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Abstract
Ultraviolet persistent luminescence technology holds potential for some new applications where ultraviolet emission is needed but constant external excitation is unavailable. Despite the promising applications, not much is known about such luminescence. Here we report ultraviolet-B (290−320 nm) persistent luminescence phenomenon in isostructural Y3Ga5O12:Bi3+ and Y3Al5O12:Bi3+ phosphors. We further investigate the luminescence by measuring thermoluminescence of the two phosphors. Our spectral results indicate that conventional thermoluminescence measurement cannot directly evaluate the electron population in the traps of Y3Ga5O12:Bi3+, in which the ultraviolet emission is suppressed at high temperature due to a thermal ionization quenching. We believe that the insight of the present trap performance is transferable to other ultraviolet persistent phosphors.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
In developing persistent luminescence technology, the persistent phosphors emitting in the visible-light and infrared regions are attracting consideration attention [1–7], while the research of ultraviolet (UV) emitting materials is relatively lacking [8–12]. Taking into account the important applications of UV light in many technological fields, such as phototherapy and light mediated treatment [13–15], the UV persistent luminescence as a novel UV emission form holds promising potential. The achievement of UV persistent luminescence will offer us a unique opportunity to use this special optical phenomenon for some new applications where UV emission is needed but constant external excitation is unavailable. In our view, the achievement of UV persistent luminescence depends primarily on two factors: (1) Appropriate emitting ions that capable of UV emissions, such as Ce3+, Pr3+, Gd3+, Pb2+ or Bi3+ in some suitable phosphor systems [16]; and (2) effective interaction between the activator (UV emitter or sensitizer) and the energy traps in phosphors. According to the prerequisites, finding suitable phosphor system containing UV emitting ion and exploring the trapping performances of the phosphor are essentially required for further developing UV persistent luminescence.
As an effective emitter for the UV luminescence, trivalent bismuth ion (Bi3+) is a good candidate due to its inter-configurational 6s6p→6s2 transition in some hosts [17–19], such as garnets. In the well-established garnet phosphors, Bi3+ generally exhibits band-like emissions peaking in the UV-B (290−320 nm) region [20–23]. However, despite the fact that steady-state photoluminescence of Bi3+-doped garnet phosphors has been studied, the UV-B persistent luminescence has not been investigated. To gain insight into the persistent luminescence, besides the luminescence property of the emitting level itself, the interaction between the Bi3+ and the traps is worth exploring. Considering that the information related to the traps can be provided by thermoluminescence (TL) technology [24,25], it is interesting to study the TL of Bi3+ doped garnet phosphors.
In this paper, the persistent luminescence and TL of Y3Ga5O12:Bi3+ and Y3Al5O12:Bi3+ garnet phosphors have been described for the first time. The effect of luminescence thermal quenching on the TL measurements for the UV emission has been revealed and discussed. It is found that the TL measurement cannot directly evaluate the population of deep traps in Y3Ga5O12:Bi3+. By combining temperature dependent photoluminescence and photostimulated TL experiments, we conclude that such an invalidity of TL measurement stems from a remarkable thermal quenching of the 3P1 emitting state.
2. Experimental
The Y3Ga5O12:0.5%Bi3+ (YGG:Bi) and the isostructural Y3Al5O12:0.5% Bi3+ (YAG:Bi) phosphors were synthesized by a solid-state reaction method. Raw materials, involving Y2O3, Bi2O3, and Ga2O3/Al2O3 powders, were mixed in stoichiometric proportions. For preparing each phosphor, after a fine ground in an agate mortar for 3 h, the chemical mixture was pressed into a disk of 1.5 cm in diameter using a 30-ton hydraulic press. The two kinds of disks were sintered for 3 h in air at different temperatures: the YGG:Bi was sintered at 1300 °C, and the YAG:Bi was sintered at 1550 °C.
Luminescence spectra were measured with a Horiba FluoroMax-Plus spectrofluorometer. TL measurements were conducted using a SL08-L TL Reader (Guangzhou Rongfan Science and Technology Co., Ltd). The TL curves were recorded for the Bi3+ emissions with a heating rate of 4 °C s−1. Photoluminescence intensity as a function of the excitation temperature was recorded using the spectrofluorometer equipped with a home-made high-temperature cell (the heating rate was fixed at 4 °C s−1). Before each measurement, the phosphor was heat-treated at 420 °C to empty all the traps. A 254 nm UV lamp (Spectroline, ENF-260C/FC) served as the primary excitation source. A red light-emitting diode (LED, output power density is 30 mW cm−2) was employed as the photostimulated illumination source.
