Effects of Silica-encapsulated CaS∶Eu，Sm Phosphors on Optically Stimulated Luminescence Properties

GAI Min-qiang，CHEN Zhao-yang，FAN Yan-wei，WANG Jun-hua，XIE Yong-xin

(1.Xinjiang Technical Institute of Physics ＆ Chemistry，Chinese Academy of Sciences，Urumqi 830011，China;2.Xinjiang Key Laboratory of Electronic Information Materials and Devices，Urumqi 830011，China;3.University of Chinese Academy of Sciences，Beijing 100049，China)

1 Introduction

Optically stimulated luminescence (OSL)technology，in which photons or charged particles stimulate luminescence，provides quick reading，resets online，and performs well in radiation dosimeters.Rare earth-doped alkaline earth sulfides(AESs)exhibit high sensitivity to radiation and high efficiency under infrared stimulation at wavelengths of approximately 1 μm［1-4］.They have been widely used in infrared detection，display applications，optical information processing，and radiation dosimetry［5-9］.Compared with the Al2O3∶C-based OSL dosimeter，AESs have more full discrimination between stimulation and emission spectra，and more broad measurement range of 0.1～185 Gy in realtime dosimeters［10］.Furthermore，their bleaching time is 10 s，which is about 10 times faster than Al2O3∶C.It makes AESs an ideal candidate for online measurements using radiation dosimeters［10］.However，the lack of stability for water and other atmospheric components significantly restrains their application in radiation dosimetry and therefore impedes their use as phosphor hosts［11-13］.In addition，the serious reunion between particles of CaS∶Eu，Sm phosphors is an obstacle the absorption of the excitation light.These features make rare earthdoped AESs insensitive to any type of ionization and distort their response to the radiation dose.The surfaces of sulfide phosphors have been coated with a transparent and uniform film layer，which would isolate the phosphors from the atmosphere and keep them away from each other to minimize the reduction of PL intensity.Studies on the moisture stability of CaS ∶Eu2+phosphors using different coating methods with silica nanoparticles were performed by Kim et al.［14］and Guo et al.［15-16］，who specifically investigated the impact of coating amount on the performance of rare earth-doped AESs and proposed that oxide coating values in the range of 2%～5%were enough to provide sufficient impermeability to moisture with minimum loss of brightness.Chao et al［17］investigated the thermal stability of SiO2-coated BaMgAl10O17∶Eu2+phosphors and found that the SiO2coating film served as a block layer to mitigate the degree of oxidation of activator Eu2+.However，to the best of our knowledge，studies on the OSL properties of oxide-coated CaS ∶Eu，Sm phosphors have not been reported.These properties are essential to online OSL dosimeters for checking the conformity between prescribed and absorbed doses delivered to patients and electronic components［7］.

The present study coated OSL materials of CaS∶Eu，Sm with SiO2were obtained by a sol-gel method.The OSL measurements were carried out as previously described［10］.To deduce the effects of encapsulation on the OSL properties of the CaS∶Eu，Sm phosphors，we investigated changes in their morphology，PL intensity，and water resistance，as well as OSL decay and dose response.

2 Experiments

CaS ∶Eu，Sm phosphors were synthesized by carbon thermal reduction，which has been discussed in detail by Wu et al.［18］.Briefly，calcium sulfate (CaSO4·2H2O)，acetylene black (C)，and the rare earth oxide activators Eu2O3and Sm2O3(purity ＞99.99%)were used as starting materials for preparation of the phosphors.The CaS∶Eu，Sm phosphors were coated with silica nanoparticles via the sol-gel technique as follows:initially，different volumes (3%，5%，and 10%)of tetraethyl orthosilicate (TEOS)(AR)were added to three beakers.Each beaker was then filled with a mixed solution containing 25 mL of ethyl alcohol and 40 mL of deionized water at the rate of 0.6 mL per minute.Prepared CaS∶Eu，Sm (20 g)phosphors of suitable quality were average-dispersed into these series of solutions，which were stirred homogenously at 60 ℃for 0.5 h.A small amount of ammonia (AR)was subsequently dripped into each solution as a catalyst for hydrolyzing TEOS.The mixture was stirred vigorously for 45 min at a pH value between 9 and 10.

