DOI: 10.1002/asia.201500580
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Doping
Investigation on Luminescence Properties of a Long Afterglow Phosphor Ca2SnO4 :Tm3 + Hai-Feng Li,[a, b] Wen-Zhi Sun,[a, b] Yong-Lei Jia,[a, b] Teng-Fei Ma,[a] Ji-Peng Fu,[a, b] Da Li,[a] Su Zhang,[a] Li-Hong Jiang,[a] Ran Pang,*[a] and Cheng-Yu Li*[a] Abstract: A series of new long afterglow phosphors Ca2SnO4 :x Tm3 + were synthesized by using traditional solidstate reactions. XRD measurements and Rietveld refinement revealed that the incorporation of the Tm3 + dopants generated no second phase other than the original one of Ca2SnO4, which indicated that the dopants completely merged into the host. The corresponding optical properties were further systematically studied by photoluminescence, phosphorescence, and thermoluminescence (TL) spectrosco-
Introduction Long afterglow phosphors (LAPs) are materials that can absorb either visible or UV light and subsequently release the stored energy as persistent visible light in ambient temperature.[1] Based on their inherent properties of energy storage, this kind of material has been widely applied to practical domains, such as the detection of high-energy irradiation, emergency illumination and display fields, three-dimensional optical memory and image storage, bioimaging, and alternating-current (AC) white light-emitting diodes (LEDs).[2] Generally, the long afterglow properties are closely related to defects in the phosphors, which can trap electrons or holes during the excitation process, and then release them under thermal stimulation to recombine automatically as emission centers, resulting in phosphorescence emissions. The production of appropriate defects in the host of a phosphor and improvements in the capability of storing the incident energy are mainly realized through inequivalent substitution, for example, doped ions as luminescence centers, especially lanthanide ions or transition-metal ions, would occupy the sites of matrix ions, which have different valence. In addition, to enhance the optic intensity of [a] Dr. H.-F. Li, Dr. W.-Z. Sun, Dr. Y.-L. Jia, Dr. T.-F. Ma, Dr. J.-P. Fu, Dr. D. Li, Dr. S. Zhang, Dr. L.-H. Jiang, Dr. R. Pang, Prof. C.-Y. Li State Key Laboratory of Rare Earth Resource Utilization Changchun Institute of Applied Chemistry Chinese Academy of Sciences Changchun 130022 (P. R. China) E-mail: [email protected] [email protected] [b] Dr. H.-F. Li, Dr. W.-Z. Sun, Dr. Y.-L. Jia, Dr. J.-P. Fu University of Chinese Academy of Sciences Beijing 100049 (P. R. China) Chem. Asian J. 2015, 10, 2361 – 2367
py. The results show that the Tm3 + -related defects account for the bright bluish green afterglow emission from the characteristic f–f transitions of Tm3 + ions. The bluish green long-lasting phosphorescence could be observed for 5 h by the naked eye in a dark environment after the end of UV irradiation. Two TL peaks at 325 and 349 K from the TL curves were adopted to calculate the depth of the traps, which were 0.45 and 0.78 eV, respectively. The mechanism of the long afterglow emission was also explored.
LAPs, another kind of ion is usually incorporated into the host as an assistant activator to modify the defects and improve the phosphorescence properties. Given the above description, the colors of LAPs are determined by the characteristics of the emissive centers, whereas the intensity and persistent time are regulated by the defects created or improved by inequivalent doping. With this method, a large number of LAPs, such as SrAl2O4 :Eu2 + ,Dy3 + ;[3] Sr2MgSi2O7:Eu2 + ,Dy3 + ;[4] CaAl2O4 :Eu2 + ,Nd3 + ;[5] Zn2P2O7:Tm3 + ,Mn2 + ;[6] Ba5Si8O21:Eu2 + ,Dy3 + ;[7] Ca9Bi(PO4)7:Eu2 + ,Dy3 + ;[8] and Ca14Mg2(SiO4)8 :Eu2 + ,Dy3 + ,[9] have been successfully synthesized. For those phosphors, the colors of the long-lasting phosphorescence are derived from luminescent centers of either Eu2 + or Mn2 + dopants, and strongly depends on the crystal-field effect of the host lattice, whereas the defects produced or improved by inequivalent doping serve as auxiliary centers to control the phosphorescence intensity and persistence time.[3b, 4, 5b, 10] However, apart from the co-doped cases, the long afterglow emission is usually observed in samples with only one activator.[11] In the forbidden band of such phosphors there are some discrete energy levels, which are pertinent to defects of the host responsible for the emission. One possible explanation for the long phosphorescence is that the trap carriers during the excitation process are generated at certain flaw sites and thermal excitation promotes the occurrence of their detrapping action at room temperature followed by energy transfer from the traps to the emissive centers, leading to the afterglow emission. In these phosphors, not only do the doped ions act as activators, but they serve as charge traps. It is known that special structures of the energy levels of some lanthanide elements make them feasible as as activators of LAPs,[12] such as europium, cerium, praseodymium, samarium, terbium, thulium,
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Full Paper and ytterbium. As a matter of fact, europium is the most popular activator for LAPs and cerium, praseodymium, terbium, samarium, and ytterbium have also been observed as luminescent centers of LAPs.[13] However, few reports on Tm3 + -doped LAPs have been published so far; this may be due to the difficulty in finding a proper host for Tm3 + ions to yield phosphorescence. Although many aluminates and silicates for LAPs have been reported,[3–5, 7] the Ca2SnO4 host has attracted more and more attention. Calcium tin oxide belongs to the orthorhombic structure with the Pbam space group,[14] which enables Ca2SnO4 to be a suitable matrix. Furthermore, Ropp studied the SnO44¢ anion and reported that it was an appropriate candidate for the host material of a phosphor.[15] In addition, recently some Ca2SnO4-based LAPs have been prepared, such as Ca2SnO4 :Eu3 + [11b] and Ca2SnO4 :Sm3 + [16] phosphors. Meanwhile, there have not been any reports on LAPs based on the Tm3 + -doped Ca2SnO4 host. Herein, we report a novel bluish green long phosphorescent material, Ca2SnO4 :Tm3 + , and further study the corresponding afterglow properties and possible mechanism with spectroscopic measurements.
Results and Discussion Phase Composition Figure 1 a shows representative XRD patterns of Ca2SnO4 :x Tm3 + (CS:x Tm3 + ; x = 0.01, 0.02, 003, and 0.04) samples, along with the standard data of Ca2SnO4. It is clear from Figure 1 a that all XRD patterns of the samples can be precisely indexed to the standard peaks of Ca2SnO4 (JCPDS no. 46-0112), which indicates that the doped Tm3 + ions have been successfully incorporated into the Ca2SnO4 host and do not generate any notable impurities. With respect to site occupancy, because of the similar radius of the Tm3 + (0.99 æ) and Ca2 + (1.06 æ) ions, compared with the Sn4 + (0.69 æ) ion, under the same coordination environment, we think that the doped Tm3 + ions tend to substitute at the sites of Ca2 + ions. Figure 1 b presents the experimental (black crosses), calculated (red dot lines), and difference (green solid lines) XRD profiles for the Rietveld refinement of the Ca2SnO4 matrix performed by using the general structure analysis system (GSAS) program.[17] The refinement finally converges to c2 = 7.924, Rwp = 11.05 %, and Rp = 8.65 %. The results further verify the phase purity of the sample and also show that Ca2SnO4 has an orthorhombic unit cell with the Pbam space group[14] and cell parameters of a = 5.753223 æ, b = 9.703403 æ, c = 3.267144 æ, and cell volume (V) = 182.391 æ3. Photoluminescence Properties of Ca2SnO4 :Tm3 + Figure 2 a exhibits the PLE spectrum of the obtained CS:0.02 Tm3 + phosphor monitored at l = 387 nm. The excitation spectrum consists of two parts. One is a broad band centered at l = 276 nm, which is due to the host charge-transfer state (CTS),[11b] whereas the other band centered at l = 339 nm is attributed to the electron transition from the ground state Chem. Asian J. 2015, 10, 2361 – 2367
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Figure 1. a) XRD patterns of CS:x Tm3 + (x = 0.01, 0.02, 0.03, and 0.04) phosphors and standard data for Ca2SnO4 (JCPDS no. 46-0112) as a reference. b) Experimental (crosses) and calculated (red dot lines) powder XRD patterns of the Ca2SnO4 matrix. The green solid lines represent the difference between experimental and calculated data and the blue bars mark the Bragg reflections.
