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Red long lasting phosphorescence in Ca2Ge7O16:Sm3+ via persistent energy transfer from the host to Sm3+
Accepted Manuscript Title: Red Long Lasting Phosphorescence in Ca2 Ge7 O16 :Sm3+ via Persistent Energy Transfer from the Host to Sm3+ Author: Ting Wang Wenjuan Bian Dacheng Zhou Jianbei Qiu Xue Yu Xuhui Xu PII: DOI: Reference:
S0025-5408(15)30162-8 http://dx.doi.org/doi:10.1016/j.materresbull.2015.10.028 MRB 8450
To appear in:
MRB
Received date: Revised date: Accepted date:
5-6-2015 12-9-2015 13-10-2015
Please cite this article as: Ting Wang, Wenjuan Bian, Dacheng Zhou, Jianbei Qiu, Xue Yu, Xuhui Xu, Red Long Lasting Phosphorescence in Ca2Ge7O16:Sm3+ via Persistent Energy Transfer from the Host to Sm3+, Materials Research Bulletin http://dx.doi.org/10.1016/j.materresbull.2015.10.028 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Red Long Lasting Phosphorescence in Ca2Ge7O16:Sm3+ via Persistent Energy Transfer from the Host to Sm3+
Ting Wanga , Wenjuan Biana, Dacheng Zhoua,b, Jianbei Qiua,b, Xue Yu*a,b, Xuhui Xu*a,b
a
School of Materials Science and Engineering, Kunming University of Science and
Technology, Xuefu RD, Kunming 650093, PR China b
Key Laboratory of Advanced Materials of Yunnan Province, Kunming 650093, PR
China
*E-mail address: [email protected]; [email protected] Fax: 86-871-65188856
1
Tel: 86-871-65188856.
Graphical abstract
2
Highlight 1. Red LLP phosphor Ca2Ge7O16:Sm3+ was synthesized for the first time. 2. The energy transfer process from Ca2Ge7O16 host to Sm3+ was confirmed. 3. The color of the LLP could be adjusted from blue to red in Ca2Ge7O16:Sm3+. 4. Ca2Ge7O16:Sm3+ also provided a possibility to be a kind of photo-storage phosphor.
3
ABSTRACT A red long-lasting phosphorescence phosphor Ca2Ge7O16:Sm3+ was synthesized by a high temperature solid state reaction. The persistent energy transfer process from Ca2Ge7O16 host to Sm3+ was confirmed to be much more efficient than that in the photoluminescence process. The color of the persistent luminescence could be adjusted from blue to red with increasing concentration of Sm3+ accordingly. Based on the analysis of the thermoluminescence curves, it was found that Sm3+ doped Ca2Ge7O16 also provided a possibility to be a kind of photo-storage phosphor. Detailed mechanism were studied and illustrated.
Keywords: A. Optical Materials; B. Optical Properties; B. Luminescence; D. Energy storage; D. phosphors
4
1. Introduction Long lasting phosphorescence (LLP) materials have a great potential application as displays in dark environment. [1, 2] Up to now, the best performance of LLP phosphors have been reported as Eu2+-doped alkaline earth aluminates or silicates, which emit in the green or blue region. [3, 4] However, the luminosity of the LLP materials which emit in the long wavelength (orange to red) region still cannot meet the needs of commercial applications. Furthermore, considering the medicine applications and biomedical researches, the emission wavelength at the longer region from 600 nm to the infrared is more suitable for use as luminescent probes in vivo imaging. [5, 6] Hence, there is a doughty desire for exploring excellent red LLP materials. In the earlier times of this research, europium ions activated sulfides such as MS:Eu2+ (M = Ca, Sr), CaS:Eu2+ ,Tm3+ ,Ce3+ are used to be red LLP phosphors, [7-9] but they are sensitive to moisture and chemically unstable. Thereafter, red LLP of MO:Eu3+ (M= Ca, Sr, and Ba) and M2SnO4:Sm3+ (M = Ca, Sr, and Ba) are reported. [10-13] Despite many progresses on studies of the red LLP, there is still a strong desire for the development of LLP phosphors with better LLP performance for practical application. In these multitudinous long afterglow systems, the germanates have attracted great attention due to their favorable luminescence and stable physicochemical properties. [14] In our previous work, it was found that Ca2Ge7O16 phosphor exhibits a self-activated LLP phenomenon, which originated from the recombination of the donors ( VO ) and acceptors (valence band). [15] Nowadays, quite a few studies on the persistent energy transfer (PET) from host matrix to emission centers have been investigated, [16-20] which encourage us to explore the further LLP properties of Ca2Ge7O16 with rare earth doping. In this paper, the red LLP properties of Ca2Ge7O16
5
were explored and illustrated with Sm3+ doping. Photoluminescence excitation (PLE), emission (PL), LLP and thermoluminescence (TL) properties of Ca2Ge7O16:Sm3+ were studied. The PET process from Ca2Ge7O16 host to Sm3+ was confirmed to be much more efficient than the photoluminescence energy transfer (ET) process. Besides, Ca2Ge7O16:Sm3+ also exhibits typical properties for photo-storage according to the TL curves.
