The phosphor Sr0.97Al2O4:0.01Eu2+, 0.02Dy3+ was synthesized by a high temperature solid state reaction in a reducing atmosphere. After charging by ultraviolet radiation, the persistent luminescence decay was divided into different time ranges and well-fitted by a biexponential function, but the two lifetimes became longer with time of persistent luminescence decay. The fractional amplitude of the shorter lifetime increased with time, whereas that of the longer lifetime decreased. The change in lifetime has been associated with the emptying of a pseudo-continuum of trap states either from different traps or different levels of the same trap species. The investigation in photostimulated persistent luminescence indicated the sample could emit bright persistent luminescence again under infrared light excitation after the UV light-excited persistent luminescence had decayed completely. The NIR photostimulation exhibited a continuous broad band, with maximum at 760 nm, representing the emptying of filled traps into the conduction band. These results also infer that the traps are pseudo-continuous but the assignment of Dy2+ as the trapped species cannot be excluded. The read-in and write-out properties of the phosphor have been elucidated and these convey applications in information storage and retrieval.
© 2016 Optical Society of America
Photostimulated persistent luminescence refers to persistent luminescence under the stimulation of a low energy photon after the excitation with high-energy radiation such as X-rays or ultraviolet (UV) radiation . This phenomenon is attributed to the recombination of trapped electrons stimulated by low energy light with holes or other centers. Using this kind of phenomenon, optical information storage can be carried on, broadening the application field of long-lasting phosphors from weak light illumination and safety signage to optical information and high energy beam survey. In conventional photostimulable storage phosphors, the stored optical information is usually read out as a transient luminescence which vanishes after the excitation stops. Research upon conventional storage phosphors has mostly focused on the superior performance of the BaFX:Eu2+ (X = Cl, Br, I) type of photostimulated phosphors, with applications in medical, computed and screen radiography [2,3]. Unlike transient luminescence, photostimulated persistent luminescent phosphors exhibit a long lasting luminescence, making it possible to replace traditional photostimulable storage phosphors in novel storage devices, high energy beam survey as well as image and optical information storage. In recent years, research interest into new photostimulable persistent phosphors has grown.
The persistent luminescence (i.e., including thermally- and photo- stimulated luminescence of materials exhibiting delayed emission of radiation after the removal of an excitation source, usually in the visible or near infrared (NIR) regions, and longer than the luminescence lifetime of the emissive state) of Eu2+-doped materials has been reviewed by Van den Eeckhout et al. .
Liu et al.  reported the photostimulated NIR persistent luminescence from LiGa5O8:Cr3+ and opened research to the study of non-halogen systems. Subsequently, much effort has been directed to the development of Cr3+ systems for bio-imaging in the first biological window [6–8].
Strontium aluminate, SrAl2O4, doped with lanthanide ions such as Eu2+ and Dy3+ exhibits long persistent luminescence and these phosphors are widely used in safety signage, displays, decorations and night-vision surveillance [9,10]. Since Matsuzawa et al.  reported on the extremely long-persistent luminescence of Eu2+ and Dy3+ co-doped SrAl2O4 in 1996, these phosphors have become a focus of research. Various synthetic methods other than solid-state reaction have been adopted in the synthesis of these phosphors [11–14]. Subsequent research has mainly concentrated upon the crystal structure [15,16], extension of the long afterglow decay time and the persistent luminescence mechanisms of SED phosphors [17–20]. Interest has also been shown in the mechanoluminescence properties of these phosphors [21–23]. However, the property of photostimulated persistent luminescence of SrAl2O4:Eu2+,Dy3+ which has potential applications in optical storage, has not been fully investigated. Jia et al.  found that the lifetime of photostimulated luminescence of this system at 80 K was three times faster than that of the normal Eu2+ luminescence and attributed this to lattice relaxation. It is pertinent to mention the observation of a Reviewer that the photostimulated upconversion emission reported for SrAl2O4:Eu2+,Dy3+  is more correctly described as photostimulated luminescence.