Figure 2. Photoluminescence excitation (PLE; a) and photoluminescence (PL; b) spectra of Ca2SnO4 :0.02 Tm3 + with the corresponding transitions of Tm3 + ions for the emissive peaks, as well as the PL spectrum (c) of undoped Ca2SnO4. The inset presents the pure emission spectrum of Tm3 + obtained by subtracting the host lattice emission from the emission spectrum of Ca2SnO4 :0.02 Tm3 + .
3
H6 of Tm3 + ions to the excited 1D2 of Tm3 + ions. In addition, there is a shoulder band at l = 260 nm, which originates from the 3H6 !3P2 transition of Tm3 + ions. Figure 2 b illustrates the PL spectrum of the CS:0.02 Tm3 + sample excited at l = 274 nm.
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Full Paper The strongest band at l = 351 nm is the organic combination of the emission from the Ca2SnO4 matrix and the 1D2 !3H6 transition of the dopant Tm3 + ions. The PL spectrum presents characteristic emission bands located at l = 467, 482, 494, 653, 664, 676, and 685 nm, which are due to the transitions of 1 G4 !3H6, 3P0 !3F2, 3P6 !3F3, 1D2 !3H4, 3P2 !3H4, 3P0 !1G4, and 3 F2 !3H6, respectively,[18] as portrayed in Figure 2. Because the l = 340 nm filter is adopted during the measurement process to better record the spectrum, there is a sharp edge of the emission spectrum at l = 340 nm. In addition, the shape and band distribution of the PL emissions for all phosphors doped by Tm3 + ions are similar to that of the CS:0.02 Tm3 + sample, which was exemplified to show the emissive properties of CS:x Tm3 + phosphors. To fully investigate the relationship between the PL and afterglow emission of Tm3 + ions, we studied the optical properties of the host Ca2SnO4. Figure 2 c displays the emission of the Ca2SnO4 host excited at l = 274 nm, based on which we obtained Figure 2 d by subtracting the host emission from the PL spectrum of the CS:0.02 Tm3 + sample, namely, the difference between Figure 2 b and c, as portrayed in the inset of Figure 2, which further attests to the PL emission from the characteristic f–f transitions of Tm3 + ions.
after the removal of the excitation source, we obtained the chromaticity coordinates of the CS:0.01 Tm3 + sample: (0.114, 0.155). From the CIE 1931 Diagram, it can be seen that the afterglow emission is located in the bright bluish green region. Long Afterglow Decay Properties of Ca2SnO4 :Tm3 + The long afterglow decay curves of CS:x Tm3 + samples were measured to clearly investigate the phosphorescence lifetime of the phosphors monitored at three different wavelengths (l = 368, 482, and 663 nm) and irradiated at l = 254 nm. Among all synthesized samples, the one with the Tm3 + concentration of x = 0.01 displays the best long, persistent emission, which can be detected with considerable intensity by using the F-7000 spectrometer for 5 h after removal of irradiation at l = 254 nm. Hence, the decay curves of the CS:0.01 Tm3 + phosphor were taken as an example to explore the long afterglow properties (Figure 4). Each decay curve of the long phosphorescence emission was fitted, and all the fitting results confirmed the third exponential decay process, and obeyed the decay function given by Equation (1): I ¼ A1 exp
Long Afterglow Properties of Ca2SnO4 :Tm3 + The highlight of this work is that we observed the bright bluish green long afterglow emission of CS:x Tm3 + samples after the removal of excitation with a UV lamp. Figure 3 demonstrates the long afterglow spectra of CS:x Tm3 + , which was first irradiated at l = 254 nm for 5 min before the measurements were taken. The profile shows that the shapes and band positions of the long afterglow spectra resemble those of the PL spectrum shown in Figure 2 d, except for the emission intensity; this seems to confirm that the long phosphorescence of CS:x Tm3 + is ascribed to the f–f transitions of Tm3 + ions, and that the host emission makes no contribution to the generation of the long afterglow emission of the samples.[18] From the results shown in Figure 3, the CS:0.01 Tm3 + sample shows the strongest intensity among the synthesized phosphors. On the basis of the long, persistent emission spectra measured
¢t ¢t ¢t þ A2 exp þ A3 exp t1 t2 t3
ð1Þ
in which I is the phosphorescence intensity; A1, A2, and A3 represent constants; t is the time; and t1, t2, and t3 are the decay constants of the exponential components. The decay times of the long afterglow emission were calculated and are shown in Table 1. The decay times of the three emission bands are different. In view of the short luminescence decay lifetimes of f–f transitions of Tm3 + ions, the observed long-lasting phosphorescence emission of the phosphor could be ascribed to energy exchange processes between traps or traps and emission centers derived from the Tm3 + dopants. Owing to various kinds of trapping centers, defects, and energy levels of the Tm3 + ions, and the complicated and obscure energy exchange processes, however, concrete reasons for the phenomenon of different decay times cannot be provided under the present experimental conditions. In addition, the bright bluish green long afterglow could last for more than 5 h, as observed by the naked eye in the dark after the excitation source was switched off. Thermoluminescence properties of Ca2SnO4 :Tm3 +
Figure 3. The long afterglow spectra of CS:x Tm3 + (0 x 0.04) irradiated for 5 min under l = 254 nm excitation. Chem. Asian J. 2015, 10, 2361 – 2367
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Generally, thermoluminescence (TL) spectroscopy is a practically useful tool for exploring defects and traps produced in insulators or semiconductors by UV light and used for energy storage in LAPs.[19] Therefore, we measured the TL spectra of CS:x Tm3 + phosphors with a 3D-TL spectrometer under the same conditions, and the CS:0.01 Tm3 + sample was used to exemplify the TL emission properties. The 3D-TL spectrum is depicted in Figure 5, in which the TL intensity is plotted against the temperature and wavelength. The spectrum illustrates the TL emission at divergent temperatures, as well as TL curves at different wavelengths. The shape and position rather than the
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Figure 4. The long afterglow decay curves of the CS:0.01 Tm3 + sample monitored at l = 368 (a), 482 (b), and 663 nm (c) under excitation at l = 254 nm.
Figure 6 shows the TL curve of the CS:0.01 Tm3 + phosphor. It can be observed that the CS:0.01 Tm3 + phosphor demonstrates a wide TL band. To comprehensively understand the TL curve, the picture was first deconvoluted based on the Gaussian function by the curve-fitting technique and finally was found to be composed of two broad bands at 349 and 325 K. Usually, the position of the TL band to some extent implies the depth of the trap. A higher TL peak temperature results in a deeper trap. An appropriate depth of trap is necessary for a potential long afterglow material. If the depth of the trap is too low, the trapped charges are easily released, which results in a short persistence time and fast decay of the long phosphorescence emission. On the other hand, if it
Table 1. The decay times of the long afterglow emission from the CS:0.01 Tm3 + phosphor calculated from Equation (1).
lem [nm]
A1
t1 [s]
A2
t2 [s]
A3
t3 [s]
368 482 663
2080.2 2756.7 188.4
19.3 15.0 13.9
535.6 638.1 845.1
108.8 91.7 88.3
2820.6 4588.0 1545.4
4.2 3.0 2.7
Figure 6. The TL curve of CS:0.01 Tm3 + phosphor and the corresponding fitting results.
