III. RESULTS AND DISCUSSION

The XRD patterns of Sr₃SiO₅: Eu²⁺, Dy³⁺ phosphors prepared by rapid and natural cooling processes are given in Fig. 1. The Sr₂SiO₄ phase appears in both rapid and natural cooled samples. However, rapid cooling treatment can reduce the relative content of Sr₂SiO₄ on the basis of the XRD intensity comparison between the Sr₂SiO₄ and Sr₃SiO₄ phases. It is believed that there is a structure transformation from Sr₃SiO₅ to Sr₂SiO₄ at 1300 °C in the process of the reaction and Sr₂SiO₄ phase forms below 1300 °C.Reference Hsu, Sheng and Tsai^9, Reference Nakamura, Watari, Torikai and Yada¹⁰ Thus, when the samples are cooled to room temperature naturally, relatively high content of Sr₂SiO₄ and other silicates are inevitable. However, the rapid cooling treatment can prevent the transformation effectively and thus improve the impurity of the final product.Reference Cheng, Liu, Cheng, Lu and Qiu¹¹

FIG. 1.

XRD patterns of Sr₃SiO₅: Eu²⁺, Dy³⁺ phosphors prepared with rapid and natural cooling processes.

The XRD patterns of synthesized Sr_2.95SiO₅: Eu²⁺, Dy³⁺, 0.05 MF₂ (M: Ca, Sr, Ba) phosphors with rapid cooling treatment are shown in Fig. 2, which indicates that most of the peaks for the tested samples agree well with standard ICDD Card No. 26-0984 for Sr₃SiO₅. Although the Sr₂SiO₄ phase is also found, its content drastically decreases comparing with the naturally cooled sample based on the XRD intensity of the two phases. Moreover, no emissions ascribing to Eu or Dy ions in Sr₂SiO₄ are found in the next spectroscopy test, meaning that the impurity Sr₂SiO₄ phase has no obvious influence on the luminescence of Sr₃SiO₅.Reference Lakshminarasimhan and Varadarajtt¹² At the meantime, the addition of Eu²⁺ and Dy³⁺ and the introduction of alkaline earth metal fluorides have no noticeable influence on the lattice structure of the phosphors. However, relative pure Sr₃SiO₅: Eu²⁺, Dy³⁺ samples can be obtained with the addition of alkaline earth metal fluorides.

FIG. 2.

XRD patterns of Sr₃SiO₅: Eu²⁺, Dy³⁺ phosphors doped with 5 mol% MF₂ (M: Ca, Sr, Ba).

The incorporation of fluoride flux can make the preparation process a solid/liquid phase synthesis, which will decrease the sintering temperature and improve the morphological characteristics of the final product.Reference Pollet, Marinel and Roulland^13, Reference Lee, Koo, Jung, Han and Kang^14, Reference Yamada, Niiori, Hirabayashi and Tanaka¹⁵ Thus, it is reliable that the introduction of alkaline earth metal fluorides can lower the synthesis temperature and is beneficial to the synthesis of pure Sr₃SiO₅. It is known that the melting points of CaF₂, SrF₂, and BaF₂ are 1423, 1473, and 1353 °C, respectively, while the melting point is 2430 °C for SrO and 1723 °C for SiO₂. During the solid-state reaction process, alkaline earth metal fluoride acts as the eutectic composition to lower the reaction temperature. In this way, the alkaline earth metal fluorides may be subject to fast decomposition, resulting in the fast nucleation and densification for Sr₃SiO₅, which is helpful to promote the formation of Sr₃SiO₅ phase.Reference Dai, Zhang, Sun and Liu¹⁶ Meanwhile, with the introduction of alkaline earth metal fluorides, it becomes easier for the activator (Eu²⁺) and sensitizer (Dy³⁺) to get into the lattice sites.