3. Results and discussion
Prior studies on YGG:Bi and YAG:Bi have shown that the two phosphors exhibit steady-state photoluminescence emissions peaking in the UV-B region at room temperature [20–22]. Besides the well-studied photoluminescence, the Bi3+ doped YGG and YAG also feature UV persistent luminescence after the stoppage of a 254 nm UV lamp illumination. Figure 1 presents the persistent luminescence emission spectra of YGG:Bi and YAG:Bi recorded at room temperature. Each spectrum consists of a UV emission band, peaking at 316 nm (YGG:Bi) and 303 nm (YAG:Bi), which may be ascribed to the Bi3+ 3P1→1S0 transition [20–22]. Insets of Figs. 1(a) and 1(b) show the corresponding persistent luminescence decay curves, which are obtained by monitoring the emission maxima of the two phosphors.
Fig. 1. Persistent luminescence of YGG:Bi and YAG:Bi at room temperature after illumination with a 254 nm UV lamp. (a) Persistent luminescence emission spectrum of YGG:Bi recorded at 30 minutes after stoppage of the illumination. Inset shows persistent luminescence decay curve of the YGG:Bi obtained by monitoring at 316 nm. (b) Persistent luminescence emission spectrum of YAG:Bi recorded with a delay time of 30 minutes. Inset shows persistent luminescence decay curve of the YAG:Bi monitored at 303 nm.
For persistent phosphors, persistent luminescence performance generally depends on the properties of traps. While the information related to the traps can be provided by TL measurement [24,25]. Accordingly, we conduct TL measurement after the 254 nm UV lamp illumination at room temperature. Figure 2(a) gives the TL curves for the YGG:Bi (monitored the 316 nm emission) and YAG:Bi (monitored at 303 nm). The two TL curves share similar spectral structure in the low-temperature region (30–180 °C). Whereas, compared with the TL curve of YAG:Bi, the high-temperature band of YGG:Bi is nearly disappeared. At first sight on the TL curves in Fig. 2(a), one may simply think there is no population in the deep traps of YGG:Bi, since the TL intensity in the high temperature region is pretty low. However, it should be realized that, compared the case of visible-light or infrared luminescence, the thermal quenching of UV luminescence is generally more remarkable and may affect the TL measurement [26,27].
Fig. 2. Thermoluminescence (TL) and photoluminescence (PL) thermal quenching of YGG:Bi and YAG:Bi. (a) TL curves monitored UV emissions of YGG:Bi (316 nm) and YAG:Bi (303 nm). The low-temperature and high-temperature bands correspond to the shallow trap (ST) and deep trap (DT), respectively. The TL intensity of YGG:Bi nearly disappears at the high temperature side (> 210 °C). Before each measurement, the phosphor was illuminated by a 254 nm ultraviolet lamp at room temperature for 10 minutes. (b) Temperature dependence of steady-state PL emission intensity for each phosphor under 254 nm excitation. The PL emission of YGG:Bi is nearly completely quenched at the high temperature side.
The apparent difference between the TL spectral shapes of YGG:Bi and YAG:Bi [Fig. 2(a)] inspires us to investigate the emission capability of the UV emitting levels at different temperatures. For each phosphor, we record the steady-state photoluminescence intensity as a function of excitation temperature. As shown in Fig. 2(b), from 30 to 270 °C the emission intensities decrease gradually upon 254 nm excitation. For visual comparison, the emission intensities of the two phosphors at room temperature are normalized. While the emission intensity of YGG:Bi decreases very steeply along the temperature, the intensity of YAG:Bi decreases slightly. By combining the observation in TL measurements of the two phosphors [Fig. 2(a)], we deduce that the difference of TL spectral shapes may stem from the different luminescence thermal quenching properties.
To further illustrate the effect of luminescence thermal quenching on the TL measurements in YGG:Bi and YAG:Bi, we propose a scheme on the competition between the thermal quenching and the TL emission. Figure 3(a) shows that the 3P1 emitting level in YGG:Bi is situated closer to the bottom of conduction band than that in YAG:Bi [17]. The smaller energy difference can explain the lower temperature at which quenching by thermal ionization starts. Accordingly, even though the deep traps of YGG:Bi are released effectively at high temperature, the thermal ionization disables the TL emission from the 3P1 level. On the contrary, in YAG:Bi, the large energy difference between the 3P1 emitting level and the bottom of conduction band inactivates the thermal ionization [Fig. 3(b)], so that the deep traps of YAG:Bi can be evaluated by the TL measurement.
