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Abstract
Persistent luminescence phosphor, as an extraordinary photonic material, is widely used in the many fields ranging from photodynamic therapy to optical data storage. The phosphor is sensitive to the ambient temperature; thus, the ambient temperature of the phosphor is a key in the widespread utilization. Here, the relation between the ambient temperature of Ba2SiO4:Eu2+/Ho3+ phosphor and the optical performances is evaluated. A warm storge condition is preferred before the excitation, while a low-temperature condition is necessary after charging for the high-intense response. The findings could make a suggestion for the preservation condition of the phosphors in the various applications.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Owing to the existence of the energy traps in the long persistent luminescence phosphors (LPPs), the phosphors exhibit many attractive features [1–5]. The phosphor can be triggered to capture and store electrons in the traps by excitation with specific light. The electrons would gradually escape from the traps after the excitation has stopped, and the phosphor exhibits the emission of light for a long time, commonly referring as persistent luminescence (PersL). The leakage only happens to a small portion of electrons and most electrons are well kept in the traps. The electrons could be actively released from the traps by applying an appropriate stimulation. The released electrons are delocalized to the conduction band, contributing to the light emission. According to the mechanism of the stimulation, the stimulated luminescence can be categorized into different properties, including thermally stimulated luminescence (TL) and optically stimulated luminescence (PSL) [6,7]. Thanks to the extraordinary features, LPPs become a promising candidate in the various interdisciplinary fields ranging from materials science to biomedical engineering and information technologies [8–11]. LPPs with a long-lasting afterglow could be employed in the long-term PersL-mediated photodynamic therapy and in vivo long-circulating bioimaging [12–15]. The properties of the storage and the controllable release of the electrons allows the phosphor to be used in the optical data-storage and anti-counterfeiting applications [16–18].
Recently, a variety of LPPs has been extensively reported. The composition and the synthesis methods determine the formation of the phosphors and the distribution of the traps, which have a significant influence over the properties. In the synthesis, divalent and trivalent rare earth ions are doped into oxide- and (oxy)nitride-hosts [19–21]. The non-equivalent substitution of the rare-earth ion induces the defects, forming traps. Besides, the doped ions could act as luminescence emission centers. The adoption of the synthesis methods, including solid-state reaction method, sol-gel method, co-precipitation method and combustion method, is related to the properties of the raw materials and the experimental conditions [22–24]. High-temperature solid-state-reaction method is a popular way to synthesize the phosphors because it is suitable for preparing most silicate, phosphate, and aluminate-host phosphors [25–28]. The proper control of the synthesis conditions could regulate the deformation of the host crystal structure and the creation of the traps [29–31]. It is necessary to optimize the synthesis condition to achieve desired afterglow properties, quantum efficiency and stimulated luminescence properties.
On the one hand, desirable intrinsic properties of the phosphors could be realized by optimizing the synthesis condition. On the other hand, the actual performance is dependent on the ambient condition in the practical applications. It has been found that the operating temperature is related to the probability of the leakage of the electrons, thus affecting the intensity and duration time of the afterglow emission [32–36]. A good performance could be achieved under a proper ambient temperature. Nevertheless, there is only few research revealing the influence of the ambient temperature over the stimulated luminescence properties of the phosphors.
In this paper, the influence of the synthesis temperature and the ambient temperature over the PersL, TL and PSL properties of the Ba2SiO4:Eu2+,Ho3+ phosphor was thoroughly investigated. A group of Ba2SiO4:Eu2+,Ho3+ phosphor was prepared under different heating temperatures. After evaluating the PersL property and the response of TL and PSL, the optimized synthesis condition was figured out. Then, the trap distribution and the trap depth of the phosphor synthesized under the optimized condition were analysed. Then, the performances of the phosphor were studied under different temperature conditions. The dependence of the ambient condition to the electron trapping and the resultant stimulated luminescence was discussed. The work could provide a guidance for the preservation of the phosphor in the different applications.
2. Materials and methods
2.1 Synthesis of the Ba2SiO4:Eu2+,Ho3+ phosphor
The preparation of the LPP (Ba2SiO4:Eu2+,Ho3+) underwent a two-step high-temperature solid-state-reaction method. The initial raw materials were BaCO3, SiO2, Eu2O3, Ho2O3, and BaF2, among which BaF2 served as the flux. The raw materials were purchased from Aladdin Biochemical Technology Co., LTD. The weight ratio of the reactants (BaCO3, SiO2, Eu2O3, Ho2O3, BaF2) was 10.578: 1.6352: 0.04789: 0.1028: 0.381. The reactants were placed in a tube furnace and firstly heated at 1000 °C for 6 h in ambient atmosphere. Then, the powders were cooled to room temperature. The powders were divided into three groups and heated for 5 hours at 1423 K, 1623 K and 1823 K respectively in the environment with a gas-mixture flow of hydrogen and nitrogen (5% H2 and 95% N2). Hydrogen atmosphere provides a condition for reduction reaction. After reduction, Eu3+ would be transformed into Eu2+. After 5 hours, the Ba2SiO4:Eu2+,Ho3+ phosphor was synthesized. Then, the powders were ground for an hour in the agate mortar. Finally, the powders were placed in the deionized water for filtering. Fine powders were obtained after passing through a membrane with pore size of 15 µm.
