White light emission and energy transfer (Dy3+ → Eu3+) in combustion synthesized YSO: Dy3+, Eu3+ nanophosphors

Optik 127 (2016) 2939–2945

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White light emission and energy transfer (Dy3+ → Eu3+ ) in combustion synthesized YSO: Dy3+ , Eu3+ nanophosphors G. Ramakrishna a , Ramachandra Naik b , H. Nagabhushana c,∗ , R.B. Basavaraj c , S.C. Prashantha d,∗ , S.C. Sharma e , K.S. Anantharaju f a

Department of Physics, University College of Science, Tumkur University, Tumkur 572 103, India Department of Physics, New Horizon College of Engineering, Bangalore 560103, India c Prof C.N.R. Rao Centre for Advanced Materials, Tumkur University, Tumkur 572 103, India d Research Center, Department of Science, East West Institute of Technology, Bangalore 560091, India e Dayananda Sagar University, Shavige Malleshwara Hills, Kumaraswamy layout, Bangalore 560078, India f Department of Chemistry, Dayand Sagar College of Engineering, Bengaluru, 560078, India, b

a r t i c l e

i n f o

Article history: Received 4 June 2015 Accepted 23 November 2015 Keywords: Y2 SiO5 :Dy3+ :Eu3+ Combustion method Nanophosphor Photoluminescence

a b s t r a c t Tunable white light emissive (YSO) Y2 SiO5 : 5Dy3+ , xEu3+ (x = 0.5 – 4.5 mol%) phosphors were synthesized by the solution combustion method. The 1000 ◦ C phosphors were characterized by powder X-ray diffraction (PXRD), Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM). The average crystallite size estimated by Debye – Scherer’s and Williamson – Hall plots were found to be in the range of 10 – 50 nm. Samples calcined at 1000 ◦ C, show pure monoclinic X1 phase. Co-doping with Eu3+ compensated the red emission component of the YSO:Dy3+ phosphor. Furthermore, the energy transfer from Dy3+ to Eu3+ was confirmed based on the luminescence spectra. The intense white light emissions are suggestive exploration for the potential phosphor for optical materials applications used in the ultraviolet excited white light emitting diodes. © 2015 Elsevier GmbH. All rights reserved.

1. Introduction Currently, in the field of luminescence, light emitting diodes (LEDs) lead over other conventional lighting devices (incandescent lamps, fluorescent lamps, etc.) with its notable features such as low power consumption, long operation lifetimes, low pollution and wide applicability as lighting, displays, etc. Today’s most common method to produce white light was invented by Nichia Chemicals Co. in 1996, in which yellow phosphor (YAG:Ce) was coated on blue chip (InGaN). However, white LEDs made by this method show poor color rendering index (CRI) because of lack of red component. This problem could be overcome either by combining red, green and blue (RGB) LEDs or UV LED chip coated with RGB tricolor phosphor. These methods enable improvement of CRI but at the cost of complicating the fabrication technology (more components of RGB LED, requiring different driving current) or relatively low efficiency in the case of UV-excited tricolor phosphor LEDs. In addition, lack of efficient red emitting phosphor limits its applicability which makes these methods less popular [1–8].

∗ Corresponding author. Tel.: +91 9945954010; fax: +91 9886021344. E-mail addresses: [email protected] (H. Nagabhushana), [email protected] (S.C. Prashantha). http://dx.doi.org/10.1016/j.ijleo.2015.11.234 0030-4026/© 2015 Elsevier GmbH. All rights reserved.

The Dy3+ doped nanophosphors have received lots of attention in white light emitting because Dy3+ ions emit two characteristic bands at 483 (4 F9/2 → 6 H15/2 , blue) and 574 nm (4 F9/2 → 6 H13/2 , yellow) [9,10]. Changing the content of Dy3+ and/or material composition, one can adjust its Y/B luminescence intensity ratio to a perfect value for white light emission [11]. For getting pure white light emission, Dy3+ /Ce4+ [12], Dy3+ /Eu3+ [13] and Dy3+ /Tm3+ [14,15] co-doped systems were investigated. In addition, the energy transfer phenomena between Tb3+ and Dy3+ [16,17], Tm3+ and Dy3+ [15,18] and Eu2+ and Dy3+ [19] were reported. To the best of our knowledge, no reports are available on generation of white light and energy transfer between Dy3+ and Eu3+ ions in Dy3+ /Eu3+ codoped Y2 SiO5 (YSO). Silicate family was an attractive class of materials among inorganic phosphors for wide range of applications due to their special properties such as water, chemical resistance and visible light transparency. Also, they show superior properties due to thermal stability, wide energy band gap, low cost, non-toxicity, chemical resistance, high temperature strength, low thermal expansion and high conductivity, multicolor phosphorescence, high resistance to acid, alkali and oxygen [20]. Among silicate phosphors, YSO doped with rare earth ions were studied extensively in display applications [21]. Synthesis of crystalline YSO phosphor were challenging because of its high crystalline temperature [22]. Typically, solution

