Tm3+ phosphors

JOURNAL OF RARE EARTHS, Vol. 34, No. 1, Jan. 2016, P. 1

Long persistent and photostimulated luminescence properties of Sr2Al2SiO7:Eu2+/Tm3+ phosphors QI Ye (齐 野) 1, 2, ZHANG Jinsu (张金苏)1,*, YU Hongquan (于洪全)2,*, SUN Jiashi (孙佳石)1, LI Xiangping (李香萍)1, CHENG Lihong (程丽红)1, CHEN Baojiu (陈宝玖)1 (1. Department of Physics, Dalian Maritime University, Dalian 116026, China; 2. College of Environmental and Chemical Engineering, Dalian Jiaotong University, Dalian 116028, China) Received 8 July 2015; revised 6 September 2015

Abstract: A series of Sr2Al2SiO7 phosphors with fixed Eu2+ concentration and various Tm3+ concentrations were synthesized via a high temperature solid state reaction. The structure and luminescence properties of the samples were characterized by X-ray powder diffraction (XRD), photoluminescence (PL) spectra, decay curves, thermoluminescence (TL) glow curves as well as the photostimulated luminescence (PSL) spectra. Sr2Al2SiO7:Eu2+,Tm3+ phosphors exhibited strong green phosphorescence and photostimulated luminescence originating from 4f65d1–4f7 transition of Eu2+ after ultraviolet light stimulation. Deep traps were found by analyses on the phosphorescence decays and thermoluminescence spectra. It was also found that with increasing the time interval between UV excitation turn-off and stimulation turn-on, the photostimulated luminescence became stronger. This phenomenon resulted from the fact that the captured electron was retrapped by the deep traps. Compared with the Tm3+-free sample, it was found that the PSL intensity was strongly enhanced in Tm3+ codoping samples. Keywords: Sr2Al2SiO7:Eu2+/Tm3+; long persistent; photostimulated luminescence; rare earths

Photostimulated luminescence (PSL) materials have attracted increasing attention owing to their promising applications in energy storage[1–3]. PSL is a phenomenon in which the materials can store light energy when they are stimulated by ultraviolet or visible light. PSL materials are effective energy storage media having been used as photostimulable phosphor plates since they were found to have the property of recording a two-dimensional image of the intensity of short-wavelength electromagnetic radiation[4]. Therefore, the research for efficient photostimulated luminescence phosphors has continuously gained popularity. Over past decades, scientists still pay their efforts to develop novel PSL materials for optical storage, and the typical PSL materials such as alkali halide (e.g., KCl)[5,6], alkaline earth metal halide (e.g., BaFBr, BaBrCl)[7,8] and sulfide (e.g., SrS, CaS)[9,10] were discovered one after another. At present the main-stream PSL phosphors are still limited to sulfides-based systems. Usually, these sulfides have a large proportion of deep traps which can store the electrons tightly at room temperature and thus they are widely regarded as potential candidates for optical storage. Nevertheless, the chemical and thermal stability of those sulfides are poor and even not environment friendly, thus restricting their practical applications[11,12].

Therefore, it is essential to search for novel PSL materials with effective excitation of infrared, easy preparation, high chemical stability and good thermal stability to replace the sulfides. In recent years, PSL was also observed in some oxide compounds, such as SrAl2O4:Eu2+/Dy3+ [13–15] , Mg2SnO4:Eu3+ [16] and Sr3Al2O5Cl2: Eu2+ codoped with Ce3+, Dy3+ or Tm3+ [17–19]. These oxide systems conquered the shortcomings of the abovementioned sulfides, herein can be used in various fields. Recently, it was discovered that Sr2Al2SiO7:Eu2+ can be efficiently excited by ultraviolet (UV) and generate green emission, indicating that it is a potential phosphor for application in white light emitting diodes[20]. Ding et al. reported that introducing Dy3+ into Sr2Al2SiO7:Eu2+ can produce deep traps capturing the electrons which cannot be released at room temperature[20]. Based on these results, in this work we attempted to explore if the other type of rare earth ions can produce deep traps in this oxide system, and study on the photosimulated luminescent properties. A series of Eu2+ and Tm3+ codoped Sr2Al2SiO7 phosphors with various Tm3+ concentrations were synthesized by a solid-state reaction in a reducing atmosphere. The XRD, photoluminescence (PL), PL excitation (PLE), afterglow decay curves and thermoluminescence (TL)

