Article pubs.acs.org/IC
Effect of Defect Distribution on the Optical Storage Properties of Strontium Gallates with a Low-Dimensional Chain Structure Ting Wang,† Xuhui Xu,*,†,‡ Dacheng Zhou,†,‡ Yong Yang,†,‡ Jianbei Qiu,†,‡ and Xue Yu*,†,‡ †
School of Materials Science and Engineering, Kunming University of Science and Technology, Xuefu RD, Kunming 650093, P. R. China ‡ Key Laboratory of Advanced of Materials Yunnan Province, Kunming 650093, P. R. China S Supporting Information *
ABSTRACT: The low-dimensional structure of the SrGa2O4 host exhibits a self-activated long persistent luminescence related to the creation of the oxygen vacancies. Because of the unique structure of the SrGa2O4 with a chain of cations along the a crystal direction, the emission and trapping centers could be introduced easily when the metal ions of Bi3+ are doped. Both the photoluminescence and long persistent luminescence are related to two efficient emission centers of Bi3+ in the two different crystallographic Sr sites, while the photostimulated luminescence spectra exhibit only one emission center of Bi1 ions under excitation at 980 or 808 nm. The results indicate that the distribution of defects in the low-chain structure of the SrGa2O4 host plays a vital role in the capture and transfer processes of carriers, which has a profound influence on the luminescence performance of SrGa2O4:Bi3+ as one of the electron-trapping materials.
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INTRODUCTION When electron-trapping materials (ETMs) are exposed to highenergy radiation, such as X-ray or ultraviolet (UV) light, some of the carriers (electrons or holes) would be captured in trapping centers that generally are lattice defects or impurities. Subsequently, the trapped carriers could be released by optical, thermal, or physical stimulations, leading to stimulated emission from the emitting center in the ETMs.1,2 The long persistent luminescence (LPL) and photostimulated luminescence (PSL) phosphors are considered to be typical ETMs. For LPL phosphors, the energy stored in suitable traps could be released, which was induced by thermal energy available at room temperature.3,4 Such LPL phosphors have long been of great research interest and have been commercialized as night or dark environment vision materials for a wide range of applications, such as safety signage, night-vision surveillance, displays, decorations, and in vivo bioimaging.5,6 However, deep and stable trapping centers can stabilize the carriers almost permanently at room temperature, and then the carriers could be steadily released by low-energy light (photo) illumination.7−9 Therefore, PSL phosphors can be applied as eliminatable and rewritable optical memory media, for infrared detection along, and in the fields of dosimetry and X-ray imaging.10 Currently, the most efficient ETMs mentioned above are alkaline sulfides, aluminates, stannates, and halides, such as SrS:Eu, Sm, CaS:Eu, Sm, BaFBr:Eu, SrAl2O4:Eu, Dy, Sr3SiO5:Eu, Tm, and Mg2SnO4.11−16 Unfortunately, the emission intensity, storage capacity, and chemical stability of © 2016 American Chemical Society
these ETMs still need to be improved substantially to satisfy the requirements for practical applications.17 More importantly, although plenty of ETMs have been developed, the details of the nature and distribution of the defects are still a subject of challenge, and the transport process of the charge carriers between trapping and emission centers remains unclear. Because both the LPL and PSL are derived from the trapping centers with different depths that originated from the lattice defects or impurities related to the host matrix, we simplified the study with a specific structure of the host to determine the nature and origin of the defects in ETMs to make it feasible and interesting. The host matrix of luminescence materials with a unique low-dimensional chain crystal structure has attracted great research interest.18 The host possesses an unusual onedimensional chain structure of cations along a certain crystal direction, which could be beneficial to the formation of the trap level and excited state level, and especially could provide an effective transmission path in one direction between luminescent and trapping centers.19 Besides, low-dimensional structural materials are easy to use to implant rare earth ions into the host lattice and create traps located at suitable depths that can store the excitation energy and emit light at room temperature.18 Hence, in this work, phosphors with lowdimensional structure are employed to clarify the capture and release process of the carriers, which would improve our Received: October 19, 2015 Published: January 7, 2016 894
DOI: 10.1021/acs.inorgchem.5b02401 Inorg. Chem. 2016, 55, 894−901
Article
Inorganic Chemistry
h, and finally stimulated by a 980 nm laser diode for 10 min. An 808 nm laser diode is used as a stimulating source.
