1. Introduction

Among up-to-date information storage technologies, the optical information storage has emerged and developed rapidly into a new research direction, owing to its significant advantages of high density, large capacity, long storage lifetime, and environmental friendliness [1–5]. Therefore, it’s an inevitable trend that novel practical optical materials need to be continuously investigated to meet rigid demands of extending storage capacity, multiplexing storage, and prolonging service life during optical storage development [6–8]. Among chemically stable inorganic materials, photo-stimulated luminescence materials (PSLs) have been paid more attention to candidates for optical storage, because PSLs are so-called electron trapping materials with capability of realizing “write-in” and “read-out” of optical information [9–12]. Obviously, there are two sorts of active centers involved in PSLs, which are traps and emitting centers [12]. The emitting centers are responsible for write-in of information in the form of lighting, while the traps act as an important role in read-out of optical information upon long-wave photo-stimulation by releasing the captured charge carriers in traps. Whether the information can be read-out strongly depends on the trap properties: trap type, concentration, and depth. However, up to now the ideal PSLs are very limited, due to the lack of PSLs with deep traps, narrow distribution of traps, and appropriate energy gap between shallow and deep traps [13]. Herein, traps tailoring strategy in PSLs is a challenging work [14].

In recent years, rare earth (RE) doped Sr₂SnO₄ (SSO) compounds (RE = Nd, Eu, Er) have attracted great attention due to their various optical functional properties, such as photoluminescence, up-conversion, and mechanoluminescence [15–17]. Rare earth doped SSO compounds possess a layered perovskite-related structure with space group I4/mmm (No. 139), in which the doped trivalent rare earth ions are mostly incorporated into Sr²⁺ sites and then at least two types of point defects, RE_sr^• and V_Sr^″. These two point defects are created in host lattice because of charge compensation mechanism, where RE_sr^• expresses RE doping defect substituted at Sr²⁺ sites and V_Sr^″ is Sr vacancy defect for two RE³⁺ located at two Sr²⁺ sites [18]. Therefore, the produced defects may play an important role in trap sites of photo-generated electrons (or charge carriers) in the compounds [18–21]. Regretfully, the PSL performance of RE single doped SSO is not ideal as expected. For example, Nd³⁺ doped SSO samples, with a major emission at ∼1079 nm (λ_ex=254 nm) and insufficient trap depth (0.51∼0.73 eV), are not suitable for intuitive optical storage [20]. To break bottlenecks, a co-doping strategy should be operated to enhance the deep trapping and shift the emission position towards visible range. The strategy really worked that recently PSL properties of some Nd³⁺ doped PSLs are enhanced by co-doping other elements, such as Y₃Al₂Ga₃O₁₂:Ce³⁺,Nd³⁺, CaAl₂O₄:Eu²⁺,Nd³⁺, and BaSi₂O₅:Eu²⁺,Nd³⁺, in which Ce³⁺ or Eu²⁺ ions act as activator centers for emission, while Nd³⁺ act as suitable auxiliary ions for inducing new traps with appropriate depth [13,22,23].

In our present research, a co-doping strategy for the compound SSO was proposed and carried out, in which SSO as host, Eu³⁺ as emission centers, and Nd³⁺ as aid ions for inducing new traps, to study the significant effects of Nd³⁺ co-doped with Eu³⁺. We report two comparable compounds SSO:0.01Eu³⁺ (SSOE) and SSO:0.02Nd³⁺,0.01Eu³⁺ (SSONE). Subsequently, the optical information storage related properties were systematically studied. The results suggest that the co-doping strategy plays an important role in optical information storage by effectively enhancing trap depth and separating shallow traps with deeper ones. Further, a demonstration experiment was performed to display the optical information write-in and read-out in the form of patterned phosphors.

