Enhanced luminescence by monovalent alkali metal ions in Sr2SiO4:Eu3+ nanophosphor prepared by low temperature solution combustion method

Journal of Alloys and Compounds 595 (2014) 192–199

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Enhanced luminescence by monovalent alkali metal ions in Sr2SiO4:Eu3+ nanophosphor prepared by low temperature solution combustion method H. Nagabhushana a,⇑, D.V. Sunitha a, S.C. Sharma b,c, B. Daruka Prasad b, B.M. Nagabhushana d, R.P.S. Chakradhar e a

Prof. C. N. R. Rao Centre for Advanced Materials Research, Tumkur University, Tumkur 572 103, India B.S. Narayan Centre of Excellence for Advanced Materials, B. M. S. Institute of Technology, Yelahanka, Bangalore 560 064, India, c Department of Mechanical Engineering, B. M. S. Institute of Technology, Yelahanka, Bangalore 560 064, India d Department of Chemistry, M. S. Ramaiah Institute of Technology, Bangalore 560 054, India e National Aerospace Laboratories (CSIR), Bangalore 560 017, India b

a r t i c l e

i n f o

Article history: Received 31 October 2013 Received in revised form 5 January 2014 Accepted 14 January 2014 Available online 21 January 2014 Keywords: Luminescence Phosphor Alkali metal ions Silicates

a b s t r a c t A series of red light emission phosphors of Sr2SiO4:Eu3+ (1–5 mol%) and Sr2SiO4:Eu3+:M+ (M+ = Na+, K+ and Li+) were prepared through simple low temperature solution combustion method and their luminescence properties were studied. The powder X-ray diffraction (XRD) results revealed the formation of highly crystalline Sr2SiO4:Eu3+ nanophosphor having an average particle size 45–60 nm and strain was measured to be (2.4–3.1)  103. The SEM pictures of Sr2SiO4:Eu3+ and Sr2SiO4:Eu3+: M+ showed porous in nature and irregular shaped particles. The phosphors can be effectively excited by the light of 395 nm and show the characteristic emission peaks assigned to transitions of 5D0 ? 4FJ (J = 0, 1, 2, 3, 4). The excitation and emission spectra indicate that the phosphors can be effectively excited by UV (395 nm) light which emits an intense red light peaked at 614 nm. The Eu3+ concentration quenching mechanism was verified to be multipole–multipole interaction and critical energy transfer distance (RC) was 16.43 nm. The effects of different concentration of Eu3+ and charge compensations on luminescence properties were discussed. The highest PL intensity was recorded for 4 mol% Eu3+ ions. Further, Li+ (0.03) has the best charge compensation effect followed by Na+ and K+ respectively. The c-induced studies showed three glow peaks at 211, 163 and 243 (shouldered peaks) respectively at a warming rate of 5 °C s1. The glow peak intensity at 163 °C increases linearly up to 3 kGy and after that it decreases. Up to 3 kGy, the phosphor was quite useful for radiation dosimetry. Ó 2014 Elsevier B.V. All rights reserved.

1. Introduction Presently white light emitting diodes (WLEDs) have attracted great attention in the area of solid state lighting (SSL) due to their superior properties such as environment-friendly, more brightness, fast response, and good reliability. Various approaches have been developed for white light generation using a LED. Among them, the phosphor-based white light generation is highly useful. However, red emitting phosphor used presently in WLEDs has some drawbacks such as low brightness and chemical instability [1,2]. Therefore, development of efficient red phosphor for its use in WLEDs is challenging for material scientists. Most of the phosphor show strong energy absorption in the range of 240–300 nm due to charge transfer band (CTB) and 4fn–4fn1 5d transition of ⇑ Corresponding author. Tel.: +91 9945954010. E-mail address: [email protected] (H. Nagabhushana). http://dx.doi.org/10.1016/j.jallcom.2014.01.094 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.

lanthanide ions [3]. Therefore, it is important to realize high brightness and efficient red phosphors used in the SSL by exciting near UV-LEDs in the range of 380–410 nm. Recent literature showed most of the silicate based phosphor used by conventional solid state technique. Liu et al. [4] studied luminescent properties of Sr2SiO4:Eu3+:M+ (M+ = Li+, Na+, K+) prepared by high temperature solid state method and pure orthorhombic phase was achieved at 1200– 1300 °C for 4 h. Yang et al. [5] studied PL and microstructure of Sr2SiO4:Eu3+ by microwave assisted sintering at 1200 °C for 1 h. Formation of pure Sr2SiO4 without secondary phase was observed. Chen et al. [6] prepared Sr2SiO4:Eu3+ phosphors using 2 wt% NH4Cl as a flux in microwave assisted sintering and observed a single phase of Sr2SiO4 at low temperature of (900 °C) and optimized properties were achieved at 1200 °C. Thermoluminescence is also known as TL studied under excitation with different types of radiations such as X, c, and heavy ions. TL is a well known and simple technique used for the estimation of

