1. Introduction

Prominent breakthrough in white LED-related devices spurred a wide interest in phosphor research [1–3]. White LEDs show many advantages over fluorescent tubes and incandescent lamps, as follows: higher power efficiency and brightness; longer life cycle; pollution-free; good stability; environment-friendly characteristics; and saves energy. Now, researchers have proposed various technologies to generate white light by coupling blue/UV LEDs with phosphors. Among various phosphors, silicate-based phosphors have been investigated because of their outstanding thermal chemical, crystal structure diversity, and mechanical stability [4–7]. Recently, Qiu’s group discovered that novel SrGa₂Si₂O₈:Mn²⁺ phosphors exhibit a yellow-green multi-band and highly thermal-stable emission, and these kinds of phosphors have a wide trap distribution that can compensate for different luminescence centers in SrGa₂Si₂O₈ [8]. Apart from the charming luminescence property of silicate-based phosphors reported by Qiu’s group, SrGa₂Si₂O₈:Mn²⁺ has other interesting optical properties. Multiple emission bands that originated from Mn²⁺ could probably be tailored through the introduction of doping ions owing to the different crystal field environments in SrGa₂Si₂O₈. Moreover, the wide trap distribution in SrGa₂Si₂O₈ may also allow energy storage, which leads to persistent luminescence. However, the report on the persistent luminescence from SrGa₂Si₂O₈ host has not been revealed so far.

Herein, we chose SrGa₂Si₂O₈ as a host to develop a series of Sr_1-xBa_xGa₂Si₂O₈:Mn²⁺ phosphors. It is found that the incorporation of Ba²⁺ drove the site-selective substitution of Mn²⁺. Furthermore, thermo-luminescence (TL) indicated a broad trap distribution ranging from 0.61 eV to 1.01 eV in SrGa₂Si₂O₈:Mn²⁺ phosphors. Interestingly, the substitution of Ba²⁺ did not bring about the change of trapping states in the phosphors. Thus, the phosphors yielded strong yellow the emission and persistent luminescence under UV light excitation.

2. Experiments

Characterization: The phases were determined by XRD on the D8 Advance Bruker Bragg-Brentano diffractometer (Cu K_α radiation) equipped with a Vantec-1 linear detector. The Rietveld refinement of XRD data was performed using TOPAS software. The emission spectra, excitation spectra, PersL spectra, and persistent luminescence decay curves were recorded at room temperature on the FLS1000 spectrophotometer (Edinburgh Instruments) using a 450 W xenon lamp. The TL curves were recorded on the FLS1000 spectrophotometer (Edinburgh Instruments) using a 450 W xenon lamp. The samples were mounted on a thermal stage (FTIR600, Linkam Scientific Instruments). In the TL measurement, the sample was heated at a rate of 1 K·s^-1. The digital photographs were captured by using a smartphone (OnePlus 6).

Materials and preparation method: Sr_1-xBa_xGa₂Si₂O₈: 1%Mn²⁺ (x = 0-0.4) and Sr_1-xBa_xGa₂Si₂O₈ (x = 0 and 0.2) samples were synthesized through high-temperature solid-state sintering. Pure ZrCO₃ (4N), Ga₂O₃ (4N), SiO₂ (4N), and MnCO₃ (4N) were used as raw materials. The powders were finely mixed in an agate mortar to form homogeneous powders for prefiring. After prefiring at 1050 °C for 2 h, the materials were ground again to fine powders. Then, all samples were sintered at 1200 °C for 2 h.

3. Results and discussion

The XRD patterns of Sr_1-xBa_xGa₂Si₂O₈: 1% Mn²⁺ (x = 0-0.4) samples are shown in Fig. 1. All the diffraction peaks coincided well with the standard data of SrGa₂Si₂O₈ (ICSD No. 34414), with no appreciable signal from impurities when x < 0.4. With increasing of Ba content, the XRD pattern gradually moved to the lower 2θ side probably due to the lattice expansion induced by Ba replacement (r = 1.38 Å, CN (coordination number) = 7) for Sr (r = 1.21 Å, CN = 7). To further explore the structure variation, XRD refinement analyses (Fig. 2(a)) were performed on both Sr_1-xBa_xGa₂Si₂O₈ (x = 0 and 0.2) samples. The structural information of SrGa₂Si₂O₈ was used as the refined model. The refinement results are listed in Table 1. The reliability parameters were as follows: R_wp = 9.28% (x = 0) and 8.23% (x = 0.2); R_p = 7% (x = 0) and 6.16% (x = 0.2); and χ² = 1.79 (x = 0) and 1.49 (x = 0.2). The compound crystallized in a monoclinic phase with space group P121/a (14). The cell parameters were as follows: a = 9.0099 Å, b = 9.4730 Å, c = 8.4039 Å, V = 717.2 ų for x = 0. Compared with the refinement result of the x = 0.2 samples (a = 9.0307 Å, b = 9.5239 Å, c = 8.4464 Å, V = 726.4 ų), the increase in cell parameters indicated that Ba²⁺ successfully occupied the Sr site by forming a solid solution. Figure 2(b) displays the crystal unit cell of the Sr_0.8Ba_0.2Ga₂Si₂O₈, which comprised two types of coordination polyhedral; the decahedral site was occupied by Sr²⁺ and Ba²⁺, whereas the tetrahedral site was occupied by Ga³⁺ and Si⁴⁺. Such low-dimensional structure is conducive to the formation of traps and enhances the interaction between trap and luminescence centers according to the studies by Long et al [8].

