Effect of synthesis methods on luminescence ...

Journal of Luminescence 178 (2016) 340–346

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Effect of synthesis methods on luminescence properties of LiCaPO4:Ce phosphor for radiation dosimetry C.B. Palan n, S.K. Omanwar Department of Physics, Sant Gadge Baba Amravati University, Amravati, Maharashtra 444602, India

art ic l e i nf o

a b s t r a c t

Article history: Received 7 March 2016 Received in revised form 18 May 2016 Accepted 25 May 2016 Available online 31 May 2016

The polycrystalline doped and un-doped LiCaPO4 phosphors were successfully prepared via solid state diffusion [SSD] and sol–gel [SG] methods. The sol–gel method was implied to decrease the processing time and heating temperature. The prepared un-doped and doped LiCaPO4 phosphors were characterized through X-ray diffraction (XRD) and scanning electron microscopy (SEM) techniques. Additionally photoluminescence (PL), thermoluminescence (TL) and optically stimulated luminescence (OSL) properties were studied. The XRD patterns of prepared LiCaPO4 and LiCaPO4:Ce phosphors were well matched with the ICDD file. The average particles size of LiCaPO4 and LiCaPO4:Ce phosphors were found to be in the range 2–10 μm by SSD method and 2-5 μm by SG method. The excitation spectra of LiCaPO4 and LiCaPO4:Ce phosphors consist of broad band in the range 200–330 nm and maximum intensity was observed at 314 nm. Also emission spectra consist of broad band in range from 330–500 nm and maximum intensity was observed at 369 nm. With the increase of Ce3 þ ions concentration, the emission spectra of LiCaPO4:Ce3 þ phosphors shifted to a longer wavelength. The prepared phosphors were showed excellent TL properties under β irradiation. The OSL sensitivity of the LiCaPO4:Ce phosphor synthesized by the SSD method was the nearly same as compared with the OSL sensitivity of LiCaPO4:Ce phosphor synthesized by the SG method. & 2016 Elsevier B.V. All rights reserved.

Keywords: LiCaPO4:Ce phosphor Solid state method Sol gel method Surface morphology Optically stimulated luminescence Thermoluminescence

1. Introduction In recent years, considerable attention has been paid to optically stimulated luminescence (OSL), which has promising applications in solid-state dosimetry [1]. The OSL technique can replace the thermoluminescence (TL) technique due to their advantages over TL technique. The advantages of OSL technique was already reported Kumar et al. [2]. Also OSL technique is more popular as compared to TL technique in radiation dosimetry fields [3]. Up to now, many materials have been studied in order to develop phosphors for radiation dosimetry applications. Among them, Ce activated phosphors have been intensively studied because of their luminescent properties. The absorption and emission spectra of Ce3 þ are observed in range 200–450 nm and this broad emission is favorable for TL/OSL reader. This limit on wavelength is due to availability of suitable filters, stimulation sources as well as sensitive PM tubes in this range and most importantly the requirement of separation of stimulating wavelength from the emission wavelength which ensures better signal to noise ratio [4]. n

Corresponding author. E-mail address: [email protected] (C.B. Palan).

http://dx.doi.org/10.1016/j.jlumin.2016.05.044 0022-2313/& 2016 Elsevier B.V. All rights reserved.

The compounds MM’PO4 [M¼Li, Na, Ka & M0 ¼Mg, Ca, Sr, Ba] have visible as main optical materials because of the excellent thermal and hydrolytic stabilities [5]. However rare earth (RE) doped LiCaPO4 phosphor showed excellent luminescence properties for various applications such as solid state lighting, thermoluminescence dosimetry etc. [6]. More et al. reported on the luminescence properties of LiCaPO4 phosphor via solid state method and time required for this method was more than 3 days [6]. Zhang et al. reported on the photoluminescence properties of Eu doped LiCaPO4 phosphor synthesized via high temperature solid state method. Also, the time required for this method was more than 16 h [5]. Han et al. reported on the LiCaPO4:Eu phosphor synthesized via sol–gel method [7]. From a literature survey it is observed that most of MM’PO4 based phosphors were synthesized by using high temperature solid state methods. The summarized data of reports tabulated in Table 1. The main drawbacks of SSD method are irregular grain size, high agglomeration, intermediate crushing required, time as well as temperature required for this reaction is high as compared to other methods and high cost [22,23]. Motivated from the above discussion, we planned to studies the luminescent (PL, TL and OSL) properties of LiCaPO4:Ce phosphor synthesized using solid state