Foundation item: Project supported by the National Natural Science Foundation of China (11374044, 11104024), Natural Science Foundation of Liaoning Province (2014025010), and Fundamental Research Funds for the Central Universities (3132014327, 3132015143) * Corresponding authors: ZHANG Jinsu, YU Hongquan (E-mail: [email protected], [email protected]; Tel.: +86-411-84728909, +86-411-84106809) DOI: 10.1016/S1002-0721(14)60569-X

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were measured. The afterglow behavior for different Tm3+ concentrations doping was discussed in detail. The PSL phenomenon in Sr2Al2SiO7:Eu2+/Tm3+ was observed under 980 nm laser stimulation after irradiation of the materials by UV light. Compared with the Eu2+ monodoped sample, the PSL intensity in the Tm3+ codoping samples was strongly enhanced. The maximum PSL intensity was found in the sample doped with Tm3+ at x=0.02. PSL mechanism was also studied.

1 Experimental Powdered Sr1.98–xAl2SiO7:0.02Eu2+/xTm3+ (here x=0, 0.001, 0.005, 0.01, 0.02, 0.04) samples were fabricated via a solid-state reaction by using the starting materials with certain chemical stoichiometry. The starting materials are SrCO3, Al2O3, SiO2, Eu2O3 and Tm2O3. Each mixture of the starting materials was ground for an hour, and then loaded into a crucible, after that sintered at 1350 ºC in a CO reducing atmosphere for 4 h. When the sample naturally cooled down to room temperature, the final product was obtained and ground again for the subsequent measurements. The crystalline structure of the samples was investigated by the X-ray diffraction (XRD) patterns, which were measured on a Shimadzu X-ray diffractometer (XRD)-6000 with a Cu target radiation source (λ= 0.154056 nm). In the measurements the scanning step size and 2θ angle region were set to be 0.02º and 10º–70º, respectively. The photoluminescence (PL), PL excitation (PLE), long lasting phosphorescence spectra and the afterglow decay curves were measured using a HITACHI F-4600 fluorescence spectrophotometer. The thermoluminescence (TL) grow curve was measured using a TL meter. The phosphors were excited by a 365 nm UV lamp for 5 min, and 3 min later the phosphorescence spectra and TL were detected. The TL curves were recorded with a heating rate of 1 K/s. In order to avoid the phosphorescence influence on the PSL spectra, the samples were pre-irradiated for 5 min with 365 nm UV lamp and then placed in the dark for 10 min before the measurements. A 980 nm laser diode was used as an infrared simulation source.

JOURNAL OF RARE EARTHS, Vol. 34, No. 1, Jan. 2016

Sr2Al2SiO7 might occupy the Sr2+ sites due to the similar ionic radius. The lattice constants for Sr1.98Al2SiO7: 0.02Eu2+ and Sr1.96Al2SiO7:0.02Eu2+/0.02Tm3+ samples were calculated by using software Jade 6.0 to be a=b=0.7818 nm, c=0.5248 nm and a=b=0.7808 nm, c=0.5242 nm, respectively. Compared with the lattice constants (a0=b0=0.7820 nm and c0=0.5264 nm) in standard XRD, the lattice constants for the studied samples decrease, thus indicating that the Eu2+ (ionic radii of 0.117 nm) and Tm3+ (ionic radii of 0.088 nm) take the place of Sr2+ (ionic radii of 0.118 nm) cites. The PLE (monitoring 510 nm) and PL (excited at 340 nm) spectra of Sr1.98–xAl2SiO7:0.02Eu2+/xTm3+ are shown in Fig. 2(a) and 2(b), respectively. In Fig. 2(a), a broadband ranging from 250 to 450 nm can be assigned to the 4f7→4f65d1 transition of the Eu2+ ions. In Fig. 2(b), all

Fig.

1

XRD patterns of Sr1.98Al2SiO7:0.02Eu2+ Sr1.96Al2SiO7:0.02Eu2+,0.02Tm3+

and

2 Results and discussion In order to characterize the phase purity and crystallization of the samples, XRD measurements for all the samples were performed. Fig. 1 shows the XRD patterns for Sr1.98–xAl2SiO7:0.02Eu2+/xTm3+ (x=0, 0.02) as examples together with the pattern reported in standard JCPDS card No. 75-1234. It can be seen that the XRD patterns for our samples are in good agreements with the one in standard JCPDS card, and no extra-peaks other than that of Sr2Al2SiO7 phase are observed. The Eu2+ and Tm3+ in

Fig. 2 PLE (a) and PL (b) spectra of Sr1.98–xAl2SiO7:0.02Eu2+, xTm3+ (x=0, 0.001, 0.005, 0.01, 0.02, 0.04)

