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Effect of Samarium on Mn Activated Zinc Borosilicate Storage Glasses Li Chengyu ($=&fip)’”, Su Qiang (8 ( 1 . Key Laboratory of Rare Earth Chemistry and Physics, Changchun Institute of Applied Chemistry, Chinese Acahmy of Sciences , Changchun 130022 , China ; 2 . State Key Laboratory of Optoelectronic Materials and Techn- ology ; School of Chemistry and Chemical Engineering, Sun Yat-Sen University, Guangzhou 510275, China)

#)’*’

Received 13 July 2005; revised 16 January 2006

. Abstract: Samarium and manganese co-doped zinc borosilicate storage glasses were prepared by high temperature solid state method. The effect of doping samarium on the defect of Mn activated sample was studied by means of thermolumines- cence spectra. It was found that the shallower traps of the sample predominate with the addition of samarium, as a result, the phosphorescence and storage properties of the manganese doped zinc borosilicate glasses were greatly changed.

Key words : electron-trapping materials ; defects ; storage glasses ; rare earths CLC number: 0614.33; 0482.31 Document code: A Article ID: 1002-0721(2006)04-0506-03

Electron-trapping materials (ETMS) are such kind of materials that can store the energy of incident light ranging from energetic radiation to visible light that can be released from the traps by the thermal or optical stim- ulation in the form of thermoluminescence (TL) or opti- cally/photo stimulated luminescence (OSUPSL) . ETMs are widely used in the filed of detecting radiation, infor- mation storage, radiology, space exploring, indicator in the dark and craft, etc[1-61.

In ETMs , in addition to the activation ion, other ions are often co-doped to improve the property of the material, such as in SrAl2O4 : E d + , Dy3+ and SrS : Eu’+ , Sm3+ . It is generally considered that the co-do- pant acts as a trapping center to improve the energy storage ability of ETMs . However, by the observation of long lasting phosphorescence (LLP) of Sm in Mn activated zinc borosilicate glasses, we suggest that the codopant can change the transfer way of the stored energy from trap to luminescent ionL7].

In this article, the effect of samarium on Mn acti- vated zinc borosilicate optical storage glasses was stud- ied mainly by the TL spectra.

1 Experimental The detail of the preparation of the samples was

described in our previous article^'^'^]. The TL curves were measured with an FJ-427A thermoluminescence- meter (Beijing Nuclear Instrument Factory) . The slice of the glass sample (about 50 mg) was firstly irradiat- ed by a UV lamp(A, = 254 nm, 2 mW.cm-*) for 1 min before the measurement of TL spectrum. For the TL measurement, the same sample was used and heat- treated at 500 “c for 30 min to get rid of the pre-cap- tured electrons remaining in deep traps. Because the temperature of the devitrification of the sample is about 800 T, the effect of the bleaching on the structure and the TL spectrum of the glass can be neglected. The heating rate is 2 “c a s - ’ .

* Corresponding author (E-mail: cyli@ciac.jl.cn; lchengyuZOOl@yahoo.com) Foundation item: Project supported by National Key Project of Basic Research of China (G1998061312) Biography : Li Chengyu ( 1973 - ) , Male, Doctor, Associate professor; Present reaearch field : Rare earth electron-trapping materials

Copyright @ZOOS, by Editorial Committee of Journal of the Chinese Rare Earthe Society. Published by Elsevier B . V . All rights reserved

Li C I’ et a1 . Effect of Samarium on Mn Activated Zinc Borosilicate Storage Glasses 507

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2 Results and Discussion 2.1 TL spectra TL spectrum of Mn doped zinc borosilicate storage glass is shown in Fig. 1, which is a broad curve with a peak at about 410 K . It is known that the TL curve is closely related with the traps in ETMs, the spectrum at lower temperature is due to shallower traps while that at higher temperature is due to the deeper ones, and the shallower traps should contribute great to the LLP property of the material. Because of the complicated structure of the glasses and various defects in the glasses[g1, it is not yet clear what kind of traps contributes to the TL curve in Fig. 1 . The broad TL curve of zinc borosilicate storage glass is related with the property of the material: The lower temperature part for the LLP property and the higher temperature part for the stored energy that can be optically released. The detail of the mechanism of the glasses were discussed in Refs. [8] and 191.

