Photoluminescence and afterglow behavior of Ce³⁺ activated Li₂Sr_0.9Mg_0.1SiO₄ phosphor

Abstract

A blue emitting phosphor Li₂Sr_0.9Mg_0.1SiO₄:Ce³⁺, with long persistence, was synthesized via a high-temperature solid phase method. According to the X-ray diffraction analysis result, the introduction of Mg²⁺ and Ce³⁺ ions has no influence on the structure of the host material. Typical 5d-²F_5/2 and 5d-²F_7/2 transitions of Ce³⁺ ions were detected by PL spectra, which corresponded to the CIE chromaticity coordinates of x = 0.1584, y = 0.0338. An optimal doping concentration of Ce³⁺ was determined as of 0.4 at%. Furthermore, the Li₂Sr_0.9Mg_0.1SiO₄:Ce³⁺ phosphor showed a typical triple-exponential afterglow behavior when the UV source was switched off. The highest lifetime of the electrons within the material reached a value of 73.9 s. Thermal stimulated luminescence study indicated that the afterglow of Li₂Sr_0.9Mg_0.1SiO₄:Ce³⁺ was due to the recombination of the electrons with holes released from the traps generated by the doping of Ce³⁺ ions in the Li₂Sr_0.9Mg_0.1SiO₄ host. The afterglow mechanism of Li₂Sr_0.9Mg_0.1SiO₄:Ce³⁺ is illustrated and discussed in detail on the basis of the experimental results.

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1. Introduction

Long-lasting phosphors (LLPs) are a kind of energy storage materials with the decay time extended to seconds, minutes, or even hours after the removal of ultraviolet (UV) and visible light at room temperature.^1–4 These materials have been widely studied by many researchers because of their excellent properties, such as long persistence time, stable crystal structure, high physical and chemical stabilities.^5–8 The most commonly used long lasting phosphors (LLPs) are CaAl₂O₄:Eu²⁺, Nd³⁺ (blue), SrAl₂O₄:Eu²⁺, Dy³⁺ (green) and Y₂O₂S:Eu³⁺, Ti⁴⁺, Mg²⁺ (red).^9,10 It is generally agreed that long luminescence lifetimes can be achieved by adjusting the host crystal bandgap and the energy level by introducing rare-earth ions. However, it is difficult to generate an appropriate trap depth in these host materials; hence, the progress in this field is quite slow. Therefore, extensive studies still need to be carried out in order to achieve the best performance of the long-lasting phosphorescent materials.

Recently, a lithium silicate system has been regarded as a suitable host with high chemical stability, and it can produce a suitable crystal environment imposing on the emission centers. Thus, such system has been widely investigated and silicate phosphors with various colors have been well recorded in documents.^11–17 We also reported some LLPs in the silicate system, where Li₂SrSiO₄ and Li₂Ca_0.6Sr_0.4SiO₄ were suggested as excellent hosts for LLPs.^18,19

It is also recommended that the phosphorescence of Ce³⁺ in most of the hosts is caused by the 5d-²F_5/2 and 5d-²F_7/2 energy-level transitions.^20–23 However, there has been no detailed report on the long lasting phosphorescence phenomenon of Ce³⁺ in lithium silicate phosphors. To understand the afterglow behavior of the Ce³⁺ doped lithium silicate phosphors, it is important to find a suitable host crystal with a certain bandgap that matches with the energy level of the Ce³⁺ ions. In this study, a series of blue emitting LLPs Li₂Sr_0.9Mg_0.1SiO₄:x% Ce³⁺ (x = 0.1, 0.2, 0.3, 0.4, and 0.5), was investigated to carry out the survey. The results demonstrated that it was the Ce³⁺ ions incorporated into the substrate that caused the afterglow. Thermal stimulated luminescence study indicated that the afterglow of Li₂Sr_0.9Mg₀.₁SiO₄:Ce³⁺ was due to the recombination of the electrons with holes released from the traps generated by the doping of Ce³⁺ ions in the Li₂Sr_0.9Mg_0.1SiO₄ host and we discussed the long-lasting phosphorescent phenomenon in detail by means of a completely trapping model.