2. Experimental Ca2-xGe7O16:xSm3+ (x=0, 0.01, 0.03, 0.05, 0.07, and 0.09) samples were synthesized by the conventional high temperature solid state reaction. The stoichiometry amounts of CaCO3(A.R), GeO2(A.R), and Sm2O3(99.99%) were mixed in an agate mortar. After fully grinding, the mixtures were put into crucibles and calcined at 1000 oC for 12 h. The crystalline structures of the prepared powders were investigated by X-ray diffraction (XRD) with Ni-filter Cu Kα radiation at a scanning stepping of 0.02o. The XRD data were collected in the range of 10o to 60o by applying a D8ADVANCE/Germany Bruker X-ray diffractometer. The PLE, PL and LLP spectra were recorded by using a Hitachi F-7000 fluorescence spectrophotometer. LLP decay curves were measured with a PR305 long afterglow instrument after the sample irradiated by UV light for about 5 min. The TL curves were measured with a FJ-427 A TL meter (Beijing Nuclear Instrument Factory). The weight of samples was remained constant (0.005g). Prior to the TL measurement, the samples were first exposed to the radiation of UV light for about 5 min, and then heated from room temperature to 600K with a rate of 1K/s.
6
3. Results and discussion The crystal phases of Ca1.95Ge7O16:0.05Sm3+ and Ca1.93Ge7O16:0.07Sm3+ are characterized by XRD measurements. All measured diffraction patterns matches those of the Ca2Ge7O16 (JCPD No. 34-0286 ) perfectly, as shown in Fig. 1S*. The PLE and PL spectra of Ca2Ge7O16 and Ca2Ge7O16:Sm3+ phosphors are shown in Fig. 1 (a)-(b). For the non-doped sample, a broad excitation band centered at 249 nm is observed when monitoring the emission at 400 nm. Under the excitation at 249 nm, the PL spectrum shows a broad band ranging from 300 to 600 nm centered at 400 nm, which is assigned to the recombination of the donors ( VO ) and acceptors (valence band). [15] For Sm3+ doped one, the PLE spectrum monitored at 600 nm contains a group of sharp peaks located at 315, 347, 357 , 370, 401 nm due to the f-f translations of Sm3+. Under the excitation at 401 nm, the PL spectrum exhibits characteristic sharp peaks located at 565, 600 and 650 nm, which are assigned to 4
G5/2-6H5/2,
4
G5/2-6H7/2,
4
G5/2-6H9/2 transitions of Sm3+, respectively. Under the
excitation at 249 nm, both the characteristic emissions of Ca2Ge7O16 host and Sm3+ are observed. In addition, a significant spectral overlap between the emission peak of Ca2Ge7O16 host and the excitation peaks of Sm3+ is observed, therefore, it can be speculated that ET process from Ca2Ge7O16 host to Sm3+ ions happens. Fig. 1c shows the PL spectra of Ca2-xGe7O16:xSm3+ (x=0, 0.01, 0.03, 0.05, 0.07, and 0.09) samples under the excitation at 249 nm. The emission intensity of the host (～400 nm) decreases remarkably with increasing concentration of Sm3+. Besides, the weak red emission peaks located around 600 nm are observed. The emission intensity of Sm3+ increases initially and reaches the maximum at x=0.05, and then decreases due to concentration quenching. These results indicate the ET process between the host and Sm3+ can be expected. In order to further illustrate the ET process, the fluorescence 7
decay curves of Ca2-xGe7O16:xSm3+ (x=0, 0.03, 0.07, and 0.09) (λex=249nm, λem=400nm) samples are recorded as shown in Fig. 1d. The values of the lifetimes are obtained by integrating the decay curves of which the initial intensities are normalized. The lifetimes of the host appear a decreasing trend with increasing concentration of Sm3+. The change strongly proves the occurrence of ET process from Ca2Ge7O16 host to Sm3+. After being irradiated by UV light for 5 min, LLP spectra of the Ca2-xGe7O16:xSm3+ (x=0, 0.01, 0.03, 0.05, 0.07, and 0.09) samples are recorded and demonstrated in Fig. 2a. The LLP spectrum of Ca2Ge7O16 host exhibits a symmetrical curve at 400 nm, which is similar to its PL spectrum. For Sm3+ doped samples, the LLP spectra not only show an extremely band, originated from the host, but also present the characteristic sharp peaks of Sm3+ located around 600 nm. In Ca2-xGe7O16:xSm3+ samples, the LLP intensities of Ca2Ge7O16 host increase gradually with increasing concentration of Sm3+, and then decrease and completely disappear when x=0.09. Meanwhile the intensities of Sm3+ are enhanced gradually and no obvious concentration quenching are observed in the current concentration range. These results confirm the PET process from the host to Sm3+ occurs. Besides, in the consideration of the fact that the ionization potential of Sm3+ appears lower than that of Ca2+, Sm3+ has a greater ability to stabilize the of the
VO
VO .
