Long-Lasting Phosphorescent Properties of Tb³⁺ Doped ZnO–P₂O₅–SiO₂ Glasses

Introduction

Luminescence has been well known and the first luminescent materials are sulfides. Since the discovery of polycrystalline SrAl₂O₄: Eu²⁺, Dy³⁺ with strong brightness and long decay time in 1996, luminescent materials have attracted attention [1].

The main type of luminescent material is polycrystalline powder with inhomogeneous shape and size; therefore, the application of luminescent material is limited [2]. Recently, some new types of luminescent materials have been prepared, such as glass and single crystal [3–5]. Glass has many characteristics such as homogeneousness, transparency, stability and isotropism. It is easily fabricated into products with various shapes such as bulk plate and fine fiber. In addition, luminescent glasses with high concentrations of earth ions are made. Therefore, luminescent glasses have many special applications, such as optical fiber, amplifier and storage devices [6–9].

In this article, the ZnO–P₂O₅–SiO₂: Tb³⁺ glass was prepared by melt-quenched method. After irradiation by 254 nm light, a visible greenish light can be observed with the naked eyes, in the dark for up to 9 h after removal of the activating light. The luminescent properties of ZnO–P₂O₅–SiO₂: Tb³⁺ glass were studied by fluorescence spectra, afterglow spectra and thermoluminescence spectra.g

Experimental

ZnO (analytically pure), NH₄H₂PO₄ (analytically pure), SiO₂ (analytically pure), Tb₄O₇ (99.99%) were used with starting materials in the present investigation. The starting materials were blended and the proportion of ZnO:P₂O₅:SiO₂ was 7:1:2 (mol) with different content (0.5, 1, 2, 5 and 10 mol%) of Tb³⁺. Then, the mixture was put into a corundum crucible to melt completely at 1,400℃ for 1 h. The liquids were poured into a stainless steel mode and then annealed at 450℃ for 0.5 h in an annealing furnace.

Emission spectra were measured at room temperature with a conventional fluorescence spectrophotometer (Hitachi model F-4500). The decay of phosphorescence in glass was monitored for 10 min with fluorescence spectrophotometer after irradiation with 254 nm light from the excitation source in the instrument. The thermoluminescence spectra were measured for 10 min with a thermoluminescent dosimeter (FJ-427A) after irradiation with 254 nm light.

Results and discussion

Figure 1 shows the excitation (a) and emission spectra (b) excited with 254 nm light of 70ZnO–10P₂O₅–20SiO₂–5Tb³⁺ glass. As shown in Figure 1(a), the broad peaks observed in the range 250–280 nm are due to the 4f⁸–4f⁷5d¹ transitions of Tb³⁺. Meanwhile, relatively sharp peaks observed in the range 280–400 nm are due to the f–f transitions of Tb³⁺. As shown in Figure 1(b), two groups of bands were observed ranging from 380 to 630 nm. These bands originated from f-f transition of Tb³⁺. A group of bands from 400 to 475 nm originated from ⁵D₃-⁷F_J (J=3, 4, 5) transitions of Tb³⁺. Four emission bands at 487, 542, 583 and 616 nm separately originate from ⁵D₄–⁷F_J（J=6, 5, 4, 3）transitions of Tb³⁺ and the band with the largest amplitude has a maximum of 542 nm. It arises from ⁵D⁴–⁷F₅ transition of Tb³⁺. A visible greenish light can be observed. The band with small widths from 300 to 600 nm originates from host glass.

Figure 1:

Excitation and emission spectra of 70ZnO–10P₂O₅–20SiO₂–5Tb³⁺ glass: (a) excitation spectra monitored with 542 nm and (b) emission spectra excited with 254 nm.

Figure 2 shows the fluorescence spectra of 70ZnO–10P₂O₅–20SiO₂–5Tb³⁺ glass measured 10 min after stopping 254 nm light-illuminations. It is evident that although the band with small widths from 300 to 600 nm is not observed, the fluorescence bands due to Tb³⁺ ions are still observable [10, 11] and the shape is almost the same as Figure 1(b).

Figure 2:

Fluorescence spectrum of 70ZnO–10P₂O₅–20SiO₂–5Tb³⁺ glass measured 20 min after stopping 254 nm light illumination.