2. Experimental procedures

2.1 Sample synthesis

The Li₂Sr_0.9Mg_0.1SiO₄:Ce³⁺ phosphor was prepared by the traditional high-temperature solid phase reaction, using the raw materials, SrCO₃, Li₂CO₃, MgO, SiO₂, and CeO₂, each with a purity of 99.99%. The Ce³⁺ doping level ranged from 0.1%, 0.2%, 0.3%, 0.4% to 0.5%. The raw materials were mixed in stoichiometric ratios, ground for 1 h and finally sintered at 850 °C under a weak reductive atmosphere (5% H₂ + 95% N₂). The samples were kept at this temperature for 10 h and then cooled to room temperature while maintaining the flow of the reducing gas till the furnace reached room temperature.

2.2 Characterization

X-ray diffraction was performed using a TD-3500 X-ray diffractometer with Cu K_α irradiation to collect the crystal information of the as-prepared phosphors. A Hitachi F7000 fluorescence spectrophotometer was used to detect the excitation and emission spectra of the samples. The afterglow behavior and the trap energy level determinations were measured using an FJ427-A1 thermally stimulated spectrometer.

3. Results and discussion

3.1 Phase identification

The phase and structure of the samples were determined using X-ray powder diffraction. Fig. 1 indicates the composition of Li₂Sr_0.9Mg_0.1SiO₄ phosphors doped with different concentrations of Ce³⁺ (0.1%, 0.3% and 0.5%), which are compared to the standard file from the JCPDS card no. 047-120. It is possible to suggest that the peaks of the samples correspond to single hexagonal phase.¹¹ Despite the increase in the Ce³⁺ dopant concentration (up to 0.5%), no other product was observed. The possible reason is that the radius of Ce³⁺ ions (r = 1.196 Å) being very close to that of Sr²⁺ (1.31 Å), the Ce³⁺ ions are expected to occupy the Sr²⁺ sites in the Li₂Sr_0.9Mg_0.1SiO₄ host lattice. It can be concluded from the results that the doped Ce³⁺ ions as well as Mg²⁺ ions, have been successfully incorporated into the lattice of the silicate compound, and the co-doping of Ce³⁺ and Mg²⁺ ions did not disturb the host crystal lattice.

Fig. 1 XRD patterns of Li₂Sr_0.9Mg_0.1SiO₄ and Li₂Sr_0.9Mg_0.1SiO₄:Ce³⁺.

3.2 Photoluminescence properties

Fig. 2 portrays the photoluminescence behavior of the Li₂Sr_0.9Mg_0.1SiO₄:0.004Ce³⁺ phosphor. A typical photoluminescence excitation and emission spectra of 0.4 at% Ce³⁺ activated Li₂Sr_0.9Mg_0.1SiO₄ phosphor is shown in Fig. 2a. From the PLE spectrum, a broad band is observed in the wavelength region of 250–350 nm, centering at about 277 nm and located at 253 nm, 276 nm and 314 nm respectively, which originates from the 5d → 4f transition of the Ce³⁺ ions.^24,25 As we can see, the emission spectra of the phosphor consist of two bands, peaking at 391 and 416 nm in the wavelength region of 350–550 nm under the excitation of 277 nm. Such emissions are typical for that of Ce³⁺ ascribed to the 5d-²F_5/2 and 5d-²F_7/2 transitions. Fig. 2b shows the corresponding CIE 1931 chromaticity diagram of the Li₂Sr_0.9Mg_0.1SiO₄:0.004Ce³⁺ phosphor. Point with chromaticity coordination (x = 0.1584, y = 0.0338) is located in the region of the blue color, indicating that the color of LLL in Li₂Sr_0.9Mg_0.1SiO₄:0.004Ce³⁺ is blue. In our study, the high quantum efficiency of the PL emission yields of up to 60% at 300 K were obtained. The value of the quantum efficiency in this case is higher than the Eu²⁺ ions as activator in the Li₂SrSiO₄ matrix in ref. 26, which means that the Ce³⁺ as an activator in the Li₂Sr_0.9Mg_0.1SiO₄ matrix is superior and valuable.

Fig. 2 PLE and PL (λ_ex = 277 nm) spectra (a), and CIE 1931 chromaticity coordinates of the Li₂Sr_0.9Mg_0.1SiO₄:0.004Ce³⁺phosphor (b).

