1. Introduction

Long-lasting phosphorescence (LLP) materials have a great potential of the device applications as displays in dark environment [1,2]. At present, some LLP materials exhibiting a high brightness of blue and green phosphorescence are commercially available [3,4]. However, red LLP phosphors are in great scarcity.

The CaS:Eu²⁺ is the early red light long-lasting phosphor, but the sulfide has serious deficiencies for its instability [5]. In 1997, Dillo et al reported red LLP phosphor CaTiO₃:Pr³⁺ [6]. In 1999, Murazaki et al. developed a red LLP materials M₂O₂S:Eu³⁺,Mg²⁺,Ti⁴⁺ (M = Y and Gd) [7]. Thereafter, Fu has reported orange and red phosphors MO:Eu³⁺ (M = Ca, Sr and Ba) [8]. In 2003, it was found that the orange LLP of Y₂O₂S:Ti⁴⁺,Mg²⁺ could be observed [9]. However, there is still not one perfect red or orange LLP phosphor that can be applied to practical applications. One of the reasons is that the role of the defects (Cation or anion vacancies) remains largely unclear because of the difficulty of identifying and properly characterizing them. Cation or anion vacancies can produce local potentials able to serve as traps for electrons or holes, which play major roles in the initial intensity and persistent time in long afterglow phosphors. Hence, there is a strong desire for studying the effect of defects on the red long lasting phosphor.

Mostly, these are lattice defects of various types always present in materials, and these traps (defects) can be strongly altered by changing the material preparation conditions, for example, the fabrication atmosphere [10]. In order to learn more about the mechanism governing the red afterglow phenomenon, in this work, a new red LLP phosphor Sr₂SnO₄:Sm³⁺ was prepared by sintering in air and vacuum atmosphere, respectively. We hoped to get some indications about the possible methods of increasing the emission intensity and afterglow time in this phosphor. In addition, we have found that Sm³⁺-doped Sr₂SnO₄ can exhibit properties typical for storage phosphors.

2. Experimental

Sr_2-xSnO₄: Sm_x ³⁺ (x = 0, 0.00125) were prepared by a solid-state reaction between SrCO₃ (99%), SnO₂ (99.99%), Sm₂O₃ (99.99%). High-purity starting powders were mixed, with further addition of alcohol, ball-milled for 40 min, then dried and calcined at 900 °C for 2 h in air. At last, the crucible with the mixture was placed in a muffle furnace and sintered at 1300 °C for 4 h in air and 10⁻² Torr vacuum atmosphere, respectively. No fluxes were used in order to get accurate experimental results.

The phases of samples were identified by X-ray powder diffraction (XRD) with Ni-filtered CuKα radiation at a scanning step of 0.02° in the 2θ range from 10° to 80 °. Emission, and excitation spectra were recorded on a FLS-920T spectrometer. Afterglow decay curve measurements were measured with a PR305 long afterglow instrument after the sample was irradiated by artificial light (1000 ± 5% lux) for 10min. The thermoluminescence(TL) curves were measured with a FJ-427A TL meter (Beijing Nuclear Instrument Factory). The sample weight was kept constant (0.002 g). Prior to the measurements, powder samples were first exposed for 10 min to standard artificial daylight (1000 lux), then heated from room temperature to 200 °C with a rate of 1 K/s.

3. Results and discussion

To make sure our results were reliable, the purity of all the prepared samples was systematically checked by XRD. All the diffraction peaks are assigned to the Sr₂SnO₄ phase and match well with JCPDS card 24-1241, as shown in Fig. 1 . Doping Sm³⁺ does not make any noticeable variation of the XRD pattern.

 

Fig. 1 XRD patterns of the air-sintered Sr₂SnO₄: Sm³⁺, the vacuum-sintered Sr₂SnO₄: Sm³⁺ and JCPDS Card No. 34-0379.