Figure 5. 3D-TL spectrum of the CS:0.01 Tm3 + sample taken as an example to show the TL properties.
intensity from the TL spectrum at disparate temperatures for the CS:0.01 Tm3 + phosphor are analogous to the corresponding ones of the long afterglow emission spectra shown in Figure 3; this substantiates that the TL emission of the sample is due to f–f transitions of the Tm3 + ions. Likewise, the shapes of TL curves at different wavelengths are identical. For further analysis, the TL curve plotted with TL intensity versus temperature is discussed in the following part. Chem. Asian J. 2015, 10, 2361 – 2367
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is too deep for the captured charges to escape at room temperature, no long afterglow light will be observed by the naked eye. According to our previous investigation, TL bands from 283 to 383 K indicate the appropriate depth of traps for the occurrence of long, persistent emission. In addition, TL peaks close to (or above) room temperature are generally expected to be crucial for the long afterglow emission.[20] Therefore, both the 349 and 325 K TL bands are consistent with a depth range suitable for a long, persistent emission at room temperature. In addition, the two different TL bands make disparate contributions to the long afterglow emission at various times. Both bands promote the occurrence of the long phosphorescent emission right after the end of excitation. However, as time passes, the TL band at 349 K continues to contribute the afterglow emission, while the 325 K band has been extin-
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Full Paper guished. Generally speaking, the traps of a phosphor as a storage material should be relatively deep to restrain the thermally stimulated release of trapped carriers, whereas the traps of the afterglow phosphor should be shallower.[21] Considering this point, it is believed that the Ca2SnO4 :Tm3 + phosphor could fulfill the requirements of both a light-stimulated (storage) phosphor and a long afterglow one. In addition, the depth of traps in the phosphor can also be obtained by the fitted TL curves of CS:x Tm3 + phosphors on the basis of Chen’s general order function given by Equations (2) and (3):[22] £ ¨ ¦¡ kB Tm2 E ¼ 2:52 þ 10:2 mg ¢ 0:42 ¢ 2kB Tm w n0 ¼
ð2Þ
wIm b½2:52 þ 10:2ðmg ¢ 0:42Þ¤
ð3Þ
In Equation (2), E is the activation energy, which indicates the depth of the trap; kB is the Boltzmann constant (1.38 Õ l0¢23 J K¢1); w is the full-width at half-maximum (FWHM); d is the high-temperature half-width; t is the low-temperature half-width; mg represents the geometrical form factor and is ded fined as mg ¼ w and w ¼ d þ t; Tm is the temperature of the glow peaks; and b is the heating rate (here, b = 1 K s¢1). In Equation (3), n0 is the concentration of trapped charges at t = 0, which greatly affects the luminescent intensity; Im is the TL intensity of the glow peaks. The estimated values of E and n0 are listed in Table 2. It is reported that the depth of the trap at 0.4–0.8 eV better matches the requirements for excellent persistent afterglow performance.[23] Therefore, the as-prepared phosphors have suitable trap depths, which are the derivation of the long afterglow phenomenon.
Table 2. Calculated results of trap depths and the TL curve parameters of CS:0.01 Tm3 + .
Sample
Peak 1 E [eV]
CS:0.01Tm3 + 0.45
Peak 1 n0 [(cm3)¢1]
Peak 2 E [eV]
Peak 2 n0 [(cm3)¢1]
6.15 Õ 106
0.78
6.51 Õ 106
With respect to the principle of the long afterglow emission, the common perspective is that during the excitation period the energy of the incident light is partly stored in the form of captured charges, which can then be thermally released from the traps to bring about the characteristic emissions of the luminescence ions. In addition, the long duration of the release of captured electrons and traps endows LAPs with the long persistence lifetime. The dopants substitute ions that have similar radii, leading to yield defects. These deficiencies serve as hole or electron traps to capture holes or electrons in the excitation process and provide the lasting energy to activated ions for the long afterglow emission. For the CS:x Tm3 + phosphors, considering the previous discussion, the doped Tm3 + ions would occupy the sites of Ca2 + ions rather than Sn4 + ions [Reaction (1)]: Chem. Asian J. 2015, 10, 2361 – 2367
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TmCa $ TmC Ca þ e0
ðR1Þ
Theoretically, the charge compensation could be realized by two possible pathways. One pathway is electronic compensation. Some Sn4 + ions could capture liberated electrons [Eq. (1)] and then be converted into Sn2 + or Sn3 + ions, which formed structure defects and were treated as sites for the localization of charge carries (electrons)[24] and played the role of hole traps. The other approach is vacancy compensation. The Tm3 + -related defects are TmCCa and V’’Ca generated by the inequivalent substitution [Reaction (2)]: 2 Tm3þ þ 3 Ca2þ ! 2 TmC Ca þ V00 Ca
ðR2Þ
Three bivalent Ca2 + ions were replaced by two trivalent Tm3 + ions, producing V’’Ca for charge compensation. The positively charged substitution deficiency TmCCa may serve as electron-trapping centers and could capture electrons, whereas the negatively charged defects V’’Ca, Sn3 + , and Sn2 + generated by the incorporation of the Tm3 + dopants may act as holetrapping sites and possess the ability to capture holes.[25] However, given the special orthorhombic structure of the Ca2SnO4 host and the central site of Sn4 + ions located in the inner SnO6 octahedron and surrounded by six oxygen atoms, it is believed that it is difficult for Sn4 + ions to change into Sn3 + and Sn2 + ions. Therefore, we maintain that charge compensation is ascribed to vacancy compensation and that the Tm3 + ion is not only the provider of traps, but also an activator itself. In addition, there are many other intrinsic defects in the phosphors, such as the oxygen vacancies, which do play a role in the generation of the long afterglow emission. However, according to studies by Jia et al. in 2014[8] and Xu et al. in 2010,[16b] the oxygen vacancies are largely produced under vacuum conditions during the synthetic process. Because the as-produced samples were synthesized in air, the number of the oxygen vacancies was small. Therefore, on the basis of their discussions on the influence of oxygen vacancies on phosphorescence, the effect of oxygen vacancies in our work may not be dominant in comparison with that of the Tm3 + ion related defects and the long-lasting phosphorescence is mainly assigned to Tm3 + ion related defects. The mechanism of the long phosphorescence emission of Ca2SnO4 :Tm3 + can be interpreted by using the graph depicted in Figure 7. After irradiation with a UV lamp (process a in Figure 7), some of the excitation energy will be transferred through the host directly to the luminescence centers of the Tm3 + ions, followed by the characteristic emission from the f–f transitions of Tm3 + ions in the form of visible light (process b in Figure 7). However, the trapping centers could store part of the excitation energy (process c in Figure 7), rather than let it directly return to the ground states. Subsequently, the captured energy will be released from the trapping centers under thermal stimulation at the appropriate temperature, and would transfer through the host to Tm3 + ions (process d in Figure 7), followed by the recombination of trapping centers and leading to the characteristic emissions of Tm3 + ions as the long afterglow emissions owing to the slow release speed of
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Full Paper Synthesis A variety of CS:x Tm3 + (x = 0–0.04) LAPs were synthesized with traditional high-temperature solid-state reactions. The starting materials CaCO3, SnO2, and Tm2O3 were weighted under a stoichiometric ratio. The raw materials were mixed and ground in an agate mortar thoroughly, and the homogenous mixtures were transferred to an alumina crucible and sintered at 1450 8C for 5 h in air. After cooling to room temperature naturally, the as-obtained phosphors were ground into powders for subsequent measurements.
Characterization
Figure 7. Schematic graph of the mechanism for the long, persistent emission of Ca2SnO4 :Tm3 + samples. CB = conduction band, VB = valence band.
the transfer of energy from the traps. However, all processes discussed above are a general description for the occurrence of the long afterglow emissions of CS:Tm3 + phosphors. Details of the mechanism remain vague. Therefore, there is a long way to go before we can completely interpret how the long phosphorescence emission is generated.
Conclusion A series of novel LAPs Ca2SnO4 :x Tm3 + were prepared with conventional high-temperature solid-state reactions. XRD measurements and Rietveld refinement demonstrated that the incorporation of the Tm3 + dopants generated no notable impurities, which indicated that the dopants merged completely into the host. The corresponding optical properties were systematically studied by using the PL, phosphorescence, and TL spectra. The results illustrated that Tm3 + -related defects were responsible for the bright bluish green afterglow emission from characteristic f–f transitions of Tm3 + ions. The radiant bluish green long-lasting phosphorescence could be observed by the naked eye and lasted for 5 h, as observed by the naked eye in a dark environment after removal of the UV light source. Two TL bands at 325 and 349 K from the TL curve responsible for the emission were adopted to calculate the depth of the traps, which were 0.45 and 0.78 eV, respectively. The mechanism of the long-lasting phosphorescence emission was also studied schematically.
