Determination of thermoluminescence parameters in nanocrystalline CaAl2O4

Journal Pre-proof Determination of thermoluminescence parameters in nanocrystalline CaAl2O4 R.K. Gartia, Moirangthem Nara Singh, Lisham Paris Chanu, Thoudam Basanta Singh PII:

S0022-2313(19)31291-8

DOI:

https://doi.org/10.1016/j.jlumin.2019.116867

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LUMIN 116867

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Journal of Luminescence

Received Date: 28 June 2019 Revised Date:

28 September 2019

Accepted Date: 1 November 2019

Please cite this article as: R.K. Gartia, M.N. Singh, L.P. Chanu, T.B. Singh, Determination of thermoluminescence parameters in nanocrystalline CaAl2O4, Journal of Luminescence (2019), doi: https://doi.org/10.1016/j.jlumin.2019.116867. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2019 Published by Elsevier B.V.

Determination of thermoluminescence parameters in nanocrystalline CaAl2O4 R.K. Gartia1*, Moirangthem Nara Singh2, Lisham Paris Chanu1, Thoudam Basanta Singh3 1

Department of Physics, Manipur University, Canchipur, Imphal, Manipur India-795003

2

Department of Radiation Oncology, Dr B Borooah Cancer Institute, Guwahati, Assam, India-781016

3

Department of Physics, Don Bosco College, Maram, Manipur, India-795015

*[email protected]

Abstract This paper is a critical investigation of trap levels present in nanocrystalline CaAl2O4. The technique of thermoluminescence (TL) is used to determine the trap levels. Unlike most workers, we use Computerized Glow Curve Deconvolution (CGCD) technique to decode the TL curves that enables us to determine all the three key trapping parameters, namely, the activation energy (E), the frequency factor (s) and the order of kinetics (b). Using these as inputs we evaluate the electron lifetime (τ) of charge trapped in different traps. This provides the vital information of the possible use of CaAl2O4 based phosphors for practical application in dosimetry, scintillation and persistent luminescence. The phosphor contains multiple discrete trap levels in the band gap (Eg = 7.4 eV) with trap-depths 0.69±0.02, 0.83±0.01, 0.99±0.02, 1.06±0.02, 1.18±0.03, 1.28±0.02 and 1.41±0.01 eV. The magnitudes of the frequency factors (s) for all the cases are ≈109 that translates the Urbach’s coefficient as 23.47±0.14. We believe this work is first of its kind in providing complete parameters of trap levels in CaAl2O4 phosphor. The life times at Room temperature (τ300) of the change in these varies are spread are a wide range of 18 mins ≤ τ300 ≤ 1007 years which show that CaAl2O4 based phosphors can be designed for specific application with appropriate trap level modification. This explains the excellent persistence luminescence in CaAl2O4: Eu2+, Nd3+ that will be discussed in detail. Keywords:

Parameterization; Trap levels; CaAl2O4; Phosphors

1.

Introduction The presence of suitable trap levels of optimum concentration in the band gap (Eg) of

insulator / semi-insulator (SI) solids are the prime criteria for excellent performance of optical / electronic devices. Therefore, determination of trap levels in solids is a field of interest in solid state physics. Amongst the various techniques used towards this end, thermoluminescence (TL) and thermally stimulated luminescence (TSC) have proved to be useful [1-4]. In this work we present not only the trap-depth of the levels but also all the other relevant parameters that are essential to evaluate the lifetime (τ) of charge in the traps. The generalized equation for electron lifetime (τ) of charge in a trap is given by Singh and Gartia [5] as

 E  exp  kT   τ= s(2 − b)

(1a)

where E is the trap-depth (eV), s the frequency factor (s-1), b the order of kinetics, T the storage temperature in absolute scale and k the Boltzmann’s constant. We note that for b = 2 we cannot evaluate τ. For such cases we assume b ≈ 1.99. Equation (1) shows that it is applicable to all the situations unlike the commonly used equation

τ = s-1exp(-E/kT)