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The sample was subjected into oxygen-deficient atmosphere (vacuum) to sinter with an attempt to evaluate the influence of the defects (oxygen vacancies) on the PL of the phosphor. The results are presented in Fig. 2 . As shown in Fig. 2, the emission transitions of Sm³⁺ are connected to the well-known intra-4f transitions of Sm³⁺ from the excited level to lower levels (⁴G_5/2 ⁶H_5/2, ⁶H_7/2, ⁶H_9/2), respectively [11]. We can note from Fig. 2 that the PL intensity for the air-sintered phosphor (curve a) is relative low. However, it increased about 200% when the sample was sintered in vacuum, as shown in Fig. 2 (curve b). The result indicates that the sintering in vacuum is quite effective to improve the red emission of the Sr₂SnO₄:Sm³⁺. The sintering in vacuum may result in two effects: (1) the creation of oxygen vacancies as proposed by many other groups and (2) the valence changing of the Sm ions from Sm³⁺ to Sm²⁺ or the Sn ions from Sn⁴⁺ to Sn³⁺. To clarify whether there is a valence changing of the Sm³⁺ and Sn⁴⁺ ions under vacuum sintering, we checked the emission spectra of our sample and found that the emission of Sm²⁺ was not observed. In addition, we adopted vacuum rather than reduction sintering for preventing the reduction of Sn⁴⁺. Because the as-synthesized sample is gray after reduction sintering, while the Sr₂SnO₄ is white after vacuum sintering. This indicates that the valence state of Sm³⁺ and Sn⁴⁺ ions changes little after the samples undergo sintering in vacuum.

 

Fig. 2 Emission(λ_ex = 254nm) and excitation(λ_em = 622nm) spectra of Sr₂SnO₄ and Sr₂SnO₄: Sm³⁺. (a) is the emission spectrum of the air-sintered Sr₂SnO₄: Sm³⁺. (b), (d) and inset are the emission spectra of Sr₂SnO₄: Sm³⁺ sintered in vacuum. (c) and (e) are the emission spectra of Sr₂SnO₄ sintered in vacuum and air, respectively. Left dash curve is the excitation spectrum of Sr₂SnO₄: Sm³⁺ sintered in vacuum.

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Based on the above analysis, we can suggest that the enhancement in the PL intensity in the vacuum-sintered Sr₂SnO₄:Sm³⁺ is related to the creation of the oxygen vacancies. To clarify the relation between the red emission and oxygen vacancies in the vacuum-sintered Sr₂SnO₄:Sm³⁺, we first examine the PL spectra of Sr₂SnO₄ without Sm³⁺ doping. As shown in Fig. 2e, a weak blue-light emission centered at 414nm was observed for Sr₂SnO₄ sintered in air atmosphere. However, in vacuum, the stronger blue emission intensity was observed in Fig. 2c. We thus conclude that the blue emission band is associated with the oxygen vacancies. The increase of oxygen vacancies made the conduction electron density rise, which contributed to the higher emission intensity. The dash curve of Fig. 2 exhibited the excitation band of the Sm³⁺ emission at 622nm. An obvious spectral overlap between the emission of oxygen-vacancy-related and the excitation band of Sm³⁺ can be observed in Fig. 2. According to Dexter theory [12], an efficient energy transfer (ET) requires a spectral overlap between the donor emission and the acceptor excitation. It is clear from the PL spectra that in the Sr₂SnO₄:Sm³⁺ phosphor, ET from the host to the Sm³⁺ ions occurs. When excited by UV light source, excitation energy is absorbed by the host and created the oxygen-vacancy-related emission, meanwhile, the absorbed energy is transfer to the Sm³⁺ ions and created the typical emissions of Sm³⁺. In addition, the ET from oxygen-vacancy to Sm³⁺ can be confirmed since the blue emission (Fig. 2d) is largely decreased after Sm³⁺ doped into Sr₂SnO₄. After sintered in vacuum, the more the oxygen vacancies exist in the phosphor, the more effective ET, thus leading to the higher red emission intensity.

A very important result of our present work is that the red long afterglow can be observed with the naked eye in the dark clearly. Afterglow characteristics, after the removal of light excitation, are shown in Fig. 3 . It is obvious that persistent time of the vacuum-sintered phosphor has been observed to increase, comparing with the air-sintered phosphor. The afterglow color photographs of the samples for 1 min after the removal of the 254nm UV lamp are also shown in Fig. 3. We can easy get that the intensity of the red afterglow of the vacuum-sintered phosphor is stronger than that of the air-sintered phosphor. Consequently, acting as the electron trap, oxygen vacancies can strongly influence the afterglow characteristics of Sr₂SnO₄:Sm³⁺.