There are a great variety of defects in oxide glass. Bridging oxygen, non- bridging oxygen, network-forming ion, network setting ion and their vacancy was considered an electron and hole-trapping center. Defects have an intimate relationship with long-lasting phosphorescent properties in glass. The species and amount of defects are different in different structures of glasses. In this study, we have studied the effect of host glass with different compositions on long-lasting phosphorescent properties of ZnO–P₂O₅–SiO₂: Tb³⁺_.

Figure 3 shows the thermal luminescence spectra of ZnO–P₂O₅–20SiO₂–5Tb³⁺ with various concentrations of ZnO from 60% to 70% and P₂O₅ from 20% to 10%, and a constant concentration of SiO₂ (20%) and Tb³⁺ (5%). It is seen from these spectra that there are two glow peaks equally located at 125℃ and 325℃. Because the location of the peak is closely associated with traps of different depths in thermal luminescence spectra [12], we think that there are two kinds of traps with different depths in samples. According to Randall and Wilkins’s formula (E = 25 kT m), it is estimated that the energy levels are 0.85 ev (125℃) and 1.290 ev (325℃) in samples. According to the intensity of the peaks, they reduce obviously as the ZnO decrease and the P₂O₅ increases. The intensity of the peak in the thermal luminescence spectra is closely associated with traps of different concentrations [12]. The concentration of the traps decreases gradually with the decreases of ZnO and the increases of P₂O₅. Zn²⁺ is a network setting ion in the system and non- bridging oxygen increase with the increases of ZnO.

Figure 3:

Thermal luminescence spectrums of (a) 70ZnO–10P₂O₅–20SiO₂–5Tb³⁺, (b) 65ZnO–15P₂O₅–20SiO₂–5Tb³⁺, (c) 60ZnO–20P₂O₅–20SiO₂–5Tb³⁺ glasses.

In summary, the process of long-lasting afterglow is considered as follows: excitant is excited into an excited state, and then it resets to a ground state and creates luminescence. The excitant is a critical factor for luminescence; therefore, the effects of Tb³⁺ contents on long-lasting phosphorescent properties of ZnO–P₂O₅–SiO₂: Tb³⁺ are studied.

Figure 4 shows thermal luminescence spectras of 70ZnO–10P₂O₅–20SiO₂–xTb³⁺ with Tb³⁺ contents from 0.5% to 5%. It is seen from these spectra that the intensities of thermal luminescence increase as the Tb³⁺ contents increase. The intensity of the peak in the thermal luminescence spectra is closely associated with the traps of different concentrations [12]. Therefore, the concentrations of traps increase as the concentration of Tb³⁺ increases. When the Tb³⁺ content reaches 5%, the intensity of the thermal luminescence is at its maximum and the long-lasting phosphorescent property of the sample is at its best. It doesn’t form glass when the Tb³⁺ content reaches 10%.

Figure 4:

The relationship of Tb³⁺ contents and thermal luminescence spectrum of 70ZnO–10P₂O₅–20SiO₂–xTb³⁺.

Ca, Sr and Ba equally belong to the second main group element. Therefore, their chemical valences are the same as the Zn. But their ionic radiuses are different from the Zn ion. The composition of 70ZnO–10P₂O₅–20SiO₂–5Tb³⁺ with the Zn ion partly replaced by Ca, Sr and Ba also can form long-lasting phosphorescent glass. Therefore, the effects of the kinds of replacing ions on long-lasting phosphorescent properties are studied.

Figure 5 shows the thermal luminescence spectra of 70ZnO–10P₂O₅–20SiO₂–5Tb³⁺glasses with 2% ZnO replaced by 2% RO (RO=CaO, SrO, BaO). It is seen that the peaks located at low temperature change from 80℃ to 160℃, but the peaks located at higher temperatures are still at 365℃, when 2% ZnO is replaced by 2% RO (RO=CaO, SrO, BaO) in 70ZnO–10P₂O₅–20SiO₂–5Tb³⁺ glasses. It indicates that the depths of traps increase after replacement. From the spectras, it is also seen that the intensities of peaks at low temperatures decreased in the order of CaO, SrO, BaO, but the intensities of the peaks are changeless at high temperatures. It indicates that the traps located at 80℃ contribute to long-lasting phosphorescent properties in the 70ZnO–10P₂O₅–20SiO₂–5Tb³⁺glasses. The chemical valences of Ca, Sr and Ba are the same as Zn²⁺ and their ionic radiuses are different from Zn²⁺. Therefore, the differences of ionic radiuses led to different depths of traps. To long-lasting phosphorescent property of the 70ZnO–10P₂O₅–20SiO₂–5Tb³⁺glasses, the traps caused by the Zn²⁺ are at their best, the traps caused by Ca²⁺, Sr²⁺ whose ionic radiuses are close to Zn²⁺are better and the traps caused by Ba²⁺ are worse.