The XRD profiles for phase identification were collected by using a D8 Focus diffractometer operating at 40 kV and 40 mA with graphite-monochromated CuKa radiation (l = 0.15405 nm). In the process, the scanning rate was 108 min¢1 with 2q = 10–758. The PLE and PL spectra of the phosphors were measured by using a Hitachi F-7000 spectrophotometer with an excitation source of a 150 W xenon lamp and a l = 340 nm filter. The long afterglow emission spectra and decay curves were also measured by using the same instrument after the samples were irradiated under l = 254 nm UV light for 5 min. The 3D-TL and phosphorescence emission spectra at different temperatures were measured with a homemade spectrometer mainly consisting of a charge-coupled device (CCD) detector and a heating apparatus. The samples were placed in a homemade sample holder and heated from RT to 573 K at a speed of 1 K s¢1, and data for the TL curves was obtained from the 3D-TL emission spectra with a data transferring technique by using computer software. All measurements were performed at RT, except those for the TL curves.
Acknowledgements We are grateful for financial aid from the Hong Kong, Macao and Taiwan Science and Technology Cooperation Special Project of Ministry of Science and Technology of China (grant no. 2014DFT10310), the Program of Science and Technology Development Plan of Jilin Province of China (grant no. 20140201007GX), the National Basic Research Program of China (973 Program, grant no.2014CB643801), and the National Natural Science Foundation of China (grant nos. 51102229, 51402288, 21401184). Keywords: doping · lanthanides · luminescence · phosphors · photochemistry
Experimental Section Chemicals All chemical reagents were used as received without further purification. Rare-earth oxide Tm2O3 (A.R., 99.99 %) was purchased from the Science and Technology Parent Company of the Changchun Institute of Applied Chemistry (P.R. China). CaCO3 (A.R., 99.9 %) and SnO2 (A.R., 99.9 %) were purchased from Beijing Chemical Works. A high-temperature furnace was purchased from Hefei Ke Jing Materials Technology Co. Ltd. Chem. Asian J. 2015, 10, 2361 – 2367
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[1] a) F. I. Vergunas, N. L. Gasting, Sov. Phys. JETP 1955, 1, 284; b) N. L. Gasting, Opt. Spektrosk. 1957, 3, 624; c) W. M. Yen, S. Shionoya, H. Yamamoto, Phosphor Handbook, CRC Press, Boca Raton, FL, 1995; d) R. C. Ropp, Luminescence and the Solid State, 2nd ed. Elsevier, Amsterdam, 2004. [2] a) T. Matsuzawa, Y. Aoki, N. Takeuchi, Y. Murayama, J. Electrochem. Soc. 1996, 143, 2670; b) J. Qiu, K. Miura, H. Inouye, Y. Kondo, T. Mitsuyu, K. Hirao, Appl. Phys. Lett. 1998, 73, 1763; c) J. Qiu, K. Hirao, Solid State Commun. 1998, 106, 795; d) H. Lin, B. Wang, J. Xu, R. Zhang, H. Chen, Y. Yu, Y. Wang, ACS Appl. Mater. Interfaces 2014, 6, 21264. [3] a) T. Aitasalo, P. Deren´, J. Hçls, H. Jungner, J. C. Krupa, M. Lastusaari, J. Legendziewicz, J. Niittykoski, W. Stre˛k, J. Solid State Chem. 2003, 171, 114; b) H. N. Luitel, T. Watari, R. Chand, T. Torikai, M. Yada, J. Mater. 2013, 613090. [4] P. Dorenbos, Phys. Status Solidi B 2005, 242, R7. [5] a) C. Chang, J. Xu, L. Jiang, D. Mao, W. Ying, Mater. Chem. Phys. 2006, 98, 509; b) A. H. Wako, B. F. Dejene, H. C. Swart, Phys. B 2014, 439, 153. Ó 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Full Paper [6] R. Pang, Y. Jia, R. Zhao, H. Li, J. Fu, W. Sun, L. Jiang, S. Zhang, C. Li, Q. Su, Dalton Trans. 2014, 43, 9661. [7] P. Wang, X. Xu, D. Zhou, X. Yu, J. Qiu, Inorg. Chem. 2015, 54, 1690. [8] Y. Jia, H. Li, R. Zhao, W. Sun, Q. Su, R. Pang, C. Li, Opt. Mater. 2014, 36, 1781. [9] W. Sun, Y. Jia, R. Zhao, H. Li, J. Fu, L. Jiang, S. Zhang, R. Pang, C. Li, Opt. Mater. 2014, 36, 1841. [10] T. Aitasalo, J. Hçls, H. Jungner, M. Lastusaari, J. Niittykoski, J. Phys. Chem. B 2006, 110, 4589. [11] a) R. Pang, C. Li, L. Jiang, Q. Su, J. Alloys Compd. 2009, 471, 364; b) T. Ishigaki, A. Torisaka, K. Nomizu, P. Madhusudan, K. Uematsu, K. Toda, M. Sato, Dalton Trans. 2013, 42, 4781; c) Z. Pan, Y.-Y. Lu, F. Liu, Nat. Mater. 2012, 11, 58. [12] S. Shionoya, W. M. Yen, Phosphor Handbook, CRC Press, Boca Raton, FL, 1999, pp. 88 and 179. [13] a) D. Jia, X. J. Wang, W. Jia, W. M. Yen, J. Appl. Phys. 2003, 93, 148; b) C. Li, J. Wang, H. Liang, Q. Su, J. Appl. Phys. 2007, 101, 113304; c) B. Lei, Y. Liu, G. Tang, Z. Ye, C. Shi, Mater. Chem. Phys. 2004, 87, 227. [14] M. Trçmel, Z. Anorg. Allg. Chem. 1969, 371, 237. [15] R. C. Ropp, Luminescence and the Solid State: Studies in Inorganic Chemistry 21, Elsevier, Amsterdam, 1991, vol. 12, p. 21. [16] a) Z. Ju, R. Wei, J. Zheng, X. Gao, S. Zhang, W. Liu, Appl. Phys. Lett. 2011, 98, 121906; b) X. Xu, Y. Wang, Y. Gong, W. Zeng, Y. Li, Opt. Express 2010, 18, 16989.
Chem. Asian J. 2015, 10, 2361 – 2367
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[17] A. C. Larson, R. B. V. Dreele, Los Alamos Natl. Lab. 1994, 1. [18] W. T. Carnall, G. L. Goodman, K. Rajnak, R. S. Rana, J. Chem. Phys. 1989, 90, 3443. [19] a) D. W. Cooke, B. L. Bennett, R. E. Muenchausen, J. K. Lee, M. A. Nastasi, J. Lumin. 2004, 106, 125; b) J. Glodo, A. J. Wojtowicz, J. Alloys Compd. 2000, 300 – 301, 289. [20] J. Trojan-Piegza, J. Niittykoski, J. Hçls, E. Zych, Chem. Mater. 2008, 20, 2252. [21] S. Schweizer, Phys. Status Solidi A 2001, 187, 335. [22] R. Chen, J. Electrochem. Soc. 1969, 116, 1254. [23] a) J. Kuang, Y. Liu, J. Electrochem. Soc. 2006, 153, G245; b) R. Sakai, T. Katsumata, S. Komuro, T. Morikawa, J. Lumin. 1999, 85, 149. [24] U. Shail, P. Om, K. Devendra, J. Phys. D 2004, 37, 1483. [25] a) F. Clabau, X. Rocquefelte, T. Le Mercier, P. Deniard, S. Jobic, M. H. Whangbo, Chem. Mater. 2006, 18, 3212; b) Y. Chen, X. Cheng, M. Liu, Z. Qi, C. Shi, J. Lumin. 2009, 129, 531.