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2. Experimental Solution combustion technique was employed for the preparation of Sr2SiO4:Eu3+:M+ (M = Na, K, Li) nanophosphor. The starting materials used for the preparation were of analar grade nitrates of strontium [Sr(NO3)2], europium [Eu(NO3)3], sodium [NaNO3] potassium [KNO3] and lithium [LiNO3]. The fuel used was laboratory-made oxalyl dihydrazide [ODH:C2H6N4O2] [13]. The stiochiometric ratio of nitrates and ODH were taken into the cylindrical petri dish of 300 ml capacity and well dissolved in de-ionized water. The aqueous redox mixture was dispersed uniformly by stirring the mixture using a magnetic stirrer for 10 min. Then the resulting mixture was placed into a muffle furnace maintained at (500 ± 10) °C. At the beginning, the solution was thermally dehydrated and ignited with the liberation of large amount of gases (N2, O2, etc.). The entire process took place within 5 min. After completion of the process, the product obtained was grinded well using mortar and pestle. In addition, the final product was heat treated at 900 °C for 2 h and then used for structural characterization and luminescence studies. The powder X-ray diffraction (XRD) measurements were made using Shimadzu made model-7000 XRD having a high precision vertical h–h goniometer at a wave0 length of 1.54 A Å. The scanning electron microscopy (SEM) pictures were taken using, Hitachi TM 3000. The high resolution transmission electron microscopy (HRTEM) pictures were taken on JEOL JEM 2100. Photoluminescence (PL) measurements were carried out using Horiba Fluoro log-3, modular spectrofluorometer. Thermoluminescence (TL) studies were measured on Nucleonix TLD reader with a heating rate of 5 °C s1. For TL measurements 0.030 mg powder was c-irradiated (60Co source) in the range 1–6 kGy.

Sr 2SiO 4:Eu

3+

5 mol%

Intensity (a.u.)

4 mol%

3 mol%

2 mol%

25

35

40

(105) (201)

Undoped

(123) (222) (204)

(203) (122) (104) (301) (220)

(212)

(211)

30

(013) (020)

(210) (103)(202)

(201) (112)

1 mol%

45

50

31.2

31.5

31.8

2θ (degree) Fig. 1. XRD patterns of Sr2SiO4:Eu3+ (1–5 mol%) nanophosphor annealed at 1000 °C for 2 h.

indicate that all the samples were orthorhombic phase and were well in agreement with standard JCPDS No. 39-1256. Further no additional peaks can be found in doped samples. Since ionic radius of Eu3+ was relatively less when compared to Sr, it was expected that the Eu3+ ion can easily enter the Sr lattice without disturbing the crystal structure. By using Debye–Scherrer’s method [14] and Williamson and Hall plot [15] (Fig. 2) the particle size and strain were estimated.

D¼

kk b cos h

ð1Þ

b cos h ¼ eð4 sin hÞ þ

k D

ð2Þ

where k is the grain constant, k is the wavelength of the X-rays used, h is the Bragg’s angle, D is the particle size, e is the strain and b is the full width at half maxima (FWHM) value of the intense peaks. In addition to this microstrain (e), dislocation density (d) [16] were also estimated by using the relations.

Dislocation densityðdÞ ¼

1

ð3Þ

2

d

5 mol% 0.024

4 mol% 0.020

βcosθ

absorbed dose [7]. Further, several workers have been used to study the electronic process, point defects as well as impurities in the phosphors. In TL, the phosphor irradiated by means of ionizing radiation a part of the energy was absorbed in the phosphors. Depending upon the nature of the excitation-free carriers/lattice defects may be created. Till date, a large number of reports are available for the study of TL in bulk and single crystals form [8,9]. However, a few reports are available on TL studies of nanomaterials. Salah et al. [9] studied TL of CaSO4 system in nanoform by inducing with c and heavy ions and found some interesting characteristic features such as good linearity, low fading, and high efficiency. Further, enhancement of TL intensity and TL glow curve structural modifications in the phosphors were achieved by doping with ions, co-dopants, sensitizers (monovalent/divalent) etc. Prokic [10] has studied the effect of Li co-dopant on TL glow curves of phosphors prepared by chemical method. The addition of Li+ ion greatly enhanced the TL intensity up to 3 times. Various methods such as sol–gel, hydrothermal, co-precipitation have been used for the preparation of silicate phosphors. So far all silicate phosphors have been prepared above 1200 °C for several hours by solid-state method [11]. For this method, high temperature calcinations were required; as a result agglomerated particles were formed. Further, sol–gel method requires longer duration and expensive precursors. Among the various methods of preparation, solution combustion method offers wide range of advantages such as simple experimental setup, high purity products, short reaction time and choice of wide variety of fuels. Further, this method makes use of the heat energy liberated by the redox exothermic reaction at a relatively low igniting temperature between oxidizer and fuels [12]. In the present paper, preparation of Sr2SiO4:Eu3+:M+ (M+ = Li, Na, K) phosphors by simple solution combustion method. Pure orthorhombic Sr2SiO4 phase is achieved at 1000 °C which is less than the calcination temperatures used for SS method. The final product was characterized by using XRD, SEM, TEM, etc. To the best of our knowledge based on the review of literature, no reports were available on the TL studies of Sr2SiO4:Eu3+:M+ phosphors. However a few reports were available on PL studies of Sr2SiO4:Eu3+:M+.