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combustion synthesis (SCS) involves a self-sustained reaction in solutions of metal nitrates and different fuels. Further, SCS contribute to the unique properties of the synthesized products. This technique is capable of producing ultra fine powders of silicates in shorter time and at a lower calcination temperature. This process provides molecular level of mixing and high degree of homogeneity, short reaction time that leads to reduction in crystallization temperature and prevents from segregation during heating [23]. In the present work, Dy3+ single doped YSO phosphor and 3+ Dy /Eu3+ co-doped phosphors with various Eu3+ ions concentrations were synthesized by solution combustion method. The energy transfer phenomenon from Dy3+ to Eu3+ in YSO:Dy3+ , Eu3+ phosphors was validated and the energy transfer mechanism was discussed in detailed. 2. Experimental The analytical reagents Yttrium nitrate Y(NO3 )2 H2 O (Sigma Aldrich), Dysprosium nitrate Dy (NO3 )2 H2 O (Sigma Aldrich),the fuel urea NH2 CONH2 (SDFCL) and flux NaCl (SDFCL) were taken as starting materials. Appropriately weighed and thoroughly mixed batches of these raw materials were taken in a petridish. The stoichiometry of composition was calculated based on the total oxidizing synthesis and reducing valences of oxidizer and the fuel [24]. The proper amount of NaCl flux was dissolved in double distilled water and well stirred for 5 – 10 min. The excess water was allowed to evaporate by heating over a hot plate until a wet powder is left out. Then the crystalline Pyrex dish is introduced into a pre-heated muffle furnace maintained at ∼400 ± 10 ◦ C. The reaction mixture undergoes thermal dehydration and ignites at one spot with liberation of gaseous products (nitrogen and carbon). The obtained product show fluffy and voluminous. Two series of powders which have the compositions of Y2 SiO5 : Dy (1 – 9 mol%) and Y2 SiO5 : Dy (5 mol%): Eu (0.5 – 4.5 mol%) were synthesized and the samples were calcined at 1000 ◦ C for 3 h. 2.1. Characterization The Shimadzu powder X-ray diffractometer (operating at 50 KV ˚ radiation with a nickel and 20 mA by means of CuK␣ (1.541 A) filter at a scan rate of 2 ◦ min−1 ) was used to study the phase purity and the crystallinity of the samples. The surface morphology

of the product was examined by Scanning Electron Microscopy (SEM) Hitachi table top, Model TM 3000 (accelerating voltage up to 20 kV using Tungsten filament). The Photoluminescence (PL) measurements were performed on a Horiba, (model fluorolog-3) spectrofluoremeter using 450 W Xenon as excitation source. Fluor EssenceTM software was used for spectral analysis. All the measurements were carried out at room temperature (RT). 3. Results and discussion Fig. 1a and b) shows the PXRD patterns of Y2 SiO5 :Dy3+ (1 – 9 mol%) and Y2 SiO5 : Dy (5 mol%): Eu (0.5 – 4.5 mol%) calcined at 1000 ◦ C for 3 h, synthesized by solution combustion technique using urea and NaCl as fuel and flux respectively. All the X-ray diffraction peaks of the sample show a monoclinic phase with X1 type Y2 SiO5 and was in good agreement with JCPDS card no. 521810 [2] (with phase group P21 /C n.14). The lattice parameters and unit cell volume for monoclinic YSO (X1) was estimated using the following relations. 2dsin  = n dhkl =



(1) 1

h2 a2

+

(2)

k2 b2

+

l2 c2

The estimated values of lattice parameters were found to be 3.78 A˚ and 414.84 A´˚ 3 , respectively. Further, no diffraction peaks from Dy2 O3 /Eu2 O3 or other impurities detected indicating that ions were homogeneously mixed and effectively doped in the host lattice in Y3+ sites. The average crystallite size (D) was estimated from the line broadening in X-ray powder using Scherrer’s formula [25] D=

