A series of novel yellow persistent luminescence Sr1-xBaxGa2Si2O8:Mn2+ phosphors were synthesized by the solid state method. To investigate the site-selective occupation of Mn2+ in Sr1-xBaxGa2Si2O8, the XRD, emission, and persistent luminescence (PersL) spectra were performed in detail for samples with different Ba2+ concentrations. With increasing Ba2+ concentrations, the Mn2+ showed an occupation preference on the decahedral sites. Furthermore, thermo-luminescence (TL) indicated a broad trap distribution ranging from 0.61 eV to 1.01 eV which endowed SrGa2Si2O8:Mn2+ a bright persistent luminescence at room temperature.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
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 . 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 Sr1-xBaxGa2Si2O8: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.
Characterization: The phases were determined by XRD on the D8 Advance Bruker Bragg-Brentano diffractometer (Cu Kα 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: Sr1-xBaxGa2Si2O8: 1%Mn2+ (x = 0-0.4) and Sr1-xBaxGa2Si2O8 (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 Sr1-xBaxGa2Si2O8: 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 Sr1-xBaxGa2Si2O8 (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: Rwp = 9.28% (x = 0) and 8.23% (x = 0.2); Rp = 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 Å3 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 Å3), 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 Sr0.8Ba0.2Ga2Si2O8, 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 .
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 . 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 .
Figure 4(a) shows the emission spectra of Sr1-xBaxGa2Si2O8: 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 . 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].
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 Sr1-xBaxGa2Si2O8: 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 Sr1-xBaxGa2Si2O8: 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].
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:7(b)). When the excitation temperature increased from 303 K to 478 K, the trap depth gradually extended from 0.61 to ∼1.01 eV, as shown in the inset of Fig. 7(c). Figure 7(c) also revealed high density of energy levels for the trap distribution in Sr1-xBaxGa2Si2O8: 1% Mn2+ (x = 0-0.3). Suitable trap level distribution and high density of energy levels yielded strong persistent luminescence at room temperature as observed in Fig. 5 [22,23]. The above result also provided further evidence that the low-dimensional structure of SrGa2Si2O8 promoted the formation of traps and enhanced the interaction between traps and luminescence center.
We successfully developed a series of Sr1-xBaxGa2Si2O8: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.
Guangdong Basic and Applied Basic Research Foundation (2020A1515010432); Project of Educational Commission of Guangdong Province of China (2019KTSCX096); Special Funds for the Cultivation of Guangdong College Students' Scientific and Technological Innovation (“Climbing Program” Special Funds) (pdjh2020b0377); Hanshan Normal University (XN201918).
The authors declare no conflicts of interest.
The data that support the findings of this study are available from the corresponding author upon reasonable request.
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