A novel orange-yellow emitting Sr3Al2O5Cl2:Eu2+, Tm3+ phosphor with bright and long persistent luminescence (LPL) has been newly developed. The incorporation of Tm3+ into the Sr3Al2O5Cl2:Eu2+ as an auxiliary activator dominates its long persistent luminescence and thermoluminescence characteristics to a large extent. The emissions in Sr3Al2O5Cl2:Eu2+, Tm3+ for both fluorescence and LPL are due to the 5d → 4f transitions of Eu2+. The orange-yellow long persistent luminescence with the chromaticity coordination of (0.53, 0.46) can persist for nearly 220 min at recognizable intensity level (≥ 0.32 mcd/m2). This investigation provides a new and efficient long persistent phosphor which enriches the color of the existing LPL.
© 2010 OSA
Long persistent phosphors have attracted considerable attention for various displays and signing applications as friendly environmental and energy economized materials [1–5]. Now the long persistent phosphors and theirs relevant applications are challenged by the generation of multicolor long persistent luminescence (LPL). In principle, we can get any color-emitting LPL by mixing the three primary color-emitting long persistent phosphors. Until now, long persistent phosphors for two of the tricolor, blue (CaAl2O4:Eu2+, Nd3+) and green (SrAl2O4:Eu2+, Dy3+), have been commercially available. But long persistent phosphors for the red color, the third of tricolor, are still in search. On the other hand, this above method is hard to work in practice because we can hardly guarantee the very consistent LPL decay process for different components to ensure the uniformity of LPL color during the fade of the LPL. In addition, most of the tricolor long persistent phosphors currently available cannot be efficiently excited by the same excitation source . So the exploration of any color-emitting long persistent phosphor is promising and thus has attracted intensive research interests in the past decades.
Nowadays, a great amount of novel long persistent phosphors based on different hosts have been reported in the literature, here we will give some typical examples, e.g. Ca2Al2SiO7:Ce3+[417 nm, >1 h] , CaMgSi2O6:Eu2+,Dy3+[447 nm, >4 h ] , Sr(Ca)2MgSi2O7:Eu2+, Dy3+[470 (516) nm, 20 h] , Sr4Al14O15: Eu2+, Dy3+[486 nm, 15 h] , Lu2O3:Tb3+, Ca2+ [green,15 h] , Y2O2S:Ti,Mg [565 nm, 5 h] , Sr3SiO5:Eu2+, Dy3+[570 nm, 6 h] , Y2O2S:Sm3+[606 nm, 1.5 h] , CaSiO3:Sm3+[pink, 5 h] , Sr2Si5N8: Eu2+,Tm3+ [612 nm, 10 min] , Y2O2S:Eu3+, Mg2+,Ti3+, [orange to red, 1 h]  and Sr2SnO4:Sm3+[reddish-orange, 1 h] . It can be noted that the performances of blue and green emitting long persistent phosphors have meet the requirement for practical applications, however, long wavelength emitting (> 600 nm) long persistent phosphor, whose persistent time last longer than 2 h, is still in great scarcity. Therefore, there is a strong desire for the development of long persistent phosphors with long wavelength emissions in recent years.
The Eu2+-doped Sr3Al2O5Cl2 produces a broadband emission peaked at 620 nm, which is a potential orange-yellow phosphor for white light emitting diodes . This long wavelength emitting phosphor attracts our intense interest. Until now, it has been recognized that Eu2+ are good activators for long persistent phosphors because its 5d electron state is usually close to the conduction band of the host, which makes trapping of electron become possible [4,18,19]. In addition, by our observation, Eu2+ activated compounds containing alkaline-earth and aluminum ions as cations usually show LPL, for example, MAl2O4:Eu2+, Dy3+(M = Ca, Sr, Ba) [1,20,21], SrAl4O7:Eu2+, Dy3+ , Sr4Al14O25:Eu2+,Dy3+ , Sr3Al2O6:Eu2+, Dy3+ . Another interesting feature is that Eu2+ activated compounds containing sulfur ions as anions also demonstrate LPL phenomenon, e.g. Y2O2S:Mg2+,Ti3+ ,CaS:Eu2+,Tm3+ . The present investigated compound Sr3Al2O5Cl2 contains strontium and aluminum ions as cations, and oxygen as well as chlorine ions as anions. Considering that chlorine and sulfur are of similar chemical properties owing to they are adjacent elements in the periodic table, hence, we can predict that Sr3Al2O5Cl2 may have the potential to serve as a new host material for Eu2+ containing long persistent phosphors. Further, the codoping of trivalent rare earth ions is generally capable of greatly improving the LPL of Eu2+ doped compounds. Usually with proper codopants, the persistent time can be increased by a factor of ten, for example, Dy3+ in SrAl2O4:Eu2+, Dy3+ . Tm3+ as codopants in sulfides and nitrides based long persistent phosphors have been reported in literatures [14,18]. An interesting point is that in these sulfides and nitrides Eu2+ emissions are in the long wavelength range. And it seems that for long wavelength emitting long persistent phosphors containing Eu2+, the best codopants is Tm3+ in order to produce excellent LPL properties. Based on the above clues, there could be a great possibility to generate efficient LPL from Eu2+, Tm3+-codoped Sr3Al2O5Cl2. This motivated us to prepare Sr3Al2O5Cl2:Eu2+, Tm3+. As expected, strong orange-yellow LPL is observed in it. Next, we mainly report on its synthesis and LPL properties.
