Persistent luminescence property of rare earth doped BaMg2Al6Si9O30 phosphor

Persistent luminescence property of rare earth doped BaMg2Al6Si9O30 phosphor

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Journal of Luminescence ∎ (∎∎∎∎) ∎∎∎–∎∎∎

Contents lists available at ScienceDirect

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

Persistent luminescence property of rare earth doped BaMg2Al6Si9O30 phosphor Shaochun Han, Yuhua Wang n, Wei Zeng, Wenbo Chen, Gen Li Department of Materials Science School of Physical Science and Technology Lanzhou University, Lanzhou 730000, PR China

art ic l e i nf o

a b s t r a c t

Article history: Received 29 August 2013 Received in revised form 30 October 2013 Accepted 31 October 2013

Several rare earth single-doped and co-doped BaMg2Al6Si9O30 phosphors were synthesized by high temperature solid-state reaction. Their persistent luminescence property was studied by means of afterglow time and thermoluminescence curves. Single-doped phosphor with 0.1 mol% ratio Eu2 þ replacing Ba2 þ obtained 143 s’ afterglow time. When doping concentration was increased from 0.1% to 0.4%, thermoluminescence peek around 75 1C rose nearly linearly, but afterglow time was shorted to 97 s at 0.2% and 74 s at 0.4%. This indicated that trap center density increased slower than luminescence center density in BaMg2Al6Si9O30:Eu phosphor. Relative low afterglow performance could be ascribed to a relative low ratio of replaceable cation (Ba2 þ in our case), and this deduction could also suit to other host matrices. & 2013 Elsevier B.V. All rights reserved.

Keywords: Afterglow Phosphor Aluminosilicate Rare earth

1. Introduction Persistent luminescence or known as long lasting phosphorescence (LLP) is a phenomenon that phosphors continue glowing for seconds or hundreds of hours even excitation sources are taken away. It is necessary to point out that persistent luminescence is a distinct physical phenomenon from phosphorescence which is associated with spin forbidden transition. This make the term “lasting phosphorescence” not that appropriate in a sense [1]. Persistent luminescence is also distinguished from the concept of fluorescence decay lifetime witch has orders of magnitudes from nanosecond to millisecond. Phenomenologically, persistent luminescence is a continuing fluorescence without external energy supply. Essentially, persistent luminescence is a thermoluminescence (TL) or known as thermal stimulated luminescence (TSL) phenomenon at a specific temperature, the room temperature. Before be stimulated by thermal perturbation, persistent luminescence phosphor need to be irradiation by external source such as an ultraviolet (UV) lamp or a radioactive source to get energy for TL to be stored. That is to say, persistent luminescence consists of stored energy and delayed luminescence. To make it more concrete, its energy storage process involves other atoms nearby the luminescent center atom. In phosphorescence, by contrast, its delay of luminescence only involves the luminescent center atom itself.

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Corresponding author. Tel.: þ 86 931 8912079. E-mail address: [email protected] (Y. Wang).

Nowadays, persistent luminescence phosphors are added to plastics, rubber, paint and other matrices serving as day and night self-luminous light source working on different occasions, such as security signs, emergency route signs, traffic signage and night displays. Broader applications of them also have been exploited, such as optical storage, radiation detection, structural damage sense and in vivo bio-imaging (infrared or near-infrared emission required) [2–6]. In the beginning, zinc sulfide was the only practical host matrix for persistent luminescence with Cu þ acting as luminescent center and Co2 þ acting as enhancer. However, this kind of persistent luminescence phosphor has several disadvantages which limited its application. It only has a dim afterglow continuing about 1 h. This leads to large amount of ZnS:Cu þ , Co2 þ or even radioactive isotopes such as 3H and 147Pm are required to meet brightness demands and persistence demands. Zinc sulfide has poor chemical stability. Photo irradiation can devitalize its luminous capacity, water vapor in the air can air-slake it, and oxygen in the air can oxidize it to zinc sulfate [6]. Till Murayama et al. reported SrAl2O4:Eu2 þ , Dy3 þ , B3 þ a persistent luminescence phosphor which can provide bright green afterglow lasting more than 10 h with no radioactive isotopes added, practicability of persistent luminescence phosphor has been advanced historically [7,8]. At the same time of SrAl2O4:Eu2 þ , Dy3 þ , B3 þ phosphor's discovery, corresponding persistent luminescence mechanism also was proposed by Matsuzawa et al. [7]. Later, several different mechanisms were proposed as well, such as Aitasalo et al.'s model [9], Beauger's model [10], Dorenbos’ model [11], Clabau et al.'s model [6] and so on. Beside various theories of persistent luminescence, various chemical compounds with afterglow lasting longer than Zinc sulfide ( 41 h) were also reported. When