Figure 3 shows the excitation spectra (monitored at 575 nm) and emission spectra (excited at 467 nm) of the phosphors. There is a wide excitation band from 250 to 550 nm containing four major peaks at 295, 350, 411, and 467 nm due to the transitions of Eu²⁺.Reference Jee, Park and Lee¹⁷ The broadband emission centered at 575 nm is also ascribed to the 4f ⁶5d ¹–4f ⁷ transition of Eu²⁺.Reference Jee, Park and Lee¹⁷ No emission peaks of Dy³⁺ are detected, meaning that Dy³⁺ does not form luminescence centers in the matrix.Reference Yamamoto and Matsuzawa¹⁸ Besides, with the introduction of Ba²⁺, Sr²⁺, and Ca²⁺ into the lattice, the shift of peak wave length is explained by the crystal field effect, which is affected by different coulomb attractive force of the various alkaline earth cation in the host lattice.Reference Nguyen, Yeo and Mho^19, Reference Chen, Hwang, Sun and Chen²⁰ When F⁻ ions enter the crystal lattice, it will occupy O²⁻ site because F⁻ and O²⁻ ions have the same electronic configuration and nearly the same ionic radius. In this case, some new centers related to island SiO₄ units can be created,Reference Huang, Cho, Jang and Jiang⁶ which may be the reason of the luminescence enhancement for the alkaline earth metal fluorides-doped phosphors.

FIG. 3.

Excitation (λem = 575 nm) and emission (λex = 467 nm) spectra of Sr₃SiO₅: Eu²⁺, Dy³⁺ phosphors doped with 5 mol% MF₂ (M: Ca, Sr, Ba).

The luminescence intensity not only depends on the concentration of rare earth ions but also on the content of alkaline earth metal fluorides. As is shown in Fig. 4, the emission intensity first increases and then decreases with the increase in the content of each kind of alkaline earth fluoride. The luminescent intensity reaches its maximum when 4 mol% of CaF₂, 4 mol% of SrF₂, or 5 mol% of BaF₂ is introduced.

FIG. 4.

Luminescence intensity as function of alkaline-earth fluoride content.

The afterglow was hardly observed in Sr₃SiO₅: Eu²⁺. But it prolongs to 5 h after doping Dy³⁺ in the phosphor.Reference Sun, Zhang, Luo and Wang^8, Reference Kuang and Liu²¹ However, the duration time of the visually recognized (≥0.32 mcd/m²) afterglow for the Sr₃SiO₅: Eu²⁺, Dy³⁺ phosphors doped with alkaline earth fluorides can be prolonged to 7–12 h after the excitation source is cut off. Especially, the afterglow time reaches 12 h when the phosphor is doped with 5 mol% of BaF₂. The afterglow decay curves for the phosphors are shown in Fig. 5. It is obvious that both the initial afterglow intensity and the duration time vary with the host composition. With the introduction of alkaline earth metal fluorides, the initial afterglow intensity increases and the afterglow decays more slowly.

FIG. 5.

Decay curves (λex = 365 nm) of Sr₃SiO₅: Eu²⁺, Dy³⁺ phosphors doped with 5 mol% MF₂ (M: Ca, Sr, Ba).

Figure 6 depicts the TL curves of Sr₃SiO₅: Eu²⁺, Dy³⁺ doped with no flux and 5 mol% MF₂ (M: Ca, Sr, Ba), respectively. Each TL curve has two peaks marked with I and II, locating at 353 ± 5 K and 475 ± 5 K, respectively, whether the flux is introduced or not. The relationship between the trap depths (E) and the peak temperature (T _m) can be estimated by the empirical equation,Reference Li, Wang, Liang and Su²² i.e., E = 25kT _m, in which k is the Boltzmann’s constant and T _m is the peak temperature of TL curves. The values for the calculated trap depth E are listed in Table I.

FIG. 6.

TL curves of Sr₃SiO₅: Eu²⁺, Dy³⁺ phosphors prepared with 5 mol% MF₂ (M: Ca, Sr, Ba) and no flux.

TABLE I.

Trap depth (E) for the Sr₃SiO₅: Eu²⁺, Dy³⁺ phosphor doped with no flux and 5 mol% MF₂.

Interestingly, the density at different depths can be adjusted by doping different alkaline earth metal fluorides. As seen from Fig. 6, the relative densities of the traps at different depths change obviously with the addition of alkaline earth metal fluorides. It is clear that the content of the trap corresponding to Peak I increases significantly when SrF₂ is introduced, comparing with the sample without flux. For the CaF₂- and BaF₂-doped samples, however, the content of the traps corresponding to Peak I remarkably decreases. On the other hand, the density of traps corresponding to Peak II all increases when the flux is introduced, in an order of BaF₂ < CaF₂ < SrF₂. Suitable trap depth can lead to the afterglow. If the depth of the trap is too shallow, the captured electrons will quickly escape from the traps, and thus, the afterglow will rapidly decay. On the contrary, if the trap levels are too deep, the escape of captured electrons becomes difficult and the phosphor will not show afterglow at room temperature either. Among the phosphors doped with alkaline earth metal fluorides, the phosphors-doped BaF₂ results in the higher content of the trap at 1.12 eV. Therefore, the afterglow time reaches 12 h when the concentration of BaF₂ is 5 mol%.