2. Experimental details

One Eu³⁺ single-doped phosphor Sr_1.99SnO₄:Eu_0.01 (SSOE or SSO:0.01Eu³⁺) and one Nd³⁺/Eu³⁺ co-doped phosphor Sr_1.97SnO₄:Nd_0.02Eu_0.01 (SSONE or SSO:0.02Nd³⁺,0.01Eu³⁺) were prepared by a two-step solid state reaction method. The doping concentrations have been optimized in our previous experiments. The raw materials SrCO₃ (99.95%), SnO₂ (AR), Nd₂O₃ (3N), Eu₂O₃ (4N) were weighed according to the stoichiometric ratio without any further treatment. These raw powder materials were thoroughly mixed and ground in an agate mortar with alcohol. Then the mixtures were transferred into crucibles, loaded into a muffle furnace, then were calcined at 600 °C for 2 h in air atmosphere. In the following procedures, the calcined mixtures were thoroughly ground again. Next, the obtained powders were further calcined at 1200 °C for 6 h in air atmosphere, cooled down to room temperature, and then finally ground in an agate mortar. The obtained powders were ready for following measurements and characterizations.

A Bruker D8 ADVANCE X-ray diffractometer (XRD) was employed to characterize phase and crystal structures of samples using Cu Kα radiation at an interval of 0.02° in the 2θ range from 10° to 80° with a scanning speed of 5°/min. UV-visible absorption spectrum was recorded by SHIMADZU Model UV-2600 to determine the excitation wavelength and calculate band gap of phosphors. X-ray photoelectron spectroscopy (XPS) measurements were performed by an electron spectroscopy for chemical analysis system (Thermo Fisher Scientific ESCALAB250) with an Al-Kα X-ray gun source, operating at 2 kV, 1µA with pass energy peaked at 50.0 eV, to confirm the valence of doped rare earth ions. Electron paramagnetic resonance (EPR) was measured by Bruker BioSpin GmbH X-band (9.89 GHz). Photoluminescence (PL) spectra, photoluminescence excitation (PLE) spectra, PSL spectra, and TSL were measured by a fluorescence spectrophotometer (Hitachi Model F-4600, Xe lamp as light source, 150 W). Before PSL measurement, a 254 nm Hg lamp (6 W) was used as the irradiation source to charge samples for 10 min, then the excitation was ceased for 1 min, and then the samples were excited again by 808 nm or 980 nm NIR laser. For TSL measurement, the samples experienced an irradiation for 10 min upon 254 nm light, then the lighting was ceased for 1 min, after that, the samples were heated to 150 °C and measured. The time-dependent TL curves were recorded from 300 K (27 °C) to 550 K (277 °C) at a heating rate of 1 K/s measured by a SL08 TL meter (Radiation Science and Technology Co. Ltd, Guangzhou, China). Before TL curves were measured, all samples were first excited by UV (254 nm) irradiation. All measurements were executed at RT except for TL measurements.

To demonstrate the capability of the prepared SSONE powders for optical information storage, an aluminum plate was used as substrate and carved into hollow information patterns (“SICCAS”, an abbreviation of our institute name, Shanghai Institute of Ceramics, Chinese Academy of Sciences). Then, the patterns were filled with the SSONE powders. After that, a Hg lamp (6W, 254 nm) was used to irradiate the patterned phosphors “SICCAS” for 10 min to display the stored (or write-in) optical information by visible light emitting (rose-red in color) from patterns. The stored optical information was retrieved (or read-out) by placing the patterns on a hot stage of 150 °C or exposing the patterns under a 980 nm laser spot.