(1b)

is true only for first order kinetics (b=1). It is common to find the arbitrary use of equation (1b) without a cross-check of the value of the order of kinetics (b) that has its origin right from the early days [6]. Unlike first order kinetics, in case of general order kinetics τ depends on not only E and s but also the order of kinetics (b) as well. Conventional techniques like initial-rise (IR) method [7] and various heating rates (VHR) method [8] used in many recent works do not help in evaluation of all the three key trapping parameters (E, s and b). This problem is solved by whole curve fitting, a technique often referred to as Computerized Glow Curve Deconvolution (CGCD). The community of TL dosimetry workers have perfected it using both b=1 and 1.0 ≤ b ≤ 2.0 [9-11]. In case of non-first order kinetics the expression derived by Chen [12] is used.

Numbers of computer programs are available in literature; some are open-source i.e. freely available. One of them is the one by Chung et al [13-15]. In this work we use the latest version of the Chung’s program [15]. TL of CaAl2O4 host-latticed phosphors have been investigated in a number of works [1628]. In quite a few of these works the analysis is rather rudimentary or incomplete. Amongst these the one by Aitasalo et al [22] has emerged as one of the best cited works on “Persistent Luminescence”. The subject has emerged as a topic in its own right because of practical applications like emergency illumination, decoration as well high end possible use in imaging [29-33]. With these in mind, in this work, we not only provide the complete parameterization of possible trap levels in nanocrystalline CaAl2O4 but also discuss their role specific traps in application in persistent luminescence, scintillation and dosimetry. It is to be noted that earlier works on persistent luminescent materials have clearly shown that activators / co-activators play major role in suppression / enhancement of specific traps having minor influence on other parameters [20, 34-36]. The present study essentially aims to provide the trap level spectroscopy of CaAl2O4. The main reason of selecting the representative materials is because of the following facts: I.

Alkaline earth aluminates with appropriate doping of rare earth are well established phosphors [37-43].

II.

TL of MAl2O4:Re3+ (M = Ca, Ba, Sr) has become an active area due to commercial use as an afterglow phosphors [34, 44-47].

III.

Trap levels in CaAl2O4 (both doped as well as undoped) has been investigated by number of workers where total curve fitting has not been done [16-22]. Finally, we give a critical account of the present state of development on use of TL as a

technique to determine trap levels in solids as a whole.

2.

Experimental

2.1.

Preparation of Phosphor

The starting materials include Aluminum Nitrate {Al(NO3)3 ▪9H2O}, Calcium Nitrate {Ca(NO3)3▪4H2O}. Urea acts as the fuel for combustion. All the reagents are weighed in stoichiometric portion. The mixture is dissolved in 10 ml of distilled water. The solution is kept in magnetic stirrer for two hours to remove excess water. The sample subsequently is transferred to a muffle furnace maintained at 500ºC±10˚C. The solution starts boiling with the evolution of large amount of gas producing a white foamy voluminous ash. The product is crushed and made to powder. It is annealed at 950ºC for 2 hours. That makes our final product. The technique of combustion synthesis of materials is well established during 1990s [4854]. This is popular because of its simplicity, cost effectiveness and rapidity that yield uncontaminated nanoparticles. The present work is based on the details provided in the paper of Fumo et al [54]. The paper incidentally is not only one of the early works but well accepted as well. 2.2.

Characterization of phosphors

The phosphors are characterized by XRD, EDX and TEM to check the crystallinity, composition and appropriate doping of the rare earth activator. Philips PAN analytical X'Pert XRD is used to analyze XRD of samples. EDX of samples is analyzed by the Element Energy Dispersive Spectroscopy (EDS) System. The size and morphology of the samples nanoparticles are also investigated using TEM images obtained by TEM, Tecnai 20 G2 under 200 KV. 2.3.

Excitations

The phosphors are irradiated with γ-ray obtained from Teletherapy Bhabhatron –II unit, located at Dr B Borooah cancer Institute, Guwahati, India.

2.3.

Thermoluminescence Data Acquisitions The irradiated samples were recorded with Nucleonix TL / OSL 1008 reader system with

a heating rate mostly with 5˚C/sec from 50˚C to 450˚C. Data for undoped CaAl2O4 are also acquired with heating rate 1 and 3°C/sec. The data presented in this work are that corrected for blackbody radiation.