2.2 Characterization of the Ba2SiO4:Eu2+,Ho3+ phosphor
The crystal phase of the fabricated phosphor was characterized by X-ray diffractometer with Cu-Kα λ=0.15418 nm radiation and a 2θ range from 20° to 70°. The morphology of phosphor was measured by scanning electron microscope. The persistent luminescence (PersL) afterglow curves and the thermoluminescence (TL) glow curves were measured by a luminescence measurement instrument. In the TL measurements, the samples were irradiated at 254 nm for 3 minutes at a specific temperature. Then, the sample was measured in the luminescence measurement instrument with the heating temperature ranging from 300 K to 550 K and the heating rate is 5 K/min.
2.3 Measurement systems for the optical performances of the Ba2SiO4:Eu2+,Ho3+ phosphor
The home-made systems for testing LPP are shown in Fig. 1. The excitation system consisted of a 254 nm LED and a lens for light collimation. The output power of the LED could be precisely controlled by a programmable voltage source. In the PersL and TL measurement system, a programmable-temperature heating platform was used to control the heating temperature. In the measurement of PersL performance, the heating platform was off and the ambient temperature was set to 300 K. A camera was focused on the phosphor powders. In the PSL measurement system, a 980 nm laser was used as the stimulated light source. The power was adjusted by an attenuator. The emission light was emitted from the powders and couped into the camera via the dichroic mirror and a filter.
Fig. 1. The schematic diagrams of the home-made systems for (a) the excitation of the LPP and the measurement of (b) PersL, (c) TL and (d) PSL properties.
3. Results and discussions
3.1 XRD pattern and SEM images of the phosphor
The properties of the Ba2SiO4:xEu2+, yHo3+ (x = 0.01, y = 0.02) powders synthesized at different temperatures (1423 K, 1623 K and 1823 K) are firstly investigated to figure out the optimized synthesis condition. The powders synthesized at different temperatures have similar appearance. The powders are light green. The surfaces of the powders are rough and the shapes are irregular, as shown in Fig. 2(a-c).
Fig. 2. (a)-(c) SEM images of the Ba2SiO4:0.01Eu2+,0.02Ho3 + phosphor powders. (d)-(f) XRD patterns of Ba2SiO4:0.01Eu2+,0.02Ho3+ samples synthesized under different firing temperatures, and (g) standard XRD pattern of the Ba2SiO4 (PDF#70-2113).
X-ray diffraction (XRD) patterns of the synthesized phosphor powders were investigated, as shown in Fig. 2(d-f). The XRD pattern (PDF#70-2113) of Ba2SiO4 which is the host of the phosphor is provided as the reference (Fig. 2(g)). The diffraction peaks in the XRD patters of the phosphors are consistent with those in the XRD pattern of Ba2SiO4. It indicates that the synthesized products are of pure Ba2SiO4 phase. Silicon-oxygen tetrahedron is a stable structure. In contrast to the crystal radius of Si4+ (0.40 Å), the crystal ionic radii of Ho3+ (1.04 Å)/Eu2+ (1.31 Å) are close to that of Ba2+ (1.49 Å). Thus, a portion of Ba2+ ions in the material lattice would be randomly replaced by Ho3+ and Eu2+ ions during the doping process. The substitution of the ions creates the potential well of holes. Eu2+ ions act as a luminescence center and green emission would be observed. The emission could be attributed to electronic transition in the dopant Eu2+ from its 4f65d1 excited state to its 4f7 ground state. The existence of the Ho3+ ions causes the charge imbalance. Thus, electron traps are formed to compensate the charge imbalance. Besides, Ho3+ co-doping plays an important role in the modulation of trap distribution and contributes to the enhancement of PersL [37].
3.2 PersL performance of the phosphor
The PersL property of the phosphors is investigated (Fig. 3). The phosphor powders were irradiated under 254 nm light for 3 minutes. After the removal of the excitation, the PersL intensity of the phosphor powders was measured at different time delays. The insets in Fig. 3 were captured by the camera. The powders emit green light and gradually fade out. After 3 hours, green light could still be clearly observed from the phosphor powder synthesized at 1623 K. The afterglow time is defined as the time interval that the intensity drops to 1/10 of the initial intensity just after the removal of the excitation. The PersL of the phosphor synthesized at 1423 K, 1623 K and 1823 K lasts for 2.5 hours, 5 hours and 3 hours, respectively.