[21, 22] Therefore, the number
increases with increasing Sm3+ concentration as well, which results in the
enhancement of LLP intensity of Ca2Ge7O16 host when x=0.01. It is notable that the LLP intensities of Sm3+ increase gradually with increasing Sm3+ concentration. Comparing the PL spectra with LLP spectra, it was found that both of them originate from the same emitting centers, but the peak shapes of which are apparently different. Intensity variations of red/blue emission depending on the Sm3+ concentration are 8
shown in Fig. S2*. The ratios are found to be 0.018, 0.200, 0.370, 0.667, 0.357, and 0.909 for PL, and 0.033, 0.435, 1.961, 2.381, 9.091, and 26.1 for LLP with increasing concentration of Sm3+. Since the ratios rise faster in LLP, it could be concluded that the PET process is much more efficient than the ET process. The decay curves of LLP of Ca2-xGe7O16:xSm3+ (x=0, 0.01, 0.03, 0.05, 0.07, and 0.09) are measured at room temperature and displayed in Fig. 2b. All curves show LLP with different persistent times, and consist of a fast decay and a consequent slow decay with a long decay tail, implying the existence of various trap depths. It is known that the lattice defects, acting as traps, play an essential role for energy storage in LLP phosphors. [23] Compared with the relatively weak LLP of the non-doped sample, longer persistent time is observed in the phosphors doping with Sm3+ and it reaches the maximum when x= 0.09. Hence, it could be safe to say that the incorporation of Sm3+ creates more defects in Ca2Ge7O16 host lattice, which acts as trapping centers and has a significant influence on the LLP performance. To characterize the traps in our samples, TL measurements of Ca2-xGe7O16:xSm3+ are performed and illustrated in Fig. 2c. For Ca2Ge7O16, only one weak peak located at 345 K is observed (TA). With the doping of Sm3+, two extra TL peaks appear. Besides, the TL intensities of Ca2-xGe7O16:xSm3+ are greatly enhanced. Above results indicate that doping with Sm3+ ions largely improves the number of trapping centers. The inset of Fig. 2d shows the Gaussian profiles of Ca1.91Ge7O16:0.09Sm3+. It could be expected that there are three types of the trap (TA, TB, TC) corresponding to three peaks (Gaussian fit), located at 345, 375 and 428 K. In the consideration of the fact that Sm3+ ions replace the Ca2+ site in Ca2Ge7O16 host matrix, two Sm3+ ions replace three Ca2+ ions to balance the charge of the phosphor, which create two positive 9
3
Sm '' defects and one negative defect ( 3Ca 2 2 ). Therefore, the electrons 2SmCa VCa traps ( SmCa ) and holes traps ( VCa'' ) are formed. The different positions of TL bands
infer that there are several traps. The optimized TL peak is situated slightly above room temperature (320-390 K) for better LLP properties. [24] Therefore, TA and TB could be responsible for the blue and red persistent luminescence. Reversely, the charge carriers that are captured by deep depths (TC) were difficult to be released at room temperature, indicating that these samples provide potential application as optical storage materials. In order to estimate the defect states in these samples, the classical fitting peak-shape methods developed by Chen et al are introduced. And the trap depth of E is calculated from the glow-peak parameters by the following equation: [25] E [2.52 10.2(u g 0.42)](
BTm2 ) 2 BTm )
(1)
Where, μg is symmetry factor, Tm, T1 and T2 are respectively the peak temperature at the maximum and the temperatures on either side of the temperature at the maximum, corresponding to half intensity. k is Boltzmann's constant. The following parameters can be defined: τ=Tm −T1 is the half width at the low temperature side of the peak, δ=T2 –Tm is the half width towards the fall-off of the glow peak, ω=T2 −T1 is the total half width, μg =δ/ω is the symmetrical geometrical factor. The calculated traps level (E) of TA, TB, TC are 0.6407, 0.7790 and 0.8853 eV for Ca2Ge7O16:Sm3+, respectively. To investigate the number of trapping centers concerned and the kinetic order of the TL, the TL curves of Ca1.91Ge7O16:0.09Sm3+ with different delay times are recorded in Fig. 2d. For a delay of 4 h, the TA peak completely decays. As time goes on, TB peak also decreases. The TB peak completely disappears as well as the afterglow subsided after 48 h. Moreover, the intensity of TC peak only decreases a little and then sustains 10
its intensity with insignificant change. Hence, it is safe to say that the presence of deep stable traps in Ca2Ge7O16:Sm3+ able to immobilize the energy permanently at room temperature. For a storage phosphor, the traps should be relatively deep to preclude the thermal release of the intercepted carriers at room temperature, whereas for the LLP phosphor, the traps should be rather shallow. [26, 27] For the deepest traps corresponding to the high-temperature band (TC), the carriers trapped there are not assumed to be released at room temperature to yield the LLP, which indicates that these samples provide potential application as optical storage materials. Therefore, it can be concluded that Ca2Ge7O16:Sm3+ fulfills the requirements of a storage phosphor as well as a LLP phosphor. On the basis of the above-mentioned results, a possible mechanism is proposed to explain the generation of red LLP in Ca2Ge7O16:Sm3+ phosphors, as shown in Fig. 3, which should be treated as a qualitative analysis. Under UV irradiation, electrons in the valence band (VB) are excited directly through the host into the conduction band (CB) (step 1). Some of excited electrons can shift freely in the CB and captured to native traps level (step 2), whereas the holes left behind move randomly in the VB. Under the thermal disturbance at room temperature, captured electrons can be easily released to create the blue LLP from the host (step 3). [15] When Sm3+ is doped,the steps 1, 2 and 3 happen under the UV excitation. Then, the electrons are relaxed from native traps to the 4F7/2 level of Sm3+ (step 4), and after nonradiatively relaxed to the 4
G5/2 level, the energy relaxes through the transitions from 4G5/2 to 6H5/2, 6H7/2 and
6
H9/2 levels and contributes to the emission located at 565, 600 and 650 nm,
respectively (step 10). Meanwhile, other excited electrons and holes captured by the electron traps (TB) and hole traps (TC), respectively (steps 5 and 6). After the UV light is switched off, under thermal agitation, the carriers (electrons and holes) will be
11
released from the trapping centers and transfer via the host to Sm3+ after a series of nonradiative transitions (steps 7, 8 and 9), followed by the recombination and contributed to the characteristic emission of Sm3+ ions as persistent phosphorescence (step 10). Thus, because the foreign defects, such as TB/TC, are served as the bridge between the host and Sm3+, the PET process efficiency is more significant, compared with ET process.
