CaTiO3:Eu3+, a potential red long lasting phosphor: Energy migration and characterization of trap level distribution

Journal of Alloys and Compounds 622 (2015) 1068–1073

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CaTiO3:Eu3+, a potential red long lasting phosphor: Energy migration and characterization of trap level distribution S. Som a,⇑, S. Dutta b, Vijay Kumar a, Anurag Pandey a, Vinod Kumar a, A.K. Kunti b, J. Priya b, S.K. Sharma b, J.J. Terblans a, H.C. Swart a,⇑ a b

Department of Physics, University of the Free State, Box 339, Bloemfontein 9300, South Africa Department of Applied Physics, Indian School of Mines, Dhanbad 826004, India

a r t i c l e

i n f o

Article history: Received 17 July 2014 Received in revised form 30 October 2014 Accepted 1 November 2014 Available online 11 November 2014 Keywords: CaTiO3:Eu3+ Long afterglow Defect levels TL Phosphorescence

a b s t r a c t This paper comprises a promising approach for the development of a red-light emitting CaTiO3:Eu3+ long-lasting phosphor. Eu3+ doped CaTiO3 phosphors were prepared by the solid state reaction method at 1000 °C. Red long-afterglow (LAG) originated from the f–f transitions of Eu3+ in the CaTiO3 and lasted for several minutes. Defect trap depth values from 0.62 up to 1.21 eV were obtained from thermoluminescence (TL) data. These defects were created due to charge compensation. These traps were responsible for the afterglow emission at room temperature. The decay curves and TL glow curves showed that the density of the traps with suitable depth has been significantly changed with an increase in dopant concentration. The depth and density of the traps can affect the LAG performance. The possible mechanism behind the LAG of Eu3+ in CaTiO3 was explained in detail on the basis of the trap distribution. Ó 2014 Elsevier B.V. All rights reserved.

1. Introduction The persistency of the long lasting phosphors (LLPs) and its mechanism have been the subject of intensive research for the last decades due to their versatile applications from commonplace decoration, luminous paints, security illumination and emergency signalization to highly skeptical uses in the detection of high energy radiation, pressure and temperature sensing, micro defect sensing and image storage appliances in different optoelectronics devices [1–3]. LLPs are a kind of storage phosphor which can absorb and store energy when it is exposed to high-energy radiation by capturing charge carriers (electrons or holes) in traps (lattice defects or impurities) [4,5]. The stored energy can then be released for several seconds to hours in the form of visible light even after the stoppage of the excitation source at room temperature by thermal, optical or other physical stimulations [5–7]. For the thermally stimulated storage phosphors the energy is released upon heating based on the principle of thermoluminescence (TL). They have found a lot of industrial applications in the fields of dosimetry, X-ray imaging and as the self-sustained night-vision luminescent materials due to the environmentally friendly nature and economical sustainability [1–7]. ⇑ Corresponding authors. Tel.: +27 514013852, +27 58 718 5308; fax: +27 58 718 444. E-mail addresses: [email protected] (S. Som), [email protected] (H.C. Swart). http://dx.doi.org/10.1016/j.jallcom.2014.11.001 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.

In recent years, a lot of research work has been carried out in developing rare-earth-doped aluminates and silicates as the long lasting phosphors due to their long duration, high luminosity, and improved chemical stability. The mechanisms of the persistent luminescence phenomenon from Eu2+ based materials have also attracted much attention [1–7]. But a very little amount of work has been reported on the mechanism of long after glow (LAG) of phosphors using RE3+ dopants as the LLPs [1] and their relevant applications are challenged by the generation of multicolor LLP using these dopants [1–7]. Theoretically, multicolor and color tunable LLPs can be obtained by mixing the three primary color-emitting LLPs. But till now the blue (CaAl2O4:Eu2+, Nd3+) and the green (SrAl2O4:Eu2+, Dy3+) LLPs are commercially available [1–9]. However, info on long lasting phosphor for red color is still very limited. Furthermore, to explain the occurrence of its persistency there is also a lack of proper explanation [1–9]. Unfortunately, there are still many unknown details such as the nature of traps and the interaction between traps which are critical to explore and design the desired LLPs. Though, most authors have reached an agreement on the general idea that charge carriers are trapped by long-lived traps inside the band gap [1–10]. Keeping this in mind, the aim of this study was to better understand the mechanism behind the LAG in a red colour emitting LLP. It is well reported that calcium titanate (CaTiO3) exhibited good chemical and thermal stability as well as better mechanical resistance [11,12]. It is widely used in electronic devices as dielectric materials. During the past few years, rare earth doped CaTiO3