3. Results and discussion

3.1 Crystal structure analysis and elemental analysis

Figure 1(a) shows XRD profiles for the two phosphors, which well matched with the standard pattern of tetragonal SSO (PDF #87-2479). Furthermore, a 2 × 2×1 super cell of SSO host is exhibited in the inset of Fig. 1(a), drawn by ourselves using VESTA software (Version 3.0.1) with relative crystal parameters downloaded from Materials Project (https://materialsproject.org/), showing a layered perovskite-related structure with unit cell parameters a = b = 8.1056 Å, c = 12.5852 Å. When smaller Eu³⁺ (0.95 Å) and Nd³⁺ (0.995 Å) ions entered Sr²⁺ (1.13 Å) sites in SSO with RE-O coordination number of nine [24], it requires charge compensation, which will create vacancies on Sr²⁺ sites. Also some oxygen will be lost to retain the overall stoichiometry during the formation of SSONE and SSOE phase. Figure 1(b) represents the measured UV-visible absorption spectrum of SSO host. Then the optical band gap energy Eg of SSO can be calculated by the Kubelka-Munk formula [25]. As indicated in Fig. 1(b), the calculated Eg value of SSO host is ∼4.49 eV. Obviously, SSO host can provide an appropriate band structure for doped ions to be effective of traps and emission centers.

 

Fig. 1. (a) XRD patterns of as-prepared SSONE and SSOE samples and the standard pattern of SSO host (PDF #87-2479). (b) Relationship between photon energy hν and plot of (Ahν)² of SSO host, E_g calculated from the Kubelka-Munk formula. Inset: the absorption spectra of SSO host.

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XPS measurements were used to analyize the chemical state of doping elements in phosphors. However, the dopant concentration is too low to get intensive peaks. Figure 2 shows XPS spectra of Eu-3d (Fig. 2(a)) and Nd-3d (Fig. 2(b)) in SSONE. The two bands around 1134.5 eV and 1168 eV are expected for Eu³⁺, resulting from the states of 3d_5/2 and 3d_3/2, respectively [26]. While, the two broad bands around 976 eV and 1004 eV are acsribed to the states of 3d_5/2 and 3d_3/2 doublet in agreement with the available data for Nd³⁺ [27,28].

 

Fig. 2. XPS spectra of (a) Eu-3d and (b) Nd-3d in SSONE sample.

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3.2 Photoluminescence properties

Figure 3(a) shows the characterized emission (PL) and excitation (PLE) spectra of SSONE and SSOE. The samples both give two narrow emission bands at 596 nm and 615 nm upon 254 nm irradiation, assigned to ⁵D₀→⁷F₁ (⁷F₂) transition of Eu³⁺ [29]. Under the monitor of 596 nm light, PLE spectra of SSONE and SSOE center at 250 nm with a narrow band from 200 nm to 300 nm. Based on the principle of energy storage and release, PL, PSL and TSL spectra properties are closely related to the optical information storage characteristic of materials, which is not only determined by the luminescent center but also depended on the appropriate trap depth in materials [9]. Therefore, to realize a simple detect of optical signals, the characteristic peak center of PL spectrum should be consistent with the peak centers of PSL and TSL spectra for optical storage materials. The obtained peak centers of PSL and TSL spectra for co-doped SSONE are both at 595 nm, extremely closed to 596 nm in PL, as seen in Fig. 3(b), attributed to the same emission center Eu³⁺. However, as depicted PSL in Fig. 3(b) and the inset (i) of Fig. 3(b), there are no any significant signals of PSL for single-doped SSOE, that means no optical information storage property in SSOE sample upon photo-stimulation. Moreover, as displayed in Fig. 3(c), the rose-red color emitting presents information write-in, from the patterned “SICCAS” filled with SSONE phosphors, upon UV irradiation. Then, after the UV irradiation source removed, the stored optical information is read-out by placing the patterned phosphors on a hot stage at 150 °C (Fig. 3(d)), with emitting the rose-red color light again. As shown in the inset (ii) of Fig. 3(b), SSONE powders (without patterned) were exposed under a 980 nm laser spot (spot size: ∼5 × 8 mm), in which the read-out information presents same color as that shown in Fig. 3(d). That means the SSONE powders can realize the optical data read-out by both thermal stimulation and optical stimulation. In the following characterizations, time-dependent TL curves were performed to further confirm the optical storage.