4. Conclusions: The red LLP of Ca2Ge7O16:Sm3+ has been realized via the PET process from the host to Sm3+, and the PET process is manifested to be more efficient than the ET process. Sm3+ ions not only act as luminescent centers, but also introduce foreign trap centers acted as the bridge, promote the PET process from the host to Sm3+ ions. The LLP colors could be adjusted from blue to red with increasing concentration of Sm3+. Besides, Sm3+ doped Ca2Ge7O16 exhibits a possibility to be a kind of storage phosphor due to the existence of the deep and stable traps, which can immobilize the carriers permanently at room temperature. The results indicate that Ca2Ge7O16:Sm3+ could be a new and efficient candidate for LLP and storage material.
Acknowledgments The project was supported by the National Nature Science Foundation of China (61308091, 61565009), the Young Talents Support Program of Faculty of Materials Science and Engineering, Kunming University of Science and Technology (14078342).
12
References: [1] T. Katsumata, T. Nabae, K. Sasajima, T. Matsuzawa, J. Cryst. Growth. 183 (1998) 361-365. [2] Y. Katayama, H. Kobayashi, S. Tanabe, Appl. Phys. Exp. 8 (2015).12102. [3] M. Kamada, J. Murakami, N. Ohno, J. Lumin. 87 (2000) 1042-1044. [4] Y. Ding, Y. Zhang, Z. Wang, W. Li, D. Mao, H. Han, C. Chang, J. Lumin. 129 (2009) 294-299 [5] AA. Lecointre, A. Bessière, A. Bos, P. Dorenbos, B. Viana, S. Jacquart, J. Phys. Chem. C 115 (2011) 4217-4227. [6] Y. Zhuang, J. Ueda, S. Tanabe, Appl. Phys. Exp. 6 (2013) 52602. [7] D. Jia, J Electronchem. Soc. 153 (2006) H198-H201. [8] X. X. Duan, S. H. Huang, F. T. You, K. Kang, J. Rare Earth 27 (2009) 43-46. [9] P. Ai, Y. Liu, L. Xiao, H. Wang, J. Meng, Sci Technol. Adv. Mat. 11 (2010) 35002. [10] X. Xu, Y. Gong, W. Zeng, Y. Wang, Phys. Procedia. 29 (2012) 62-64. [11] D. Jia, J. Wilhelm, J. Freed, D. E. Best, A. Goonewardene, ECS Transac. 2 (2007) 7-13. [12] J. Fu, Electrochem. Solid-State Lett. 3 (2000) 350-351. [13] B. Lei, H. Zhang, W. Mai, S. Yue, Y. Liu, S. Man , Solid State Sci. 13 (2011) 525-528. [14] T. E. Peters, J. R. McColl. Goole. Patents 1982. [15] T. Wang, J. Gou, X. H. Xu, D. C. Zhou, J. B. Qiu, X. Yu, Opt. Exp. 23 (2015) 12595-12604. [16] Y. Jin, Y. Hu, Y. Fu, G. Ju, Z. Mu, R. Chen, J. Lin, Z. Wang, J Am. Ceram. Soc. 98 (2015) 1555-1561. [17] Y. Jin, Y. Hu, H. Duan, L. Chen, X. Wang, RSC Adv. 4 (2014) 11360-11366. [18] D Jia, R. S. Meltzer, W.M. Yen, X. J. Wang, Appl. Phys. Lett. 9 (2002) 1535-1537. [19] X. J. Wang, D. D. Jia, W. M. Yenb, J. Lumin. 102 (2003) 34-37. [20] D Jia, X. J. Wang, W. Jia, W. M. Yen, J. Appl. Phys. 93 ( 2003) 147-152. [21] F. Clabau, X. Rocquefelte, S. Jobic, P. Deniard, M. Whangbo, A. Garcia, T. Le Mercier, Chem. Mater. 17 (2005) 3904-3912. [22] Y. Guan, L. Qin, Y. Huang, C. Qin, D. Wei, H. J. Seo, Mater. Res. Bull. 54 (2013) 24-27. [23] T. Maldiney, A. Bessière, J. Seguin, E. Teston, S. K. Sharma, B. Viana, A. J. Bos, P. Dorenbos, M. Bessodes, D. Gourier,, Nat. Mater. 13 (2014) 418-426. [24] B. Wang, H. Lin, Y. Yu, D. Chen, R. Zhang, J. Xu, Y. Wang, J A. Ceram. Soc. 97 (2014) 2539-2545. [25] B. Qu, B. Zhang, L. Wang, R. Zhou, X. C. Zeng, Chem. Mater. 27 (2015) 2195-2202. [26] Y. Li, S. Zhou, Y. Li, K. Sharafudeen, Z. Ma, G. Dong, M. Peng, J. Qiu, J Mater. Chem. C 2 (2014) 2657-2663. [27] X. Xu, X. Zhang, T. Wang, J. Qiu, X. Yu, Mater. Lett. 127 (2014).