3 mol%

0.016

2 mol%

0.012

1 mol%

0.008

undoped

3. Characterization

0.004

3.1. Powder X-ray diffraction (XRD)

0.9 3+

Fig. 1 shows the XRD patterns of pure and 1–5 mol% Eu doped Sr2SiO4 nanophosphor calcined at 900 °C for 2 h. The XRD patterns

1.0

1.1

1.2

1.3

1.4

1.5

4sinθ Fig. 2. W–H plots of Sr2SiO4:Eu3+ annealed at 1000 °C for 2 h.

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Table 1 Estimated parameters of Sr2SiO4:Eu3+ (0–5 mol%) nanophosphor. Sr2SiO4:Eu3+

Average particle size D (nm)

Microstrain e (103 lin2 m4)

Dislocation density d (1014 lin m2)

Undoped 1 mol% 2 mol% 3 mol% 4 mol% 5 mol%

30.43 28.27 28.00 26.56 25.37 23.04

1.99 2.07 1.91 1.05 1.30 1.50

10.79 12.75 15.53 12.51 14.17 18.83

MicrostrainðeÞ ¼

b  cos hB 4

ð4Þ

where b is the FWHM and d is the average grain size of nanocrystallite and hB is the Bragg’s angle. The average crystallite size estimated by Debye–Scherrer’s method and Williamson and Hall plot for Sr2SiO4:Eu3+ (1– 5 mol%) were found to be 45–60 and 48–65 nm respectively. The estimated strain was found to increase from 2.4  103 to 3.1  103 with increase in Eu3+ concentration. The increase in strain results due to the broadening and shifting of XRD peak with Eu3+ ion concentration is given in Table 1. Further, the strain values were observed to be more in Li, Na and K co-doped phosphors compared to pure and without addition of codopants. The SEM micrographs of Sr2SiO4:Eu3+:M+ co-doped phosphors were shown in Fig. 3(a–d). As clearly seen from the pictures, the product is highly porous and agglomerated. From Fig. 3(e and f) it is observed that the crystallites are of uniform shape and orientations in different directions show the polycrystalline nature of Sr2SiO4:Eu3+ nanophosphor. 3.2. UV–Visible studies Fig. 4 shows the UV–Vis absorption spectra of Eu3+ doped (1– 5 mol%) Sr2SiO4 phosphor calcined at 900 °C for 2 h. A strong absorption band in the range 250–320 nm and the broad band in the range of 340–560 nm are ascribed to the intra configurational 4f–4f transitions from the ground 4F0 level, which corresponds to the excitation spectra. These results are in good agreement with those reported in the literature [17]. The direct optical energy band gap (Eg) of un-doped and Eu3+ doped (2, 5 mol%) Sr2SiO4 were estimated using Tauc relation [18]

aðhmÞ  ðhm  Eg Þ1=k

ð5Þ

where ‘ht’ is the photon energy and ‘a’ is the optical absorption coefficient near the fundamental absorption edge. The absorption coefficients were calculated from these optical absorption spectra. The values of the optical band gaps of the un-doped and Eu3+ doped Sr2SiO4 were obtained by plotting (aE)2 vs ‘E’ in the high-absorption range followed by extrapolating the linear region of the plots to (aE)2 = 0 (Fig. 4). The analysis of the present data showed that the plots give the linear relations, which can be fitted with the above equation with k = 2 for both the un-doped and Eu3+ doped samples. This indicates that allowed direct transitions are responsible for the inter band transitions in the un-doped and the doped samples. The optical energy band gap for the un-doped sample was found to be 5.29 eV and for the Eu3+ doped (2 and 5 mol%) phosphors were 5.35 and 5.46 eV (Fig. 4). It is noticed that the optical band gap is lower in the un-doped sample when compared to the Eu3+-doped Sr2SiO4 nanophosphor. 4. Photoluminescence (PL) studies The excitation spectra of Sr2SiO4:Eu3+ (1–5 mol%) monitored at 612 nm emission was shown in Fig. 5. The excitation spectra