QI Ye et al., Long persistent and photostimulated luminescence properties of Sr2Al2SiO7:Eu2+/Tm3+ phosphors

the Sr1.98–xAl2SiO7:0.02Eu2+/xTm3+ phosphors show a broad band emission centered at 510 nm attributed to the typical 4f65d1→4f7 transition of Eu2+. The intrinsic Tm3+ emission peaks are not observed in the emission spectra of the phosphors, because it acts as trap centers in the present system. The optimum doping concentration of Tm3+ for achieving maximum emission intensity is x= 0.01. The emission intensity of optimum Eu2+ and Tm3+ co-doping Sr2Al2SiO7 phosphor is higher than that of Eu2+ single doped Sr2Al2SiO7. Moreover, with increasing the Tm3+ concentration, the green emission intensity increases firstly and then decreases. To examine the long afterglow luminescence, all the samples were exposed to UV light for 5 min, and then the long afterglow decays were measured under the same experimental conditions and are depicted in Fig. 3(a). It can be seen from the curves that the phosphors show green long-lasting phosphorescence after being irradiated by UV light. The initial intensity enhances strongly as the Tm3+ codoping concentration increases, and reaches its maximal value at the Tm3+ concentration of 2 mol.%. It is well known that the phosphorescence decay behavior for long afterglow phosphors can be expressed by following exponential equation[21]: I=At–α (1) where I represents the phosphorescence intensity, A stands for a constant, t is the time, α is a constant for a certain system and decides the decay rate of the phosphorescence. It should be mentioned that Eq. (1) does not fit the initial period of the decay. Therefore, the phosphorescence decays in Fig. 3(a) were measured 3 min later after the UV irradiation. Eq. (1) was used to fit the data in Fig. 3(a), and the fitting results of A and α are listed in Table 1. It should be mentioned that Eq. (1) fits well the experimental data, and the correlative coefficient derived from each fitting process is larger than 0.994 which are listed in the last column of Table 1. From Table 1 it can be found that the decay rate of all the samples with Tm3+ is larger than that of Eu2+ single doped one, and the minimum value of α was obtained to be 0.3311 for the sample with x=0.02. Under 340 nm excitation, the afterglow spectra of Sr1.96Al2SiO7:0.02Eu2+,0.02Tm3+ samples were recorded at different time and are shown in Fig. 3(b). Clearly, the line shape for the afterglow spectra is in agreement with that shown in Fig. 2(b). Though there are not any afterglow peaks originating from Tm3+, the Tm3+ codoping could greatly improve the afterglow property of Sr2Al2SiO7:Eu2+ phosphor due to the contribution of traps produced by adding Tm3+. Generally, the mechanism for the long afterglow generation can be deduced from the TL measurement[22–25]. It is suggested that certain trap depth and trap concentrations are essential for phosphors to present good long persistence and energy storage[22,26]. In the case of

3

Fig. 3 Decay curves of Sr1.98Al2SiO7:0.02Eu2+/xTm3+ (x=0, 0.001, 0.005, 0.01, 0.02, 0.04) (a) and afterglow spectra of Sr1.96Al2SiO7:0.02Eu2+,0.02Tm3+ (b) Table 1 Fitting results of A and α fitted according to the decay curves of Sr1.98–xAl2SiO7:0.02Eu2+/xTm3+ (x=0, 0.001, 0.005, 0.01, 0.02, 0.04) x values

Initial intensity

A

α

0

942

1.1823

0.4263

0.001

1008

1.1672

0.3994

0.005

1161

1.1704

0.4208

0.01

1236

1.1316

0.4228

0.02

1939

1.7787

0.3311

0.04

1038

1.2037

0.4233

Sr1.98–xAl2SiO7:0.02Eu2+/xTm3+ phosphors, the deep traps are formed by the codoped Tm3+ ions that replace the Sr2+ ions in the host lattice. The replacement caused positive centers around Tm3+ ions and therefore became electron traps. Therefore, a comparison between the trap depth and trap concentration will reveal the difference in afterglow behavior for the Eu2+ single doped and Eu2+/ Tm3+ codoped samples. Fig. 4 shows the TL spectra of Sr1.98–xAl2SiO7:0.02Eu2+/xTm3+ phosphors and thus there appear two peaks at ~360 and ~490 K, respectively. Tm3+ would form electron-trapping centers, and the defect level is deeper than that caused by Dy3+ [27]. The remarkable growth of trap around 490 K with codoping Tm3+ is pronounced[28]. The maximum TL intensity for trap 490 K occurs at Tm3+ concentration of x=0.02. To examine the PSL all the samples were exposed by UV light for 5 min, and then placed in dark for 10 min.