In this work, the persistent luminescence and NIR photostimulated luminescence of Eu2+,Dy3+-codoped SrAl2O4 have been investigated in order to gain further insight concerning the nature of the trap states. The application of this phosphor in information storage and retrieval has been investigated.
The strontium aluminate phosphors with bright green light emission and long persistent luminescence were prepared by a solid state reaction method under reducing conditions. The raw materials SrCO3 (AR) and Al2O3 (A.R: Tianjin Damou and Kemiou Chemical Reagent Companies), Eu2O3 (99.95%) and Dy2O3 (99.99%) (Shanghai Aladdin Chemical Company) and small quantities (1%) of H3BO3 (A.R) were mixed intimately in stoichiometric amounts with alcohol in a mortar for an hour and then annealed in a high-temperature tubular resistance furnace with 10% H2 and 90% N2 as a reducing atmosphere. The mixture was maintained at 1250 °C for 4 h to obtain the product powders. Pellets (0.5g, diameter 15 mm) were formed from the powder mixtures with a uniaxial loading of 30 MPa in a stainless steel mold. In this report we focus upon the sample with the composition Sr0.97Al2O4:0.01Eu2+,0.02Dy3+, subsequently abbreviated to SED throughout.
2.2. Characterization of samples
Powder diffraction patterns of SED phosphors were recorded by a MSAL-XD-2 X-ray diffractometer using Cu Kα radiation, λ = 1.5418 Å, at 36 kV tube voltage and 20 mA tube current. A Hitachi F-7000 Fluorescence Spectrophotometer equipped with a 150 W xenon lamp as excitation source was used to measure the optical properties of the phosphors Thermoluminescence analysis of bleached samples was carried out by a Pyroelectric Spectrograph Thermoanalyzer from room temperature to 430 °C at a heating rate of 1 °C s−1.
3. Results and discussion
3.1 X-ray diffraction patterns
The composition and phase purity of samples were examined by X-ray powder diffraction patterns. The powder pattern for SED is shown in Fig. 1 and is compared with the standard file. The weak peak at 21.0° is due to Sr4Al14O25 impurity. At room temperature, SrAl2O4 crystallizes in the monoclinic space group P21 (No. 4: Z = 4), and Shi et al.  have provided evidence that Eu2+ ions substitute at the two 9-coordinate Sr2+ sites, with preference of the tripositive lanthanide ions for the slightly smaller site.
3.2 Photoluminescence emission, excitation spectrum and persistent luminescence spectrum
The room temperature photoluminescence excitation (a), emission (b) and persistent luminescence (c) spectra of the SED phosphor were measured and are depicted in Fig. 2. The green emission in both photoluminescence and persistent luminescence is due to the 4f65d1 → 8S7/2 4f7 transition of Eu2+ situated at one of the two Sr2+ sites . The assignment of this emission to a specific site has received much attention, as summarized by Botterman et al. . The low temperature emission spectra of these authors exhibit a broad feature with peak maximum at 521 nm and another weaker, sharper band with maximum intensity at 445 nm. Our 80 K emission spectra (not shown) are in agreement with this. On the basis of an empirical theory, first equating the configuration of Eu2+ to 5d1, then coupling each 5d1 Kramers level with 7 (instead of 3003) 4f levels, these authors associated the lower energy emission to that of Eu2+ situated at the Sr(1) site, which has larger average bond distances and a slightly larger coordination polyhedron volume. Subsequently, following the analysis of the spectra of Ce3+ diluted into the SrAl2O4 host from first principles calculations, a contrary conclusion was established: the green emission originates from Eu2+ situated at the smaller Sr(2) site .
3.3 Persistent luminescence decay curve
In the dark, the green persistent luminescence (upper inset in Fig. 3) can be seen by the naked eye for one hour after stoppage of UV excitation. The decay curve in Fig. 3 depicts the persistent luminescence decay of the SED disc monitored at 510 nm following 20 min excitation at 356 nm. The tail of the curve after 4000 s indicates the background level.
