Study of the thermoluminescence mechanisms in LaAlO3: Ce, Dy crystals

Accepted Manuscript Title: Study of the thermoluminescence mechanisms in LaAlO3 : Ce, Dy crystals Author: Kadari Ahmed Kadari Dahane Nicholas M. Khaidukov Neriene Alves Luiz O. Faria PII: DOI: Reference:

S0030-4026(16)30354-0 http://dx.doi.org/doi:10.1016/j.ijleo.2016.04.079 IJLEO 57562

To appear in: Received date: Accepted date:

23-2-2016 18-4-2016

Please cite this article as: Kadari Ahmed, Kadari Dahane, Nicholas M.Khaidukov, Neriene Alves, Luiz O.Faria, Study of the thermoluminescence mechanisms in LaAlO3: Ce, Dy crystals, Optik - International Journal for Light and Electron Optics http://dx.doi.org/10.1016/j.ijleo.2016.04.079 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Study of the thermoluminescence mechanisms in LaAlO3:Ce, Dy crystals

Ahmed Kadari a,b,*, Dahane Kadri b, Nicholas M. Khaidukov c, Neriene Alves d, Luiz O. Fariae

a

Faculté des Sciences de la Matière, Laboratoire de Synthèse et Catalyse, Université Ibn

Khaldoun de Tiaret, BP 78 Zaaroura , Tiaret , Algeria. b

Department of Physics, Electronic Microscopy and Materials Sciences Laboratory

(EMMSL), B.P.1505 El M'Naouar, Oran, Algeria. c

Institute of General and Inorganic Chemistry, Leninskii Prospect 31, 119991 Moscow,

Russia d

Depto. de Engenharia Nuclear (DEN/UFMG-MG), Av. Antônio Carlos 6627, 31270-970

Belo Horizonte, MG, Brazil e

Centro de Desenvolvimento da Tecnologia Nuclear, Av. Antonio Carlos 6627, C.P.941,

30161-970 Belo Horizonte, MG, Brazil

Corresponding author: Tel: 00213775955627 E-mail address: [email protected], [email protected] (A. Kadari)

1

Abstract

In the present paper a complementary investigation about the mechanisms behind the thermoluminescence of LaAlO3:Ce,Dy crystals have been performed. The aim of this study is to present a model that, by using as guess values the parameters obtained by employing the GOK model, explain the role of Ce3+ and Dy3+ in the huge increase of the thermoluminescence signal of undoped LaAlO3 crystals. In the first section of this paper, the previous recorded TL glow peaks were further analyzed and their corresponding kinetic parameters such as order of kinetics (b), trap depth (E) and frequency factor (s) were determined; in this approach, the GCD analysis is mainly used to find the individual glowpeaks of a composite TL glow-curve and further to evaluate the sets of trapping parameters of each glow-peak.

Keywords: Thermoluminescence; LaAlO3:Ce,Dy crystals; TL Modeling; Traps and centers