The results of TL test in Sr₃SiO₅: Eu²⁺ show only one TL peak (Peak I) located in the range of 350–375 K in Fig. 6, which may be ascribed to the doped Eu²⁺ ions.Reference Yamaga, Masui, Sakuta, Kodamaand and Kaminaga²³ The shallow traps corresponding to Peak I are responsible for the afterglow in the initial period. As it is reported before, Dy³⁺ acts as a supplier of traps, in which its replacement of Sr site can result in defects in the crystal. The process can be expressed as 3Sr²⁺ = 2Dy_Sr^* + V_Sr. Each substitution of two Dy³⁺ ions would create two positive defects of Dy_Sr^* capturing electrons and one negative vacancy of V_Sr″.Reference Huang, Cui and Wang^24, Reference Lin, Dang, Deng and Nan²⁵ The doped Dy³⁺ would produce the deeper traps that determine the afterglow in the later period. Besides, the introduction of alkaline earth metal fluorides can deepen the traps and enhance the trap density, which can capture much more electrons and results in longer and stronger afterglow.

To better explain the afterglow process, a simple model in Sr₃SiO₅: Eu²⁺, Dy³⁺ is proposed, as shown in Fig. 7. As discussed above, there are Trap I and Trap II corresponding to TL peaks I and II in Fig. 6. Trap I is produced by the lattice point defects, while Trap II is produced by the unequal charge substitution of Dy³⁺ to Sr²⁺ ion. The transition of the ground state 4f ⁷ to lower and higher 4f ⁶5d ¹ excited states of Eu²⁺ (transitions 8 and 1) consists the broad excitation band. Under ultraviolet (UV) excitation, part of electrons in higher 4f ⁶5d ¹ excited state may enter the conduction band (transition 2), making the electrons captured by the Trap I (transition 4) and Trap II (transition 3). Thus, the light energy is stored in the Trap I and II. When the excitation source is removed, the electron can be released thermally to the conduction band through transitions 5 and 6. Then, the electrons transit from the conduction band to higher 4f ⁶5d ¹ excited state level of Eu²⁺ and relax to its lower 4f ⁶5d ¹ state (transition 7). The radiative transition from the lower 4f ⁶5d ¹ excited state to the ground state spontaneously causes the yellow afterglow (transition 9).Reference Sun, Zhang and Wang^26, Reference Sun, Zhang, Zhang, Luo and Wang²⁷ In fact, the electrons captured by the deeper traps (see TL curves II in Fig. 6) can escape at room temperature but at a much slower rate than that of the electrons in shallower traps (see TL curves I in Fig. 6), which makes a dominant contribution to the long afterglow.

FIG. 7.

Model of afterglow process in Sr₃SiO₅: Eu²⁺, Dy³⁺ phosphors.

It is noted that though the long afterglow can be observed under UV excitation, the initial afterglow intensity becomes very weak and the afterglow also decays quickly when the phosphor is excited at 411 nm. While excited at 467 nm, the luminescence can be obtained but no afterglow is observed. The Eu²⁺ is excited to its lower 4f ⁶5d ¹ state rather than the higher 4f ⁶5d ¹ state under the excitation of 467 nm, which is far from the conduction band. The electrons in the lower 4f ⁶5d ¹ state are difficult to enter the conduction band and fall into the ground state spontaneously. Therefore, no afterglow can be observed when the energy of the exciting photon is too low. When the excitation wave length is shorter than 411 nm, however, the electrons are pumped to the higher 4f ⁶5d ¹ state very close to the conduct band and are possible to enter the conduct band by absorbing several phonons and can be finally captured by the traps. Hence, only excited by UV photons (250–400 nm) with enough energy, the electrons can be captured by the traps with the help of the conduction band, producing the afterglow.