The effects of doping concentration on the PL properties were further investigated. When being excited by 277 nm, the emission spectra of Li₂Sr_0.9Mg_0.1SiO₄:Ce³⁺ phosphors with different concentrations of Ce³⁺ are depicted in Fig. 3. It can be observed that the emission intensity of Li₂Sr_0.9Mg_0.1SiO₄:Ce³⁺ increases initially with an increase in the Ce³⁺ concentration, and then decreases with a maximum intensity reaching at a doping concentration of 0.4 at%. This phenomenon results from the concentration quenching, which was studied by Chen.²⁷ These results demonstrate that the optimal doping concentration of Ce³⁺ in Li₂Sr_0.9Mg_0.1SiO₄ experimentally determined to be 0.4 at%.

Fig. 3 PL (λ_ex = 290 nm) spectra of Li₂Sr_0.9Mg_0.1SiO₄:Ce³⁺ phosphors.

3.3 Long afterglow properties

As mentioned above, the Li₂Sr_0.9Mg_0.1SiO₄:0.004Ce³⁺ phosphors show a blue emission when excited under an ultraviolet source of 277 nm and exhibit long-lasting phosphorescence. The room-temperature decay curves of the long afterglow phosphorescence of Li₂Sr_0.9Mg_0.1SiO₄:0.004Ce³⁺ are depicted in Fig. 4. The decay characteristics of the Li₂Sr_0.9Mg_0.1SiO₄:0.004Ce³⁺ phosphor can be roughly divided into two processes, the rapid-decay process and the slow-decay process. The decay curves of the afterglow of the phosphors can be evaluated by the curve fitting technique. In this study, a triple-exponential equation, which has been used widely, can fit the experimental decay curves very well:²⁸where I₀ is the persistent luminescence intensity at time 0; A₁, A₂ and A₃ are the constants; t is the time, and τ₁, τ₂, τ₃ are the decay times for the rapid-decay and slow-decay components. Fig. 4 reveals the results of the fitting curve, with the maximum value (τ₃) reaching 73.9 s. It is known that the bigger the value of the decay time is, the slower the decay rate and the better the afterglow properties are.^29,30 The results imply that the Ce³⁺ ion-doped phosphor possesses a good persistence although the phosphor showed a rapid decay at the initial process.

Fig. 4 Afterglow curve of the Li₂SrSiO₄:0.004Ce³⁺ phosphor.

3.4 Thermoluminescence performance

In order to further investigate the nature of the trap in Li₂Sr_0.9Mg_0.1SiO₄:0.004Ce³⁺, thermoluminescence studies were carried out. The thermoluminescence curves indicating the integrated emission intensity is depicted in Fig. 5. As we all know, a proper energy level of the traps is essential for good afterglow properties and the depth of traps and the trap density in the phosphors can be dealt with the equation:^31,32where I(T) is the TL intensity, s the frequency factor, n₀ is the trap concentration of trap charges t = 0, E_t is the trap depth, k is the Boltzmann's constant, l is the kinetics order and b is the heating rate (1 °C s⁻¹ in the experiment). In our study, the parameters s, n₀, E_t and l were obtained by a computer fitting technique.³⁰ The thermoluminescence behavior can be analyzed by curve fitting as referred by Chen, relying on the following exponential equation:^33,34j is conceded to be 3 in this case and we got another eqn (4):

Fig. 5 TSL curve of the Li₂Sr_0.9Mg_0.1SiO₄:0.004Ce³⁺ phosphor.