Manuscript received: June 4, 2015 Accepted Article published: July 16, 2015 Final Article published: August 26, 2015
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Doping
Investigation on Luminescence Properties of a Long Afterglow Phosphor Ca2SnO4 :Tm3 + Hai-Feng Li,[a, b] Wen-Zhi Sun,[a, b] Yong-Lei Jia,[a, b] Teng-Fei Ma,[a] Ji-Peng Fu,[a, b] Da Li,[a] Su Zhang,[a] Li-Hong Jiang,[a] Ran Pang,*[a] and Cheng-Yu Li*[a] Abstract: A series of new long afterglow phosphors Ca2SnO4 :x Tm3 + were synthesized by using traditional solidstate reactions. XRD measurements and Rietveld refinement revealed that the incorporation of the Tm3 + dopants generated no second phase other than the original one of Ca2SnO4, which indicated that the dopants completely merged into the host. The corresponding optical properties were further systematically studied by photoluminescence, phosphorescence, and thermoluminescence (TL) spectrosco-
Introduction Long afterglow phosphors (LAPs) are materials that can absorb either visible or UV light and subsequently release the stored energy as persistent visible light in ambient temperature.[1] Based on their inherent properties of energy storage, this kind of material has been widely applied to practical domains, such as the detection of high-energy irradiation, emergency illumination and display fields, three-dimensional optical memory and image storage, bioimaging, and alternating-current (AC) white light-emitting diodes (LEDs).[2] Generally, the long afterglow properties are closely related to defects in the phosphors, which can trap electrons or holes during the excitation process, and then release them under thermal stimulation to recombine automatically as emission centers, resulting in phosphorescence emissions. The production of appropriate defects in the host of a phosphor and improvements in the capability of storing the incident energy are mainly realized through inequivalent substitution, for example, doped ions as luminescence centers, especially lanthanide ions or transition-metal ions, would occupy the sites of matrix ions, which have different valence. In addition, to enhance the optic intensity of [a] Dr. H.-F. Li, Dr. W.-Z. Sun, Dr. Y.-L. Jia, Dr. T.-F. Ma, Dr. J.-P. Fu, Dr. D. Li, Dr. S. Zhang, Dr. L.-H. Jiang, Dr. R. Pang, Prof. C.-Y. Li State Key Laboratory of Rare Earth Resource Utilization Changchun Institute of Applied Chemistry Chinese Academy of Sciences Changchun 130022 (P. R. China) E-mail: [email protected] [email protected] [b] Dr. H.-F. Li, Dr. W.-Z. Sun, Dr. Y.-L. Jia, Dr. J.-P. Fu University of Chinese Academy of Sciences Beijing 100049 (P. R. China) Chem. Asian J. 2015, 10, 2361 – 2367
py. The results show that the Tm3 + -related defects account for the bright bluish green afterglow emission from the characteristic f–f transitions of Tm3 + ions. The bluish green long-lasting phosphorescence could be observed for 5 h by the naked eye in a dark environment after the end of UV irradiation. Two TL peaks at 325 and 349 K from the TL curves were adopted to calculate the depth of the traps, which were 0.45 and 0.78 eV, respectively. The mechanism of the long afterglow emission was also explored.
LAPs, another kind of ion is usually incorporated into the host as an assistant activator to modify the defects and improve the phosphorescence properties. Given the above description, the colors of LAPs are determined by the characteristics of the emissive centers, whereas the intensity and persistent time are regulated by the defects created or improved by inequivalent doping. With this method, a large number of LAPs, such as SrAl2O4 :Eu2 + ,Dy3 + ;[3] Sr2MgSi2O7:Eu2 + ,Dy3 + ;[4] CaAl2O4 :Eu2 + ,Nd3 + ;[5] Zn2P2O7:Tm3 + ,Mn2 + ;[6] Ba5Si8O21:Eu2 + ,Dy3 + ;[7] Ca9Bi(PO4)7:Eu2 + ,Dy3 + ;[8] and Ca14Mg2(SiO4)8 :Eu2 + ,Dy3 + ,[9] have been successfully synthesized. For those phosphors, the colors of the long-lasting phosphorescence are derived from luminescent centers of either Eu2 + or Mn2 + dopants, and strongly depends on the crystal-field effect of the host lattice, whereas the defects produced or improved by inequivalent doping serve as auxiliary centers to control the phosphorescence intensity and persistence time.[3b, 4, 5b, 10] However, apart from the co-doped cases, the long afterglow emission is usually observed in samples with only one activator.[11] In the forbidden band of such phosphors there are some discrete energy levels, which are pertinent to defects of the host responsible for the emission. One possible explanation for the long phosphorescence is that the trap carriers during the excitation process are generated at certain flaw sites and thermal excitation promotes the occurrence of their detrapping action at room temperature followed by energy transfer from the traps to the emissive centers, leading to the afterglow emission. In these phosphors, not only do the doped ions act as activators, but they serve as charge traps. It is known that special structures of the energy levels of some lanthanide elements make them feasible as as activators of LAPs,[12] such as europium, cerium, praseodymium, samarium, terbium, thulium,
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Full Paper and ytterbium. As a matter of fact, europium is the most popular activator for LAPs and cerium, praseodymium, terbium, samarium, and ytterbium have also been observed as luminescent centers of LAPs.[13] However, few reports on Tm3 + -doped LAPs have been published so far; this may be due to the difficulty in finding a proper host for Tm3 + ions to yield phosphorescence. Although many aluminates and silicates for LAPs have been reported,[3–5, 7] the Ca2SnO4 host has attracted more and more attention. Calcium tin oxide belongs to the orthorhombic structure with the Pbam space group,[14] which enables Ca2SnO4 to be a suitable matrix. Furthermore, Ropp studied the SnO44¢ anion and reported that it was an appropriate candidate for the host material of a phosphor.[15] In addition, recently some Ca2SnO4-based LAPs have been prepared, such as Ca2SnO4 :Eu3 + [11b] and Ca2SnO4 :Sm3 + [16] phosphors. Meanwhile, there have not been any reports on LAPs based on the Tm3 + -doped Ca2SnO4 host. Herein, we report a novel bluish green long phosphorescent material, Ca2SnO4 :Tm3 + , and further study the corresponding afterglow properties and possible mechanism with spectroscopic measurements.
Results and Discussion Phase Composition Figure 1 a shows representative XRD patterns of Ca2SnO4 :x Tm3 + (CS:x Tm3 + ; x = 0.01, 0.02, 003, and 0.04) samples, along with the standard data of Ca2SnO4. It is clear from Figure 1 a that all XRD patterns of the samples can be precisely indexed to the standard peaks of Ca2SnO4 (JCPDS no. 46-0112), which indicates that the doped Tm3 + ions have been successfully incorporated into the Ca2SnO4 host and do not generate any notable impurities. With respect to site occupancy, because of the similar radius of the Tm3 + (0.99 æ) and Ca2 + (1.06 æ) ions, compared with the Sn4 + (0.69 æ) ion, under the same coordination environment, we think that the doped Tm3 + ions tend to substitute at the sites of Ca2 + ions. Figure 1 b presents the experimental (black crosses), calculated (red dot lines), and difference (green solid lines) XRD profiles for the Rietveld refinement of the Ca2SnO4 matrix performed by using the general structure analysis system (GSAS) program.[17] The refinement finally converges to c2 = 7.924, Rwp = 11.05 %, and Rp = 8.65 %. The results further verify the phase purity of the sample and also show that Ca2SnO4 has an orthorhombic unit cell with the Pbam space group[14] and cell parameters of a = 5.753223 æ, b = 9.703403 æ, c = 3.267144 æ, and cell volume (V) = 182.391 æ3. Photoluminescence Properties of Ca2SnO4 :Tm3 + Figure 2 a exhibits the PLE spectrum of the obtained CS:0.02 Tm3 + phosphor monitored at l = 387 nm. The excitation spectrum consists of two parts. One is a broad band centered at l = 276 nm, which is due to the host charge-transfer state (CTS),[11b] whereas the other band centered at l = 339 nm is attributed to the electron transition from the ground state Chem. Asian J. 2015, 10, 2361 – 2367
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Figure 1. a) XRD patterns of CS:x Tm3 + (x = 0.01, 0.02, 0.03, and 0.04) phosphors and standard data for Ca2SnO4 (JCPDS no. 46-0112) as a reference. b) Experimental (crosses) and calculated (red dot lines) powder XRD patterns of the Ca2SnO4 matrix. The green solid lines represent the difference between experimental and calculated data and the blue bars mark the Bragg reflections.