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classified by chemical composition, we can see that the most extraordinary types of them are Eu2 þ -rare earth co-doped alkaline earth aluminates. After them, Eu2 þ -rare earth co-doped alkaline earth silicates also perform favorable afterglow quality. And then, Eu2 þ -rare earth co-doped alkaline earth aluminosilicates are preferable persistent luminescence phosphors either [6]. So it is reasonable to pursue persistent luminescence via conventional way by co-doping other elements and lowering concentrations of dopants [2] in this aluminosilicates, the BaMg2Al6Si9O30 (BMAS). BMAS is a color tunable full color host matrix for UV light excited phosphor converted light emitting diode (LED) and fluorescent lamps (FLs). When triple-doped with Eu2 þ , Tb3 þ and Mn2 þ , BMAS can emit light from pink to blue via photoluminescence. And a white light emitting with chromaticity coordinates of (0.31, 0.30), color render index of Ra ¼90, and correlated color temperature (CCT) of 5374 K can also be achieved by tuning relative concentrations of Tb3 þ and Mn2 þ [1].

2. Experimental All BMAS samples were synthesized by the conventional high temperature solid-state method from reactants BaCO3 (A. R.), Al2O3 (A. R.), H2SiO3 (A. R.), Mg(NO3)2  6 H2O (A. R.) and rare earth oxides (99.99%). The reactants were wet grinded by adding several mL of dehydrated alcohol till obtaining homogeneously mixed fine powder in an agate mortar. Then, the obtained powder was placed into alumina crucibles and sintered at 1300 1C for 8 h under reducing atmosphere (10 mol% H2 and 90 mol% N2) in a tube furnace. At the end, resultants were air cooled to room temperature and grinded into powder again for subsequent using. Powder X-ray diffraction was tested by using a RINT2000 X-ray diffractometer (Rigaku Corporation) with Ni-filtered Cu Kα radiation at a scanning step of 0.021 in the 2θ range from 101 to 801. Afterglow decay curves were recorded using a PR-305 Phosphorophotometer (Hangzhou Zhejiang University Sensing Instruments Co., Ltd) after the samples were irradiated by an artificial sunlight light source with illuminance of 1100 lx for 15 min. Thermoluminescence (TL) curves were measured by a FJ427A Thermoluminescent Dosimeter (CNNC Beijing Nuclear Instrument Factory) with a heating rate of 1 1C/s. All the data were measured at room temperature except the TL curves’ which demands temperature rising from 20 1C to 400 1C.

3. Results and discussion From Fig. 1 we can see that as long as doping density increases, diffraction maximums show an obvious tendency to shift right [12]. When doping density reaches 1.0 mol% ratio, an unexpected maximum shows up near 2-Theta¼ 231. For this reason, all samples’ doping density in our experiment does not exceed 1.0 mol% ratio. As shown in Fig. 2, afterglow decay times of BMAS powders with Eu2 þ single-doped from 0.1 mol% to 1.0 mol% ratio is reducing when concentration of Eu2 þ increases. Decay times (cut off at Luminance¼0.00032 cd/m2) are 143 s, 97 s, 74 s and 18 s for samples which have Eu2þ concentrations are 0.1 mol%, 0.1 mol% and 0.1 mol% ratio correspondingly. Joanna Trojan-Piegza et al. reported in phosphor Lu2O3:Tb3 þ , Ca2þ afterglow decay times can be shorted when concentration of Tb2þ increases [13] just like our situation in the BMAS. Reasons of this phenomenon could be ascribed to energy transfer in the persistent luminescence process [14,15]. Dexter et al. estimated maximum effective range of energy transfer in crystal lattice by using electric multipole moment and magnetic multipole moment models. They proposed that this maximum

Fig. 1. XRD patterns of BMAS powders with doping density varying from 0.0 mol% to 1.0 mol% ratio.