Fig. 3. The PersL decay of the phosphors synthesized at three different temperatures after 254 nm UV light excitation. Insets: the images of the phosphors detected at different time after the cease of the excitation.
Then, the TL property of the phosphors was studied. The phosphor powders were excited with 254 nm light for 3 minutes, and then kept in dark for 30 minutes. The excitation process and the storage were operated at room temperature. Then, the phosphors were heated on a heating platform with the temperature of 323 K or 373 K. The measured TL responses are shown in Fig. 4(a) and Fig. 4(b). When the phosphor powders were heated at 323 K, the emission could last for 30 minutes and gradually decays. The emission of the phosphor powders at 373 K was much stronger, but the decay was faster. The high-temperature stimulation resulted in an energetic transition of electrons to conduction band. The phosphor powders fabricated at 1623 K exhibited strong and long emission in comparison with those obtained at 1423 K and 1823 K.
Fig. 4. The TL decay of the phosphors synthesized at different firing temperatures. The measurement was conducted with the heating temperature of (a) 323 K and (b) 373 K after the UV excitation.
The investigation of the PersL and TL properties show that the phosphor powder synthesized at 1623 K has strong and persistent emission, indicating that a large number of electrons are captured by the traps during the excitation stage. The superior performances indicate that the temperature setting at 1623 K in the Ba2SiO4:Eu2+,Ho3+ phosphor synthesis is an optimized condition. The following work focuses on the study about the phosphor synthesized at 1623 K.
3.3 PSL performance of the phosphor
In the analysis of the PSL property, the phosphor powder was illuminated under 254 nm UV light for excitation. The excitation time varied from 30 s to 420 s. After the excitation, the phosphor powder was kept in dark for 1 hour. Then, the phosphor powder was exposed under 980 nm NIR light stimulation. The optical power changed within the range of 1 mW to 24 mW in each measurement. Figure 5 shows the PSL response of the phosphor powder under different stimulation power and duration. If the excitation time exceeds 2 minutes, the intensity of the emission is almost same under the identical NIR stimulation, and higher than those obtained with the short excitation time (e.g. 30 seconds and 1 minute) especially for the cases with high-power photostimulation. It is worth noting that the simultaneous release of the electrons occurs during the charging process. It hints that the balance for filling up and emptying the traps with the electrons could be achieved after 2-minute UV excitation. In addition, with the increase of the NIR stimulation power, the emission becomes intense. When the stimulation power is at a low level, only a small number of electrons could be released. The influence of the excitation time over the emission is very low in the cases of low-power photostimulation. When the stimulation power is high, the stimulation intensely drives the transition of the electrons to the conduction band. A low-intensity emission occurs due to the limited number of electrons captured by the traps during the short exposure time. It has significant influence of the excitation time over the emission in the cases of high-power photostimulation.
3.4 Trap distribution of the phosphor and mechanism of luminescence
The TL glow curves are illustrated in Fig. 6(a). The phosphor powders of the phosphor synthesized at 1623 K were heated at different temperatures ranging from 303 K to 453 K when exposed at 254 nm irradiation. After 1 minute cooling at room temperature, the phosphor powders were heated from 300 K to 550 K. The luminance was measured during the heating. The TL glow curves have symmetric profiles. The peaks of the TL bands shift from 366 K to 448 K with the increase of the temperature in the excitation. The trap distribution is calculated as E = TPeak/500, where E is the trap depth and TPeak is the temperature at the peak intensity of the TL band. The trap density, as shown in Fig. 6(b), is calculated as the difference between the areas of the adjacent TL bands. The traps are in the range of 0.73 eV to 0.90 eV with the high density around 0.75 eV.
Fig. 6. (a) The TL glows measured after the excitation at different temperatures. (b) The depth and the distribution of the traps.
The TL glow curves are illustrated in Fig. 6(a). The phosphor powders of the phosphor synthesized at 1623 K were heated at different temperatures ranging from 303 K to 453 K when exposed at 254 nm irradiation. After 1 minute cooling at room temperature, the phosphor powders were heated from 300 K to 550 K. The luminance was measured during the heating. The TL glow curves have symmetric profiles. The peaks of the TL bands shift from 366 K to 448 K with the increase of the temperature in the excitation. The trap distribution is calculated as E = TPeak/500, where E is the trap depth and TPeak is the temperature at the peak intensity of the TL band. The trap density, as shown in Fig. 6(b), is calculated as the difference between the areas of the adjacent TL bands. The traps are in the range of 0.73 eV to 0.90 eV with the high density around 0.75 eV.