K ˇ cos 

(3)

where, ‘K’; constant, ‘’; wavelength of X-rays, and ‘ˇ’; FWHM] was found to be in the range 25 – 50 nm. Further, strain present in the YSO:Dy3+ (1 – 9 mol%) and YSO:Dy3+ (5 mol%):Eu (0.5 – 4.5) nanoparticles prepared by combustion method was calculated using the W – H plots. ˇ cos  =

(a)

0.9 + 4ε sin  D 3+

Dy:Eu(5.5 mol %)

3+

Dy:Eu(4.5 mol %)

3+

5 mol % Dy

3+

3 mol % Dy

Intensity (arb.units)

Intensity (arb.units)

7 mol % Dy

Dy:Eu(3.5 mol %)

Dy:Eu(2.5 mol %)

Dy:Eu(1.5 mol %)

3+

1 mol % Dy

Dy:Eu(0.5 mol %)

Undoped 30

40

2θ(degree)

50

60

(b)

(5 mol% Dy )

3+

9 mol % Dy

20

(4)

20

30

40

50

60

2θ(degree)

Fig. 1. (a) PXRD patterns of undoped and (1 – 9 mol%)Dy3+ doped Y2 SiO5. (b) PXRD patterns of 5 mol% Dy3+ :Eu3+ (0.5 – 4.5 mol%) doped Y2 SiO5.

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3+

(b)

(5 mol% Dy )

(a)

3+

9 mol % Dy

Dy:Eu(5.5 mol %)

3+

7 mol % Dy

Dy:Eu(4.5 mol %)

3+

Dy:Eu(3.5 mol %)

β cos θ

β cos θ

5 mol % Dy

3+

3 mol % Dy

Dy:Eu(2.5 mol %)

3+

Dy:Eu(1.5 mol %)

1 mol % Dy

Undoped 0.95

Dy:Eu(0.5 mol %)

1.00

1.05

1.10

1.15

0.8

1.0

1.2

1.4

1.6

4Sin θ

4Sin θ

Fig. 2. (a) W – H plots of un-doped and (1 – 9 mol%)Dy3+ doped Y2 SiO5. (b) W – H plots of 5 mol% Dy3+ : Eu3+ (0.5 – 4.5 mol%) doped Y2 SiO5.

where, ‘ε’; the strain associated with the nanoparticles [26]. The above equation represents a straight line between ‘4sin ’ (x-axis) and ‘ˇcos ’ (y-axis) (shown in Fig. 2a and b). The slope of the line gives the strain and intercept of this line on y-axis gives grain size (D). The detailed values were given in Table 1. The surface morphology of pure, Dy3+ (1 – 9 mol%) and Dy3+ (5 mol%): Eu (0.5 – 4.5) YSO nanophosphors prepared via combustion method was studied (Fig. 3). Combustion derived as formed YSO product showed highly porous, many agglomerates with an irregular morphology, large voids, cracks, pores and shape. Further, the dopant concentration does not influence the morphology of the sample [21]. TEM studies were carried out to understand the crystalline characteristics of the nanoparticles. Fig. 4(a – c) illustrates the TEM, HRTEM and SAED images of 5 mol% Dy3+ doped YSO nanoparticles. The particles were observed to be irregular shaped, highly dispersed and the average size was found to be in the range 20 – 50 nm. The selected area electron diffraction (SAED) image provide distinct ring pattern which was evidence for the polycrystalline behavior of the as-prepared nanoparticles. Further, a well defined lattice fringes obtained by high resolution TEM (HRTEM) pattern reveal the formation of highly crystalline nanophosphor [27]. Fig. 5 shows the excitation spectra of YSO:Dy3+ (5 mol%) and YSO:Dy3+ (5 mol%): Eu3+ (3.5 mol%) nanophosphors with emission wavelength 575 nm. The spectra consists of series of excitation peaks at 388, 423, 451 and 468 nm correspond to the electron transitions of Dy3+ from the ground state 6 H15/2 to higher levels 4 I13/2 , 4G 4 4 11/2 , I15/2 , and F9/2 , respectively [28]. The emission spectra of YSO:Dy3+ (1 – 9 mol%) nanophosphor excited at 388 nm was shown in the Fig. 6. The PL spectra consist of three main peaks in blue 485 nm, yellow 575 nm and red 636 nm