Fig. 2. Afterglow decay curves of BMAS powders with Eu2 þ single-doped from 0.1 mol% to 1.0 mol% ratio. Our meter uses a xenon lamp as artificial sunlight light source. This lamp has an afterglow about 3 s, which would cause confusion in the early period of decay patterns. After 10 s, decay patterns changed regularly.

effective range is about 30 sites [15]. So, if we imagine a cube which takes 30 sites as its edge length, then we can calculate out simply that this maximum effective range is equivalent to the situation that only one luminescent center atom should be contained in one cube. In other words, when luminescent center atom's mole ratio is less than 1/27,000 among all kind of atoms, energy transfer between luminescent center atoms will not take place effectively. For our BMAS samples which include 48 atom/mol, when Eu2 þ doping density is at 0.1 mol% ratio its luminescent center atom's mole ratio is 1/48,000 which means energy transfer is inactive. And when Eu2 þ doping density is at 1.0 mol% ratio its luminescent center atom's mole ratio is 1/4800 which means energy transfer is active. Associate this difference with the tendency shown in Fig. 2, we can say that doping density related degradation of persistent luminescence in Eu2 þ doped BMAS is at least partially caused by energy transfer between luminescent center atoms. It is worth noting that the reason we use a cube model to expand a one-dimensional range (30 sites) to a three-dimensional range (1/48,000 atom's mole ratio) rather than use a ball mode is that cubes can fill crystal lattice without leaving blank but balls cannot. We know that persistent luminescence is a specific thermoluminescence at room temperature, so it is conventional to investigate persistent luminescence properties by analyzing TL

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Fig. 3. showed TL curves of BMAS powders. In (A), Eu2 þ was single-doped at 1.0 mol% ratio and Eu2 þ , Dy2 þ were co-doped at 1.0 mol% ratio. In (B), Eu2 þ was single-doped from 0.1 mol% to 0.4 mol% ratio. Fig. 3(C) showed TL curves of BMAS: 0.1 mol%Eu, 0.1 mol%Dy, 15 mol%H3BO3 and BMAS: 0.1 mol%Eu, 0.1 mol%Dy, 30 mol%H3BO3. Fig. 3(D) showed TL curves of double-doped and triple-doped BMAS phosphors. In the legend of Fig. 3(D), data lied before @ sign are persistent luminescence duration times of corresponding samples in unit of seconds. Every dopant element has a concentration of 0.1 mol% ratio. TL intensities do not match afterglow times point for point. But when arranged in ascending order of TL intensity, afterglow time order is 31 s, 41 s, 41 s, 46 s, 41 s, 39s, 79 s, 53 s. In consideration of measuring error of TL mentioned in this paper, we can find a positive correlation between TL intensities and afterglow times. Generally, all TL curves have peak near 150 1C.

