were fabricated by coating the phosphor onto near UV- or blue-emitting InGaN chips. & 2011 Elsevier B.V. All rights reserved.

Keywords: Phosphor Luminescence Crystallographic sites Sr3Al2O6 Eu2 þ LED

1. Introduction Since Nakamura and his co-workers fabricated a blue-emitting InGaN light-emitting diode (LED) in 1993 [1], increasing interest has been focused on white LEDs because of their attractive properties such as energy saving, compactness, light weight, long lifetime, mercury-free, quick response, etc [2,3]. White LEDs can be produced from a combination of a blue LED chip and a broad-band yellowemitting phosphor, or a near UV LED (NUV LED) chip and multichromatic phosphors. These are called phosphor-converted LEDs (pc-LEDs). Today, the former dominates the white LED market due to the demand for backlighting liquid crystal displays, car illumination, solid state lighting, etc., while the latter is also of broad interest, mainly because it allows the application of more types of luminescent materials in fabrication of LEDs than the former. Furthermore, NUV-based pc-LED shows some other advantages such as high color-rendering performance, uniformity in the emitted white color and wider chromaticity [4]. Therefore, NUV-based pc-LED is expected to have great potential applications in the field of solid state lighting. Due to the lack of red emitting, the combination of a blue LED chip and a broad-band yellow-emitting phosphor emits a cold light. One way to generate a warm-white light is adding a red phosphor into a blue chip based pc-LED. Sr3Al2O6:Eu2 þ is a phosphor of great interest. Zhang et al. [5] synthesized Sr3Al2O6:Eu2 þ by a sol–gel method assisted by

microwave irradiation, and followed by a treatment at a high temperature in CO atmosphere, and the phosphor emitted a broad-band red light peaking at 612 nm under a blue light excitation. Akiyama et al. [6] reported the mechanoluminescence of Sr3Al2O6:Eu2 þ with a peak at 510 nm, and the photoluminescence spectrum is similar to the mechanoluminescence spectrum under the excitation of 365 nm. The phosphor was obtained under a reducing atmosphere of 95% Ar and 5% H2. Furthermore, there were several reports on the Eu2 þ and Dy3 þ co-doped Sr3Al2O6, which emitted a red light [7–9]. To our best knowledge, there is no report of Sr3Al2O6:Eu2 þ application in LED until now. Chakoumakos et al. [10] reported that there are six crystallographic sites for Sr2 þ in Sr3Al2O6. Therefore, it is possible for Sr3Al2O6:Eu2 þ to emit light with different wavelengths. There are no reports on the study of the relationship between the emission wavelengths and the crystallographic sites of Sr2 þ occupied by Eu2 þ ions in this matrix. In the present work, Sr3Al2O6:Eu2 þ was synthesized by a conventional solid-state reaction under either a reductive atmosphere of 10% H2 and 90% N2 or a CO atmosphere. The relationship between the emission wavelengths of the phosphor and the crystallographic sites occupied by Eu2 þ ion was studied. Greenemitting and red-emitting Sr3Al2O6:Eu2 þ converted LEDs were fabricated with the synthesized phosphor.

2. Experimental section n

Corresponding authors. Tel.: þ 86 20 8411 2830; fax: þ 86 20 8411 2245. E-mail addresses: [email protected] (J. Shi), [email protected] (M. Gong). 0022-2313/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2011.05.064

The powder samples with the general formula of Sr3 xEuxAl2O6 (x¼0.03–0.42) were prepared using a conventional solid-state