region. Upon 388 nm wavelength excitation, Dy3+ ions jumps resonantly to 6 P7/2 state and quickly releases non-radiatively to 4 F9/2 level. Radiative emission takes place from 4 F9/2 → 6 H15/2 (485 nm, blue), 4 F9/2 → 6 H13/2 (575 nm, yellow) and 4 F9/2 → 6 H11/2 (636 nm, red). The blue emission corresponds to the magnetic dipole transition and the yellow emission belongs to the hypersensitive (forced electric dipole) transition with the selection rule J = 2 [29–31]. It was observed that PL intensity increases up-to 5 mol% Dy3+ ions and above this intensity decreases due to concentration quenching. The increase in PL intensity was due to cross relaxation between Dy3+ ions, consequently with increasing Dy3+ concentration the generation of luminescent centers increases as a result PL intensity enhances. The decrease of PL intensity may be due to creation of point defects due to imbalance charges. The prominent emission peaks at 485, 583 and 612, 651, 688, 702 nm were observed from the PL spectra of YSO: 5 mol% Dy3+ , xEu3+ (x = 0.5, 1.5, 2.5, 3.5, 4.5) phosphors (Fig. 8) excited at 393 nm. The peaks at 485, 583 and 688 nm corresponding to the transitions of Dy3+ ions, and others are assigned to the transitions of Eu3+ ions. It is clear that the introduction of Eu3+ to the YSO:5 mol% Dy3+ added several regions of characteristic emissions located at red region 612 (5 D0 → 7 F2 ), 651 (5 D0 → 7 F3 ) and 702 (5 D0 → 7 F4 ) nm [32]. With increase of Eu3+ ions concentration the cross-relaxation between two neighbouring Eu3+ ions takes place and hence it quenches the emission intensity. The concentration quenching might be explained on the basis of following two factors: (i) the excitation migration due to resonance between the activators was enhanced when the doping concentration was increased, and thus the excitation energy reaches quenching centers; and (ii) the activators were paired or coagulated and were changed to quenching centers [33,34].

Table 1 Estimated crystallite parameters of YSO:Dy3+ :Eu3+ nanophosphor. Dy3+ (mol%)

0 1 3 5 7 9

Crystallite size (nm) Scherrer method

W – H plots

38 39 38 38 37 36

62 59 54 44 36 30

Strain × 10−3

Eu3+ (mol%)

5.46 6.39 4.13 1.34 2.8 3.23

0.5 1.5 2.5 3.5 4.5

Strain × 10−3

Crystallite size (nm) Scherrer method

W – H plots

38 38 38 34 35

73 82 80 69 65

4.13 1.6 2.11 1.11 2.2

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Fig. 3. SEM pictures of un-doped (a) 1 mol% Dy3+ , (b) 5 mol% Dy3+ , (c) 9 mol%, (d) 5 mol% Dy3+ :0.5 Eu3+ , (e) 5 mol% Dy3+ :1.5 Eu3+ , (f) 5 mol% Dy3+ :2.5 Eu3+ , (g) 5 mol% Dy3+ :3.5 Eu3+ , (h) 5 mol% Dy3+ :4.5 Eu3+ and (i) doped Y2 SiO5 .

Fig. 4. (a) TEM, (b) HRTEM, (c) SAED images of (5 mol% Dy3+ :3.5 Eu3+ ) doped Y2 SiO5 .

comparable emission intensities for blue, yellow and red emissions were achieved at 1.5 mol% Eu3+ concentration. Further, it was observed that the emission intensity of 3.5 mol% Eu3+ was greater than that of 5 mol% Dy3+ which clearly explains energy transfer between the activators. Therefore the co-doped phosphors was highly efficient and white light was possible to be combined by

4 6 F9/2−− H13/2 6 4 F9/2−− H11/2

4 6 F9/2−− H15/2

450

500

550

600

650

0.0 (9 mol%) (7 mol%) (5 mol%) (3 mol%) (1 mol%)

PL Intensity(arb.units)

It was observed that the red emission intensity increases with the increase of Eu3+ concentration and reaches a maximum value at Eu3+ concentration of 3.5 mol% accompanied by the intensity decrease of the Dy3+ blue emission after 1.5 mol% of Eu3+ . The

wavelength (nm) Fig. 5. Excitation spectra of (5 mol%) Dy3+ and 5 mol% Dy3+ :Eu3+ (3.5 mol%) doped Y2 SiO5. nanophosphors emission at 575 nm.

Fig. 6. Emission spectra of Y2 SiO5 : Dy3+ (1 – 9 mol%) excited at 388 nm.