S. Som et al. / Journal of Alloys and Compounds 622 (2015) 1068–1073

phosphors have been the subject of great interest in research community due to their enhanced luminescence properties which makes them promising material in field emission display applications [11,13]. Some research work has been carried out on the red LLPs taking CaTiO3 as host materials doped with Pr3+. Diallo et al. reported that CaTiO3 doped with Pr3+ exhibited red cathodoluminescence [14,15]. Improvement of the luminescence properties of Pr doped CaTiO3 has been studied by Chen et al. [16]. UV excitation and trapping centres in CaTiO3:Pr3+ phosphors have been studied by Jia et al. [17]. All these studies indicate that the existence of traps is mainly responsible for LLP. According to them, these traps are associated with electron trapping, which they further related to Pr4+ and oxygen vacancies existing in the samples. But, a few reports exist in the literature on the luminescence properties of Eu3+ doped CaTiO3 phosphors [11,18–22]. However, most of the work reported so far was on the optical and luminescence properties of Eu3+ doped CaTiO3. The optical performances of CaTiO3:Eu3+ depend critically on charge compensation [21]. In donor perovskites such as CaTiO3:Eu3+, the charge compensation occurs by the formation of intrinsic defects such as negatively charged Ca vacancies and/or positively charged oxygen vacancies [23]. CaTiO3:Eu3+ red phosphors were prepared using H3BO3 assisted solid state synthesis by Liu et al. [24]. The added H3BO3 improved the crystallinity, and increased the color purity, implying the potential to be a promising red phosphor in white light emitting diodes (WLEDs). To the best of our knowledge no investigation has been performed on the LLP performance of Eu3+ doped CaTiO3 phosphor based on thermoluminescence (TL) properties. To prepare phosphors, a number of synthetic approaches such as solid state reaction [5,7,10], solution combustion [11], polymer precursor [12], sol–gel [13,14], flame transportation [25,26], rapid thermal oxidation [27] and hydrothermal methods [21] have been investigated. In our previous work, the defect correlated fluorescent quenching and electron phonon coupling in the spectra transition of doped and codoped Eu3+ and K+ in CaTiO3 for red emission display application have been described [22]. In the present work, an LLP based on Eu3+ doped CaTiO3 was synthesized. The lattice structure, luminescence spectra, decay curves and TL spectra were used to characterize the synthesized phosphors. The generation mechanism of LAG was also discussed in detail. 2. Experimental The Eu3+ doped CaTiO3 phosphors were prepared by the solid state reaction method as per the formula Ca1xEuxTiO3 [22,28] by varying the Eu3+ concentration from 1–5 mol%. The starting raw materials CaCO3, TiO2, K2CO3 and Eu2O3 (99.99%) used in this synthesis method were taken in stoichiometric ratio as per formula Ca:Eu:Ti = (1x): x: 1 where the amount of x is varied from 0.01 to 0.05. These powders were blended with absolute ethyl alcohol and grounded thoroughly in an agate mortar for 3 h for proper mixing. The obtained homogeneous mixture was added into an alumina crucible and sintered at 1000 °C for 5 h and allowed to cool to room temperature. Finally the product was ground into fine powders and thus the desired phosphors were obtained [22]. The X-ray diffraction (XRD) pattern of the prepared phosphors were recorded in a wide range of Bragg angles 2h (10° 6 2h 6 90°) using a Bruker D8 advanced X-ray diffractometer with Cu target radiation (k = 0.15405 nm). The photoluminescence (PL) phosphorescence studies and decay kinetics were carried out on a Cary Eclipse Fluorescence Spectrophotometer in the range 220–750 nm. TL glow curves were recorded using a Harshaw TLD reader (model 3500). The heating rate was kept fixed at 5 °C/s.