The TL spectrum of the sample doped with Sm is shown in Fig. 2 . Compared with the TL curve in Fig. 1, the peak position of the curve shifts to lower tempera- ture , from 410 to 340 K , with the addition of Sm ; as well as an obvious change in the curve shape. It indi- cates that the co-doping of Sm is beneficial to the for- mation of shallower traps in the glass.

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Fig. 1

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TL spectrum of Mn activated zinc borosilicate storage glass

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300 400 500 600

TIK

Fig. 2 TL, spectrum of Srn and Mn co-doped zinc borosilicate glass

2 .2 Effect on phosphorescence The phosphores- cence decay curves of Mn-doped and Mn, Sm co- doped zinc borosilicate glasses are shown in Fig. 3. The two curves were measured at the same condition. As shown by the two curves, the addition of Sm im- proves the phosphorescence intensity and extends the luminescent life. According to our observation, the life time of the LLP of the co-doped sample is about 10 h , far exceeding the Mn-doped sample, which is about 6 h . However, according to our observation, the property of the photostimulated LLP of the sample greatly decreased, namely, at the cost of the optical storage time, the LLP property of the material is im- proved by the addition of Sm. 2 .3 Study on mechanism For Mn activated zinc borosilicate storage glass, a mechanism is suggested as given in Fig. 4 . The explanation of the mechanism is as follows. Trap 1 represents the traps from which the captured electrons could be thermally released to tun- neling state while electrons at trap 2 could not be re- leased at room temperature. After the irradiation, as shown by the case a in Fig.4 the captured electrons in trap 1 were thermally excited to tunneling state and then recombine with (M) , eventually leading to the characteristic emission of M , i . e . , Mn . When trap 1 was emptied, the latent electrons in trap 2 could be photo-released by photon with a lower energy, viz. stimulation light, and the emptied trap 1 was subse- quently re-filled. The electrons re-trapped by trap 1 underwent the above-described LLP process after stop- ping the stimulation light, resulting in optically stimu- lated LLP. During the stimulation process, there exist three cases for the electrons released from trap 2: b directly stimulated to tunneling state, c re-captured by trap 1, and d re-trapped by trap1 and then optically stimulated to tunneling state. The cases b and d lead to the OSL of the sample. After stopping the stimula- tion , the electrons re - captured by trap 1 ( case c in

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Fig. 3 Phosphorescence decay curves of Mn-doped ( dashed line) and Mn. Srn co-doped (solid line) zinc borosili- cate glasses

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Conduction band

JOURNAL OF RARE EARTHS, Vol . 24 , N o . 4 , Aug . 2006

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Tunneling state

'Trap I gb : Stimulation

4 : light hv 4.AA 'M" T Trap2 2 M L

Valence band

Fig. 4 Mechanism of Mn activated zinc borosilicate storage glass

Fig.4) repeat the LLP process resulting in LLF' that was called optically (or photo) stimulated LLP.

As mentioned above, the shallower traps of Mn activated zinc borosilicate glass contribute to the LLP property and the deeper ones to the information storage ability of the material. The addition of Sm obviously increases the intensity of the TL at lower temperature, meaning the increasing of the amount of the shallower traps. On the supposition that the energy stored by all the traps of the material is fixed, the improvement of the property of LLP should mean the weakening of op- tical storage ability. It is consistent with the result in our experiment. Therefore, the mechanism of the Mn and Sm co-doped zinc borosilicate glass should be the one in Fig. 5. Compared with Fig. 4, the relative amount of trap 1 in Fig.5 increases but the amount of trap 2 decreases (the thick and thin of the line repre- sent much or less of amount of the traps). This is also consistent with the deduction obtained from TL spec- trum in Fig.2. By analysis of the spectrum, the LLP and photostimulated LLP of the sample are due to the emission of Sm3+ and Mn2+ . Unfortunately, the OSL spectrum is not obtained because there is a lack of suitable apparatus. The letter M in Fig. 5 represents Sm and Mn.

In general, the properties of ETMs is determined by the depth of its traps. We also observed that the LLP of Mn doped zinc borosilicate glass is improved at the cost of optical storage ability by the addition of Sm. Whether it is controllable to obtain a wanted property of ETMs by adjusting its trap? It is a very in- teresting question that worths further study.

Valence band

Fig. 5 Schematic diagram of mechanism of Sm and Mn co-doped zinc borosilicate glass

3 Conclusion The addition of Sm is beneficial to the formation

of relatively shallower traps in Mn activated zinc boro- silicate glass, greatly improving its LLP property while weakening the optical storage ability.

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