understanding of the trapping mechanism of ETMs. We preliminarily aimed at SrGa2O4, which possesses a lowdimensional structure, as the ideal host for the low synthesis temperature, high stability, and reasonable conductivity. In addition, SrGa2O4 possessed an open structure with tunnel-like cavities running throughout the crystal lattice. The tunnel was generated through the link of Sr2+ sites, which was located in the one-dimensional space structure surrounded by a GaO4 tetrahedral. It was reasonable that the conduction along the tunnel (a crystal direction) is much larger than that along other directions (b and c directions),20,21 which could be beneficial for the formation of the trap level and excited state level and especially could provide an effective transmission path in one direction between luminescent centers and trapping centers. Moreover, considering the fact that yellow emission matches the maximal sensitivity of human vision in the mesopic and scotopic regime,22,23 the development of efficient yellowemitting storage ETM phosphors is significant for practical applications. As a special metal ion, Bi3+ ion photoluminescence usually covers a spectral range from UV to yellow with different crystal fields. With an increase in the crystal field, the emission of Bi3+ might red shift.24,25 It could be suggested that the stronger crystal fields in the a crystal direction of SrGa2O4 may provide a suitable environment for Bi3+ ions to realize longwavelength emission. Therefore, in this work, Bi3+ metal ions rather than rare earth ions are selected as the emission center to realize the goal mentioned above. The influence of the defect distribution on the ETMs with the low-chain structure is investigated. A possible mechanism responsible for the electron trapping process has also been proposed.
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RESULTS AND DISCUSSION The experimental values, calculated values, peak positions, and differences in Rietveld refinement XRD patterns of the Sr0.99Ga2O4:0.01Bi3+ sample are shown in Figure 1a. The
EXPERIMENTAL SECTION
Sr1−xGa2O4:xBi3+ (x = 0, 0.001, 0.005, 0.01, 0.02, and 0.03) samples were synthesized by the conventional high-temperature solid state reaction. Stoichiometric amounts of Sr2CO3(A.R), Ga2O3(A.R), and Bi2O3(99.99%) were mixed in an agate mortar with ethanol. After being fully ground, the mixtures were put into crucibles and calcined at 1200 °C for 6 h in air and a reducing atmosphere (95:5 N2:H2), and all of the Sr1−xGa2O4:xBi3+ (x > 0) samples are sintered in an air atmosphere. After cooling to room temperature naturally, the asobtained samples were ground into a powder for the following measurements. The crystalline structures of the prepared powders were investigated by X-ray diffraction (XRD) with Ni-filter Cu Kα radiation (λ = 0.154056 nm) at a scanning step of 0.02°. The XRD data were collected in the range of 10−60° by applying a D8ADVANCE/ Germany Bruker X-ray diffractometer. The photoluminescence excitation (PLE), PL, LPL, and PSL spectra were recorded by using a Hitachi F-7000 fluorescence spectrophotometer. The EPR experiment was performed at 100 K by using a Bruker E-500 spectrometer operated in the X-band (9.77 GHz) with 100 kHz field modulation. The power and amplitude modulation are 12.7 mW and 0.1 mT, respectively. The PL decay curves were measured with an FS980 fluorescence spectrophotometer. The LPL lifetime curves were measured with a PR305 long afterglow instrument after the sample had been irradiated with UV light for ∼20 min. The thermoluminescence (TL) curves were measured with a FJ-427 A TL meter (Beijing Nuclear Instrument Factory). The weight of the measured samples was constant (0.002 g). Prior to the TL measure, the samples were first exposed to the radiation from the UV light for ∼20 min and then heated from room temperature to 650 K at a rate of 1 K/s. Before the PSL had been measured (λex = 980 nm), the samples were preirradiated with UV light for 20 min and then placed in the dark for 10 h. A 980 nm laser diode is used as a stimulating source. Before the PSL had been measured (λex = 808 nm), first, the samples were preirradiated with UV light for 20 min, then placed in the dark for 10
Figure 1. (a) XRD refinement of Sr0.99Ga2O4:0.01Bi3+. The inset is the crystal structure of the SrGa2O4 host. (b) XRD patterns of Sr0.98Ga2O4:0.02Bi3+ and SrGa2O4 obtained in different atmospheres (air and the reducing atmosphere) and the JCPDS Card No. 72-0222 of SrGa2O4.