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397 nm in the range 500–750 nm. All of the emissions observed were due to the 4f–4f transitions of Eu3+. Different distinct emission bands lying throughout the visible region were observed due to the transitions from the excited 5D2 to 7F3 (514 nm), 5D1 to 7 F1 (540 nm) and 5D0 to the 7Fj (j = 0–2 and 4) levels of Eu3+ ions. A luminescence photograph of the corresponding luminescence is also shown in Fig. 1(b). Keeping the emission wavelength fixed at 617 nm, the excitation spectra were recorded in the wavelength range 220–560 nm. It consists of different sharp peaks which are associated with the typical intra-4f transitions of the Eu3+ ion. The characteristic peaks at about 360, 380, 397, 417, 465 and 535 nm were observed due to the 7F0 ? 5D4, 7F0 ? 5L7, 7F0 ? 5L6, 7 F0 ? 5D3, 7F0 ? 5D2 and 7F0 ? 5D1 electronic transitions of the Eu3+ ion, respectively. It indicates that this phosphor could strongly be excited by near-UV and blue light, which has a potential as a near-UV light converted phosphor in solid state lighting applications [29]. Monitoring the 617 nm emission of the 5D0 excited level in the CaTiO3 host, the luminescence decay behaviour of the Eu3+ ions at different concentrations were recorded as shown in Fig. 1(b). A luminescence photograph of the corresponding luminescence is also shown in Fig. 1(c) as representative of the colour coming from the phosphor when excited with 397 nm. To understand the behaviour of the luminescence decay [30], the decay data were fitted with different decay equations. It was found that the curves in the present case follow bi exponential decay behaviour st

IðtÞ ¼ I1 e

1

st

þ I2 e

2

ð1Þ

where s1 is the fast component and s2 is the slow component and are generally known as the attenuation times from which the decay rate for corresponding exponential components can be determined. Here, I1 and I2 are the fitting parameters [30]. The average lifetime in the case of a bi-exponential decay can be calculated using the equation

s¼

I1 s21 þ I2 s22 I 1 s1 þ I 2 s2

ð2Þ

The fitting parameters of all decay curves are listed in Table 1. The fitting results indicate that there were two decay processes involved including a rapid decay process at first and then a slow decay process. Due to the second significant slow decay

3. Results and discussions 3.1. Photoluminescence properties of CaTiO3:Eu3+ Fig. 1 shows the room-temperature (a) PL emission and excitation spectra of the CaTiO3:Eu3+ (3 mol%) phosphor [22]. The PL emission spectra were recorded at an excitation wavelength of

Fig. 1. (a) PL, (b) decay kinetics and (c) a luminescence photograph of the CaTiO3 phosphor.

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Table 1 Fitting parameters for PL decay curve. Eu3+ concentration (mol%)

s1 (s)

I1

s2 (s)

I2

1 2 3 4 5

54 46 32 16 13

13,861 9475 3479 938 354

392 374 321 274 191

2176 1289 639 326 89

component, the phosphorescence of the Eu3+-doped CaTiO3 phosphors could be seen with the naked eyes in the dark for several minutes after the irradiation source has been removed. Generally in phosphorescent materials, different defects inside the materials store the energy by trapping and de-trapping of charge carriers [31]. This trapping and de-trapping of charge carriers by various traps affects the LAG of the phosphors. The trap nature and the structure play the crucial role in the generation of good LAG performance. The trap nature includes the knowledge of suitable trap depth and trap density which characterizes the traps present inside the materials. So, the knowledge of the structure and the nature of the trap are highly desirable. The idea about the structure is revealed by refining the X-ray diffraction data using Rietveld refinement method. Since persistent emission is a special case of thermally stimulated luminescence, it can provide vital information about the nature of the traps and the trap levels. So, for the characterization of the traps, TL measurements were carried out on each sample.

Fig. 2. Refinement and crystal structure of CaTiO3 phosphor.

3.2. Structure and phase identification of CaTiO3:Eu3+ From our previous report in Ref. [22] it is already established that the XRD pattern of the prepared phosphors were well matches with JCPDS file No. 86-1393 of CaTiO3 [22] and indicated the presence of an orthorhombically distorted perovskite structure with the lattice parameter a = 5.380 Å, b = 5.440 Å and c = 7.639 Å. A structural refinement by the Rietveld method using the Expo Program [32] was performed to analyze the structure and unit cell of the prepared phosphor and is shown in Fig. 2. The results indicate a good agreement between the observed and calculated XRD patterns. The illustration of the CaTiO3 unit cell is also shown in this figure. The unit cell was modelled through a program called Visualization for Electronic and Structural Analysis (VESTA) [33] using Rietveld refinement data. This shows that CaTiO3 phosphor has an orthorhombically distorted perovskite structure. The unit cell contains 4 formula units. The Ca2+ ion in the CaTiO3 is coordinated with eight O atoms, forming a distorted dodecahedron and the Ti4+ ion is coordinated with six O atoms, forming an octahedron. The Eu3+ is substituted in the dodecahedron sites (CaO8) of Ca2+ while the Ti4+ remains unchanged in the octahedral sites (TiO6) as shown in Fig. 2. The substitution of Eu3+ in the CaTiO3 lattice is mainly based on the charge compensation, occurring through the creation of intrinsic defects of negatively charged Ca vacancies (VCa) and/or positively charged oxygen vacancies (Vo):3Ca2+ ? 2Eu3+ + VCa2+ [22,28,29,34]. The presence of charge carriers (electrons or holes) trapped in these defects in the host structure may be the cause for the persistency. However, in the Eu3+ doped CaTiO3, there is no need for co-doping because Eu3+ acts both as a luminescence center and a creator of the energy storing defects.

