Analysis of (Ba,Ca,Sr)3MgSi2O8:Eu2+, Mn2+ phosphors for application in solid state lighting

Journal of Luminescence 148 (2014) 1–5

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Analysis of (Ba,Ca,Sr)3MgSi2O8:Eu2 þ , Mn2 þ phosphors for application in solid state lighting J.K. Han a,1, A. Piqutte b, M.E. Hannah b, G.A. Hirata c, J.B. Talbot a,d, K.C. Mishra b, J. McKittrick a,e,n a

University of California, San Diego, Materials Science and Engineering Program, La Jolla, CA 92093, USA OSRAM SYLVANIA Central Research, 71 Cherry Hill Drive Beverly, MA 01915, USA c Centro de Nanociencias y Nanotecnolgía, Universidad Nacional Autónoma de México, Km. 107 Carretera Tijuana-Ensenada Apdo, Ensenada MX CP 22860, Mexico d University of California, San Diego, Department of Nanoengineering, La Jolla, CA 92093, USA e University of California, San Diego, Department of Mechanical and Aerospace Engineering, La Jolla, CA 92093, USA b

art ic l e i nf o

a b s t r a c t

Article history: Received 28 August 2013 Received in revised form 31 October 2013 Accepted 2 November 2013 Available online 28 November 2013

The luminescence properties of Eu2 þ and Mn2 þ co-activated (Ba,Ca,Sr)3MgSi2O8 phosphors prepared by combustion synthesis were studied. Eu2 þ -activated (Ba,Ca,Sr)3MgSi2O8 has a broad blue emission band centered at 450–485 nm and Eu2 þ –Mn2 þ -activated (Ba,Ca,Sr)3MgSi2O8 exhibits a red emission around 620–703 nm, depending on the relative concentrations of Ba, Ca and Sr. The particle size of Eu2 þ and Mn2 þ co-activated (Ba,Ca)3MgSi2O8 ranges from 300 nm to 1 μm depending on the metal ion and are agglomerated due to post-synthesis, high temperature annealing. The green emission of Ba3MgSi2O8 originates from secondary phases (Ba2SiO4 and BaMgSiO4) confirmed by emission spectra and X-ray diffraction patterns. The secondary phases of Ba3MgSi2O8 are removed by the addition of Sr. The quantum efficiencies range from 45% to 70% under 400 nm excitation and the lifetime of red emission of Ba3MgSi2O8 decreases significantly with increasing temperature, which is 54% at 400 K of that at 80 K compared to that of blue emission (90% at 400 K of that at 80 K). & 2013 Elsevier B.V. All rights reserved.

Keywords: Silicate phosphors Energy transfer Particle size Thermal stability White-emitting LEDs

1. Introduction Solid-state lighting (SSL) has the potential to substantially improve energy efficiency in lighting [1–4]. The function of a phosphor in light emitting diodes (LEDs) for SSL is to absorb the primary ultraviolet (UV) or blue-emission from the LED chips and convert it into visible light. For this application, silicate-based phosphors activated with Eu2 þ are very suitable, as they have a rigid, stable structure and find application in fluorescent lamps, cathode ray tubes, plasma displays and as scintillators [5–7]. These phosphors have intense, broad emission bands characteristic of the parity-allowed 4f–5d transition in Eu2 þ activators and emit over a range of wavelengths, depending on the host lattice [8,9]. Phosphors with the composition M3MgSi2O8 (M: Ba,Sr,Ca) have been developed as host materials for Eu2 þ -activated blue-emitting phosphors. Ba3MgSi2O8:Eu2 þ has been used for color correction in lamps because of the intense blue emission band, and in displays

n Corresponding author at: University of California, San Diego, Department of Mechanical and Aerospace Engineering, La Jolla, CA 92093, USA. Tel.: þ 1 858 534 5425; fax: 1 858 534 5698. E-mail address: [email protected] (J. McKittrick). 1 Now at Brookhaven National Laboratory, Upton, NY 11973, USA.

0022-2313/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jlumin.2013.11.022