 

Fig. 3. (a) PL and PLE spectra of SSONE and SSOE (λex = 254 nm, λem = 596 nm). (b) PL, PSL (808 nm excitation) and TSL (150 °C thermal stimulation) spectra of SSONE sample, and PSL (808 nm excitation) spectrum of SSOE. Inset (i) and (ii) present information read-out figures of SSOE and SSONE upon 980 nm laser spot. (c) Information write-in (photo-stimulated under 254 nm UV light) and (d) read-out (thermal-stimulated at a hot stage of 150 °C) of SSONE phosphor patterned as “SICCAS” on an aluminum plate.

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3.3 Thermoluminescence properties

In addition to PSL and TSL spectra showing the capability of optical dada read-out, TL glow curves can give more accurate description about the “read-out” mechanism, in which the captured charge carriers overcome the energy barrier to escape from traps by absorbing heat energy. TL curve measurement is widely considered as one of the most powerful techniques for exact insights into the traps, including trap distribution and trap depth [30]. Figure 4(a) depicts the TL curves of either SSONE or SSOE samples measured with a time interval of 1 min after removal irradiation of 254 nm light for 5 min, covering a broad range from 300 K to 550 K. It’s obvious that the TL curve of SSONE phosphor shows two bands centered at 320 K (peak 1) and 388 K (peak 2), while the TL curve of SSOE phosphor only displays one band centered at 362 K. This result demonstrates the introduced deeper traps and separated shallow and deep traps in the co-doped system, that should be caused by extra addition of activator Nd³⁺ for creating more defects or traps. These phenomena are very crucial for optical information storage to obtain deep traps and avoid interference between traps. As mentioned above, when smaller Eu³⁺ and Nd³⁺ ions enter larger Sr²⁺ sites in SSO, it requires charge compensations, with creating vacancies on Sr²⁺ sites for the incorporation of Eu³⁺ and Nd³⁺. Also some oxygen will be lost to retain the overall stoichimmetry in the formation of SSONE and SSOE. The created vacancies and lost oxygen may be in charge of new defects for modulating trap and optical storage properties. The created traps can be described by the following Eq. (1):

 

Fig. 4. (a) Normalized TL glow curves of SSONE and SSOE. After 254 nm irradiation for 5 min at delay time t = 1 min and a heating range from 300 K to 550 K with a heating rate of 1 K/s. Inset is Gaussian Fitting peaks of TL glow curve of SSONE. (b) Peak 2’s temperature and relative intensity of TL at different delay durations. Inset is TL glow curves of SSONE at various delay duration, after excitation by 254 nm irradiation for 5 min.

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As for the evidence of defects existence, the preliminary EPR measurement of SSONE sample was carried out. The relative values of g-factor in EPR spectrum are calculated to be 1.96, 2.11, and 2.35. The g of 1.96 is rationally speculated that there are some unpaired electrons trapped in oxygen vacancies in SSONE sample [20,31,32]. The other g values of 2.11 and 2.35 are possibly related to Nd_Sr•/Eu_Sr• and V_Sr″ [19–21], respectively. The EPR results to some extent confirmed that there existed traps from defects in SSONE. Other evident experiments on defects are in progress.

And as depicted Gaussian fitting in the inset of Fig. 4(a), the TL curve of SSONE sample can be further divided into five bands (marked as 1, 2, 3, 4, 5) centered at about 325 K, 361 K, 387 K, 432 K and 493 K, respectively. By using the Urbach empirical equation [33], the trap depth E can be approximately calculated from the TL curve. Accordingly, the traps depth can be roughly calculated to be 0.650 eV, 0.722 eV, 0.774 eV, 0.864 eV and 0.986 eV, most of which are obviously higher than the measured trap depth of SSOE (around 0.724 eV) and the reported value 0.808 eV in the single-doped SSO:Nd³⁺ [20].