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diffusion[SSD] and Sol–Gel [SG] method. Also Effective atomic number of LiCaPO4:Ce phosphor is 13.66 and it used for radiation dosimetry applications. However, to the best of our knowledge, there are no reports on the TL/OSL properties of LiCaPO4:Ce phosphor. Table 1 Summarized review on MM’PO4 materials. Phosphors

Synthesis method

LiCaPO4:Eu2 þ

Polymerizable complex method Solid-state method LiCaPO4:Eu2 þ NaCaPO4:Dy Solid-state reaction KCaPO4:Mn Solid-state reaction KMgPO4:Eu2 þ Solid-state reaction 2þ Melt synthesis technique NaMgPO4:Eu LiMgPO4:Eu2 þ Solid-state reaction LiBaPO4:Tb3 þ Solution combustion KBaPO4:Eu Solid-state reaction NaBaPO4:Eu2 þ Solid-state reaction LiMgPO4:Tb3, þ B Modified solid-state reaction Solid-state reaction KMgPO4:Tb3 þ KSrPO4:Eu2 þ Solid-state reaction KCaPO4:Ce Modified solid-state reaction

Required time (h)

References

20

[8]

8 6 420 420 46 430 44 420 420 44

[9] [10] [11] [12] [13] [14] [15] [16] [17] [18]

411 410 49

[19] [20] [21]

Fig. 1. XRD diffraction patterns of LiCaPO4 and LiCaPO4:Ce phosphors were prepared by using SSD and SG method with ICDD file.

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2. Experimental details 2.1. Solid state diffusion (SSD) method The polycrystalline LiCaPO4 and LiCa(1  x)PO4:xCe (x ¼ 0.001,0.005 and 0.01) phosphors were prepared by solid-state diffusion [SSD] method as previously reported [24]. The white powders of doped and un-doped LiCaPO4 phosphors were used for characterizations. It was well discussed by the researchers that SSD not only needs highly sophisticated equipment but also produces non-uniform particles due to high temperature diffusion. The intermediate grinding required for the fusion of materials produces lots of physical defects and increases the time of reaction. To overcome the possible drawbacks of solid state diffusion method [SSD], we have tried Sol–Gel [SG] method. 2.2. Sol–Gel (SG) method The stearic acid sol–gel route was employed for the synthesis of LiCaPO4 and LiCa(1  x)PO4:xCe (x¼ 0.001,0.005 and 0.01) phosphors. The precursors LiNO3, Ca(NO3), NH4H2PO4, Ce(NO3) and stearic acid were used as initiative materials. The composition of each chemical weighed in proper stoichiometric ratio. Stearic acid (0.5 M) in stoichiometric proportion was slowly heated to 60 °C. It molted within 5 min. The dried precursors LiNO3, Ca(NO3), NH4H2PO4 and Ce(NO3) were added to the molten stearic acid with 2-3 drops of acetic acid. This mixture was heated at 70 °C with continuously stirring till the precursors dissolved partially leading to a solution. The solutions further heated to 100 °C and then allow cooling. Transparent gel formed after cooling. The dry gel was then slowly heated which burnt slowly into red flame. The remaining residue was sintered at 800 °C for 2h and 950 °C for 1h yielding a white fine powder of prepared phosphor. Phase purity of un-doped and doped LiCaPO4 phosphors were checked by means of X-ray diffraction (XRD) using a Rigaku miniflex II diffractometer with Cu Kα (λ ¼ 1.5405 Å) operated at 5 kV. The data were collected in a 2θ range from 10 to 70. The morphological characteristic was studied by using scanning electron microscope (SEM). The measurements were taken using a ZEISS EVO/18 Research at Department of Physics, RTM University, Nagpur. Irradiations of all the samples were performed at room temperature using a calibrated 90Sr/90Y beta source in-housed in RISO TL/OSL Reader (DA-15 Model). The activity of the source was 40mCi and the dose rate was 20 mGy/s. All TL/OSL measurements were carried out using an automatic RISO TL/OSL-DA-15. The PL and PL excitation (PLE) spectra were measured on (Hitachi F-7000) fluorescence spectrophotometer with a 450 W Xenon lamp, in the

Fig. 2. Surface morphology of (a) LiCaPO4-SG method, (b) LiCaPO4:Ce - SG method, (c) LiCaPO4-SSD method and (d)LiCaPO4:Ce - SSD method.