Rietveld method was used to refine the XRD data using the MAUD refinement program. The reliability parameters of refinement are Rwp = 11.28% and χ2 = 1.06, which verifies the phase purity of the as-prepared sample. As shown in the inset of Figure 1a, SrGa2O4 possesses a monoclinic phase, in space group P21/C (14).23 The unit cell parameters are as follows: a = 8.3920 Å, b = 9.0180 Å, c = 10.6970 Å, β = 93.9000°, V = 807.66 Å3, and Z = 8. In the crystal lattice, SrGa2O4 possesses a distinct open structure with tunnel-like cavities (a crystal direction) running throughout the host lattice. In addition, there are three crystallographically independent cation sites, namely, two different sites of Sr2+ (marked as Sr1 and Sr2) and one of Ga3+. Sr1 ions occupy a tetrahedral site with four oxygen atoms surrounding it, and the average Sr−O bond length is 2.51 Å; Sr2 ions occupy a hexahedron site with five oxygen atoms, and the average band distance is 2.53 Å. Ga atoms occupy a tetrahedral site with four coordinated oxygen atoms. Because of the similar effective ion radii [r(Bi3+) = 1.08 Å, and r(Sr2+) = 1.13 Å], Bi3+ ions are supposed to occupy Sr2+ sites in the SrGa2O4 host. Figure 1b shows the typical XRD patterns of 895
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Figure 2. (a) PLE and PL spectra of the SrGa2O4 sample. The inset shows the LPL spectrum of SrGa2O4. (b) PL spectra of SrGa2O4 samples in the air and reducing atmospheres. The inset shows the LPL spectra of SrGa2O4 samples.
Figure 3. (a) PLE and PL spectra of SrGa2O4:Bi3+ (λem = 540 nm, and λex = 255 and 330 nm). The dotted lines are Gaussian profiles, and the inset shows the photograph of SrGa2O4:Bi3+ under 365 nm excitation with an UV lamp. (b) PL spectra of Sr1−xGa2O4:xBi3+ (x = 0, 0.001, 0.005, 0.01, 0.02, and 0.03) under 255 nm excitation. The inset shows the fluorescence decay curves of Sr1−xGa2O4:xBi3+ (x = 0, 0.001, 0.005, and 0.01) (λem = 401 nm, and λex = 255 nm).
Sr0.98Ga2O4:0.02Bi3+ and SrGa2O4 obtained in different atmospheres (the air and reducing atmosphere). Here, all the diffraction peaks could be exactly indexed to the phase of SrGa2O4 registered in JCPDS Card No. 72-0222, which indicates that all samples are identified as SrGa2O4 phase. Figure 2a shows the PLE and PL spectra of the nondoped SrGa2O4 sample. SrGa2O4 shows a blue broad band centered at 401 nm under the excitation at 255 nm, which is termed the 4 T1−4A1 transition of electrons in d orbits of Ga3+.20 Besides, the LPL phenomenon is observed in the SrGa2O4 host matrix after the UV excitation sources had been switched off, as shown in the inset of Figure 2a. The LPL permanence possibly resulted from the intrinsic defects, such as the oxygen vacancies (V•• O ) and strontium vacancies (V″ Sr). In the synthesis of arising from the break of Sr−O bands would SrGa2O4, V•• O inescapably be produced, because of long-term calcinations at high temperatures. It should be noticed that more abundant V•• O can be created when samples are sintered under an oxygendeficient atmosphere as proposed by many other groups.26−29 Therefore, the SrGa2O4 sample calcined in a reducing atmosphere was prepared to clarify the effect of V•• O on the LPL phenomenon. Figure 2b shows the PL spectra of SrGa2O4 phosphors obtained in the reducing and air atmosphere as a comparison. Under 255 nm excitation, SrGa2O4 shows the