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Figure caption:
Fig. 1 PL and PLE spectra of Ca2Ge7O16 ( λex=249nm, λem=400nm) (a), and Ca2Ge7O16:Sm3+ (λex=401nm, λex=249nm, λem=600nm) (b); PL spectra of Ca2-xGe7O16:xSm3+ (x=0, 0.01, 0.03, 0.05, 0.07, and 0.09) excited at 294nm (c); fluorescence decay curves of Ca2-xGe7O16:xSm3+ (x=0, 0.03, 0.07, and 0.09) (d).
14
Fig. 2 LLP spectra of Ca2-xGe7O16:xSm3+ (x= 0, 0.01, 0.03, 0.05, 0.07, and 0.09) (a); LLP decay curves of Ca2-xGe7O16:xSm3+ samples (b); TL curves of Ca2-xGe7O16:xSm3+ samples (c); TL curves of Ca1.91Ge7O16:0.09Sm3+ placed in dark room for different times, the inset shows the Gaussian profiles of Ca1.91Ge7O16:0.09Sm3+ and its LLP photographs (d).
15
Fig. 3 The schematic diagram of the phosphorescence mechanism for Ca 2Ge7O16:Sm3+.
16
S0025-5408(15)30162-8 http://dx.doi.org/doi:10.1016/j.materresbull.2015.10.028 MRB 8450
To appear in:
MRB
Received date: Revised date: Accepted date:
5-6-2015 12-9-2015 13-10-2015
Please cite this article as: Ting Wang, Wenjuan Bian, Dacheng Zhou, Jianbei Qiu, Xue Yu, Xuhui Xu, Red Long Lasting Phosphorescence in Ca2Ge7O16:Sm3+ via Persistent Energy Transfer from the Host to Sm3+, Materials Research Bulletin http://dx.doi.org/10.1016/j.materresbull.2015.10.028 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Red Long Lasting Phosphorescence in Ca2Ge7O16:Sm3+ via Persistent Energy Transfer from the Host to Sm3+
Ting Wanga , Wenjuan Biana, Dacheng Zhoua,b, Jianbei Qiua,b, Xue Yu*a,b, Xuhui Xu*a,b
a
School of Materials Science and Engineering, Kunming University of Science and
Technology, Xuefu RD, Kunming 650093, PR China b
Key Laboratory of Advanced Materials of Yunnan Province, Kunming 650093, PR
China
*E-mail address: [email protected]; [email protected] Fax: 86-871-65188856
1
Tel: 86-871-65188856.
Graphical abstract
2
Highlight 1. Red LLP phosphor Ca2Ge7O16:Sm3+ was synthesized for the first time. 2. The energy transfer process from Ca2Ge7O16 host to Sm3+ was confirmed. 3. The color of the LLP could be adjusted from blue to red in Ca2Ge7O16:Sm3+. 4. Ca2Ge7O16:Sm3+ also provided a possibility to be a kind of photo-storage phosphor.
3
ABSTRACT A red long-lasting phosphorescence phosphor Ca2Ge7O16:Sm3+ was synthesized by a high temperature solid state reaction. The persistent energy transfer process from Ca2Ge7O16 host to Sm3+ was confirmed to be much more efficient than that in the photoluminescence process. The color of the persistent luminescence could be adjusted from blue to red with increasing concentration of Sm3+ accordingly. Based on the analysis of the thermoluminescence curves, it was found that Sm3+ doped Ca2Ge7O16 also provided a possibility to be a kind of photo-storage phosphor. Detailed mechanism were studied and illustrated.