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reaction method. The starting materials were SrCO3 (AR), Al(OH)3 (AR), Eu2O3 (99.99%) and (NH4)2CO3 (AR). In a typical process, the raw materials with a stoichiometric ratio were mixed thoroughly by grinding in an agate mortar and heated at 1200 1C under a reductive atmosphere of 10% H2 and 90% N2 in a tube furnace for 2 h and then cooled to room temperature naturally. Other series of the samples were also prepared by calcination of the mixtures at 1100, 1300 and 1400 1C in the same way. When using CO as the reductive atmosphere, a small amount of (NH4)2CO3 was added into the mixture. The phases of the as-prepared samples were identified by X-ray powder diffraction spectroscopy (XRD) with a Rigaku D/max 2200 ˚ at 40 kV and X-ray diffractometer with CuKa radiation (l ¼1.5406 A) 20 mA. The photoluminescence (PL), PL-excitation (PLE) spectra of the solid samples were measured by a Fluorolog 3-21 spectrofluorometer (Jobin Yvon Inc/specx) equipped with a 450 W Xe lamp and double excitation monochromators. All the measurements were carried out at room temperature.

3. Results and discussion 3.1. Sr3  xEuxAl2O6 obtained under H2 and N2 atmosphere The samples were synthesized by calcination at 1100–1400 1C for 2 h. Fig. 1 shows the XRD patterns of Sr3 xEuxAl2O6 (x¼0.03–0.42) with different Eu2 þ -doping concentrations obtained at 1200 1C. The results show that pure Sr3Al2O6 (JCPDS card no. 81-0506) phase was obtained. An impurity phase, Sr2EuAlO5 (JCPDS card no. 70-2197), formed when Eu-content in the raw material was more than 6 mol% (x¼0.18). The XRD results of the samples obtained at 1100, 1300 and 1400 1C are all similar to that of the sample obtained at 1200 1C. The PLE and PL spectra are similar for different temperature, but the relative emission intensity of the sample obtained at 1200 1C has the maximum value under the same Eu-doping concentration. Therefore, the samples obtained at 1200 1C are chosen in the following study. The PLE and PL spectra of the samples are shown in Fig. 2. The PLE spectra show that the excitation band ranges from 240 to 475 nm for all the doping values monitored at 513 nm. Akiyama et al. [6] reported that the band gap of Sr3Al2O6 was found to be 6.3 eV. This means the absorption band of the host lattice is peaking at 194 nm. Therefore, the excitation band (240–475 nm) is assigned to the 4f 7–4f 65d1 transition of Eu2 þ ions. The broad and strong excitation band indicates that the samples are adapted to being excited by 350–395 nm-emitting NUV chips. The PL spectra show a broad-band green emission of the samples. The maximum peaks are located at 513 nm for all the

Fig. 1. XRD patterns of Sr3  xEuxAl2O6 synthesized at 1200 1C under H2 and N2 atmosphere.

Fig. 2. PLE (a) and PL(b) spectra of Sr3  xEuxAl2O6 synthesized at 1200 1C under H2 and N2 atmosphere. Insert in b: emission intensities of Sr3  xEuxAl2O6 as a function of the doped Eu2 þ content.

samples, which is assigned to the transition from 4f 65d1 to 4f 7 levels of Eu2 þ ions. When x is equal to 0.09 or greater, weak red emission peaks were observed due to the existence of a small quantity of remaining Eu(III). The inset in Fig. 2b is the curve of integrated emission intensity versus doped Eu2 þ content. The integrated emission intensity increases intensively when x increases from 0.03 to 0.15, then decreases slightly with the further increase of doped Eu2 þ content. The maximum is at 0.15. Because of the existence of impurity phase at higher doped Eu2 þ content, the doping of Eu2 þ ions into Sr3Al2O6 must be saturated at 5 mol%. Bachmann et al. reported that the relaxation or energy transfer of the higher 5d levels of Eu2 þ ions to the lower 5d levels is always accompanied with a slight red-shift of the emission wavelength [11–14]. However, no shift of the emission wavelength was observed in our Sr3  xEuxAl2O6 series. Therefore, the decrease in emission intensity may mainly originate from the consumption of Eu by the formation of Sr2EuAlO5, and not from the energy transfer among the Eu2 þ ions. By varying the doped Eu2 þ content in the host lattice, the influence of the doping content on the excitation spectra was studied. The red-shift in the excitation band upon raising the Eu2 þ concentration from 1 to 7 mol% is shown in Fig. 3. Further increasing the Eu2 þ concentration does not make the red-shift in the excitation band. It is thought that the 5d electron of Eu2 þ ion has more probabilities to stay at a higher 5d level upon excitation when the Eu2 þ concentration is low, while it has more probabilities to stay at a lower 5d level when the Eu2 þ concentration is high. On the other hand, the excited electron will relax to the lowest level of the excited state and then return to the ground state f levels, regardless of which level the electron stay upon excitation. As a result, the emission peak keeps unchanged with different doped Eu2 þ concentrations.