Next，we obtained a series of CaS ∶Eu，Sm phosphors coated with various ratios of silica after drying the powders at 80 ℃for 2 h in a vacuum drying oven and heating them at 500 ℃for 1 h by a muffle furnace.The phase composition of the composite was determined by X-ray diffraction (XRD;BRUKER D8-ADVANCE，Germany)with CuKα1(λ=0.154 06 nm)radiation in the 2θ range of 10°～80°.The microstructure and elemental compositions of the selected samples were observed using scanning electron microscopy (SEM，LEO1430VP，Germany)in combination with energy-dispersive spectroscopy (OXFORD INCA200，England).Their PL properties were analyzed in a WGY-10 fluorescence spectrometer，in which a 150 W tungsten lamp was used as excitation source and a slit width of 5 nm on both excitation and emission sides.A PHS-3B acidometer (Shanghai LEICI Instrument Inc.，China)was used to measure the pH value.

Discs made from the phosphors were used for OSL measurements.The phosphors were uniformly mixed with poly (methyl methacrylate)，dissolved in chloroform，at a ratio of 1∶3.The mixture was then placed in an air chamber for several hours to produce discs with a size of 4 mm × 4 mm and with 0.3 mm thickness.OSL measurements were carried out using an indigenously developed OSL reader with a P30A-05 photomultiplier tube operating at 500 V and a DSO5012A oscilloscope (Agilent，USA).A green long pass filter (PIN-10AP)is incorporated in front of the photomultiplier tube to minimize the amount of directly scattered red.For γ irradiation，a γ chamber containing60Co of 150 000 Curie was used as source.All measurements were performed at room temperature.

3 Results and Discussion

3.1 XRD Spectrum

Fig.1 XRD patterns of CaS∶Eu，Sm phosphors:(a)uncoated，(b)coated with 5% SiO2，and JCPDS Card(no.75-0261)of CaS.

3.2 SEM Analysis

Panels (a)and (b)of Fig.2 represent the SEM micrographs of the uncoated and SiO2-coated CaS∶Eu，Sm phosphors，respectively.However，after sol-gel treatment，the surface became rough，which proves the success of ammonia-catalyzed TEOS hydrolysis.The SEM results indicated that a SiO2coating layer could protect the original unstable phosphor particles from atmospheric components.From Fig.2(a)we can see that the particle morphology is not obvious.That is because when the CaS∶Eu，Sm phosphors are prepared by the method of carbon-thermal reduction，oxygen can not be discharged well from the reactants.If other reduction of gas in vacuum purging the reactants，a big improvement of reunion may occur.

Fig.2 SEM micrographs of CaS∶Eu，Sm phosphors particles:(a)uncoated and (b)coated with 5% SiO2.

3.3 Photoluminescence Spectra of CaS∶Eu，Sm

Changes in the PL emission intensities of the coated phosphors are depicted in Fig.3.Under an excitation wavelength of 466 nm，the emission spectra consisted of a broad band from 580 to 750 nm with a maximum wavelength at 645.9 nm which can be ascribed to the 4f65d→4f7(8S7/2)allowed transition of Eu2+ions.As shown in Fig.3，the peak position of the emission spectra is at approximately 645.9 nm，which suggests that the surface coating does not affect the luminescence properties of the phosphors.The emission intensities decreased as the coating amount of SiO2increased，because the thicker SiO2layer and greater hydrolysis of AESs impeded the light emission of the CaS∶Eu，Sm phosphors.

Fig.3 PL spectra of CaS∶Eu，Sm phosphors:(a)uncoated，(b)coated with 3% SiO2，(c)coated with 5%SiO2，(d)coated with 10% SiO2.

3.4 Stability Analysis of Coated Phosphors

To identify and confirm the protective function of the phosphor coating，we carried out stability testing of different coating amounts of CaS ∶Eu，Sm phosphors.We dispersed 0.35 g of uncoated phosphors and 0.35 g of SiO2-coated ones in different volumes (3%，5%，and 10%)into four beakers filled with 50 mL of deionized water.As shown in Fig.4，the pH value variations for both coated and uncoated phosphor solutions shared a similar changing trend versus soaking time and only differed in numerical value.The pH value of the uncoated phosphors was higher than 13.2，whereas that of the 5% SiO2-coated samples was approximately 9.Research has also shown that CaS∶Eu，Sm phosphors have extremely low water-resistance stability，which due to theformation of hydroxide in water［20］.Furthermore，as illustrated in Fig.4，the pH value of the experimental system decreased as soaking time progressed，which can be attributed to the H2S，the phosphors hydrolyzing side，dissolved in water.This also implied that 5% of silica coating can effectively improve water resistance at minimum loss of light emission (Figs.3 and 4).This result is consistent with previous findings［15］.