[10]. Blasse and co-workers [9,11] studied the photoluminescence (PL) properties from the Eu2 þ -activated M3MgSi2O8 ternary system (M: Ba,Sr,Ca), which showed a systematic decrease of peak emission wavelength depending on the M cation size. Barry [12] studied mixtures of Eu2 þ -activated Ba3MgSi2O8, Sr3MgSi2O8 and Ca3MgSi2O8 and found Ba3MgSi2O8 had the highest PL emission intensity and shortest emission wavelength (437 nm). Ca3MgSi2O8 had the lowest intensity and longest peak emission wavelength (475 nm) with Sr3MgSi2O8 falling between the two (458 nm). Sr3MgSi2O8 formed ideal solid solutions with both Ba3MgSi2O8 and Ca3MgSi2O8, but Ba3MgSi2O8 and Ca3MgSi2O8 did not, leading to an intermediate compound, BaCa2Si2O8. Yonesaki [13] further elucidated the structural aspects by finding a solubility limit of x  0.33 for either (Sr1  xBax)3MgSiO8 or (Ca1  xBax)3MgSiO8, similar to the limit in (Ca1  xBax)3MgSiO8 found by Barry [12] The Ba-rich compositions were determined to have the trigonal glaserite-type structure while the Ba-poor compositions have the monoclinic merwinite-type structure. Co-activation with Mn2 þ results in phosphors with blue- and red-emission bands that have application in agricultural lamps [12,14,15]. Recently, the luminescent properties and energy transfer of Eu2 þ –Mn2 þ under near-UV light excitation have been investigated for single composition white-emission for SSL devices

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J.K. Han et al. / Journal of Luminescence 148 (2014) 1–5

diluted nitric acid, a stoichiometric amount of carbohydrazide was added and mixed thoroughly with a magnetic stirrer. Note that high purity Ba(NO3)2 (above 99.99%) is required for dissolution to be complete in water or nitric acid solutions. The mixture was then placed in a muffle furnace at 500 1C in which the combustion reaction occurred by M(NO3)2 þ2SiO2 þ3.5CH6N4OM3Si2O8 þ3.5CO2 þ10.5H2O þ10N2

Fig. 1. (a) X-ray diffraction patterns of (a) (Ba0.97Eu0.03)3(Mg0.95Mn0.05)Si2O8 and (Ba0.735Sr0.235Eu0.03)3(Mg0.95Mn0.05)Si2O8 (b) (Ca0.97Eu0.03)3(Mg0.95Mn0.05)Si2O8.

[16–20], but there are contradictory reports for the green-emission band [12,16,17,21]. Kim et al. [16] reported that the green band is attributed to Eu2 þ substituting for Ba(II, III) sites. Wang et al. [21] reported an increase in the green emission with additions of Al. In contrast, the green band was not observed in other works [12,15,18]. Moreover, excellent thermal stability, up to 150 1C, was found for Ba3MgSi2O8:Eu2 þ ,Mn2 þ [18]. In this work, the luminescence properties of Eu2 þ and Mn2 þ co-activated (Ba,Ca,Sr)3MgSi2O8 phosphors prepared by combustion synthesis are presented. Combustion synthesis is advantageous for preparing microcrystalline or sub-microcrystalline materials at low-cost, high production yield and produce highpurity single or multiphase complex oxides [22–24]. The effects of the different crystalline structures and the influence of additions of Sr on the PL spectra and thermal stability properties were investigated.

2. Experimental Combustion synthesis was used to fabricate powders of (Ba0.97 Eu0.03)3(Mg0.95Mn0.05)Si2O8 (Ba-silicate), (Ba0.735Sr0.235Eu0.03)3(Mg0.95 Mn0.05)Si2O8 (Ba,Sr-silicate) and (Ca0.97Eu0.03)3(Mg1 0.95Mn0.05)Si2O8 (Ca-silicate). The reagents were Sr(NO3)2 (99.99%, Alfa Aesar), Ba (NO3)2 (99.999%, Alfa Aesar), Ca(NO3)2  4H2O (99.9%, Alfa Aesar), Mn (NO3)2 (99.99%, Alfa Aesar), Eu2O3 (99.999%, Alfa Aesar) and fumed SiO2 (Sigma-Aldrich) with carbohydrazide (CH6N4O, 97%, Alfa Aesar) as the fuel. As soon as the reagents were completely dissolved in

where M¼ Ca, Ba, Sr, Mg, Mn or Eu. Twenty-four moles of gas are produced per mole of solid, resulting in a porous, fluffy product. After combustion, the resultant powders were lightly crushed with a mortar and pestle, placed in a 50 ml alumina crucible, and heat treated for 6 h at 1200 1C with forming gas (95% N2, 5% H2) to reduce the europium from 3þ to 2þ and further crystallize the powders. The crystalline phases of the annealed powders were identified by X-ray diffraction (XRD) and the amount of each phase was determined by an XRD analysis program (JADE, Materials Data Inc.). Jade is a program that uses least squares regression to fit a curve to each peak in the diffraction pattern to determine the peak areas, thereby computing the amount of each phase present. The particle morphology and size were determined by using a field emission scanning electron microscope (FESEM, XL30, Philips). PL properties were measured using a Jobin-Yvon Triax 180 monochromator and SpectrumOne charge-coupled device detection system, which was shared with a PL system that uses a 450 W Xe lamp as the excitation source. The low temperature measurements were performed with a Jobin-Yvon Fluorolog-3 system with a 450 W xenon excitation lamp. Both the emission and the excitation monochromators were standard double grating 1680B monochromators (Jobin-Yvon Spex). To investigate the temperature dependence of the photoluminescence spectra, the spectrometer was configured to allow a cryostat to be lowered into the sample chamber. The cryostat had a refrigerated cold head attached to a cold finger sample mount. The signal was detected with a cooled R928P PMT (Hamamatsu). Quantum efficiency (QE) measurements were made using a 400 nm laser diode as an excitation source. Powdered phosphor samples were dispersed in a silicone gel and cured. The samples were then placed in a 10.2 cm sphere and three measurements (of which the average is reported) were taken following the method outlined in [25].