3.

Results and discussions

3.1.

Phosphor Characterization The XRD pattern

of the phosphor is shown in Fig. 1. The diffraction pattern of the phosphor is matched with that of the standard data (ICDD 00023-1036) confirming the formation

of

CaAl2O4.Particle sizes of the

phosphor

are

determined by using the Scherer Formula

Fig. 1.XRD pattern of Calcium Aluminate nanoparticles.

Dp =

0.94 λ β Cosθ

(2)

These are in the ranges of 42±3 nm that confirms the formation to the nanocrystalline. These facts are also checked by TEM and electron diffraction studies. One of them is shown in Fig. 2. TEM data shows the particles size to be ~ 30 nm (Fig. 2a). The electron diffraction patterns show that the nanoparticles are randomly oriented (Fig.2b). The purity of the phosphor is checked by the EDX technique.

3.2.

TL Curve and their deconvolution The TL curves of undoped

CaAl2O4 samples are recorded with various doses of γ-ray (1 to 20Gy). They are shown in Fig.3. The results are consistent in the sense that the patterns are similar. It confirms the findings of earlier results where the existence of TL peaks at different temperatures right from ~ 90°C to ~ 400°C depending upon the activator and co-activators [16-19, 21, 47, 55]. In

order

to

make

a

complete

parameterization of the TL curves in terms of number of trap-levels, their trap-depth as well as other relevant parameters to predict the potential development phosphors

of

CaAl2O4

for

luminescence

as

based

persistent well

as

Fig. 2. TEM and diffraction pattern image of Calcium Aluminate nanoparticles.

TL

dosimetry [23] the TL curves are deconvoluted and the results are shown in Fig. 4. The parameters are presented in Table 1 and 2.The present

analysis

shows

the

followings. I.

A number of discrete trap levels are present in the large band gap (Eg= 7.4 eV) of CaAl2O4 based

Fig. 3. TL curves of Calcium Aluminate with different doses.

phosphors that are pretty close to each other, a fact not reported earlier. This has led to the misconception of the possibility of distributed traps [22, 56]. II.

The TL peaks are found to be non-first order kinetics (b ≠ 1.0).

III.

The frequency factors of all the TL peaks are almost of same order (~ 109 s-1). This lies very well

within

the

realistic

range[57]. IV.

Since the lifetime at room temperature (τ300) are in the range right from minutes to 107 years as a host material CaAl2O4 has full potentials to be designed for all possible TL applications like persistence luminescence and dosimetry.

The acceptability of a deconvolution today is generally decided by a low value of Figure-of-merit

(FOM)

[58-59].

defined as:FOM =

j stop

∑ j start

100 y j − y (x j ) A

It

is

Fig. 4. CGCD of TL curve of undoped Calcium Aluminate with doses (a) 1 Gy, (b) 5 Gy and (c) 20 Gy.

(3)

where jstart is the initial temperature in the fit region, jstop the final temperature in the fit region, yj the

experimental

TL

intensity at temperature j,

Table 1 Trapping Parameters of CGCD of CaAl2O4 with different Dose

y(xj) the value of the fit found at temperature j, A is the integral of the fitted glow curve. The magnitudes of the

FOM

of

our

deconvolutions show that the fittings are excellent in all cases. Recently a number of workers have used the Urbach’s formula [60] for evaluation of the material a topic discussed recently by our group [61-62]. Therefore,

we

have

evaluated the Urbach’s constant

for

CaAl2O4

based on our extensive data of Table 2. The Urbach’s coefficient is found to be 23.47± 0.14 (Fig.5). This shows that for most practical purposes one can use Urbach’s formulae for phosphors based on CaAl2O4 based lattice. As a further test to check the reliability of the values of the trap parameters we have used the various heating rates method (VHR) [8] based on the data shown in Fig.6. Three trap levels of trap parameters (E,s) with values (1.2 eV,1.29 ×1016s-1), (1.95 eV, 1.42 × 1014s-1) and (1.64 eV, 1.86× 1011s-1) respectively. These values are not in agreement with that of Table 2. The

magnitude of the frequency factor for CaAl2O4:Eu2+, Re3+ (Re = Rare Earths) as shown by Singh Table 2 Summary Trapping Parameters of CGCD of CaAl2O4 with different Dose

et al [63] is 108 s-1. As for CaAl2O4:Dy3+ it has been report to be 109 s-1 [64]. The large uncertainty in VHR method

is

expected

for

highly overlapped TL peaks as in the present case (Fig. 4) since the method is highly

sensitive

to

the

reliability of the TL peak maximum. On the other hand in case CGCD all the peaks irrespective of their relative intensity are treated