consist of sharp and intense peaks at 360, 380, 395, 416, and 463 nm. These excitation peaks which correspond to 7F0 ? 5D4, 7 F0 ? 5L7, 7F0 ? 5L6, 7F0 ? 5D3 and 7F0 ? 5D2 transitions were attributed to the 4f transitions of Eu3+ ions [19]. The strong and intense absorption peak at 395 nm was located in UV-region along with another peak at 464 nm at blue region whose intensity was less than that of 395 nm. For 395 nm excitation the Eu3+ doped phosphor materials exhibits higher luminescence efficiency, therefore these materials are highly useful for UV-LEDs light systems [20]. Figs. 6 and 7 illustrate the PL spectra of Sr2SiO4:Eu3+ (1–5 mol%) nanophosphor excited at 395 nm and 464 nm respectively. For 395 nm excitation, emission peaks were found at 534, 590, 612, 654 and 705 nm corresponding to 5D0 ? 7FJ (J = 0, 1, 2, 3 and 4) transitions respectively [21]. The strong and highest emission peak at 612 nm was assigned to electric dipole transition which shows that the color purity of the phosphor was very good and Eu3+ ions occupied the lower symmetry site. In addition, as can be seen from Fig. 6 it was observed that red emission 612 nm was stronger than the orange emission (654 nm), indicating that Eu3+ is located in a noncentrosymmetric position of the Sr2SiO4 matrix. Further, positions of fluorescent emission peaks remain the same but the intensity varies with Eu3+ ion concentration. As the Eu3+ concentration increases, the emission intensity also increases up to 4 mol% beyond this critical concentration, the peak intensity gradually decreases (Inset of Fig. 8). The energy transfer mechanism in phosphors is essential to obtain the critical distance (Rc) i.e. the critical separation between Eu3+ and the quenching site. Blesse suggested that Rc can be estimated by the critical concentration of the activator ion (Eu3+) using the relation

 Rc ¼ 2

3V 4pNX c

1=3 ð6Þ

where Xc is the critical concentration of the Eu3+ ion, N is the number of cation in the unit cell and V is the volume of the unit cell. For Sr2SiO4:Eu3+ nanophosphor the values of N, V and Xc were 4, 391 and 4 mol% respectively. Using these parameters, the estimated Rc 0 was found to be 16.71 A Å. According to Blesse theory [22] non-radiative transfer between different Eu3+ ions in Sr2SiO4 phosphor may occur by radiative re-absorption exchange interaction, or multipole–multipole interaction. Usually, radiative re-absorption mechanism comes into effect only when there was a broad overlap of emission peaks of the sensitizer and activator. In the present case radiative–reabsorption is completely ruled out as there were no broad overlapping peaks. The luminescence intensity of the phosphor is strongly influenced by the activator concentration. Therefore, effect of doping concentration is studied to optimize the PL of the Sr2SiO4:Eu3+. Inset of Figs. 8 and 9 show the emission intensity as a function of different Eu3+ concentration. It is found that shape and profile of the emission spectra do not change with varying Eu3+ concentration but PL intensity greatly changes. The PL intensity increases with increasing Eu3+ concentration, reaching a maximum at 4 mol% and then it decreases. Thus the optimum concentration was determined to be 4 mol%. The concentration quenching might be due to

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Fig. 3. SEM micrographs of (a) Sr2SiO4:Eu3+, (b) Sr2SiO4:Eu3+:K+, (c) Sr2SiO4:Eu3+:Na+, (d) Sr2SiO4:Eu3+:Li+, (e) TEM image of Sr2SiO4:Eu3+ and (f) HRTEM of Sr2SiO4:Eu3+nanophosphor annealed at 1000 °C for 2 h.

3+

Sr2SiO4:Eu Undoped

3+

7

5 mol%

(e) (c) (b) (a)

5

F0−> D2

(d)

7

5

F0−> D3

7

5

F0−> L7

7

5

F0−> D4

Eg= 5.535 eV

λemi = 612 nm

(a) 1 mol% (b) 2 mol% (c) 3 mol% (d) 4 mol% (e) 5 mol%

7

2

2 mol%

PL Intensity (a.u.)

(αhν) (eVcm )

-1 2

Absorbance (a.u.)

Sr2SiO4:Eu

5

F0−> L6

Eg= 5.529 eV

Eg= 5.546 eV 360 200 300 400 500 600 700

Wavelength (nm)

3.0

3.5

4.0

4.5

5.0

5.5

6.0

400

440

480

Wavelength (nm)

hν (eV)

Fig. 4. UV–Vis spectra and band gap of Sr2SiO4:Eu3+ (0, 2 and 5 mol%) nanophosphor.