ultraviolet and blue emission located at 373 and 401 nm, respectively. In addition, the PL band of SrGa2O4 sintered under the reducing condition blue-shifts compared with that of the air-sintered sample. It has been reported that a formation of V•• O in the Ga−O tetrahedron causes a decrease in site symmetry, contributing to a decrease in the crystal field.30 Then, the decrease in the crystal field results in a blue-shift from 401 to 373 nm in the SrGa2O4 sintered under reducing conditions. The inset of Figure 2b shows the LPL spectra of SrGa2O4 phosphors sintered under reducing conditions, which exhibit a shape and position similar to those of samples prepared in an air atmosphere, because trapping centers are closer to the lowest level of the excited state in Ga3+. It indicates that the PL and LPL originate from the same emitting centers. Furthermore, the corresponding LPL intensity of the SrGa2O4 phosphor sample prepared in a reducing atmosphere (red curve) is much stronger than that of that prepared in an air atmosphere (black curve), implying that the LPL properties are associated with V•• O , as well. The ESR spectra are employed to monitor the processes of energy storage during continuous excitation with an UV light and release by afterglow emission associated with V•• O . In Figure S1, the EPR spectra of the SrGa2O4 host at different time intervals after the stoppage of irradiation at 100 K are measured. g = 2.00041 signals ascribed 896
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Figure 4. (a) LPL spectra of Sr1−xGa2O4:xBi3+ (x = 0, 0.001, 0.005, 0.01, 0.02, and 0.03). (b) LPL lifetime curves of Sr1−xGa2O4:xBi3+ (x = 0, 0.001, 0.005, 0.01, and 0.02) samples. The inset shows the phosphorescence chromaticity coordinates of Sr1−xGa2O4:xBi3+ (x = 0 and 0.01) phosphors and their LPL photographs.
samples can be fitted well by the single-exponential decay mode as the following equation:35
to the hole center on oxygen31−33 could be obviously detected after the irradiation. It is distinctly evident that the intensity of ESR is increased as a result of UV irradiation, which may be caused by photon-generated carriers trapped at the defect centers. Considering that the positive effective charges should •• mainly come from V•• O , VO defects may play a key part in the persistent luminescence. The results described above indicate that the LPL properties are predominantly attributed to V•• O. However, the influence of hole trap VSr ″ on the LPL phenomenon cannot be ignored. Figure 3a illustrates the PLE and PL spectra of SrGa2O4:Bi3+ phosphors. Under 255 nm excitation, the emission originated from the SrGa2O4 host (401 nm) and a band peak (540 nm) ascribed to the 3P1−1S0 transition of Bi3+ ions is detected in the PL spectrum. It indicates that Bi3+ ions could be excited by the energy effectively absorbed via the SrGa2O4 host. Monitored at 540 nm, the spectrum shows an extra excitation peak of Bi3+ ions located at 330 nm.25 Only an asymmetric band peaking at 504 nm as the characteristic emission of Bi3+ (3P1−1S0 transition) is observed under 330 nm excitation. As mentioned above, there are two crystallographically nonequivalent Sr2+ sites that can be substituted with the Bi3+ ions to produce two nonequivalent emission centers of Bi3+ ions in the SrGa2O4 host. Thus, the asymmetric emission of SrGa2O4:Bi3+ phosphor is fitted well by two Gaussian profiles, located at 498 and 557 nm (dotted line). The average band length between oxygen and strontium at the Sr1 site (four-coordinated site) is shorter than that at the Sr2 site (five-coordinated site). Hence, the covalent bonding and crystal field strength are stronger for the Sr1 site than for the Sr2 site, resulting in longer wavelength emission for Bi3+ on the Sr1 site.34 Therefore, we deduce that the emission derived from Bi1 and Bi2 is located at 557 and 498 nm, respectively. Figure 3b portrays the PL of Sr1−xGa2O4:xBi3+ (x = 0, 0.001, 0.005, 0.01, 0.02, and 0.03) phosphors under 255 nm excitation. The emission intensity of SrGa2O4 (401 nm) decreases remarkably with an increasing concentration of Bi3+ ions. The emission intensities of Bi3+ increase initially, reach the maximum at x = 0.02, and then decrease because of concentration quenching. On one hand, these results indicate the energy transfer process between the SrGa2O4 host and Bi3+ can be expected. The fluorescence decay curves of the SrGa2O4 host in Sr1−xGa2O4:xBi3+ (x = 0, 0.001, 0.005, and 0.01) (λex = 255 nm; λem = 401 nm) samples are recorded as shown in the inset of Figure 3b. The decay curves of Sr1−xGa2O4:xBi3+