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Fig. 3. Normalized PLE spectra of Sr3  xEuxAl2O6 synthesized at 1200 1C under H2 and N2 atmosphere.

Fig. 5. PLE and PL spectra of Sr2.91Eu0.09Al2O6 measured at room temperature: (a) lem ¼ 604 nm and lex ¼460 nm, (b) lem ¼514 nm and lex ¼ 354 nm.

Fig. 4. XRD patterns of the samples obtained at 1300 1C for 1 h under CO atmosphere.

3.2. Sr3Al2O6:Eu2 þ obtained under CO atmosphere In this series, samples are obtained at 1200 1C for 2 h, or at 1300 1C for 1 h. Fig. 4 shows the XRD patterns of the samples obtained at 1300 1C for 1 h under CO atmosphere. The main phase of the products is Sr3Al2O6 (JCPDS no. 81-0506), and there are also some weak peaks, which belong to Sr12Al14O33 (JCPDS no. 40-0025). Sr12Al14O33 is stable between 900 and 1040 1C, therefore, the final composition of the product is dependent on the cooling rate. The experiment showed that increase in the cooling rate resulted in the reduction of Sr12Al14O33. The XRD results of the samples obtained at 1200 1C, which are not given here, are similar to that of the samples obtained at 1300 1C. Excited by 460 nm-light, the samples obtained at either 1200 or 1300 1C all show a broad-band red emission. PLE and PL spectra of Sr2.91Eu0.09Al2O6 obtained at 1300 1C are shown in Fig. 5. Fig. 5a shows that the sample has broad red emission peaking at 604 nm under the excitation wavelength of 460 nm. The broad emission band is assigned to the d–f transition of Eu2 þ . No emission peaks that belong to Eu3 þ are observed. Monitored at 604 nm, there is a broad and strong excitation band between 390 and 550 nm, while several weak sharp peaks around 468 nm appear due to the 7 F0-5D2 transition of Eu3 þ , which suggests there remains a little Eu3 þ . When the phosphor is excited at 468 nm, only very weak sharp peaks of Eu3 þ emission are observed besides the intense wide Eu2 þ emission band. Monitored at 514 nm, the excitation band of Sr2.91Eu0.09Al2O6 obtained at 1300 1C is between 250 and 450 nm. The sample has two emission bands under the excitation wavelength of 354 nm,

namely 514 and 604 nm, respectively (Fig. 5b). The excitation band and the emission peak at 514 nm are in accord with that of the samples obtained under H2 and N2 atmosphere, but these emission peaks are very weak compared to the red emission under the excitation of 460 nm. 3.3. Relationship between the emission wavelengths and the crystallographic sites occupied by Eu2 þ in Sr3  xEuxAl2O6 Chakoumakos et al. [10] reported that Sr2 þ ions took six different crystallographic sites in Sr3Al2O6, three different SrO6 (Sr1, Sr2 and Sr3), SrO9 (Sr4), SrO8 (Sr5) and SrO7 (Sr6) polyhedra. Fig. 6 shows these polyhedra. The emission peak of Eu2 þ at different crystallographic sites can be estimated using the following relation given by Van Uitert [15]: E ¼ Q ½1ðV=4Þ1=V 10ðn ea rÞ=80 