 

Fig. 1. XRD patterns of Sr_1-xBa_xGa₂Si₂O₈: 1% Mn²⁺ (x = 0-0.4) samples and the standard data for the SrGa₂Si₂O₈ phase.

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Fig. 2. (a) Rietveld refinement of powder XRD profile of Sr_1-xBa_xGa₂Si₂O₈ (x = 0 and 0.2) samples. (b) Crystal structure of Sr_0.8Ba_0.2Ga₂Si₂O₈.

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Table 1. The refined positions of all atoms and the lattice parameters of SrGa₂Si₂O₈ and Sr_0.8Ba_0.2Ga₂Si₂O₈

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Figure 3 shows the excitation and emission spectra of SrGa₂Si₂O₈ matrix. The excitation band was observed at about 270 nm when the emission at 695 nm was monitored. Under excitation at 270 nm, SrGa₂Si₂O₈ matrix exhibited a broad emission band at around 695 nm. The same photoluminescence was observed in β-Ga₂O₃ single crystals and β-Ga₂O₃ nanowires [9,10]. The experiment of Víllora et al. indicated that the excitation band at 270 nm was due to the transition from the valence band perturbed by Ga³⁺ vacancies to the conduction band [11]. The emission band at around 695 nm was due to an electron–hole recombination, which was related to the recombination of electrons trapped by oxygen vacancies and holes captured by deep level acceptor impurity [9].

 

Fig. 3. Excitation and emission spectra of SrGa₂Si₂O₈ samples.

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Figure 4(a) shows the emission spectra of Sr_1-xBa_xGa₂Si₂O₈: 1% Mn²⁺ (x = 0-0.3) samples under 270 nm excitation. The broad emission band was obtained from 475 nm to 850 nm, and the broad band can be fitted by three Gaussian peaks. The peak 1 centered at 525 nm and originated from the ⁴T₁→⁶A₁ transition of the Mn²⁺ in the tetrahedral site. The peak 2 centered at 585 nm was caused by the ⁴T₁→⁶A₁ transition of the Mn²⁺ in the decahedral site [8]. The peak 3 centered at 695 nm was the same as the emission band of SrGa₂Si₂O₈ matrix, which was due to an electron–hole recombination via the donor and acceptor levels. The Mn²⁺ can replace the Ga³⁺ in the tetrahedral site, because the ion radius of Ga³⁺ (r = 0.47 Å, CN = 4) was more similar to Mn²⁺ (r = 0.66 Å, CN = 4) than Si⁴⁺ (r = 0.26 Å, CN = 4). Moreover, Mn²⁺ (r = 1.04 Å, CN = 7) can also substitute Sr²⁺ (r = 1.21 Å, CN = 7) in the decahedral site. In addition, the ratios of the peak 2 integrated intensity to the peak 1 integrated intensity increased linearly with x, as shown in Fig. 4(b). This phenomenon indicated that the incorporation of Ba²⁺ drove the site selection of Mn²⁺, which transferred from the tetrahedral site to the decahedral site. Figure 4(c) shows the excitation spectrum monitored at 525 nm. The band at 270 nm was the same as the excitation band in the SrGa₂Si₂O₈ matrix, which was attributed to the recombination of electrons trapped by oxygen vacancies and holes captured by deep level acceptor impurity. The other excitation bands ranged from 350 nm to 470 nm and corresponded to the spin-forbidden d–d transitions of Mn²⁺ from the ground state ⁶A₁(⁶S) to the excited states, ⁴E(⁴D), ⁴T₂(⁴D), [⁴A₁(⁴G),⁴E(G)], and ⁴T₂(⁴G). The Ba²⁺ doping did not lead to the excitation band shift, and the incorporation of Ba²⁺ did not affect the crystal field strength of Mn²⁺ [12,13].

 

Fig. 4. (a) Emission spectra of Sr_1-xBa_xGa₂Si₂O₈: 1% Mn²⁺ (x = 0-0.3) excited at 270 nm. (b) Intensity ratios of the Peak 2 to Peak 1 as functions of x in the Sr_1-xBa_xGa₂Si₂O₈: 1% Mn²⁺ (x = 0-0.3) samples. (c) Excitation spectra of Sr_1-xBa_xGa₂Si₂O₈: 1% Mn²⁺ (x = 0, 0.1 and 0.3).