∞
τ=
∫0 tI(t ) dt ∞
∫0 I(t ) dt
(1)
where I(t) represents the luminescence intensity. On the basis of eq 1, the average decay time τ of the SrGa2O4 host with an increased concentration of Bi3+ can be calculated to be 1.75, 1.54, 1.29, and 1.09 μs, respectively. Besides, the fluorescence decay time of acceptor (Bi3+) Sr0.99Ga2O4:0.01Bi3+ (λem = 540 nm, and λex = 255 nm) is measured, as shown in Figure S2. The average decay time τ of Bi3+ can be calculated to be 2.01 μs, which is longer than that of SrGa2O4. Because the decay time of the donors is shorter than that of the acceptors, it is possible that the energy transfer process from SrGa2O4 host to Bi3+ could occur less effectively. On the other hand, considering that fact, with the doping of Bi3+, Bi3+ ions replace the Sr2+ site in the SrGa2O4 host matrix. Thus, some foreign defects are introduced into the phosphors lattice, which contribute to the decrease in the donor’s emission intensity. After the samples had been irradiated with 254 nm UV light for 20 min, an obvious LPL phenomenon is observed in Sr1−xGa2O4:xBi3+ (x = 0, 0.001, 0.005, 0.01, 0.02, and 0.03), and the LPL spectra are depicted in Figure 4a. Different from the LPL located at 401 nm in the SrGa2O4 host, a yellow emission broad band centered at 540 nm of Bi3+ is detected, while the emission band of SrGa2O4 host is mostly insignificant. It indicates that Bi3+ ions act as dominate emission centers in LPL properties, and the emission derives from the 3P1−1S0 transition of Bi3+ ions in the two crystallographic Sr sites mentioned above. With an increasing concentration of Bi3+, the yellow emission intensities at 540 nm are gradually enhanced and reach the maximum value at y = 0.01, which results in the phenomenon in which the LPL color changes from blue to yellow accordingly. The LPL CIE chromaticity diagram of SrGa2O4 and Sr0.99Ga2O4:0.01Bi3+ is marked in the inset of Figure 4b. It is found that the chromaticity coordinates of the samples vary from (0.1784, 0.1548) to (0.3891, 0.4784). The results indicate that the persistent energy transfer process from the host to Bi3+ occurs, and the energy transfer efficiency of LPL is more significant than that of the PL process. It could be suggested that Bi3+ ions introduce foreign trapping centers that acted as the bridge, promoting the energy transfer efficiency 897
DOI: 10.1021/acs.inorgchem.5b02401 Inorg. Chem. 2016, 55, 894−901
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Inorganic Chemistry from the SrGa2O4 host to Bi3+ of the LPL process. The detailed proof will be given in TL curves. The LPL lifetime curves of Sr1−xGa2O4:xBi3+ (x = 0, 0.001, 0.005, 0.01, and 0.02) phosphors are shown in Figure 4b. The LPL lifetime curves show that these samples exhibit different persistent times that consist of a fast decay and a consequent slow decay with a long decay tail, implying the existence of various trap depths. Compared with the relatively weak LPL of the host, a remarkable longer persistent time is observed in the Bi3+doped phosphors. Hence, it is safe to say that the incorporation of Bi3+ creates more defects (the intrinsic defects or foreign defects) in the SrGa2O4 host lattice, which act as trapping centers and have a significant influence on the LPL performance. The LPL lifetime curves show that the optimized persistent luminescence lasts for more than 7 h. Trapping centers play a notable role in the photoenergy storage of persistent and thermostimulated phosphors.36,37 LPL or PSL phenomena are critically dependent on trap depth. The kinetics, density, and depth of the traps may be evaluated using the TL technique. Figure 5a shows the TL curves of the