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1. Introduction

Materials whose cubic crystalline structure follows the general formula ABX3, where 'A' and 'B' are two cations of very different sizes, and X is an anion that bonds to both, are known as perovskites. They can be found in various distinct forms such as zirconates, titanates and aluminates. Perovskites find application in very different fields such as piezoelectric devices (Pb(Zr, Ti)O3) and ferromagnetism ((Ca, La)MnO3) [1,2]. Recently, the lanthanum aluminum oxide (LaAlO3), which is well known as superconductive substrates [3], have been reported to find application also in radiation dosimetry. When LaAlO3 crystals are co-doped with Ce and Dy [4] or doped with C impure atoms [5], they show very high thermoluminescent (TL) response to ultraviolet (UV) photon fields, comparable to the TL output of the best TL dosimeters ever reported in literature, i.e. Al2O3:C and ZrO2 crystals [6,7]. It is well known that UV radiation can induce certain deleterious effects, such as erythema, painful inflammation of the membrane of the eye and skin cancer [8]. TL materials are quite useful for UV dosimetry purposes, owing to simplicity of the sample readout compared to other techniques. Thus, the research for producing new and high-performance TL materials sensible to UV radiation has been encouraged around the world. The optical band-gap of LaAlO3 crystal is 5.6 eV [9] The reported thermoluminescence of LaAlO3:Ce,Dy and LaAlO3:C crystals reveal a contradictory behavior between the TL output for undoped and doped crystals. In fact, in LaAlO3 crystals batch co-doped with 5.0 at.% Ce3+ and 1.0 at.% Dy3+, the undoped crystal has no significant TL signal when compared to the doped ones [4]. Otherwise, in LaAlO3 crystals batch doped with 0.5 at. % C, which is the best signal for all doping levels investigated, the TL output signal of the undoped sample is a bit bigger than the doped one [5]. Then, if we are interested in the enhancement of the TL output of LaAlO3 crystals, it is crucial to understand the role of dopants in its crystalline structure. In order to start to unravel this problem, in this work we have performed a complementary investigation about the mechanisms behind the thermoluminescence of LaAlO3:Ce,Dy crystals. The aim of this paper is to present a model that, by using as guess values the parameters obtained by employing the GOK model, explain the role of Ce3+ and Dy3+ in the huge increase of the thermoluminescence signal of undoped LaAlO3 crystals. In this approach, the GCD analysis is mainly used to find the individual glow-peaks of a composite TL glow-curve and further to evaluate the trapping parameters E and s of each glow-peak. 3

2. Experimental LaAlO3 crystals doped with 1.0 and 5.0 at.% Ce3+, co-doped with 5.0 at.% Ce3+ - 1.0 at.% Dy3+ as well as undoped ones were synthesized by hydrothermal technique [10]. Before studying the dosimetric characteristics, samples in the powder form were annealed by heating from room temperature to 603K at a heating rate of 288 K.s-1 and subsequent quenching to 298K. This time temperature profile was used to readout samples after UV irradiation in a Harshaw-Bicron 3500 TL reader. The UV irradiation was performed using a commercial 8W UV fluorescent lamp. The spectral irradiance at the lamp surface was 2.98 mJ.cm-2, measured using a calibrated radiometer UVX100 E-22476 with a 254 nm sensor. The samples were exposed to values of spectral irradiance ranging from 0.042 to 740 mJ.cm-2 and stored in a dark ambient to prevent against UV-Vis induced fading [4].

3. Glow curve analysis

3.1. Curve deconvolution

In order to separate thermoluminescent glow curves into their individual glow peaks, the glow-curve deconvolution (GCD) technique has been applied widely since the 80s. [11]. The initial values of the fitted parameters, i.e the maximum peak intensity (Im) and the maximum peak temperature (Tm) (which can be obtained experimentally) are placed in the Input files of the GCD program. Its graphic interface enables easy intuitive manipulation of glow-peaks, at the initial stage (parameter initialization) and at the final stage (manual adjustment) of fitting peak parameters to the glow curves.

3.2. Trap parameters determination After the deconvolution stage, the obtained individuals’ glow peaks were analyzed by using the Chen’s peak shape method [12]. This method has been widely used for analyzing TL glow curves in order to ascertain the following kinetic parameters: activation energy (E), the frequency factor (s), and the kinetics order (b). This method is useful for a broad range of energies ranging between 0.1 eV and 2.0 eV, and for values of the frequency factors between

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105 s-1 and 1023 s-1. When applying this method, the trap depth is given by the following equation:

 kT 2  E  c  m   b (2kTm )   

(1)

where α corresponds to  = (Tm-T1),  = (T2-Tm) and  = (T2-T1). Here, Tm is the maximum peak temperature, T1 and T2, respectively are the temperatures on either side of Tm, corresponding to the half intensity of the glow peak and k is the Boltzmann constant (k = 8.610-5 eV.K-1). The values of Cα and bα are summarized below. C = 1.51+3.0(g - 0.42), b =1.58 + 4.2(g - 0.42) C= 0.976+7.3(g - 0.42), b =0 C = 2.52+10.2(g - 0.42), b =1

Chen’s method does not require knowledge of the kinetic order, which is found by using the symmetry factor (µg) described by the equation:

g 

T2  Tm T2  T1

The frequency factor (s) is given by the following relationship:

 E  2kTm   2 1  (b  1) s exp 2 E  kTm  kTm 

E

1

(2)

where is the linear heating rate (β = 288 K.s-1).