The investigated phosphors in this work were synthesized through the solid-state reaction with SrCO3 (A. R.), Al2O3 (A. R.), SrCl2·6H2O (A. R.), Eu2O3 (99.99%) and Tm2O3 (99.99%) as raw materials. Stoichiometric mixtures of raw materials were homogeneously mixed and ground, and subsequently the mixture were placed in alumina crucibles with covers and sintered at 1100 °C for 4 h under ambient atmosphere in an electric tube furnace, and the sintered products were ground again in an agate mortar. Then the powder products were sintered at 900 °C for 2 h in a reducing atmosphere (N2:H2= 95:5) to reduce the Eu from its trivalent state to its divalent state. After firing, the samples were cooled to room temperature in the furnace, and ground again into powder for subsequent use.
The phase identification of samples was carried out by a Rigaku D/Max-2400 X-ray diffractometer with Ni-filter Cu Kα radiation. The excitation and emissison spectra were measured by a FLS-920T fluorescence spectrophotometer with Xe 900 (450W xenon arc lamp) as the light source. The decay curves were recorded by a PR305 Phosphorophotometer. The thermoluminescence (TL) curves were measured with a FJ-427 TL meter (Beijing Nuclear Instrument Factory) with a heating rate of 1°C/s in the temperature range from 20 to 400 °C. Prior to the TL measurement, 0.0080 g samples were exposed to radiation for 10 min by 365 nm UV light from a low pressure Hg lamp with a power of 30 W. All measurements were carried out at room temperature except the TL curves.
3. Results and discussion
In our work, the effect of Eu2+ and Tm3+ doping concentration on LPL properties were investigated. The optimum phosphor is Sr2.955Al2O5Cl2:Eu2+ 0.015, Tm3+ 0.03 in which the persistent time of LPL achieves maximum. Figure 1 shows the XRD patterns of Sr2.985Al2O5Cl2:Eu2+ 0.015 and Sr2.955Al2O5Cl2:Eu2+ 0.015, Tm3+ 0.03. All the observed peaks can be indexed to the pure phase of Sr3Al2O5Cl2 and match well with JCPDS card 80-0564, indicating the high purity and crystalline of the samples in this work. The doping of Eu2+ and Tm3+ does not make any noticeable variation of the XRD patterns.
Both Sr2.985Al2O5Cl2:Eu2+ 0.015 and Sr2.955Al2O5Cl2:Eu2+ 0.015, Tm3+ 0.03 produce bright and efficient orange-yellow emission under UV excitation. The excitation and emission spectra of Eu2+ doped Sr3Al2O5Cl2 have been reported by Tang et al . Our photoluminescence results about Sr2.985Al2O5Cl2:Eu2+ 0.015 agree well with theirs and thus it will not be discussed here. Figure 2 displays the excitation and emission spectra of Sr2.955Al2O5Cl2:Eu2+ 0.015, Tm3+ 0.03. The excitation spectrum is a broadband consisting of unresolved bands from 200 to 450 nm, which is due to the 4f7-4f65d1 of Eu2+. The emission spectrum exhibits a broad emission band peaked at 620 nm, originating from the typical 4f65d1-4f7 transition of Eu2+. The typical Tm3+ emission are not observed, indicating that it may not serve as luminescence centers in Sr2.955Al2O5Cl2:Eu2+ 0.015, Tm3+ 0.03 and may play a role of trapping centers as that of in CaS:Eu2+, Tm3+ .