 

Fig. 3 Afterglow decay curves of Sr₂SnO₄: Sm³⁺ sintered in air and vacuum. Inset: long afterglow photographs of Sr₂SnO₄: Sm³⁺ sintered in air and vacuum. The photographs were taken in the darkroom for 1 min after the removal of the 254-nm ultraviolet lamp.

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The thermoluminescence (TL) technique is a very useful tool for revealing the nature of defects produced in insulators or semiconductors by UV light [13, 14]. The aim of this section is to gain insight into the influence of oxygen vacancies on the origin of the afterglow of the Sr₂SnO₄:Sm³⁺ phosphor and to study the nature of the traps created under UV radiation by performing TL measurements. The TL glow curve of the vacuum-sintered Sr₂SnO₄:Sm³⁺ was shown in Fig. 4 , perhaps we can say that there exist three types of the traps (T₁, T₂, T₃) present in the Sr₂SnO₄:Sm³⁺ phosphor corresponding to three peaks (Gaussian fit) in the TL grow curve, located at 325, 385 and 443k, respectively. The black dash curve in Fig. 4 represents the TL grow curve of the vacuum-sintered Sr₂SnO₄. Comparing with curves T and T₁, we can get that both samples have the same TL position (325K), indicating the presence of the same defects in the Sr₂SnO₄:Sm³⁺ and undoped sample lattice. In general, the TL peaks close to (or above) the room temperature are expected to be essential to the long afterglow [15]. As discussed above, it was manifested that the red afterglow from Sr₂SnO₄:Sm³⁺ was closely related to the oxygen vacancies. Based on the above results, for the TL peak (T₁) at 325k, the corresponding defect may be ascribed to the oxygen vacancies in the sample host. To argue this assignation, the TL glow curves (Fig. 5 ) were measured with different delay times (30min, 2h, 72h,) after ceasing the UV irradiation. With the delay time increases, the intensity of peak T₁ decreases, and the band T₁ completely disappear after delay 2 hours. Consequently, we can affirm that the afterglow is mainly attributed to the contribution of traps T₁. Moreover, the shapes of peak T₂ and T₃ were found not to be change as the delay time increases. Trap T₂ and T₃, responsible for TL peak T₂ and T₃, respectively, are not considered to be thermally released at room temperature to yield the afterglow because peak T₂ and T₃ occur at the relative higher temperature. Accordingly, the trap T₁ arising from oxygen defects (oxygen vacancies) is responsible for the afterglow performance of the Sr₂SnO₄:Sm³⁺ phosphor.

 

Fig. 4 Thermoluminescence glow curves of the vacuum-sintered Sr₂SnO₄ and Sr₂SnO₄: Sm³⁺.

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Fig. 5 Thermoluminescence glow curves of the vacuum-sintered Sr₂SnO₄: Sm³⁺ recorded with different delay times (30min, 2h, 72h) after the UV irradiation.

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Figure 5 evidences another very interesting effect. Namely, for a delay of 2h, although the T₁ peak has completely decayed as well as the afterglow subsided, other two peaks remain the original intensity. Hence, the presence of deep stable traps in Sr₂SnO₄:Sm³⁺ able to immobilized the energy permanently at room temperature, since the TL bands at 385 and 443k remained the original intensity after a delay of 72h in dark. For a storage phosphor, the traps should be relatively deep to preclude the thermal release of the intercepted carriers at room temperature, whereas for the afterglow phosphor, the traps should be rather shallow [16]. Taking into account these finding and considerations, it can be concluded that Sr₂SnO₄:Sm³⁺ fulfill the requirements of either a photostimulated (storage) phosphor or a long-afterglow phosphor.

4. Conclusion

A new red afterglow phosphor Sr₂SnO₄:Sm³⁺ was synthesized by solid-state reaction. Comparing with the air-sintered sample, the emission intensity and afterglow time of which sintered in vacuum atmosphere increased significantly. It is manifested that the enhancement in the emission intensity and afterglow time for the vacuum-sintered Sr₂SnO₄:Sm³⁺ phosphor is related to the creation of the oxygen vacancies. In addition, this phosphor can be used as the storage phosphor, since the presence of deep stable traps able to immobilize the energy permanently at room temperature.