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properly designing the doping concentrations of Eu3+ and Dy3+ [35]. 3.1. Energy transfer between Dy3+ and Eu3+ in Dy3+ /Eu3+ co-doped YSO phosphors To examine the energy transfer mechanism from Dy3+ to Eu3+ in YSO, the concentration of the Dy3+ was fixed at the optimum doping of 5 mol% with a changing amount of Eu3+ . It was obvious that the emission intensity of Dy3+ decreases gradually with the increasing Eu3+ concentration. In the interim, the characteristic emission of Eu3+ ions increases gradually, which further supports the occurrence of the energy transfer from Dy3+ to Eu3+ in YSO host. The equivalent energy levels diagram and the probable optical transition which was involved in the energy transfer processes from Dy3+ to Eu3+ was described in Fig. 7. Upon excitation at 393 nm, the photon energy was absorbed by the host and was transferred to 4f shell of Dy3+ and Eu3+ . Thus, the electrons of Dy3+ at 4 F9/2 and Eu3+ at 5 D excited state populates both from the nonradiative charge feed0 ing and nonradiative transitions from higher excited states, then emit their characteristic light, respectively. On the other hand, the photon energy was absorbed by Dy3+ ions inducing the transitions from ground state to the metastable states that were low, and part of the excited electrons in these metastable excited states were repopulated into 4 F9/2 level by multiphonon assisted nonradiative transitions [35]. The transitions from 4 F9/2 to 6 Hj (j = 15/2, 13/2, 11/2) were radiative and produce the emission lines of 485 nm and 583 nm, respectively. Huang and Lou et al. [36] proposed that concentration quenching in many cases was due to the energy transfer from one activator to another until an energy drop in the lattice was reached. As suggested by Blasse, the average separation RDy − Eu can be expressed by [37] (Fig. 8)

 3V 1/3

RDy−Eu ≈ 2

4Xc N

(5)

where, N; the number of Z ions in the unit cell, and V; the volume of the unit cell. For YSO host, N = 4 and V = 289 A˚ 3 , Xc; the total concentration of the doping content of the Dy3+ and Eu3+ . Thus, the critical concentration RDy − Eu (Rc) was determined to be 15.76 A˚ when the 612 nm emission reaches to the maximum value at the Eu3+ doping concentration of 3.5% hence, the mechanism of exchange interaction was ineffective. As a result, the process of energy transfer of would be due to electric multipolar interaction. Multipolar interaction involves several types of interaction such as dipole – dipole (d – d), dipole – quadropole (d – q), quadropole – quadropole (q – q)

Fig. 7. Partial energy level diagram of Dy3+ and Eu3+ ions.

Fig. 8. Emission spectra of 5 mol% Dy3+ : Eu3+ (0.5 – 4.5 mol%) excited at 393 nm.

interaction. In order to determine the type of interaction involved in the energy transfer Van-Uitert’s [38] proposed an equation:



I = k 1 + ˇ(X)Q/3 X

−1

(6)

where, I; the integral intensity of emission spectra from 450 to 750 nm, X; the activator concentration, I/X; the emission intensity per activator (X), ˇ and K; constants for a given host under same excitation condition. According to above equation,  = 3 for the energy transfer among the nearest neighbor ions, while  = 6, 8 and 10 for d – d, d – q and q – q interactions, respectively [39,40]. Assuming that ˇ(X)/3 > >1, above equation can be written as log

I X

= K −

 log X(K  = log K − log ˇ) 3

(7)

From Eq. (6), the multipolar character () can be obtained by plot log (I/X) vs log (X) as shown in Fig. 9a and b. The multipolar character  for 5 mol% Dy3+ : Eu3+ (0.5 – 4.5 mol%) and Y2 SiO5 : Dy3+ (1 – 9 mol%) was found to be ∼7.016 and 7.30 which was close to 8. Therefore, the in both the cases mechanism of interaction was due to dipole – quadrupole interaction. The Commission International De I-Eclairage (CIE) chromaticity coordinates [34] for YSO:Dy3+ (1 – 9 mol%) and YSO:Dy3+ (5 mol%): Eu3+ (0.5 – 4.5 mol%) phosphors was calculated as a function of Dy3+ concentration (Fig. 10a and b). The CIE coordinates of white emission of Dy3+ ions not only depend upon the asymmetric ratio but also on the higher energy emission levels. It was observed that in both the cases the CIE co-ordinates falls in white region which clearly represents both the phosphors can be used for white light

Fig. 9. (a) Relation between log(x) and log (I/x) in Y2 SiO5 : 5 mol% Dy3+ :Eu3+ (0.5 – 4.5 mol%) nanophosphors. (b) Relation between log(x) and log (I/x) in Y2 SiO5 : Dy3+ (1 – 9 mol%) nanophosphors.