Sr1−xGa2O4:xBi3+ (x = 0, 0.005, 0.01, and 0.02) phosphors recorded immediately after being excited with an UV lamp for 20 min. Compared with those of other samples, the TL signal of the SrGa2O4 host is so weak that it seems like a nearly straight line, which is shown alone in Figure 5b for greater detail. Figure 5b shows that there are two very weak bands located at TA (362 K) and TB (414 K), which might be due to the intrinsic defects of the SrGa2O4 host. The suitable TL peak is slightly above room temperature (320−393 K) for the excellent LPL performance.38,39 Therefore, the TL bands centered at around 362 K contribute to the LPL phenomenon. Four TL bands are detected, when Bi3+ ions are doped in SrGa2O4. Besides the appearance of the two extra TL bands located at TC (501 K) and TD (553 K), a remarkable enhancement of the TL intensities of TA and TB is observed with increasing concentrations of Bi3+. It indicates that the defect types and the number of the trapping centers greatly increased with the introduction of Bi3+. The increasing number of trapping centers would be the dominant reason that the persistent energy transfer from host to Bi3+ becomes more efficient. Moreover, the charge carriers captured by deep depths of TB, TC, and TD are difficult to release at room temperature, indicating that these samples provide potential application as optical storage materials.40,41 To investigate the number of trapping centers concerned and the kinetic order of these samples, TL curves of Sr0.99Ga2O4:0.01Bi3+ with different delay times are recorded in Figure 5c. The TA band was greatly depleted after a delay of 5 h and completely disappears, and the afterglow subsided after 24 h. However, the intensities of TB, TC, and TD bands sustain the intensity without significant changes. Hence, it is safe to say that the relatively shallow traps of TA could be ascribed to the LPL phenomenon, while the presence of deep stable traps in SrGa2O4:Bi3+ can immobilize the energy permanently at room temperature. These relatively deep traps preclude the thermal release of the intercepted carriers at room temperature, which is an essential factor required for PSL phosphors. The detailed PSL properties for SrGa2O4:Bi3+ were investigated as follows. As shown in Figure 6a, it is interesting to find that the Sr0.99Ga2O4:0.01Bi3+ phosphor exhibits strong PSL upon 980 nm stimulation after being preirradiated with UV light for 20 min and then placed in the dark for 10 h. An evident difference between the PSL (λex = 980 nm) and LPL spectrum is
Figure 5. (a and b) TL curves of Sr1−xGa2O4:xBi3+ (x = 0, 0.005, 0.01, and 0.02) phosphors recorded immediately after UV lamp irradiation for 30 min. (c) TL curves of Sr0.99Ga2O4:0.01Bi3+ placed in a dark room for different periods of time.
Figure 6. (a) PSL emission spectra of Sr0.99Ga2O4:0.01Bi3+ taken under varying stimulation times (5, 20, 40, 60, and 80 s). The inset shows that the corresponding photographs of PSL with different delay times. (b) TL curves of Sr0.99Ga2O4:0.01Bi3+ placed in the dark for 24 h at room temperature after preirradiation with UV excitation for 20 min stimulated by a 980 nm laser diode after varying periods of time (0, 3, 6, and 15 min). 898
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Figure 7. (a) TL spectra of Sr0.99Ga2O4:0.01Bi3+ placed in the dark for 24 h at room temperature after preirradiation with UV excitation for 20 min stimulated by an 808 nm laser diode after varying periods of time (0, 3, 6, and 15 min). (b) PSL emission spectra of Sr0.99Ga2O4:0.01Bi3+ taken after varying stimulation times (5, 20, 40, 60, and 80) after preirradiation with a 980 nm laser diode for 15 min. The inset shows the corresponding photographs of PSL with different delay time.