3. Results and discussion The XRD patterns of (Ba0.97Eu0.03)3(Mg0.95Mn0.05)Si2O8, (Ba0.735 Sr0.235Eu0.03)3(Mg0.95Mn0.05) Si2O8 powders are shown in Fig. 1(a). For Ba-silicate, the major peaks are shifted  0.11 to larger 2θ values than shown on the JCPDS card 10-0074 (Ba3MgSi2O8). This small peak shift is significant even though the activators Eu and Mn are substituted for Ba and Mg, respectively, and do not occupy interstitial sites. The shift can be attributed to the substitution of the smaller ion Eu2 þ (0.131 nm) for Ba2 þ (0.149 nm) and Mn2 þ (0.081 nm) for Mg2 þ (0.086 nm) in the host lattice. Additionally there are substantial amounts of secondary phases: Ba2SiO4 ( 20 wt%) and BaMgSiO4 (  15 wt%). However, by adding Sr, a single phase is obtained, as shown in Fig. 1(a). The peak shift to larger angles is due to the substitution of Sr with a smaller ionic radius (0.132 nm) for Ba in the primary structure. The secondary phases are present in amounts o5 wt%. The XRD patterns of (Ca0.97Eu0.03)3(Mg0.95Mn0.05)Si2O8 are shown in Fig. 1(b). The peaks match with JCPDS card 25-0161, however there is a small amount (o 5 wt%) of secondary phase (Ca2MgSi2O7). The  24% substitution of Sr for Ba in the structure stabilizes the primary phase. The XRD peaks for the Ca-silicate are very broad, indicating some degree of structural disordering. The smaller ionic

J.K. Han et al. / Journal of Luminescence 148 (2014) 1–5

radius of Sr increases the crystal field strength and can thus improve the structural arrangement. Fig. 2 shows the PL excitation and emission (λex ¼380) spectra of Ba-silicate and Ba,Sr-silicate. The excitation peaks of both phosphors are near 330 nm; the 446 nm emission bands are attributed to the 4f– 5d transitions of Eu2 þ . The existence of secondary phases in the XRD pattern can be confirmed by the PL emission spectra, because Ba2SiO4: Eu2 þ and BaMgSiO4:Eu2þ show bright green bands centered at 510 nm and 502 nm, respectively, under near UV excitation [26,27] in agreement with Umetsu et al. [18]. It appears that the green emission band in Ba-silicate is from the secondary phases, and not from the primary structure, since it is not present in the Ba, Sr-silicate emission pattern. Thus, the addition of Sr in Ba3MgSi2O8 helps to form the single phase. The 621 nm emission band originates from the 4T–6A transition of Mn2þ ion located at Mg2 þ sites. With the addition of Sr, both the blue and red emission bands are red-shifted, which is attributed to the crystal field effect. The crystal field strength in Basilicate increases with decreasing bond length through the replacement of smaller Sr cations; Dq p1/R5 where Dq is the crystal field and R is the bond length between a center ion and ligand ions [9]. SEM micrographs of Ba-silicate and Ca-silicate powders are shown in Fig. 3(a) and (b), respectively. The particle size of Ba-silicate is 1 μm and that of Ca-silicate powders is smaller (from 300 nm to 1 μm). However, both particles are severely agglomerated and thus form aggregates, mainly due to high annealing temperature (1200 1C) for a long annealing time (6 h).

Fig. 2. Photoluminescence excitation and emission spectra of (a) (Ba0.97Eu0.03)3 (Mg0.95Mn0.05)Si2O8 and (b) (Ba0.735Sr0.235Eu0.03)3(Mg0.95Mn0.05)Si2O8.

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Fig. 4(a) shows the PL emission spectra of (Ba1 xEux)3 (Mg1 yMny)Si2O8 (x¼ 0 or 0.03) as a function of Mn concentration. For x¼ 0 and y¼0.05 there is negligible emission in the visible region. For x¼0.03, and as y increases, the intensity of blue-emission band decreases significantly with a drastic increase of the red-emission band. This is mainly due to the energy transfer between Eu2 þ and Mn2þ that arises from the large spectral overlap between the

Fig. 4. (a) Photoluminescence emission spectra of (Ba1  xEux)3(Mg1  yMny)Si2O8 for x, y¼ 0, 0.05; 0.03, 0; 0.03, 0.01 and 0.03, 0.05 and (b) photoluminescence excitation and emission spectra of (Ca0.97Eu0.03)3(Mg1  yMny)Si2O8 for y¼0, 0.01 and 0.05.