Table 1 presents the calculated trapping parameters for the deconvoluted peaks of LaAlO3:Ce, Dy crystals. It can be seen that most of the peaks are characterized by general-order kinetics. The average value of the activation energy varies from 0.835 to 1.2171eV, which corresponds to the frequency factors ranging between 3.60×1010 and 5.66×1010 s-1.

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4. The model

The thermoluminescence process occurring in LaAlO3:Ce, Dy crystals was simulated by resorting to the Four traps and one kind of recombination center model. For easy reference, Fig. 2 schematically shows the transitions involved in our proposed model. The corresponding rate equations describing the traffic of electrons during excitation stage are the following (in this stage we assumed that the transitions of the traps to the conduction band are not possible): dni  nc ( N i  ni ) Ai , dt

(3)

For (i=1,…,4) dm  nv ( M  m) B  Am mnc , dt

(4)

dnv  X  B( M  m)nv , dt

(5)

4 dnc dm dnv dn    i. dt dt dt i 1 dt

(6)

The governing equations for the heating stage are:

 E  dni  nc ( N i  ni ) Ai  ni si exp   i , dt  k BT 

(7)

For (i=1,…,4) dm   Am mnc , dt

(8)

dnc dm 4 dni   . dt dt i 1 dt

(9)

The thermoluminescence intensity is given by the following expression:

I (T )  nc m.B.

(10)

where ni(i=1,…,4) are the concentrations of electrons in traps, Ni (i=1,…,4) are the corresponding concentrations of electron traps, M is the concentration of recombination centres, si (i=1,…,4) are the corresponding frequency factors, Ei (i=1,…,4) are the activation energies of the traps, kB is the Boltzmann constant, and m is the concentration of holes in the recombination centers, nc and nv are the concentration of electrons and holes in the conduction

6

and valance bands, respectively. Ai (i=1,…,4) are the retrapping probabilities, B(cm3.s-1) is the trapping coefficient of free holes in centers, Am (cm3.s-1) is the recombination coefficient for free electrons with holes in centers and T is the temperature. In what follows, we will assume that T=T0+βt, where T0 is the initial temperature for recording the TL glow curve, β is the constant heating rate and t is the time. X(cm-3s-1) is the rate of production of electron–hole pairs, which is proportional to the excitation dose rate. Thus, if the length of excitation is tD (s), the total concentration of produced electron–hole pairs is X.tD (cm-3), which is proportional to the imparted dose. The constant heating rate used in this work was 15°C.s-1. A significant point worth mentioning regards the fact that all simulations were started with empty traps and centers. Besides, it is important to remark that the quasi-equilibrium approximation was not taken into account. The simulation begins by assuming some initial guess values for the free parameters Ai and B, which cannot be extracted directly from the experimental TL curve. These guess values were set manually in such manner that the computed glow curve matched the experimental one as much as possible.

5. Numerical results

In this paper, an excellent numerical result is presented. We start with numerical investigations of the electron capture process by calculations performed using the sets of differentials equations cited above. The traps distribution in the forbiden energy gap of our sample has been studied and it has been represented by the energy levels diagram (Fig. 2) cited in the previouse section. The thermoluminescence (TL) glow curve recorded experimentally shows the presence of four TL glow peaks cited respectively at the following temperatures: 398, 425, 484 and 562K; this mean that our sample contains four electrons traps. So it is necessary to use the four electrons traps and one kind of recombination center. Several authors [13,14] were tried to identify the role of Ce3+ and Dy3+ ions in the LaAlO3 using the absorption and the luminescence properties of these last. For the Ce3+ ions A number of electrons and holes will be produced under excitation of electromagnetic radiation (λ < 220 nm). These centers will transfer their energy to Ce3+ through the recombination process which is based on a sequential charge carrier capture where Ce3+ acts as a hole trap first and electron is captured in the second step by an already created Ce4+ [15] . Concerning the Dy3+ ions It is known that the emission peaks from blue to red came from main emitting level of dysprosium 4F9/2 to the ground and other excited levels of Dy3+ ions [14, 16]. In order to run our model, the relevant sets of differential equations (Eqs. 3 to 10) were solved using 7

the MATLAB ode23 solver. Figure 3 shows the good agreement between the calculated and the experimental TL glow curves, this result confirm the validity of our proposed model. The sets of trapping parameters used in our simulations are given in Table 2.