curves. It can be seen that no typical peak appears on any of the curves in Fig. 3(A). So we use Urbach's empirical formula ET ¼Tm/ 500 [16] which do not need shape of TL peak involved to estimate trap depth. In this formula ET is estimated trap depth and Tm is corresponding temperature of a maximum point of the curve. We obtain trap depths for BMAS:Eu and BMAS:Eu, Dy are 0.70–0.85 eV and 0.85–1.05 eV, respectively. For peak position can shift with variation of heating rate [17,18], we fixed heating rate at 1 1C/s for all decay curves we tested in order to make comparisons more reliable. By comparing TL curves of two samples we can say that co-doping of Dy3 þ can increase ratios of deeper trap but cannot increase total number of all traps in BMAS phosphors. Because abscissa of the maximum point shifted to the high temperature end with no area under the curve changed. In Fig. 3(B) ordinate of the maximum point increases along with growth of doping density of Eu2 þ nearly linearly leaving shapes and abscissa of the maximum point almost unchanged. For the purpose of oneness for conclusion, Urbach's empirical formula is still used to estimate trap depth. And result is 0.68 eV for all of curves. From Fig. 3 (C) it is safe to say that TL curves shapes are obviously changed by adding fluxing agent H3BO3. Though content of H3BO3 were doubled from 15 mol% ratio to 30 mol% ratio, two curves just sharing almost

same shape and same height. This phenomenon can be ascribed to volatility of H3BO3. When amount of H3BO3 exceed some certain level (such as 15 mol% ratio in our case) its volatility leads to superfluous H3BO3 leaving the reacting sample and then make no difference from not exceed ones in the end. By using Urbach's empirical formula again we get trap depths in Fig. 3(C) are 0.67 eV, 0.85 eV and 1.05 eV. The first one is very close to Eu2 þ single-doped samples. The last two are influenced by adding H3BO3. From Fig. 3(D) we can see that Eu, Tm double-doped simple and Eu, Sr double-doped simple have arresting TL peaks around 150 1C. Other samples have wide flat TL peaks with ranges from 75 1C to 275 1C. None of them has TL peaks around 75 1C. which are deemed to be most beneficial to persistent luminescence [19,20]. This result is consistent with relatively short persistent luminescence duration times of these samples. 4. Conclusion Minutes of persistent luminescence can be observed when Eu doping density in BMAS host matrix is at level of 0.1 mol% ratio. For our BMAS samples which include 48 atoms per mol, when Eu2 þ doping density is at 0.1 mol% ratio its luminescent center

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atom's mole ratio is 1/48,000 and when Eu2 þ doping density is at 1.0 mol% ratio its luminescent center atom's mole ratio is 1/4800. By evoking Dexter et al.'s effective range of energy transfer in crystal lattice (30 sites in one-dimensional and 1/48,000 atom's mole ratio in three-dimensional) we can say that doping density related degradation of persistent luminescence in Eu2 þ doped BMAS is at least partially caused by energy transfer between luminescent center atoms. And other reason restricting BMAS phosphor's persistent luminescence performance traced to the crystal structure of BMAS. Both Ba and Mg atoms, which are located at 12 coordination and 8 coordination in frame structures constituted by AlO4 tetrahedrons and SiO4 tetrahedrons, are candidates for doping atoms to replace in BMAS host [1]. This kind of frame structures constituted by two kinds of polyhedrons may have disadvantageous effects to persistent luminescence, as we can see that SrAl2O4 which has only one kind of AlO4 tetrahedrons emerges much more superior persistent luminescence performance than Sr4Al14O25 which have both AlO4 tetrahedrons and AlO6 octahedrons [21]. By comparing crystal radius of Ba2 þ ions and Mg2 þ ions with rare earth ions which we doped [22], we can see that all these rare earth ions have much bigger radius than Mg2 þ ions have, which means it is hard for those rare earth ions to replace Mg2 þ ions in this crystal host. On the contrary, Ba2 þ ions have similar crystal radius to these rare earth ions. So we can infer that rare earth ions would mostly occupy Ba2 þ ions’ sites. In this situation, we can calculate available atom sites ratio in BMAS host for rare earth atom to replace which is only 1/48. Even when Mg2 þ ions’ sites are counted, this ratio is 1/16, which is still much smaller than remarkable persistent luminescence phosphor SrAl2O4's 1/7. At this point, we deem that low available atom sites ratio is adverse to persistent luminescence.

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Acknowledgements This work was supported by the National Science Foundation for Distinguished Young Scholars (No. 50925206).

Please cite this article as: S. Han, et al., J. Lumin. (2013), http://dx.doi.org/10.1016/j.jlumin.2013.10.068i