Fig.4 pH variation of CaS∶Eu，Sm phosphors soaked in water:(a)coated with 10% SiO2，(b)coated with 5%SiO2，(c)coated with 3% SiO2，(d)uncoated.

To carry out the reusability study，12 discs prepared from the same batch of CaS:Eu，Sm phosphor were completely bleached and exposed to 1 Gy of60Co gamma rays.From Fig.5，we can see that the 5% SiO2-coated CaS∶Eu，Sm phosphors can be reused for 12 cycles and disc to disc variation of OSL signal was about 2% in the OSL output，which is better than the uncoated phosphors.

Fig.5 The reusability study of CaS∶Eu，Sm phosphors:(a)uncoated，(b)coated with 5% SiO2.

3.5 OSL in CaS∶Eu，Sm

The CaS∶Eu，Sm phosphors are protected by a sleeve rigidly fixed inside the cable and affixed near the end of the optic fiber (Fig.6).According to the stimulation spectrum and OSL spectrum of CaS∶Eu，Sm materials in our previous work［4］，an Nd laser(1 064 nm，MW=150)was used as the infrared excitation source for the dosimeter readout and the stimulation time is about 3 s.The probe was subjected to60Co irradiation to measure different exposure doses，and the OSL was collected and guided back along the optic fiber to a wavelength-selective photomultiplier tube through adequate optical filters.The radiation dose-response curve of the OSL film exposed to60Co radiation at the total ionizing dose of 10 Gy is shown in Fig.7.Note that the shapes of the two curves are not very similar over the entire range.As expected，the amount of coating had a pronounced effect on the OSL signal and the OSL intensities of the coated CaS∶Eu，Sm phosphors were obviously lower than those of the uncoated phosphors，especially at the SiO2coating level of 10%.These findings suggest that the OSL intensities are proportional to the trapped electrons induced by the60Co radiation and that the SiO2-coated core-shell structure may absorb and transport the photons generated on impact away from the phosphors［21］，although silica is transparent.Hence，the trapped electrons and OSL intensities decreased as the amount of SiO2increased.

Fig.6 OSL dosimetry system transmitted by optical fiber and profiles of the three ends of the optical coupler

Fig.7 OSL decay curve of CaS∶Eu，Sm phosphor at 10 Gy ionizing dose:(a)uncoated，(b)coated with 5%SiO2，(c)coated with 10% SiO2.

The data shown in Fig.8 were obtained from materials subjected to different surface treatments with respect to dose across 4 orders of magnitude by varying the source-to-sample distance from 16 to 253 cm and exposure time from 1 to 300 s respectively.And the dose rates are from 0.99 to 100.62 rad/s.The real-time dosimeter exhibited exceptional linearity in the dose range of 0.1～300 Gy.Fig.7 also indicates that 5% of coating significantly improved the stability of the material with minimum loss of OSL signal.The obvious effect of the increase in coating amount on the OSL signal and this phenomenon were particularly significant with an increase in the radiation dose.This might be due to the possibility that a higher amount of SiO2affects sensitivity by providing a protective insulating barrier around the CaS∶Eu，Sm phosphors.Further investigations to understand this phenomenon are in progress.

4 Conclusion

In conclusion，we have synthesized CaS ∶Eu，Sm phosphors by carbon thermal reduction.SiO2-coated CaS ∶Eu，Sm phosphors were obtained by the sol-gel method with TEOS as the silica coating precursor to improve the stability and reproducibility of CaS∶Eu，Sm phosphors in a real-time dosimetry system.The phase structure of CaS∶Eu，Sm did not change after the coating.The results of stability testing demonstrated that the water resistance of the CaS∶Eu，Sm phosphors had been enhanced by coating a thin layer of SiO2on their grain surface.This study found that 5% of silica coating was beneficial to the formation of the SiO2-coated core-shell structure during the sol-gel process and that the amount of surface coating significantly affected the OSL properties of the CaS∶Eu，Sm phosphors，particularly with increasing radiation dose.The proposed realtime dosimetry system exhibited exceptional linearity in the dose range of 0.1～300 Gy regardless of whether the phosphors were coated or not.These findings render coated CaS∶Eu，Sm phosphors a suitable candidate for real-time dosimetry as used in environmental，medical，and food irradiation.

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