The decay curve is not well-fitted in the region from 0 to 1750 s by the equation I(t) = atn, with n = −1.0 (R2adj = 0.9845). However, the fitting improves for longer times (1020-3900 s) with n = −1.7 (R2adj = 0.9984). Kamiyanagi et al.  have stated that this type of fitting indicates that the afterglow originates in the tunneling recombination within defect pairs of trapped holes and trapped electrons.
The decay can be fitted in the range from 0 to 4000 s by a biexponential function with lifetimes of 9.9 ± 0.06 s and 74.7 ± 0.6 s (R2adj = 0.9972). As pointed out by Korthout et al. , the origin of the two components is not known but presumably corresponds to different types of trap center, different levels of the same trap species and/or retrapping.
From 0 to 400 s after stopping excitation, the decay curve in Fig. 3(a) is poorly fitted by a monoexponential decay function (R2adj = 0.9567) and more accurately fitted by a bi-exponential decay function (R2adj = 0.9972) with the two lifetimes of 9.91 ± 0.06 s and 74.9 ± 0.6 s. From 400 s to 2000 s delay after excitation, the monoexponential fit is much poorer and the biexponential fit (R2adj = 1.0000) gives two different lifetimes of 148.5 ± 0.5 s and 497.5 ± 1.6 s. The lifetimes therefore increase with time but the ratio of the normalized amplitudes of the lifetimes changes. In fact, when fitting the persistent luminescence decay from 2000 s to 3950 s delay after excitation with biexponential decay (R2adj = 0.9992), the lifetimes become even longer: 217 ± 44 s and 988 ± 26 s. Biexponential fittings of the lifetimes τ1 and τ2 of the persistent luminescence decay over various time periods, as indicated by horizontal bars with the corresponding R2adj values above, are displayed graphically in Fig. 3(b). The fraction of the total emission intensity for the components τ1 and τ2 (as defined in ) is indicated at the side of each data point. The fractional amplitude of the shorter lifetime increases with time, whereas that of the longer lifetime decreases. The plots of τ1 and τ2 against time are each fitted by exponential growth functions as shown by solid lines:Fig. 3(b) indicate that we are not dealing with discrete traps but with multiple, or a continuum, of traps, or continuum of levels of the same trap species.
3.4. Persistent luminescence excitation spectrum
To evaluate the effectiveness of different excitation wavelengths for persistent luminescence, the excitation wavelength-persistent luminescence intensity of the SED disc was investigated to discern the most efficient excitation wavelength. Figure 4 displays the persistent luminescence intensity at 510 nm for different exciting wavelengths, after ceasing illumination for 10 s, normalized by the intensity of the exciting xenon lamp radiation. First, the persistent luminescence was not detected at the range of excitation wavelengths longer than 479 nm, which corresponds to the absence of absorption by Eu2+. The maximum intensity of persistent luminescence is observed under excitation at 335-365 nm, corresponding to the maximum absorption of Eu2+ at both sites. From the low temperature excitation spectrum (Fig. 3 in ), the excitation wavelength range from 432 nm to 470 nm (Fig. 4) is associated only with absorption by only the Sr(2) site. Aitasalo et al.  have commented that the excitation processes for the photoluminescence and persistent luminescence of SrAl2O4:Eu2+ are significantly different but the result in Fig. 4 shows distinct similarity between the two processes in the wavelength region that we have investigated.
Figure 5 depicts the thermoluminescence spectrum over the temperature region from 40 °C to 430 °C with different waiting times of 10 s, 10 min and 40 hours respectively after pre-irradiation by ultraviolet radiation for 2 min. The three curves all contain two broad bands at ~130 °C and 380 °C. However, the lower temperature peak comprises several unresolved components. The shift of the peaks with varying delay time is small. When the waiting time increased, the intensity of the peak at ~130 °C decreased whereas that of the peak at 380 °C did not change. This indicates the preferential release of electrons in shallow traps.