Figure 2. Photoluminescence excitation (PLE; a) and photoluminescence (PL; b) spectra of Ca2SnO4 :0.02 Tm3 + with the corresponding transitions of Tm3 + ions for the emissive peaks, as well as the PL spectrum (c) of undoped Ca2SnO4. The inset presents the pure emission spectrum of Tm3 + obtained by subtracting the host lattice emission from the emission spectrum of Ca2SnO4 :0.02 Tm3 + .
3
H6 of Tm3 + ions to the excited 1D2 of Tm3 + ions. In addition, there is a shoulder band at l = 260 nm, which originates from the 3H6 !3P2 transition of Tm3 + ions. Figure 2 b illustrates the PL spectrum of the CS:0.02 Tm3 + sample excited at l = 274 nm.
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Full Paper The strongest band at l = 351 nm is the organic combination of the emission from the Ca2SnO4 matrix and the 1D2 !3H6 transition of the dopant Tm3 + ions. The PL spectrum presents characteristic emission bands located at l = 467, 482, 494, 653, 664, 676, and 685 nm, which are due to the transitions of 1 G4 !3H6, 3P0 !3F2, 3P6 !3F3, 1D2 !3H4, 3P2 !3H4, 3P0 !1G4, and 3 F2 !3H6, respectively,[18] as portrayed in Figure 2. Because the l = 340 nm filter is adopted during the measurement process to better record the spectrum, there is a sharp edge of the emission spectrum at l = 340 nm. In addition, the shape and band distribution of the PL emissions for all phosphors doped by Tm3 + ions are similar to that of the CS:0.02 Tm3 + sample, which was exemplified to show the emissive properties of CS:x Tm3 + phosphors. To fully investigate the relationship between the PL and afterglow emission of Tm3 + ions, we studied the optical properties of the host Ca2SnO4. Figure 2 c displays the emission of the Ca2SnO4 host excited at l = 274 nm, based on which we obtained Figure 2 d by subtracting the host emission from the PL spectrum of the CS:0.02 Tm3 + sample, namely, the difference between Figure 2 b and c, as portrayed in the inset of Figure 2, which further attests to the PL emission from the characteristic f–f transitions of Tm3 + ions.
after the removal of the excitation source, we obtained the chromaticity coordinates of the CS:0.01 Tm3 + sample: (0.114, 0.155). From the CIE 1931 Diagram, it can be seen that the afterglow emission is located in the bright bluish green region. Long Afterglow Decay Properties of Ca2SnO4 :Tm3 + The long afterglow decay curves of CS:x Tm3 + samples were measured to clearly investigate the phosphorescence lifetime of the phosphors monitored at three different wavelengths (l = 368, 482, and 663 nm) and irradiated at l = 254 nm. Among all synthesized samples, the one with the Tm3 + concentration of x = 0.01 displays the best long, persistent emission, which can be detected with considerable intensity by using the F-7000 spectrometer for 5 h after removal of irradiation at l = 254 nm. Hence, the decay curves of the CS:0.01 Tm3 + phosphor were taken as an example to explore the long afterglow properties (Figure 4). Each decay curve of the long phosphorescence emission was fitted, and all the fitting results confirmed the third exponential decay process, and obeyed the decay function given by Equation (1): I ¼ A1 exp
Long Afterglow Properties of Ca2SnO4 :Tm3 + The highlight of this work is that we observed the bright bluish green long afterglow emission of CS:x Tm3 + samples after the removal of excitation with a UV lamp. Figure 3 demonstrates the long afterglow spectra of CS:x Tm3 + , which was first irradiated at l = 254 nm for 5 min before the measurements were taken. The profile shows that the shapes and band positions of the long afterglow spectra resemble those of the PL spectrum shown in Figure 2 d, except for the emission intensity; this seems to confirm that the long phosphorescence of CS:x Tm3 + is ascribed to the f–f transitions of Tm3 + ions, and that the host emission makes no contribution to the generation of the long afterglow emission of the samples.[18] From the results shown in Figure 3, the CS:0.01 Tm3 + sample shows the strongest intensity among the synthesized phosphors. On the basis of the long, persistent emission spectra measured
¢t ¢t ¢t þ A2 exp þ A3 exp t1 t2 t3
ð1Þ
in which I is the phosphorescence intensity; A1, A2, and A3 represent constants; t is the time; and t1, t2, and t3 are the decay constants of the exponential components. The decay times of the long afterglow emission were calculated and are shown in Table 1. The decay times of the three emission bands are different. In view of the short luminescence decay lifetimes of f–f transitions of Tm3 + ions, the observed long-lasting phosphorescence emission of the phosphor could be ascribed to energy exchange processes between traps or traps and emission centers derived from the Tm3 + dopants. Owing to various kinds of trapping centers, defects, and energy levels of the Tm3 + ions, and the complicated and obscure energy exchange processes, however, concrete reasons for the phenomenon of different decay times cannot be provided under the present experimental conditions. In addition, the bright bluish green long afterglow could last for more than 5 h, as observed by the naked eye in the dark after the excitation source was switched off. Thermoluminescence properties of Ca2SnO4 :Tm3 +
Figure 3. The long afterglow spectra of CS:x Tm3 + (0 x 0.04) irradiated for 5 min under l = 254 nm excitation. Chem. Asian J. 2015, 10, 2361 – 2367
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Generally, thermoluminescence (TL) spectroscopy is a practically useful tool for exploring defects and traps produced in insulators or semiconductors by UV light and used for energy storage in LAPs.[19] Therefore, we measured the TL spectra of CS:x Tm3 + phosphors with a 3D-TL spectrometer under the same conditions, and the CS:0.01 Tm3 + sample was used to exemplify the TL emission properties. The 3D-TL spectrum is depicted in Figure 5, in which the TL intensity is plotted against the temperature and wavelength. The spectrum illustrates the TL emission at divergent temperatures, as well as TL curves at different wavelengths. The shape and position rather than the
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Figure 4. The long afterglow decay curves of the CS:0.01 Tm3 + sample monitored at l = 368 (a), 482 (b), and 663 nm (c) under excitation at l = 254 nm.
Figure 6 shows the TL curve of the CS:0.01 Tm3 + phosphor. It can be observed that the CS:0.01 Tm3 + phosphor demonstrates a wide TL band. To comprehensively understand the TL curve, the picture was first deconvoluted based on the Gaussian function by the curve-fitting technique and finally was found to be composed of two broad bands at 349 and 325 K. Usually, the position of the TL band to some extent implies the depth of the trap. A higher TL peak temperature results in a deeper trap. An appropriate depth of trap is necessary for a potential long afterglow material. If the depth of the trap is too low, the trapped charges are easily released, which results in a short persistence time and fast decay of the long phosphorescence emission. On the other hand, if it
Table 1. The decay times of the long afterglow emission from the CS:0.01 Tm3 + phosphor calculated from Equation (1).
lem [nm]
A1
t1 [s]
A2
t2 [s]
A3
t3 [s]
368 482 663
2080.2 2756.7 188.4
19.3 15.0 13.9
535.6 638.1 845.1
108.8 91.7 88.3
2820.6 4588.0 1545.4
4.2 3.0 2.7
Figure 6. The TL curve of CS:0.01 Tm3 + phosphor and the corresponding fitting results.