Fig. 3. Scanning electron microscopy images of (a) (Ba0.97Eu0.03)3(Mg0.95Mn0.05)Si2O8 and (b) (Ca0.97Eu0.03)3(Mg1  0.95Mn0.05)Si2O8.

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Fig. 5. Lifetime of (a) blue and (b) red emission of (Ba0.97Eu0.03)3(Mg0.95Mn0.05)Si2O8 as a function of temperature under 380 nm excitation.

excitation spectrum of Mn2þ and the emission spectrum of Eu2þ near 400–450 nm, as shown in Figs. 2 and 4(a). The quantum efficiency for x¼0.03 and y¼ 0 is 70% and for x¼ 0.03 and y¼0.05 is 45% under 380 nm excitation. In the case of the green emission, there is no clear dependence on activator concentration. This suggests that the green emission is only dependent on the amount of secondary phases: Ba2SiO4:Eu2þ and BaMgSiO4:Eu2 þ . Fig. 4(b) shows the PL excitation and emission spectra of Ca-silicate for x ¼0.03, y¼ 0. 0.01 and 0.05. The excitation maximum occurs at  300 nm,  25 nm lower than for Ba-silicate. As with Ba-silicate, the 485 nm emission band is attributed to 4f–5d transitions of Eu2 þ and 703 nm emission band originates from the 4 T–6A transition of Mn2 þ . The quantum efficiency for x¼0.03 and y¼0.05 is 43% under 380 nm excitation, similar to Ba-silicate. Both emission bands are red-shifted compared to Ba-silicate because of the larger crystal field effect from the smaller Ca2 þ . At x ¼0.03, and y¼0.05, the intensity of red-emission band compared to the blue-emission band of the Ca-silicate is smaller than that of Basilicate due to the smaller spectral overlap. These results demonstrate that although red-emission can be generated in these compositions, the quantum efficiency is sacrificed. The temperature dependent lifetimes of Ba-silicate for blue and red emission at 380 nm excitation are shown in Fig. 5(a) and (b), respectively. In the case of blue emission, the lifetime varies little with temperature. This corroborates the results of Umetsu et al. [18], who found stable blue emission up to 150 1C. The blue emission lifetime at 400 K is 90% of that at 80 K. However, the lifetime for red emission decreases drastically with increasing temperature as shown in Fig. 5(b). The lifetime for red emission at 400 K is 54% of that at 80 K. This is in contrast to Umetsu et al. [18] and Geng et al. [28] who found the red emission intensity was stable up to 150 1C in Ba3MgSi2O8:Eu2 þ ,Mn2þ and Na2SrMg(PO4)2:Eu2þ , Mn2þ , respectively. The decrease in red-emission can be explained by the lifetime scales of the blue and red emission. There are two paths for the electrons to take to the ground state, a radiative path and a non-radiative path. The radiative path is always longer (4nano-seconds) whereas the nonradiative lifetimes are all much shorter (onano-seconds) [7]. Electrons with longer radiative lifetime are more likely to return to the ground state through a non-radiative path than those with a shorter radiative lifetime. As temperature increases, since lifetime for red emission ( ms) is much longer than blue emission ( ns), the redemission decreases more significantly with increasing temperature.

4. Conclusions (Ba,Ca,Sr)3MgSi2O8:Eu2 þ ,Mn2 þ phosphors were prepared by a combustion synthesis method. The emission spectra consist of

broad blue-emission band centered between 450 nm and 485 nm and red-emission band centered between 620 nm and 703 nm with quantum efficiencies between 45% and 70%, depending on the relative concentrations of Ba, Ca and Sr. The emission peak is red-shifted with increasing size of the alkaline earth ions because of the crystal field effect. The particle size of the phosphor with Ba is larger than that with Ca and the size is in the range from 300 nm to 1 μm. The Ba3MgSi2O8:Eu2 þ ,Mn2 þ has two secondary phases, which show a green emitting band centered at 510 nm. The secondary phases were eliminated by additions of Sr. The lifetime of the red-emission decreases significantly with increasing temperature compared to that of the blue-emission, which is 54% at 400 K of that at 80 K. This phenomenon can be explained by the longer radiative lifetime of the Mn2 þ emission. These results suggest that these phosphors may be useful for agricultural lamp applications, but not for solid state lighting due to the low quantum efficiency and thermal stability.

Acknowledgments This work is supported by the U.S. Department of Energy of Grant DE-EE0002003.

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