6. The dose dependence of the signals Fig. 4 shows the evolution of the TL glow curves as a function of the dose, the selected dose level ranging between 6.6109 and 9.6109 cm-3. The results show an increase of TL intensity with the increasing of the irradiation dose. Fig. 5 presents the simulated dose dependence of the maximum TL intensity. For doses range cited above the TL response is linear and no saturation can be observed.

7. Conclusion In this work the thermoluminescent mechanisms of LaAlO3 nanocrystals doped with Ce3+ and Dy3+ ions have been analyzed and modelled using the four electrons traps and one kind of recombination center. The thermoluminescent glow curve has been separated into their individual glow peaks using the glow-curve deconvolution (GCD) technique [11]. Most of the LaAlO3:Ce3+, Dy3+ glow peaks obeyed to the general order kinetics, whereas their activation energy average values varied between 0.8357 eV and 1.2171 eV. The charge transfer and relaxations mechanisms have also been studied and the simulation of the TL mechanism occurring in these samples was conducted using the 4T1R model. The results showed the good agreement between the calculated and the experimental TL glow curves, this result confirm the validity of our proposed model.

Acknowledgment The corresponding author Dr. Ahmed KADARI would like to thank the People's Democratic Republic of Algeria, for her help. The second part of this work was supported by Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), Fundação de Amparo à Pesquisa do Estado de Minas Gerais (FAPEMIG), Comissão Nacional de Energia Nuclear (CNEN) and the Russian Foundation for Basic Research (RFBR Grants 10-02-91167 and 1003-90305).

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References

[1] Warren, W.L., Robertson, J., Dimos, D., Tuttle, B.A., Pike, G.E., Payne, D.A., 1996. Pb displacements in Pb(Zr, Ti)O3 perovskites. Phys. Rev. B 53 (1996) 3080-3087. [2] Kowalczyk, A., Baszynski, J., Szajek, A., Slebarski, A, Tolinski, T, Electronic structure of doped LaMnO3 perovskite studied by x-ray photoemission spectroscopy. J. Phys.: Condens. Matter 13 (2001) 5519-5525. [3] Dere, P.J, Krupa, J.C, Spectroscopic investigations of LaAlO3:Eu3+. J. Lumin.102 (2003) 386-390. [4] Oliveira V.H, Khaidukov N.M, Silva E.C, Faria L.O, Study of TL properties of LaAlO3:Ce,Dy crystals for UV dosimetry. Rad. Measurements 46 (2011) 1173-1175. [5] Alves, N, Ferraz W.B, Faria L.O, Synthesis and investigation of the luminescent properties of carbon doped lanthanum aluminate (LaAlO3) for application in radiation dosimetry. Rad. Measurements 71 (2014) 90-94. [6] Chang, S.C, Su, C.S, Influence of the sintering process of ZrO2 pellets on thermoluminescence induced by ultraviolet radiation. Radiat. Prot. Dosim. 47 (1993) 689-692. [7] Colyott, L.E, Akselrod, M.S, McKeever, S.W.S, An integrating ultraviolet-B dosemeter using photo-transferred thermoluminescence from α-Al2O3:C. Radiat. Prot. Dosim. 72 (1997) 87-94. [8] Blum, H.F, Carcinogenesis by Ultraviolet Light. University Press, Princeton (1959). [9] Peacock, P.W, Robertson, J, Band offsets and schottky barrier heights of high dielectric constant oxides. J. Appl. Phys. 92 (2002) 4712-4721. [10] Yoshimura, M., 1998. Importance of soft solution processing for advanced inorganic materials. J. Mater. Res. 13 (1998) 796-802. [11] Kitis, G, Gomez-Ros, G.M, Tuyn, J.W.N, thermoluminescence glow-curve deconvolution functions for first, second and general orders of kinetics. J. Phys. D: Appl. Phys. 31 (1998) 2636–2641 [12] Chen, R., Mckeever, S.W.S., 1997. Theory of thermoluminescence and related phenomena. World Scientific Press, Singapore. [13] Wang, X.D, Pan.T, Zang T.C, et al. Comparison of energy structure and spectral properties of Ce:LaAlO3 and Ce: Lu2(SiO4)O. Sci. China Ser. E-Tech Sci, 52(12) (2009) 3678-3682, doi: 10.1007/s11431-009-0377-9. [14] Lemanski, K, Deren, P.J. Luminescent properties of dysprosium (III) ions in LaAlO3 nanocrystallites. Journal of Rare Earths, 29 (2011) 1195-1197. 9