Figure 5:

(a) Thermal luminescence spectrum of 70ZnO–10P₂O₅–20SiO₂–5Tb³⁺ and (b, c, d) thermal luminescence spectrum of 2% ZnO replaced by 2% CaO, SrO, BaO in 70ZnO–10P₂O₅–20SiO₂–5Tb³⁺.

The long-lasting phosphorescent properties of glasses appear regularly variable, when the Zn is replaced by Ca, Sr and Ba. In this article, the effects of the concentration of replaced ions on long-lasting phosphorescent properties are also studied.

Figure 6 shows the thermal luminescence spectra of (70–x) ZnO–10P₂O₅–20SiO₂–xBaO–5Tb³⁺ with BaO contents from 1 to 10 mol%. It is seen that the intensities of thermal luminescence rapidly decrease and the peaks located at low temperatures changed from 80℃ to 160℃ as the BaO contents increase to 10%. The ionic radiuses of Ba and Zn are different, the concentrations of traps reduce rapidly and the depths of traps increase after being replaced by BaO. When the ZnO is replaced by CaO, and SrO, the regularity is the same as that of BaO. Long-lasting luminescence materials must have proper depths of traps. According to other studies, the peaks located at low temperature were benefit to the long-lasting luminescence [7]; therefore, the level of traps at 80℃ is helpful to long-lasting luminescence in 70ZnO–10P₂O₅–20SiO₂: 5Tb³⁺ glass.

Figure 6:

(a) Thermal luminescence spectrum of 70ZnO–10P₂O₅–20SiO₂–5Tb³⁺ and (b, c, d and e) 1% BaO, 2% BaO, 5% BaO, 10% BaO replace the same content of ZnO.

The mechanism of long-lasting afterglow is considered as follows: electrons excited by UV light are trapped at electron trapping centers and released by the thermal energy to be transferred to an emissive ion of Tb³⁺. Finally, the ion emits a long-lasting afterglow [7]. There are two kinds of oxygen vacancies: one is associated with network-forming ions, and the other is associated with network modifying ions. The oxygen vacancies which are associated with network modifying ions can be used as electron trapping centers [3]. In ZnO–P₂O₅–SiO₂:Tb³⁺ glass, there are oxygen vacancies which are associated with network modifying ions. When the glass is excited by UV light, it generates free electrons and electron holes, then the free electrons are captured by oxygen vacancies and the electron holes are captured by other traps. When the UV light is removed, some free electrons and electron holes are thermally released. The free electrons are again captured by electron holes following a release of energy. Then the energy is absorbed by Tb³⁺ and the Tb³⁺ transfers from a ground state to an excited state. Therefore, a long-lasting afterglow is generated. The process of releasing energy is very slow. Therefore, the long-lasting afterglow of ZnO–P₂O₅–SiO₂:Tb³⁺ glass can be observed with the naked eyes in the dark for up to 9 h.

Conclusions

In conclusion, the long-lasting phosphorescent glasses with the composition of ZnO–P₂O₅–SiO₂: Tb³⁺ was prepared by a conventional melt-quenched method. After irradiation by 254 nm light, a visible greenish light can be observed with the naked eyes in the dark for up to 9 h after removal of the activating light. The afterglow phosphorescence spectra with a dominant peak at 542 nm are almost identical in peak position and shape to the emission spectra of the sample. The peak at 542 nm of the phosphorescence spectra is associated with the ⁵D₄→⁷F₅ transition of Tb³⁺. When the mass of SiO₂ is definite, the luminescent properties of glasses became better with increase of ZnO contents. When the Tb³⁺ contents reached 5%, the luminescent property of glasses is the best. When 2% of ZnO is replaced by 2% of RO (RO=CaO, SrO, BaO), the luminescent properties of glasses become bad in the order of CaO, SrO, BaO. The higher contents of RO (RO=CaO, SrO, BaO) are, the worse luminescent properties of the glasses become.