Fig. 3. Schematic illustrations of TL and thermal ionization quenching at high temperature for YGG:Bi and YAG:Bi. (a) While the deep traps in YGG:Bi are being released at high temperature, the 3P1 emitting level has been quenched by thermal ionization. (b) The energy difference between the 3P1 emitting level and the bottom of conduction band is large in YAG:Bi, so that the luminescence thermal quenching is not remarkable at the present high temperature. For clarity, only the emitting state 3P1 and ground state 1S0 of the Bi3+ ion are presented. CB, ST and DT represent the conduction band, shallow traps and deep traps, respectively. Straight-line arrows and curved-line arrows stand for the optical transitions and thermal excitations, respectively. Cross marks indicate that the present high temperature inactivates the transition or excitation.
To gain more insight into the deep traps in YGG:Bi, we design an extended TL experiment. Figures 4(a) and 4(b) show the experimental condition and outline. The YGG:Bi phosphor is initially illuminated at 200 °C by a 254 nm UV lamp for 60 s, followed by a fast cooling down step (in 5 s). Subsequently, the TL measurement starts at 180 s after stoppage of the illumination. Figure 4(c) gives the resulting TL curve (dashed-line curve). Compared with the curve of YGG:Bi shown in Fig. 2(a), a majority of the low-temperature TL band disappears in Fig. 4(c), indicating that the shallow traps are inactive (i.e., emptied) during the illumination due to the thermal energy available at 200 °C. That is, the shallow traps do not contribute to the TL under such an excitation condition. For the deep traps, as mentioned above, their storage capability cannot be directly evaluated by the corresponding TL intensity due to the effect of remarkable thermal quenching.
Fig. 4. Photostimulated thermoluminescence (PSTL) of YGG:Bi. (a) Temperature profile for the experiment. (b) Experimental outline. (top) A charging step using a 254 nm UV lamp for 60 s at 200 °C, followed by a fast cooling down to room temperature without excitation. The TL measurement starts at 240 s. (bottom) The sample is successively illuminated by a 254 nm UV lamp at 200 °C and by a 630 nm red LED at room temperature. The PSTL measurement starts at 240 s. (c) TL (dashed-line) and PSTL (solid-line) curves obtained according to the approach outlined in (b). (d) Schematic illustration of the PSTL. Upon the 200 °C illumination, the ST is thermally emptied, while the DT is filled. Subsequently, upon the red LED illumination, the electron in the DT is promoted to the conduction band. Consequently, the conduction electron may be captured by the ST, and released during heating.
Subsequently, we conduct another extended TL experiment, in which photostimulation approach is introduced [28], as outlined in the bottom of Fig. 4(b). Firstly, we illuminate the YGG:Bi phosphor for 60 s at 200 °C using the 254 nm UV lamp to fill the deep traps (note: shallow traps have been thermally emptied during the excitation). After cooling down to room temperature, we illuminate the charged phosphor using a 630 nm red LED. Compared with the dashed-line curve shown in Fig. 4(c), the red LED illumination significantly changes the TL curve profile [solid-line curve presented in Fig. 4(c)], i.e., the low-temperature TL band reappears. It indicates that the shallow traps are refilled upon the LED illumination, and thus a photostimulated thermoluminescence (PSTL) phenomenon occurs. The PSTL curve exhibits a similar spectral shape with that in Fig. 2(a). Such a result fits well our prediction. That is, there is remarkable population of deep traps in YGG:Bi, even though the high-temperature TL intensity looks very weak. Accordingly, we put forward a schematic illustration accounting for the reappeared PSTL intensity of YGG:Bi in the low-temperature region. As illustrated in Fig. 4(d), upon the red-light illumination at room temperature, some of electrons in the deep traps are photo-released and the thermally emptied shallow traps are refilled. Subsequently, all the traps are released during heating, followed by the PSTL emission from the 3P1 energy level in YGG:Bi.
4. Conclusion
We have achieved UV-B persistent luminescence in both Y3Ga5O12:Bi3+ and Y3Al5O12:Bi3+ phosphors. Our investigations indicate TL measurement cannot directly evaluate the population of deep traps in YGG:Bi due to the remarkable thermal quenching of the 3P1 emitting state. We believe that the result on the TL measurement is not an individual phenomenon and may appear in other UV persistent phosphors.
Funding
National Natural Science Foundation of China (11774046, 51732003); Department of Science and Technology of Jilin Province (20180414082GH).
Disclosures
The authors declare that there are no conflicts of interest related to this article.
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