The calculated trap depth and density of Li₂Sr_0.9Mg_0.1SiO₄:0.004Ce³⁺ phosphors are shown in Fig. 5. The trap level and trap concentrations are two useful indicators to evaluate the long duration phosphorescence of different phosphors. Having an electron trap of suitable energy level is necessary to create the afterglow behavior. The asymptotes can be found practically in all experiments, where the values of I, S and E are between 0.7 and 2.5, between 10⁵ s⁻¹ and 10¹³ s⁻¹, and between 0.1 and 1.6 eV, respectively. In this case, the corresponding values were in these ranges. It is well known that the trap created by the lattice defects play a very important role on the afterglow properties of the phosphors. According to Kuang's report, the trap depths should lie between 0.4 and 0.6 eV if the materials are to show excellent persistent phosphorescence performance.³⁵ The calculated trap concentration was 1.14 × 10¹⁰ mol⁻¹ and the depth of the trap energy level of Li₂Sr_0.9Mg_0.1SiO₄:0.004Ce³⁺ was 0.46 eV, which were very close to the values reported. We observed that the trap depth of 0.46 eV for 0.4 at% Ce³⁺ doped Li₂Sr_0.9Mg_0.1SiO₄ is suitable for excellent afterglow at room temperature.

3.5 Possible mechanism of persistence

Based on the above description and analyses, we are trying to elucidate the phosphorescence mechanism of Li₂Sr_0.9Mg_0.1SiO₄:Ce³⁺. In Ce³⁺-doped phosphor, it seems reasonable that the Ce³⁺ (r = 1.196 Å) ions are expected to occupy the incorporated Sr²⁺ (r = 1.31 Å) sites of the Li₂Sr_0.9Mg_0.1SiO₄ host due to their close ionic radius. The replacement can be explained by the mechanism: . In order to maintain the charge balance, it is the only replacement way possible, as three Sr²⁺ ions can be replaced by two Ce³⁺ ions, which induce two positive defects and one negative defect in the host. In our case, on the basis of our PL spectra and TL analysis results, Ce³⁺ not only acts as the luminescent center in the host lattice, but also serves as the trap center.

The detailed mechanism of the long-lasting phosphorescence is yet to be known. The afterglow of Ce³⁺ doped in the Li₂Sr_0.9Mg_0.1SiO₄ phosphor and the cause of such phenomenon was assumed to be due to the thermo-stimulated recombination of the holes at the traps induced by irradiation, which leave the holes or the electrons in a meta-stable excited state at room temperature. Based on the above results, a simple mechanism for the LLP of Li₂Sr_0.9Mg_0.1SiO₄:Ce³⁺ is proposed, which is presented in Fig. 6. Upon UV-irradiation (step 1), the electrons in the valence band are excited to the conduction band, resulting in the formation of free electrons and holes at the same time in the phosphor. Then, the energy associated with the excited electrons is transferred to the 5d level of Ce³⁺. The subsequent jumping of electron traps to the ground level ²F_5/2 and ²F_7/2 states of Ce³⁺ and the recombination of holes give rise to the characteristic emission of Ce³⁺ ions (step 2 and step 5). In this process, a number of free holes and excited electrons can be captured by the and trapping centers (step 3). After the UV irradiation is removed, these electrons trapped by Ce³⁺ will be released from the traps and transferred via the host to the luminescence center (step 4), followed by the recombination of free electrons and holes, leading to the characteristic emission of Ce³⁺ ions (step 5). When the decay ratio of the carries released from the electron traps to the 5d state of the Ce³⁺ ions is proper, the blue-emitting LLP of Ce³⁺ can be obtained.

Fig. 6 Schematic graph of phosphorescence mechanism. ○ represent hole; ● represent electron.

4. Conclusions

In summary, a kind of blue-emitting long afterglow phosphor Li₂Sr_0.9Mg_0.1SiO₄:Ce³⁺ was successfully synthesized by the traditional high-temperature solid phase reaction. The doping of Ce³⁺ had no influence on the Li₂Sr_0.9Mg_0.1SiO₄ lattice. Upon UV illumination, typical 5d-²F_5/2 and 5d-²F_7/2 transitions from Ce³⁺ ions were observed by PL spectra, and the optimal doping concentration for Ce³⁺ of 0.4 at% was determined. After the UV irradiation was removed, an afterglow was observed from the Li₂Sr_0.9Mg_0.1SiO₄:Ce³⁺ phosphor, which was later confirmed in terms of the triple-exponential model with a maximum lifetime value (τ₃) of 73.9 s. The LLP phenomenon of the matrix was supposed to be caused by the thermo-stimulated recombination of holes and free electrons, which were released from the trapped metastable state at room temperature.

Conflicts of interest

There are no conflicts to declare.

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