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The SrGa₂Si₂O₈: 1% Mn²⁺ samples emitted yellow fluorescence under 254 nm light, and also exhibited bright yellow afterglow after the removal of 254 nm light, as shown in Figs. 5(a)–(c). Figure 5(d) shows the persistent decay curves of Sr_1-xBa_xGa₂Si₂O₈: 1% Mn²⁺ (x = 0-0.3) samples after the irradiation of 270 nm was stopped for 3 min. The persistent luminescence intensity barely varied with increasing Ba²⁺ concentration. Persistent luminescence intensity and time were determined by the trapping state (such as trap depth and distribution) in persistent phosphors [14–17]. Hence, the incorporation of Ba²⁺ had little impact on the trapping states in the samples. Figure 6(a) shows the PersL spectra of Sr_1-xBa_xGa₂Si₂O₈: 1% Mn²⁺ (x = 0-0.3) irradiated by 270 nm light for 5 min. The persistent luminescence bands that ranged from 475 nm to 700 nm can be divided into two Gaussian peaks, namely, peaks 1 and peak 2. These two Gaussian peaks are similar to the emission Gaussian peaks in Fig. 4(a), which corresponded to ⁴T₁→⁶A₁ transition of the Mn²⁺ in the tetrahedral and decahedral sites, respectively. The inexistence of the persistent luminesce peak at around 695 nm showed that the recombination via the donors and acceptors did not occur in the persistent luminescence procedure. The ratios of the peak 2 integrated intensity to the peak 1 integrated intensity also increased linearly with x, as shown in Fig. 6(b), which further demonstrated that the incorporation of Ba²⁺ drove the site selection of Mn²⁺ from the tetrahedral site to the decahedral site. The ratios of persistent luminescence varied from the ratios of emission, which can be explained by the different energy transfer mechanisms between photoluminescence and persistent luminescence. The higher ratios of persistent luminescence than those of emission implied that defects related to trap states may be located near the Mn²⁺ in the decahedral site [18,19].

 

Fig. 5. Image of SrGa₂Si₂O₈: 1% Mn²⁺ samples (a) under white-LED illumination, (b) under 254 nm light and (c) 1 min after 254 nm light ceased. (d) Persistent luminescence decay curve of the Sr_1-xBa_xGa₂Si₂O₈: 1% Mn²⁺ (x = 0-0.3) samples monitoring at 585 nm.

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Fig. 6. (a) PersL spectra of Sr_1-xBa_xGa₂Si₂O₈: 1% Mn²⁺ (x = 0-0.3). (b) Emission intensity ratios and persistent luminescence intensity ratios of the Peak 2 to Peak 1 as functions of x in the Sr_1-xBa_xGa₂Si₂O₈: 1% Mn²⁺ (x = 0-0.3) samples.

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The persistent luminescence originated from the energy released from traps. We performed a series of TL experiments on SrGa₂Si₂O₈: 1%Mn²⁺ at varying excitation temperatures (Fig. 7(a)). The TL peak gradually moved to a higher temperature region at a higher excitation temperature. The initial rise analysis method was used to estimate the trap depths [20,21]. This approach assumed that the concentration of trapped electrons on the low-temperature side of a TL glow curve was relatively constant. Thus, the TL intensity can be written as follows:

 

Fig. 7. (a) TL curves of SrGa₂Si₂O₈: 1% Mn²⁺ samples monitoring at 585 nm after initially being excited by 270 nm light for 60 s at various temperatures. (b) Initial rise method used to estimate the trap depths of SrGa₂Si₂O₈: 1% Mn²⁺ samples. (c) The density of energy levels for the trap distribution in SrGa₂Si₂O₈: 1% Mn²⁺ samples. Inset: The estimated trap depth in SrGa₂Si₂O₈: 1% Mn²⁺ as a function of excitation temperature.

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4. Conclusions

We successfully developed a series of Sr_1-xBa_xGa₂Si₂O₈:Mn²⁺ phosphors with yellow persistent luminescence. The results of XRD and Rietveld refinement indicated that the samples possessed the pure SrGa₂Si₂O₈ phase, and the incorporated Ba²⁺ occupied the Sr site by forming a solid solution. When the Sr/Ba ratio varied, the emission and persistent luminescence behavior of the sample changed, which was due to the fact that the site selection of Mn²⁺ changed after the incorporation of Ba²⁺. Interestingly, the substitution of Ba²⁺ did not bring about the change of trapping states in the samples. Thus, the samples yielded strong yellow emission and persistent luminescence under UV light excitation.