ð1Þ

where E is the position in energy of the lower d-band edge for Eu2 þ in a simple three-dimensional structure, Q is the position in energy for the lower d-band edge for the free Eu2 þ ion (34,000 cm  1), V is the valence of the ‘‘active’’ cation, n is the number of anions in the immediate shell about this ion, ea is the electron affinity of the atoms that form anions, and r is the radius of the host cation replaced by the ‘‘active’’ cation. The electron affinity for oxygen in simple compounds is 1.17 eV, while a correction of times 1/0.73 should be made for the non-polarized one. The calculated emission wavelengths of Eu2 þ at different SrOn sites in Sr3Al2O6 were 609, 566, 531 and 505 nm for n equals 6, 7, 8 and 9, respectively. Furthermore, based on the data from a large number of literatures, Dorenbos [16–19] concluded that the larger the size of the coordinating polyhedron, the smaller the crystal field splitting of 5d levels and shorter emission wavelength. The influence of the polyhedron on the crystal field splitting of the 5d levels can be

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Fig. 6. Schematic diagram of six different SrOn (n¼ 6, 7, 8 and 9) polyhedra.

Fig. 7. Schematic diagram of representative energy levels and d–f transitions of Eu2 þ in SrO9 and SrO6 sites of the matrix.

expressed [19] as

ecfs ¼ bQpoly R2 av

Fig. 8. Decay curves of Eu2 þ green emission of Sr3  xEuxAl2O6: (a) x ¼ 0.15, H2 and N2 atmosphere with monitoring wavelength of 513 nm and (b) x ¼0.09, CO atmosphere with monitoring wavelength of 602 nm.

ð2Þ

Q

where bpoly is a constant that depends on the type of the coordination polyhedron and Q¼2þ for Eu2 þ . The ratio bocta ð6Þ2 þ:

bcubal ð8Þ2 þ: bcubo ð12Þ2 þ equals 1:0.89:0.42, where the numbers in the brackets stand for the coordination number. The mean Sr–O distance for n¼6, 7, 8 and 9 in Sr3Al2O6 are (2.470, 2.494, 2.478), 2.639, 2.748 and 2.794, respectively. Therefore, the green emission of the samples should be from the doped Eu2 þ ions in the Sr2 þ sites of SrO9, SrO8 and SrO7 polyhedra, and the broad-band red emission should be from the Eu2 þ ions in the Sr2 þ sites of SrO6 polyhedra. Fig. 7 is the schematic diagram of representative energy levels and d–f transitions of Eu2 þ in SrO9 and SrO6 sites of the matrix. To understand the relationship between the emission spectrum of Sr3  xEuxAl2O6 and the crystallographic sites of Sr2 þ occupied by Eu2 þ ions, the luminescent lifetimes of the samples obtained under H2 or CO atmosphere were measured. Fig. 8a is the decay curve of the green emission of Sr2.85Eu0.15Al2O6 obtained under H2 and N2 atmosphere with monitoring wavelength of 513 nm. The curve can be well fitted by three-exponential fittings. The lifetime values are 11, 107 and 791 ns, respectively. Fig. 8b is the decay curve of the red emission of Sr2.91Eu0.09Al2O6 obtained under CO atmosphere with monitoring wavelength of 602 nm. The curve can be well fitted by three-exponential fittings too. The lifetime values are 13, 71 and 401 ns, respectively. Therefore, the green and the red emissions are both originated from three different crystallographic sites for Eu2 þ . These results are consistent with the calculation above. Why Eu2 þ doped Sr3Al2O6 tends to emit green light obtained under H2 þN2 atmosphere, while it tends to emit red light under CO atmosphere, with otherwise identical conditions. This phenomenon can be explained by analyzing the crystal structure of