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Fig. 10. (a) CIE diagram of Y2 SiO5 :Dy3+ (1 – 9 mol%) nanophosphor. [Inset (x,y) co-ordinate values]. (b) CIE diagram of 5 mol% Dy3+ : Eu3+ (0.5 – 4.5 mol%) nanophosphor. [Inset (x,y) co-ordinate values].

Fig. 11. (a) CCT diagram of Y2 SiO5 :Dy3+ (1 – 9 mol%) nanophosphor. (b) CCT diagram of 5 mol% Dy3+ : Eu3+ (0.5 – 4.5 mol%) nanophosphor.

emitting diodes (WLEDs) and solid state lighting devices. Correlated color temperature (CCT) can be estimated by Planckian locus, which was only a small portion of the (x, y) chromaticity diagram and there exist many operating points outside the Planckian locus. If the coordinates of a light source do not fall on the Planckian locus, CCT was used to define the color temperature of a light source. CCT was obtained by transforming the (x, y) coordinates of the light source to (U , V ) by using following equations, and by determining the temperature of the closest point of the Planckian locus to the light source on the (U , V ) uniform chromaticity diagram (Fig. 11a and b)) [41,42]. U =

4x −2x + 12y + 3

(8)

V =

9y −2x + 12y + 3

(9)

Fig. 11a and b shows the CCT diagram of YSO:Dy3+ (1 – 9 mol%) and YSO:Dy3+ (5 mol%):Eu3+ (0.5 – 4.5 mol%) nanophosphors. The average CCT of 5740 K was observed in YSO:Dy3+ (1 – 9 mol%) and 4308 K in YSO:Dy3+ (5 mol%):Eu3+ (0.5 – 4.5 mol%) nanophosphors. Further, it was observed that in both the cases coordinates falls on the Planckian locus. The average CCT 4308 K of YSO:Dy3+ (5 mol%):Eu3+ (0.5 – 4.5 mol%) nanophosphors indicates warm white light emission can be achieved and phosphors could be potentially used in white light emission displays and near ultraviolet pumped white LEDs [43]. 4. Conclusions Single-phased white light emitting phosphors YSO: Dy3+ and YSO: 5 mol% Dy3+ , xEu3+ were prepared by the solution combustion method. Further, the calcinations temperature of YSO was less

(1000 ◦ C) when compared to solid state route. PXRD results of calcined samples at 1000 ◦ C show pure monoclinic X1 phase. It was found that doping Eu3+ can enhance the red emission of the phosphors. The energy transfer from Dy3+ to Eu3+ was observed through the combination of the PLE and PL spectra. The CIE chromaticity coordinates for both doped and codoped phosphors suggested that the emission color of the obtained phosphors can be tunable from white to reddish by simply adjusting the amount of Eu3+ ions which indicates that the phosphors could be potentially used in white light emission displays and near ultraviolet pumped white LEDs. Acknowledgments Authors are thankful to the Department of Science and Technology (DST), New Delhi, India for providing financial assistance under Project No. SR/NM/NS-48/2010. The authors RN and SCP thanks to VGST, Govt. of Karnataka, India (VGST/K-FIST-L1/2015-16/GRD-360) for sanctioning the research project. References [1] K.V. Dabre, S.J. Dhoble, J. Lumin. 150 (2014) 55–58. [2] G. Ramakrishna, H. Nagabhushana, D.V. Sunitha, S.C. Prashantha, S.C. Sharma, B.M. Nagabhushana, Spectr. Acta Part A: Molec. Biomol. Spectr. 127 (2014) 177–184. [3] B.M. Manohara, H. Nagabhushana, D.V. Sunitha, K. Thyagarajan, B. Daruka Prasad, S.C. Sharma, B.M. Nagabhushana, R.P.S. Chakradhar, J. Alloy Compd. 592 (2014) 319–327. [4] S.A. Hassanzadeh-Tabrizi, E. Taheri-Nassaj, Ceram. Inter. 39 (2013) 6313–6317. [5] B.N. Lakshminarasappa, S.C. Prashantha, Fouran Singh, Curr. Appl. Phys. 11 (2011) 1274–1277. [6] D.V. Sunitha, C. Manjunatha, C.J. Shilpa, H. Nagabhushana, S.C. Sharma, B.M. Nagabhushana, N. Dhananjaya, C. Shivakumara, R.P.S. Chakradhar, Spectr. Acta Part A: Molec. Biomol. Spectr. 99 (2012) 279–287.

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