Keywords: A. Optical Materials; B. Optical Properties; B. Luminescence; D. Energy storage; D. phosphors
4
1. Introduction Long lasting phosphorescence (LLP) materials have a great potential application as displays in dark environment. [1, 2] Up to now, the best performance of LLP phosphors have been reported as Eu2+-doped alkaline earth aluminates or silicates, which emit in the green or blue region. [3, 4] However, the luminosity of the LLP materials which emit in the long wavelength (orange to red) region still cannot meet the needs of commercial applications. Furthermore, considering the medicine applications and biomedical researches, the emission wavelength at the longer region from 600 nm to the infrared is more suitable for use as luminescent probes in vivo imaging. [5, 6] Hence, there is a doughty desire for exploring excellent red LLP materials. In the earlier times of this research, europium ions activated sulfides such as MS:Eu2+ (M = Ca, Sr), CaS:Eu2+ ,Tm3+ ,Ce3+ are used to be red LLP phosphors, [7-9] but they are sensitive to moisture and chemically unstable. Thereafter, red LLP of MO:Eu3+ (M= Ca, Sr, and Ba) and M2SnO4:Sm3+ (M = Ca, Sr, and Ba) are reported. [10-13] Despite many progresses on studies of the red LLP, there is still a strong desire for the development of LLP phosphors with better LLP performance for practical application. In these multitudinous long afterglow systems, the germanates have attracted great attention due to their favorable luminescence and stable physicochemical properties. [14] In our previous work, it was found that Ca2Ge7O16 phosphor exhibits a self-activated LLP phenomenon, which originated from the recombination of the donors ( VO ) and acceptors (valence band). [15] Nowadays, quite a few studies on the persistent energy transfer (PET) from host matrix to emission centers have been investigated, [16-20] which encourage us to explore the further LLP properties of Ca2Ge7O16 with rare earth doping. In this paper, the red LLP properties of Ca2Ge7O16
5
were explored and illustrated with Sm3+ doping. Photoluminescence excitation (PLE), emission (PL), LLP and thermoluminescence (TL) properties of Ca2Ge7O16:Sm3+ were studied. The PET process from Ca2Ge7O16 host to Sm3+ was confirmed to be much more efficient than the photoluminescence energy transfer (ET) process. Besides, Ca2Ge7O16:Sm3+ also exhibits typical properties for photo-storage according to the TL curves.
2. Experimental Ca2-xGe7O16:xSm3+ (x=0, 0.01, 0.03, 0.05, 0.07, and 0.09) samples were synthesized by the conventional high temperature solid state reaction. The stoichiometry amounts of CaCO3(A.R), GeO2(A.R), and Sm2O3(99.99%) were mixed in an agate mortar. After fully grinding, the mixtures were put into crucibles and calcined at 1000 oC for 12 h. The crystalline structures of the prepared powders were investigated by X-ray diffraction (XRD) with Ni-filter Cu Kα radiation at a scanning stepping of 0.02o. The XRD data were collected in the range of 10o to 60o by applying a D8ADVANCE/Germany Bruker X-ray diffractometer. The PLE, PL and LLP spectra were recorded by using a Hitachi F-7000 fluorescence spectrophotometer. LLP decay curves were measured with a PR305 long afterglow instrument after the sample irradiated by UV light for about 5 min. The TL curves were measured with a FJ-427 A TL meter (Beijing Nuclear Instrument Factory). The weight of samples was remained constant (0.005g). Prior to the TL measurement, the samples were first exposed to the radiation of UV light for about 5 min, and then heated from room temperature to 600K with a rate of 1K/s.
6
3. Results and discussion The crystal phases of Ca1.95Ge7O16:0.05Sm3+ and Ca1.93Ge7O16:0.07Sm3+ are characterized by XRD measurements. All measured diffraction patterns matches those of the Ca2Ge7O16 (JCPD No. 34-0286 ) perfectly, as shown in Fig. 1S*. The PLE and PL spectra of Ca2Ge7O16 and Ca2Ge7O16:Sm3+ phosphors are shown in Fig. 1 (a)-(b). For the non-doped sample, a broad excitation band centered at 249 nm is observed when monitoring the emission at 400 nm. Under the excitation at 249 nm, the PL spectrum shows a broad band ranging from 300 to 600 nm centered at 400 nm, which is assigned to the recombination of the donors ( VO ) and acceptors (valence band). [15] For Sm3+ doped one, the PLE spectrum monitored at 600 nm contains a group of sharp peaks located at 315, 347, 357 , 370, 401 nm due to the f-f translations of Sm3+. Under the excitation at 401 nm, the PL spectrum exhibits characteristic sharp peaks located at 565, 600 and 650 nm, which are assigned to 4
G5/2-6H5/2,
4
G5/2-6H7/2,
4
G5/2-6H9/2 transitions of Sm3+, respectively. Under the
excitation at 249 nm, both the characteristic emissions of Ca2Ge7O16 host and Sm3+ are observed. In addition, a significant spectral overlap between the emission peak of Ca2Ge7O16 host and the excitation peaks of Sm3+ is observed, therefore, it can be speculated that ET process from Ca2Ge7O16 host to Sm3+ ions happens. Fig. 1c shows the PL spectra of Ca2-xGe7O16:xSm3+ (x=0, 0.01, 0.03, 0.05, 0.07, and 0.09) samples under the excitation at 249 nm. The emission intensity of the host (～400 nm) decreases remarkably with increasing concentration of Sm3+. Besides, the weak red emission peaks located around 600 nm are observed. The emission intensity of Sm3+ increases initially and reaches the maximum at x=0.05, and then decreases due to concentration quenching. These results indicate the ET process between the host and Sm3+ can be expected. In order to further illustrate the ET process, the fluorescence 7