To obtain more details, the TL intensity recorded at various delay duration and band center are indicated in Fig. 4(b), using peak 2 as example. After 254 nm irradiation for 5 min, the TL fading curves of SSONE are recorded in an ever-increasing time span (1 min ∼ 48 h). With delay duration less than 3 h, TL intensity slowly decreased and finally about 67% intensity of original TL intensity (delay 1 min) retained. Even though after 48 h delay duration, there is still 21% intensity of original TL intensity left. TL intensity slightly decreases while the relative peak of TL moves to higher temperature with increasing delay duration. The reason of the TL peak shift is that the charge carriers gradually escaped from shallow traps with delay increasing, owing to the disturbing of hot energy from environment temperature. As delay duration prolonged, shallower traps were gradually empty and then charge carriers would turn to escape from deeper traps. It is reasonable to speculate that there exists a continuous trap distribution in SSONE. And it also indicates that the captured charge carriers easily escape from shallow traps with increasing delay duration and shallow traps are easily prone to empty, causing the TL intensity decrease and temperature shift up. Yahong Jin’s group has also showed the same phenomena in their research, agreeing with our results [25].

In order to obtain more insights into the trap properties, the time-dependent TL glow curves with details about the capture and release behaviors of charge carriers by trapping or de-trapping in SSONE have been investigated. First of all, TL curves of SSONE under 254 nm irradiation for various duration time (1 ∼ 300 s) are analyzed and illustrated in Fig. 5(a). In Figs. 5(b) and 5(c), they show that the intensity of peak 1 and peak 2 gradually increases while excitation duration extends. It reveals that traps in the phosphors can be only filled with charge carriers under a longer-time (>10 s) irradiation. It can also suggest that there may exist multiple trap levels with a continuous distribution rather than discrete trap levels for each TL peak in SSONE sample [16].

 

Fig. 5. (a) TL glow curves of SSONE sample measured with a delay interval of 1 min after excitation by 254 nm light at various duration. (b) Peak 1: temperature and relative intensity of TL at different excitation duration. (c) Peak 2: temperature and relative intensity of TL at different excitation duration.

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3.4 Optical information storage mechanism

As discussed above, a possible schematic illustration of optical information storage, as well as PSL and TSL mechanism in SSONE is proposed in Fig. 6, referred to the works of Dorenbos [34,35]. Under UV light excitation, some electrons located at the ground state of Eu³⁺ would be promoted to ⁵D₀ excited state and then to CB (pathway (i)). Then a part of the excited electrons (or charge carriers) transition occurs from ⁵D₀ to ⁷F₁ or ⁷F₂ (pathway (ii)), realizing PL emission of Eu³⁺, while the other part of charge carriers can be captured by traps with different levels (pathway (iii)), complying information write-in. After removing UV excitation, the charge carriers stored in shallow traps can easily jump to the CB and then transfer back to the ground state of Eu³⁺ at RT condition (pathway (iv)), emitting same color luminescence.

 

Fig. 6. Schematic illustration of optical information storage, PSL and TSL mechanism possibly happened in SSONE phosphor under thermal- and photo-stimulation. The energy level diagram was constructed in a host-referred binding energy (HRBE) scheme.

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When the shallow traps are almost empty, the charge carriers captured in deep traps can be gradually released to the CB and transfer to the ground state of Eu³⁺ (pathway (v) or (vi)) with the assistance of thermal stimulation (150 °C) or NIR light (808/980 nm) stimulation, resulting in PSL or TSL emission and decoding (or read-out) optical information stored before in the form of luminescence. Through the above processes, the optical information writing-in and reading-out are finally realized.

4. Conclusion

In conclusion, the prepared SSONE phosphor was found to be a promising phosphor for optical information storage, with its band center of PL under UV identical with PSL upon NIR laser and TSL under heating, attributed to the same emission center of Eu³⁺. Remarkably, the Nd³⁺ and Eu³⁺ co-doping strategy enhances the deep traps and also separates the deep traps with shallow traps, which are very important for realizing optical information storage, with appropriate trap depth and no interference between traps. Finally, the time-dependent TL glow curves reveal the existence of multiple trap levels with continuous distributions rather than discrete trap levels in the co-doped phosphor. In addition, an evidence of potential applications in optical information storage is shown in a demonstration experiment. The presented SSONE phosphor is obviously a kind of promising candidate material in optical storage fields.