Figure 5. 3D-TL spectrum of the CS:0.01 Tm3 + sample taken as an example to show the TL properties.
intensity from the TL spectrum at disparate temperatures for the CS:0.01 Tm3 + phosphor are analogous to the corresponding ones of the long afterglow emission spectra shown in Figure 3; this substantiates that the TL emission of the sample is due to f–f transitions of the Tm3 + ions. Likewise, the shapes of TL curves at different wavelengths are identical. For further analysis, the TL curve plotted with TL intensity versus temperature is discussed in the following part. Chem. Asian J. 2015, 10, 2361 – 2367
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is too deep for the captured charges to escape at room temperature, no long afterglow light will be observed by the naked eye. According to our previous investigation, TL bands from 283 to 383 K indicate the appropriate depth of traps for the occurrence of long, persistent emission. In addition, TL peaks close to (or above) room temperature are generally expected to be crucial for the long afterglow emission.[20] Therefore, both the 349 and 325 K TL bands are consistent with a depth range suitable for a long, persistent emission at room temperature. In addition, the two different TL bands make disparate contributions to the long afterglow emission at various times. Both bands promote the occurrence of the long phosphorescent emission right after the end of excitation. However, as time passes, the TL band at 349 K continues to contribute the afterglow emission, while the 325 K band has been extin-
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Full Paper guished. Generally speaking, the traps of a phosphor as a storage material should be relatively deep to restrain the thermally stimulated release of trapped carriers, whereas the traps of the afterglow phosphor should be shallower.[21] Considering this point, it is believed that the Ca2SnO4 :Tm3 + phosphor could fulfill the requirements of both a light-stimulated (storage) phosphor and a long afterglow one. In addition, the depth of traps in the phosphor can also be obtained by the fitted TL curves of CS:x Tm3 + phosphors on the basis of Chen’s general order function given by Equations (2) and (3):[22] £ ¨ ¦¡ kB Tm2 E ¼ 2:52 þ 10:2 mg ¢ 0:42 ¢ 2kB Tm w n0 ¼
ð2Þ
wIm b½2:52 þ 10:2ðmg ¢ 0:42Þ¤
ð3Þ
In Equation (2), E is the activation energy, which indicates the depth of the trap; kB is the Boltzmann constant (1.38 Õ l0¢23 J K¢1); w is the full-width at half-maximum (FWHM); d is the high-temperature half-width; t is the low-temperature half-width; mg represents the geometrical form factor and is ded fined as mg ¼ w and w ¼ d þ t; Tm is the temperature of the glow peaks; and b is the heating rate (here, b = 1 K s¢1). In Equation (3), n0 is the concentration of trapped charges at t = 0, which greatly affects the luminescent intensity; Im is the TL intensity of the glow peaks. The estimated values of E and n0 are listed in Table 2. It is reported that the depth of the trap at 0.4–0.8 eV better matches the requirements for excellent persistent afterglow performance.[23] Therefore, the as-prepared phosphors have suitable trap depths, which are the derivation of the long afterglow phenomenon.
Table 2. Calculated results of trap depths and the TL curve parameters of CS:0.01 Tm3 + .
Sample
Peak 1 E [eV]
CS:0.01Tm3 + 0.45
Peak 1 n0 [(cm3)¢1]
Peak 2 E [eV]
Peak 2 n0 [(cm3)¢1]
6.15 Õ 106
0.78
6.51 Õ 106
With respect to the principle of the long afterglow emission, the common perspective is that during the excitation period the energy of the incident light is partly stored in the form of captured charges, which can then be thermally released from the traps to bring about the characteristic emissions of the luminescence ions. In addition, the long duration of the release of captured electrons and traps endows LAPs with the long persistence lifetime. The dopants substitute ions that have similar radii, leading to yield defects. These deficiencies serve as hole or electron traps to capture holes or electrons in the excitation process and provide the lasting energy to activated ions for the long afterglow emission. For the CS:x Tm3 + phosphors, considering the previous discussion, the doped Tm3 + ions would occupy the sites of Ca2 + ions rather than Sn4 + ions [Reaction (1)]: Chem. Asian J. 2015, 10, 2361 – 2367
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TmCa $ TmC Ca þ e0
ðR1Þ
Theoretically, the charge compensation could be realized by two possible pathways. One pathway is electronic compensation. Some Sn4 + ions could capture liberated electrons [Eq. (1)] and then be converted into Sn2 + or Sn3 + ions, which formed structure defects and were treated as sites for the localization of charge carries (electrons)[24] and played the role of hole traps. The other approach is vacancy compensation. The Tm3 + -related defects are TmCCa and V’’Ca generated by the inequivalent substitution [Reaction (2)]: 2 Tm3þ þ 3 Ca2þ ! 2 TmC Ca þ V00 Ca
ðR2Þ
Three bivalent Ca2 + ions were replaced by two trivalent Tm3 + ions, producing V’’Ca for charge compensation. The positively charged substitution deficiency TmCCa may serve as electron-trapping centers and could capture electrons, whereas the negatively charged defects V’’Ca, Sn3 + , and Sn2 + generated by the incorporation of the Tm3 + dopants may act as holetrapping sites and possess the ability to capture holes.[25] However, given the special orthorhombic structure of the Ca2SnO4 host and the central site of Sn4 + ions located in the inner SnO6 octahedron and surrounded by six oxygen atoms, it is believed that it is difficult for Sn4 + ions to change into Sn3 + and Sn2 + ions. Therefore, we maintain that charge compensation is ascribed to vacancy compensation and that the Tm3 + ion is not only the provider of traps, but also an activator itself. In addition, there are many other intrinsic defects in the phosphors, such as the oxygen vacancies, which do play a role in the generation of the long afterglow emission. However, according to studies by Jia et al. in 2014[8] and Xu et al. in 2010,[16b] the oxygen vacancies are largely produced under vacuum conditions during the synthetic process. Because the as-produced samples were synthesized in air, the number of the oxygen vacancies was small. Therefore, on the basis of their discussions on the influence of oxygen vacancies on phosphorescence, the effect of oxygen vacancies in our work may not be dominant in comparison with that of the Tm3 + ion related defects and the long-lasting phosphorescence is mainly assigned to Tm3 + ion related defects. The mechanism of the long phosphorescence emission of Ca2SnO4 :Tm3 + can be interpreted by using the graph depicted in Figure 7. After irradiation with a UV lamp (process a in Figure 7), some of the excitation energy will be transferred through the host directly to the luminescence centers of the Tm3 + ions, followed by the characteristic emission from the f–f transitions of Tm3 + ions in the form of visible light (process b in Figure 7). However, the trapping centers could store part of the excitation energy (process c in Figure 7), rather than let it directly return to the ground states. Subsequently, the captured energy will be released from the trapping centers under thermal stimulation at the appropriate temperature, and would transfer through the host to Tm3 + ions (process d in Figure 7), followed by the recombination of trapping centers and leading to the characteristic emissions of Tm3 + ions as the long afterglow emissions owing to the slow release speed of
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Full Paper Synthesis A variety of CS:x Tm3 + (x = 0–0.04) LAPs were synthesized with traditional high-temperature solid-state reactions. The starting materials CaCO3, SnO2, and Tm2O3 were weighted under a stoichiometric ratio. The raw materials were mixed and ground in an agate mortar thoroughly, and the homogenous mixtures were transferred to an alumina crucible and sintered at 1450 8C for 5 h in air. After cooling to room temperature naturally, the as-obtained phosphors were ground into powders for subsequent measurements.
Characterization
Figure 7. Schematic graph of the mechanism for the long, persistent emission of Ca2SnO4 :Tm3 + samples. CB = conduction band, VB = valence band.
the transfer of energy from the traps. However, all processes discussed above are a general description for the occurrence of the long afterglow emissions of CS:Tm3 + phosphors. Details of the mechanism remain vague. Therefore, there is a long way to go before we can completely interpret how the long phosphorescence emission is generated.
Conclusion A series of novel LAPs Ca2SnO4 :x Tm3 + were prepared with conventional high-temperature solid-state reactions. XRD measurements and Rietveld refinement demonstrated that the incorporation of the Tm3 + dopants generated no notable impurities, which indicated that the dopants merged completely into the host. The corresponding optical properties were systematically studied by using the PL, phosphorescence, and TL spectra. The results illustrated that Tm3 + -related defects were responsible for the bright bluish green afterglow emission from characteristic f–f transitions of Tm3 + ions. The radiant bluish green long-lasting phosphorescence could be observed by the naked eye and lasted for 5 h, as observed by the naked eye in a dark environment after removal of the UV light source. Two TL bands at 325 and 349 K from the TL curve responsible for the emission were adopted to calculate the depth of the traps, which were 0.45 and 0.78 eV, respectively. The mechanism of the long-lasting phosphorescence emission was also studied schematically.