Fig. 3. TL glow curves of CaTiO3 phosphor varying Eu3+ concentration.

bands increased continuously with the increase of the Eu3+ doping concentration up to 3 mol% and then decreases as shown in the inset of Fig. 3. Since the trap density is approximately proportional to the integral intensity of the TL band, the traps of the 3 mol% doped CaTiO3 phosphor possessed the biggest capacity to capture charge carriers. The TL response of the phosphor studied over the c-dose range from 20 to 100 Gy for 3 mol% doped phosphor is shown in Fig. 4. It indicates that the TL intensity varies about linearly in the studied dose range as shown in the inset of Fig. 4. The maximum intensity is observed in the case of 100 Gy irradiated phosphor. Now, in order to achieve a quantitative analysis of the TL curves, the deconvolution method was used based on the glow curve deconvolution functions (GCD) [35] for general order kinetics glow curves suggested by Kiti’s et al. for the maximum glow peak:

3.3. Thermoluminescence characterization

! " E T  Tm T2 ðb  1Þð1  DÞ 2 IðtÞ ¼ Im b exp kT T m Tm   b=b1 ! E T  Tm þ Zm  exp kT T m

Fig. 3 shows the TL glow curves of Eu3+ doped CaTiO3 by varying the rare earth concentration. It shows an asymmetry broad TL band between 50 and 150 °C with the peak predominating at 98 °C. The TL intensities for the Eu3+-doped samples corresponding to these

where I(t) = TL intensity at any temperature T(K), Im = maximum peak intensity, E = activation energy (eV), b = order of kinetics, k = Boltzman constant, Tm = peak temperature, D = 2kT/E, Zm = 1+(b1)Dm and Dm = 2kTm/E. This yielded a good agreement

b=b1



ð3Þ

S. Som et al. / Journal of Alloys and Compounds 622 (2015) 1068–1073

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Fig. 6. Schematic diagram of LAG mechanism based on trap.

Fig. 4. TL glow curves of CaTiO3 phosphor varying gamma dose and TL response.

between the experimental and calculated glow curves by using four traps (four deconvoluted peaks) as shown in Fig. 5. The experimental glow curves were fitted well with the theoretically generated glow curves. The quality of fitting was checked by calculating the figure of merit (FOM), as defined by [33]:

P FOM ¼

TLExp  TLThe P TLThe

Here, TLExp and TLThe represent the TL intensity of the experimental and theoretical glow curves, respectively. The trap depth [30] for each deconvoluted peak was estimated from the following equation suggested by Chen:

Ea ¼ c a k

T 2m

a

!  ba ð2kT m Þ

ð4Þ

where Cs = 1.51 + 3.0(lg 0.42), Cd = 0.976 + 7.3(lg 0.42), Cx = 2.52 + 10.2(lg 0.42) and bs = 1.58 + 4.2(lg 0.42), bd = 0, bx = 1 [35]. Here a stands for s, d and x respectively. The trap depth of the deconvoluted peaks was obtained in between the depths 0.62 and 1.21 eV. Only the TL bands peaking above room temperature are responsible for the afterglow at room temperature. In the present case the TL bands were centred at 75 °C, 90 °C, 105 °C and 115 °C all are

well above the room temperature and hence may be responsible for the red LAG. It is also commonly considered that the lower and higher temperature of the TL bands is related to the shallower and deeper traps, respectively. The suitable TL peak is situated slightly above room temperature (50–120 °C) for excellent LAG performance [1–7,31]. In all cases, the lowest temperature bands are close to the ideal temperature (100 °C) can contribute mainly to the energy storage capability of the material that ensures the long duration of persistent luminescence [1–7,31]. These traps were present in this phosphor due to the charge compensation defects related to the presence of Eu3+ in the Ca2+ sites described before. The working process of the LLP is similar to the rechargeable battery and in general can be divided into three sequential steps, i.e., energy storage, energy transfer, and energy release [1–7,31]. In order to interpret the generation process of the LAG emission in CaTiO3:Eu3+ clearly, the schematic of phosphorescence mechanism based on the above analysis is illustrated in Figs. 6 and 7 for two different possible ways. Fig. 6 explains the first possible mechanism [1–7,31]: (i) Under UV excitation, electrons are excited to the higher excited states of Eu3+ (as shown in Fig. 6 as a violet arrow). (ii) As the conduction band (CB) of the host and the higher excited state (sky blue1 levels in Fig. 6) of Eu3+ overlap, some electrons escape to the host’s conduction band, along with a simultaneous formation of the pairs Eu3+–h+ as shown in Fig. 6. (iii) Part of these free electrons are trapped to the defects created by charge compensation from CB and thus stored some of the excitation energy. The presence of these traps in between the depths 0.62 and 1.21 eV is already proved from the previous section. (iv) The thermal stimulation can free the electrons from the traps to the excited states in the reverse discharging process via CB. (v) It precedes by the radiative relaxation of the system back to the ground states of Eu3+ via the excited states 5D0. Thus this total process occurs via trapping of electron which delayed its stay in the traps and then relaxes to the ground state