Fig. 5. PL excitation spectra of Sr2SiO4:Eu3+ (1–5 mol%) nanophosphor annealed at 1000 °C for 2 h at 612 nm emission wavelength.

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7

D0−> F2

λexi = 395 nm

5

8.0x10

4.0x10

7

D0−> F1 5

5

7

D0−> F3

7

D0−> F0

5

5

5

7

D0−> F4

)

5

ol %

4

Eu 3+ co n. (m

3

PL Intensity (a.u.)

3+

Sr2SiO4:Eu

PL Intensity (a.u.)

5

2x10

7

1.8x10

7

1.6x10

7

7.50x10

5

5.00x10

5

2.50x10

5

λexi= 464 nm

590 nm 612 nm 643 nm 702 nm

2

1 550

600

650

700

0.00

Wavelength (nm)

1

Fig. 6. PL emission spectra of Sr2SiO4:Eu3+ (1–5 mol%) nanophosphor annealed at 1000 °C for 2 h at a excitation wavelength 395 nm.

2

3

4

5

Eu3+ Concentration (mol%)

622 nm

590 nm

PL Intenstiy (a.u.)

612 nm

λexi = 464 nm

2.0x10 1.5x10

(d)

7

7

(e) (c) (a)

673 nm

(b)

1.0x10

702 nm

5.0x10 595

630

665

7

PL Intensity (a.u.)

3+

Sr2SiO4:Eu

643 nm

Fig. 9. Variation of PL intensity with Eu3+ concentration in Sr 2SiO4 nanophosphor excited at 464 nm.

6

700

720

m .(

on 3+

Eu

680

C

2

640

ol

4 3 1

600

%

643 nm

0.0 5

)

Wavelength (nm)

Wavelength (nm) Fig. 7. PL emission spectra of Sr2SiO4:Eu3+ (1–5 mol%) nanophosphor annealed at 1000 °C for 2 h at a excitation wavelength 464 nm.

8.0 PL Intensity (a.u.)

Slope = -0.7446 Q = 5.5889 7.6

8.0x10

5

4.0x10

5

1 I ¼ K½1 þ bðXÞh=3  X

534 nm 590 nm 612 nm 654 nm 705 nm

1 0.57

2 3 4 3+ Eu Concentration (mol%)

5

I590/I612

log (I/x)

0.0

0.60

0.54

6.4

log 0.51

1

6.0

3+

2

3

4

5

Eu concentration (mol%)

-2.0

-1.8

-1.6

-1.4

ð7Þ

where I is the integral intensity of emission spectra from 550 to 750 nm, X is the activator concentration, I/X is the emission intensity per activator (X), b and K are constants for a given host under same excitation condition. According to the above equation, h = 3 for the energy transfer among the nearest-neighbor ions, while h = 6, 8 and 10 for d–d, d–q and q–q interactions respectively [25]. Assuming that b(X)h/3  1, above reason can be written as:

7.2

6.8

Eu3+ doped Sr2SiO4. This indicates that the mechanism of exchange interaction is ineffective in energy transfer between Eu3+ ions in Sr2SiO4:Eu3+ phosphor. Therefore, multipolar interaction is important to explain the energy transfer mechanism. As Rc of Eu3+–Eu3+ 0 for Sr2SiO4:Eu3+ phosphor was estimated to be 16.71 Å A, the multipolar interaction was the dominant mechanism of concentration quenching. It is well known that the 5D0 ? 7F2 (612 nm) transition of Eu3+ is an electric dipole transition, so the process of energy transfer should be well controlled by electric multipole–multipole interaction according to Dexter’s theory [24]. When electric multipolar interaction is involved in the energy transfer, several types of interaction such as dipole–dipole (d–d), dipole–quadropole (d–q), quadrupole–quadropole (q–q) interactions occur. It is necessary to know the type of interaction involved in the energy transfer. According to Vanuitert’s same sorts of activators the strength of the multipolar interaction can be determined from the change of the emission intensity level which has the multipolar interaction. The emission intensity (I) per activator ion follows the equation:

-1.2

log (x) Fig. 8. Relationship between log (I/x) and log x of Sr2SiO4:Eu3+ nanophosphor. (Inset: variation of PL intensity and symmetry ratio with Eu3+ concentration).

radiative energy transfer from one Eu3+ ion to another Eu3+ ion. Non-radiative energy transfer usually occurs as a result of exchange interaction, radiation reabsorption or multipole–multipole interaction. The mechanism0 of energy interaction was possible only when the typical Rc  5 Å A [23], was far less than the calculated result of