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Fig. 3. (a) Excitation and emission spectra of un-doped LiCaPO4 phosphor was synthesized via SSD and SG method. (b) Excitation and emission spectra of LiCaPO4 phosphor for different concentration of Ce ions was synthesized via (a) SSD and (b) SG method.

range 200–500 nm, with spectral slit width of 1 nm and PMT voltage at 700 V at room temperature.

3. Results and discussion

diffraction data) file with card no. 01-079-1396 and found to be in agreement confirming the formation of the materials. The structure of LiCaPO4 and LiCaPO4:Ce phosphors were hexagonal system with space group P-3m1(164) and lattice parameters a¼5.5085, b¼5.5085, c¼7.5020 and α ¼ β ¼90, γ ¼ 120. From XRD patterns were observed that synthesis methods are not affected on crystal structure.

3.1. X-ray diffraction patterns 3.2. Surface morphology The X-ray diffraction patterns of LiCaPO4 and LiCaPO4:Ce phosphors were prepared by using SSD and SG methods as shown in Fig. 1. The XRD patterns were well match with ICDD (International center for

The SEM micro images of LiCaPO4 and LiCaPO4:Ce phosphors were synthesized via SSD and SG methods as shown in Fig. 2. It

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was observed that microstructure of the phosphor consist of irregular grains with heavy agglomeration. The average size particles of un-doped and doped LiCaPO4 phosphor synthesized via SSD method was found to be in the range 2–10 μm and via SG method it was found to be in the range 2–5 μm. 3.3. Photoluminescence properties (PL) The combined excitation and emission spectra of un-doped LiCaPO4 phosphor was synthesized via SSD and SG method as shown in Fig. 3(a). The excitation and emission spectra were observed under 369 and 314 nm respectively. The PL emission intensity of SG method was very poor as compared with SSD method. The emission spectra of un-doped LiCaPO4 phosphor consists broad band emission in 330–500 nm region. The combined excitation and emission spectra of LiCaPO4 doped with different concentration of Ce3 þ ions were synthesized via SSD and SG method as shown in Fig. 3(b). The excitation spectra of LiCa0.999PO4:0.001Ce phosphor was consist broad band in the range 200–330 nm and maximum intensity was observed at 314 nm and emission spectra consist of broad band in range from

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330–500 nm and maximum intensity was observed at 369 nm. The Ce3 þ luminescence in LiCaPO4 was caused by the transitions from the lowest energy level of the Ce3 þ 5d configuration to the spin-orbit split 2F5/2 and 2F7/2 levels [25]. The nature of excitation and emission spectra of LiCaPO4:Ce phosphor was synthesized via SSD and SG method was same. However concentrations of Ce3 þ were varies form 0.001-0.01 moles. From Fig. 3(b) it was observed that 0.005 mol was the optimum concentration for the prepared phosphor. The luminescence intensity was observed to be decreased when the concentration of Ce3 þ ions was increased beyond this optimum value, due to well known phenomenon of concentration quenching. Mean while, the peak wavelength of Ce3 þ ions shifts to a long wavelength compared with that of LiCaPO4:Ce3 þ phosphors (x ¼0.001). The peak wavelength of LiCa0.999PO4:0.001Ce3 þ phosphor is located at 369 nm for SSD & SG methods, while the addition of Ce ion (0.01 mol.) makes the peak wavelength shift to 382 nm for SSD method and 378 nm for SG method. The shift of emission spectra is up to about 13 nm for SSD Table 2 Values of cγ and bγ depending on τ , δ , or ω. Values

τ

δ

ω

cγ bγ

1.81 2

1.71 0

3.54 1

Table 3 Kinetic parameter of LiCaPO4:Ce phosphor was synthesized via SSD and SG methods.

Fig. 4. TL glow curve of LiCaPO4:Ce phosphor prepared by SSD method compared with TL glow curve of LiCaPO4:Ce phosphor prepared by SG method.