decay curves of Ca2-xGe7O16:xSm3+ (x=0, 0.03, 0.07, and 0.09) (λex=249nm, λem=400nm) samples are recorded as shown in Fig. 1d. The values of the lifetimes are obtained by integrating the decay curves of which the initial intensities are normalized. The lifetimes of the host appear a decreasing trend with increasing concentration of Sm3+. The change strongly proves the occurrence of ET process from Ca2Ge7O16 host to Sm3+. After being irradiated by UV light for 5 min, LLP spectra of the Ca2-xGe7O16:xSm3+ (x=0, 0.01, 0.03, 0.05, 0.07, and 0.09) samples are recorded and demonstrated in Fig. 2a. The LLP spectrum of Ca2Ge7O16 host exhibits a symmetrical curve at 400 nm, which is similar to its PL spectrum. For Sm3+ doped samples, the LLP spectra not only show an extremely band, originated from the host, but also present the characteristic sharp peaks of Sm3+ located around 600 nm. In Ca2-xGe7O16:xSm3+ samples, the LLP intensities of Ca2Ge7O16 host increase gradually with increasing concentration of Sm3+, and then decrease and completely disappear when x=0.09. Meanwhile the intensities of Sm3+ are enhanced gradually and no obvious concentration quenching are observed in the current concentration range. These results confirm the PET process from the host to Sm3+ occurs. Besides, in the consideration of the fact that the ionization potential of Sm3+ appears lower than that of Ca2+, Sm3+ has a greater ability to stabilize the of the
VO
VO .
[21, 22] Therefore, the number
increases with increasing Sm3+ concentration as well, which results in the
enhancement of LLP intensity of Ca2Ge7O16 host when x=0.01. It is notable that the LLP intensities of Sm3+ increase gradually with increasing Sm3+ concentration. Comparing the PL spectra with LLP spectra, it was found that both of them originate from the same emitting centers, but the peak shapes of which are apparently different. Intensity variations of red/blue emission depending on the Sm3+ concentration are 8
shown in Fig. S2*. The ratios are found to be 0.018, 0.200, 0.370, 0.667, 0.357, and 0.909 for PL, and 0.033, 0.435, 1.961, 2.381, 9.091, and 26.1 for LLP with increasing concentration of Sm3+. Since the ratios rise faster in LLP, it could be concluded that the PET process is much more efficient than the ET process. The decay curves of LLP of Ca2-xGe7O16:xSm3+ (x=0, 0.01, 0.03, 0.05, 0.07, and 0.09) are measured at room temperature and displayed in Fig. 2b. All curves show LLP with different persistent times, and consist of a fast decay and a consequent slow decay with a long decay tail, implying the existence of various trap depths. It is known that the lattice defects, acting as traps, play an essential role for energy storage in LLP phosphors. [23] Compared with the relatively weak LLP of the non-doped sample, longer persistent time is observed in the phosphors doping with Sm3+ and it reaches the maximum when x= 0.09. Hence, it could be safe to say that the incorporation of Sm3+ creates more defects in Ca2Ge7O16 host lattice, which acts as trapping centers and has a significant influence on the LLP performance. To characterize the traps in our samples, TL measurements of Ca2-xGe7O16:xSm3+ are performed and illustrated in Fig. 2c. For Ca2Ge7O16, only one weak peak located at 345 K is observed (TA). With the doping of Sm3+, two extra TL peaks appear. Besides, the TL intensities of Ca2-xGe7O16:xSm3+ are greatly enhanced. Above results indicate that doping with Sm3+ ions largely improves the number of trapping centers. The inset of Fig. 2d shows the Gaussian profiles of Ca1.91Ge7O16:0.09Sm3+. It could be expected that there are three types of the trap (TA, TB, TC) corresponding to three peaks (Gaussian fit), located at 345, 375 and 428 K. In the consideration of the fact that Sm3+ ions replace the Ca2+ site in Ca2Ge7O16 host matrix, two Sm3+ ions replace three Ca2+ ions to balance the charge of the phosphor, which create two positive 9
3
Sm '' defects and one negative defect ( 3Ca 2 2 ). Therefore, the electrons 2SmCa VCa traps ( SmCa ) and holes traps ( VCa'' ) are formed. The different positions of TL bands
infer that there are several traps. The optimized TL peak is situated slightly above room temperature (320-390 K) for better LLP properties. [24] Therefore, TA and TB could be responsible for the blue and red persistent luminescence. Reversely, the charge carriers that are captured by deep depths (TC) were difficult to be released at room temperature, indicating that these samples provide potential application as optical storage materials. In order to estimate the defect states in these samples, the classical fitting peak-shape methods developed by Chen et al are introduced. And the trap depth of E is calculated from the glow-peak parameters by the following equation: [25] E [2.52 10.2(u g 0.42)](
BTm2 ) 2 BTm )
(1)