  I h ¼ K 0 log ðXÞ X 3

ð8Þ

where (K0 = logK-logb); from Eq. (8), the multipolar character h can be obtained from the slope (h/3) of the plot log (I/X) vs log (X). From Fig. 8 the slope value obtained was 0.744. The h values can be calculated and found to be around 5.58 which are close to 6. It means that the concentration quenching for the Eu3+ is caused by the energy transfer among the d–d interaction in Sr2SiO4:Eu3+ phosphor. The ratio of the radiative intensities of the 612/596 nm (7F2/7F1) transitions (Inset of Fig. 8), understood to be reliable spectroscopic probe for the degree of symmetry around the Eu3+ ion. This is observed to be around 0.65 and it is comparable with the typical value higher than three characteristic of low symmetry sites. These observations constitute a further demonstration that Eu3+ ions were embedded in their crystalline surroundings.

197

In order to improve the luminescence intensity, different charge compensation (Li+, Na+ and K+) were added to Sr2SiO4:Eu3+ (Fig. 10). As can be seen from Fig. 11, the PL intensity is found to be higher in Li+, Na+ and K+ with Eu3+ when compared to Eu3+ ions. The main reason is the site of the defects are occupied by Li, Na and K ions and changes the symmetry of local environment of Eu3+ ion crystal field. Further, PL intensity is enhanced in Li+ than Na+ and K+ since Li+ has less radius (0.076 nm) than Na+ (0.102 nm) and K+ (0.138 nm) [20]. In Eu3+ doped Sr2SiO4, Eu3+ entered the Sr2SiO4 and substituted for Sr2+ ion, resulting in charge imbalance. Further, this charge imbalance could induce defects in the Sr2SiO4 host and leads to the decrease of PL/TL intensity. The incorporation of Li+, Na+ and K+ might compensate the charge imbalance, reduce the lattice distortion and enhance luminescence intensity without creating any impurities in the host structure. CIE chromaticity diagrams of Sr2SiO4:Eu3+ phosphor excited at 293 and 464 nm are shown in Fig. 12(a and b) and their corresponding chromaticity coordinate values were given in Table 2. As can be seen from Fig. 12 with increase of Eu3+ concentration, red emission dominates. Further, the co-ordinate values for 4 mol% Eu doped in Sr2SiO4 phosphor excited at 464 nm (0.6674, 0.3274), which is close to the NTSC red standard (0.67, 0.33).

PL Intensity (a.u.)

H. Nagabhushana et al. / Journal of Alloys and Compounds 595 (2014) 192–199

1.6x10

6

1.2x10

6

8.0x10

5

4.0x10

5

534 nm 590 nm 612 nm 654 nm 705 nm

0.0 3+ 0.04

Eu

3+ + ;K 0.03 0.04

3+ + ;Na 0.03 0.04

Eu

Eu

3+ + ;Li 0.03 0.04

Eu

Co-dopants Fig. 11. Variation of PL intensity with co-dopants.

0.9

(a) λexi = 395 nm

5. Thermoluminescence (TL) studies

0.6

5

7

D0−> F2

Sr2SiO4 λexi = 393 nm

1.6x10

6

1.2x10

6

8.0x10

5

4.0x10

5

7

5

D0−> F1 5

5

5

7

D0−> F3

7

D0−> F0

7

D0−> F4

3+

PL Intensity (a.u.)

+

Eu :Li

600

640

680

pa n

oC

3+

Eu 560

Y 0.0

0.0

0.2

0.4

0.6

0.8

X 0.9

(b) λexi = 464 nm 0.6

0.3

1- 5 mol%

0.0

0.0

0.2

0.4

0.6

0.8

X Fig. 12. CIE diagram of (a) 395 and (b) 464 nm excited Sr2SiO4:Eu3+ (1–5 mol%) nanophosphor.

+

do

+

0.3

ts

3+

Eu :Na 3+

Eu :K

1- 5 mol%

Y

TL glow spectra of 3 kGy c-irradiated Sr2SiO4:Eu3+ (1–5 mol%) heated at a warming rate of 3 °C s1 was shown in Fig. 13. As can be seen from inset of Fig. 13, the highest TL intensity was observed for 4 mol% Eu3+ doped Sr2SiO4 and decreases with further increase of Eu3+ ion concentration. Fig. 14 shows the c-irradiated (1–6 kGy) Sr2SiO4, with optimized concentration of Eu3+ ions (4 mol%). Three TL glow peaks at 211 (intense), 163 and 243 °C (shouldered) were recorded at a warming rate of 3 °C s1. Variation of TL intensity (Inset of Fig. 14) with c-dose was studied and observed that TL glow peak intensity increases linearly up to 3 kGy for 211 °C and sub linear for 163 and 243 °C peaks. The linear behavior can be explained on the basis of track interaction model (TIM) [26,27]. According to this model, the traps/ luminescent centers created by c-irradiation depends upon the cross section and the track length inside the Sr2SiO4:Eu3+ matrix. The length of the track generated in the nanomaterials is of the order of few tenths of nanometers. At lower c-doses, a few traps/ luminescent centers exist and for higher c-doses some particles that would have been missed while targeted by the c-doses still exist, owning to the small size of the particles. This gives linearity over a wide range of c-dose. Effect of addition of different monova-