Phosphor

Peaks Activation energy (eV)

Frequency factors (s  1)

Peak Temp (°C)

Shape factors (μg)

LiCaPO4:Ce (SSD) method

P1 P2

0.543 0.917

1.38  105 1.91  107

179 293

0.5 0.5

LiCaPO4:Ce (SG) method

P1 P2

0.599 0.599

9.48  105 6.59  105

169 296

0.5 0.5

Fig. 5. TL deconvolution curve of LiCaPO4 phosphor synthesized Via (a) SSD and (b) SG method.

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and 9 nm for SG method. The LiCa(0.995)PO4:(0.005)Ce3 þ phosphor was used for the remaining studies.

3.4. Thermoluminescence (TL) properties of LiCaPO4: Ce phosphor Fig. 4 represents the TL glow curve of LiCaPO4: Ce phosphor prepared by using SSD and SG method under 100 mGy of β irradiations. The TL sensitivity of SSD method was 1.3 time than TL sensitivity of SG method. From Fig. 4 observed that the TL glow curve consist overlapping peaks in temperature rang 50–450 °C and also observed that area under glow curve was same. These overlapping peaks were deconvoluted by using peaks fit software. The deconvoluted TL glow curve of LiCaPO4:Ce phosphor was represent in Fig. 5. The kinetic parameters such as activation

energy, frequency factor and order of kinetics were calculated by using peak shape method [26–28]. It is known that the geometrical factor mg of a TL peak is determined with the Eq. (1) [29]. mg ¼ ðT 2 –T m Þ=ðT 2 –T 1 Þ

ð1Þ

Where, T1, Tm and T2 represent the temperature of half intensity at low temperature side, peak temperature and temperature of half intensity at high temperature side of TL peak, respectively. With the above values fitted into Eq. (1), the geometrical factor is about P1 ¼0.5 and P2 ¼ 0.5.The Peaks Obey second order kinetics. Therefore, the activation energy (E) can be estimated with the following equation where γ stands for τ, δ, or ω [29].   E ¼ cγ kðT m Þ2 =γ –bγ ð2kT m Þ

ð2Þ

The values of τ, δ, and ω are respectively determined by lowtemperature half-width (τ ¼Tm–T1), high-temperature half-width (δ ¼T2–Tm) and full width (ω ¼T2–T1). For second-order kinetics, the values of the cγ and bγ depending on τ, δ, or ω are given in Table 2 [25]. The k is the Boltzmann constant. Therefore, according to Eq. (2), different values of E remarked as Eτ, Eδ and Eω can be obtained when γ stands for τ, δ, or ω, respectively. The activation energy (E) is calculated with the mean value of Eτ, Eω and Eδ. The value of frequency factor s can be obtained with the following Table 4 Photoionization cross-section of LiCaPO4:Ce phosphor was synthesized via SSD and SG methods.

Fig. 6. OSL sensitivity of LiCaPO4:Ce phosphor synthesized via (a) SSD method compared with OSL sensitivity of LiCaPO4:Ce phosphor synthesized via (b) SG method.

Phosphors

OSL components

Coefficients Decay constant (s)

Photo-ionization cross section σ (cm2)

LiCaPO4:Ce SSD

Fast Medium Slow

285,000 100,000 45,000

0.4 1 2.7

0.353  10  17 0.884  10  17 2.38  10  17

LiCaPO4:Ce SG

Fast Medium Slow

278,000 150,000 50,000

0.5 1.5 6.8

0.442  10  17 1.327  10  17 6.01  10  17

Fig. 7. 3rd order exponentially decay curve of LiCaPO4:Ce phosphor was synthesized via (A) SSD method and (B) SG method.

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Fig. 8. Reusability study of LiCaPO4: Ce phosphor was synthesized via SG and SSD method.

equation where

β is the heating rate (4 °C/s).

βE=kðT m Þ ¼ s exp E=kT m 2




ð3Þ

The values of sτ, sδ and sω are obtained with the values of Eτ , Eδ and Eω fitted into Eq. (3), respectively. The frequency factor s is calculated with the mean value of sτ, sδ and sω. The results are shown in Table 3. Tm represents the peak temperature of P1 and P2. 3.5. Optically stimulated luminescence (OSL)

3.7. Reusability

The CW-OSL response of LiCaPO4:Ce phosphor synthesized via SSD and SG method was as shown in Fig. 6. The OSL response of LiCaPO4:Ce phosphor was measured under 20mGy of β irradiation for 60 s stimulation of blue LED. From Fig. 5 observed that OSL sensitivity of LiCaPO4:Ce phosphor synthesized via SSD method was nearly same as compared with OSL sensitivity of LiCaPO4:Ce phosphor synthesized via SG method. The decay curve can be fitted with the Eq. (4) [30] and Fig. 7 represent 3rd order exponentially decay curve of LiCaPO4:Ce was synthesized via SSD and SG method.