Where, μg is symmetry factor, Tm, T1 and T2 are respectively the peak temperature at the maximum and the temperatures on either side of the temperature at the maximum, corresponding to half intensity. k is Boltzmann's constant. The following parameters can be defined: τ=Tm −T1 is the half width at the low temperature side of the peak, δ=T2 –Tm is the half width towards the fall-off of the glow peak, ω=T2 −T1 is the total half width, μg =δ/ω is the symmetrical geometrical factor. The calculated traps level (E) of TA, TB, TC are 0.6407, 0.7790 and 0.8853 eV for Ca2Ge7O16:Sm3+, respectively. To investigate the number of trapping centers concerned and the kinetic order of the TL, the TL curves of Ca1.91Ge7O16:0.09Sm3+ with different delay times are recorded in Fig. 2d. For a delay of 4 h, the TA peak completely decays. As time goes on, TB peak also decreases. The TB peak completely disappears as well as the afterglow subsided after 48 h. Moreover, the intensity of TC peak only decreases a little and then sustains 10
its intensity with insignificant change. Hence, it is safe to say that the presence of deep stable traps in Ca2Ge7O16:Sm3+ able to immobilize the energy permanently at room temperature. For a storage phosphor, the traps should be relatively deep to preclude the thermal release of the intercepted carriers at room temperature, whereas for the LLP phosphor, the traps should be rather shallow. [26, 27] For the deepest traps corresponding to the high-temperature band (TC), the carriers trapped there are not assumed to be released at room temperature to yield the LLP, which indicates that these samples provide potential application as optical storage materials. Therefore, it can be concluded that Ca2Ge7O16:Sm3+ fulfills the requirements of a storage phosphor as well as a LLP phosphor. On the basis of the above-mentioned results, a possible mechanism is proposed to explain the generation of red LLP in Ca2Ge7O16:Sm3+ phosphors, as shown in Fig. 3, which should be treated as a qualitative analysis. Under UV irradiation, electrons in the valence band (VB) are excited directly through the host into the conduction band (CB) (step 1). Some of excited electrons can shift freely in the CB and captured to native traps level (step 2), whereas the holes left behind move randomly in the VB. Under the thermal disturbance at room temperature, captured electrons can be easily released to create the blue LLP from the host (step 3). [15] When Sm3+ is doped,the steps 1, 2 and 3 happen under the UV excitation. Then, the electrons are relaxed from native traps to the 4F7/2 level of Sm3+ (step 4), and after nonradiatively relaxed to the 4
G5/2 level, the energy relaxes through the transitions from 4G5/2 to 6H5/2, 6H7/2 and
6
H9/2 levels and contributes to the emission located at 565, 600 and 650 nm,
respectively (step 10). Meanwhile, other excited electrons and holes captured by the electron traps (TB) and hole traps (TC), respectively (steps 5 and 6). After the UV light is switched off, under thermal agitation, the carriers (electrons and holes) will be
11
released from the trapping centers and transfer via the host to Sm3+ after a series of nonradiative transitions (steps 7, 8 and 9), followed by the recombination and contributed to the characteristic emission of Sm3+ ions as persistent phosphorescence (step 10). Thus, because the foreign defects, such as TB/TC, are served as the bridge between the host and Sm3+, the PET process efficiency is more significant, compared with ET process.
4. Conclusions: The red LLP of Ca2Ge7O16:Sm3+ has been realized via the PET process from the host to Sm3+, and the PET process is manifested to be more efficient than the ET process. Sm3+ ions not only act as luminescent centers, but also introduce foreign trap centers acted as the bridge, promote the PET process from the host to Sm3+ ions. The LLP colors could be adjusted from blue to red with increasing concentration of Sm3+. Besides, Sm3+ doped Ca2Ge7O16 exhibits a possibility to be a kind of storage phosphor due to the existence of the deep and stable traps, which can immobilize the carriers permanently at room temperature. The results indicate that Ca2Ge7O16:Sm3+ could be a new and efficient candidate for LLP and storage material.
Acknowledgments The project was supported by the National Nature Science Foundation of China (61308091, 61565009), the Young Talents Support Program of Faculty of Materials Science and Engineering, Kunming University of Science and Technology (14078342).
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Figure caption:
Fig. 1 PL and PLE spectra of Ca2Ge7O16 ( λex=249nm, λem=400nm) (a), and Ca2Ge7O16:Sm3+ (λex=401nm, λex=249nm, λem=600nm) (b); PL spectra of Ca2-xGe7O16:xSm3+ (x=0, 0.01, 0.03, 0.05, 0.07, and 0.09) excited at 294nm (c); fluorescence decay curves of Ca2-xGe7O16:xSm3+ (x=0, 0.03, 0.07, and 0.09) (d).
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Fig. 2 LLP spectra of Ca2-xGe7O16:xSm3+ (x= 0, 0.01, 0.03, 0.05, 0.07, and 0.09) (a); LLP decay curves of Ca2-xGe7O16:xSm3+ samples (b); TL curves of Ca2-xGe7O16:xSm3+ samples (c); TL curves of Ca1.91Ge7O16:0.09Sm3+ placed in dark room for different times, the inset shows the Gaussian profiles of Ca1.91Ge7O16:0.09Sm3+ and its LLP photographs (d).
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Fig. 3 The schematic diagram of the phosphorescence mechanism for Ca 2Ge7O16:Sm3+.
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