[15] Glodo J, Wojtowicz A.J. Thermoluminescence and scintillation properties of LuAP and YAP. J Alloys Comp, 300-301 (2000) 289-294. [16] Su Q, Liang H, Li C, He H, Lu Y, Li J, Tao Y. Luminescent materials and spectroscopic properties of Dy3+ ion. J. Lumin., 122-123 (2007) 927-930.

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Figure captions

Figure 1: Thermoluminescence glow curve deconvolution of LaAlO3:Ce, Dy using the GCD program.

Figure 2: Energy level diagram of the four-trap one recombination centre model.

Figure 3: Comparison between the experimental and the simulated TL glow curves of LaAlO3:Ce, Dy crystal.

Figure 4: Simulated TL glow curves after different dose irradiations.

Figure 5: Simulated dose dependence of the maximum TL intensity.

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Figure 1.

TL Intensity (arb. units)

40 35 30 25 20 15 10 5 0 350

400

450

500

550

600

Temperature (K)

12

Figure 2.

nc

E1, s1 A1, N1

Am E2, s2 A2, N2

E3, s3 A3, N3

E4, s4 A4, N4

TL

X M, m B

nv

13

Figure 3.

Experimental Calculated

TL Intensity (arb. units)

40 35 30 25 20 15 10 5 0 350

400

450

500

550

600

650

700

750

Temperature (K)

14

Figure 4.

9

-3

9

-3

9

-3

9

-3

(a) : D1 = 6.6x10 cm

(b) : D2 = 7.6x10 cm

TL Intensity (arb. units)

4

(c) : D3 = 8.6x10 cm 3

(d) : D4 = 9.6x10 cm

(g)

2

(a)

1

0 300

350

400

450

500

550

600

650

700

750

Temperature (K)

15

Figure 5.

Peak 1: (398K) Peak 2: (425K) Peak 3: (484K) Peak 4: (562K)

TL max (arb. units)

48

40

32

24

6.5

7.0

7.5

8.0

8.5 6

9.0

9.5

10.0

-3

Dose (x10 cm )

16

Table 1: Kinetic parameters of deconvoluted TL glow peaks obtained for LaAlO3:Ce, Dy crystals.. Samples

Tmax

Ug

b

(K)

Eω

Eτ

Eδ

Eavr

β

(ev)

(eV)

(eV)

(eV)

(K/s)

s (s)

3.60×1010

LaAlO3:Ce, 398

0.48 1.60

0.8385 0.8168 0.8517 0.8357

Dy3+

425

0.51 1.83

0.8871 0.8740 0.8921 0.8844

484

0.49 1.60

1.0249 1.0008 1.0387 1.0214

3.29×1010

562

0.48 1.60

1.2213 1.1929 1.2371 1.2171

5.66×1010

288

2.45×1010

17

Table 2: Set of trapping parameters used in our simulation work. Samples

Parameters Levels Ni (cm-3)

Ei (eV)

si (s-1)

Ai (cm3.s-1)

Bi (cm3.s-1)

1 (398K)

9.00×106

0.835

3.60×1010 1.00×10-6

0

LaAlO3:Ce,

2 (425K)

5.00×106

0.884

2.45×1010 0.50×10-7

0

Dy3+

3 (484K)

1.11×108

1.021

3.29×1010 1.10×10-7

0

4 (562K)

2.67×108

1.217

4.50×1010 0.10×10-8

0

L- centre

1.00×1010

4.000

2.00×1010 1.00×10-10

0.29×10-9

18