SrGa2O4:Bi3+, we took Sr0.99Ga2O4:0.01Bi3+ as an example. 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:17
observed. The PSL (λex = 980 nm) spectrum exhibits only a symmetric emission band that peaked at ∼557 nm, which is different from the LPL and PL. The PSL intensities decline with an increased stimulation times. It confirms that it is not an up-conversion process but the release of the carriers in the traps of this phosphor under 980 nm light stimulation. As presented in Figure 6b, time-varying TL curves of Sr0.99Ga2O4:0.01Bi3+ phosphors are recorded after stimulation with a 980 nm laser diode. It is obviously observed that the relative TL intensity of TB gradually decreases with a prolonged stimulating time. It suggests that the co-dopant of Bi3+ increases the number of the traps corresponding to the TB band, which could be responsible for the performance of PSL (λex = 980 nm). There is no significant change in the bands of TC and TD with a prolonged stimulating time, indicating that the carriers captured by deep traps (TC and TD) that cannot be released with 980 nm laser diode may be due to the low energy. Thus, a higher-energy 808 nm laser diode is employed. The TL curves of Sr0.99Ga2O4:0.01Bi3+ phosphors are measured after stimulation by an 808 nm laser diode (Figure 7a). It is found that the relative TL intensities of TB, TC, and TD gradually decrease with prolonged stimulating times, indicating that the carriers captured by deep traps (TB, TC, and TD) could be released with an 808 nm laser diode. To investigate the carriers captured by deep traps of TC and T D , the corresponding PSL (λ ex = 808 nm) spectra of the Sr0.99Ga2O4:0.01Bi3+ phosphor are measured (Figure 7b). Before the measurement, the samples were preirradiated with UV light for 20 min, then placed in the dark for 10 h, and finally stimulated by a 980 nm laser diode for 15 min to empty the carriers trapped in TA and TB, and then a 808 nm laser diode is used as a stimulating source. In this approach, the released carriers from TA and TB are precluded, and the PSL (λex = 808 nm) comes only from the release of the carriers in the traps of TC and TD. The PSL (λex = 808 nm) spectrum exhibits a symmetric emission band that peaked at 557 nm, which would be attributed to the cation sites of Sr1 occupied by Bi3+. From the results described above, it is safe to say that the PSL spectra (λex = 980 and 808 nm) derive from only one emission center of Bi1 in the crystallographic Sr1 sites. To explore the distribution and interaction of trapping centers and emitting centers during the processes of PSL of
E = [2.52 + 10.2(μg − 0.42)]
κBTm 2 − 2κBTm ω
(2)
where μg is a symmetry factor; Tm, T1, and T2 are the peak temperature at the maximum and the temperatures on either side of the temperature at the maximum, corresponding to halfintensity, respectively; and k is Boltzmann’s constant. The following parameters can be defined: τ = Tm − T1 is the halfwidth at the low-temperature side of the peak, δ = T2 − Tm is the half-width toward the falloff of the glow peak, ω = T2 − T1 is the total half-width, and μg = δ/ω is the symmetrical geometrical factor. The calculated trap levels (E) of TA, TB, TC, and TD are 0.8429, 0.9432, 1.331, and 1.344 eV, respectively, for SrGa2O4:0.01Bi3+ (Table S1 and Figure S3). Thereby, the carriers captured at deep traps (TB) and the deepest traps (TC and TD) could be released under 980 nm (1.26 eV) and 808 nm (1.53 eV) stimulation, respectively. These results clearly show that SrGa2O4:Bi3+ exhibits an excellent storage memory in luminescence properties. All of the experiments indicate that highly efficient trapping levels in this material exist. In the SrGa2O4 host, some intrinsic defects, such as V•• ″ , exist. With the doping of Bi3+, considering the O and VSr fact that Bi3+ ions replace the Sr2+ site in the SrGa2O4 host matrix, two Bi3+ ions replace three Sr2+ ions to balance the charge of the phosphor, which creates two positive defects and 2Bi 3 +
one negative defect (3Sr 2 + ⎯⎯⎯⎯→ 2Bi•Sr + V″Sr ). Therefore, two extra electron traps Bi•Sr1 and Bi•Sr2 are formed. Low-dimensional structure provides one-dimensional tunnel-like cavities running throughout the crystal lattice, which could be propitious to the interaction among these defect states.42,43 The interaction among these defect states would form defect clusters when Bi3+ ions were doped in SrGa2O4, and the defect clusters could trap carriers more effectively than intrinsic defects, V•• ″. O and VSr Therefore, different types of defect states with different trap depths are created. Figure 8 shows a possible mechanism of the LPL and PSL model in SrGa2O4:Bi3+, which should be treated as a qualitative analysis. After UV light excitation, the electrons 899
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which could also provide potential applications in the fabrication of optical memory.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.5b02401. Electron spin resonance spectra of the SrGa2O4 host measured at 100 K, before irradiation, and after the stoppage of irradiation (5, 20, and 60 min) (Figure S1); fluorescence decay curves of Sr0.99Ga2O4:0.01Bi3+ (λem = 540 nm; λex = 255 nm) (Figure S2); TL and fitting curves of the Sr0.99Ga2O4:0.01Bi3+ phosphor (Figure S3); and the parameters of the TL curves of Sr0.99Ga2O4:0.01Bi3+ (Table S1) (PDF)
Figure 8. Possible schematic representation of the LPL and PSL model in SrGa2O4:Bi3+.