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Fig. 4 Thermoluminescence (TL) patterns of Sr1.98–xAl2SiO7: 0.02Eu2+/xTm3+ (x=0, 0.001, 0.005, 0.01, 0.02, 0.04)

All samples were observed PSL phenomenon under the 980 nm infrared excitation with 0.5 A current, as shown in Fig. 5. When the infrared stimulation turns on, the PSL rapidly appears with an initial intensity and subsequently decays slowly. With increasing of Tm3+ concentrations, PSL intensity decreases as the value of x beyond the optimal concentration. The maximum PSL intensity also occurs at Tm3+ concentration of x=0.02, implying the correlation with the TL intensity. The relationship among TL intensity and PSL intensity is shown in Fig. 6. The samples are labeled from 1 to 6 as the concentration of Tm3+ increases. It can be seen that the variations of TL intensity and PSL intensity keep consistent with the increase of Tm3+ concentration from sample 1 to 6. The TL and PSL intensities enhanced 8.2 and 40 times than Eu2+ single doped sample, respectively. The possible reason is described as follows: when trivalent Tm 3+ ions act as an auxiliary doped into Sr2Al2SiO7:Eu2+ phosphor and substitute Sr2+ sites resulting in chemically nonequivalent substitutions. Because of these nonequivalent substitutions, an excess of positive charge in the lattice must be compensated. Additionally, the reducing atmosphere is introduced during the materials synthesis process, there have not enough

Fig. 5 PSL of Sr1.98–xAl2SiO7:0.02Eu2+,xTm3+ (x=0, 0.001, 0.005, 0.01, 0.02, 0.04) under the 980 nm excitation

JOURNAL OF RARE EARTHS, Vol. 34, No. 1, Jan. 2016

Fig. 6 PSL and TL (at 490 K) intensities of Sr1.98–xAl2SiO7: 0.02Eu2+/xTm3+ (x=0, 0.001, 0.005, 0.01, 0.02, 0.04) labeled from sample 1 to 6

oxygen elements to form the crystal, leading to the appearance of oxygen vacancies, and each oxygen vacancy is two positive charged. By and large, the replacement way is in the form of 2Tm3++3Sr2+→2Tm•Sr+ VSr'' , which created the positive defects and negative defects can serve as electron and hole trap centers, respectively. The oxygen vacancies which can be regarded as electron trap centers are generated for charge compensation[29]. VSr'' can act as quenching centers that reduce PSL intensity[30]. Therefore, it was observed that the PSL intensity decreases when the concentration of Tm3+ is higher than 2 mol.%. Another phenomenon with applied value we observed is that with increasing the time interval between UV excitation turn-off and stimulation turn-on, the PSL intensity became stronger, as shown in Fig. 7. The thermally released electrons from the shallow trap can be retrapped by deep trap through the conduction band[28]. This result implies that once the electrons are retrapped by the deep trap, they could hardly be released at room temperature. Nonetheless, they can be excited by infrared light resulting in PSL[31]. When the infrared stimulation is turned on, the PSL appears and rises up subsequently reaching the maximum intensity. As the UV excitation turn-off period

Fig. 7 PSL intensity within different time intervals between UV excitation turn-off and stimulation turn-on in Sr1.96Al2SiO7:0.02Eu2+,0.02Tm3+

QI Ye et al., Long persistent and photostimulated luminescence properties of Sr2Al2SiO7:Eu2+/Tm3+ phosphors

prolongs, more accumulation performs; hence, further enhancement of PSL intensity is expected. All these indicate the re-trapping of charge carriers by deep traps. Hence, the conclusion that the deeper TL trap is responsible for the PSL can be confirmed.

3 Conclusions In summary, a series of Sr2Al2SiO7 phosphors with fixed Eu2+ concentration and various Tm3+ concentrations were successfully synthesized via a high temperature solid state reaction. They exhibited strong green phosphorescence and photostimulated luminescence originating from 4f65d1→4f7 transition of Eu2+. The relationship between TL intensity and PSL intensity were studied. TL glow curves presented that two dominated traps with shallow depth and deep depth were responsible for the long persistent phosphorescence and photostimulated luminescence, respectively. The strongest intensity of PSL was gained with the optimal Tm3+ concentration at x=0.02. Compared with the Eu2+ single doped sample, the TL and PSL intensity of Sr1.96Al2SiO7:0.02Eu2+, 0.02Tm3+ enhanced more than 8.2 and 40 times, respectively.

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