Figure 7 depicts the possible mechanism of the luminescence. The electrons located at ground state of Eu2+ are excited to a higher 5d excited level (Fig. 7-➀). Meanwhile, the electron transition from 5d level to 4f level occurs, emitting green light (Fig. 7-➁). A portion of electrons shift through the conduction band (CB) and are captured by the traps with different depth (Fig. 7-➂). In the initial stage of the afterglow, the electrons in the deep trap are recombined with the luminescence center (Fig. 7-➃), and the electrons in the shallow trap fill in the vacancy of the deep trap via CB (Fig. 7-➄) [38]. To maintain persistent luminescence, the electrons in the closest trap to the luminescence center continue to recombine with the center. The electrons in the deep traps transfer to the vacancy through the tunneling effect. PersL process only requires low energy for electron transition and could happen at room temperature. If the phosphor is under optical or thermal stimulation, the electrons in the traps could gain enough energy to escape the traps and move to the luminescence center via CB (Fig. 7-➅ and Fig. 7-➆) [10,38,39].
3.5 Influence of ambient temperature over the optical performances of the phosphor
The influence of the ambient temperature during the excitation and the storage on the PersL, TL and PSL properties was investigated. In the PersL measurement, the phosphor powder was divided into 3 groups and stored at 273 K, 298 K and 323 K for 2 hours before the experiments. Then, the phosphors were exposed under the 254 nm excitation for 2 minutes at room temperature. After the cease of the 254 nm excitation, the PersL decay was measured. Figure 8(a) depicts the PersL curves. The PersL gradually decays and the afterglow lasts for more than 40 minutes. The phosphor powder stored at 298 K presents the highest luminance and the longest afterglow time. The luminance of the phosphor stored at 323 K is relatively high in the initial stage, but drops rapidly. It could be attributed to the quick leakage of the electrons due to the high residual temperature. The PersL of the phosphor stored at 273 K is low. The low temperature might lead to an inactive transition of the electrons. Thus, the ambient temperature for the storage of the phosphors before the excitation would be a key factor to affect the intensity and the duration of the PersL.
Fig. 8. (a) The PersL decay of the phosphors stored at 273 K, 298 K and 323 K and excited by 254 nm UV light at room temperature. (b) The TL decay of the phosphors under 323 K heating after the UV excitation and the storage at different temperatures for 30 minutes. (c) The PSL response of the phosphors under 980 nm NIR stimulation after the UV excitation and the storage at different temperatures for 1 hour.
In the investigation of the TL property, the three groups of phosphor powders kept at room temperature were excited by the UV light for 2 minutes. Then, the powders were stored at different temperatures respectively. After 30 minutes, the phosphors were heated at 373 K. The luminance of the TL response is inversely proportional to the ambient temperature, as shown in Fig. 8(b). The phosphor stored at 273 K has an intense response and the phosphor stored at 323 K has a weak response. During the UV excitation, electrons are delocalized from the conduction band and captured by the traps. After that, the leakage of the electrons from the traps occurs. It has a high probability for the trapped electrons, especially those in the shallow traps, to escape when the phosphor is exposed to the high ambient temperature. Thus, the phosphor stored at a high temperature experiences a significant leakage. As a result, only few electrons remain in the traps and the TL response is weak.
Figure 8(c) shows the PSL response of the phosphors. After the cease of the UV excitation, the three groups of the phosphor powders were stored at different temperatures for 1 hour. Next, the phosphors were stimulated by 16 mW 980 nm NIR light. The experiments were repeated for three times. The similar trend was obtained in the PSL response as that in the TL response. The phosphor stored at low temperature exhibited a high PSL response. The same reason could be given as that for the TL response. When the ambient temperature was low, only few electrons escaped from the traps and most electrons remained in the traps. Hence, under the photostimulation, more electrons could be released from the phosphor stored at a low temperature, resulting in a relatively stronger PSL response.
4. Conclusions
In summary, a photoluminescent material, Ba2SiO4:Eu2+,Ho3+, was synthesized. During the doping of lanthanides, i.e. Eu and Ho, Ba2+ was non-equivalently replaced Eu2+ in Ba2SiO4 matrix, which would induce defects in the lattice structure of the material. These defects, forming traps, could capture free electrons. The intrinsic properties were optimized by adjusting the firing temperature in the synthesis. In addition to the common characterization of the phosphors, including optical properties (PersL, PSL and TL) and trap analysis (trap depth and trap distribution), the influence of the ambient temperature over the optical performances was investigated. A mild condition, e.g. room temperature, before the excitation was suitable for the phosphor to trap more free electrons because the electron transition was active and the leakage was not significant. After the excitation, a low-temperature condition was preferred, under which a long-duration high-intensity PSL and TL responses could be achieved. The present work ascertains the influence of the ambient temperature condition and could provide storage strategies to various applications such as preservation of LPP based storage media and drug carriers.
Funding
National Special Fund for the Development of Major Research Equipment. Instrument (2020YFF01014503); Shanghai Rising-Star Program (20QA1407000).
Disclosures
The authors declare no conflicts of interest
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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