720

Wavelength (nm) Fig. 10. PL spectra of Sr2SiO4:Eu3+ nanophosphor co-doped with Li+, Na+ and K+ metal ions.

lent ions (Na, Li, K) on TL properties for the dose of 3 kGy is as shown in Fig. 15. Li doped Sr2SiO4 is found to be higher intensity when compared to Na+ and K+ (Inset of Fig. 15). The characteristics of dosimeters used in TL process were related to kinetic parameters and these parameters qualitatively describe the trapping-emitting centers. Detailed studies of the kinetic

H. Nagabhushana et al. / Journal of Alloys and Compounds 595 (2014) 192–199

Table 2 CIE co-ordinate values for 395 and 464 nm excited Sr2SiO4:Eu3+ nanophosphor.

0.4672 0.4738 0.5007 0.5316 0.5421

0.5056 0.4992 0.4763 0.4502 0.4414

0.6289 0.6357 0.6518 0.6674 0.6722

0.3620 0.3557 0.3410 0.3274 0.3232

5000

4000

+

co-dopants

3000

2000

6000

γ - irradiation (3kGy)

4000

3+

Sr2SiO4:Eu

4800

3000

3600

2000 1000 0

0

1 2 3 4 3+ Eu Concentration (mol%)

2400

5

1200

150

200

)

m .( 3000 2000

4

5

6

γ-dose (kGy)

Experimental curve Fitted curve Deconvoluted peaks

2 1500

1000

1 500

3

0 80

1000

120

160

200

240

Temperature (ºC)

200

250

io n

γ-

150

irr ad

5

ia t

3 4

(k G y)

o

2

6 100

Fig. 16. Deconvoluted glow curve of Sr2SiO4:Eu3+:Li+ nanophosphor.

1

243 C

o

163 C

o

211 C

TL Intensity (a.u.)

TL Intensity (a.u.)

4000

TL Intensity (a.u.)

5000

3+

Sr2SiO4:Eu (4 mol%)

3

240

C

Fig. 13. TL glow curves of Sr2SiO4:Eu3+ (1–5 mol%) nanophosphor irradiated with 3 kGy c-dose at a heating rate of 3 °C s1. (Inset: Variation of TL glow peaks intensity with Eu3+ concentration).

2

200

3+

2000

250

Temperature (ºC)

1

160

Temperature (ºC)

Eu

1 0

120

on

2

o

163 C

80

ol

4 3

100

%

5

o

0

Fig. 15. Effect of Co-dopants on the TL spectra of Sr2SiO4:Eu3+ nanophosphor irradiated with 3 kGy c-dose at a heating rate of 3 °C s1 (Inset: variation of TL intensity with co-dopants).

o

211 C 243 C

1000

TL Intensity (a.u.)

TL Intensity (a.u.)

o

162 C o 211 C o 243 C

5000

Eu:K 3+ Eu

o

Y

Eu:Li + Eu:Na

243 C

X

+

Eu : K 3+

Eu

o

Y

+

+

Eu : Na

211 C

X

+

o

1 2 3 4 5

kexi = 464 nm

3+

163 C

kexi = 395 nm

TL Intensity (a.u.)

Eu3+ conc. (mol%)

3+

Eu : Li 3+

6000 TL Intensity (a.u.)

198

Temperature (ºC) Fig. 14. TL glow curves of Sr2SiO4:Eu3+ (3 mol%) nanophosphor irradiated with crays in the dose range 1–6 kGy. (Inset: Variation of TL intensity with c-dose).

parameters provide valuable information regarding the TL mechanism responsible for the dosimetric applications. A reliable dosimetric study of a TL material should be based on a good knowledge of the kinetic parameters [28]. The evolution of kinetic parameters (E & s) associated with the TL glow peak is one of the important parameters. Any complete description of TL characteristics of the material requires the knowledge of these parameters. TL glow curve analysis leads to the estimation of localized trap depth. The analysis further leads to the estimation of the frequency factor (s), which gives information about the electrons that are released from the trap due to thermal energy, that is, released electrons may get retrapped at the trapping center, which is known as second order kinetics (b = 2). On the other hand, the thermally released electrons may reach the

conduction band without getting retrapped and it is known as first order kinetics (b = 1). Further, it has been observed that the glow curves were more symmetric in nature on a wide temperature range. This is one of the characteristic features of a second order kinetics and it is due to the fact that in second order kinetics, significant concentration of released electrons were retrapped before they recombine with hole centers. This might be the reason for the delay in the luminescence emission and hence spreading out the emission over a wide temperature range [29]. Therefore, the kinetic parameters like activation energy and frequency factor were estimated by Luschiks method [30]. Before estimation of the kinetic parameters, the TL glow curves are deconvoluted as shown in Fig. 16 using ORIGIN 8.1 software [31]. The equations used for estimation were as follow 2