Fig. 5.Urbach’s plot using data of Table 1.

with equal footing. Therefore, weak TL peaks in general are missed in analysis of TL by methods other than CGCD.

5.

Conclusions In this work for the first

time we present a complete parameterization

of

the

key

trapping parameters of CaAl2O4. Based

on

the

following

results

conclusions

the are

drawn:i)

In

nanocrystalline

CaAl2O4phosphors, there exist a number of discrete trapping

Fig. 6. TL curves of Calcium Aluminate with β = 1, 3,

levels. In our material it is seven in number. ii) The phosphor contains both shallow (0.6 ≤ E ≤ 1.0 eV) as well as deep traps (1.26 ≤ E ≤ 1.40 eV). This shows the possibility of suitably activated CaAl2O4 for many practical applications. In CaAl2O4: Eu2+, Nd3+ afterglow phosphor this has been optimized. iii) As for the magnitude of the trap-depth namely, 0.69±0.02, 0.83±0.01, 0.99±0.02, 1.06±0.02, 1.18±0.03, 1.28±0.02, 1.41±0.01 eV, they are in perfect agreement to the mechanistic study by Qu et al [65] on CaAl2O4: Eu2+, Nd3+ wherein their theoretical estimation of Nd impurity induced defect levels lie in the range 0.55 to 1.45 eV below the conduction band. It seems that the long persistent luminescence in CaAl2O4: Eu2+, Nd3+ is combination where the shallow traps below 1.0 eV are optimized while the deeper ones (E > 1.0 eV) are suppressed. iv) Unlike most workers who have taken a casual approach to the magnitude of the frequency factor (s) we did focus on that aspect; our works clearly show that the magnitude of s is ≈ 109s-1. This translate the Urbach’s coefficient to be 23.47 ± 0.14. v) As many as four trap levels are located in the ~ 0.70 eV to ~ 1.20 eV. This has given to the false notion of possible trap distribution. The presence of continuous trap distribution in crystalline material is physically unsound.

vi) Logically the concept can be extended to all TL emitting crystalline materials i.e., the concept of trap distribution in crystalline materials is physically unrealistic. Such works need careful scrutiny. Acknowledgements This communication is based on the work carried out by the corresponding author (RKG) under the UGC, New Delhi Emeritus Fellowship Programme where Lisham Paris Chanu worked as an assistant. *On behalf of all authors, the corresponding author (RKG) states that there is no conflict of interest.

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Table 1 Trapping Parameters of CGCD of CaAl2O4 with different Dose Dose

1 Gy

3 Gy

5 Gy

10 Gy

20 Gy

Peak Peak 1 Peak 2 Peak 3 Peak 4 Peak 5 Peak 6 Peak 7 Peak 1 Peak 2 Peak 3 Peak 4 Peak 5 Peak 6 Peak 7 Peak 1 Peak 2 Peak 3 Peak 4 Peak 5 Peak 6 Peak 7 Peak 1 Peak 2 Peak 3 Peak 4 Peak 5 Peak 6 Peak 7 Peak 1 Peak 2 Peak 3 Peak 4 Peak 5 Peak 6 Peak 7