Fig. 5. TL glow curve deconvolution of CaTiO3 (3 mol% Eu3+) phosphor.

1 For interpretation of color in Fig. 6, the reader is referred to the web version of this article.

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4. Conclusion In conclusion, CaTiO3:Eu3+ exhibited red-light long lasting afterglow from the transitions between the 4f levels of Eu3+. The afterglow lasted over 4 min after 397 nm irradiation and its decay process met the bi exponential behaviour. The traps above room temperature have depths between 0.62 and 1.21 eV, with the corresponding TL peaks at 75 °C, 90 °C, 105 °C and 115 °C, respectively. A possible afterglow mechanism was proposed and the generation processes of LAG were illustrated in detail. From the above discussion it was concluded that CaTiO3:Eu3+ is a promising material as a red-light emitting long-lasting phosphor although more systematic investigation is yet to be carried out to improve its afterglow even further.

Acknowledgments Fig. 7. Schematic diagram of LAG mechanism based on energy transfer.

instead of direct relaxation [36]. It delays the luminescence to occur; creates the persistent luminescence. The second mechanism [1–7,34] is shown in Fig. 7. (i) According to this model under UV excitation, electrons are excited to the higher excited states of Eu3+ and hence electron are available in the CB and holes are generated in the valence band (VB) in CaTiO3 host. (ii) Some excited electrons move through the CB and some holes move through VB freely to the native defects. (iii) However, the excited electrons and holes can be captured by the electron and hole traps, respectively. Here Vca can act as a hole trap and Vo can act as electron trap. (iv) Under the activation of thermal motion, these trapped charge carriers (electrons and holes) can recombine with a slow rate. (v) The energy of this recombination will be transferred to the excited energy levels of Eu3+. Meanwhile, on the other hand, those gradually released electrons and holes from traps can also transfer to the excited energy level and ground state of Eu3+, respectively. (vi) The process of charge carrier’s motion occurs with a delay time, which causes the red LAG phenomenon from the Eu3+ ion. The luminescence decay curves of CaTiO3:0.5%Er3+/0.5%Tm3+/ 5%Yb3+ and CaTiO3:0.5%Tm3+/5%Yb3+ were determined by Chen et al. [37]. All of the decay curves were fitted with a single exponential function I(t) = I0exp(t/s), where s is 1/e lifetime of the Tm3+ ion. The lifetimes for the 1G4 and 3H4 states of Tm3+ in CaTiO3:0.5%Er3+/0.5%Tm3+/5%Yb3+ were 319 and 326 ms, respectively, and were shortened comparing to that of CaTiO3:0.5%Tm3+/ 5%Yb3+ (351 and 410 ms, respectively). Zhang et al. [38] reported that the afterglow of CaTiO3:Pr3+ with 1 mol% Zr addition was seen over 100 s with the naked eyes, while the Zr free sample can only have a persistent time of 30 s for the red afterglow after the irradiation with an ultraviolet lamp. The results clearly showed that the phosphorescence lifetime was increased by Zr substitution for Ti. The results of Huong et al. [39] showed that two lifetimes, a fast one of 0.194 ms, and a slow one of 0.919 ms have been observed for the 5D0 ? 7F2 emission of Eu3+ in CaTiO3 prepared by a sol– gel method. The fact that their CaTiO3:Eu3+ samples did not exhibit a long afterglow luminescence indicated there were no metastable traps in their samples. The decay values obtained in this case with single doped Eu3+ ions are much longer due to the presence of the defects.

The research is also supported by the South African Research Chairs Initiative of the Department of Science and Technology and National Research Foundation of South Africa. The financial support from the University of the Free State is highly recognized.

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