Aitasalo et al.  reported two peaks within the temperature range from 40 to 240 °C: located at 80 °C and 160 °C in the thermoluminescence spectrum of SrAl2O4:Eu2+ after pre-irradiation and delay of 3 min. The initial rise method showed that the lower temperature peak corresponded to at least three traps, with depths of 0.55-0.65 eV. The relative intensities of the peaks in Fig. 5 differ considerably from those in Fig. 4 of  – notably that the 80 °C peak is much weaker herein and the ~130 °C peak appears. By contrast, Chang et al.  observed a broad band with maximum at 103 °C in the thermoluminescence spectrum of SED:xEu2+,yDy3+. Takasaki et al.  calculated trap depths of 0.3 eV and 0.6 eV. The results differ not only because of differences in synthesis but also instrumental parameters. However it is clear that trap depths of 0.3-0.5 eV occur.
3.6 NIR photostimulated luminescence
The inset of Fig. 6 shows the excitation spectrum of photostimulated persistent luminescence when monitoring the emission at 510 nm and stimulating the SED disc with near infrared (NIR) radiation after 30 mins of ceasing pre-irradiation of the sample with 420 nm UV excitation for 5 min. The maximum emission intensity occurs for the wavelength range between 740 and 780 nm. This photostimulation corresponds to the elevation of an electron from a filled trap level to the conduction band, from whence recombination occurs at a Eu3+ ion, leading to 4f65d1 → 4f7 emission of Eu2+. From the figure, the smallest trap depth in the measurement range is then less than 1.4 eV and the traps form a continuum, with maximum density at 1.6 eV. These experimental results agree with the report of Ji et al. . However, energy labeled E2 at 2.6 eV in , assigned to the removal of an electron from the valence band to the ground state of Dy3+, in fact corresponds to the commencement of the 4f7 → 4f65d absorption spectrum of Eu2+.
Although it is contended that Dy3+ is not the trap , the current model of persistent luminescence involves the ionization of the Eu2+ 5d electron to conduction band states and the subsequent trapping by Dy3+ [30,31], The model is partly based upon the vacuum referred binding energy (VRBE) diagram with the electronic ground state of Dy2+ situated slightly below the bottom of the conduction band. Firstly, the band gap of SrAl2O4 (or the co-doped system) has been determined to be 4.98 eV , 6.5 eV , 6.6 eV [19,33] and 7.28 eV  thus making the trap location below the conduction band uncertain. Secondly, the charge transfer energy of Eu3+ in SrAl2O4 has been given as 4.96 eV , 4.68 eV  and as 4.77 eV in CaAl2O4:Eu3+,Li+ . The electronic ground state of Dy2+ is then located at 2.2-2.4 eV higher than this value above the valence band maximum. Hence there are some uncertainties in the construction of this diagram. Furthermore, the location of the Dy2+ ground state is affected by the environment of this ion – for example being lowered with respect to the conduction band when adjacent to an oxygen ion vacancy. If we accept the VRBE explanation [30,31] then the pseudo-continuous trap distribution highlighted in the present study must be accounted for. The 4f10 Dy2+ ion has a similar electronic structure to Ho3+, although the larger size of Dy2+ causes the inter-electronic repulsion to be less so that the terms are closer in energy. In fact, Dy2+ has many (>30) energy levels extending from the electronic ground state up to ~0.7 eV and the de-trapping rates from these (intermediate) levels may differ, thereby accounting to some extent for the change in decay lifetimes with time of persistent luminescence decay. The 5I8 levels will be in thermal equilibrium. The energy separation of Dy2+ 5I8-5I7 trap states is estimated to be ~0.6 eV by comparison with 4f10 Ho3+. Moreover, the Dy2+ ground state energy at a specific site is subject to the particular environment, which is expected to vary due to the absence of charge compensation and the presence of random charges. First principles density functional calculations of the electron trap(s) would be helpful in identification, as demonstrated in .