E (eV) 0.68 0.84 0.96 1.06 1.16 1.27 1.40 0.68 0.83 0.93 1.05 1.16 1.26 1.41 0.68 0.81 0.92 1.04 1.20 1.29 1.42 0.72 0.84 0.96 1.09 1.23 1.29 1.41 0.68 0.84 0.96 1.06 1.16 1.27 1.40

s (s-1) 3.24 × 1009 6.50 × 1009 1.65 × 1010 8.21 × 1009 2.56 × 1009 1.61 × 1009 2.56 × 1009 3.63 × 1009 5.15 × 1009 5.79 × 1009 3.63 × 1009 2.56 × 1009 1.27 × 1009 2.56 × 1009 5.79 × 1009 3.13 × 1009 5.15 × 1009 7.31 × 1009 3.24 × 1009 2.03 × 1009 3.63 × 1009 1.16 × 1010 5.15 × 1009 1.04 × 1010 1.16 × 1010 5.79 × 1009 1.81 × 1009 2.28 × 1009 3.24 × 1009 6.50 × 1009 1.65 × 1010 8.21 × 1009 2.56 × 1009 1.61 × 1009 2.56 × 1009

b 1.95 1.97 1.88 1.78 1.91 1.42 1.40 1.86 2.00 1.92 1.98 1.22 1.35 1.94 1.95 2.00 1.92 1.55 1.86 2.00 1.74 2.04 1.98 1.86 1.98 1.80 1.45 2.00 1.72 1.97 1.88 1.78 1.91 1.42 1.40

Tm (˚C) 68 132 175 233 305 372 420 64 134 179 247 307 370 422 58 134 177 225 317 372 420 70 136 183 237 317 376 424 68 132 175 233 305 372 420

I (Rel.) 10 11 26 12 15 34 100 3 18 11 4 28 59 100 2 22 9 2 31 56 100 3 18 11 4 28 59 100 0.27 28 18 5 52 54 100

T300 31.18 Mins 6.21 Days 81.2 Days 1.19 × 1001 Years 1.45 × 1004 Years 3.56 × 1005 Years 9.08 × 1006 Years 7.5 Min 19.5 Days 99.5 Days 3.04 × 1002 Years 5.93 × 1002 Years 5.37 × 1004 Years 8.71 × 1007 Years 16 min 39.7 Days 80.21 Day 2.98 × 1001 Years 2.76 × 1004 Years 5.28 × 1007 Years 1.17 × 1007 Years 7.5 Mins 9.74 Days 56.84 Days 3.04 × 1002 Years 5.93 × 1002 Years 5.37 × 1004 Years 8.71 × 1007 Years 5.14 Mins 6.21 Days 81.2 Days 1.19 × 1001 Years 1.45 × 1004 Years 3.56 × 1005 Years 9.08 × 1006 Years

FOM

0.34

0.30

0.32

0.27

0.32

Table 2 Summary Trapping Parameters of CGCD of CaAl2O4 with different Dose Peak

E s Tm b (eV) (s-1) (˚C) Peak 1 0.69±0.02 5.51±3.59 ×1009 1.94±0.05 65±4.8˚C 09 Peak 2 0.83±0.01 5.29±1.38 ×10 1.98±0.02 134±1.7˚C 10 Peak 3 0.99±0.02 1.09±0.53 ×10 1.89±0.03 178±3.4˚C 09 Peak 4 1.06±0.02 7.86±2.86 ×10 1.82±0.18 236±8.0˚C 09 Peak 5 1.18±0.03 3.54±1.40 ×10 1.74±0.30 312±6.3˚C 09 Peak 6 1.28±0.02 1.67±0.28 ×10 1.53±0.27 372±2.2˚C 09 Peak 7 1.41±0.01 2.72±0.53 ×10 1.70±0.29 421±1.8˚C *Peaks relevant to persistent luminescence are highlighted.

τ300 18.67±11.93 Mins 16.27±14.18 Days 79.79±15.16 Days 1.32±1.57×1002 Years 1.16±1.13×1004 Years 1.07±2.35×1007 Years 4.08±4.23×1007 Years

• • • •

Thermoluminescence of CaAl2O4 shows a number of discrete trap levels. These trap levels can be anywhere between 0.69 eV to 1.41 eV. Lifetime (τ) of charge in these varies from hours to 107 years. CaAl2O4 lattice has potentiality for both persistent luminescence as well as dosimetry.

On behalf of all authors, the corresponding author (RKG) states that there is no conflict of interest.