As indicated above, when exposed to a 365 nm UV lamp, both Sr2.985Al2O5Cl2:Eu2+ 0.015 and Sr2.955Al2O5Cl2:Eu2+ 0.015, Tm3+ 0.03 emit intense orange-yellow light, however, after the excitation source is switched off, only Sr2.955Al2O5Cl2:Eu2+ 0.015, Tm3+ 0.03 yields orange-yellow LPL. An encouraging result of the present work is that the orange-yellow LPL in Sr2.955Al2O5Cl2:Eu2+ 0.015, Tm3+ 0.03 can be clearly observed with the naked eye in the dark, as seen in the inset A of Fig. 3 which was taken at 1 min after the exposure to the 365 nm UV light. The LPL decay curve of the Sr2.955Al2O5Cl2:Eu2+ 0.015, Tm3+ 0.03 is show in Fig. 3. It can be seen that the initial LPL intensity of Sr2.955Al2O5Cl2:Eu2+ 0.015, Tm3+ 0.03 can reach about 5000 mcd/m2 and its LPL can last about 220 min at recognizable intensity level (≥ 0.32 mcd/m2). The inset B in Fig. 3 shows the LPL spectra of Sr2.955Al2O5Cl2:Eu2+ 0.015, Tm3+ 0.03 measured at different times (t = 1, 8 min) after removal of the excitation source. For comparison, its UV-excited emission spectrum is also show in the inset B of Fig. 3. It is obvious that the shape and bandwidth of the UV-excited emission spectrum and the LPL spectra is similar, suggesting the LPL is due to the 4f65d1→4f7 transitions of Eu2+ as well.
Figure 4 is the corresponding CIE chromaticity diagram of the Sr2.955Al2O5Cl2:Eu2+ 0.015, Tm3+ 0.03 at 1 min after the removal of the excitation source. Point with chromaticity coordination of (0.53, 0.46) in Fig. 4 locates in the region of yellowish-orange color, indicating that the LPL in Sr2.955Al2O5Cl2:Eu2+ 0.015, Tm3+ 0.03 is orange-yellow.
The significant role of traps has been recognized in the field of LPL. In order to characterize the traps, TL measurements are performed on Sr2.985Al2O5Cl2:Eu2+ 0.015 and Sr2.955Al2O5Cl2:Eu2+ 0.015, Tm3+ 0.03 and their TL curves are illustrated in Fig. 5 . For Sr2.985Al2O5Cl2:Eu2+ 0.015, clearly, there is almost no TL phenomenon. For the codoped sample, one peak predominates at 61 °C and another two weak bands exist at 128 and 305 °C, respectively, suggesting that Tm3+ may serves as trapping centers. For long persistent phosphors, one critical factor is the suitable trap depth correlated to the TL peaks [1,24]. The optimized TL peak is situated slightly above room temperature (50-120 °C) for better LPL properties [3,5]. Therefore, the predominating peak at 61 °C in Sr2.955Al2O5Cl2:Eu2+ 0.015, Tm3+ 0.03 may be responsible for its orange-yellow LPL. However, the detailed mechanism of LPL in Sr3Al2O5Cl2:Eu2+, Tm3+ is needed to be further investigated.
Orange-yellow LPL was obtained from Sr3Al2O5Cl2:Eu2+, Tm3+ prepared via a solid state reaction. The intense orange-yellow LPL predominates at 620 nm, due to the 4f65d1→ 4f7 transition of Eu2+. The initial intensity of the orange-yellow LPL can reach nearly 5000 mcd/m2 and its LPL can last about 220 min at recognizable intensity level (≥ 0.32 mcd/m2). The incorporation of Tm3+ into the matrix is essential to achieve the trapping centers associated with the dominant TL peak at 61 °C, which is responsible for the intense orange-yellow LPL. The present Sr3Al2O5Cl2:Eu2+, Tm3+ phosphor enriches the color of LPL and is a new member in the family of long persistent phosphors.
This work is supported by the National Science Foundation for Distinguished Young Scholars (No. 50925206) and Key Science and Technology Project of Gansu Province (Grant no. 2GS064-A52-036-03).
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