The main panel of Fig. 6 shows the effect of photostimulation by 760 nm radiation upon the persistent luminescence decay of the SED disc. When the radiation is switched on (“On”) the persistent luminescence intensity increases rapidly and then reaches saturation. As noted by Ji et al. , the function of the NIR radiation is to empty filled traps and not to fill new traps. The rise portions of Fig. 6 can be fitted by biexponential functions with lifetimes of 29.3 ± 0.5 s and 4.7 ± 0.4 s. The rise curves in Fig. 6 might be explained by a partial (re)trapping in shallower traps. For the “On” phase, the deep trap release by the NIR radiation does not yield immediate recombination, but partial retrapping in shallower traps, leading to a certain extent a redistribution in trap depth occupation. For the “Off” phase, the persistent luminescence intensity increases compared with a normal persistent luminescence decay curve due to retrapping in shallower traps after NIR photostimulation.
When turning off the NIR excitation source, the green luminescence decays slowly, exhibiting a long-lasting luminescence. After many times of turning on and off NIR photostimulation, the sample still shows good photostimulated persistent luminescence. This means that the SED sample can repeatedly read information using NIR light after one period of UV excitation, indicating its potential application in optical information storage.
3.7 The write-in and read-out properties of the SED phosphor
The SED phosphor has a potential application in optical storage. The ranges of write-in and read-out wavelengths for this phosphor are summarized in Figs. 7(a),(b), respectively. Figures 7(a) and (b) are similar to Fig. 4 and the inset of Fig. 6, respectively, but the experiments utilized to construct these figures were different. The method of construction of the Figs. 7(a),(b) was as follows. For the write-in Fig. 7(a), the sample was irradiated for 2 min by different monochromatic ultraviolet wavelengths between 240 nm to 490 nm. Then the pre-irradiation was stopped for 30 s. Immediately, radiation with the NIR wavelength of 760 nm was employed to stimulate the sample for 70 s. The emission intensity at 510 nm after 10 s of ceasing NIR stimulation was then plotted against the ultraviolet irradiation wavelength. The NIR photostimulation raises electrons from deep traps populated by pre-irradiation. Wavelengths beyond 430 nm up to 490 nm are not efficient in populating these traps and the most efficient write-in wavelengths are between 330 and 360 nm. Figure 7(c) shows the persistent luminescence decay at 510 nm as above when using the different write-in wavelengths of 290 nm, 350 nm and 450 nm and ceasing the pre-irradiation, prior to photostimulation. Fitting these decays by biexponential functions over the same time duration shows that the short decay lifetimes are the same within experimental error, whilst the long decay lifetime is rather longer for 450 nm charging. Figure 7(b) shows the read-outs (also quantified by emission intensity at 510 nm) following charging by 430 nm radiation for 2 min, decay for 30 s, then stimulation by NIR radiation between 600 and 900 nm (separately, using 10 nm steps) for a further 70 s, and taking the intensity reading after the NIR stimulation stopped for 10 s. The phosphor has good read-out performance when using wavelengths between 630 and 850 nm, with the optimum value at 760 nm. Figure 7(d) depicts the NIR photostimulation part of the read-out experiment, with duration of 70 s. Based upon the NIR photostimulated persistent luminescence phenomena, the write-in and read-out capabilities of the phosphor can be applied in optical information storage.
Numerous reports have been previously made concerning the persistent luminescence phosphor SrAl2O4:Eu2+,Dy3+. In the present work the phosphor SED has been synthesized by a high temperature solid state reaction and the persistent- and photo- luminescence and excitation spectra, thermoluminescence and photostimulated properties have been investigated. The major new findings are that the persistent luminescence decay curve, as fitted by a biexponential function, exhibits a change in lifetimes with time of persistent luminescence. The fraction of the total emission intensity increases for the shorter lifetime component with increasing time. The change in lifetime has been associated with the emptying of a pseudo-continuum of trap states, either from different traps or from different levels of the same trap species. Secondly, photostimulation of the phosphor can take place over a wide range of NIR wavelengths, which is consistent with this type of distribution of traps over a wide energy range. However, as argued above, these facts do not exclude the postulate that Dy3+ is the electron trap species. Finally, the use of the phosphor in information storage and retrieval has been emphasized.
National Natural Science Foundation of China (Grant No. 11574058); Science and Technology Program of Guangzhou City of China (Grant No. 201607010102).
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