The XRD profiles for phase identification were collected by using a D8 Focus diffractometer operating at 40 kV and 40 mA with graphite-monochromated CuKa radiation (l = 0.15405 nm). In the process, the scanning rate was 108 min¢1 with 2q = 10–758. The PLE and PL spectra of the phosphors were measured by using a Hitachi F-7000 spectrophotometer with an excitation source of a 150 W xenon lamp and a l = 340 nm filter. The long afterglow emission spectra and decay curves were also measured by using the same instrument after the samples were irradiated under l = 254 nm UV light for 5 min. The 3D-TL and phosphorescence emission spectra at different temperatures were measured with a homemade spectrometer mainly consisting of a charge-coupled device (CCD) detector and a heating apparatus. The samples were placed in a homemade sample holder and heated from RT to 573 K at a speed of 1 K s¢1, and data for the TL curves was obtained from the 3D-TL emission spectra with a data transferring technique by using computer software. All measurements were performed at RT, except those for the TL curves.
Acknowledgements We are grateful for financial aid from the Hong Kong, Macao and Taiwan Science and Technology Cooperation Special Project of Ministry of Science and Technology of China (grant no. 2014DFT10310), the Program of Science and Technology Development Plan of Jilin Province of China (grant no. 20140201007GX), the National Basic Research Program of China (973 Program, grant no.2014CB643801), and the National Natural Science Foundation of China (grant nos. 51102229, 51402288, 21401184). Keywords: doping · lanthanides · luminescence · phosphors · photochemistry
Experimental Section Chemicals All chemical reagents were used as received without further purification. Rare-earth oxide Tm2O3 (A.R., 99.99 %) was purchased from the Science and Technology Parent Company of the Changchun Institute of Applied Chemistry (P.R. China). CaCO3 (A.R., 99.9 %) and SnO2 (A.R., 99.9 %) were purchased from Beijing Chemical Works. A high-temperature furnace was purchased from Hefei Ke Jing Materials Technology Co. Ltd. Chem. Asian J. 2015, 10, 2361 – 2367
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[1] a) F. I. Vergunas, N. L. Gasting, Sov. Phys. JETP 1955, 1, 284; b) N. L. Gasting, Opt. Spektrosk. 1957, 3, 624; c) W. M. Yen, S. Shionoya, H. Yamamoto, Phosphor Handbook, CRC Press, Boca Raton, FL, 1995; d) R. C. Ropp, Luminescence and the Solid State, 2nd ed. Elsevier, Amsterdam, 2004. [2] a) T. Matsuzawa, Y. Aoki, N. Takeuchi, Y. Murayama, J. Electrochem. Soc. 1996, 143, 2670; b) J. Qiu, K. Miura, H. Inouye, Y. Kondo, T. Mitsuyu, K. Hirao, Appl. Phys. Lett. 1998, 73, 1763; c) J. Qiu, K. Hirao, Solid State Commun. 1998, 106, 795; d) H. Lin, B. Wang, J. Xu, R. Zhang, H. Chen, Y. Yu, Y. Wang, ACS Appl. Mater. Interfaces 2014, 6, 21264. [3] a) T. Aitasalo, P. Deren´, J. Hçls, H. Jungner, J. C. Krupa, M. Lastusaari, J. Legendziewicz, J. Niittykoski, W. Stre˛k, J. Solid State Chem. 2003, 171, 114; b) H. N. Luitel, T. Watari, R. Chand, T. Torikai, M. Yada, J. Mater. 2013, 613090. [4] P. Dorenbos, Phys. Status Solidi B 2005, 242, R7. [5] a) C. Chang, J. Xu, L. Jiang, D. Mao, W. Ying, Mater. Chem. Phys. 2006, 98, 509; b) A. H. Wako, B. F. Dejene, H. C. Swart, Phys. B 2014, 439, 153. Ó 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Full Paper [6] R. Pang, Y. Jia, R. Zhao, H. Li, J. Fu, W. Sun, L. Jiang, S. Zhang, C. Li, Q. Su, Dalton Trans. 2014, 43, 9661. [7] P. Wang, X. Xu, D. Zhou, X. Yu, J. Qiu, Inorg. Chem. 2015, 54, 1690. [8] Y. Jia, H. Li, R. Zhao, W. Sun, Q. Su, R. Pang, C. Li, Opt. Mater. 2014, 36, 1781. [9] W. Sun, Y. Jia, R. Zhao, H. Li, J. Fu, L. Jiang, S. Zhang, R. Pang, C. Li, Opt. Mater. 2014, 36, 1841. [10] T. Aitasalo, J. Hçls, H. Jungner, M. Lastusaari, J. Niittykoski, J. Phys. Chem. B 2006, 110, 4589. [11] a) R. Pang, C. Li, L. Jiang, Q. Su, J. Alloys Compd. 2009, 471, 364; b) T. Ishigaki, A. Torisaka, K. Nomizu, P. Madhusudan, K. Uematsu, K. Toda, M. Sato, Dalton Trans. 2013, 42, 4781; c) Z. Pan, Y.-Y. Lu, F. Liu, Nat. Mater. 2012, 11, 58. [12] S. Shionoya, W. M. Yen, Phosphor Handbook, CRC Press, Boca Raton, FL, 1999, pp. 88 and 179. [13] a) D. Jia, X. J. Wang, W. Jia, W. M. Yen, J. Appl. Phys. 2003, 93, 148; b) C. Li, J. Wang, H. Liang, Q. Su, J. Appl. Phys. 2007, 101, 113304; c) B. Lei, Y. Liu, G. Tang, Z. Ye, C. Shi, Mater. Chem. Phys. 2004, 87, 227. [14] M. Trçmel, Z. Anorg. Allg. Chem. 1969, 371, 237. [15] R. C. Ropp, Luminescence and the Solid State: Studies in Inorganic Chemistry 21, Elsevier, Amsterdam, 1991, vol. 12, p. 21. [16] a) Z. Ju, R. Wei, J. Zheng, X. Gao, S. Zhang, W. Liu, Appl. Phys. Lett. 2011, 98, 121906; b) X. Xu, Y. Wang, Y. Gong, W. Zeng, Y. Li, Opt. Express 2010, 18, 16989.
Chem. Asian J. 2015, 10, 2361 – 2367
www.chemasianj.org
[17] A. C. Larson, R. B. V. Dreele, Los Alamos Natl. Lab. 1994, 1. [18] W. T. Carnall, G. L. Goodman, K. Rajnak, R. S. Rana, J. Chem. Phys. 1989, 90, 3443. [19] a) D. W. Cooke, B. L. Bennett, R. E. Muenchausen, J. K. Lee, M. A. Nastasi, J. Lumin. 2004, 106, 125; b) J. Glodo, A. J. Wojtowicz, J. Alloys Compd. 2000, 300 – 301, 289. [20] J. Trojan-Piegza, J. Niittykoski, J. Hçls, E. Zych, Chem. Mater. 2008, 20, 2252. [21] S. Schweizer, Phys. Status Solidi A 2001, 187, 335. [22] R. Chen, J. Electrochem. Soc. 1969, 116, 1254. [23] a) J. Kuang, Y. Liu, J. Electrochem. Soc. 2006, 153, G245; b) R. Sakai, T. Katsumata, S. Komuro, T. Morikawa, J. Lumin. 1999, 85, 149. [24] U. Shail, P. Om, K. Devendra, J. Phys. D 2004, 37, 1483. [25] a) F. Clabau, X. Rocquefelte, T. Le Mercier, P. Deniard, S. Jobic, M. H. Whangbo, Chem. Mater. 2006, 18, 3212; b) Y. Chen, X. Cheng, M. Liu, Z. Qi, C. Shi, J. Lumin. 2009, 129, 531.
Manuscript received: June 4, 2015 Accepted Article published: July 16, 2015 Final Article published: August 26, 2015
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