E¼

2kT m d

s¼

    b Tm exp d d

ð9Þ

ð10Þ

where Tm is the maximum glow peak temperature, T1 and T2 are the temperatures corresponding to FWHM, k is the Boltzmann’s

199

H. Nagabhushana et al. / Journal of Alloys and Compounds 595 (2014) 192–199 Table 3 The estimated kinetic parameters of Sr2SiO4:Eu3+ and Li+, K+ and Na+ co-doped nanophosphor irradiated with 3 kGy c-dose at a heating rate of 3 °C s1. Peak

Tm

b (lg)

E

no

s

1 2 3

179 215 239

2(0.48) 2(0.49) 2(0.49)

1.665 1.278 1.383

2.1E+07 8.1E+07 3.9E+06

1.0E+20 2.9E+14 9.2E+15

Sr2SiO4:Eu3+:K+

1 2 3

194 218 245

2(0.48) 2(0.50) 2(0.49)

1.184 1.505 1.487

7.5E+07 1.1E+08 7.9E+06

1.4E+12 6.1E+16 3.1E+15

Sr2SiO4:Eu3+:Na+

1 2 3

194 223 247

2(0.52) 2(0.49) 2(0.48)

1.582 1.578 1.865

1.4E+08 8.9E+07 2.0E+07

3.0E+18 2.4E+17 1.6E+19

Sr2SiO4:Eu3+:Li+

1 2 3

176 211 237

2(0.50) 2(0.49) 2(0.49)

1.922 2.105 1.075

3.6E+07 5.6E+07 2.7E+08

4.9E+17 6.2E+20 5.8E+11

Phosphors Sr2SiO4:Eu

3+

constant, d = T2  Tm is the high temperature half width and b is the heating rate. The kinetic parameters estimated for different co-dopants Sr2SiO4:Eu3+ are given in Table 3. Further, the concentration of trapped charges (no) [32] is determined by using equation

Acknowledgement The authors would like to thank the DST-Nanomission for the financial support. References

x  Im no ¼ bð2:52 þ 10:2ðlg  0:42ÞÞ

ð11Þ

where Im is the glow peak intensity, lg is the asymmetric parameter and x is the shape parameter. From Table 3, it is observed that the trap depth and concentration of the charge carriers is more in Li+ codoped Sr2SiO4:Eu3+ nanophosphors. This may be due to the effect of co-dopant size on the TL properties.

6. Conclusions Sr2SiO4:Eu3+ (1–5 mol%) and Sr2SiO4:Eu3+:M+ (Na+, K+ and Li+) nanophosphors are synthesized by combustion technique. The formation of highly crystalline Sr2SiO4:Eu3+ nanophosphor is confirmed by XRD. The particle size and strain are estimated by Debye Scherrer’s formula and W–H plots. The SEM images of codoped Sr2SiO4:Eu3+ show high porosity. The PL spectra reveal strong red emission under the excitation wavelength of 395 nm. The PL intensity is optimized for Eu3+ concentration of 4 mol%. The concentration quenching is due to multipole–multipole interaction and the critical energy transfer distance was estimated to be 16.71 nm. The PL intensity is enhanced by the addition of codopants due to charge compensation where Li+ codoped Sr2SiO4:Eu3+ showed the maximum PL intensity. Therefore, Sr2SiO4:Eu3+:Li+ is a potential phosphor which can be used in UV LED because of the strong excitation peak 395 nm. TL spectra of 3 kGy c-irradiated Sr2SiO4:Eu3+ nanophosphor showed three glow peaks one main peak at 211 °C and two shouldered peaks at 163 and 243 °C. Here also 4 mol% Eu3+ doped Sr2SiO4 showed maximum TL intensity. The effect of c-dose showed that the TL intensity increased up to 3 kGy and there after it decreases due to non-radiative transition. In case of TL also, the effect of Li+ co-doped Sr2SiO4:Eu3+ showed the maximum TL intensity and also the glow curve is simple with a glow peak at 214 °C. The kinetic parameters are estimated using thermal peak temperature formulae and Kiti’s equation.

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