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AUTHOR INFORMATION
Corresponding Authors
excited from the 1S0 ground level are captured by trapping centers (shallow and depth traps). After the UV light is switched off, carriers captured in the shallow traps (TA) will be easily released under thermal stimulation at room temperature to generate LPL (498 and 557 nm). Under 808 or 980 nm stimulation, the electrons immobilized in deep traps (TB, TC, and TD) near the emitting center (Bi1) are released to generate PSL successfully, which results in the emission band that peaked at 557 nm. Although the electrons from Bi3+ cannot escape conveniently because of the existence of barriers around the deep traps, the persistent energy transfer from Bi2 to Bi1 facilitates the more efficient PSL (557 nm). As it is impracticable that the reversal of energy transfer occurs from Bi1 to Bi2, the electrons released by 808 or 980 nm stimulation from the deep traps cannot be delivered to Bi2 to yield any PSL. The particular distribution and interaction of trapping and emitting centers explain the distinguishable difference among the PL, LPL, and PSL (λex = 980 and 808 nm) spectra, and the deep trapping centers located at Bi1 sites contribute to only the PSL emission. We think that the energy transfer process occurs from Bi2 to Bi1 in a single direction because the transmission path is in one direction in the low-dimensional structure of SrGa2O4.
*School of Materials Science and Engineering, Kunming University of Science and Technology, Xuefu RD, Kunming 650093, P. R. China. E-mail:
[email protected]. *E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The project was supported by the National Nature Science Foundation of China (61308091 and 61565009) and the Young Talents Support Program of Faculty of Materials Science and Engineering, Kunming University of Science and Technology (14078342).
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ABBREVIATIONS LPL, long persistent luminescence; PL, photoluminescence; PSL, photostimulated luminescence; ETMs, electron-trapping materials; UV, ultraviolet; TL, thermoluminescence
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CONCLUSIONS The LPL properties of the SrGa2O4 host matrix with a lowdimensional structure of cations were studied, which are related to the creation of the oxygen vacancies. The energy transfer processes from the SrGa2O4 host to Bi3+ ions in the PL and LPL process are identified, resulting in the corresponding emitting color changing from blue to yellow. The PL and LPL emission bands of Bi3+ exhibit as asymmetric band derived from the 3P1−1S0 transitions of Bi3+ ions in the two different crystallographic Sr sites, while PSL spectra of SrGa2O4:Bi3+ exhibit only one efficient emission center of the Bi1 site. It indicates that the distribution of defects and interaction of trapping and emitting centers play a vital role in the capture and release process of carriers in ETMs. Our result indicates that the low-dimensional structure of the SrGa2O4 host provides an efficient tunnel for the introduction of emission and trapping centers, which could brief the captured and released processes of carriers. The results indicate that SrGa2O4:Bi3+ could be a new member of the ETM family, 900
DOI: 10.1021/acs.inorgchem.5b02401 Inorg. Chem. 2016, 55, 894−901
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DOI: 10.1021/acs.inorgchem.5b02401 Inorg. Chem. 2016, 55, 894−901