Sr3Al2O6. First, in the Sr3Al2O6 lattice viewed along [1 1 1], Sr1, Sr2 and Sr3 ions are all situated at chains contain only Sr ions with more space left around, while 3 quarters of Sr4, Sr5 and Sr6 ions are situated at chains containing both Sr and Al ions with less space left around. Second, Eu2 þ ion with a large radius tends to stabilize with more oxygen ions coordinate and there are more SrO9 (Sr4) and SrO8 (Sr5) sites than SrO6 in Sr3Al2O6. Therefore, in the Sr3Al2O6:Eu2 þ system, Eu2 þ occupies the SrO6 sites more easily than the SrOn (n¼7, 8 and 9) sites, but will be stable in SrOn site with more oxygen coordinated, such as SrO9 site. And the substitution rate of Eu2 þ into Sr2 þ sites under H2 þN2 atmosphere is larger than that under CO atmosphere, with otherwise identical conditions. Therefore, Sr3Al2O6:Eu2 þ obtained at 1200 1C for 2 h under H2 þN2 atmosphere emits green light, but it emits red light obtained under CO atmosphere. With the increase of reaction time under CO atmosphere, the emission of Sr3Al2O6:Eu2 þ changes from red to green. The PLE and PL spectra of the green emission are similar as what obtained under H2 þN2 atmosphere. 3.4. Fabrication of LEDs According to the above discussions, the excitation spectrum of Sr3 xEuxAl2O6 obtained under H2 and N2 atmosphere matches well with an NUV-emitting chip, while that of Sr3 xEuxAl2O6 obtained under CO atmosphere matches well with a blue-emitting chip. A pc-LED was fabricated by coating a silica gel containing Sr2.85Eu0.15Al2O6 obtained under H2 and N2 atmosphere onto a 370 nm-emitting chip. Fig. 9a shows the luminescence spectrum of the fabricated LED under excitation of 20 mA forward bias. Two emission bands are observed and combined to give intense green light with CIE color coordinates, x¼ 0.3240 and y ¼0.5334.

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4. Conclusions In summary, Sr3  xEuxAl2O6 was synthesized by a solid-state reaction under either H2 and N2 atmosphere or CO atmosphere. The relationship between the emission wavelengths and the occupation of Eu2 þ at different crystallographic sites was studied. When the reaction temperature and times were the same, Eu2 þ ions prefer to occupy Sr2 þ in SrOn (n¼ 9, 8 and 7) polyhedra under H2 and N2 atmosphere, and the phosphor emitted green light under NUV excitation, while Eu2 þ ions prefer to occupy Sr2 þ in SrO6 polyhedra under CO atmosphere, and the phosphor showed red-emission under blue light excitation. The preferential substitution of Eu2 þ into different Sr2 þ cites at different reaction periods and the substitution rates under different atmospheres were discussed. Both emissions belong to the d–f transition of Eu2 þ . Finally, green- and red-emitting pc-LEDs were fabricated, and the results indicate that Eu2 þ -doped Sr3Al2O6 synthesized in different reducing atmosphere may be a candidate as a green or red component in fabrication of white LEDs.

Acknowledgments This work was financially supported by grants from the Natural Science Foundation of China (No. 50672136) and the Natural Science Foundation of Guangdong Province (No. 9151027501000047).

References

Fig. 9. The emission spectra of the pc-LEDs under a forward bias of 20 mA: (a) NUV chip combined with a green-emitting Sr2.85Eu0.15Al2O6 and (b) blue chip combined with a red-emitting Sr2.91Eu0.09Al2O6.

The 370 nm band is attributed to the NUV chip, while the 513 nm band is ascribed to the emission of Sr2.85Eu0.15Al2O6. The remaining NUV light may be used to excite other color-emitting phosphors and give white light by a multi-color combination, which reveals that Sr3 xEuxAl2O6 obtained under H2 and N2 atmosphere is a good candidate as a green component for fabrication of white LEDs with NUV chips. Another pc-LED was also fabricated by a combination of a blue-emitted InGaN chip and Sr2.91Eu0.09Al2O6 phosphor obtained under CO atmosphere. Fig. 9b shows the luminescence spectrum of the fabricated LED under excitation of 20 mA forward bias. The peak at 460 nm belongs to the blue chip, while the peak at 604 nm is ascribed to the emission of the phosphor. The remaining blue light can be used to excite other color-emitting phosphor (such as green and yellow) and give white light.

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