ð4Þ I OSL ¼ A1 exp  t 1 =τ þA2 exp  t 2 =τ þ A3 expð  t 3 =τÞ Where IOSL is the initial OSL intensity, A1, A2, A3 are constant coefficients and τ1, τ2, τ3 are the decay constants of the respective OSL traps. This indicates the presence of wide distribution of traps having different optical trap depths and photoionization cross sections. The constant coefficients, photoionization cross sections and decay constants for the LiCaPO4:Ce phosphor was given in Table 4. 3.6. Minimum detectable dose (MDD) In order to determine MDD of LiCaPO4:Ce phosphor by using Eq. (5) MDD ¼

3σ Integrated CW  OSL signal per unit dose at given ϕ

Where σ is the standard deviation in background counts integrated for time. For background variation measurement three powder disc (same weight) of LiCaPO4:Ce phosphor were used. The OSL reader was calibrated using three discs which were irradiated with 100 mGy. The MDD was found out to be 18.137 μGy for SSD method and 18.57 μGy for SG method (dose corresponding to 3σ of the background).

ð5Þ

The reusability studied of LiCaPO4:Ce phosphor was synthesized by using SSD and SG methods as shown in Fig. 8. This study was carried out by exposing the discs (in powder form) under 90 Sr/90Y beta source; the OSL was taken after the disc was optically annealed. The remaining few dose remove by using optical bleaching. The discs were optically bleached for different times after the OSL readouts until the entire OSL signal was erased. The OSL (after bleaching) was again taken to ensure that no signal was left inside the material. Ten such cycles were carried out. The study results show that the phosphor can be reused for ten cycles without changes in the OSL output signal.

4. Conclusions In this work, we report first time TL/OSL properties of LiCaPO4: Ce phosphor and this phosphor was successfully synthesized via solid state diffusion [SSD] and sol–gel [SG] methods. The X-ray diffraction patterns of prepared doped and un-doped phosphors were well match with ICDD card with file no. 01-079-1396. The average particles size of prepared phosphor was in sub-micron range. The experimental results indicated that temperature of sol gel method was lower and it was 55% less time consuming as compared to solid state diffusion method. The PL excitation and emission spectra of prepared LiCaPO4 and LiCaPO4:Ce phosphors were observed at 314 and 369 nm, respectively. With the increase of Ce3 þ concentration, the emission spectra of LiCaPO4:Ce3 þ

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phosphors shifted to a longer wavelength. The luminescence sensitivity of LiCaPO4:Ce phosphor prepared by SSD method was more than that of luminescence sensitivity of LiCaPO4:Ce phosphor prepared by SG method. Because average particle size of LiCaPO4:Ce phosphor prepared by SG method was found to 2– 5 μm which is less than that of LiCaPO4:Ce phosphor prepared by SSD method (2–10 μm). The TL glow curve of LiCaPO4:Ce phosphor consist overlapping peaks in temperature rang 50–450 °C and kinetic parameters were calculated by using peaks shape method. The CW-OSL curved fitted with 3rd order exponentially decay and photoionization photoionization cross sections were calculated. The minimum detectable dose was found out to be 18.137 μGy for SSD method and 18.57 μGy for SG method. Effective atomic number (Zeff) of prepared LiCaPO4 phosphor is 13.66 and phosphor show not only showed good TL response but also good OSL response under β irradiation. Also phosphor showed excellent reusability properties under β irradiations. Due to these reasons prepared LiCaPO4:Ce phosphor is one of candidate for radiation dosimetry.

Acknowledgments One of the authors CBP is very much thankful to Head, RPAD, Radiological Physics and Advisory Division, Bhabha Atomic Research Centre, Mumbai-400085, India for providing the necessary facilities for the analysis of OSL and TL results. The author also thanks to Dr. S.B. Kondawar, Department of Physics RTM University Nagpur-440013, India for providing the access of SEM.

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